Chapter 2 Datalink Layer & LAN Protocols. 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
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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 Physical Media
Coaxial cable: wire (signal carrier) within a wire (shield) baseband: single channel on cable broadband: multiple channel on cable bidirectional common use in 10Mbs Ethernet Physical Media: coax, fiber Fiber optic cable: • glass fiber carrying light pulses • high-speed operation: • 100Mbps Ethernet • high-speed point-to-point transmission (eg, 40 Gps) • very low error rate
signal carried in electromagnetic spectrum no physical “wire” bidirectional propagation environment effects: reflection obstruction by objects interference Physical media: Wireless Wireless link types: • microwave • e.g. up to 45 Mbps channels • LAN (e.g., 802.11b/g) • 11/54 Mbps • wide-area (e.g., cellular) • e.g. CDPD, 10’s Kbps • 3G ~ 2.4 Mbps • satellite • up to 50Mbps channel • multiple smaller channels • 270 Msec end-end delay • geosynchronous versus LEOS (low earth orbit)
Point to point link Shared medium link- also called: Broadcast link Multi-access link LAN Physical link types: Shared medium link: • Many stations on same medium segment • Intermittent transmission: only when needed • Qn: WHY? • Collisions occur • unless protocol makes special arrangements for co-ordination of transmission • Bit synchronization done per frame • Point to point link • Two stations only • Continuous transmission • Needed to keep bit clock synchronization • Sends filler when no data • Full duplex
Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing instantiation and implementation of various link layer technologies Overview: link layer services error detection, correction multiple access protocols and LANs link layer addressing specific link layer technologies: Ethernet The Data Link Layer
Recap: The Hourglass Architecture of the Internet Telnet Email FTP WWW TCP UDP IP Ethernet Wireless FDDI 8
M H H H H H H H H H t n l t t n l t n M M application transport network link physical M Link Layer: setting the context • two physically connected devices: • host-router, router-router, host-host • unit of data: frame network link physical data link protocol M frame phys. link adapter card
Link layer: Context • Data-link layer has responsibility of transferring datagram from one node to another node over a link • Datagram transferred by different link protocols over different links, e.g., • Ethernet on first link, • frame relay on intermediate links • 802.11 on last link transportation analogy • trip from New Haven to San Francisco • taxi: home to union station • train: union station to JFK • plane: JFK to San Francisco airport • shuttle: airport to hotel
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! • (OPTIONAL) Reliable data delivery: • seldom used on low bit-error link • E.g., fiber, twisted pair • wireless links: high error rates • Qn: why both link-level and end-end reliability?
Link Layer Services (more) • (OPTIONAL) 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(depending on protocol) • (OPTIONAL) Error Correction: • receiver identifies and corrects bit error(s) without resorting to retransmission
frame frame Adaptors Communicating datagram receiving node link layer protocol sending node adapter adapter • sending side: • encapsulates datagram in a frame, delimits frame • adds error checking bits, rdtparam’s, flow ctrl, etc. • receiving side • recognizes frame start /end • checks errors, rdt, flow ctrl, .. • extracts datagram, passes to L3 • link layer implemented in “adaptor” (aka NIC) • Ethernet card, modem, 802.11 card • adapter is semi-autonomous, implementing link & physical layers
M H H H H H H H H H t n l t t n l t n M M application transport network link physical M Link Layer: Implementation • implemented in “adapter” • e.g., PCMCIA card, Ethernet card • typically includes: RAM, DSP chips, host bus interface, and link interface network link physical data link protocol M frame phys. link adapter card
Error Detection • EDC= Error Detection and Correction bits (redundancy) • D = Data protected by error checking, may include header fields • Error detection not 100% reliable! Qn: why? • protocol may miss some errors, but rarely • larger EDC field yields better detection and correction Checksum Generator =? EDC” Checksum Generator
0 0 Parity Checking Two dimensional Bit Parity: Correct all single bit errors, Detect all X bit errors X=? Single Bit Parity: Detect all single bit errors Parity bit=1 iff Number of 1’s even
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? Internet checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)
Checksumming: Cyclic Redundancy Check • choose a (r+1) bit pattern (generator), G • G is fixed, known to Sender & Receiver • Sender: Wants to send data bits D • Finds r CRC bits, R, such that • (D || R) is exactly divisible by G (viewed as modulo 2 polynomials (*)) • Sends D and R • Receiver:divides (D || R) by G. • If remainder ≠ 0 : error detected! • can detect all burst errors less than r+1 bits • widely used in practice (Ethernet, ATM, HDLC) (*) This means that addition and subtraction use bitwise XOR
CRC Example Want: D.2rXOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want remainder R D.2r G R = remainder[ ]
