5.6. Round- Robin -Access MACUnlike contention -access MAC, the round-robin -access scheme is deterministic, and there is no random allocation of a time slot to a frame. Although this scheme is not as popular as the contention-access LANs, some practical applications are associated with this scheme. Round-robin-access MAC is effective when most users have large amounts of data to transmit, such as in stream traffic. Each user is given a chance to transmit data in a round-robin fashion. Each user may transmit data; if it has no data to transmit, the user passes its turn to the next user. Round-robin access can be implemented in a couple of ways. One method is known as the token-ring access . 5.6.1. Token-Ring Access ProtocolThe token-ring configuration (see Figure 5.8) is based on the round-robin-access MAC. Token-ring passing is a method used for accessing the medium. The ring comprises repeaters interconnected with unidirectional links to form a closed loop. Each user is connected to a repeater. Repeaters perform data insertion, data reception , and data removal. A token is a small frame that is injected into the ring and circulated through the ring. Normally, one user is responsible for this task. Once a new token is circulated through the ring, the user needing to transmit captures the token and then starts transmitting data. Figure 5.8. The movement of a frame in a ring-access MAC networkWhen a user transmits a frame, its repeater forwards the frame to the ring. The frame is circulated completely through the ring. During the circulation of the frame in the ring, each repeater copies the destination address of the frame and examines it. At each stop, if a repeater is the destination, it copies the entire frame. The data is removed by the sender after a complete circulation in the ring. The transmitting user also releases the token to the ring when the following two conditions are met: (1) the transmitting user has finished transmitting the frame and (2) the leading edge of the frame has finished one circulation through the ring and has reached the transmitting user. The advantages of this approach are
A repeater plays an important role in the operation of the ring. The repeater goes to the listen state when passing the data through the ring and then goes to the transmit state when transmitting and receiving data. The repeater can also be in the bypass state when its associated user is down or offline. To provide fairness in the system, an upper limit is set on the amount of data each user can transmit at a time. The control mechanism for this scheme can be either centralized or decentralized. The token-ring scheme is simple, promotes fairness, and provides priority and guaranteed bandwidth. The major flaw with this scheme is maintenance of the token. If the token is lost, the operation of the entire ring is halted. On the other hand, the duplication of token results in frame collisions. For successful ring operations, one user is selected as a monitor to replace a lost token and to ensure that only one token is circulated in the ring.
5.6.2. Token-Ring: IEEE 802.5 StandardThe IEEE 802.5 standard defines the token-ring topology. IEEE 802.5 has also introduced the dedicated token ring, which makes use of a star topology. Users are directly connected to a central hub (concentrator) through full-duplex point-to-point links. This scheme eliminates the use of tokens. |