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Computer NetworksToken Rings 802.5

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Token Rings

• IBM Token Ring & IEEE 802.5• FDDI• Resilient Packet Ring IEEE 802.17…

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IBM Token Rings are the most famous of this series

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About IBM Token Rings

• Network consists of a set of nodes connected in a ring & data always flows in particular direction

• Each node receives frames from its upstream node and transmits to its downstream node

• Token rings share two features with Ethernets– Requires an algorithm that controls when each node is allowed

to transmit

– All nodes see all frames with destination saving a copy of it

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Tokens

• Special sequence of bits that circulates around the ring– 24 bits in case of 802.5

• When node with a frame to transmit sees the token, it takes the token off the ring and instead inserts its own frame on the network– Frame is forwarded by all nodes while destination too makes a

copy of it and forwards it

– Packet is received again by sender which strips its frame off the ring and reinserts the token

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Physical Properties

• Link failure in ring may disrupt transmission of the whole network

• Electromechanical relay is used to overcome this problem

• As long as the station is healthy, the relay is open• When the stations stops powering the relay, it

gets closed and the station is removed from the network

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Use of Electromechanical relay

From Previous Host To Next Host

Host

Relay

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Use of Electromechanical relay

From Previous Host To Next Host

Host

Relay

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Multi Station Access Unit… MSAU

• Several Electromechanical relays packed into a single box

• It makes token rings look like a star topology• Systems are plugged in and out of MSAU• One of the main differences between 802.5 and

IBM Token Rings is that the latter requires MSAU– The other difference however can be that that IBM Token

Rings can have up to 260 stations per ring while 802.5 supports 250

– Physical media for IBM is twisted pair but not defined for 802.5

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Multi Station Access Unit… MSAU

Previous MAUS

Next MAUS

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Token Ring Media Access Control

• Any station that wants to transmit data, has to seize the token from the network

• Once the token is seized, transmission can be done• In 802.5, seizing process involves simply modifying

1 bit in the token, and it acts as a preamble for the subsequent data packet

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Data Transmission

• Each transmitted packet contains a destination address

• As the packet flows past each node on the ring, each node looks inside the packet to see if it is the intended recipient

• If so, it copies the packet into its buffer as it flows through the network adapter

• But it does not remove the packet form the network

• The sending station has the responsibility of removing the packet from the ring

• Other stations don’t harm the packet

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Token Holding Time

• Setting THT to infinity will be dangerous because other devices wont have fair access to the ring

• Setting THT to one message will be silly – If node has more packets to send, after every THT it would let

the token circulate around the whole ring before seizing it again

• Hence it is set to 10ms– A device should take care of the fact that before transmission

of a packet, whether it got sufficient THT left to transmit this packet

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Token Rotation Time

• Time required by token to traverse the ring• TRT < ActiveNodes * THT + RingLatency • Active Nodes are the number of nodes that have

data to transmit and Ring Latency is the time a packet takes to traverse the ring when no node has data to send… Ideal

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Reliable Delivery

• A & C bits contained in frame status byte• Both set to 0 by transmitter• As the destination station sees the packet, it set

the A-bit to 1• When the destination station copies the packet

into its buffer successfully, the C bit is set to 1• At Sender again

– A=1 & C=1 means Successful Delivery

– A=1 & C=0 means Host is alive but frame could not be copied

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Priority

• Priority allows a particular host to access the token

• Token contains a 3-bit priority field• Token has certain priority n at any time• Each host that wants to transmit a packet, has

to assign a priority to the data packet• The host that holds the token, before releasing

the token sets its priority to the priority of the data packet it received

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Token Ring Maintenance

• Monitor Station• A monitor periodically announces its presence by

special control messages• If these periodic messages stop arriving, the

monitor will be assumed to have failed• And a new monitor is elected

– Each station that wants to be monitor sends a claim frame

– If the token circulates back to the sender, it will be the monitor

– If more than one claim frames, break the tie by some algorithm (say the highest address wins)

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Monitor

• Monitor is to take care of the health of the token ring network– Clocking… In case of Token

– Setting bits… In Case of Data Frame

• Reinserts a new token if token is not seen by monitor after the interval– (NumStations * THT) + RingLatency

• Checks for orphaned & Corrupted frameso M-bit is set to 0 by transmitter and set to 1 as it passes the

monitoro If a token passes the monitor with the m-bit already set to 1, it is

removed from the network

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Frame Format

• Take it as a Reading Assignment

– Included in Exam

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Recommended Text

• Computer Networks: A System Approach by Lary Peterson and Bruce Davie