can create problems for the entire network. This can be solved by using a dual ring or a switch that closes off the break. Moving, adding and changing the devices can affect the network Communication delay is directly proportional to number of nodes in the network Bandwidth is shared on all links between devices More difficult to configure than a Star: node adjunction = Ring shutdown and reconfiguration 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 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. Misconceptions "Token Ring is an example of a ring topology." 802.5 (Token Ring) networks do not use a ring topology at layer 1. As explained above, IBM Token Ring (802.5) networks imitate a ring at layer 2 but use a physical star at layer 1. "Rings prevent collisions." The term "ring" only refers to the layout of the cables. It is true that there are no collisions on an IBM Token Ring, but this is because of the layer 2 Media Access Control method, not the physical topology (which again is a star, not a ring.) Token passing, not rings, prevent collisions. "Token passing happens on rings." Token passing is a way of managing access to the cable, implemented at the MAC sublayer of layer