Computer Networks Medium Access Control. Mostafa Salehi Fall 2008

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1 Computer Networks Medium Access Control Mostafa Salehi Fall

2 Outline Issues ALOHA Network Ethernet Token Ring Wireless 2

3 Main Issues Local Area Network (LAN) : Three or more machines are physically connected and communicating. Problems: How to connect them? Topology Sharing links How to address each machine? Addressing How to regulate accessing to the media? MAC (Media Access method or protocol) Collision! Different technology address each problem in different way

4 LAN Technologies Application Presentation Session Transport Network Link Physical The 7-layer OSI Model Telnet FTP SMTP IP TCP HTTP NNTP LAN-LINK TFTP UDP The 4-layer Internet Model Link layer can have two types of technologies; Point to point link like PPP where there are only 2 nodes. Broadcast link like Ethernet when there are more than nodes.

5 Data link sublayers Multiplexing Media Access (MAC) Error Detection Framing Our focus will be on MAC sublayer in this slide. The link is shared among different sender and receivers. Since every frame is simultaneously l accessed by different nodes; They are called multi access links. They are called broadcast links. (important) LAN because of limited area. We need some type of medium access rules to avoid collision

6 Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. When one node wants to transmit, it can send at rate R. 2. When M nodes want to transmit, each can send at average rate R/M 3. Fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots 4. Simple

7 Goals of MAC Protocols MAC Protocols arbitrate access to a common shared channel among a population of nodes Goals: 1. Fair among users 2. High efficiency 3. Low delay 4. Fault tolerant 5. Easy to implement

8 Simple Random Complex Deterministic Examples of MAC Protocols Packet-Switched Radio Network Aloha Carrier Sense Multiple Access/Collision Detection Ethernet (IEEE 802.3) Token Passing Wireless Token Ring (IEEE 802.5) Fiber Distributed Data Interface (FDDI)

9 MAC Protocols Three broad classes: Channel Partitioning divide channel into smaller pieces (time slots, frequency, code) allocate piece to node for exclusive use Random Access channel not divided, allow collisions recover from collisions Taking turns Nodes take turns, but nodes with more to send can take longer turns Channel Reservation

10 Channel Partitioning: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load

11 Channel Partitioning: FDMA FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle frequency bands

12 Random Access Protocols When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes two or more transmitting nodes collision, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: ALOHA slotted ALOHA 2008 CSMA, CSMA/CD, CSMA/CA 12

13 Central Node f0 f1 Implemented Aloha All nodes transmit on one freq. Central node relays packets on the other frequency If more than one node transmit at the same time Collision! If there is a collision, both nodes re-transmit packets

14 Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization when frame first arrives transmit immediately collision probability increases: frame sent at ttt 0 collides with other frames sent in [t 0-1,t 1t+1]

15 Assumptions all frames have same size time is divided into equal size slots, time to transmit 1 frame nodes start to transmit frames only at beginning of slots nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Slotted ALOHA Operation when node obtains fresh frame, it transmits in next slot no collision, node can send new frame in next slot if collision, node retransmits frame in each subsequent slot with prob. p until success

16 Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Slotted ALOHA Cons collisions, wasting slots idle slots

17 Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [t 0-1,t 0 ]. P(no other node transmits in [t 0,t 0 +1] = p. (1-p) N-1. (1-p) N-1 = p. (1-p) 2(N-1) choosing optimum p and then letting n -> infty... = 1/(2e) =

18 Slotted Aloha efficiency Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send Suppose N nodes with many frames to send, each transmits in slot with probability p prob that node 1 has success in a slot = p(1-p) N-1 prob that any node has a success = Np(1-p) N-1 For max efficiency with N nodes, find p* that maximizes Np(1-p) N-1 For many nodes, take limit of Np*(1-p*) N-1 as N goes to infinity, gives 1/e = 0.37 At best: channel used for useful transmissions 37% of time!

19 How to improve ALOHA Aloha is not efficient due to collisions. How to reduce collision and increase efficiency? Do not send any data if somebody else is already transmitting. Carrier Sense. While sending if you recognized somebody else is also transmitting, then, there is a collision. Please stop. Collision Detection

20 CSMA/CD Protocol All nodes transmit & receive on one channel Packets are of variable size. 1. Carrier Sense: Check if the line is idle before transmitting. 2. Collision Detection: If more than one node transmit. Collision! All nodes detect collision, wait for random delay. Goto 1. binary exponential backoff

21 CSMA/CD Network Size Restriction Node must be able to hear that there is a collision before its packet is transmitted completely. i.e. Packet Transmission Time > Round trip propagation time i.e. TRANSP > 2.PROP

22 Performance of CSMA/CD Assume time-slotted channel 1. Find : Probability that exactly one node transmits in a given slot, where: p = Prob{a node tries to transmit a packet in a time slot}, N = number of nodes N N 1 α( p) p(1 p) 1 dα N 1 = N(1 p) pn( N dp α max 36% 40% 1)(1 when : p) p N 2 = 1/ N

23 1-persistent CSMA CSMA Protocols Send frame whenever the channel is free if the channel is not free wait for a random time. More propagation delay results in less efficiency. Nonpersistent Sense the channel randomly. if the channel is not free wait for a random time. P-persistent Sense the channel at slot times. Whenever the channel is free send frame with p probability if the channel is not free wait for a random time

