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Physical Media physical link: transmitted data bit propagates across link guided media: signals propagate in solid media: copper, fiber unguided media: signals propagate freely, e.g., radio Twisted Pair (TP) two insulated copper wires Category 3: traditional phone wires, 10 Mbps ethernet Category 5 TP: 100Mbps ethernet 1 Physical Media: coax, fiber Coaxial cable: wire (signal carrier) within a wire (shield) baseband: single channel on cable broadband: multiple channel on cable bidirectional common use in 10Mbs Fiber optic cable: glass fiber carrying light pulses high-speed operation: 100Mbps Ethernet high-speed point-to-point transmission (e.g., 5 Gps) very low error rate Ethernet 2 Physical media: radio signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference Radio link types: microwave e.g. up to 45 Mbps channels LAN (e.g., 802.11b/g) 11Mbps wide-area (e.g., cellular) e.g. CDPD, 10’s Kbps satellite up to 50Mbps channel (or multiple smaller channels) 270 Msec end-end delay geosynchronous versus LEOS (low earth orbit) 3 The Data Link Layer Our goals: Overview: understand principles link layer services behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing error detection, correction multiple access protocols and LANs link layer addressing specific link layer technologies: Ethernet instantiation and implementation of various link layer technologies 4 Link Layer: setting the context 5 Link Layer: setting the context two physically connected devices: host-router, router-router, host-host unit of data: frame M Ht M Hn Ht M Hl Hn Ht M application transport network link physical data link protocol phys. link network link physical Hl Hn Ht M frame adapter card 6 Link Layer Services Framing, link access: encapsulate datagram into frame, adding header, trailer implement channel access if shared medium, ‘physical addresses’ used in frame headers to identify source, destination • different from IP address! Reliable delivery between two physically connected devices: seldom used on low bit error link (fiber, some twisted pair) wireless links: high error rates • Q: why both link-level and end-end reliability? 7 Link Layer Services (more) Flow Control: pacing between sender and receivers Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors: • signals sender for retransmission or drops frame Error Correction: receiver identifies and corrects bit error(s) without resorting to retransmission 8 Link Layer: Implementation implemented in “adapter” e.g., PCMCIA card, Ethernet card typically includes: RAM, DSP chips, host bus interface, and link interface M Ht M Hn Ht M Hl Hn Ht M application transport network link physical data link protocol phys. link adapter card network link physical Hl Hn Ht M frame 9 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields • Error detection not 100% reliable! Q: why? • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction 10 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors Parity bit=1 iff Number of 1’s even 0 0 11 Internet checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only) Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? 12 Checksumming: Cyclic Redundancy Check view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that <D,R> exactly divisible by G (modulo 2) receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! can detect all burst errors less than r+1 bits widely used in practice (ATM, HDCL) 13 CRC Example Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want reminder R R = remainder[ D.2r G ] 14 Multiple Access Links and Protocols Three types of “links”: point-to-point (single wire, e.g. PPP, SLIP) broadcast (shared wire or medium; e.g, Ethernet, Wavelan, etc.) switched (e.g., switched Ethernet, ATM etc) 15 Multiple Access protocols single shared communication channel two or more simultaneous transmissions by nodes: interference only one node can send successfully at a time multiple access protocol: distributed algorithm that determines how stations share channel, i.e., determine when station can transmit communication about channel sharing must use channel itself! what to look for in multiple access protocols: • synchronous or asynchronous • information needed about other stations • robustness (e.g., to channel errors) • performance 16 Multiple Access protocols claim: humans use multiple access protocols all the time class