1 Physical Media  physical link: transmitted data bit propagates across link  guided media: m signals propagate in solid media: copper, fiber  unguided media: m signals propagate freely, e.g., radio Twisted Pair (TP)  two insulated copper wires m Category 3: traditional phone wires, 10 Mbps ethernet m Category 5 TP: 100Mbps ethernet

2 Physical Media: coax, fiber Coaxial cable:  wire (signal carrier) within a wire (shield) m baseband: single channel on cable m broadband: multiple channel on cable  bidirectional  common use in 10Mbs Ethernet Fiber optic cable:  glass fiber carrying light pulses  high-speed operation: m 100Mbps Ethernet m high-speed point-to-point transmission (e.g., 5 Gps)  very low error rate

3 Physical media: radio  signal carried in electromagnetic spectrum  no physical “wire”  bidirectional  propagation environment effects: m reflection m obstruction by objects m interference Radio link types:  microwave m e.g. up to 45 Mbps channels  LAN (e.g., b/g) m 11/54 Mbps  wide-area (e.g., cellular) m e.g. CDPD, 10’s Kbps  satellite m up to 50Mbps channel (or multiple smaller channels) m 270 Msec end-end delay m geosynchronous versus LEOS (low earth orbit)

4 The Data Link Layer Our goals:  understand principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m 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: m Ethernet

5 Link Layer: setting the context

6  two physically connected devices: m host-router, router-router, host-host  unit of data: frame application transport network link physical network link physical M M M M H t H t H n H t H n H l M H t H n H l frame phys. link data link protocol adapter card

7 Link Layer Services  Framing, link access: m encapsulate datagram into frame, adding header, trailer m implement channel access if shared medium, m ‘physical addresses’ used in frame headers to identify source, destination different from IP address!  Reliable delivery between two physically connected devices: m seldom used on low bit error link (fiber, some twisted pair) m wireless links: high error rates Q: why both link-level and end-end reliability?

8 Link Layer Services (more)  Flow Control: m pacing between sender and receivers  Error Detection: m errors caused by signal attenuation, noise. m receiver detects presence of errors: signals sender for retransmission or drops frame  Error Correction: m receiver identifies and corrects bit error(s) without resorting to retransmission

9 Link Layer: Implementation  implemented in “adapter” m e.g., PCMCIA card, Ethernet card m typically includes: RAM, DSP chips, host bus interface, and link interface application transport network link physical network link physical M M M M H t H t H n H t H n H l M H t H n H l frame phys. link data link protocol adapter card

10 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

11 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0 Parity bit=1 iff Number of 1’s even

12 Internet checksum 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: m NO - error detected m YES - no error detected. But maybe errors nonetheless? Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only)

13 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 m exactly divisible by G (modulo 2) m receiver knows G, divides by G. If non-zero remainder: error detected! m can detect all burst errors less than r+1 bits  widely used in practice (ATM, HDCL)

14 CRC Example Want: D. 2 r XOR R = nG equivalently: D. 2 r = nG XOR R equivalently: if we divide D. 2 r by G, want reminder R R = remainder[ ] D.2rGD.2rG

15 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)

16 Multiple Access protocols  single shared communication channel  two or more simultaneous transmissions by nodes: interference m only one node can send successfully at a time  multiple access protocol: m distributed algorithm that determines how stations share channel, i.e., determine when station can transmit m communication about channel sharing must use channel itself! m what to look for in multiple access protocols: synchronous or asynchronous information needed about other stations robustness (e.g., to channel errors) performance

17 Multiple Access protocols  claim: humans use multiple access protocols all the time  class can "guess" multiple access protocols m multiaccess protocol 1: m multiaccess protocol 2: m multiaccess protocol 3: m multiaccess protocol 4:

18 MAC Protocols: a taxonomy Three broad classes:  Channel Partitioning m divide channel into smaller “pieces” (time slots, frequency) m allocate piece to node for exclusive use  Random Access m allow collisions m “recover” from collisions  “Taking turns” m tightly coordinate shared access to avoid collisions Goal: efficient, fair, simple, decentralized

19 MAC Protocols: Measures  Channel Rate = R bps  Efficient: m Single user: Throughput R  Fairness m N users m Min. user throughput R/N  Decentralized m Fault tolerance  Simple

20 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.

21 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. frequency bands time

22 TDMA & FDMA: Performance  Channel Rate = R bps  Single user m Throughput R/N  Fairness m Each user gets the same allocation m Depends on maximum number of users  Decentralized m Requires resource division  Simple

23 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 almost “orthogonal”)

24 CDMA - Basics  Orthonormal codes: m =0 i≠j m =1  Encoding at user i: m Bit 1 send +c i m Bit 0 send -c i  Decoding (at user i): m Receive a vector r i m Compute t= m If t=1 THEN bit=1 m If t=-1 THEN bit=0  Correctness of decoding m Single user m Multiple users Assume additive channel. R = c 1 – c 2 Output = + = = 1

25 CDMA Encode/Decode

26 CDMA: two-sender interference

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

28 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

29 Slotted Aloha efficiency Q: what is max fraction slots successful? A: Suppose N stations have packets to send m each transmits in slot with probability p m 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!

