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Token Ring

After Ethernet, Token Ring is the second most widely used LAN technology, and its second rank is due mostly to its cost factor. Ethernet technology is simply cheaper to implement than is Token Ring. Token Ring is a ring topology created by IBM in the 1970s. The IEEE 802.5 subcommittee—with help from IBM representatives—developed a set of standards that described a token-passing network in a ring topology.

Token-passing networks move a stream of data, called a token, around the network. Any station with a message or data to transmit waits until it receives a free token. It then changes the free token to a busy token and transmits a block of data called a frame. The frame contains the data that needs to be sent to the rest of the network. The token circulates around the ring, passing through as many as three stations at a time until it finds the receiving station.

The receiving station copies the data from the frame, and the frame continues around the ring, making a complete round trip back to the original transmitting station. The transmitting station now knows the frame has been received, and the station then purges the busy token it has been keeping and inserts a new free token on the ring for others to use.

The use of token-passing prevents messages from interfering with one another by guaranteeing that only one station at a time is transmitting. Therefore, collisions cannot occur on the network. Unlike Ethernet, token passing ensures the delivery of the frame.

The Token Ring topology runs at the Physical and Data Link layers of the OSI model, and it is modeled as a star topology using STP wiring. Each station is connected to a central hub called a multistation access unit (MSAU) that houses electromechanical relays to make the physical star into a logical ring. The logical ring is where each station receives signals from its nearest active upstream neighbor (NAUN) and repeats these signals to its downstream neighbors.

Token Ring networks use a priority system that permits certain user- designated, high-priority stations to use the network more frequently. Priority levels are configured by the network administrator. A Token Ring network has two fields inside the frame of the token, as shown in Figure 3.5, that control priority: the priority field and the reservation field. When a priority token is transmitted, it can be seized by only those stations with a priority that is equal to or higher than the priority value contained in that token. The station then seizes the token and changes it to an information frame, and only stations with a priority value higher than that of the transmitting station can reserve the token for the next pass around the network. When the next token is generated, it includes the higher priority of the reserving station. Stations that raise a token's priority level must reinstate the previous priority after the transmission is complete.

The original IBM Token Ring product ran at speeds of 4Mbps. In 1989, IBM released a faster version of Token Ring, which ran at 16Mbps. Over time, a high-speed Token Ring (HSTR) was released that operated at 100Mbps and led to 1Gbps Token Ring. The speed has maintained with the evolving technology and makes Token Ring topology a serious contender against Ethernet.

Frame Format of Token Ring

Token Ring is similar to FDDI frames, in that they both support token formats, as well as data. Figure 3.5 illustrates the frame format of both the token being passed and a frame sent from a node on the ring during a Token Ring network communication.

Here is a brief explanation of each field in a Token Ring frame as shown in Figure 3.5.

  • Start Delimiter—Alerts each station of a token and uses a unique coding for the frame.

  • Access-Control Byte—Contains a series of bits that circulate throughout the ring and are used by the active monitor to ensure delivery: a Priority bit indicates the priority of the frame or token; a Reservation bit indicates the priority required for the next token to gain access to the ring; a Token bit differentiates a token from a data or command frame; and a Monitor bit determines whether a frame is circling the ring endlessly. Active monitor employs a mechanism for detecting and compensating for network fault.

  • Frame Control—Indicates the frame type and contains the Frame type bit, the Reserved bit, and the Control bits.

  • Destination Address—Indicates the address of the receiver.

  • Source Address—Identifies the address of the sender.

  • Data—Indicates the actual data being sent from the upper layers.

  • Frame Control Sequence—Ensures that all the frames are delivered without damage.

  • End Delimiter—Defines the end of the token or frame and contains bits to indicate if a frame is damaged.

  • Frame Status—Terminates the frame and ensures that the frame has been copied to the destination address.

Figure 3.5Figure 3.5 The Token Ring Frame formats.

Active Monitor

Token Ring networks employ a mechanism for detecting and compensating for network faults. Any station on a Token Ring network can be selected to be the active monitor. This active monitor station acts as a centralized source of timing information for the Token Ring stations and makes sure that there isn't more than one token on the ring at any given time. Also, when a sending device fails, its frame may continue to circle the ring. This can prevent other stations from transmitting their frames, which may lock up the network. The active monitor can detect such frames, remove them from the ring, and generate a new token. The active monitor also has several standby monitors that act as backups in case the active monitor goes offline.

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