Data Transmission And Data Networking

Data transmission at physical level involves the hardware required for handling individual bits and encoding bits in signals. The details of the underlying hardware are generally handled by the engineers who design the hardware.

Any two devices directly linked via a communication medium (point to point communication) can send and receive data, to and from each other respectively. If a large number of computers need to interact with each other, point to point communication will require direct link between all the computers. This is not a practical solution. The communication circuits and the associated hardware required for communication (like modem) are expensive. Moreover, there may not be a need to transmit data all the time, which will result in the communication medium lying idle for most of the time. For long distance communication, instead of point to point connection, a network of nodes is used as a communication medium. The different computers attached to the network share the communication facility

(a) Point-to-point communication (b) Switching

The computer network provides a convenient interface that handles sending of multiple bytes of data across the network instead of handling data transmission at physical level.

Switching

A network cannot allow or deny access to a shared communication facility. All computers attached to the network can use it to send and receive data. Networks allow sharing of communication medium using switching. Switching routes the traffic (data traffic) on the network. It sets up temporary connections between the network nodes to facilitate sending of data. Switching allows different users, fair access to the shared communication medium. There are three kinds of switching techniques—(1) Packet switching, (2) Circuit switching, and (3) Message switching. Computer networks generally use packet switching, occasionally use circuit switching but do not use message switching.

Circuit Switching

Circuit switching sets up end-to-end communication path between the source and the destination, before the data can be sent. The path gets reserved during the duration of the connection. Circuit switching is commonly used in the telephone communication network.

                                              Circuit switching

Message Switching

Message switching does not establish a physical path in advance, between the sender and the receiver. It uses the ‘store and forward’ mechanism. In this mechanism, the network nodes have large memory storage. The message is received from the sender and stored in the network node, and when it finds a free route, it forwards the message to the next node till it reaches the destination. Message switching requires large data storage capacity and incurs delay in storing and forwarding of message. Message switching may block the network nodes for a long time. They are thus not suitable for interactive communication. Message switching is no more used in computer networks.

                                 Message switching

Packet Switching

Like message switching, packet switching does not establish a physical path between the sender and the receiver, in advance. Packet switching also uses the ‘store and forward’ mechanism. However, instead of a complete message, packets are sent over the network. Packet switching splits a message into small “packets” of defined size to be sent over the network. Each packet is numbered.

                                              Packet switching

· A packet is a self-contained part of data that can be sent over the network. A packet contains the data to be transmitted and a header that contains information about the packet, like the source and destination addresses, size of packet, error checking bytes etc.

· Since the path through which the packets travel is not reserved, the packets may travel through different paths in the network and may not reach the destination in order. At the destination, the received packets are reassembled (according to the packet number), and the complete message is constructed.

· Packet switching is suited for interactive traffic. Packet switching limits the size of the packet and does not block a network node for a long time. Moreover, a node can transmit a packet before the arrival of another full packet, thus reducing the delay.

· Packet switching does not require dedicated communication link, and shares the underlying resources. Packet switching is commonly used for computer networks, including the Internet.