Time Token Rotation Protocol

The Time Token Rotation Protocol (TTRP) is a token passing procedure from network technology . It is used in FDDI systems.

In contrast to CSMA / CD , as it is used with Ethernet , the token-passing procedure explicitly gives permission to send. Only the stations that receive the send token are allowed to send. In order not to bring the entire system to a standstill in the event of a token loss, suitable protocols must be implemented in order to be able to detect and remedy the loss.

In the TTRP, the mean cycle time of the token is determined between the stations, this time is then doubled and set as the maximum limit value for the cycle time of the transmit token. If this time is exceeded, there is a fault in the ring and appropriate measures are taken.

Process in detail

The TTRP, also called the Target Token Rotation Protocol, should guarantee an average token rotation time between 4 ms and 165 ms. The token rotation time is formed by doubling the target token rotation time (TTRT) determined in the ring. In addition to the synchronous and asynchronous service classes, the token types Restricted and Nonrestricted are also part of the protocol.

In the send mode in FDDI, the early token release is used, which means that a sending station forwards the token immediately after sending the last data packet.

Target Token Rotation Time (TTRT)

The target token rotation time (TTRT) is determined by the so-called claim process while the ring is being set up. The triggering element of this process is the initial commissioning of the ring or the insertion of new stations into the ring. The individual MACs of the connected stations negotiate the TTRT with one another. After the initialization process, each individual station has a so-called operative TTRT in its MAC, which is copied into the respective Token Rotation Time (TRT). You can simply imagine a counter that is regularly decremented.

Token Rotation Time (TRT)

The token rotation time (TRT) is used to measure the current token rotation time and to determine the difference to the planned rotation time (TTRT). The value of TRT that was copied from TTRT is counted down evenly. When a token arrives, the current value of TRT is copied into the Token Holding Time (THT) counter and the TRT is set again with the value of TTRT.

If the token arrives in good time before the TRT expires, it is the above-mentioned early token, but if the token arrives after the TRT has expired once, it is called a late token and a corresponding late flag is set. While the early token can be used for synchronous and asynchronous data transfers, the late token can only be used for synchronous data transfers. If the TRT reaches more than twice the value of the TTRT, an error has occurred.

Token Holding Time (THT)

The token holding time (THT) is a counter that is decremented linearly and forces the token to be released when it expires. The token holding timer takes over the remaining time from the TRT and determines how long a station can still send asynchronously.

Nonrestricted Token

If the optional nonrestricted token is used, there are 7 priority levels (0–6). If no priority is set, the T_Pri variable contains the value zero, which stands for the lowest priority. Correspondingly, if T_Pri has the value 6, the highest priority applies and the maximum transmission time is automatically available for the transmission of the data.

Restricted token

By controlling a higher protocol layer, the Restricted Token allows the entire bandwidth to be used in an asynchronous transmission. With the restricted token, the normal control mechanisms of the FDDI are switched off. The requesting station takes a nonrestricted token from the ring and sends the data packets to one or more stations. The data packets must contain the initialization process required by the recipient for the restricted mode. After the data packets have been sent, a restricted token is released, which in turn lets the receiving stations go into restricted mode. All stations involved in this extended dialogue now exchange data and only use the restricted token. The initializing station usually ends the extended dialog and puts the nonrestricted token back into circulation. No further asynchronous data traffic is possible during restricted mode; synchronous data traffic is not affected by this, as synchronous data traffic does not know any classification of tokens. The restricted mode process is monitored by the station management (SMT) of the FDDI and can end the restricted mode by sending a MAC frame.