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5: DataLink Layer5-1 Chapter 5: The Data Link Layer Our goals: r understand principles behind data link layer services: m error detection, correction m.

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Presentation on theme: "5: DataLink Layer5-1 Chapter 5: The Data Link Layer Our goals: r understand principles behind data link layer services: m error detection, correction m."— Presentation transcript:

1 5: DataLink Layer5-1 Chapter 5: The Data Link Layer Our goals: r understand principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing m reliable data transfer, flow control: done! r instantiation and implementation of various link layer technologies

2 5: DataLink Layer5-2 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-layer Addressing r 5.5 Ethernet r 5.6 Link-layer switches r 5.7 PPP r 5.8 Link virtualization: ATM, MPLS

3 5: DataLink Layer5-3 Link Layer: Introduction Some terminology: r hosts and routers are nodes r communication channels that connect adjacent nodes along communication path are links m wired links m wireless links m LANs r 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

4 5: DataLink Layer5-4 Link layer: context r datagram transferred by different link protocols over different links: m e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link r each link protocol provides different services m e.g., may or may not provide rdt over link transportation analogy r trip from Princeton to Lausanne m limo: Princeton to JFK m plane: JFK to Geneva m train: Geneva to Lausanne r tourist = datagram r transport segment = communication link r transportation mode = link layer protocol r travel agent = routing algorithm

5 5: DataLink Layer5-5 Link Layer Services r framing, link access: m encapsulate datagram into frame, adding header, trailer m channel access if shared medium m “MAC” addresses used in frame headers to identify source, dest different from IP address! r reliable delivery between adjacent nodes m we learned how to do this already (chapter 3)! 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?

6 5: DataLink Layer5-6 Link Layer Services (more) r flow control: m pacing between adjacent sending and receiving nodes r error detection: m errors caused by signal attenuation, noise. m receiver detects presence of errors: signals sender for retransmission or drops frame r error correction: m receiver identifies and corrects bit error(s) without resorting to retransmission r half-duplex and full-duplex m with half duplex, nodes at both ends of link can transmit, but not at same time

7 5: DataLink Layer5-7 Where is the link layer implemented? r in each and every host r link layer implemented in “adaptor” (aka network interface card NIC) m Ethernet card, PCMCI card, 802.11 card m implements link, physical layer r attaches into host’s system buses r combination of hardware, software, firmware controller physical transmission cpu memory host bus (e.g., PCI) network adapter card host schematic application transport network link physical

8 5: DataLink Layer5-8 Adaptors Communicating r sending side: m encapsulates datagram in frame m adds error checking bits, rdt, flow control, etc. r receiving side m looks for errors, rdt, flow control, etc m extracts datagram, passes to upper layer at receiving side controller sending host receiving host datagram frame

9 5: DataLink Layer5-9 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-layer Addressing r 5.5 Ethernet r 5.6 Link-layer switches r 5.7 PPP r 5.8 Link Virtualization: ATM. MPLS

10 5: DataLink Layer5-10 Error Detection EDC= Error Detection and Correction bits (redundancy) D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction otherwise

11 5: DataLink Layer5-11 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0

12 5: DataLink Layer5-12 Internet checksum (review) Sender: r treat segment contents as sequence of 16-bit integers r checksum: addition (1’s complement sum) of segment contents r sender puts checksum value into UDP checksum field Receiver: r compute checksum of received segment r 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 packet (note: used at transport layer only)

13 5: DataLink Layer5-13 Checksumming: Cyclic Redundancy Check r view data bits, D, as a binary number r choose r+1 bit pattern (generator), G r 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 r widely used in practice (Ethernet, 802.11 WiFi, ATM)

14 5: DataLink Layer5-14 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 remainder R R = remainder[ ] D.2rGD.2rG

15 5: DataLink Layer5-15 CRC calculation r Calculation of the polynomial codes data: 1101011011 G: 10011 CRC: ?

