Communication and Networking





Definition of Network:
A computer network is a set of computers or devices that are connected with each other to carry on data and share information. In computing, it is called a network as a way to interconnect two or more devices to each other using cables, signals, waves or other methods with the ultimate goal of transmitting data, share information, resources and services.


 
Purpose of networking:
The purpose of a network is, generally, to facilitate and expedite communications between two or more instances on the same physical space or connected remotely. Such systems also allow cost savings and time.
The most known type of network is the Intranet, which is a private network that uses Internet as a basic architecture in order to connect various devices. Internet, however, is a technology that connects devices throughout the world, and that is why it is called “network of networks.”

Classifications of Networks:
The networks are classified by range (personal, local, campus, metropolitan or wide area), as well as by method of connection (cable, fiber optics, radio, infrared, wireless, etc..) or by functional relationship (client – server or peer-to-peer). Also in the topology field there is a classification to be aware of (bus, star, ring, mesh, tree etc.) and directional (simplex, half duplex or full duplex).

Use of a network:
The use of a network in an office, for example, in which all employees have the same access to resources such as programs and applications or devices like a printer or scanner. Moreover, configuring a large-scale network facilitates communication among different geographic locations, so a company with multiple branches in the world can keep in communication with its members in a simple and quick. Finally, a network can be used as a home to share files or maximize the available space.

Analog Network Signaling:
An analog signal is best compared to a wave.  It has similar properties to an ocean wave, and can be described using three specific characteristics: amplitude, frequency, and wavelength.
To use the ocean wave analogy an analog signal's amplitude is like the height of a wave rolling in onto the beach. The frequency of an analog signal can be compared to how fast the waves roll in.  Wavelength can be compared to the distance between one wave and the next wave.  Wavelength is measured as the distance between the peak of one wave and the next.


Advantages and Disadvantages of Analog Signals
Analog signals are variable and can convey more subtly than a digital signal.  For example the human voice is analog, and has more tone than a digital representation of the same voice.  However, analog signals are very vulnerable to interference from outside forces and other waves which can cancel them out.

Digital Network Signaling:
A digital signal is made up of on/off states.  Unlike the smooth curve of an analog wave, the digital signal cuts on and off.  This happens to perfectly fit the type of communication inside a computer, which is made up of on/off states as well.

Advantages and Disadvantages of Digital Signals:
Digital signals are much more reliable than analog signals because they are less vulnerable to interference and errors. However, digital equipment costs more and is much more complex.


Modulation (AM, FM, PM)
In telecommunications, modulation is the process of conveying a message signal, for example a digital bit stream or an analog audio signal, inside another signal that can be physically transmitted. Modulation of a sine waveform is used to transform a baseband message signal into a pass band signal, for example low-frequency audio signal into a radio-frequency signal (RF signal). In radio communications, cable TV systems or the public switched telephone network for instance, electrical signals can only be transferred over a limited pass band frequency spectrum, with specific (non-zero) lower and upper cutoff frequencies. Modulating a sine-wave carrier makes it possible to keep the frequency content of the transferred signal as close as possible to the centre frequency (typically the carrier frequency) of the pass band.
A device that performs modulation is known as a modulator and a device that performs the inverse operation of modulation is known as a demodulator (sometimes detector or demod). A device that can do both operations is a modem (modulator–demodulator).

Amplitude Modulation (AM)
Amplitude modulation (AM) is a method of impressing data onto an alternating-current (AC) carrier waveform. The highest frequency of the modulating data is normally less than 10 percent of the carrier frequency. The instantaneous amplitude(overall signal power) varies depending on the instantaneous amplitude of the modulating data. In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of signal components at frequencies slightly higher and lower than that of the carrier. These components are called sidebands. The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB) appears at frequencies above the carrier frequency. The LSB and USB are essentially "mirror images" of each other in a graph of signal amplitude versus frequency, as shown in the illustration. The sideband power accounts for the  variations in the overall amplitude of the signal.
When a carrier is amplitude-modulated with a pure sine wave, up to 1/3 (33percent) of the overall signal power is contained in the sidebands. The other 2/3 of the signal power is contained in the carrier, which does not contribute to the transfer of data. With a complex modulating signal such as voice, video, or music, the sidebands generally contain 20 to 25 percent of the overall signal power; thus the carrier consumes75 to 80 percent of the power. This makes AM an inefficient mode. If an attempt is made to increase the modulating data input amplitude beyond these limits, the signal will become distorted, and will occupy a much greater bandwidth than it should. This is called over modulation, and can result in interference to signals on nearby frequencies.

Frequency Modulation (FM)
Frequency modulation (FM) is a method of impressing data onto an alternating-current (AC) wave by varying the instantaneous (immediate) frequency of the wave. This scheme can be used with analog or digital data.
Analog FM
In analog FM, the frequency of the AC signal wave, also called the carrier, varies in a continuous manner. Thus, there are infinitely many possible carrier frequencies. In narrowband FM, commonly used in two-way wireless communications, the instantaneous carrier frequency varies by up to 5 kilohertz (kHz, where 1 kHz = 1000 hertz or alternating cycles per second) above and below the frequency of the carrier with no modulation. In wideband FM, used in wireless broadcasting, the instantaneous frequency varies by up to several megahertz (MHz, where 1 MHz = 1,000,000 Hz). When the instantaneous input wave has positive polarity, the carrier frequency shifts in one direction; when the instantaneous input wave has negative polarity, the carrier frequency shifts in the opposite direction. At every instant in time, the extent of carrier-frequency shift (the deviation) is directly proportional to the extent to which the signal amplitude is positive or negative.
Digital FM
In digital FM, the carrier frequency shifts abruptly, rather than varying continuously. The number of possible carrier frequency states is usually a power of 2. If there are only two possible frequency states, the mode is called frequency-shift keying (FSK). In more complex modes, there can be four, eight, or more different frequency states. Each specific carrier frequency represents a specific digital input data state.

Phase Modulation (PM):
Phase modulation (PM) is a method of impressing data onto an alternating-current (AC) waveform by varying the instantaneous phase of the wave. This scheme can be used with analog or digital data.
Analog PM, The phase of the AC signal wave, also called the carrier, varies in a continuous manner. Thus, there are infinitely many possible carrier phase states. When the instantaneous data input waveform has positive polarity, the carrier phase shifts in one direction; when the instantaneous data input waveform has negative polarity, the carrier phase shifts in the opposite direction. At every instant in time, the extent of carrier-phase shift(the phase angle) is directly proportional to the extent to which the signal amplitude is positive or negative.
Digital PM
In digital PM, the carrier phase shifts abruptly, rather than continuously back and forth. The number of possible carrier phase states is usually a power of2. If there are only two possible phase states, the mode is called biphase modulation. In more complex modes, there can be four, eight, or more different phase states. Each phase angle (that is, each shift from one phase state to another)represents a specific digital input data state.
Phase modulation is similar in practice to frequency modulation (FM). When the instantaneous phase of a carrier is varied, the instantaneous frequency changes as well. The converse also holds: When the instantaneous frequency is varied, the instantaneous phase changes. But PM and FM are not exactly equivalent, especially in analog applications. When an FM receiver is used to demodulate a PM signal, or when FM signal is intercepted by a receiver designed for PM, the audio is distorted. This is because the relationship between phase and frequency variations is not linear; that is, phase and frequency do not vary in direct proportion.


Direction of communication flow(Simplex, Halfduplex, FullDuplex)

In data communications, flow control is the process of managing the pacing of data transmission between two nodes to prevent a fast sender from outrunning a slow receiver. It provides a mechanism for the receiver to control the transmission speed, so that the receiving node is not overwhelmed with data from transmitting node. Flow control should be distinguished from congestion control, which is used for controlling the flow of data when congestion has actually occurred. Flow control mechanisms can be classified by whether or not the receiving node sends feedback to the sending node.
Flow control is important because it is possible for a sending computer to transmit information at a faster rate than the destination computer can receive and process them. This can happen if the receiving computers have a heavy traffic load in comparison to the sending computer, or if the receiving computer has less processing power than the sending computer.

