IEEEIEEE

IEEE has had the greatest IEEE has had the greatest impact on Layer 2 standards.impact on Layer 2 standards.

IEEE divided Layer 2 into two IEEE divided Layer 2 into two sublayers.sublayers.

IEEE—Layers 1 & 2IEEE—Layers 1 & 2

The Logical Link Control sublayer was The Logical Link Control sublayer was created to provide different Layer 1 created to provide different Layer 1 technologies a single method of technologies a single method of accessing the higher layers.accessing the higher layers.

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LLC sublayer of Layer 2

BASIC OF TOKEN-RINGBASIC OF TOKEN-RING

VariantsVariants

IBM developed the first IBM developed the first Token Ring network in Token Ring network in

the 1970s. It is still the 1970s. It is still IBM's primary LAN IBM's primary LAN technology, and is technology, and is

second only to Ethernet second only to Ethernet (IEEE 802.3) in terms of (IEEE 802.3) in terms of

LAN implementation. LAN implementation.

Ring TopologyRing Topology

Data passingData passing• When a station has information to transmit, it When a station has information to transmit, it

seizesseizes the the tokentoken and sends data frame to and sends data frame to the next station.the next station.

• When frame reaches the When frame reaches the destination destination stationstation, the data is , the data is copiedcopied for for processingprocessing..

• Frame continues to circle the ring until it Frame continues to circle the ring until it returnsreturns to the to the sendingsending station station..

• Sending station Sending station removes the frameremoves the frame from from the ring, the ring, verifies receiptverifies receipt, and , and releasesreleases the the tokentoken..

Token-Ring frame formatToken-Ring frame format

Start DelimiterStart Delimiter and and End DelimiterEnd Delimiter StartStart delimiterdelimiter..

– AlertAlert for the for the arrivalarrival of a token. of a token.– Includes an Includes an identification symbolidentification symbol..– Violates Violates thethe encoding system encoding system to to

differentiatedifferentiate it from it from other frame fieldsother frame fields.. EndEnd DelimiterDelimiter

– CompletesCompletes the token or data/command the token or data/command frameframe..

– Contains Contains damage indicatordamage indicator..– LastLast of of logical sequencelogical sequence..

Access controlAccess control

• P: Priority bitsP: Priority bits• T: Token bitT: Token bit• M: Monitor bitM: Monitor bit• R: Reservation bitsR: Reservation bits• (this is the second field in the frame. It is one (this is the second field in the frame. It is one

byte, or 8 bits; each bit has it’s own function.)byte, or 8 bits; each bit has it’s own function.)

P P P T M R R R

Frame controlFrame control

• 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 Destination and Source addressesaddresses

• Universal Address.• Local Administered Address.• Broadcast Address (D).• Functional Address (0x0C0000 00XXXX) (D).

DataData

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

Frame checksumFrame 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 statusFrame status

• Address recognized / frame copied indicator.

Management mechanismsManagement mechanisms Active MonitorActive Monitor

– One stationOne station acts as acts as centralized sourcecentralized source of of timingtiming informationinformation for other stations. for other stations.

– RemovesRemoves continuously circulating framescontinuously circulating frames by by set monitor bitset monitor bit to to 1 1 (i.e., ends “loops”).(i.e., ends “loops”).

– StartStart a a tokentoken, when token have been , when token have been lostlost.. BeaconingBeaconing

– DetectsDetects and and repairsrepairs network network faultsfaults..– Initiates Initiates auto-reconfigurationauto-reconfiguration..

Physical topologyPhysical topology Physical topology : Star. Logical topology : Ring. IBM Token Ring network

stations are connected to MSAU (Multi-Station Access Unit).

Many MSAU can be wired together to form one large ring.

Multi-MSAUMulti-MSAU