16 5: DataLink Layer5-16 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-layer Addressing r 5.5 Ethernet r 5.6 Link-layer switches r 5.7 PPP r 5.8 Link Virtualization: ATM, MPLS

17 5: DataLink Layer5-17 Multiple Access Links and Protocols Two types of “links”: r point-to-point m PPP for dial-up access m point-to-point link between Ethernet switch and host r broadcast (shared wire or medium) m old-fashioned Ethernet m upstream HFC m 802.11 wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., 802.11 WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical)

18 5: DataLink Layer5-18 Multiple Access protocols r single shared broadcast channel r two or more simultaneous transmissions by nodes: interference m collision if node receives two or more signals at the same time multiple access protocol r distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit r communication about channel sharing must use channel itself! m no out-of-band channel for coordination

19 5: DataLink Layer5-19 Ideal Multiple Access Protocol Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: m no special node to coordinate transmissions m no synchronization of clocks, slots 4. simple

20 5: DataLink Layer5-20 MAC Protocols: a taxonomy Three broad classes: r Channel Partitioning m divide channel into smaller “pieces” (time slots, frequency, code) m allocate piece to node for exclusive use r Random Access m channel not divided, allow collisions m “recover” from collisions r “Taking turns” m nodes take turns, but nodes with more to send can take longer turns

21 5: DataLink Layer5-21 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access r access to channel in "rounds" r each station gets fixed length slot (length = pkt trans time) in each round r unused slots go idle r example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 1 3 4 1 3 4 6-slot frame

22 5: DataLink Layer5-22 Channel Partitioning MAC protocols: FDMA FDMA: frequency division multiple access r channel spectrum divided into frequency bands r each station assigned fixed frequency band r unused transmission time in frequency bands go idle r example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle frequency bands time FDM cable

23 5: DataLink Layer5-23 Random Access Protocols r 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”, r random access MAC protocol specifies: m how to detect collisions m how to recover from collisions (e.g., via delayed retransmissions) r Examples of random access MAC protocols: m slotted ALOHA m ALOHA m CSMA, CSMA/CD, CSMA/CA

24 5: DataLink Layer5-24 Slotted ALOHA Assumptions: r all frames same size r time divided into equal size slots (time to transmit 1 frame) r nodes start to transmit only slot beginning r nodes are synchronized r if 2 or more nodes transmit in slot, all nodes detect collision Operation: r when node obtains fresh frame, transmits in next slot m if no collision: node can send new frame in next slot m if collision: node retransmits frame in each subsequent slot with prob. p until success

25 5: DataLink Layer5-25 Slotted ALOHA Pros r single active node can continuously transmit at full rate of channel r highly decentralized: only slots in nodes need to be in sync r simple Cons r collisions, wasting slots r idle slots r nodes may be able to detect collision in less than time to transmit packet r clock synchronization

26 5: DataLink Layer5-26 Slotted Aloha efficiency r suppose: N nodes with many frames to send, each transmits in slot with probability p r prob that given node has success in a slot = p(1-p) N-1 r prob that any node has a success = Np(1-p) N-1 r max efficiency: find p* that maximizes Np(1-p) N-1 r for many nodes, take limit of Np*(1-p*) N-1 as N goes to infinity, gives: Max efficiency = 1/e =.37 Efficiency : long-run fraction of successful slots (many nodes, all with many frames to send) At best: channel used for useful transmissions 37% of time! !

27 5: DataLink Layer5-27 Pure (unslotted) ALOHA r unslotted Aloha: simpler, no synchronization r when frame first arrives m transmit immediately r collision probability increases: m frame sent at t 0 collides with other frames sent in [t 0 -1,t 0 +1]

28 5: DataLink Layer5-28 Pure Aloha efficiency P(success by given node) = P(node transmits). P(no other node transmits in [p 0 -1,p 0 ]. P(no other node transmits in [p 0 -1,p 0 ] = p. (1-p) N-1. (1-p) N-1 = p. (1-p) 2(N-1) … choosing optimum p and then letting n -> infty... = 1/(2e) =.18 even worse than slotted Aloha!

29 5: DataLink Layer5-29 Pure ALOHA Throughput versus offered traffic for ALOHA systems.

30 5: DataLink Layer5-30 CSMA (Carrier Sense Multiple Access) CSMA: listen before transmit: If channel sensed idle: transmit entire frame r If channel sensed busy, defer transmission r human analogy: don’t interrupt others!