Example G(x) 16 bits CRC: CRC-16: x16+x15+x2+1, CRC-CCITT: x16+x12+x5+1 both can catch all single or double bit errors all odd number of bit errors all burst errors of length 16 or less >99.99% of the 17 or 18 bits burst errors CRC-CCITT hardware implementation Using shift and XOR registers http://en.wikipedia.org/wiki/CRC-32#Implementation
Multiple Access Links and Protocols Three types of “links”: • point-to-point (single wire, e.g. PPP, SLIP, HDLC) • broadcast (shared wire or medium; e.g, Ethernet, Token Ring, WiFi, WaveLAN, etc.) • switched (e.g., switched Ethernet, ATM etc)
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
Multiple Access protocols • claim: humans use multiple access protocols all the time • class can "guess" multiple access protocols • multiaccess protocol 1: • multiaccess protocol 2: • multiaccess protocol 3: • multiaccess protocol 4:
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
MAC Protocols: Measures • Channel Rate = R bps • Efficient: • Single user:Throughput R • Fairness • N users • Min. user throughput R/N • Decentralized • Fault tolerance • Simple
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 • TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. • FDM (Frequency Division Multiplexing): frequency subdivided.
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 bands 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. • FDM (Frequency Division Multiplexing): frequency subdivided. time frequency bands
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 resource division • Simple
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
Slotted Aloha [Norm Abramson] • 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
At best: channel use for useful transmissions 37% of time! 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 of the nodes transmits) = N p (1-p)(N-1) … choosing optimum p =1/N as N -> infinity ... S≈ 1/e = .37 as N -> infinity
Goodput vs. Offered Load Slotted Aloha S = throughput = “goodput” (success rate) 1.5 2.0 0.5 1.0 G = offered load = Np • when pN < 1, as p (or N) increases • probability of empty slots reduces • probability of collision is still low, thus goodput increases • when pN > 1, as p (or N) increases, • probability of empty slots does not reduce much, but • probability of collision increases, thus goodput decreases • goodput is optimal when pN = 1
At best: channel use for useful transmissions 37% of time! Maximum Efficiency vs. n 1/e = 0.37
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]
0.4 0.3 Slotted Aloha protocol constrains effective channel throughput! 0.2 0.1 Pure Aloha 1.5 2.0 0.5 1.0 G = offered load = Np 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 S = throughput = “goodput” (success rate)
Aloha: Performance • Channel Rate = R bps • Single user • Throughput R • Fairness • Multiple users • Combined throughput only 0.37*R • Decentralized • Slotted Aloha needs slot synchronization • Simple
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!
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.
CSMA/CD: Collision Detection spatial layout of nodes along Ethernet spatial layout of nodes along Ethernet D D A A B C B C t0 t0 time time B detects collision, aborts D detects collision, aborts instead of wasting the whole packettransmission time, abort after detection. CSMA CSMA/CD
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 • in wireless LAN: • receiver closed when transmitting • the interfering station may not be heard by contender • human analogy: the polite conversationalist
CDMA/CD • Channel Rate = R bps • Single user • Throughput R • Fairness • Multiple users • Depends on Detection Time • Decentralized • Completely • Simple • Needs collision detection hardware
“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!
Token passing: • control token passed from one node to next sequentially. • token message • concerns: • token overhead • latency • single point of failure (token) “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)
Reservation-based protocols Distributed Polling: • time divided into slots • begins with N short dedicatedreservation slots • reservation slot time equals to channel end-end propagation delay Qn: WHY? • station with message to send posts reservation • reservation seen by all stations • after reservation slots, message transmissions ordered by known priority
Summary of MAC protocols • What do you do with a shared media? • Channel Partitioning: by time, frequency or code • Time Division, Frequency Division, Code 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
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
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 at production time
LAN Address (more) • MAC address allocation administered by IEEE • manufacturer buys portion of MAC address space (to assure uniqueness) • Analogy: (a) MAC address: like ID number תעודת זהות (b) IP address: like postal address כתובת מגורים • 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
Comparison of IP address and MAC Address IP address is hierarchical for routing scalability IP address needs to be globally unique (if no NAT) IP address depends on IP network to which an interface is attached NOT portable • MAC address is flat • MAC address: no need for global uniqueness, but in fact is globally unique • MAC address is assigned to a device • portable