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Internet Protocols Fall 2006 Lectures 5 and 6 MAC Layer Andreas Terzis Outline • MAC Protocols – MAC Protocol Examples • Channel Partitioning – TDMA/FDMA – Token Ring • Random Access Protocols – Aloha and Slotted Aloha – CSMA(/CD) – CSMA/CA • Connecting Layer 2 networks • Interface to layer 3 CS 349/Fall05 2 MAC Protocols • Problem: How to share medium among multiple senders • Different Solutions – Channel Partitioning (FDMA, TDMA,CDMA) – Taking Turns (Token Ring) – Random Access (Aloha, CSMA, CSMA/CD, CSMA/CA) • Parameters – Physical medium characteristics – End-point capabilities CS 349/Fall05 3 Goals of MAC Protocols • MAC Protocols arbitrate access to a common shared channel among a population of users • Goals – – – – – – – Fairness High efficiency Low delay Fault tolerance Low overhead Low complexity … CS 349/Fall05 4 ALOHA • Packet radio network created by the University of Hawaii in the 70’s – Star topology – Two channels: Broadcast channel, Random Access channel • Basic Idea: – Let a node transmit when it has data to send – If frame was destroyed then retransmit after a random period of time • Feedback about packet reception status is sent over the broadcast channel CS 349/Fall05 5 ALOHA Efficiency • Assumptions – Infinite number of users – Frame arrivals are modeled by a Poisson process with rate λ – Retransmissions can also be modeled by a Poisson process with rate G > λ – Throughput S=GP0 • P0 = probability frame does not suffer a collision CS 349/Fall05 6 ALOHA: Collision Probability • Frame tx time:t • Vulnerable interval:2t • Prob[k arrivals]: 2G k e −2G Pr[k ] = k! • P0=e-2G • S=Ge-2G • Max throughput is 0.184 when G=0.5 t0 t0+t t0+2t t0+3t Vulnerable Interval CS 349/Fall05 7 Slotted ALOHA • Time is divided into slots equal to the frame transmission time – Central station sends clock tick signal – Satellite stations are allowed to send only immediately after hearing clock tick • Vulnerable time is equal to t • Throughput S=Ge-G (max =0.37, G=1) • Expected number of transmissions: ∞ ∞ E = ∑ kPk = ∑ ke −G (1 − e −G ) k −1 = 1 k =1 k =1 CS 349/Fall05 e −G = eG 8 Comparison CS 349/Fall05 9 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: • If channel sensed idle: transmit entire frame • If channel sensed busy, defer transmission • Human analogy: don’t interrupt others! CS 349/Fall05 10 CSMA collisions collisions can still occur: spatial layout of nodes propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability CS 349/Fall05 11 CSMA Variants • Persistent CSMA: Backlogged nodes will immediately transmit after channel becomes idle – Higher probability of collisions at high loads • P-Persistent CSMA: After channel is idle transmit with probability p – Higher delay at low loads • Non-persistent CSMA: Transmit with probability p even when the channel is idle CS 349/Fall05 12 Performance CS 349/Fall05 13 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA – Collisions detected within short time – Colliding transmissions aborted, reducing channel wastage • Collision detection: – Easy in wired LANs: measure signal strengths, compare transmitted, received signals – Difficult in wireless LANs: receiver shut off while transmitting • Human analogy: the polite conversationalist CS 349/Fall05 14 CSMA/CD collision detection CS 349/Fall05 15 Ethernet uses CSMA/CD • Adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense • Transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection CS 349/Fall05 • Before attempting a retransmission, adapter waits a random time, that is, random access 16 Ethernet CSMA/CD algorithm 1. Adapter gets datagram from upper layer and creates frame 2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff: after the m-th collision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K*512 bit times and returns to Step 2 CS 349/Fall05 17 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Bit time: .1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec Exponential Backoff: • Goal: adapt retransmission attempts to estimated current load – heavy load: random wait will be longer • first collision: choose K from {0,1}; delay is K x 512 bit transmission times • after second collision: choose K from {0,1,2,3}… • after ten collisions, choose K from {0,1,2,3,4,…,1023} CS 349/Fall05 18 CSMA/CD efficiency • Assume: