Link Layer: Introduction link Some terminology: hosts and routers - PDF document

Link Layer: Introduction “link” Some terminology: ❒ hosts and routers are nodes Link Layer – Error ❒ communication channels that connect adjacent nodes along Detection/Correction and MAC communication path are links ❍ wired links ❍ wireless links ❍ LANs ❒ layer-2 packet is a frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 5: DataLink Layer 5-1 5: DataLink Layer 5-2 Adaptors Communicating Error Detection datagram EDC= Error Detection and Correction bits (redundancy) rcving link layer protocol sending D = Data protected by error checking, may include header fields node node frame frame • Error detection not 100% reliable! adapter adapter • protocol may miss some errors, but rarely ❒ receiving side • larger EDC field yields better detection and correction ❒ link layer implemented in “adaptor” (aka NIC) ❍ looks for errors, rdt, flow control, etc ❍ Ethernet card, PCMCI card, 802.11 card ❍ extracts datagram, passes to rcving node ❒ sending side: ❒ adapter is semi- ❍ encapsulates datagram in autonomous a frame ❍ adds error checking bits, ❒ link & physical layers 5: DataLink Layer 5-3 5: DataLink Layer 5-4 rdt, flow control, etc. Parity Checking Internet checksum Two Dimensional Bit Parity : Single Bit Parity: Goal: detect “errors” (e.g., flipped bits) in Detect and correct single bit errors Detect single bit errors transmitted segment (note: used at transport layer only ) Receiver: Sender: compute checksum of received ❒ ❒ treat segment contents segment as sequence of 16-bit check if computed checksum ❒ integers equals checksum field value: ❒ checksum: addition (1’s ❍ NO - error detected complement sum) of ❍ YES - no error detected. segment contents But maybe errors 0 0 ❒ sender puts checksum nonetheless? More later …. value into UDP checksum field 5: DataLink Layer 5-5 5: DataLink Layer 5-6 1

CRC Example Checksumming: Cyclic Redundancy Check Want: ❒ view data bits, D, as a binary number ❒ choose r+1 bit pattern (generator), G D . 2 r XOR R = nG ❒ goal: choose r CRC bits, R, such that equivalently: ❍ <D,R> exactly divisible by G (modulo 2) D . 2 r = nG XOR R ❍ receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! equivalently: ❍ can detect all burst errors less than r+1 bits if we divide D . 2 r by ❒ widely used in practice (ATM, HDCL) G, want remainder R D . 2 r R = remainder[ ] G 5: DataLink Layer 5-7 5: DataLink Layer 5-8 Multiple Access Links and Protocols Multiple Access protocols Two types of “links”: ❒ single shared broadcast channel ❒ point-to-point ❒ two or more simultaneous transmissions by nodes: ❍ PPP for dial-up access ❍ point-to-point link between Ethernet switch and host interference ❒ broadcast (shared wire or medium) ❍ collision if node receives two or more signals at the same time ❍ traditional Ethernet multiple access protocol ❍ upstream HFC ❍ 802.11 wireless LAN ❒ distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit ❒ communication about channel sharing must use channel itself! ❍ no out-of-band channel for coordination 5: DataLink Layer 5-9 5: DataLink Layer 5-10 Ideal Mulitple Access Protocol MAC Protocols: a taxonomy Three broad classes: Broadcast channel of rate R bps ❒ Channel Partitioning 1. When one node wants to transmit, it can send at ❍ divide channel into smaller “pieces” (time slots, rate R. frequency, code) 2. When M nodes want to transmit, each can send at ❍ allocate piece to node for exclusive use average rate R/M ❒ Random Access 3. Fully decentralized: ❍ channel not divided, allow collisions ❍ “recover” from collisions ❍ no special node to coordinate transmissions ❒ “Taking turns” ❍ no synchronization of clocks, slots 4. Simple ❍ Nodes take turns, but nodes with more to send can take longer turns 5: DataLink Layer 5-11 5: DataLink Layer 5-12 2