31 5: DataLink Layer5-31 CSMA collisions collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted spatial layout of nodes note: role of distance & propagation delay in determining collision probability

32 5: DataLink Layer5-32 Nonpersistent CSMA 1. If medium is idle, transmit; otherwise, go to 2 2. If medium is busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1 r Random delays reduces probability of collisions m Consider two stations become ready to transmit at same time While another transmission is in progress m If both stations delay same time before retrying, both will attempt to transmit at same time r Capacity is wasted because medium will remain idle following end of transmission m Even if one or more stations waiting r Nonpersistent stations deferential

33 5: DataLink Layer5-33 1-persistent CSMA r To avoid idle channel time, 1-persistent protocol used r Station wishing to transmit listens and obeys following: 1. If medium idle, transmit; otherwise, go to step 2 2. If medium busy, listen until idle; then transmit immediately r 1-persistent stations selfish r If two or more stations waiting, collision guaranteed m Gets sorted out after collision

34 5: DataLink Layer5-34 P-persistent CSMA r Compromise that attempts to reduce collisions m Like nonpersistent r And reduce idle time m Like1-persistent r Rules: 1. If medium idle, transmit with probability p, and delay one time unit with probability (1 – p) m Time unit typically maximum propagation delay 2. If medium busy, listen until idle and repeat step 1 3. If transmission is delayed one time unit, repeat step 1 r What is an effective value of p?

35 5: DataLink Layer5-35 Value of p? r Avoid instability under heavy load r n stations waiting to send r End of transmission, expected number of stations attempting to transmit is number of stations ready times probability of transmitting m n x p r If n x p > 1 on average there will be a collision r Repeated attempts to transmit almost guaranteeing more collisions r Retries compete with new transmissions r Eventually, all stations trying to send m Continuous collisions; zero throughput r So nxp < 1 for expected peaks of n r If heavy load expected, p small r However, as p made smaller, stations wait longer r At low loads, this gives very long delays

36 5: DataLink Layer5-36 Persistent and Nonpersistent CSMA Comparison of the channel utilization versus load for various random access protocols.

37 5: DataLink Layer5-37 Which Persistence Algorithm? r IEEE 802.3 uses 1-persistent r Both nonpersistent and p-persistent have performance problems r 1-persistent (p = 1) seems more unstable than p-persistent m Greed of the stations m But wasted time due to collisions is short (if frames long relative to propagation delay m With random backoff, unlikely to collide on next tries m To ensure backoff maintains stability, IEEE 802.3 and Ethernet use binary exponential backoff

38 5: DataLink Layer5-38 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 r collision detection: m easy in wired LANs: measure signal strengths, compare transmitted, received signals m difficult in wireless LANs: received signal strength overwhelmed by local transmission strength r human analogy: the polite conversationalist

39 5: DataLink Layer5-39 CSMA/CD collision detection

40 5: DataLink Layer5-40 CSMA/CD (Collision Detection) r With CSMA, collision occupies medium for duration of transmission r Stations listen whilst transmitting 1. If medium idle, transmit, otherwise, step 2 2. If busy, listen for idle, then transmit 3. If collision detected, jam then cease transmission 4. After jam, wait random time then start from step 1

41 5: DataLink Layer5-41 “Taking Turns” MAC protocols channel partitioning MAC protocols: m share channel efficiently and fairly 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!

42 5: DataLink Layer5-42 “Taking Turns” MAC protocols Polling: r master node “invites” slave nodes to transmit in turn r typically used with “dumb” slave devices r concerns: m polling overhead m latency m single point of failure (master) master slaves poll data

43 5: DataLink Layer5-43 “Taking Turns” MAC protocols Token passing: r control token passed from one node to next sequentially. r token message r concerns: m token overhead m latency m single point of failure (token) T data (nothing to send) T

44 5: DataLink Layer5-44 Summary of MAC protocols r channel partitioning, by time, frequency or code m Time Division, Frequency Division r random access (dynamic), m ALOHA, S-ALOHA, CSMA, CSMA/CD m carrier sensing: easy in some technologies (wire), hard in others (wireless) m CSMA/CD used in Ethernet m CSMA/CA used in 802.11 r taking turns m polling from central site, token passing m Bluetooth, FDDI, IBM Token Ring

