FDDI: Token Passing Ring
FDDI 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 strips the token replacing it by a data frame (MAC data FDDI SMT PCI or LLC data LLC PCI).
Then the adapter immediately releases a new token after frame transmission (early token release, ETR); the frame will be stripped when returning.
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 SMT 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.
If a ring station detects frame errors it will set the error detect indicator (E bit) within the frame status field (E=1).
Ending Delimiter (4 bits) (FDDI)
Because the FDDI MAC function depends on a passing token frame, all stations monitor the token frame. 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.
SMT - Station Management Protocol
If not frames are received any more, FDDI adapters start the beacon process.