CS 716 Advanced Computer Networks By Dr Amir

CS 716 Advanced Computer Networks By Dr. Amir Qayyum

Lecture No. 11 2

Review Lecture 10 • • CSMA/CD MAC algorithm Constraints on collision detection Min frame size – RTT 51. 2 us Retries after collision Binary exp backoff: Min value unique Frame reception: acceptable frames Bus and Hub topology: CD handle Exp: nodes, dist, RTT, 30% load, largeframe 3

Review Lecture 10 • • Token Ring MAN backbone IBM, 802. 5 token ring, FDDI Rationale: emulate shared med p 2 p links Directional data flow Shared med: see all frames, distr MAC algo Token – special bit pattern Capture and release – round robin service 4

Physical Properties • A node is connected into the ring using a relay – Relay is open as long as the station is alive (a) – Relay closes and bypasses the station if it is dead (b) Host From previous host Host To next host From previous host Relay (a) To next host Relay (b) 5

Physical Properties • Several relays are packed into a single box: multi Station Access Unit (MSAU) – Stations can be added/removed by plugging/unplugging – IBM token ring requires MSAU but 802. 5 does not 6

MSAU Host From previous MSAU To next MSAU Host 7

Physical Properties • Data rate can be 4 Mbps or 16 Mbps • Encoding of bits uses differential Manchester • Ring may have up to 250 (802. 5) or 260 (IBM) nodes • Physical medium is twisted pair (IBM Token Ring) 8

Token Ring MAC • Network adaptor contains receiver, transmitter and some storage of bits between them • Token circulates if no station has anything to send – Ring must have enough capacity to store entire token – At least 24 stations with 1 -bit storage for 24 -bit long token (if propagation delay is negligible) – This situation is avoided by designating a monitor 9

Token Ring MAC • Any station that has a data to send can seize token • In 802. 5, simply 1 bit in second byte token is modified • First two bytes of modified token become preamble for the next frame 10

Token Ring MAC • The station is then allowed to send one or more frames • Destination address may be a multicast or a broadcast address • Intended recipient copies the frame into a buffer 11

Token Ring MAC • Sender is responsible for removing these frames • Frame longer than the capacity of ring is drained by the sender while still transmitting its later part 12

Frame Format • “Illegal” Manchester codes in the start and end delimiters • Frame priority and reservation bits in access control byte • Demux key in frame control byte • A and C bits for reliable delivery, in status byte 8 8 8 48 48 Start delimiter Access control Frame control Dest addr Src addr Variable Body 32 8 8 CRC End delimiter Frame status 13

Token Ring MAC Issue • A node captures the token and sends as much data as it has … – A node could keep the token for arbitrarily long time, and other nodes are not given the turn – Favors nodes with large data over nodes having a small message to send • How much data a node is allowed to transmit each time it capture the token ? 14

Timed Token Algorithm • Token Holding Time (THT) – Upper limit on how long a station can hold the token – A node checks before putting each frame on ring that its transmit time would not cause THT to exceed – Long THT achieves better utilization with few senders – Short THT helps when multiple nodes 15 have data to send

Timed Token Algorithm • Token Rotation Time (TRT) – How long it takes the token to traverse the ring. –TRT <= Active. Nodes x THT + Ring. Latency 16

Reliable Delivery • The A and C bit in the packet trailer for reliability • Both bits are initially set to 0 • Destination sets A bit if it sees the frame and sets C bit if it copies the frame into its adaptor 17

Reliable Delivery • If sender receives bit A=0, it assumes destination is absent (not functioning) • If A bit is set but not bit C, it means destination is unable to accept the frame – Sender retransmits this frame later 18

Token Ring Packet Priorities • Token contains 3 -bit priority field • Token priority changes over time by using reservation bits in the frame header 19

Token Ring Packet Priorities • A station willing to send priority n packet can set reservation bits to n, if this makes it lower in value – it captures the token when the current sender releases it with priority set to n • Strict priority scheme: no lowerpriority packets get sent when higher priority packets are waiting 20

Token Maintenance • Token rings have a designated monitor node • Any station can become the monitor according to a well defined procedure • Monitor is elected when the ring is first connected, or when the current monitor fails 21

Token Maintenance • Monitor periodically announces its presence • Claim token sent by a station seeing no monitor – if the sender receives back the claim token, it becomes monitor – if another station is also contending for monitor, some rule defines the monitor 22

Monitor’s Role • May additional delay into the ring • Assures that a valid token is present in the ring because token may be lost: – No token when initializing the ring – Bit error corrupts token pattern – Node holding the token crashes 23

Monitor’s Role • Maintains a timer to detect the missing token – Timer = Num. Stations x THT + Ring. Latency – When times expires, it creates a new token • Checks for corrupted frames – checksum error, invalid format, may circulate forever – Monitor drains them off the ring 24

Monitor’s Role • Checks for orphaned frames – sending station died before removing the frame – “Monitor” bit is used to detect orphaned frames – Initially 0, and set to 1 when a frame passes the monitor – If the monitor sees a frame with this bit set, 25 it drains it off

Review Lecture 11 • • Token Ring phy properties: MSAU, speed, # MAC, storage, circulation, seize+ modify Send 1+ frame, recipient cpy, remove frame Frame format, resv, priority, A, C bits MAC issues, THT, TRT Reliable delivery, A, C Frame priorities, resv 26 Maintenance, monitor, claim, role