Medium Access Control

In large networks there are many differernt paths for communication and if two or more nodes wish to transmit at the same time they can use different network paths, thus avoid collisions.
In a Bus or Ring topology all notes share the same transmission media and therefore they use MAC methods to prevent data collision.

MAC is used to arbitrate which transmitter in a network gain access to the transmission media

It is used to prevent two or more nodes from trying to transmit information at the same time --interfering electrical signals may cause data loss or corruption.

  Poll/Select

MASTER

In a poll/Select mechanism only one station on the network is allowed to transmittions.

The master polls devices for information and will only request information from a device once its previous request has been successfully completed.

An error detecion machanism is inherernt with this method of communication: a note failing to reply to a request after a certain period time --out period may have failed.

This type of communication can be made quite fast to small networks.
 There have some disadvantages to this system:
1,time ,if note B is broken ,no respone to B
2,large number of data,to the master.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)
The CSMA/CD method is illustrated in
In a CSMA/CD MAC network, transmitting nodes first check the network to see if the bus is clear  i .e there are no electrical signals on the bus (Carrier sense)

If the bus is idle,  a node begins transmittion of its frame or message , other nodes will see this transmission and will not transmit themselves .

The transmitting node also'listen' to the bus at the same time. Therefore, if two nodes try to transmit data smultaneously ,remember propagation delays may have an effect  the actual signal on the bus will differ from the transmitted one (intended signal)

Both transmitting nodes will detect this variation and stop their transmission.

After a certain period, the nodes will try to re-transmitt again,provided the bus is free, this period is sometimes randomly defined, as in the case of Ethernet.


Token Ring and Token Bus MAC
 A control token is a special kind of frame used to arbitrate bus access. In this scheme the control token is passed from one node to the next and only when a node has possession of the token may it transmit data.

In a real time control enviroment small amounts of data need to be transmitted at regular intervals.

To prevent a node from continually transmitting data and monopolising the token each node is configured to hold the token for a limited period of time ,known as Token Hold Time.

Care must be taken to make the THT small enought to give the network fast response times but large enough to allow node to tramsmit its data;otherwise a data build up can occur on the node(profibus).


Slotted Ring
Slotted Ring MAC is primarily used in Ring networkss.
A slotted Ring MAC is initialised with a fixed number of bits for transmission by a special mode known as the Monitor.

The bits stream is continually shifted around the ring from one node to another.The ring contains a fixed number of transmission slots . each made up of  singal frame of information.


Initial all slots are marked as empty.

When a note detects an empty slot it fills in the frame with data and destination address for the data it then marks the slot as full.

The slot is then circulated through each node until the destination address match the address if the noteIf the address do not match ,the node simply ignores the slot. If the address match, the node reads the data and sets the acknowledge field.

Then the slot re-circuit until it reaches the source which detects the acknowledge and marks the slot as emptyIf there is no acknowledge, an error has occurred .