IEEE l IEEE has had the greatest impact on Layer 2 standards. l IEEE divided Layer 2 into two sublayers.

IEEE—Layers 1 & 2 l The Logical Link Control sublayer was created to provide different Layer 1 technologies a single method of accessing the higher layers. IEEE Base TIEEE 802.5FDDI IEEE Ethernet These sample technologies include Layer 1 & the MAC sublayer of Layer 2 LLC sublayer of Layer 2

BASIC OF TOKEN-RING

Variants IBM developed the first Token Ring network in the 1970s. It is still IBM's primary LAN technology, and is second only to Ethernet (IEEE 802.3) in terms of LAN implementation.

Ring Topology

Data passing When a station has information to transmit, it seizes the token and sends data frame to the next station. When a station has information to transmit, it seizes the token and sends data frame to the next station. When frame reaches the destination station, the data is copied for processing. When frame reaches the destination station, the data is copied for processing. Frame continues to circle the ring until it returns to the sending station. Frame continues to circle the ring until it returns to the sending station. Sending station removes the frame from the ring, verifies receipt, and releases the token. Sending station removes the frame from the ring, verifies receipt, and releases the token.

Token-Ring frame format

Start Delimiter and End Delimiter l Start delimiter. –Alert for the arrival of a token. –Includes an identification symbol. –Violates the encoding system to differentiate it from other frame fields. l End Delimiter –Completes the token or data/command frame. –Contains damage indicator. –Last of logical sequence.

Access control P: Priority bits P: Priority bits T: Token bit T: Token bit M: Monitor bit M: Monitor bit R: Reservation bits R: Reservation bits (this is the second field in the frame. It is one byte, or 8 bits; each bit has it’s own function.) (this is the second field in the frame. It is one byte, or 8 bits; each bit has it’s own function.) PPPTMRRR

Frame control Only present in data/command frames. Indicates whether frame contains data or control information. If control, this byte specifies type of control information. Only present in data/command frames. Indicates whether frame contains data or control information. If control, this byte specifies type of control information.

Destination and Source addresses Universal Address. Local Administered Address. Broadcast Address (D). Functional Address (0x0C XXXX) (D). Universal Address. Local Administered Address. Broadcast Address (D). Functional Address (0x0C XXXX) (D).

Data Length limited by the maximum time a station may hold the token.

Frame checksum Frame Check Sequence. Source fills field with calculated value dependent on frame contents. Destination recalculates to check data integrity. Frame is discarded if damaged. Frame Check Sequence. Source fills field with calculated value dependent on frame contents. Destination recalculates to check data integrity. Frame is discarded if damaged.

Frame status Address recognized / frame copied indicator.

Management mechanisms l Active Monitor –One station acts as centralized source of timing information for other stations. –Removes continuously circulating frames by set monitor bit to 1 (i.e., ends “loops”). –Start a token, when token have been lost. l Beaconing –Detects and repairs network faults. –Initiates auto-reconfiguration.

Physical topology l l Physical topology : Star. l l Logical topology : Ring. l l IBM Token Ring network stations are connected to MSAU (Multi-Station Access Unit). l l Many MSAU can be wired together to form one large ring.

Multi-MSAU