Token Ring

Token Ring

Developed by IBM, Token Ring, is standardized to IEEE 802.5. Token Ring uses a star topology, but it is wired so the signal will travel from hub to hub in a logical ring. These networks use a data token passed from computer to computer around the ring to allow each computer to have network access. The token comes from the nearest active upstream neighbor (NAUN). When a computer receives a token, if it has no attached data and the computer has data for transmission, it attaches its data to the token then sends it to its nearest active downstream neighbor (NADN). Each computer downstream will pass the data on since the token is being used until the data reaches its recipient. The recipient will set two bits to indicate it received the data and transmit the token and data. When the computer that sent the data receives the package, it can verify that the data was received correctly. It will remove the data from the token and pass the token to its NADN.

It can use UTP, STP, or fiber optic cable. Maximum cable length is 45 meters with UTP and 101 meters with STP. Topology is star-wired ring. Uses type 1 STP and type 3 UTP. Connectors are RJ-45 or IBM type A. Minimum length between nodes is 2.5 meters. Maximum number of hubs or segments is 33. Maximum nodes per network is 72 nodes with UTP and 260 nodes with STP. Speed is 4 or 16 Mps. IEEE 802.5 defines token ring. Data frames may be 4,000 to 17,800 bytes long.

Hubs

A token ring network uses a multistation access unit (MAU) as a hub. It may also be known as a Smart Multistation Access Unit (SMAU). A MAU normally has ten ports. Two ports are Ring In (RI) and Ring Out (RO) which allow multiple MAUs to be linked to each other. The other 8 ports are used to connect to computers.

Cables

UTP or STP cabling is used as a media for token ring networks. Token Ring uses an IBM cabling system based on American Wire Gauge (AWG) standards that specify wire diameters. The larger the AWG number, the small diameter the cable has.

Cable types:

• Type 1 - Two 22 AWG solid core pair of STP cable with a braided shield. This is the normal cable between MAUs and computers.
• Type 2 - Two 22 AWG solid core pair with four 26 AWG solid core of STP cable.
• Type 3 - Four 22 or 24 AWG UTP cable. This is voice-grade cable and cannot transmit at a rate above 4Mbps.
• Type 4 - Not defined.
• Type 5 - Fiber-optic cable. Usually used to link MAUs.
• Type 6 - Two 26 AWG stranded core pair of STP cable with a braided shield. The stranded-core allows more flexibility but limits the transmission distance to two-thirds that of type 1.
• Type 7 - Not defined.
• Type 8 - Type 6 cable with a flat casing to be used under carpets.
• Type 9 - Type 6 cable with plenum-rating for safety.

Token ring networks normally use Type 1, type 3 or regular UTP similar to that used with ethernet installations. If electrical interference is a problem, the type 1 cable is a better choice.

Beaconing

The first computer turned on on a token ring will be the active monitor. Every seven seconds it sends a frame to its nearest active downstream neighbor. The data gives the address of the active monitor and advertised the fact that the upstream neighbor is the active monitor. That station changes the packets upstream address and sends it to its nearest

active downstream neighbor. When the packet has traveled around the ring, all stations know the address of their upstream neighbor and the active monitor knows the state of the network. If a computer has not heard from its upstream neighbor after seven seconds, it will send a packet that announces its own address, and the NAUN that is not responding. This packet will cause all computers to check their configuration. The ring can thereby route around the problem area

giving some fault tolerance to the network.

Troubleshooting

An IBM PS2 reference disk will set speed to 4 Mbps. Three card settings may be changed which are address, shared RAM address, and early token release feature. The same brand MAUs should be used through the system.