agation delay on the channel throughput for 1-persistent CSMA 5-00"11.01 = (13.24) (13.26) This equation holds for slotted or unslotted channels, it being Figure 13.15 shows the throughput for various CSMA systems assumed that for slotted channels the slot time is much shorter than and compares this with slotted ALOHA. The delay curves are given the packet time. in Figure 13.16. Pure contention protocols are best at low loads and Figure 13.14 shows the variation of throughput with offered load have high channel utilisation and low delay. Contention minimis- for different channel delays, including zero delay. ation systems are best at high loads, when they have higher effi- ciency, but also higher delay at low loads. 13.6.2.2 Non-persistent CSMA 13.6.2.4 CSMA/CD In a non-persistent CSMA a user who is ready to send data senses the line, and if it is free it commences transmission. Provided the In the previous CSMA systems once transmission commences it is completed even if a collision occurs on the first bit. This is clearly delay in the transmission line is small (given as a low a) then there wasteful of bandwidth. In a modification, known as Carrier Sense is good chance of success. Multiple Access with Collision Detection (CSMA/CD), a user If the line is busy then the user does not continue sensing, but monitors the line even when it commences transmission and stops backs off for a random time before sensing it again, and so on. This once a collision is detected (Metcalfe and Boggs, 1976). After that method therefore gives a better utilisation of the line, but also results the user waits a random time before sensing the line again. This in larger delays than the 1-persistent CSMA. multiple access method has been standardised by the IEEE as 802.3 (IEEE, 1985). It is also not always possible for a sender to effectively sense the 13.6.2.3 p-persistent CSMA line during its transmission since the strength of the transmitted signal may be SO strong as to swamp any signal returned back. Often This system is usually used