Data flow is the flow of data between two points. The direction of the data flow can be described as:

Simplex:
Data flows in only one direction on the data communication line (medium). Examples are radio and television broadcasts. They go from the TV station to your home television.

HALF-DUPLEX
Half-Duplex: data flows in both directions but only one direction at a time on the data communication line. For example, a conversation on walkie-talkies is a half-duplex data flow. Each person takes turns talking. If both talk at once - nothing occurs!
Bi-directional but only 1 direction at a time!


Full-Duplex:
Data flows in both directions simultaneously. Modems are configured to flow data in both directions.
Bi-directional both directions simultaneously!



Simplex vs. Duplex
SIMPLEX
Simplex communication is permanent unidirectional communication. Some of the very first serial connections between computers were simplex connections. For example, mainframes sent data to a printer and never checked to see if the printer was available or if the document printed properly since that was a human job. Simplex links are built so that the transmitter (the one talking) sends a signal and it's up to the receiving device (the listener) to figure out what was sent and to correctly do what it was told. No traffic is possible in the other direction across the same connection.
You must use connectionless protocols with simplex circuits as no acknowledgement or return traffic is possible over a simplex circuit. Satellite communication is also simplex communication. A radio signal is transmitted and it is up to the receiver to correctly determine what message has been sent and whether it arrived intact. Since televisions don't talk back to the satellites (yet), simplex communication works great in broadcast media such as radio, television and public announcement systems.

HALF DUPLEX
A half-duplex link can communicate in only one direction, at a time. Two way communication is possible, but not simultaneously. Walkie-talkies and CB radios sort of mimic this behavior in that you cannot hear the other person if you are talking. Half-duplex connections are more common over electrical links. Since electricity won't flow unless you have a complete loop of wire, you need two pieces of wire between the two systems to form the loop. The first wire is used to transmit, the second wire is referred to as a common ground. Thus, the flow of electricity can be reversed over the transmitting wire, thereby reversing the path of communication. Electricity cannot flow in both directions simultaneously, so the link is half-duplex.

FULL DUPLEX
Full duplex communication is two-way communication achieved over a physical link that has the ability to communicate in both directions simultaneously. With most electrical, fiber optic, two-way radio and satellite links, this is usually achieved with more than one physical connection. Your telephone line contains two wires, one for transmit, the other for receive. This means you and your friend can both talk and listen at the same time.
Half or Full-Duplex is required for connection-oriented protocols such as TCP. A duplex circuit can be created by using two separate physical connections running in half duplex mode or simplex mode. Two way satellite communications is achieved using two simplex connections.


Types of Network
The types of network are categorized on the basis of the number of systems or devices that are under the networked area. Computer Networking is one of the most important wings of computing. Networking is the process by which two or more computers are linked together for a flawless communication. By creating a network, devices like printers and scanners, software, and files and data that are stored in the system can be shared. It helps the communication among multiple computers easy. By computer networking the user access may be restricted when necessary.
There three types of networks:

Local Area Network (LAN):
A local area network (LAN) is a computer network that connects computers and devices in a limited geographical area such as home, school, computer laboratory or office building. The defining characteristics of LANs, in contrast to wide area networks (WANs), include their usually higher data-transfer rates, smaller geographic area, and lack of a need for leased telecommunication lines. The Local Area Network is also referred as LAN. This system spans on a small area like a small office or home. The computer systems are linked with cables. In LAN system computers on the same site could be linked.

Wide Area Network (WAN):
A wide area network (WAN) is a computer network that covers a broad area (i.e., any network whose communications links cross metropolitan, regional, or national boundaries). This is in contrast with personal area networks (PANs), local area networks (LANs), campus area networks (CANs), or metropolitan area networks (MANs) which are usually limited to a room, building, campus or specific metropolitan area (e.g., a city) respectively. A Wide Area Network or WAN is a type of networking where a number of resources are installed across a large area such as multinational business. Through WAN offices in different countries can be interconnected. The best example of a WAN could be the Internet that is the largest network in the world. In WAN computer systems on different sites can be linked.

Metropolitan area network (MAN):
A metropolitan area network (MAN) is a computer network that usually spans a city or a large campus. A MAN usually interconnects a number of local area networks (LANs) using a high-capacity backbone technology, such as fiber-optical links, and provides up-link services to wide area networks (or WAN) and the Internet.
The IEEE 802-2002 standard describes a MAN as being:
                A MAN is optimized for a larger geographical area than a LAN, ranging from several blocks of buildings to entire cities. MANs can also depend on communications channels of moderate-to-high data rates. A MAN might be owned and operated by a single organization, but it usually will be used by many individuals and organizations.  MANs might also be owned and operated as public utilities. They will often provide means for internetworking of local networks.

The types of networks can be further classified into two more divisions:
Peer to peer Network:
Peer to peer is an approach to computer networking where all computers share equivalent responsibility for processing data. Peer-to-peer networking (also known simply as peer networking) differs from client-server networking, where certain devices have responsibility for providing or "serving" data and other devices
Consume or otherwise act as "clients" of those servers.


Characteristics of a Peer Network:
Peer to peer networking is common on small local area networks (LANs), particularly home networks. Both wired and wireless home networks can be configured as peer to peer environments.
Computers in a peer to peer network run the same networking protocols and software. Peer networks are also often situated physically near to each other, typically in homes, small businesses or schools. Some peer networks, however, utilize the Internet and are geographically dispersed worldwide.
Home networks that utilize broadband routers are hybrid peer to peer and client-server environments. The router provides centralized Internet connection sharing, but file, printer and other resource sharing is managed directly between the local computers involved.

Peer to Peer and P2P Networks:
Internet-based peer to peer networks emerged in the 1990s due to the development of P2P file sharing networks like Napster. Technically, many P2P networks (including the original Napster) are not pure peer networks but rather hybrid designs as they utilize central servers for some functions such as search.

Peer to Peer and Ad Hoc Wi-Fi Networks
Wi-Fi wireless networks support so-called ad hoc connections between devices. Ad hoc Wi-Fi networks are pure peer to peer compared to those utilizing wireless routers as an intermediate device.
Benefits of a Peer to Peer Network:
You can configure computers in peer to peer workgroups to allow sharing of files, printers and other resources across all of the devices. Peer networks allow data to be shared easily in both directions, whether for downloads to your computer or uploads from your computer.
On the Internet, peer to peer networks handle a very high volume of file sharing traffic by distributing the load across many computers. Because they do not rely exclusively on central servers, P2P networks both scale better and are more resilient than client-server networks in case of failures or traffic bottlenecks.

Client Server Networks
The term client-server refers to a popular model for computer networking that utilizes client and server devices each designed for specific purposes. The client-server model can be used on the Internet as well as local area networks (LANs). Examples of client-server systems on the Internet include Web browsers and Web servers, FTP clients and servers, and DNS.

Client and Server Devices:
Client/server networking grew in popularity many years ago as personal computers (PCs) became the common alternative to older mainframe computers. Client devices are typically PCs with network software applications installed that request and receive information over the network. Mobile devices as well as desktop computers can both function as clients. A server device typically stores files and databases including more complex applications like Web sites. Server devices often feature higher-powered central processors, more memory, and larger disk drives than clients.

Client-Server Applications:
The client-server model distinguishes between applications as well as devices. Network clients make requests to a server by sending messages, and servers respond to their clients by acting on each request and returning results. One server generally supports numerous clients, and multiple servers can be networked together in a pool to handle the increased processing load as the number of clients grows.
A client computer and a server computer are usually two separate devices, each customized for their designed purpose. For example, a Web client works best with a large screen display, while a Web server does not need any display at all and can be located anywhere in the world. However, in some cases a given device can function both as a client and a server for the same application. Likewise, a device that is a server for one application can simultaneously act as a client to other servers, for different applications.
[Some of the most popular applications on the Internet follow the client-server model including email, FTP and Web services. Each of these clients features a user interface (either graphic- or text-based) and a client application that allows the user to connect to servers. In the case of email and FTP, users enter a computer name (or sometimes an IP address) into the interface to set up connections to the server.

