FREE online courses on Information Technology - Chapter 8 NETWORK &
COMMUNICATIONS IN I T - Ethernet and Token ring
At a low level, the LAN must transmit data over the network.
How is transmission coordinated so every workstation does not try to send data
at the same time and thus block the data from other PC's? One solution to this
contention problem is called carrier sense-multiple access with collision
detection (CSMA/CD). The transmitting
station checks whether a channel is clear by listening for a carrier signal. If
the net is busy, the station waits until it is clear and then send a message
while listening for collision with other stations that might have started to
send at the same time. If a collision is detected, the station stops sending and
waits a random time interval before starting to send again.
Ethernet is the best example of a
CSMA/CD protocol.
An alternative scheme is to use a token that a pass along the
network from node to node, an approach adopted by IBM. A station can transmit
when it sees a free token come by; this approach is more complicated to
implement than CSMA/CD, but it reduces the collision problem. FDDI uses a token ring approach.
As you might expect, LAN's grew in organizations without a
lot of planning. Different departments are even different floors in buildings
have their own LANs. Sometimes these LANs use different standards, one might use
Apple Talk and serve Macintosh computers while another features IBM PCs and a
token ring architecture. Sooner or later, users on these networks want to be
connected to each others. The solution is to use a bridge or a router. A bridge has a very little logic. It
connects similar networks. A router
contains logic and serves to interface two or more networks to each other and
possibly to a wide area network. Routers have enough logic to select the best
path between two nodes on a network.
All of the wires for a network come together in some kind of wiring hub. You
might use a bridge or a router to connect hubs on different floors of a building
“smarter”. An organization might route all of the wires from the local wiring
hub to an intelligent hub. This smart hub contains bridges and routers and is
controlled by software, since the hub itself is a computer.
