Advanced Computer Networks: Lecture 11. This lecture will cover the following: physical properties; multi station access unit (MSAU); token ring MAC; token ring MAC issue; timed token algorithm with token rotation time and token holding time; token ring packet priorities; monitor’s role;...
CS716 Advanced Computer Networks By Dr. Amir Qayyum 1 Lecture No. 11 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 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 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) 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 MSAU Host MSAU Host Host From previous MSAU To next MSAU Host 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) 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 1bit storage for 24bit long token (if propagation delay is negligible) – This situation is avoided by designating a 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 • 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 48 48 Start delimiter Access control Frame control Dest addr Src addr Variable Body 32 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