Definition

Data communication are the exchange of data between two devices via some form of transmission medium such as wire cable. For communication to occur, the communicating devices must be part of a communication system made up of a combination of hardware i.e. physical equipment and software i.e. programs.

The effectiveness of a data communication system depends on four fundamental characteristics:

1. Delivery: The system must deliver data to the correct destination. Data must be received by the intended device or user and only by that device or user.
2. Accuracy: The system must deliver that data accurately data that have been altered during transmission and left uncorrected and unusable.
3. Timelines: The system must deliver data in a timely manner. Data delivered late are useless. Video and audio data should be delivered in a timely manner i.e. delivering data as they are produced, in the same order that they are produced, and without significant delay. This kind of delivery is called real time transmission.
4. Jitter: Jitter refers to the variation in the packet arrival time. It is the uneven delay in the delivery of audio or video packets. Eg: Let us assume that video packets are sent every 30 ms. If some of the packets arrive with 30 ms delay and other with 40 ms delay, an uneven quality in the video is the result.

Transmission Hierarchy

The OSI Reference Model:

The OSI model is based on a proposal developed by the International Standards Organization (ISO) as the first step toward international standardization of the protocols used in various layers. The model is called the ISO OSI (Open Systems Interconnection) Reference Model because it deals with connecting open systems i.e., systems that are open for communication with other systems. The OSI model itself is not a network architecture because it does not specify the exact services and protocols to be used in each layer. It just tells what each layer should do. However, ISO has also produced standards for all the layers, although these are not part of the reference model itself. Each one has been published as a separate international standard. The OSI model has seven layers.

The Physical Layer

The major function of the physical layer is to move data in the form of electromagnetic signals across the transmission medium. It is responsible for transmission of individual bits from sender to receiver. During this transmission, the sender and receiver clocks must be synchronized. The stream of bits 0’s and 1’s are encoded into signals, without any interpretation.

The Data Link Layer

The Data link layer is responsible for moving the frame from one node to other. The stream of data is framed into manageable data units. The physical addressing is done at the sublayer of the data link layer, the medium access control sublayer using MAC address. For flow control, the mechanism used is to send the frame to the receiver continuously thus flooding the receiver node. Also for error control trailer is added to each frame so that the receiver understands that it is the end of the frame. Frames can also be retransmitted from the sender in case the acknowledgement is not received from the receiver.

The Network Layer

The network layer is responsible for the delivery of individual packets from the source host to destination host. The IP address is used for logical addressing in the network layer. If too many packets are present in the subnet at the same time, they will get in one another’s way, forming bottlenecks. Handling congestion is also a responsibility of the network layer, in conjunction with higher layers that adapt the load they place on the network. More generally, the quality of service provided (delay, transit time, jitter, etc.) is also a network layer issue. When a packet has to travel from one network to another to get to its destination, many problems can arise. The addressing used by the second network may be different from that used by the first one. The second one may not accept the packet at all because it is too large. The protocols may differ, and so on. It is up to the network layer to overcome all these problems to allow heterogeneous networks to be interconnected.

The Transport Layer

The transport layer is responsible for delivery of a message from one process to another. It accepts data from the above layer, split it up into smaller units if needed and then pass these to the network layer. It ensures that the units arrive correctly at the other end. Furthermore, all this must be done efficiently and in a way that isolates the upper layers from the inevitable changes in the hardware technology over the course of time. The most popular type of transport connection is an error-free point-to-point channel that delivers messages or bytes in the order in which they were sent. The transport layer is a true end-to-end layer; it carries data all the way from the source to the destination. In other words, a program on the source machine carries on a conversation with a similar program on the destination machine, using the message headers and control messages.

The Session Layer

The session layer allows users on different machines to establish sessions between them. Sessions offer various services, including dialog control (keeping track of whose turn it is to transmit), token management (preventing two parties from attempting the same critical operation simultaneously), and synchronization (check pointing long transmissions to allow them to pick up from where they left in the event of a crash and subsequent recovery).

The Presentation Layer

The presentation layer is responsible for translation, compression and encryption. Translation of the data i.e interoperability between different encoding systems is needed. Also, data compression and encryption is required r a safe transmission.

The Application Layer

The application layer is responsible for providing service to the user. The application layer contains a variety of protocols that are commonly needed by users. One widely used application protocol is HTTP (HyperText Transfer Protocol), which is the basis for the World Wide Web. When a browser wants a Web page, it sends the name of the page it wants to the server hosting the page using HTTP. The server then sends the page back. Other application protocols are used for file transfer, electronic mail, and network news.

The TCP/IP Reference Model:

When satellite and radio networks were added later, the existing protocols had trouble interworking with them, so a new reference architecture was needed. Thus, from nearly the beginning, the ability to connect multiple networks in a seamless way was one of the major design goals. This architecture later became known as the TCP/IP Reference Model.

Figure 1.2

The Link Layer

It is the lowest layer in the TCP/IP reference model. Protocols are used to connect to the host so that the packets can be sent over it. It varies from host to host and network to network.

The Internet Layer

It is the layer which holds the whole architecture together. It helps the packet to travel independently to the destination. Order in which packets are received is different from the way they are sent. The internet layer defines an official packet format and protocol called IP(Internet Protocol), plus a companion protocol called ICMP (Internet Control Message Protocol) that helps it function. The various functions performed by the Internet Layer are includes Delivering IP packets, performing routing, avoiding congestion

The Transport Layer

The layer above the internet layer in the TCP/IP model is now usually called the transport layer. It is designed to allow peer entities on the source and destination hosts to carry on a conversation, just as in the OSI transport layer. Two end-to-end transport protocols have been defined here. The TCP (Transmission Control Protocol), is a reliable connection-oriented protocol that allows a byte stream originating on one machine to be delivered without error on any other machine on the internet. It segments the incoming byte stream into discrete messages and passes each one on to the internet layer. At the destination, the receiving TCP process reassembles the received messages into the output stream. TCP also handles flow control to make sure a fast sender cannot swamp a slow receiver with more messages than it can handle. The UDP (User Datagram Protocol), is an unreliable, connectionless protocol for applications that do not want TCP’s sequencing or flow control and provide their own. It is also widely used for one-shot, client-server-type request-reply queries and applications in which prompt delivery is more important than accurate delivery, such as transmitting speech or video.

The Application Layer

The TCP/IP model does not have session or presentation layers. On top of the transport layer is the application layer. It contains all the higher-level protocols. The early ones included a virtual terminal (TELNET), file transfer (FTP), and electronic mail (SMTP). Many other protocols have been added to these over the years. The Domain Name System (DNS), for mapping host names onto their network addresses, HTTP, the protocol for fetching pages on the World Wide Web, RTP, the protocol for delivering real-time media such as voice or movies.

Merits of TCP/IP model

1. It operated independently.
2. It is scalable.
3. Client/server architecture.
4. Supports a number of routing protocols.
5. Can be used to establish a connection between two computers.

Demerits of TCP/IP

1. In this, the transport layer does not guarantee delivery of packets.
2. The model cannot be used in any other application.
3. Replacing protocol is not easy.
4. It has not clearly separated its services, interfaces and protocols.

Comparison between OSI reference model and TCP/IP reference model