1. 11 CS716 Advanced Computer Networks By Dr. Amir Qayyum

2. 2 Lecture No. 11

3. 3 Review Lecture 10 CSMA/CD MAC algorithm Constraints on collision detection Min frame size – RTT 51.2us 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

4. 4 Review Lecture 10 Token Ring MAN backbone IBM, 802.5 token ring, FDDI Rationale: emulate shared med p2p links Directional data flow Shared med: see all frames, distr MAC algo Token – special bit pattern Capture and release – round robin service

5. 5 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 To next host Relay (a) Host From previous host To next host Relay (b)

6. 6 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

7. 7 MSAU Host From previous MSAU To next MSAU

8. 8 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)

9. 9 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

10. 10 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

11. 11 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

12. 12 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

13. 13 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 BodyCRC Src addr Variable48 Dest addr 4832 End delimiter 8 Frame status 8 Frame control 8 Access control 8 Start delimiter 8

14. 14 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 ?

15. 15 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 have data to send

16. 16 Timed Token Algorithm Token Rotation Time (TRT) –How long it takes the token to traverse the ring. –TRT <= ActiveNodes x THT + RingLatency

17. 17 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

18. 18 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

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

20. 20 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 lower- priority packets get sent when higher priority packets are waiting

21. 21 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

22. 22 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

23. 23 Monitor’s Role May add 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

24. 24 Monitor’s Role Maintains a timer to detect the missing token –Timer = NumStations x THT + RingLatency –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

25. 25 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, it drains it off

26. 26 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 Maintenance, monitor, claim, role