Token-Ring: Token Passing Ring
Token-Ring media access control is based upon a token that passes all stations connected to the ring. Only that station that holds the token is allowed to transmit a frame.
A station that captured the token in order to transmit a frame converts the so-called token bit from '0' to '1' (access control protocol field), then protocol and user data are appended to the token (MAC data Token-Ring MAC Data PCI (n bytes) or LLC data LLC PCI).
Then (a) the adapter waits for its own frame in order to strip the frame off the ring and to release a new token, or (b) the adapter immediately releases a new token after frame transmission (early token release, ETR); the frame will be stripped as with (a). Early token release is not used with 4 Mbps token-ring (only with 16 Mbps token-ring, not mandatory).
The token consists of three protocol fields only:
> starting delimiter
> access control
> ending delimiter
Data frames (upper layer protocols as well as user data) are indicated as LLC frames within the frame control field; the same with MAC frames (with vector data).
The frame control field is followed by destination address and source address protocol fields. If a ring station recognizes its own MAC address within the destination address field, it will copy the frame into its buffer; then the AC bits are set to one (AC=11).
> A bit: address recognized;
> C bit: frame copied.
If a token ring adapter suffers from congestion, it well set A=1 and C=0 (address recognized, frame not copied). The source station will detect the destination's congestion.
Frame Status (1 byte) (Token-Ring)
If a ring station detects frame errors it will set the error detect indicator (E bit) within the ending delimiter field (E=1).
Ending Delimiter (1 byte) (Token-Ring)
Because the token-ring MAC function depends on a passing token frame, a designated station, called the active monitor, monitors the token; the active monitor itself is monitored by all other adapters, called standby monitors. If any adapter misses the token (token or data frames don't pass within the valid rotation time), it will initiate the claim token process (also referred to as monitor contention process) in order to designate a new active monitor that has to release a new token.
Monitor Contention Process: Claim Token
There are different access priorities assigned to ring stations: servers or bridges are allowed to reserve extra tokens in order to have higher throughput.
Therefore the access control reservation bits of any passing frame are set according to the adapter's allowed access priority (as long as the frame's current reservation value is less than the adapter's allowed access priority). The next token frame with a priority not higher than the adapter's allowed access priority may be captured for frame transmission. Access priority values may be assigned to ring stations by LAN management.
Access Control (1 byte) (Token-Ring)
The allowed access priority is assigned by the ring parameter server (RPS) during station insertion by station initialization request/reply.
Station Initialization Process
If no more frames are received, ring stations will assume hardware errors and will start the beacon process in order to notify all other ring stations as well as to recover the ring.