Local Client-Server Networks:
Many home networks utilize client-server systems without even realizing it. Broadband routers, for example, contain DHCP servers that provide IP addresses to the home computers (DHCP clients). Other types of network servers found in home include print servers and backup servers.

Client-Server vs Peer-to-Peer and Other Models:
The client-server model was originally developed to allow more users to share access to database applications. Compared to the mainframe approach, client-server offers improved scalability because connections can be made as needed rather than being fixed. The client-server model also supports modular applications that can make the job of creating software easier. In so-called "two-tier" and "three-tier" types of client-server systems, software applications are separated into modular pieces, and each piece is installed on clients or servers specialized for that subsystem.
Client-server is just one approach to managing network applications The primary alternative, peer-to-peer networking, models all devices as having equivalent capability rather than specialized client or server roles. Compared to client-server, peer to peer networks offer some advantages such as more flexibility in growing the system to handle large number of clients. Client-server networks generally offer advantages in keeping data secure.



Network topology
Network topology is the layout pattern of interconnections of the various elements (links, nodes, etc.) of a computer or biological network. Network topologies may be physical or logical. Physical topology refers to the physical design of a network including the devices, location and cable installation. Logical topology refers to how data is actually transferred in a network as opposed to its physical design. In general physical topology relates to a core network whereas logical topology relates to basic network.

Topology can be understood as the shape or structure of a network. This shape does not necessarily correspond to the actual physical design of the devices on the computer network. The computers on a home network can be arranged in a circle but it does not necessarily mean that it represents a ring topology.
Any particular network topology is determined only by the graphical mapping of the configuration of physical and/or logical connections between nodes. The study of network topology uses graph theory. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ in two networks and yet their topologies may be identical.

A local area network (LAN) is one example of a network that exhibits both a physical topology and a logical topology. Any given node in the LAN has one or more links to one or more nodes in the network and the mapping of these links and nodes in a graph results in a geometric shape that may be used to describe the physical topology of the network. Likewise, the mapping of the data flow between the nodes in the network determines the logical topology of the network. The physical and logical topologies may or may not be identical in any particular network.



Bus Topology:
In local area networks where bus topology is used, each node is connected to a single cable. Each computer or server is connected to the single bus cable. A signal from the source travels in both directions to all machines connected on the bus cable until it finds the intended recipient. If the machine address does not match the intended address for the data, the machine ignores the data. Alternatively, if the data does match the machine address, the data is accepted. Since the bus topology consists of only one wire, it is rather inexpensive to implement when compared to other topologies. However, the low cost of implementing the technology is offset by the high cost of managing the network. Additionally, since only one cable is utilized, it can be the single point of failure. If the network cable breaks, the entire network will be down.

Advantages and disadvantages of a bus network:

Advantages
·         Easy to implement and extend.
·         Easy to install.
·         Well-suited for temporary or small networks not requiring high speeds (quick setup), resulting in faster networks.
·         less expensive than other topologies (But in recent years has become less important due to devices like a switch)
·         Cost effective; only a single cable is used.
·         Easy identification of cable faults.

Disadvantages
·         Limited cable length and number of stations.
·         If there is a problem with the cable, the entire network breaks down.
·         Maintenance costs may be higher in the long run.
·         Performance degrades as additional computers are added or on heavy traffic (shared bandwidth).
·         Proper termination is required (loop must be in closed path).
·         Significant Capacitive Load (each bus transaction must be able to stretch to most distant link).
·         It works best with limited number of nodes.
·         Commonly has a slower data transfer rate than other topologies.
·         Only one packet can remain on the bus during one clock pulse

Star Topology:
Star networks are one of the most common computer network topologies. In its simplest form, a star network consists of one central switch, hub or computer, which acts as a conduit to transmit messages. This consists of a central node, to which all other nodes are connected; this central node provides a common connection point for all nodes through a hub.  Thus, the hub and leaf nodes, and the transmission lines between them, form a graph with the topology of a star. If the central node is passive, the originating node must be able to tolerate the reception of an echo of its own transmission, delayed by the two-way transmission time (i.e. to and from the central node) plus any delay generated in the central node. An active star network has an active central node that usually has the means to prevent echo-related problems.
The star topology reduces the chance of network failure by connecting all of the systems to a central node. When applied to a bus-based network, this central hub rebroadcasts all transmissions received from any peripheral node to all peripheral nodes on the network, sometimes including the originating node. All peripheral nodes may thus communicate with all others by transmitting to, and receiving from, the central node only. The failure of a transmission line linking any peripheral node to the central node will result in the isolation of that peripheral node from all others, but the rest of the systems will be unaffected.
It is also designed with each node (file servers, workstations, and peripherals) connected directly to a central network hub, switch, or concentrator.
Data on a star network passes through the hub, switch, or concentrator before continuing to its destination. The hub, switch, or concentrator manages and controls all functions of the network. It is also acts as a repeater for the data flow. This configuration is common with twisted pair cable. However, it can also be used with coaxial cable or optical fiber cable.


Advantages
·         Better performance: star topology prevents the passing of data packets through an excessive number of nodes. At most, 3 devices and 2 links are involved in any communication between any two devices. Although this topology places a huge overhead on the central hub, with adequate capacity, the hub can handle very high utilization by one device without affecting others.
·         Isolation of devices: Each device is inherently isolated by the link that connects it to the hub. This makes the isolation of individual devices straightforward and amounts to disconnecting each device from the others. This isolation also prevents any non-centralized failure from affecting the network.
·         Benefits from centralization: As the central hub is the bottleneck, increasing its capacity, or connecting additional devices to it, increases the size of the network very easily. Centralization also allows the inspection of traffic through the network. This facilitates analysis of the traffic and detection of suspicious behavior.
·         Easy to detect faults and to remove parts.
·         No disruptions to the network when connecting or removing devices.

Disadvantages
·         High dependence of the system on the functioning of the central hub
·         Failure of the central hub renders the network inoperable


Ring Topology:
A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node - a ring. Data travels from node to node, with each node along the way handling every packet. Because a ring topology provides only one pathway between any two nodes, ring networks may be disrupted by the failure of a single link.[1] A node failure or cable break might isolate every node attached to the ring.
Fiber Distributed Data Interface(FDDI) networks overcome this vulnerability by sending data on a clockwise and a counterclockwise ring: in the event of a break data is wrapped back onto the complementary ring before it reaches the end of the cable, maintaining a path to every node along the resulting "C-Ring". Many ring networks add a "counter-rotating ring" to form a redundant topology. Such "dual ring" networks include Spatial Reuse Protocol, Fiber Distributed Data Interface (FDDI), and Resilient Packet Ring.


Advantages
·         Very orderly network where every device has access to the token and the opportunity to transmit
·         Performs better than a bus topology under heavy network load
·         Does not require network server to manage the connectivity between the computers

Disadvantages
·         One malfunctioning workstation can create problems for the entire network
·         Moves, adds and changes of devices can affect the network
·         Network adapter cards much more expensive than Ethernet cards and hubs
·         Much slower than an Ethernet network under normal load

Tree Topology:
The type of network topology in which a central 'root' node (the top level of the hierarchy) is connected to one or more other nodes that are one level lower in the hierarchy (i.e., the second level) with a point-to-point link between each of the second level nodes and the top level central 'root' node, while each of the second level nodes that are connected to the top level central 'root' node will also have one or more other nodes that are one level lower in the hierarchy (i.e., the third level) connected to it, also with a point-to-point link, the top level central 'root' node being the only node that has no other node above it in the hierarchy (The hierarchy of the tree is symmetrical.) Each node in the network having a specific fixed number, of nodes connected to it at the next lower level in the hierarchy, the number, being referred to as the 'branching factor' of the hierarchical tree. This tree has individual peripheral nodes.
A network that is based upon the physical hierarchical topology must have at least three levels in the hierarchy of the tree, since a network with a central 'root' node and only one hierarchical level below it would exhibit the physical topology of a star.
A network that is based upon the physical hierarchical topology and with a branching factor of 1 would be classified as a physical linear topology.
The branching factor, f, is independent of the total number of nodes in the network and, therefore, if the nodes in the network require ports for connection to other nodes the total number of ports per node may be kept low even though the total number of nodes is large – this makes the effect of the cost of adding ports to each node totally dependent upon the branching factor and may therefore be kept as low as required without any effect upon the total number of nodes that are possible.
The total number of point-to-point links in a network that is based upon the physical hierarchical topology will be one less than the total number of nodes in the network.
If the nodes in a network that is based upon the physical hierarchical topology are required to perform any processing upon the data that is transmitted between nodes in the network, the nodes that are at higher levels in the hierarchy will be required to perform more processing operations on behalf of other nodes than the nodes that are lower in the hierarchy. Such a type of network topology is very useful and highly recommended.