45 5: DataLink Layer5-45 LAN technologies Data link layer so far: m services, error detection/correction, multiple access Next: LAN technologies m addressing m Ethernet m hubs, switches m PPP

46 5: DataLink Layer5-46 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing r 5.5 Ethernet r 5.6 Link-layer switches r 5.7 PPP r 5.8 Link Virtualization: ATM, MPLS

47 5: DataLink Layer5-47 MAC Addresses and ARP r 32-bit IP address: m network-layer address m used to get datagram to destination IP subnet r MAC (or LAN or physical or Ethernet) address: m function: get frame from one interface to another physically-connected interface (same network) m 48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable

48 5: DataLink Layer5-48 LAN Addresses and ARP Each adapter on LAN has unique LAN address Broadcast address = FF-FF-FF-FF-FF-FF = adapter 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN (wired or wireless)

49 5: DataLink Layer5-49 LAN Address (more) r MAC address allocation administered by IEEE r manufacturer buys portion of MAC address space (to assure uniqueness) r 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 r IP hierarchical address NOT portable m address depends on IP subnet to which node is attached

50 5: DataLink Layer5-50 ARP: Address Resolution Protocol r Each IP node (host, router) on LAN has ARP table r ARP table: IP/MAC address mappings for some LAN nodes m TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) Question: how to determine MAC address of B knowing B’s IP address? 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN 137.196.7.23 137.196.7.78 137.196.7.14 137.196.7.88

51 5: DataLink Layer5-51 ARP protocol: Same LAN (network) r A wants to send datagram to B, and B’s MAC address not in A’s ARP table. r A broadcasts ARP query packet, containing B's IP address m dest MAC address = FF- FF-FF-FF-FF-FF m all machines on LAN receive ARP query r B receives ARP packet, replies to A with its (B's) MAC address m frame sent to A’s MAC address (unicast) r A caches (saves) IP-to- MAC address pair in its ARP table until information becomes old (times out) m soft state: information that times out (goes away) unless refreshed r ARP is “plug-and-play”: m nodes create their ARP tables without intervention from net administrator

52 5: DataLink Layer5-52 Addressing: routing to another LAN R 1A-23-F9-CD-06-9B 222.222.222.220 111.111.111.110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D 111.111.111.112 111.111.111.111 A 74-29-9C-E8-FF-55 222.222.222.221 88-B2-2F-54-1A-0F B 222.222.222.222 49-BD-D2-C7-56-2A walkthrough: send datagram from A to B via R assume A knows B’s IP address r two ARP tables in router R, one for each IP network (LAN)

53 5: DataLink Layer5-53 r A creates IP datagram with source A, destination B r A uses ARP to get R’s MAC address for 111.111.111.110 r A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram r A’s NIC sends frame r R’s NIC receives frame r R removes IP datagram from Ethernet frame, sees its destined to B r R uses ARP to get B’s MAC address r R creates frame containing A-to-B IP datagram sends to B R 1A-23-F9-CD-06-9B 222.222.222.220 111.111.111.110 E6-E9-00-17-BB-4B CC-49-DE-D0-AB-7D 111.111.111.112 111.111.111.111 A 74-29-9C-E8-FF-55 222.222.222.221 88-B2-2F-54-1A-0F B 222.222.222.222 49-BD-D2-C7-56-2A This is a really important example – make sure you understand!

54 5: DataLink Layer5-54 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing r 5.5 Ethernet r 5.6 Link-layer switches r 5.7 PPP r 5.8 Link Virtualization: ATM and MPLS

55 5: DataLink Layer5-55 Ethernet r History m Developed by Bob Metcalfe and others at Xerox PARC in mid- 1970s m Roots in Aloha packet-radio network m Standardized by Xerox, DEC, and Intel in 1978 m LAN standards define MAC and physical layer connectivity IEEE 802.3 (CSMA/CD - Ethernet) standard – originally 2Mbps IEEE 802.3u standard for 100Mbps Ethernet IEEE 802.3z standard for 1,000Mbps Ethernet  CSMA/CD: Ethernet ’ s Media Access Control (MAC) policy m CS = carrier sense Send only if medium is idle m MA = multiple access m CD = collision detection Stop sending immediately if collision is detected