k stations, prob transmit = p Pr[node acquires channel] = A = kp(1 − p ) k −1 p = 1 then A → 1 as k → ∞ k e Pr[contention interval has j slots] = A(1 − A) j −1 w= 1 A mean contention interval = 2τ S= S= F F + 2τ A 1 1 + 2 BLe A F F ≈ = F + 2eτ F + 2eBL c Notation: • L : link length • B : link speed • c : speed of light • F : frame size • τ : slot length in bits cF CS 349/Fall05 19 Efficiency as a function of τ CS 349/Fall05 20 Ethernet “Classic” • 10Base2, 10Base5 • 10: 10Mbps; 2: under 200 meters max cable length • thin coaxial cable in a bus topology • • Repeaters used to connect up to multiple segments • Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! • Has become a legacy technology CS 349/Fall05 21 Newer Ethernet Variants: 10BaseT and 100BaseT • 10/100 Mbps rate; latter called “fast ethernet” • T stands for Twisted Pair • Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub • Hubs are essentially physical-layer repeaters: – – – – bits coming in one link go out all other links no frame buffering no CSMA/CD at hub: adapters detect collisions provides net management functionality CS 349/Fall05 nodes hub 22 Gbit Ethernet • Use standard Ethernet frame format • Allows for point-to-point links and shared broadcast channels • In shared mode, CSMA/CD is used; short distances between nodes to be efficient • Uses hubs, called here “Buffered Distributors” • Full-Duplex at 1 Gbps for point-to-point links • 10 Gbps now • Q: Who needs all this bandwidth? CS 349/Fall05 23 Token Ring Overview • Examples – 16Mbps IEEE 802.5 (based on earlier IBM ring) – 100Mbps Fiber Distributed Data Interface (FDDI) – Resilient Packet Ring MAN (802.17) CS 349/Fall05 24 Token Ring (cont) • Idea – – – – Frames flow in one direction: upstream to downstream special bit pattern (token) rotates around ring must capture token before transmitting release token after done transmitting • immediate release • delayed release – remove your frame when it comes back around – stations get round-robin service Host Host Host MSAU Host From previous host Host To next host From previous host Relay (a) Host Host To next host Relay (b) From previous MSAU To next MSAU CS 349/Fall05 Host 25 Timed Token Algorithm • Token Holding Time (THT) – Upper limit on how long a station can hold the token • Token Rotation Time (TRT) – Upper limit on how long it takes the token to traverse the ring – TRT <= ActiveNodes x THT + RingLatency CS 349/Fall05 26 Additional Features • Successful delivery notification – Frame returning to sending host contains ACK • Different levels of service – Token contains priority field (3-bit) – Only frames with at least as high priority can be transmitted – Priority field is adjusted through reservation mechanism 8 8 8 48 48 Start delimiter Access control Frame control Dest addr Src addr Variable Body CS 349/Fall05 32 8 8 Checksum End delimiter Frame status 27 Token Maintenance • Lost Token – No token when initializing ring – Bit error corrupts token pattern – Node holding token crashes • Generating a Token (and agreeing on monitor) – Execute when join ring or suspect a failure – Send a claim frame that includes the node’s MAC address – When receive claim frame forward if local MAC address smaller – If your claim frame makes it all the way around the ring: • Your are the ring monitor • You insert new token CS 349/Fall05 28 Maintenance (cont) • Monitor duties – Regenerate token if current one is destroyed – Remove corrupted or orphaned frames CS 349/Fall05 29 When to send token? Early Release am Fr e Late Release Token Token Fra me (b) (a) Relative advantages and drawbacks? CS 349/Fall05 30 FDDI • Physical Properties – 100 Mbps, commonly uses fiber (although CDDI exists) – Two independent rings that transmit data in opposite directions – Second ring is used only when primary ring fails • Tolerate failure of a stations or single cable break – 500 hosts max, 2 km between any pair of hosts, 100 km total network size (a) (b) CS 349/Fall05 31 Wireless LANs • IEEE 802.11 • Bandwidth: 1 - 54 Mbps • Physical Media – spread spectrum radio (2.4GHz) – diffused infrared (10m) CS 349/Fall05 32 Spread Spectrum • Idea – spread signal over wider frequency band than required – originally designed to thwart jamming • Frequency Hopping – transmit over random sequence of frequencies – sender and receiver share… • pseudorandom number