Tree topology advantages:
·         It is the best topology for a large computer network for which a star topology or ring topology are unsuitable due to the sheer scale of the entire network. Tree topology divides the whole network into parts that are more easily manageable.
·         Tree topology makes it possible to have a point to point network.
·         All computers have access to their immediate neighbors in the network and and also the central hub. This kind of network makes it possible for multiple network devices to be connected with the central hub.
·         It overcomes the limitation of star network topology, which has a limitation of hub connection points and the broadcast traffic induced limitation of a bus network topology.
·         A tree network provides enough room for future expansion of a network.

Tree topology disadvantages:
·         Dependence of the entire network on one central hub is a point of vulnerability for this topology. A failure of the central hub or failure of the main data trunk cable, can cripple the whole network.
·         With increase in size beyond a point, the management becomes difficult.


Mesh Topology:
Mesh networking (topology) is a type of networking where each node must not only capture and disseminate its own data, but also serve as a relay for other sensor nodes, that is, it must collaborate to propagate the data in the network.
A mesh network can be designed using a flooding technique or a routing technique. When using a routing technique, the message propagates along a path, by hopping from node to node until the destination is reached. To ensure all its paths' availability, a routing network must allow for continuous connections and reconfiguration around broken or blocked paths, using self-healing algorithms. A mesh network whose nodes are all connected to each other is a fully connected network. Mesh networks can be seen as one type of ad hoc network. Mobile ad hoc networks (MANET) and mesh networks are therefore closely related, but MANET also have to deal with the problems introduced by the mobility of the nodes.
The self-healing capability enables a routing based network to operate when one node breaks down or a connection goes bad. As a result, the network is typically quite reliable, as there is often more than one path between a source and a destination in the network. Although mostly used in wireless scenarios, this concept is also applicable to wired networks and software interaction.


Advantages of Mesh Topology
·         There are dedicated links used in the topology, which guarantees, that each connection is able to carry its data load, thereby eliminating traffic problems, which are common, when links are shared by multiple devices.
·         It is a robust topology. When one link in the topology becomes unstable, it does not cause the entire system to halt.
·         If the network is to be expanded, it can be done without causing any disruption to current users of the network.
·         It is possible to transmit data, from one node to a number of other nodes simultaneously
·         Troubleshooting, in case of a problem, is easy as compared to other network topologies.
·         This topology ensures data privacy and security, as every message travels along a dedicated link.

Disadvantages of Mesh Topology
·         The first disadvantage of this topology is that, it requires a lot more hardware (cables, etc.) as compared to other Local Area Network (LAN) topologies.
·         The implementation (installation and configuration) of this topology is very complicated and can get very messy. A large number of Input / Outout (I/O) ports are required.
·         It is an impractical solution, when large number of devices are to be connected to each other in a network.
·         The cost of installation and maintenance is high, which is a major deterrent.










Transmission Media

Various physical media can be used to transport a stream of bits from one device to another. Each has its own characteristics in terms of bandwidth, propagation delay, cost, and ease of installation and maintenance. Media can be generally classified as guided (e.g. copper and fiber cable) and unguided (wireless) media. The main categories of transmission media used in data communications networks. Some Bound Media are Coaxial Cable, Twisted Pair cable and Optical Fiber Cable.

Coaxial cable:


A coaxial cable has a central copper wire core, surrounded by an insulating (dielectric) material. Braided metal shielding surrounds the dielectric and helps to absorb unwanted external signals (noise), preventing it from interfering with the data signal travelling along the core. A plastic sheath protects the cable from damage. A terminating resistor is used at each end of the cable to prevent transmitted signals from being reflected back down the cable. The following diagram illustrates the basic construction of a coaxial cable.

Construction of coaxial cable
Coaxial cable has a fairly high degree of immunity to noise, and can be used over longer distances (up to 500 meters) than twisted pair cable. Coaxial cable has, in the past, been used to provide network backbone cable segments. Coaxial cable has largely been replaced in computer networks by optical fiber and twisted pair cable, with fiber used in the network backbone, and twisted pair used to connect workstations to network hubs and switches.
Thick net cable (also known as 10Base5) is a fairly thick cable (0.5 inches in diameter). The 10Base5 designation refers to the 10 Mbps maximum data rate , baseband signaling and 500 meter maximum segment length . Thick net was the original transmission medium used in Ethernet networks, and supported up to 100 nodes per network segment. An Ethernet transceiver was connected to the cable using a vampire tap , so called because it clamps onto the cable, forcing a spike through the outer shielding to make contact with the inner conductor, while two smaller sets of teeth bite into the outer conductor. Transceivers could be connected to the network cable while the network was live. A separate drop cable with an attachment unit interface (AUI) connector at each end connected the transceiver to the network interface card in the workstation (or other network device). The drop cable was typically a shielded twisted pair cable, and could be up to 50 meters in length. The minimum cable length between connections ( taps ) on a cable segment was 2.5 meters.
Thick net (10Base5) coaxial cable
Thin net cable (also known as 10Base2) is thinner than Thick net (approximately 0.25 inches in diameter) and as a consequence is cheaper and far more flexible. The 10Base2 designation refers to the 10 Mbps maximum data rate , baseband signaling and 185 (nearly 200) meter maximum segment length . A T-connector is used with two BNC connectors to connect the network segment directly to the network adapter card. The length of cable between stations must be at least 50 centimeters, and Thin net can support up to 30 nodes per network segment.
Thin net (10Base2) coaxial cable



Coaxial cable has the following advantages and disadvantages:
Advantages
·         Highly resistant to EMI (electromagnetic interference)
·         Highly resistant to physical damage
Disadvantages
·         Expensive
·         Inflexible construction (difficult to install)
·         Unsupported by newer networking standards


Twisted pair cable:
Twisted pair copper cable is still widely used, due to its low cost and ease of installation. A twisted pair consists of two insulated copper cables, twisted together to reduce electrical interference between adjacent pairs of wires. This type of cable is still used in the subscriber loop of the public telephone system (the connection between a customer and the local telephone exchange), which can extend for several kilometers without amplification. The subscriber loop is essentially an analogue transmission line, although twisted pair cables are also be used in computer networks to carry digital signals over short distances.
The bandwidth of twisted pair cable depends on the diameter of wire used, and the length of the transmission line. The type of cable currently used in local area networks has four pairs of wires. Until recently, category 5 or category 5E cable has been used, but category 6 is now used for most new installations. The main difference between the various categories is in the data rate supported - category 6 cable will support gigabit Ethernet. The main disadvantage of UTP cables in networks is that, due to the relatively high degree of attenuation and a susceptibility to electromagnetic interference, high speed digital signals can only be reliably transmitted over cable runs of 100 metres or less.
Unshielded twisted pair cable

Shielded Twisted Pair (STP) cable was introduced in the 1980s by IBM as the recommended cable for their Token Ring network technology. It is similar to unshielded twisted pair cable except that each pair is individually foil shielded, and the cable has a braided drain wire that is earthed at one end during installation. The popularity of STP has declined for the following reasons:
High cost of cable and connectors
More difficult to install than UTP
Ground loops can occur if incorrectly installed
There is still a cable length limitation of 100 meters

