Tokenring Networks
A token passing ring LAN is a group of computers connected in a loop. The group uses a token passing access mechanism. A computer wishing to send data should first receive permission. When it gets control of the network it may transmit a frame. Each frame transmitted on the ring is transmitted from one computer to the next, until it ultimately returns to the initiator of the transmission.
The Token Passing Ring Network was originally developed by IBM and only Ethernet LANs are more popular. The IEEE 802.5 specification which was modeled after IBM's Token Ring is almost identical and the term Token Ring is used to refer both specifications. FDDI (Fiber Distributed Data Interconnect) is an improved token ring specification based on fiber as the physical medium. As opposed to Token Ring's single ring, FDDI, uses two to achieve better results. CDDI, yet another standard, resembles FDDI, but uses a copper wire for it's ring.
Characteristics
Comparison of Basic Characteristics
| Technology |
Data Rate (Mbps) |
Maximum Segment Length (m) |
Media |
Rings |
Recovery |
| IBM Token Ring |
4/16 |
250 Shielded 72 Unshielded |
Twisted pair |
1 |
Can handle a computer failure but cant recover from a broken connection. |
| IEEE 802.5 |
4/16 |
250 |
Not specified |
1 |
Can handle a computer failure but cant recover from a broken connection. |
| CDDI |
4/16 |
250 Shielded 72 Unshielded |
Twisted pair |
2 |
Can recover from a broken connection (Self healing). |
| FDDI |
100 |
Unlimited |
Optical fiber |
2 |
Can recover from a broken connection (Self healing). |
Fault Management Mechanisms
In order to detect and correct network faults Token Ring networks may dedicate a station for monitoring frames which are circling around without being dealt with. This monitor removes such frames and allow the network to function in a normal manner all over again.
As described above FDDI networks implements a recovery mechanism which enable the
network to function properly even under a broken ring. FDDI uses two rings to achieve
recovery capabilities. As shown a token is passed simultaneously on the network's inner and outer rings which backup each other.
As shown in the figure below, in case of broken connection or station malfunction, the closest station closes the network loop by sending the token it received from the outer/inner ring back using the inner/outer ring. This feature is called Self healing.
Frame Format
Token Ring Frame Formats (tokens and data/command)
Tokens consist of:
- Start delimiter - which alerts the stations of a token arrival (or data/command frame).
- Access control byte - which contains the priority and reservation fields, a token bit to differentiate token from data/command frame and a monitor bit checking whether a frame is circling the ring endlessly.
- End delimiter - which signals the end of a frame, end of a logical sequence and damaged frames. Data/Command Frames carry information for upper-layer protocols.
After the Access control byte, a frame control byte arrives and indicates whether it is a data or control information (and which) frame. Then arrives two address fields (source & destination) each 6 bytes long. Data follows these fields (its length depends on the time each station can hold a token) and then a FCS (frame check sequence) field. At the end as in tokens, an end delimiter completes the frame.
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