can "guess" multiple access protocols multiaccess protocol multiaccess protocol multiaccess protocol multiaccess protocol 1: 2: 3: 4: 17 MAC Protocols: a taxonomy Three broad classes: Channel Partitioning divide channel into smaller “pieces” (time slots, frequency) allocate piece to node for exclusive use Random Access allow collisions “recover” from collisions “Taking turns” tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralized 18 MAC Protocols: Measures Channel Rate = R bps Efficient: Single user: Throughput R Fairness N users Min. user throughput R/N Decentralized Fault tolerance Simple 19 Channel Partitioning MAC protocols: 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 20 Channel Partitioning MAC protocols: 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 frequency bands bands 2,5,6 idle 21 TDMA & FDMA: Performance Channel Rate = R bps Single user Throughput R/N Fairness Each user gets the same allocation Depends on maximum number of users Decentralized Requires division Simple 22 Channel Partitioning (CDMA) CDMA (Code Division Multiple Access) unique “code” assigned to each user; ie, code set partitioning used mostly in wireless broadcast channels (cellular, satellite, etc) all users share same frequency, but each user has own “chipping” sequence (ie, code) to encode data encoded signal = (original data) X (chipping sequence) decoding: inner-product of encoded signal and chipping sequence allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”) 23 CDMA - Basics Orthonormal codes: <ci,cj> =0 i≠j <ci,ci> =1 Encoding at user i: Bit 1 send +ci Bit 0 send -ci Decoding (at user i): Receive a vector ri Compute t=<ri,ci> If t=1 THEN bit=1 If t=-1 THEN bit=0 Correctness of decoding Single user Multiple users • Assume additive channel. • R = c1 – c2 • Output <R,c1> = <c1,c1> + <-c2,c1> = 1 + 0 = 1 24 CDMA Encode/Decode 25 CDMA: two-sender interference 26 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: slotted ALOHA ALOHA CSMA and CSMA/CD 27 Slotted Aloha time is divided into equal size slots (= pkt trans. time) node with new arriving pkt: transmit at beginning of next slot if collision: retransmit pkt in future slots with probability p, until successful. Success (S), Collision (C), Empty (E) slots 28 Slotted Aloha efficiency Q: what is max fraction slots successful? A: Suppose N stations have packets to send each transmits in slot with probability p prob. successful transmission S is: by single node: S= p (1-p)(N-1) by any of N nodes S = Prob (only one transmits) = N p (1-p)(N-1) … choosing optimum p =1/N as N -> infty ... S≈ 1/e = .37 as N -> infty At best: channel use for useful transmissions 37% of time! 29 Pure (unslotted) ALOHA unslotted Aloha: simpler, no synchronization pkt needs transmission: send without awaiting for beginning of slot collision probability increases: pkt sent at t0 collide with other pkts sent in [t0-1, t0+1] 30 Pure Aloha (cont.) P(success by given node) = P(node transmits) . P(no other node transmits in [t0-1,t0] . P(no other node transmits in [t0,t0+1] = p . (1-p)N-1 . (1-p)N-1 P(success by any of N nodes) = N p . (1-p)N-1 . (1-p)N-1 … choosing optimum p=1/(2N-1) as N -> infty ... S≈ 1/(2e) = .18 0.4 0.3 Slotted Aloha 0.2 0.1 protocol constrains effective channel throughput! Pure Aloha 0.5 1.0 1.5 2.0 G = offered load = Np 31 Aloha: Performance Channel Rate = R bps Single user Throughput R! Fairness Multiple users Combined throughput only 0.37*R Decentralized Slotted needs slot synchronization Simple 32 CSMA: Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire pkt If channel sensed busy, defer transmission Persistent CSMA: retry immediately with probability p when channel becomes idle Non-persistent CSMA: retry after random interval human analogy: don’t interrupt others! 33 CSMA collisions spatial layout of nodes along ethernet collisions can occur: propagation delay means two nodes may not yet hear each other’s transmission collision: entire packet transmission time wasted note: role of distance and propagation delay in determining collision prob. 34 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage persistent or non-persistent retransmission 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 35 CSMA/CD collision detection 36 CDMA/CD Channel Rate = R bps Single user Throughput Fairness R Multiple users Depends on Detection Time Decentralized Completely Simple Needs collision detection hardware 37 “Taking Turns” MAC protocols channel partitioning MAC protocols: share channel efficiently 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! 