TOKEN RING

Starting Delimiter - consists of a special bit pattern denoting the beginning of the frame. The bits from most significant to least significant are J,K,0,J,K,0,0,0. J and K are code violations. Since Manchester encoding is self clocking, and has a transition for every encoded bit 0 or 1, the J and K codings violate this, and will be detected by the hardware.

Access Control - this byte field consists of the following bits from most significant to least significant bit order: P,P,P,T,M,R,R,R. The P bits are priority bits, T is the token bit which when set specifies that this is a token frame, M is the monitor bit which is set by the Active Monitor (AM) station when it sees this frame, and R bits are reserved bits.

Ending Delimiter - The counterpart to the starting delimiter, this field marks the end of the frame and consists of the following bits from most significant to least significant: J,K,1,J,K,1,I,E. I is the intermediate frame bit and E is the error bit.

Starting Delimiter - as described above.

Access Control - as described above.

Frame Control - a one byte field that contains bits describing the data portion of the frame contents.

Destination address - a six byte field used to specify the destination(s).

Source address - a six byte field that is either the locall assigned address (LAA) or universally assigned address (UAA) of the sending station adapter.

Data - a variable length field of 0 or more bytes, the maximum allowable size depending on ring speed containing MAC management data or upper layer information.

Frame Check Sequence - a four byte field used to store the calculation of a CRC for frame integrity verification by the receiver.

Ending Delimiter - as described above.

Frame Status - a one byte field used as a primitive acknowledgement scheme on whether the frame was recognized and copied by its intended receiver.

The monitor contention process is initiated when

i) a loss of signal on the ring is detected,

ii) an active monitor station is not detected by other stations on the ring or

iii) when a particular timer on an end station expires such as the case when a station hasn't seen a token frame in the past 7 seconds.

The active monitor performs a number of ring administration functions. The first function is to operate as the master clock for the ring in order to provide synchronization of the signal for stations on the wire. Another function of the AM is to insert a 24-bit delay into all frame transmissions. A third function for the AM is to ensure that a frames on the ring is present or to detect a broken ring. Lastly, the AM is responsible for removing circulating frames from the ring.

Phase 0 (Lobe Check) - A station first performs a lobe media check. A station is wrapped at the MSAU and is able to send 2000 test frames down its transmit pair which will loop back to its receive pair. The station checks to ensure it can receive these frames without error.

Phase 1 (Physical Insertion) - A station then sends a 5 volt signal to the MSAU to open the relay.

Phase 2 (Address Verification) - A station then transmits MAC frames with its own MAC address in the destination address field of a token ring frame. When the frame returns and if the address copied and frame recognized bits are set, the station knows there is another station using its MAC address on the ring and will then de-insert.

Phase 3 (Participation in Ring Poll) - Before transmitting any data, the station must participate in the periodic (every 7 seconds) ring poll process. This is where stations identify themselves on the network as part of the MAC management functions.

Phase 4 (Request Initialization) - Finally a station sends out a special request to a parameter server to obtain configuration information. This frame is sent to a special functional address, typically a token ring bridge, which may hold timer and ring number information with which to tell the new station about.