early token release (TR) (ETR)
The early token release procedure is a variant of the well-known token procedure in which several frames can be placed on the token ring( TR) at the same time. This procedure is only provided in the 16 Mbit/s version.
The advantage of the early token release procedure over the known token passing procedure is the increased data throughput. New tokens can be placed on the ring without having to wait for the arrival of each individual frame. By using multiple frames, the early token release method compensates for possible run times and dwell times of the token in the ring or in the connected stations. It therefore offers particular advantages for large token ring installations that can extend over several kilometers, for example in a token ring network as a backbone.
According to the procedure, a first transmitting station generates a frame consisting of header, data part and trailer. After a short waiting period, it generates a new free token, which it places on the ring. The following station that is willing to send uses the free token to generate its own frame, which it also sends to the network. The second station then generates a new free token, which it places on the ring, again after a short waiting period. The next station willing to send uses the free token in the same way.
The individual transmitting stations therefore do not wait for acknowledgement of receipt, but generate a new free token immediately after sending the frame. In this procedure, there is at most only one token in the ring and each transmitting station receives its own message as the next frame, provided there is no error, to take it off the ring. The early token release procedure is optionally included in the token ring for 16 Mbit/s by IEEE 802.5.