egment exclusively to a user. This is shown pictor- Ring architectures are very powerful and often used in networks. ially in Figure 13.18. Guard bands are used between each segment The property of a ring is that the communication line passes through of the frequency band to prevent interference between users. the nodes, and these are not just interfacing to it. The information The advantage of the FDMA system is its simplicity, since once passing through a node is stored a bit at a time and then forwarded. the channel capacity is divided amongst users each can operate There is therefore a one bit delay, which can become significant if there are many users on the ring. Contention rings use a token, similar to the token passing method described later. A token is a unique combination of bits which gives User D a user authorisation to transmit its data. There is only one token on the line at any time and the only user who can transmit data is the User C one holding the token, SO contention is avoided. Unlike the token passing method, a contention ring does not have User B Guard bands a continuously circulating token, SO when none of the users has data to transmit the ring is empty. If a user wishes to transmit it checks User A to see if a bit is passing though it. If not it commences transmission and at the end of this it puts a token onto the ring. When the token again comes round to the originator he removes it from the ring. If there is traffic on the ring when a user wishes to commences transmission it waits until the token passes through it. This is then Figure 13.18 User assignment within FDMA converted to a 'connector', usually by changing the polarity of the last bit, and then it commences transmission. Since there is now no token on the line no other station can transmit. If two stations start transmission simultaneously, because they independently of each other. Since each user has exclusive use of its both checked the line and did not find a bit passing them, then they allocated bandwidth there is no cont