Shielded twisted pair cable

Advantages
·         It is a thin, flexible cable that is easy to string between walls.
·         More lines can be run through the same wiring ducts.
·         UTP costs less per meter/foot than any other type of LAN cable.
·         Electrical noise going into or coming from the cable can be prevented.
·         Cross-talk is minimized.
 Disadvantages
·         Twisted pair’s susceptibility to electromagnetic interference greatly depends on the pair twisting schemes (usually patented by the manufacturers) staying intact during the installation. As a result, twisted pair cables usually have stringent requirements for maximum pulling tension as well as minimum bend radius. This relative fragility of twisted pair cables makes the installation practices an important part of ensuring the cable’s performance.
·         In video applications that send information across multiple parallel signal wires, twisted pair cabling can introduce signaling delays known as skew which results in subtle color defects and ghosting due to the image components not aligning correctly when recombined in the display device. The skew occurs because twisted pairs within the same cable often use a different number of twists per meter so as to prevent common-mode crosstalk between pairs with identical numbers of twists. The skew can be compensated by varying the length of pairs in the termination box, so as to introduce delay lines that take up the slack between shorter and longer pairs, though the precise lengths required are difficult to calculate and vary depending on the overall cable length.
·

Optical fiber:
Optical fibers are thin, solid strands of glass that transmit information as pulses of light. The fiberd has a core of high-purity glass, between 6μm and 50μm in diameter, down which the light pulses travel. The core is encased in a covering layer made of a different type of glass, usually about 125 μm in diameter, known as the cladding. An outer plastic covering, the primary buffer, provides some protection, and takes the overall diameter to about 250 μm. The structure of an optical fiber is shown below.

The basic construction of an optical fiber
The cladding has a slightly lower refractive index than the core (typical values are 1.47 and 1.5 respectively), so that as the pulses of light travel along the fiber they are reflected back into the core each time they meet the boundary between the core and the cladding. Optical fibers lose far less of its signal energy than copper cables, and can be used to transmit signals of a much higher frequency. More information can be carried over longer distances with fewer repeaters. The bandwidth achievable using optical fiber is almost unlimited, but current signaling technology limits the data rate to 1 Gbps due to time required to convert electronic digital signals to light pulses and vice versa. Digital data is converted to light pulses by either a light emitting diode (LED) or a laser diode. Although some light is lost at each end of the fiber, most is passed along the fiber to the receiver, where the light pulses are converted back into electronic signals by a photo-detector.
As the ray passes along the fiber it meets the boundary between the core and the cladding at some point. Because the refractive index of the cladding is lower than that of the core, the ray is reflected back into the core material, as long as the angle of incidence ( θ i ) is greater than the critical angle ( θ c ). The critical angle depends on the refractive indices of the two materials. In the case of an optical fiber, the values are chosen so that almost all of the light is reflected back into the fiber, and there is virtually no loss through the walls of the fiber. This is called total internal reflection . The critical angle for a particular fiber can be calculated using Snell's Law. This states that:
n 1 sin θ 1 = n 2 sin θ 2
where θ 1 is the angle of incidence, θ 2 is the angle of refraction, and n 1 and n 2 are the refractive indices of the core and cladding respectively. The effect on a ray of light passing along the fiber is shown below.

Light transmission in an optical fiber
In step-index fibers, the refractive index of both the core and the cladding has a constant value, so that the refractive index of the fiber steps from one value to the next.
A step-index fiber
If the core diameter of the fiber is such that it allows light to enter at different angles and follow multiple paths, it is said to be a multi-mode fiber. The number of times the light is internally reflected will vary according to the angle at which the light initially enters the fiber, which will in turn determine the path length of the light as it travels along the fiber. Over long distances, there will be a significant difference in path length between light rays that enter the fiber at different angles. They will, as a consequence, arrive at slightly different times, causing distortion of the transmitted signal - an effect known as modal dispersion . For this reason, multi-mode fibers are only used for short-haul applications such as LAN backbone connections, where the distances involved are likely to be considerably less than one kilometre.

A multi-mode step-index fiber
If the core diameter of the fiber is made small enough, the angle at which light can enter the fiber can be limited such that most of the light travels down a single path, effectively eliminating modal dispersion. This type of fiber is called a mono-mode fiber, and is commonly used for long-haul applications such as long distance telecommunications. Distances of many kilometres are possible with mono-mode fiber before a repeater is needed.

Multi-mode and mono-mode fibers
One way to improve the performance of multi-mode fiber is to use a graded index fiber instead of a step index fiber. The refractive index of this type of fiber varies across the diameter of the core in such a way that light is made to follow a curved path along the fiber (see below). Light near the edges of the core travels faster than light at the centre of the core, so although some rays follow a longer path than others, they all tend to arrive at the same time, resulting in far less modal dispersion than would occur in a step-index multi-mode fiber.
A graded index fiber
Light paths in a graded index fiber

Advantages
·         Better security - very hard to tap a fibre without being noticed.
·         Longer cable runs
·         Greater bandwidth.
·         Not affected by electromagnetic interference.
·         Can connect between buildings with different earth potentials. These would could cause problems with a copper wired system.
·         Not effected by near-miss lightning strikes
·         Lower cost for 2 to 3 km fibre runs. CAT5 twisted-pair is limited to 100 metres.
·         Carrier signals on different frequencies (colours) can be used to increase the capacity (frequency division multiplexing).
·         Single mode or monomode fibre has a very thin inner glass layer.
·         This is so thin that it behaves like a wave guide and the light can't follow different paths.
·         This reduces the dispersion and makes longer fibres possible. 30km.

Disadvantages
·         Attenuation is still a problem and this limits the maximum cable length
·         Dispersion is still a problem and this also limits the maximum cable length
·         Scattering occurs when there are imperfections in the fibre. This causes attenuation or energy loss.
·         Higher installation cost for small networks. For major backbones fibre works out cheaper per megabit of bandwidth.
·         Optical fibres can be fragile although they are reinforced with kevlar fibres and an outer protective plastic layer
·         Optical fibres are difficult to connect to the transmitting light source and the receiving light detector. A complex cutting and polishing operation is needed to make the fibre ends flat and free from dirt or imperfections.

Unbound Media or Unguided Media:
Unguided Media: It is one that does not guide the data signals instead it uses the multiple paths for transmitting data signals. In this type the data cable are not bounds to a cable media. So it is called “Unbound media” basically there are 2 types.
a) Microwave 
b) Satellite Technology.

a) Microwave:
Microwaves are radio waves that are used to provide high-speed transmission. Both voice and data can be transmitted through microwave. Data is transmitted through the air form one microwave station to other similar to radio signals.
Microwave uses line-of-sight transmission. It means that the signals travel in straight path and cannot bend. Microwave stations or antennas are usually installed on high towers or buildings. Microwave stations are placed within 20 to 30 miles to each other. Each station receives signal from previous station and transfer to next station. In this way, data transferred from one place to another. There should be no buildings on mountains between microwave stations.

 Advantages:
1. It has the medium capacity slightly higher than “Bound Media”
2. Medium cost.
3. It can cover longer distance that cannot be possible by bound media.
Disadvantages:
1. Noise interference is more.
2. Since it uses less susceptible signal so it has got greater influence from rain & fog.
3. It is not secure & reliable.

b) Satellite Communication:
Communication satellite is a space station. It receives microwave signals from earth station. It amplifies the signal and retransmits them back to earth. Communication satellite is established in space about 22,300 miles above the earth. The data transfer speed of communication satellite is very high.
The transmission from earth station to satellite is called uplink. The transmission from satellite to earth station is called downlink. An important advantage at satellite is that a large volume of data can be communicated at once. The disadvantage is that bad weather can severely affect the quality of satellite transmission.

 Advantages:
1. Low cost per user (for pay TV)
2. High Capacity
3. Very large coverage area.
Disadvantages:
1. High Installing & managing cost.
2. Receive dishes & decoders required.
3. Delays involved in the reception of the signal.

Wireless Media:
Wireless telecommunications, is the transfer of information between two or more points that are physically not connected. Distances can be short, as a few meters as in television remote control; or long ranging from thousands to millions of kilometers for deep-space radio communications. It encompasses various types of fixed, mobile, and portable two-way radios, cellular telephones, personal digital assistants (PDAs), and wireless networking. Other examples of wireless technology include GPS units, garage door openers and or garage doors, wireless computer mice, keyboards and headsets, satellite television and cordless telephones.

