Access Methods
‘Access method’ means how the computers on a network share the cable. How do they know when they may use the cable?
The Problem:
Only one station can send data on the cable at any time, yet many computers must share one cable. How do we avoid collisions?
Solution 1 - CSMA/CD:
This is the system where a station wishing to send listens to the cable. If the cable is free, the station sends. If the cable is busy moving someone else’s packet, then the station waits a moment then tries again.
It is possible, however, that two stations check the wire at exactly the same time, and both see that it is free. In this case they would both send at the same time. Their packets would then collide.
Each NIC is able to tell when a collision has happened. In this case both stations wait a random amount of time, then try again.
Collision detection only works if the two sending PCs are less than 2500m apart. This gives the maximum size for a LAN using this system.
CS - Carrier Sense
Listen to the cable. Carrier is the name for the electrical signal which represents the data. If you hear (sense) the carrier, the cable is busy.
MA - Multiple Access
Many computers wish to access the wire at the same time.
CD - Collision Detect
A collision is when two packets bump into each other. The NICs are able to detect (tell) when this has happened.
CSMA/CD is a contention protocol. Contention means a disagreement or argument. The NICs are not patient, they argue about who will use the cable next.
Solution 2- Token Passing
This is a non-contention system. Here the NICs do not argue, they wait patiently until it is their turn to send.
A token ring network must use a special hub that makes a star topology behave like a ring.
A special packet called a token is passed around the ring by the hub, in order, from station to station. A computer wishing to send must wait until the token comes around. When it gets the token it eats it, and sends out its data packet instead. The data packet is passed by the hub in turn to each station in the ring. The station that the packet is addressed to copies the data, then passes the packet back to the hub and so on to the next station. All other stations simply pass it back to the hub.
When the data packet returns to the sender, the sender sends out a new token which can be used by anyone else waiting to send.
Solution 3 - Demand Priority
This is a new system used with high speed (100 Mbps) networks called 100VG-anyLAN.
It is built around highly intelligent hubs and cables that allow sending and receiving at the same time. Instead of a sender broadcasting its data to all stations, the hub routes the packet only to its addressee.
Under this system, different types of data can be assigned different priorities, and higher priority packets will always be sent before lower priority packets.
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