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Token rings have an optional priority feature, for which the 8 access control bits in the token ring non-token frame are used.

Suppose we are working with the delayed token reinsertion scheme. How does the priority scheduling for the token work?

Also, what is the difference in meaning and use between the 3 priority bits and the 3 reservation bits?

I suppose here, the non-destination stations in the ring will have to copy the frame too, since they have to change the priority/reservation bits and recalculate the CRC. Am I correct?

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  • I have to ask, who's still using Token Ring these days?
    – Ricky
    Nov 5 '15 at 21:59
  • 1
    No one I know...this question is mainly to sate curiosity about a thing of historical significance, and ace the coursework :) Nov 6 '15 at 21:19
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Cisco has a pretty good explanation:

Priority System

Token Ring networks use a sophisticated priority system that permits certain user-designated, high-priority stations to use the network more frequently. Token Ring frames have two fields that control priority: the priority field and the reservation field.

Only stations with a priority equal to or higher than the priority value contained in a token can seize that token. After the token is seized and changed to an information frame, only stations with a priority value higher than that of the transmitting station can reserve the token for the next pass around the network. When the next token is generated, it includes the higher priority of the reserving station. Stations that raise a token's priority level must reinstate the previous priority after their transmission is complete.

If a non-destination station wants to reserve the token for the next pass, it must have a higher priority, and it uses that to reserve the token by putting its priority into the reservation bits. The Frame Delimiter and Access Control are not included in the Frame Check Sequence, so changing the reservation bits doesn't require a new CRC.

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