IEEE 802.5 - Token Ring
A special bit pattern token circulates around ring when stations are idle.
When station wants to transmit, it must take token from ring and then transmit.
A single bit in the 3byte token is inverted and this changes it into the first 3 bytes of a normal data frame.
There being only one token only one channel can transmit at any time.
Thus the ring must have sufficient delay to let token circulate when all stations are idle