polling. In token passing a special bit pattern, called a token (for example In token passing three strategies may be used for the transmitting eight logical 11111111) circulates amongst users. This special user regenerating the token. These are: pattern is prevented from being generated by a user, as part of its normal data, by bit stuffing techniques. Users operate in one of two 1. Multiple token operation, where the token is sent straight after mode, listen and transmit, as in Figure 13.30. During the listen mode the user has completed his data transmission. This allows the all the bits are read off the line and then regenerated, one bit at a next user to 'piggy-back' his data onto it, and is a useful time. If the data is for the user, as indicated by the address, then it is technique to use if the ring is long and has appreciable delays. read, otherwise it is just passed on. The user knows a token is present 2. Single token operation, where the user who has sent data (after by the special sequence of bits. having sent the busy token) waits to receive the busy token and his original transmission back again and following the last bit of his data he regenerates the token. This is the system used in the IEEE 802. standards. From line Delay To line 3. Single packet operation, where the token is regenerated after the last bit of the busy token is received back (the user not waiting for his original transmission to be received). Sense The time for which a token is held by a user can be limited to prevent 'hogging'. If this is long the user will be able to empty his buffers and this is known as exhaustive transmission. If it is very short he will only be able to send a packet every time he receives the token. A master clock is required in all instances to synchronise all users. This clock can be maintained by one of the users (who is referred to as a monitor station). This station also carries out other functions, such as checking for loss of token. From line To line Priority operation is achieved by having a priorit