In switched digital networks
the switching nodes can operate in three different ways:
-
Circuit Switching
The switches on a route establish a
physical circuit between two hosts for the duration of the
communication session. This is how the telephone system
operates.
-
Message Switching
The sending host parcels up data into long
messages which are transferred step by step over each link between
the switches and delivered to the recipient.
-
Packet Switching
As message switching, but the parcels are
short, so that the switching nodes can store numbers of them in
memory.
Packet switching is favored for computer communication:
-
It makes good use of the links. Between bursts of data
between one pair of hosts. other hosts may use the same links
and switches.
-
Delays due to storing short packets in switches are short
compared with storing long messages.
-
Switches need less memory/buffer space to hold packets for
forwarding than for messages.
-
Hardware to implement packet switches is now relatively cheap.
-
Switches and links may break during a communication session, but
alternative routes be found.
-
Packet switched networks have been in use for over 15 years.
-
Early examples are the now decommissioned US ARPANET and the UK
National Physical Laboratory Network.
-
The PNOs provide international packet switched networks.
-
National networks now exist with millions of attached
computers.
For a packet to get from one host to another through a series
of switches, each switch on the route requires to know where
the packet is going.
The range of network characteristics is wide and
WAN and LAN Characteristics are varied:
-
WAN data rates go from 300 bps up to 100s Mbps
-
Error rates of order 1 in #tex2html_wrap_inline4196#
-
Delays from 100msecs to Seconds.
-
Bit serial interfaces.
-
LAN data rates from 10 kbps to 140Mbps
-
Error rates of order 1 in #tex2html_wrap_inline4198#
-
Delays of order msecs.
-
Frame or byte level interfaces.