Circuit Switching, Packet Switching, and Message Switching

Circuit Switching — In circuit-switched networks , a dedicated path
is set up between the two parties. This path remains for the exclusive use of both
parties for the duration of the call, and is therefore not available to any other users.
This method has traditionally been used within standard telephone exchanges and
networks since telephony was first developed.
There is a delay involved in setting up circuit-switched calls because each of the
switching nodes has to route the call. However, the actual delays encountered once
the connection has been established are minimal. This makes circuit switching
ideal for voice and other real-time applications.
Circuit switching can be inefficient in its use of network resources. During a
voice conversation there are periods when neither party is talking but the connection
is still tied up and unavailable for other users. Similarly, bursty data, which has
gaps between the data, is not efficiently carried over circuit-switching networks.
Charging for circuit-switched services is generally based on the duration of the
call. The PSTN and ISDN are examples where circuit switching is employed.
Packet Switching — Packet switching involves dividing the data into packets (or
cells or frames) prior to transmission. The length of the packets varies enormously,
depending on the technology employed.
Added to each packet is the destination address, together with other control
information. The packets are then transmitted across the network. This addressing
means there is no requirement to set up a pre-established link. To some extent, each
individual packet can be viewed as being able to find its own way to its destination.
In a packet-switched network (Figure 2.23), the resources are shared between
many users. This leads to more efficient use of these resources than provided for by
circuit-switching techniques. However, with packet switching there is a danger of
congestion occurring. The subsequent delays are both variable and unpredictable.
Much effort has been put into reducing these delays for applications that require
near-real-time transmission such as voice telephony.
When data is divided into packets, it does not follow that all the packets that
contain the original data will follow the same route to the destination. This can
order before delivery to the recipient. Examples of packet-switching technologies
include X25, FR (Frame Relay), ATM (Asynchronous Transfer Mode), and IP
(Internet Protocol).
Because it is possible to charge for data throughput rather than for the duration
of the connection within a packet-switched network, it is more feasible to have permanent
online connections than can be provided for by traditional circuit-switched
networks. Most commentators see packet switching as the future backbone of new
high-bandwidth telecommunications services.
Message Switching — It is not always necessary to establish an end-to-end circuit
for the transmission of data. So-called store and forward techniques can be
applied.
In Figure 2.24, an e-mail message is transmitted between nodes A and D.
Because no circuit is established, the message is carried in stages over the links
between the nodes. At each stage, the message is stored within the node while the
next link is established. This does lead to queuing delays at each stage; but to the
applications utilizing message switching, these small delays are unimportant.
Another example of the use of message switching is for SMS text messaging
within a GSM network (Figure 2.25.). When a user sends a text message, it is
transmitted across the air interface into nodes within the GSM network. The entire
message is stored within the node responsible for SMS messaging. Here, the message
is stored before it can be forwarded to its destination. Should a node be unable
to forward a message for any reason, it will retry until an expiry time or number of
attempts is reached. Depending on the application, a failure message can be transmitted
to the originating subscriber to inform them that the message transmission
has failed.