Editing Ring network (section)

==Access protocols==
Rings can be used to carry circuits or packets or a combination of both.  SDH rings carry circuits.  Circuits are set up with out-of-band signalling protocols, whereas packets are usually carried via a [[Medium access control|Medium Access Control Protocol]] (MAC).

The purpose of media access control is to determine which station transmits when.  As in any MAC protocol, the aims are to resolve contention and provide fairness. There are three main classes of media access protocol for ring networks: slotted, token and register insertion.

The slotted ring treats the latency of the ring network as a large shift register that permanently rotates.  It is formatted into so-called slots of fixed size.  A slot is either full or empty, as indicated by control flags in the head of the slot.  A station that wishes to transmit waits for an empty slot and puts data in. Other stations can copy out the data and may free the slot, or it may circulate back to the source who frees it.  An advantage of source-release, if the sender is banned from immediately re-using  it, is that all other stations get the chance to use it first, hence avoiding bandwidth hogging.  The pre-eminent example of the slotted ring is the [[Cambridge Ring (computer network)|Cambridge Ring]].