30 Pure (unslotted) ALOHA  unslotted Aloha: simpler, no synchronization  pkt needs transmission: m send without awaiting for beginning of slot  collision probability increases: m pkt sent at t 0 collide with other pkts sent in [t 0 -1, t 0 +1]

31 Pure Aloha (cont.) 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(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) G = offered load = Np Pure Aloha Slotted Aloha protocol constrains effective channel throughput!

32 Aloha: Performance  Channel Rate = R bps  Single user m Throughput R !  Fairness m Multiple users m Combined throughput only 0.37*R  Decentralized m Slotted needs slot synchronization  Simple

33 CSMA: Carrier Sense Multiple Access CSMA: listen before transmit:  If channel sensed idle: transmit entire pkt  If channel sensed busy, defer transmission m Persistent CSMA: retry immediately with probability p when channel becomes idle m Non-persistent CSMA: retry after random interval  human analogy: don’t interrupt others!

34 CSMA collisions collisions can occur: propagation delay means two nodes may not yet hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes along ethernet note: role of distance and propagation delay in determining collision prob.

35 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA m collisions detected within short time m colliding transmissions aborted, reducing channel wastage m persistent or non-persistent retransmission  collision detection: m easy in wired LANs: measure signal strengths, compare transmitted, received signals m difficult in wireless LANs: receiver shut off while transmitting  human analogy: the polite conversationalist

36 CSMA/CD collision detection

37 CDMA/CD  Channel Rate = R bps  Single user m Throughput R  Fairness m Multiple users m Depends on Detection Time  Decentralized m Completely  Simple m Needs collision detection hardware

38 “Taking Turns” MAC protocols channel partitioning MAC protocols: m share channel efficiently at high load m inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols m efficient at low load: single node can fully utilize channel m high load: collision overhead “taking turns” protocols look for best of both worlds!

39 “Taking Turns” MAC protocols Polling:  master node “invites” slave nodes to transmit in turn  Request to Send, Clear to Send msgs  concerns: m polling overhead m latency m single point of failure (master) Token passing:  control token passed from one node to next sequentially.  token message  concerns: m token overhead m latency m single point of failure (token)

40 Reservation-based protocols Distributed Polling:  time divided into slots  begins with N short reservation slots m reservation slot time equal to channel end-end propagation delay m station with message to send posts reservation m reservation seen by all stations  after reservation slots, message transmissions ordered by known priority

41 Summary of MAC protocols  What do you do with a shared media? m Channel Partitioning, by time, frequency or code Time Division,Code Division, Frequency Division m 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 m Taking Turns polling from a central cite, token passing Popular in cellular 3G/4G networks where base station is the master

42 LAN technologies Data link layer so far: m services, error detection/correction, multiple access Next: LAN technologies m addressing m Ethernet m hubs, bridges, switches m m PPP m ATM

43 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

44 LAN Addresses Each adapter on LAN has unique LAN address

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 m can move LAN card from one LAN to another  IP hierarchical address NOT portable m depends on network to which one attaches  ARP protocol translates IP address to MAC address

46 Ethernet “dominant” LAN technology:  cheap $20 for 10/100/1000 Mbs!  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

47 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble:  7 bytes with pattern followed by one byte with pattern  used to synchronize receiver, sender clock rates

48 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

49 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}

50 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 m 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,…, 2 n -1}  after ten or more collisions, choose K from {0,1,2,3,4,…,1023}

51 Exponential Backoff (simplified)  N users  Interval of size 2 n  Prob Node/slot is 1/2 n  Prob of success N(1/2 n )(1 – 1/2 n ) N-1  Average slot success N(1 – 1/2 n ) N-1  Intervals size: 1, 2, 4, 8, 16 …  Fraction (out of N) of success: m 2 n = N/8 -> 0.03 % 2 n = N/4 -> 2% m 2 n = N/2 -> 15% 2 n = N -> 37 % m 2 n = 2N -> 60%

52 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!

53 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

54 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

55 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

56 Token Rings (IEEE 802.5)  A ring topology is a single unidirectional loop connecting a series of stations in sequence  Each bit is stored and forwarded by each station’s network interface

57 Token Ring: IEEE802.5 standard  4 Mbps (also 16 Mbps)  max token holding time: 10 ms, limiting frame length  SD, ED mark start, end of packet  AC: access control byte: m token bit: value 0 means token can be seized, value 1 means data follows FC m priority bits: priority of packet m reservation bits: station can write these bits to prevent stations with lower priority packet from seizing token after token becomes free

58 Token Ring: 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 m set to indicate destination up, frame copied OK from ring m DLC-level ACKing