38 “Taking Turns” MAC protocols Polling: master node “invites” slave nodes to transmit in turn Request to Send, Clear to Send msgs concerns: polling overhead latency single point of failure (master) Token passing: control token passed from one node to next sequentially. token message concerns: token overhead latency single point of failure (token) 39 Reservation-based protocols Distributed Polling: time divided into slots begins with N short reservation slots reservation slot time equal to channel end-end propagation delay station with message to send posts reservation reservation seen by all stations after reservation slots, message transmissions ordered by known priority 40 Summary of MAC protocols What do you do with a shared media? Channel Partitioning, by time, frequency or code • Time Division,Code Division, Frequency Division Random partitioning (dynamic), • ALOHA, S-ALOHA, CSMA, CSMA/CD • carrier sensing: easy in some technologies (wire), hard in others (wireless) • CSMA/CD used in Ethernet Taking Turns • polling from a central cite, token passing • Popular in cellular 3G/4G networks where base station is the master 41 LAN technologies Data link layer so far: services, error detection/correction, multiple access Next: LAN technologies addressing Ethernet hubs, bridges, switches 802.11 PPP ATM 42 LAN Addresses 32-bit IP address: network-layer address used to get datagram to destination network LAN (or MAC or physical) 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 ROM 43 LAN Addresses Each adapter on LAN has unique LAN address 44 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 network to which one attaches ARP protocol translates IP address to MAC address 45 Ethernet “dominant” LAN technology: cheap $20 for 100Mbs! first widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 1, 10, 100, 1000 Mbps Metcalfe’s Etheret sketch 46 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock rates 47 Ethernet Frame Structure (more) Addresses: 6 bytes, frame is received by all adapters on a LAN and dropped if address does not match Type: indicates the higher layer protocol, mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped 48 Ethernet: uses CSMA/CD A: sense channel, if idle then { transmit and monitor the channel; If detect another transmission then { abort and send jam signal; update # collisions; delay as required by exponential backoff algorithm; goto A } else {done with the frame; set collisions to zero} } else {wait until ongoing transmission is over and goto A} 49 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; 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 n-th collision: choose K from {0,1,…, 2n-1} after ten or more collisions, choose K from {0,1,2,3,4,…,1023} 50 Exponential Backoff (simplified) N users Interval of size 2n Prob Node/slot is 1/2n Prob of success N(1/2n)(1 – 1/2n)N-1 Average success N(1 – 1/2n)N-1 Intervals size: 1, 2, 4, 8, 16 … Fraction (out of N) of success: 2n = N/8 -> 0.03 % 2n = N/4 -> 2% 2n = N/2 -> 15% 2n = N -> 37 % 2n = 2N -> 60% 51 Ethernet Technologies: 10Base2 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! 52 10BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Hub to which nodes are connected by twisted pair, thus “star topology” CSMA/CD implemented at hub 53 10BaseT and 100BaseT (more) Max distance from node to Hub is 100 meters Hub can disconnect “jabbering adapter Hub can gather monitoring information, statistics for display to LAN administrators 54 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 55 Token Passing: IEEE802.5 standard 4 Mbps max token holding time: 10 ms, limiting frame length SD, ED mark start, end of packet AC: access control byte: token bit: value 0 means token can be seized, value 1 means data follows FC priority bits: priority of packet reservation bits: station can write these bits to prevent stations with lower priority packet from seizing token after token becomes free 56 Token Passing: IEEE802.5 standard FC: frame control used for monitoring and maintenance source, destination address: 48 bit physical address, as in Ethernet data: packet from network layer checksum: CRC FS: frame status: set by dest., read by sender set to indicate destination up, frame copied OK from ring DLC-level ACKing 57