Packet Switching

Packet Switching

Long-haul circuit-switching telecommunications networks were originally

designed to handle voice traffic, and the majority of traffic on these networks continues

to be voice. A key characteristic of circuit-switching networks is that

resources within the network are dedicated to a particular call. For voice connections,

the resulting circuit will enjoy a high percentage of utilization since, most of

the time, one party or the other is talking. However, as the circuit-switching network

began to be used increasingly for data connections, two shortcomings

became apparent:

• In a typical terminal-to-host data connection, much of the time the line is idle.

Thus, with data connections, a circuit-switching approach is inefficient.

• In a circuit-switching network, the connection provides for transmission at a

constant data rate. Thus each of the two devices that are connected must transmit

and receive at the same data rate as the other, which limits the utility of the

network in interconnecting a variety of host computers and terminals.

To understand how packet switching addresses these problems, let us briefly

summarize packet-switching operation. Data are transmitted in blocks, called

packets. A typical upper bound on packet length is 1000 octets (bytes). If a source

has a longer message to send, the message is broken up into a series of packets

(Figure 3.6). Each packet consists of a portion of the data (or all of the data for a

short message) that a station wants to transmit, plus a packet header that contains

control information. The control information, at a minimum, includes the information

that the network requires in order to be able to route the packet through the

network and deliver it to the intended destination. At each node en route, the

packet is received, stored briefly, and passed on to the next node.

The packet-switching approach has a number of advantages over circuit switching:

s Line efficiency is greater, since a single node-to-node link can be dynamically

shared by many packets over time. The packets are queued up and transmitted

as rapidly as possible over the link. By contrast, with circuit switching, time on

a node-to-node link is preallocated using synchronous time division multiplexing.

Much of the time, such a link may be idle because a portion of its time is

dedicated to a connection that is idle.

s A packet-switching network can carry out data-rate conversion. Two stations

of different data rates can exchange packets, since each connects to its node at

its proper data rate.

• When traffic becomes heavy on a circuit-switching network, some calls are

blocked; that is, the network refuses to accept additional connection requests

until the load on the network decreases. On a packet-switching network, packets

are still accepted, but delivery delay increases.

• Priorities can be used. Thus, if a node has a number of packets queued for

transmission, it can transmit the higher-priority packets first. These packets

will therefore experience less delay than lower-priority packets.

Packet switching also has disadvantages relative to circuit switching:

.. Each time a packet passes through a packet-switching node it incurs a delay not

present in circuit switching. At a minimum, it incurs a transmission delay equal to

the length of the packet in bits divided by the incoming channel rate in bits per

second; this is the time it takes to absorb the packet into an internal buffer. In addition,

there may be a variable delay due to processing and queuing in the node.

s Because the packets between a given source and destination may vary in length,

may take different routes, and may be subject to varying delay in the switches

they encounter, the overall packet delay can vary substantially. This phenomenon,

called jitter, may not be desirable for some applications (for example, in

real-time applications, including telephone voice and real-time video).

41 To route packets through the network, overhead information, including the

address of the destination, and often sequencing information must be added to

each packet, which reduces the communication capacity available for carrying

user data. This is not needed in circuit switching once the circuit is set up.

• More processing is involved in the transfer of information using packet switching

than in circuit switching at each node. In the case of circuit switching, there

is virtually no processing at each switch once the circuit is set up.

Basic Operation

A station has a message to send through a packet-switching network that is of

greater length than the maximum packet size. It therefore breaks the message into

packets and sends these packets, one at a time, to the network. A question arises as

to how the network will handle this stream of packets as it attempts to route them

through the network and deliver them to the intended destination. Two approaches

are used in contemporary networks: datagram and virtual circuit.