Packet Switching

Packet Switching

Allocated applications exchange messages in completing their task. Messages can contain anything the protocol designer desires. Messages may carry out a control function (far example, the "Hi" messages in our handshaking example) or can include data, such as an e-mail message, a JPEG image. or an MP3 audio file. In modern computer networks, the source breaks long messages into smaller chunks of data known as packets. Between source and destination, each of these packets travels through communication links and packet switches (for which there are two major types, routers and link-layer switches). Packets are transmitted over each communication link at a rate equal to the full transmission rate of the link.

The majority of packet switches use store-and-forward transmission at the inputs to the links. Store-and-forward transmission means that the switch must receive the entire packet before it can begin to transmit the first bit of the packet onto the outbound link. In this way store-and-forward packet switches introduce a store-and-forward delay at the input to each link along the packet's route. Think how long it takes to send a packet of L bits from one host to another host across a packet-switched network, Let's suppose that there are Q links between the two hosts, each of rate R bps.  Assume that this is the only packet in the network. The packet must first be transmitted onto the first link originating from Host A; this takes L/R seconds. It must then be transmitted on each of the Q-1 remaining links; that is, it must be stored and forwarded Q-1 times, each time with a store-and-forward delay of L/R. In this way the total delay is QL/R.

Each packet switch has numerous links connected to it. For each attached link, the packet switch has an output buffer (also called an output queue), which stores packets that the router is about to send into that link. The output buffers play a key role in packet switching. lf an arriving packet requires to be transmitted across a link but finds the link busy with the transmission of another packet, the arriving packet must wait in the output buffer. Therefore, in addition to the store-and-forward delays, packets suffer output buffer queuing delays. These delays are variable and depend on the level of congestion in the network. Since the amount of buffer space is limited, an arriving packet may find that the buffer is totally filled with other packets waiting for transmission. In this case, packet loss will occur - either the arriving packet or one of the already-queued packets will be dropped. Returning to our restaurant analogy from earlier in this section, the queuing delay is similar to the amount of time you spend waiting at the restaurant's bar for a table to become free. Packet loss is similar to being told by the waiter that you must leave the premises because there are already too many other people waiting at the bar for a table.

The following figure demonstrates a simple packet-switched network. In this and subsequent figures, packets are represented by three-dimensional slabs.  The width of a slab represents the number of bits in the picket. In this figure, all packets have the same width and hence the same length. Suppose Hosts A and B are sending packets to Host E. Hosts A and B first send their packets along 10 Mbps Ethernet links to the first packet switch. The packet switch then directs these packets to the 1.5 Mbps link.

Packet Switching

If the arrival rate of packets to the switch exceeds the rate at which the switch can forward packets across the 1.5 Mbps output link, congestion will occur as packets queue in the link's output buffer before being transmitted onto the link. We'll look at this queuing delay in more detail in "Delay, Loss and Throughput in Packet-Switched Networks".



Tags

communication links, packet switches, link-layer switches

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