Device Network Connecting Device: (Modem, NIC, Switch / Hub, Router, Gateway, Repeater, Bluetooth, IR, WiFi):
Computer networking devices are units that mediate data in a computer network. Computer networking devices are also called network equipment, Intermediate Systems (IS) or InterWorking Unit (IWU). Units which are the last receiver or generate data are called hosts or data terminal equipment.

Modem
A modem (modulator-demodulator) is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data. Modems can be used over any means of transmitting analog signals, from light emitting diodes to radio. The most familiar example is a voice band modem that turns the digital data of a personal computer into modulated electrical signals in the voice frequency range of a telephone channel. These signals can be transmitted over telephone lines and demodulated by another modem at the receiver side to recover the digital data.
Modems are generally classified by the amount of data they can send in a given unit of time, usually expressed in bits per second (bit/s, or bps). Modems can alternatively be classified by their symbol rate, measured in baud. The baud unit denotes symbols per second, or the number of times per second the modem sends a new signal. For example, the ITU V.21 standard used audio frequency-shift keying, that is to say, tones of different frequencies, with two possible frequencies corresponding to two distinct symbols (or one bit per symbol), to carry 300 bits per second using 300 baud. By contrast, the original ITU V.22 standard, which was able to transmit and receive four distinct symbols (two bits per symbol), handled 1,200 bit/s by sending 600 symbols per second (600 baud) using phase shift keying.

Network interface controller(NIC)
A network interface controller (also known as a network interface card, network adapter, LAN adapter and by similar terms) is a computer hardware component that connects a computer to a computer network.
Whereas network interface controllers were commonly implemented on expansion cards that plug into a computer bus, the low cost and ubiquity of the Ethernet standard means that most newer computers have a network interface built into the motherboard.
The network controller implements the electronic circuitry required to communicate using a specific physical layer and data link layer standard such as Ethernet, Wi-Fi, or Token Ring. This provides a base for a full network protocol stack, allowing communication among small groups of computers on the same LAN and large-scale network communications through routable protocols, such as IP.

Switch / Hub
A network switch or switching hub is a computer networking device that connects network segments.
The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer (layer 2) of the OSI model. Switches that additionally process data at the network layer (Layer 3) and above are often referred to as Layer 3 switches or multilayer switches.
The network switch plays an integral part in most modern Ethernet local area networks (LANs). Mid-to-large sized LANs contain a number of linked managed switches. Small office/home office (SOHO) applications typically use a single switch, or an all-purpose converged device such as a gateway to access small office/home broadband services such as DSL or cable internet. In most of these cases, the end-user device contains a router and components that interface to the particular physical broadband technology. User devices may also include a telephone interface for VoIP.
An Ethernet switch operates at the data link layer of the OSI model to create a separate collision domain for each switch port. With 4 computers (e.g., A, B, C, and D) on 4 switch ports, A and B can transfer data back and forth, while C and D also do so simultaneously, and the two conversations will not interfere with one another. In the case of a hub, they would all share the bandwidth and run in half duplex, resulting in collisions, which would then necessitate retransmissions. Using a switch is called microsegmentation. This allows computers to have dedicated bandwidth on a point-to-point connections to the network and to therefore run in full duplex without collisions.

Router
A router is a device that forwards data packets between telecommunications networks, creating an overlay internetwork. A router is connected to two or more data lines from different networks. When data comes in on one of the lines, the router reads the address information in the packet to determine its ultimate destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey or drops the packet. A data packet is typically forwarded from one router to another through networks that constitute the internetwork until it gets to its destination node.
 The most familiar type of routers are home and small office routers that simply pass data, such as web pages and email, between the home computers and the owner's cable or DSL modem, which connects to the Internet (ISP). However more sophisticated routers range from enterprise routers, which connect large business or ISP networks up to the powerful core routers that forward data at high speed along the optical fiber lines of the Internet backbone.

Gateway
A network gateway is an internetworking system capable of joining together two networks that use different base protocols. A network gateway can be implemented completely in software, completely in hardware, or as a combination of both. Depending on the types of protocols they support, network gateways can operate at any level of the OSI model.
Because a network gateway, by definition, appears at the edge of a network, related capabilities like firewalls tend to be integrated with it. On home networks, a broadband router typically serves as the network gateway although ordinary computers can also be configured to perform equivalent functions.

(1) A node on a network that serves as an entrance to another network. In enterprises, the gateway is the computer that routes the traffic from a workstation to the outside network that is serving the Web pages. In homes, the gateway is the ISP that connects the user to the internet.
In enterprises, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which use headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.
(2) A computer system located on earth that switches data signals and voice signals between satellites and terrestrial networks.
(3) An earlier term for router, though now obsolete in this sense as router is commonly used.

Repeater
A repeater is an electronic device that receives a signal and retransmits it at a higher level and/or higher power, or onto the other side of an obstruction, so that the signal can cover longer distances. The term "repeater" originated with telegraphy and referred to an electromechanical device used by the army to regenerate telegraph signals. Use of the term has continued in telephony and data communications.
In telecommunication, the term repeater has the following standardized meanings:
An analog device that amplifies an input signal regardless of its nature (analog or digital).
A digital device that amplifies, reshapes, retimes, or performs a combination of any of these functions on a digital input signal for retransmission. Because repeaters work with the actual physical signal, and do not attempt to interpret the data being transmitted, they operate on the Physical layer, the first layer of the OSI model.



Bluetooth
Bluetooth is a proprietary open wireless technology standard for exchanging data over short distances (using short wavelength radio transmissions in the ISM band from 2400-2480 MHz) from fixed and mobile devices, creating personal area networks (PANs) with high levels of security. Created by telecoms vendor Ericsson in 1994,[1] it was originally conceived as a wireless alternative to RS-232 data cables. It can connect several devices, overcoming problems of synchronization.
Bluetooth is managed by the Bluetooth Special Interest Group, which has more than 14,000 member companies in the areas of telecommunication, computing, networking, and consumer electronics.[2] The SIG oversees the development of the specification, manages the qualification program, and protects the trademarks.[3] To be marketed as a Bluetooth device, it must be qualified to standards defined by the SIG. A network of patents are required to implement the technology and are only licensed to those qualifying devices; thus the protocol, whilst open, may be regarded as proprietary.

Infrared
Infrared (IR) light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.7 micrometers, and extending conventionally to 300 micrometres. These wavelengths correspond to a frequency range of approximately 1 to 430 THz, and include most of the thermal radiation emitted by objects near room temperature. Microscopically, IR light is typically emitted or absorbed by molecules when they change their rotational-vibrational movements.
Sunlight at zenith provides an irradiance of just over 1 kilowatt per square meter at sea level. Of this energy, 527 watts is infrared radiation, 445 watts is visible light, and 32 watts is ultraviolet radiation.

WiFi
Wi-Fi is a wireless standard for connecting electronic devices. A Wi-Fi enabled device such as a personal computer, video game console, smartphone, and digital audio player can connect to the Internet when within range of a wireless network connected to the Internet. A single access point (or hotspot) has a range of about 20 meters indoors. Wi-Fi has a greater range outdoors and multiple overlapping access points can cover large areas.
"Wi-Fi" is a trademark of the Wi-Fi Alliance and the term was originally created as a simpler name for the "IEEE 802.11" standard. Wi-Fi is used by over 700 million people, there are over 4 million hotspots (places with Wi-Fi Internet connectivity) around the world, and about 800 million new Wi-Fi devices every year.[citation needed] Wi-Fi products that complete the Wi-Fi Alliance interoperability certification testing successfully can use the Wi-Fi CERTIFIED designation and trademark.




OSI Reference Model
The Open Systems Interconnection model (OSI model) was a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a way of sub-dividing a communications system into smaller parts called layers. Similar communication functions are grouped into logical layers. A layer provides services to its upper layer while receiving services from the layer below. On each layer, an instance provides service to the instances at the layer above and requests service from the layer below.
For example, a layer that provides error-free communications across a network provides the path needed by applications above it, while it calls the next lower layer to send and receive packets that make up the contents of that path. Two instances at one layer are connected by a horizontal connection on that layer.
 


