Frame types Token When no station is sending a frame, a special token frame circles the loop. This special token frame is repeated from station to station until arriving at a station that needs to send data. Tokens are three octets in length and consist of a start delimiter, an access control octet, and an end delimiter.
Abort frame Used by the sending station to abort transmission.
Data Data frames carry information for upper-layer protocols, while command frames contain control information and have no data for upper-layer protocols. Data and command frames vary in size, depending on the size of the Information field. ; Starting delimiter: The 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 of
Differential Manchester encoding. Differential Manchester encoding has a mid symbol transition for every coded 0 or 1, however the J and K codes do not have a mid symbol transition. Both the Starting Delimiter and Ending Delimiter fields are used to mark frame boundaries. : ; 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 reservation bits, which indicate that the next token should be issued with that priority. : ; Frame control: A one-byte field that contains bits describing the data portion of the frame contents which indicates whether the frame contains data or control information. In control frames, this byte specifies the type of control information. : :Frame type –
01 indicates LLC frame IEEE 802.2 (data) and ignore control bits; :
00 indicates MAC frame and control bits indicate the type of
MAC control frame ; Destination address: A six-byte field used to specify the destination(s) physical address. ; Source address: Contains physical address of sending station. It is a six-byte field that is either the local 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. Maximum length of 4500 bytes. ; Frame check sequence: A four-byte field used to store the calculation of a CRC for frame integrity verification by the receiver. ; 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. : ; Frame status: A one-byte field used as a primitive acknowledgment scheme on whether the frame was recognized and copied by its intended receiver. : :
A = 1, Address recognized :
C = 1, Frame copied
Active and standby monitors Every station in a Token Ring network is either an active monitor (AM) or standby monitor (SM) station. There can be only one active monitor on a ring at a time. The active monitor is chosen through an election or
monitor contention process. The monitor contention process is initiated when the following happens: • a loss of signal on the ring is detected. • an active monitor station is not detected by other stations on the ring. • 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. When any of the above conditions take place and a station decides that a new monitor is needed, it will transmit a
claim token frame, announcing that it wants to become the new monitor. If that token returns to the sender, it is OK for it to become the monitor. If some other station tries to become the monitor at the same time then the station with the highest
MAC address will win the election process. Every other station becomes a standby monitor. All stations must be capable of becoming an active monitor station if necessary. 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 the ring, to ensure that there is always sufficient buffering in the ring for the token to circulate. A third function for the AM is to ensure that exactly one token circulates whenever there is no frame being transmitted, and to detect a broken ring. Lastly, the AM is responsible for removing circulating frames from the ring.
Token insertion process Token Ring stations must go through a 5-phase ring insertion process before being allowed to participate in the ring network. If any of these phases fail, the Token Ring station will not
insert into the ring and the Token Ring driver may report an error. • 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 Recognized (AR) and Frame Copied (FC) bits in the frame-status are set to 0 (indicating that no other station currently on the ring uses that address), 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 3 (Participation in ring poll) – A station learns the address of its Nearest Active Upstream Neighbour (NAUN) and makes its address known to its nearest downstream neighbour, leading to the creation of the ring map. Station waits until it receives an AMP or SMP frame with the AR and FC bits set to 0. When it does, the station flips both bits (AR and FC) to 1, if enough resources are available, and queues an SMP frame for transmission. If no such frames are received within 18 seconds, then the station reports a failure to open and de-inserts from the ring. If the station successfully participates in a ring poll, it proceeds into the final phase of insertion, request initialization. • 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 the new station needs to know.
Optional priority scheme In some applications there is an advantage to being able to designate one station having a higher priority. Token Ring specifies an optional scheme of this sort, as does the
CAN Bus, (widely used in automotive applications) – but Ethernet does not. In the Token Ring priority MAC, eight priority levels, 0–7, are used. When the station wishing to transmit receives a token or data frame with a priority less than or equal to the station's requested priority, it sets the priority bits to its desired priority. The station does not immediately transmit; the token circulates around the medium until it returns to the station. Upon sending and receiving its own data frame, the station downgrades the token priority back to the original priority. Here are the following eight access priority and traffic types for devices that support
802.1Q and
802.1p: ==Interconnection with Ethernet==