y slot in the token and assigning each user a priority level. When a user wishes to Control transmit it waits for a token to pass and, if it is busy, it puts its priority number into the priority slot, if its priority is higher than any Remove which may already be in this slot. The token is then allowed to Insert circulate round the ring. When the original user receives this busy token back it generates a free token and marks it with the priority level which it received. Now stations with this priority level, or above, are only allowed to seize the token. Figure 13.30 Interface to a token passing network: (a) Figure 13.31 shows typical throughput curves for token passing, listen mode; (b) transmit mode comparing this with a contention system such as CSMA/CD. Al- though the throughput is relatively constant as the number of users increases, above a certain minimum, the delay also increases, as for Because of the requirement to read and regenerate each bit, there polling. is a bit delay at each user node, which can become quite significant if many users are connected to the line. In addition the delay in the overall line must be high enough to permit a whole token to circulate at any time. If the user wishes to transmit data then it waits for a token to Token passing appear. It knows this by the unique bit pattern, and as soon as the token passes through the user's node it is marked as busy. This can Token passing Decreasing a be done, for example, by reversing the polarity of the last bit (i.e. 11111110). The user is now in the transmit mode. All the data input CSMA/CD from the line is absorbed (since it is not relevant, no user being permitted to transmit without the token) and the data from the user's buffer is emptied out onto the line, immediately after the busy token. Decreasing a After all the data has been transmitted the user regenerates the token which can now be seized by the next ready user in the chain. CSMA/CD When the traffic is light the token circulates from user to user around the ring. When the t