Channel Partitioning MAC protocols: TDMA Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access TDMA: time division multiple access ❒ channel spectrum divided into frequency bands ❒ access to channel in "rounds" ❒ each station assigned fixed frequency band ❒ each station gets fixed length slot (length = pkt trans time) in each round ❒ unused transmission time in frequency bands go idle ❒ unused slots go idle ❒ example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle ❒ example: 6-station LAN, 1,3,4 have pkt, slots time frequency bands 2,5,6 idle 5: DataLink Layer 5-13 5: DataLink Layer 5-14 Random Access Protocols Slotted ALOHA ❒ When node has packet to send Assumptions Operation ❍ transmit at full channel data rate R. ❒ all frames same size ❒ when node obtains fresh ❍ no a priori coordination among nodes ❒ time is divided into equal frame, it transmits in next slot ❒ two or more transmitting nodes ➜ “collision”, size slots, time to transmit ❒ no collision, node can send new 1 frame ❒ random access MAC protocol specifies: frame in next slot ❒ nodes start to transmit ❍ how to detect collisions ❒ if collision, node retransmits frames only at beginning of ❍ how to recover from collisions (e.g., via delayed frame in each subsequent slot slots retransmissions) with prob. p until success ❒ nodes are synchronized ❒ Examples of random access MAC protocols: ❍ slotted ALOHA ❒ if 2 or more nodes transmit in slot, all nodes detect ❍ ALOHA collision ❍ CSMA, CSMA/CD, CSMA/CA 5: DataLink Layer 5-15 5: DataLink Layer 5-16 Slotted ALOHA Slotted Aloha efficiency ❒ For max efficiency with N Efficiency is the long-run nodes, find p* that fraction of successful slots maximizes when there are many nodes, each Np(1-p) N-1 with many frames to send ❒ For many nodes, take limit Suppose N nodes with many of Np*(1-p*) N-1 as N goes ❒ Pros Cons frames to send, each to infinity, gives 1/e = .37 transmits in slot with single active node can ❒ collisions, wasting slots ❒ probability p continuously transmit at full ❒ idle slots rate of channel prob that node 1 has success ❒ ❒ nodes may be able to in a slot = p(1-p) N-1 highly decentralized: only ❒ At best: channel detect collision in less slots in nodes need to be in prob that any node has a ❒ used for useful than time to transmit sync success = Np(1-p) N-1 transmissions 37% packet simple ❒ of time! ❒ clock synchronization 5: DataLink Layer 5-17 5: DataLink Layer 5-18 3

Pure (unslotted) ALOHA CSMA (Carrier Sense Multiple Access) ❒ unslotted Aloha: simpler, no synchronization CSMA : listen before transmit: ❒ when frame first arrives If channel sensed idle: transmit entire frame ❍ transmit immediately ❒ If channel sensed busy, defer transmission ❒ collision probability increases: ❍ frame sent at t 0 collides with other frames sent in [t 0 -1,t 0 +1] ❒ Human analogy: don’t interrupt others! 5: DataLink Layer 5-19 5: DataLink Layer 5-20 CSMA collisions CSMA/CD (Collision Detection) spatial layout of nodes CSMA/CD: carrier sensing, deferral as in CSMA collisions can still occur: propagation delay means ❍ collisions detected within short time two nodes may not hear ❍ colliding transmissions aborted, reducing channel each other’s transmission wastage collision: ❒ collision detection: entire packet transmission time wasted ❍ easy in wired LANs: measure signal strengths, note: compare transmitted, received signals role of distance & propagation delay in determining collision ❍ difficult in wireless LANs: receiver shut off while probability transmitting ❒ human analogy: the polite conversationalist 5: DataLink Layer 5-21 5: DataLink Layer 5-22 CSMA/CD collision detection “Taking Turns” MAC protocols channel partitioning MAC protocols: ❍ share channel efficiently and fairly at high load ❍ inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols ❍ efficient at low load: single node can fully utilize channel ❍ high load: collision overhead “taking turns” protocols look for best of both worlds! 5: DataLink Layer 5-23 5: DataLink Layer 5-24 4

“Taking Turns” MAC protocols Summary of MAC protocols Token passing: Polling: ❒ What do you do with a shared media? ❒ control token passed from one ❒ master node node to next sequentially. ❍ Channel Partitioning, by time, frequency or code “invites” slave ❒ token message • Time Division, Frequency Division nodes to transmit ❒ concerns: ❍ Random partitioning (dynamic), in turn ❍ token overhead • ALOHA, S-ALOHA, CSMA, CSMA/CD ❒ concerns: ❍ latency • carrier sensing: easy in some technologies (wire), ❍ single point of failure (token) ❍ polling overhead hard in others (wireless) ❍ latency • CSMA/CD used in Ethernet ❍ single point of • CSMA/CA used in 802.11 failure (master) ❍ Taking Turns • polling from a central site, token passing 5: DataLink Layer 5-25 5: DataLink Layer 5-26 5