Direct Link Networks: Multiaccess Protocols (2.7) CS/ECpE 5516: - PowerPoint PPT Presentation

Direct Link Networks: Multiaccess Protocols (2.7) CS/ECpE 5516: Computer Networks Originally by Scott F. Midkiff (ECpE) Modified by Marc Abrams (CS) Virginia Tech courses.cs.vt.edu/~cs5516

Lecture Topics I Multiaccess control I IEEE 802.5 Token Ring and FDDI ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 2

Multiaccess Communication (1) I Previous discussion considered point-to-point links G Received signal is transmitted signal (plus noise) I Many networks are such that received signal at one node depends on transmitted signal at two or more other nodes G Satellite systems G Radio networks G Multi-tap bus systems ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 3

Multiaccess Communication (2) I Multiaccess media are communication media where received signal is sum of attenuated transmitted signals plus effects of delay, distortion, and noise I Examples: Multitap bus (Ethernet) Radio (wireless) network ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 4

Medium Access Control -- MAC (1) I With multiaccess media, protocol is needed to coordinate sharing of media I Medium access control (MAC) protocol performs this function I MAC is sublayer between data link control (DLC) layer and physical layer (usually grouped with DLC) ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 5

Medium Access Control -- MAC (2) I LLC provides “link” to adjacent node I MAC coordinates access to shared media I Physical provides hardware interface Data LLC Link MAC physical ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 6

Medium Access Control -- MAC (3) I Separation of layer functions in multiaccess networks is not as well-defined as in networks with point-to-point links G Feedback about errors is part of ARQ strategy of DLC, but may depend on how media is shared G Flow and congestion control needed to provide fair, efficient access to shared media G Broadcast nature of shared media implements some routing functions ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 7

Token Ring Networks Token ring networks are common form of LAN & MAN G IEEE 802.5 (Token Ring): 4 Mbps or 16 Mbps G Fiber Distributed Data Interface (FDDI): 100 Mbps ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 8

Nodes Arranged in Ring Topology 1 G Node… G receives bit stream 8 2 from last node G relays bit stream to next node 7 3 6 4 5 Point-to-point links between stations ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 9

Nodes Arranged in Ring Topology Node can repeat or replace each 1 bit 8 2 interface 7 3 logic 6 4 5 At least 1 bit delay at each node: N Propagation N Processing N Regeneration & transmission ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 10

Token (1) I To transmit its own data, node must discard input & output its data interface logic I But we can't discard data until it has reached its destination G Token is used to coordinate use of ring G Ring is shared medium, so network is multiaccess system ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 11

Token (2) I Conceptually, token is passed from node to node G Only send your data when you've got token G Pass token when data reaches destination or you've got no data to send I So what is a token anyway? G Special pattern -- distinguished from data G Similar to framing flags � ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 12

Token (3) I Token can be in 2 states G Free token (or idle token ): ring available N Discard bits following free token G Busy token : ring in use N Follow busy token with data I Token indicates G upcoming data (if busy ), as well as G permission to transmit (if free ) ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 13

Basic Token Ring Operation (1) I When node with data to transmit receives free token, it marks token as busy and appends its own data I Subsequent nodes forward data since token is marked busy G Destination node both forwards and stores data G Destination node may mark data as received, but token is still busy I Data returns to originating node where it is discarded ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 14

Basic Token Ring Operation (2) I After node finishes transmission, it G marks token as free G forwards token next node G follows token with idle fill (until it sees busy or idle token again) ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 15

Ring Example (1) data busy 1 1 4 2 4 2 3 3 Node 1 transmits busy token Node 1 receives followed by data for node 3 free token ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 16

Ring Example (2) 1 1 4 2 4 2 3 3 Busy token followed by Busy token completes data continues around round trip and is ring, node 3 stores data stripped at node 1 ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 17

Ring Example (3) idle free 1 1 token 4 2 4 2 3 3 Node 1 strips old data When finished, node 1 from ring and puts free token on transmits new data ring, followed by idle until finished fill ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 18

Ring Example (4) 1 1 4 2 4 2 3 3 Node 2 forwards free Node 3 receives all of token (no data to data from 1, forwards send), node 3 still free token (no data to storing data send) ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 19

Ring Example (5) 1 1 4 2 4 2 3 3 Node 4 receives free Node 1 forwards bits token, transmits busy (busy token) after its token followed by data last data bit arrives ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 20

Additional Details of Ring Operation I Propagation delay around ring must be long enough to “store” complete token G Otherwise first part of free token would be discarded to transmit last part I Error detection G Receiving node can check CRC and put an ACK or NAK in packet trailer on its way back to sender G Sending node can also check CRC since it sees all transmitted data I Numerous variations are possible in ring operation ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 21

Token Holding Time I How long can node hold free token? I Option 1: Transmit only 1 packet G Lets token rotates at maximum rate G Minimizes latency I Option 2: Transmit all waiting packets G Reduces token transmission overhead G Maximizes throughput I 3: Transmit waiting packets up to time limit G Best of both worlds ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 22

Retransmission Schemes (1) I 2 options in handling retransmissions… G Selfless operation (FDDI): Give up token when done transmitting; if error detected, reacquire token & retransmit G Selfish (IEEE 802.5): Hold token for round trip time, to be sure receiver got data correctly ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 23

Retransmission Schemes (2) I Pros/Cons G Selfless (FDDI): N Penalty: higher latency on error G Selfish (802.5): N Ring transmits idle fill until sender gets ack N Penalty: lower throughput for low error rates N Advantage: Lower latency for retransmissions ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 24

Type of Token Failures (1) I Lost token -- no node can transmit! G Corrupted by noise (bit errors alter token code) G Node holding token fails I Token is permanently marked busy -- no node can transmit G Idle token corrupted by noise (is marked busy) I Multiple tokens created -- conflicts for access G Non-token corrupted by noise to become token G Node failure I Ring protocol recognizes token failures & recovers (e.g. generating new token) ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 25

Fiber Distributed Data I nterface -- FDDI I 100 Mbps timed token ring network based on fiber optics I Developed under auspices of ANSI committee X3T9 formed in 1982 I Limited popularity G Lack of high BW apps in 1980's G High cost of NIC's ($5000) and concentrators ($$) G Was popular for backbones & switching fabric ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 26

FDDI Standard LLC Data Link MAC PHY SMT Physical PMD LLC Logical Link Control MAC Media Access Control PHY Physical PMD Physical Media Dependent SMT Station Management ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 27

FDDI Versus Token Ring I Token Ring: sender waits until all of transmitted data goes round ring before releasing token I FDDI: sending node releases token after sending last bit of data G Busy token not sent G Data frame header recognized as “busy token” G Improves FDDI’s throughput I FDDI supports low-priority (asynchronous) and high-priority (synchronous) packets G Guarantees throughput and latency G Suitable for digitized voice, real-time control, etc. ECPE/CS 5516 (1/31/00) Direct Link Networks: Multiaccess Protocols - 28