Layer 1: Physical Layer
The Physical Layer defines electrical and physical specifications for devices. In particular, it defines the relationship between a device and a transmission medium, such as a copper or optical cable. This includes the layout of pins, voltages, cable specifications, hubs, repeaters, network adapters, host bus adapters (HBA used in storage area networks) and more.
To understand the function of the Physical Layer, contrast it with the functions of the Data Link Layer. Think of the Physical Layer as concerned primarily with the interaction of a single device with a medium, whereas the Data Link Layer is concerned more with the interactions of multiple devices (i.e., at least two) with a shared medium. Standards such as RS-232 do use physical wires to control access to the medium.
The major functions and services performed by the Physical Layer are:
Establishment and termination of a connection to a communications medium.
Participation in the process whereby the communication resources are effectively shared among multiple users. For example, contention resolution and flow control.
Modulation, or conversion between the representation of digital data in user equipment and the corresponding signals transmitted over a communications channel. These are signals operating over the physical cabling (such as copper and optical fiber) or over a radio link.

Layer 2: Data Link Layer
The Data Link Layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical Layer. Originally, this layer was intended for point-to-point and point-to-multipoint media, characteristic of wide area media in the telephone system. Local area network architecture, which included broadcast-capable multiaccess media, was developed independently of the ISO work in IEEE Project 802. IEEE work assumed sublayering and management functions not required for WAN use. In modern practice, only error detection, not flow control using sliding window, is present in data link protocols such as Point-to-Point Protocol (PPP), and, on local area networks, the IEEE 802.2 LLC layer is not used for most protocols on the Ethernet, and on other local area networks, its flow control and acknowledgment mechanisms are rarely used. Sliding window flow control and acknowledgment is used at the Transport Layer by protocols such as TCP, but is still used in niches where X.25 offers performance advantages.

Layer 3: Network Layer
The Network Layer provides the functional and procedural means of transferring variable length data sequences from a source host on one network to a destination host on a different network, while maintaining the quality of service requested by the Transport Layer (in contrast to the data link layer which connects hosts within the same network). The Network Layer performs network routing functions, and might also perform fragmentation and reassembly, and report delivery errors. Routers operate at this layer—sending data throughout the extended network and making the Internet possible. This is a logical addressing scheme – values are chosen by the network engineer. The addressing scheme is not hierarchical.
Careful analysis of the Network Layer indicated that the Network Layer could have at least three sublayers:
Subnetwork Access – that considers protocols that deal with the interface to networks, such as X.25;
Subnetwork Dependent Convergence – when it is necessary to bring the level of a transit network up to the level of networks on either side;
Subnetwork Independent Convergence – which handles transfer across multiple networks.
The best example of this latter case is CLNP, or IPv7 ISO 8473. It manages the connectionless transfer of data one hop at a time, from end system to ingress router, router to router, and from egress router to destination end system. It is not responsible for reliable delivery to a next hop, but only for the detection of erroneous packets so they may be discarded. In this scheme, IPv4 and IPv6 would have to be classed with X.25 as subnet access protocols because they carry interface addresses rather than node addresses.
A number of layer management protocols, a function defined in the Management Annex, ISO 7498/4, belong to the Network Layer. These include routing protocols, multicast group management, Network Layer information and error, and Network Layer address assignment. It is the function of the payload that makes these belong to the Network Layer, not the protocol that carries them.

Layer 4: Transport Layer
The Transport Layer provides transparent transfer of data between end users, providing reliable data transfer services to the upper layers. The Transport Layer controls the reliability of a given link through flow control, segmentation/desegmentation, and error control. Some protocols are state- and connection-oriented. This means that the Transport Layer can keep track of the segments and retransmit those that fail. The Transport layer also provides the acknowledgement of the successful data transmission and sends the next data if no errors occurred.
Although not developed under the OSI Reference Model and not strictly conforming to the OSI definition of the Transport Layer, typical examples of Layer 4 are the Transmission Control Protocol (TCP) and User Datagram Protocol (UDP).
Of the actual OSI protocols, there are five classes of connection-mode transport protocols ranging from class 0 (which is also known as TP0 and provides the least features) to class 4 (TP4, designed for less reliable networks, similar to the Internet). Class 0 contains no error recovery, and was designed for use on network layers that provide error-free connections. Class 4 is closest to TCP, although TCP contains functions, such as the graceful close, which OSI assigns to the Session Layer. Also, all OSI TP connection-mode protocol classes provide expedited data and preservation of record boundaries, both of which TCP is incapable.

Layer 5: Session Layer
The Session Layer controls the dialogues (connections) between computers. It establishes, manages and terminates the connections between the local and remote application. It provides for full-duplex, half-duplex, or simplex operation, and establishes checkpointing, adjournment, termination, and restart procedures. The OSI model made this layer responsible for graceful close of sessions, which is a property of the Transmission Control Protocol, and also for session checkpointing and recovery, which is not usually used in the Internet Protocol Suite. The Session Layer is commonly implemented explicitly in application environments that use remote procedure calls.



Layer 6: Presentation Layer
The Presentation Layer establishes context between Application Layer entities, in which the higher-layer entities may use different syntax and semantics if the presentation service provides a mapping between them. If a mapping is available, presentation service data units are encapsulated into session protocol data units, and passed down the stack.
This layer provides independence from data representation (e.g., encryption) by translating between application and network formats. The presentation layer transforms data into the form that the application accepts. This layer formats and encrypts data to be sent across a network. It is sometimes called the syntax layer.[5]
The original presentation structure used the basic encoding rules of Abstract Syntax Notation One (ASN.1), with capabilities such as converting an EBCDIC-coded text file to an ASCII-coded file, or serialization of objects and other data structures from and to XML.

Layer 7: Application Layer
The Application Layer is the OSI layer closest to the end user, which means that both the OSI application layer and the user interact directly with the software application. This layer interacts with software applications that implement a communicating component. Such application programs fall outside the scope of the OSI model. Application layer functions typically include identifying communication partners, determining resource availability, and synchronizing communication. When identifying communication partners, the application layer determines the identity and availability of communication partners for an application with data to transmit. When determining resource availability, the application layer must decide whether sufficient network or the requested communication exist. In synchronizing communication, all communication between applications requires cooperation that is managed by the application layer.



Communication Protocol: TCP/IP, SMTP, POP3, FTP, HTTPs, Telnet protocol

A communications protocol is a formal description of digital message formats and the rules for exchanging those messages in or between computing systems and in telecommunications.
Protocols may include signaling, authentication and error detection and correction capabilities.
A protocol defines the syntax, semantics, and synchronization of communication, and the specified behaviour is typically independent of how it is to be implemented. A protocol can therefore be implemented as hardware or software or both.
Communicating systems use well-defined formats for exchanging messages. Each message has an exact meaning intended to provoke a defined response of the receiver. A protocol therefore describes the syntax, semantics, and synchronization of communication. A programming language describes the same for computations, so there is a close analogy between protocols and programming languages: protocols are to communications what programming languages are to computations. (A less technical reader might appreciate this similar analogy: protocols are to communications what grammar is to writing.)
The communications protocols in use on the Internet are designed to function in very complex and diverse settings. To ease design, communications protocols are structured using a layering scheme as a basis. Instead of using a single universal protocol to handle all transmission tasks, a set of cooperating protocols fitting the layering scheme is used.
The layering scheme in use on the Internet is called the TCP/IP model. The actual protocols are collectively called the Internet protocol suite. The group responsible for this design is called the Internet Engineering Task Force (IETF).

