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A Token Ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time.
A D V E R T I S E M E N T
The Token Ring protocol is the second most widely-used protocol on local area networks after Ethernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 Token Ring technology provides for data transfer rates of either 4 or 16 megabits per second. Very briefly, here is how it works:
Empty information frames are continuously circulated on the ring.
When a computer has a message to send, it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame.
The frame is then examined by each successive workstation. If the workstation sees that it is the destination for the message, it copies the message from the frame and changes the token back to 0.
Characteristics
Maximum cable length is 45 meters when UTP cable is used and 101
meters when STP is used. Topology is star-wired ring. It 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. 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. Token ring networks normally use type 1, type 3 or regular UTP like cable
used on ethernet installations. If electrical interference is a problem, the
type 1 cable is a better choice. Cable types:
| Type |
Description |
| 1 |
Two 22 AWG solid core pair of STP cable with a braided shield. This
cable is normally used between MAUs and computers. |
| 2 |
Two 22 AWG solid core pair with four 26 AWG solid core of STP cable. |
| 3 |
Four 22 or 24 AWG UTP cable. This is voice-grade cable and cannot
transmit at a rate above 4Mbps. |
| 4 |
Undefined. |
| 5 |
Fiber-optic cable. Usually used to link MAUs. |
| 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. |
| 7 |
Undefined. |
| 8 |
Type 6 cable with a flat casing to be used under carpets. |
| 9 |
Type 6 cable with plenum-rating for safety. |
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.
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