). Furthermore, the control mechanism for allocation of slots can be centralised or distributed. In the slotted ring (Pierce, 1972) the ring is divided into a number In distributed systems each user needs to maintain knowledge of the of slots. These slots can hold packets of data, and contain an queue length and its position in the queue. Often all users transmit indication bit which shows whether the slot is empty or full. The information of the queue status within their packet headers, so that ring needs to have a considerable amount of delay, often introduced new users who join the queue can acquire its history. For centrally artificially, in order to hold a reasonable number of circulating slots. controlled systems this function would be performed by the master When a user wants to transmit data it waits for an empty slot to station. arrive. This is then marked as busy and the data in placed in it. The In distributed control it is also required that data and reservation receiver retrieve the data from the slot and once again marks it as requests from each user is broadcast, SO that it is received by all free. Clearly the data needs to be the same size as the slot, so this other users, and they know the state of the queue. In a centrally system uses fixed slot sizes. controlled system it is only necessary for the central controller to know about the reservations being made, and for it to be able to instruct the other users. 13.7.2.4 Register insertion ring The efficiency of a reservation system is closely related to the overhead which is used to make reservations. If, for example, the Operation of this method (Liu, 1978) is best explained by the fraction of the bandwidth is used for making reservations, as given simplified representation of the interface of a node to the ring, as in by Equation 13.34, then the maximum throughput of a reservation Figure 13.33. Data from the line feeds into a shift register and its channel is given by Equation 13.35. Input to Bandwidth occupied by reservation slots (13.34) Point