Information on Switches:
In a telecommunications network, a switch is a
device that channels incoming data from any of
multiple input ports to the specific output port
that will take the data toward its intended destination.
In the traditional circuit-switched telephone
network, one or more switches are used to set
up a dedicated though temporary connection or
circuit for an exchange between two or more parties.
On an Ethernet local area network (LAN), a switch
determines from the physical device (Media Access
Control or MAC) address in each incoming message
frame which output port to forward it to and out
of. In a wide area packet-switched network such
as the Internet, a switch determines from the
IP address in each packet which output port to
use for the next part of its trip to the intended
destination.
In the Open Systems Interconnection (OSI) communications
model, a switch performs the layer 2 or Data-Link
layer function. That is, it simply looks at each
packet or data unit and determines from a physical
address (the "MAC address") which device a data
unit is intended for and switches it out toward
that device. However, in wide area networks such
as the Internet, the destination address requires
a look-up in a routing table by a device known
as a router. Some newer switches also perform
routing functions (layer 3 or the Network layer
functions in OSI) and are sometimes called IP
switches.
On larger networks, the trip from one switch
point to another in the network is called a hop.
The time a switch takes to figure out where to
forward a data unit is called its latency. The
price paid for having the flexibility that switches
provide in a network is this latency. Switches
are found at the backbone and gateway levels of
a network where one network connects with another
and at the subnetwork level where data is being
forwarded close to its destination or origin.
The former are often known as core switches and
the latter as desktop switches.
In the simplest networks, a switch is not required
for messages that are sent and received within
the network. For example, a local area network
may be organized in a Token Ring or bus arrangement
in which each possible destination inspects each
message and reads any message with its address. |
