Local area networks. Copyright

Transcription

1 Local area networks Telecommunication Networks Group COMPUTER NETWORKS LANs - 1 Copyright Quest opera è protetta dalla licenza Creative Commons NoDerivs-NonCommercial. Per vedere una copia di questa licenza, consultare oppure inviare una lettera a: Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. This work is licensed under the Creative Commons NoDerivs-NonCommercial License. To view a copy of this license, visit: or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA. COMPUTER NETWORKS LANs - 2 Pag. 1

2 LANs Small geographical extension Shared transmission medium (originally) only one node can transmit at a time Multiple access problem Motivation: bursty traffic Dedicated channel would be wasted When sending, each node would like a high tx speed Useful for broadcast-multicast transmission Need to use address to identify node for unicast traffic Many topologies bus,ring, star COMPUTER NETWORKS LANs - 3 Multiplexing and multiple access (recap) Problem: channel sharing Multiplexing: centralized All flows available in a single access point to the channel router, antenna point to point radio link, satellite, base station in a cellular network Multiple access: distributed Flows access the channel using different transmitters LANs, mobile terminal in a cellular network, earth stations in a satellite network, COMPUTER NETWORKS LANs - 4 Pag. 2

3 Multiple access in LANs Static channel division Fixed assignement of portion of channels Time Division Frequency Division Code Division Not suited to bursty traffic (N queues and servers at speed C are worse that 1 queue and server at speed NC) Could extend to a dynamic assignment scenario Suppose a centralized controller Need to collect node tx needs (according to which access scheme?) Need to send allocation decision to nodes (according to which access scheme?) Complexity and increase in delay Solution: rely on distributed, access protocols Goal: to emulate statistical multiplexing COMPUTER NETWORKS LANs - 5 Multiple access protocols Multiple access protocols inspired by human behaviour Examples: Moderator gives permissions Reservation (rising hands to make it) Free access Free access, but polite (do not talk if somebody else is speaking) Cyclic passing a permission to talk COMPUTER NETWORKS LANs - 6 Pag. 3

4 LAN access protocols: taxonomy Three main families: Random access (CSMA/CD, Ethernet) Ordered access (Token Ring, Token Bus, FDDI) Slotted access, with reservation (DQDB) How to evaluate LAN access protocols performance Throughput Fairness Access delay Number of nodes, network size, reliability, ease of deployment COMPUTER NETWORKS LANs - 7 Random access protocols Free access Each node send at the channel speed R No coordination among nodes If two concurrent transmissions collision MAC (Medium Access Control) random access protocols specify: How to detect a collision How to recover after a collision has been detected Basic idea: random transmission. If collision is detected, retransmit after a random delay COMPUTER NETWORKS LANs - 8 Pag. 4

5 Slotted Aloha Time organized in slot (all nodes synchronized on the same slot) All nodes start packet transmission at the beginning of the slot If a collision occurs, data are re-transmitted with probability p in the next slot (or after a random number of slots) Success (S), Collision (C), Empty (E) slots COMPUTER NETWORKS LANs - 9 ALOHA Simpler than Slotted Aloha, Synchronization not needed When a packet becomes available, nodes attempt transmission Higher collision probability Yellow packet collides with other packets transmitted in the interval [t0-1, t0+1] COMPUTER NETWORKS LANs - 10 Pag. 5

6 Performance (infinite users) v COMPUTER NETWORKS LANs - 11 Simple protocols Observations Low throughput due to collisions For uniformly generated Poisson traffic and infinite users capacity 18% (ALOHA) or 37% (SLOTTED ALOHA) It depends on the type of traffic! Unstable protocols At low load, small access delay It is not possible to control access delays a priori No support for priorities Throughput improvments not difficult to be obtained COMPUTER NETWORKS LANs - 12 Pag. 6

7 CSMA: Carrier Sense Multiple Access Sense the channel before transmission If the channel is sensed free, transmit a packet If the channel is busy, defer transmission to avoid collision 1-persistent CSMA: retry transmission as soon as channel sensed free 0-persistent CSMA : retry transmission after a random time p-persistent CSMA: with p behave as 1-persistent, with probability (1-p) behave as 0-persistent COMPUTER NETWORKS LANs - 13 Collisons occur due to propagation delay If a collision occurs, a full packet transmission time is wasted The propagation delay (distance) plays a fundamental role in collision probability Vulnerability period depends on propagation delay CSMA: collisions? COMPUTER NETWORKS LANs - 14 Pag. 7

8 CSMA/CD (Collision Detection) CSMA/CD adds to CSMA If a collision is (quickly) detected, packet transmission is suspended Reduce the waste due to useless transmission Collision detection: Compare the tx signal with the rx signal Easy in wired LANs Almost impossible in wireless LANs: half duplex (when tx the rx is disbled) COMPUTER NETWORKS LANs - 15 CSMA/CD collision detection COMPUTER NETWORKS LANs - 16 Pag. 8

9 CSMA/CD: performance Throughput performance strongly depend on the end to end propagation delay More precisely, on the ratio between packet transmission time and the propagation delay Very good throughput performance on small size networks (with respect to packet size) and with relatively small transmission speed Large packets, much larger than network size! Constraint on the minimum packet size to detect collisions (a node must transmit when detecting a collision) COMPUTER NETWORKS LANs - 17 CSMA/CD: performance 1 persistent is preferred for the better performance at low load Networks must run at low loads! Reduced access delay Collision cost not too high in small size networks Unstable Exponential backoff on retransmission Difficult to separate traffic on a priority basis Adopted in Ethernet networks COMPUTER NETWORKS LANs - 18 Pag. 9

10 Random access protocols performance COMPUTER NETWORKS LANs - 19 Pag. 10