TCP/IP
TCP/IP (Transmission Control Protocol/Internet Protocol) is the basic communication language or protocol of the Internet. It can also be used as a communications protocol in a private network (either an intranet or an extranet).
TCP/IP is a two-layer program. The higher layer, Transmission Control Protocol, manages the assembling of a message or file into smaller packets that are transmitted over the Internet and received by a TCP layer that reassembles the packets into the original message. The lower layer, Internet Protocol, handles the address part of each packet so that it gets to the right destination. Each gateway computer on the network checks this address to see where to forward the message. Even though some packets from the same message are routed differently than others, they'll be reassembled at the destination.
As with all other communications protocol, TCP/IP is composed of layers:
IP - is responsible for moving packet of data from node to node. IP forwards each packet based on a four byte destination address (the IP number). The Internet authorities assign ranges of numbers to different organizations. The organizations assign groups of their numbers to departments. IP operates on gateway machines that move data from department to organization to region and then around the world.
TCP - is responsible for verifying the correct delivery of data from client to server. Data can be lost in the intermediate network. TCP adds support to detect errors or lost data and to trigger retransmission until the data is correctly and completely received.
Sockets - is a name given to the package of subroutines that provide access to TCP/IP on most systems.

TCP/IP uses the client/server model of communication in which a computer user (a client) requests and is provided a service (such as sending a Web page) by another computer (a server) in the network. TCP/IP communication is primarily point-to-point, meaning each communication is from one point (or host computer) in the network to another point or host computer. TCP/IP and the higher-level applications that use it are collectively said to be "stateless" because each client request is considered a new request unrelated to any previous one (unlike ordinary phone conversations that require a dedicated connection for the call duration). Being stateless frees network paths so that everyone can use them continuously. (Note that the TCP layer itself is not stateless as far as any one message is concerned. Its connection remains in place until all packets in a message have been received.)
Many Internet users are familiar with the even higher layer application protocols that use TCP/IP to get to the Internet. These include the World Wide Web's Hypertext Transfer Protocol (HTTP), the File Transfer Protocol (FTP), Telnet (Telnet) which lets you logon to remote computers, and the Simple Mail Transfer Protocol (SMTP). These and other protocols are often packaged together with TCP/IP as a "suite."
Personal computer users with an analog phone modem connection to the Internet usually get to the Internet through the Serial Line Internet Protocol (SLIP) or the Point-to-Point Protocol (PPP). These protocols encapsulate the IP packets so that they can be sent over the dial-up phone connection to an access provider's modem.
Protocols related to TCP/IP include the User Datagram Protocol (UDP), which is used instead of TCP for special purposes. Other protocols are used by network host computers for exchanging router information. These include the Internet Control Message Protocol (ICMP), the Interior Gateway Protocol (IGP), the Exterior Gateway Protocol (EGP), and the Border Gateway Protocol (BGP).

Simple Mail Transfer Protocol (SMTP)
SMTP (Simple Mail Transfer Protocol) is a TCP/IP protocol used in sending and receiving e-mail. However, since it is limited in its ability to queue messages at the receiving end, it is usually used with one of two other protocols, POP3 or IMAP , that let the user save messages in a server mailbox and download them periodically from the server. In other words, users typically use a program that uses SMTP for sending e-mail and either POP3 or IMAP for receiving e-mail. On Unix-based systems, sendmail is the most widely-used SMTP server for e-mail. A commercial package, Sendmail, includes a POP3 server. Microsoft Exchange includes an SMTP server and can also be set up to include POP3 support.
SMTP usually is implemented to operate over Internet port 25. An alternative to SMTP that is widely used in Europe is X.400. Many mail servers now support Extended Simple Mail Transfer Protocol (ESMTP), which allows multimedia files to be delivered as e-mail.

POP3 (Post Office Protocol 3)
In computing, the Post Office Protocol (POP) is an application-layer Internet standard protocol used by local e-mail clients to retrieve e-mail from a remote server over a TCP/IP connection. POP and IMAP (Internet Message Access Protocol) are the two most prevalent Internet standard protocols for e-mail retrieval. Virtually all modern e-mail clients and servers support both. The POP protocol has been developed through several versions, with version 3 (POP3) being the current standard. Like IMAP, POP3 is supported by most webmail services such as Hotmail, Gmail and Yahoo! Mail.
POP3 is designed to delete mail on the server as soon as the user has downloaded it. However, some implementations allow users or an administrator to specify that mail be saved for some period of time. POP can be thought of as a "store-and-forward" service.
An alternative protocol is Internet Message Access Protocol (IMAP). IMAP provides the user more capabilities for retaining e-mail on the server and for organizing it in folders on the server. IMAP can be thought of as a remote file server.
POP and IMAP deal with the receiving of e-mail and are not to be confused with the Simple Mail Transfer Protocol (SMTP), a protocol for transferring e-mail across the Internet. You send e-mail with SMTP and a mail handler receives it on your recipient's behalf. Then the mail is read using POP or IMAP.

File Transfer Protocol(FTP)
File Transfer Protocol (FTP) is a standard network protocol used to transfer files from one host to another over a TCP-based network, such as the Internet. FTP is built on a client-server architecture and utilizes separate control and data connections between the client and server. FTP users may authenticate themselves using a clear-text sign-in protocol but can connect anonymously if the server is configured to allow it.
The first FTP client applications were interactive command-line tools, implementing standard commands and syntax. Graphical user interface clients have since been developed for many of the popular desktop operating systems in use today.

HTTPS (HTTP over SSL or HTTP Secure)
HTTPS (HTTP over SSL or HTTP Secure) is the use of Secure Socket Layer (SSL) or Transport Layer Security (TLS) as a sublayer under regular HTTP application layering. HTTPS encrypts and decrypts user page requests as well as the pages that are returned by the Web server. The use of HTTPS protects against eavesdropping and man-in-the-middle attacks. HTTPS was developed by Netscape.
HTTPS and SSL support the use of X.509 digital certificates from the server so that, if necessary, a user can authenticate the sender. Unless a different port is specified, HTTPS uses port 443 instead of HTTP port 80 in its interactions with the lower layer, TCP/IP.
Suppose you visit a Web site to view their online catalog. When you're ready to order, you will be given a Web page order form with a Uniform Resource Locator (URL) that starts with https://. When you click "Send," to send he page back to the catalog retailer, your browser's HTTPS layer will encrypt it. The acknowledgement you receive from the server will also travel in encrypted form, arrive with an https:// URL, and be decrypted for you by your browser's HTTPS sublayer.
The effectiveness of HTTPS can be limited by poor implementation of browser or server software or a lack of support for some algorithms. Furthermore, although HTTPS secures data as it travels between the server and the client, once the data is decrypted at its destination, it is only as secure as the host computer. According to security expert Gene Spafford, that level of security is analogous to "using an armored truck to transport rolls of pennies between someone on a park bench and someone doing business from a cardboard box."
HTTPS is not to be confused with S-HTTP, a security-enhanced version of HTTP developed and proposed as a standard by EIT.


Getting started with HTTPS
To explore how HTTPS is used in the enterprise, here are some additional resources for learning about HTTPS and Web page security:
Enabling HTTPS in J2EE Web components: The HTTPS protocol is a valuable security feature for J2EE Web components. Expert Ramesh Nagappan explains how to implement HTTPS in JSPs and servlets.
Authentication and authorization for Web applications: Web applications need robust authentication and authorization mechanisms, such as HTTPS. Expert Ramesh Nagappan explains what measures are needed before you deploy Web apps.
How to create a secure login page using ASP.NET: A secure ASP.NET login page is easier to create than one might assume. Expert Dan Cornell explains how to use authentication, authorization and HTTPS to ensure your login page is safe.

TELNET Protocol
Telnet is a user command and an underlying TCP/IP protocol for accessing remote computers. Through Telnet, an administrator or another user can access someone else's computer remotely. On the Web, HTTP and FTP protocols allow you to request specific files from remote computers, but not to actually be logged on as a user of that computer. With Telnet, you log on as a regular user with whatever privileges you may have been granted to the specific application and data on that computer.
A Telnet command request looks like this (the computer name is made-up):
>telnet the.libraryat.whatis.edu
The result of this request would be an invitation to log on with a userid and a prompt for a password. If accepted, you would be logged on like any user who used this computer every day.
Telnet is most likely to be used by program developers and anyone who has a need to use specific applications or data located at a particular host computer.

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