24 Persistent and Nonpersistent CSMA

25 CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, or idle

26 Collision-Free Protocols CSMA/CA N node sharing a single channel The basic bit-map protocol

27 CSMA/CA The binary countdown protocol. A dash indicates silence

28 Ethernet History developed by Xerox PARC in mid-1970s roots in Aloha packet-radio network standardized by Xerox, DEC, and Intel in 1978 similar to IEEE standard Uses CSMA/CD technique for Media access. Uses 10Mbps physical link originally and now extended to 100Mbps, Fast Ethernet, and recently to 1000Mbps, Gigabit Ethernet. Uses variable frame length, bytes

29 The Original Ethernet PROP TRANSP > 10Mb/s 2 PROP TRANSP l < = l / c = 1500 / µ s max = Packetsize m > 12µ s ( 12µ s) 10Mb / s = 120bits Thick copper coaxial cable In practice, minimum packet size = 512 bits. allows for extra time to detect collisions. allows for repeaters that can boost signal. Repeaters every 500m

30 The Original Ethernet Original picture drawn by Bob Metcalfe, inventor of Ethernet (1972 Xerox PARC) The Ethernet protocol is implemented in Contoroler (Adaptor)

31 Bytes: Ethernet Frame Format PreambleSFD e DA SA Type Data Pad CRC 1. Preamble: trains clock-recovery circuits 2. Start of Frame Delimiter: indicates start of frame 3. Destination Address: 48-bit globally unique address assigned by manufacturer. 1b: unicast/multicast 1b: local/global address 4. Type: Indicates protocol of encapsulated data (e.g. IP = 0x0800) 5. Pad: Zeroes used to ensure minimum frame length 6. Cyclic Redundancy Check: check sequence to detect bit errors

32 Ethernet Addresses Unique, 6 bytes or 48-bit address assigned to each adapter by manufacturer. An address with all 1s is a broadcast address. multicast: first bit is 1 In order to make the address unique, first 24 bits are assigned to manufacturers and the last 24 bits are assigned locally. Each adaptor accept the packet if the destination address is its own address, broadcast address or multicast to which this adaptor belongs

33 The 10Mb/s Ethernet Standard IEEE Different physical layer options Ethernet MAC Protocol 10Base-5 10Base-2 10Base-T 10Base-F physical layer 10: 10Mbs Base: baseband 5: 500 Meter 10Base-5: Original Ethernet: large thick coaxial cable. 10Base-2: Thin coaxial cable version. 10Base-T: Voice-grade unshielded twisted-pair Category-3 telephone cable. 10Base-F: Two optical fibers in a single cable

34 Ethernet Cabling Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T

35 Cable topologies (a) Linear, (b) Spine, (c) Tree, (d) Segmented

36 Transmit Algorithm If line is idle send immediately upper bound message size of 1500 bytes must wait 9.6us between back-to-back frames If line is busy wait until idle and transmit immediately called 1-persistent

37 Algorithm (cont) If collision jam for 32 bits, then stop transmitting frame (minimum frame is 64 bytes (header + 46 bytes of data)) delay and try again 1st time: 0 or 51.2us 2nd time: 0, 51.2, or 102.4us 3rd time51.2, 102.4, or 153.6us nth time: k x 51.2us, for randomly selected k=0..2 n -1 give up after several tries (usually 16) exponential backoff

38 Increasing the data rate Increasing the data rate create the following Problem: E.g. CSMA/CD at 100Mb/s over 1500m of cable: TRANSP > 2PROP PROP = 1500 / = 6µ s TRANSP > 12µ s Packetsize 1200bits To overcome this two techniques used: Cable length limited to 100m: 8 PROP = 200 / Packetsize 160bits Increase the minimum packet length

39 4-32 plug-in line card Switched Ethernet 1-8 connector for each line card

40 Ethernet Switch Router Ethernet Switch/Bridge If only one computer per port, no collisions can take place (each cable is now a self-contained point-to-point Ethernet link). Capacity is increased: the switch can forward multiple frames to different computers at the same time. An Ethernet switch must contain buffers to hold frames during times of congestion

41 Ethernet Hub Extending LANs Combinations of Hub, switch and router Broadcasts by Hub is sensed by switch Ethernet Switch/Bridge Router

42 IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats

43 Wireless LAN (a) Hidden station problem (C B) (b) Exposed station problem (C D)

44 Collision Avoidance in Wireless MACA RTS (request to send) CTS (clear to send) (a) A sending an RTS to B. (b) B responding with a CTS to A

45 The MAC Sublayer Protocol The use of virtual channel sensing using CSMA/CA

46 Local Internetworking A configuration with four LANs and two bridges

47 Backward Learning Flooding algorithm for the first run If source LAN is the same as Destination LAN discard frame If source LAN as Destination LAN are different transmit to Destination LAN If Destination LAN is unknown use flooding algorithm

48 Spanning Tree Bridges Two parallel transparent bridges

49 Spanning Tree Bridges (a) Interconnected LANs. (b) A spanning tree covering the LANs. The dotted lines are not part of the spanning tree

50 Repeaters, Hubs, Bridges, Switches, Routers and Gateways

51 Repeaters, Hubs, Bridges, Switches, Routers and Gateways

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