ver a given number of The Carrier Sense Multiple Access (CSMA) technique relies on the frames. They can then transmit over these slots without risk of sender sensing the state of the transmission channel and basing its contention from other users. It is therefore required that each user actions on this. It can therefore only be effectively used in channels keeps a record of the queue associated with each slot so that they which have short propagation delays, since for channels with long know when to transmit their data. This mode of operation is called delays (e.g. satellite) the sensed data is considerably out of date. the reservation mode. This is usually specified as having a small a as given by Equation When there are no remaining reserved slots in the queue (i.e. no 13.7. In a perfect channel (zero transmission delay) the sender could stations waiting to transmit data) the whole frame is divided into listen to the state of the channel when it was ready to transmit and smaller reservation slots, and it is now in the ALOHA mode. Figure only send when the channel was free, so avoiding any collision. 13.12 illustrates the reservation and ALOHA modes. Let t be the time from the start of a transmission from one user to when all users sense the presence of the signal on the line, i.e. the Slot number propagation delay on the network plus the sense time. The vulner- able period when collisions can occur is then 2t, i.e. t after the end of one transmission and t to complete a transmission. This is shown in Figure 13.13. If the channels are slotted into t second time slots, Data slots Reservation slots All users detect end of Slot number transmission from A End of Start of transmission Reservation slots transmission from user B from user A Figure 13.12 Illustration of an explicit reservation ALOHA system: (a) reservation mode; (b) ALOHA mode All users detect start of transmission from B 13.6.1.3 Analysis of reservation ALOHA Figure 13.13 Vulnerable period in CSMA The throughput (SR) of a reservation ALOHA system is af