Packet Switching

You are familiar with the-concept of switching. Circuit switching is the study of data link layer. The services of networks are provided by using the concept of packet switching.

Packet switching can be used as an alternate to circuit switching. In the packet switched networks, data is sent in discrete units that have variable length. They are called as packets. There is a strict upper bound limit on the size of packets in a packet switch network. The packet contains data and various control information. The packet switched networks allow any host to send data to any other host without reserving the circuit. Multiple paths between a pair of sender and receiver may exist in a packet switched network. One path is selected between source and destination. Whenever the sender has data to send, it converts them into packets and forwards them to next computer or router. The router stores this packet till the output line is free.

Then, this packet is transferred to next computer or router (called as hop). This way, it moves to the destination hop by hop. All the packets belonging to a transmission may or may not take the same route. The route of a packet is decided by network layer protocols.

Advantages of packet Switching

• The main advantage of packet switching is the efficiency of the network. In circuit switching network, a reserved circuit cannot be used by others, till the sender and receiver leave it. Even if no data is being sent on a reserved circuit, no one else can access the circuit. This results in network bandwidth wastage. The packet switching reduces network bandwidth wastage.

• The other advantage is that the packet switching is more faults tolerant. In case of circuit switching, all the packets are lost if a router in the circuit is down as all the packets follow the same route. But, in case of packet switching network, the packets can be routed over the malfunctioning component of the network. This is because all the packets may follow a different route to the destination.

Advantages of circuit Switching

• The circuit switching bills the user depending on the distance and duration of connection whereas packet switching network bill users only on the basis of duration of connectivity.

• The advantage of circuit switching network over packet switching network is that the circuit switching network provides ordered delivery of packets. As all the packets follow the same route. They arrive in correct order at destination.

 Types of Packet Switching

The packet switching has two approaches: Virtual Circuit approach and Datagram approach. WAN, ATM, frame relay and telephone networks use connection oriented virtual circuit approach; whereas internet relies on connectionless datagram based packet switching.

 (i) Virtual Circuit Packet Switching: In virtual circuit packet switching, a single route is chosen between the sender and receiver and all the packets are sent through this route. Every packet contains the virtual circuit number. As in circuit switching, virtual circuit needs call setup before actual transmission can be started. He routing is based on the virtual circuit number.

 (ii) Datagram Packet Switching: In datagram packet switching each packet is transmitted without any regard to other packets. Every packet contain full packet of source and destination. Every packet is treated as individual, independent transmission.

The datagram can arrive at the destination with a different order from the order in which they where sent. The source and destination address are used by the routers to decide the route for packets. Internet use datagram approach at the network layer.

LAN switches rely on packet-switching. The switch establishes a connection between two segments just long enough to send the current packet. Incoming packets (part of an Ethernet frame) are saved to a temporary memory area (buffer); the MAC address contained in the frame's header is read and then compared to a list of addresses maintained in the switch's lookup table. In an Ethernet-based LAN, an Ethernet frame contains a normal packet as the payload of the frame, with a special header that includes the MAC address information for the source and destination of the packet.
Packet-based switches use one of three methods for routing traffic:

• Cut-through
• Store-and-forward
• Fragment-free

Cut-through switches read the MAC address as soon as a packet is detected by the switch. After storing the 6 bytes that make up the address information, they immediately begin sending the packet to the destination node, even as the rest of the packet is coming into the switch.

A switch using store-and-forward will save the entire packet to the buffer and check it for CRC errors or other problems before sending. If the packet has an error, it is discarded. Otherwise, the switch looks up the MAC address and sends the packet on to the destination node. Many switches combine the two methods, using cut-through until a certain error level is reached and then changing over to store-and-forward. Very few switches are strictly cut-through, since this provides no error correction.

A less common method is fragment-free. It works like cut-through except that it stores the first 64 bytes of the packet before sending it on. The reason for this is that most errors, and all collisions, occur during the initial 64 bytes of a packet.