(i) Token ring

(ii) FDDI

(i) Token ring:

When a station has some data to send, it waits until it receives the token from its predecessor. It then holds the token and sends its data. When the station has no more data to send, it releases the token, passing it to the next logical station in the ring. The station cannot send data until it receives the token again in the next round. In this process, when a station receives the token and has no data to send, it just passes the data to the next station. Token management is needed for this access method. Stations must be limited in the time they can have possession of the token. The token must be monitored to ensure it has not been lost or destroyed. Another function of token management is to assign priorities to the stations and to the types of data being transmitted. And finally, token management is needed to make low-priority stations release the token to high­ priority stations. In the physical ring topology, when a station sends the token to its successor, the token cannot be seen by other stations; the successor is the next one in line. This means that the token does not have to have the address of the next successor. The problem with this topology is that if one of the links-the medium between two adjacent stations ­fails, the whole system fails. The dual ring topology uses a second (auxiliary) ring which operates in the reverse direction compared with the main ring. The second ring is for emergencies only (such as a spare tire for a car). If one of the links in the main ring fails, the system automatically combines the two rings to form a temporary ring. After the failed link is restored, the auxiliary ring becomes idle again. Note that for this topology to work, each station needs to have two transmitter ports and two receiver ports. The high-speed Token Ring networks called FDDI (Fiber Distributed Data Interface) and CDDI (Copper Distributed Data Interface) use this topology. In the bus ring topology, also called a token bus, the stations are connected to a single cable called a bus. They, however, make a logical ring, because each station knows the address of its successor (and also predecessor for token management purposes). When a station has finished sending its data, it releases the token and inserts the address of its successor in the token. Only the station with the address matching the destination address of the token gets the token to access the shared media. The Token Bus LAN, standardized by IEEE, uses this topology.

(ii) FDDI:

Fibre Distributed Data Interface is high performance fibre optic token ring. It is similar to 802.5 and IBM token rings, except the difference that FDDI runs on fiber, not copper. FDDI cabling consists of two fibre rings, one transmitting clockwise and the other transmitting counter clockwise. If either one breaks, the other end can be used as a backup. If both break at the same point, due to a fire or other accident in cable duct, the two rings can be joined into a single ring. Each station contains relays that can be used to join the two rings or bypass the station in the event of the station problems. To transmit data, a station must first capture the token. Then it transmits a frame and removes it when it comes round again, one more difference between FDDI and 802.5 is that in 802.5 a station may not generate a new token until its frame has gone all the way around and come back. The FDDI Mac protocol uses three times. The token holding timer determines how long a station may continue to transmit once it has acquired the token. This timer prevents a station from hogging the ring forever. The token rotation timer is restarted every time the token is seen. If its timer expires, it means that the token has not been sighted for long on interval, it has been lost, so the token recovery procedure is initiated. Finally, the valid transmission timer is used to time out and recover from certain transient on a given token pass. If the token is ahead of schedule, all the priorities may transmit, but if it is behind schedule, only the highest ones may send.