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Most of us are familiar with our own Local Area Networks (LANs) that use the Ethernet protocol, which operates at the Data Link Layer of the OSI model. What may not be familiar is the fact that LANs may also be designed around the Token Ring and Fiber Distributed Data Interface (FDDI) protocols.
Token Ring
Token ring is a Local Area Network protocol that uses a token-passing network-access technology. It is implemented in a physical ring or physical star topology that creates a logical ring topology. This protocol was initially developed by IBM and was considered to be proprietary. However, the protocol was later codified by the IEEE with the IEEE 802.5 specification.
With token passing, a three-byte token that has a specific and special bit pattern is inserted into a frame and passed in a single direction from one node to another until it forms a complete loop. The node that has current possession of the token is the only one that is allowed to send data at any given time on that LAN. This is considered to be a form of network contention and, because only one node can send data at a time, collisions such as those experienced with Ethernet are avoided.
Rather than using a hub or switch, as does the Ethernet protocol, token ring uses a Multistation Access Unit (MAU) to send a token across the network. The MAU has ports identified as Ring In (RI) and Ring Out (RO). The RO of the first MAU in the ring is connected to the RI of the next MAU. The sequence of token passing continues until the final MAU in the ring, which connects back to the first MAU RI port via its own RO port, is reached. It is important that each device in the ring must be configured for the same speed. Otherwise, the token passing will not function.
One of the primary reasons that Token Ring is considered to be a legacy protocol and rarely used in modern networks is the low bandwidths available. A token-ring LAN can only run at either 4Mbs or 16Mbs. In addition, although this protocol provides a collision-free network, it is more expensive to implement than Ethernet.
FDDI
Another token-passing protocol is known as Fiber Distributed Data Interface (FDDI). This protocol was developed by the American National Standards Institute (ANSI) and is defined in the ANSI X3T9.5 specification. With FDDI, data transmission occurs on fiber-optic cables at a rate of 100Mbs. FDDI was primarily designed and developed to transfer data across the network backbone of a large company.
Instead of using a single copper-media ring, as does token ring, FDDI uses a dual fiber-optic ring that provides both redundancy and fault-tolerance. In addition, since FDDI uses fiber-optic cable, it is not susceptible to Electromagnetic Interference (EMI) as are other media options.
When FDDI devices detect a failure on a network, they use a process called beaconing to send a signal informing the other devices on that LAN that token passing has stopped. The beacon itself travels around the ring from one device to the next until it reaches the last device in that ring. All the network administrator needs to do to troubleshoot a failed network is to identify the beacon at that last device and then check the connection between that device and the next connected device on the FDDI network.
FDDI is also considered to be a legacy protocol and rarely used in modern networks. Although this protocol provides a collision-free network, it is more expensive to implement than Ethernet and is limited to a 100Mbs network speed.
Author: David Stahl
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