generator • seed – 802.11 uses 79 x 1MHz-wide frequency bands CS 349/Fall05 33 Spread Spectrum (cont) • Direct Sequence – – – – for each bit, send XOR of that bit and n random bits random sequence known to both sender and receiver called n-bit chipping code 802.11 defines an 11-bit chipping code 1 0 Data stream: 1010 1 0 Random sequence: 0100101101011001 1 0 XOR of the two: 1011101110101001 CS 349/Fall05 34 Collisions Avoidance • Similar to Ethernet • Problem: hidden and exposed nodes A B C CS 349/Fall05 D 35 MACAW • Sender transmits RequestToSend (RTS) frame • Receiver replies with ClearToSend (CTS) frame • Neighbors… – see CTS: keep quiet – see RTS but not CTS: ok to transmit • Receive sends ACK when has frame – neighbors silent until see ACK • Collisions – no collisions detection – known when don’t receive CTS – exponential backoff CS 349/Fall05 36 Interconnecting LANs • Bridges (aka Ethernet switches) were introduced to allow the interconnection of several local area networks (LANs) without a router. • By partitioning a large LAN into multiple smaller networks, there are fewer collisions, and more parallel communications. • It is now common for the port of an Ethernet switch to connect to just one (or a small number of) hosts. CS 349/Fall05 37 An Ethernet Network Router Router Problem: Outside world Shared network limits throughput. Lots of collisions reduces efficiency. CS 349/Fall05 38 Ethernet Switching Ethernet Switch Benefits: Number of collisions is reduced. 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. CS 349/Fall05 Outside Router Router world 39 One Ethernet Switch CS 349/Fall05 40 A typical LAN (IP network) Shared CS 349/Fall05 41 Ethernet Switching 1. Examines the header of each arriving frame. 2. If the Ethernet DA is in its table, it forwards the frame to the correct output port(s). 3. If the Ethernet DA is not in its table, it broadcasts the frame to all ports (except the one through which it arrived). 4. The table is learned by examining the Ethernet SA of arriving packets. CS 349/Fall05 42 Ethernet Switching Learning addresses B A C D E 2 3 1 Ethernet Switch 6 I J 4 5 K F G Ethernet Switch J F Q Router Router Switch learns that ‘F’ is reachable through port 5 L M N H O Outside world P Ethernet Switch Ethernet Switch CS 349/Fall05 43 Ethernet address Port A 1 B 2 C 3 Ethernet Switching Learning addresses B A C D 4 E, F, G, H, Q 5 I, J, K, L, M, N, O, P 6 D E F 2 3 1 Ethernet Switch 6 I J Ethernet Switch 4 5 K G L M N H Outside Q Router Router world O P Ethernet Switch Ethernet Switch Q: How do we prevent loops? CS 349/Fall05 44 LAN Addresses and ARP 32-bit IP address: • network-layer address • used to get datagram to destination IP network (recall IP network definition) LAN (or MAC or physical or Ethernet) address: • used to get datagram from one interface to another physically-connected interface (same network) • 48 bit MAC address (for most LANs) “burned” in the adapter EPROM CS 349/Fall05 45 LAN Address (more) • MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) • Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address • MAC flat address => portability – can move LAN card from one LAN to another • IP hierarchical address NOT portable – depends on IP network to which node is attached CS 349/Fall05 46 ARP: Address Resolution Protocol Question: how to determine MAC address of B knowing B’s IP address? • Each IP node (Host, Router) on LAN has ARP table • ARP Table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> – CS 349/Fall05 TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) 47 ARP protocol • • • • A wants to send datagram to B, and A knows B’s IP address. Suppose B’s MAC address is not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address – all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address • A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) – soft state: information that times out (goes away) unless refreshed • ARP is “plug-and-play”: – frame sent to A’s MAC address (unicast) CS 349/Fall05 – nodes create their ARP tables without intervention from net administrator 48 ARP Protocol (II) • Proxy ARP – Reply on behalf of another node • Gratuitous ARP – Node sends ARP asking for its own IP address • Find out if address has been claimed • Change the mapping between MAC<->IP addr • Do you see any problem with this? CS 349/Fall05 49