56 5: DataLink Layer5-56 Ethernet “dominant” wired LAN technology: r cheap $20 for NIC r first widely used LAN technology r simpler, cheaper than token LANs and ATM r kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch

57 5: DataLink Layer5-57 Star topology r bus topology popular through mid 90s m all nodes in same collision domain (can collide with each other) r today: star topology prevails m active switch in center m each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other) switch bus: coaxial cable star

58 5: DataLink Layer5-58 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: r 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 r used to synchronize receiver, sender clock rates

59 5: DataLink Layer5-59 Ethernet Frame Structure (more) r Addresses: 6 bytes m if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to network layer protocol m otherwise, adapter discards frame r Type: indicates higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) r CRC: checked at receiver, if error is detected, frame is dropped

60 5: DataLink Layer5-60 Ethernet Frame Structure (more)

61 5: DataLink Layer5-61 Ethernet Frame Structure (more)

62 5: DataLink Layer5-62 Ethernet: Unreliable, connectionless r connectionless: No handshaking between sending and receiving NICs r unreliable: receiving NIC doesn’t send acks or nacks to sending NIC m stream of datagrams passed to network layer can have gaps (missing datagrams) m gaps will be filled if app is using TCP m otherwise, app will see gaps

63 5: DataLink Layer5-63 Ethernet uses CSMA/CD r No slots r adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense r transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection r Before attempting a retransmission, adapter waits a random time, that is, random access

64 5: DataLink Layer5-64 Ethernet CSMA/CD algorithm 1. NIC receives datagram from network layer, creates frame 2. If NIC senses channel idle, starts frame transmission If NIC senses channel busy, waits until channel idle, then transmits 3. If NIC transmits entire frame without detecting another transmission, NIC is done with frame ! 4. If NIC detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, NIC enters exponential backoff: after mth collision, NIC chooses K at random from {0,1,2,…,2 m -1}. NIC waits K·512 bit times, returns to Step 2

65 5: DataLink Layer5-65 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: r Goal: adapt retransmission attempts to estimated current load m heavy load: random wait will be longer r first collision: choose K from {0,1}; delay is K· 512 bit transmission times r after second collision: choose K from {0,1,2,3}… r after ten collisions, choose K from {0,1,2,3,4,…,1023} See/interact with Java applet on AWL Web site: highly recommended !

66 5: DataLink Layer5-66 Exponential Backoff r If a collision is detected, delay and try again r Delay time is selected using binary exponential backoff m 1st time: choose K from {0,1} then delay = K * 51.2us m 2nd time: choose K from {0,1,2,3} then delay = K * 51.2us  nth time: delay = K x 51.2us, for K=0..2 n – 1 Note max value for k = 1023 m give up after several tries (usually 16) Report transmit error to host r If delay were not random, then there is a chance that sources would retransmit in lock step r Why not just choose from small set for K m This works fine for a small number of hosts m Large number of nodes would result in more collisions

67 5: DataLink Layer5-67 State Diagram for CSMA/CD Packet? Sense Carrier Discard Packet Send Detect Collision Jam channel b=CalcBackoff(); wait(b); attempts++; No Yes attempts < 16 attempts == 16

68 5: DataLink Layer5-68 Collisions AB AB Collisions are caused when two adaptors transmit at the same time (adaptors sense collision based on voltage differences) Both found line to be idle Both had been waiting to for a busy line to become idle A starts at time 0 Message almost there at time T when B starts – collision! How can we be sure A knows about the collision?

69 5: DataLink Layer5-69 Collision Detection r How can A know that a collision has taken place? m There must be a mechanism to insure retransmission on collision  A ’ s message reaches B at time T  B ’ s message reaches A at time 2T m So, A must still be transmitting at 2T r IEEE 802.3 specifies max value of 2T to be 51.2us m This relates to maximum distance of 2500m between hosts m At 10Mbps it takes 0.1us to transmit one bit so 512 bits (64B) take 51.2us to send m So, Ethernet frames must be at least 64B long 14B header, 46B data, 4B CRC Padding is used if data is less than 46B r Send jamming signal after collision is detected to insure all hosts see collision m 48 bit signal

70 5: DataLink Layer5-70 Collision Detection contd. AB AB AB time = 0 time = T time = 2T

71 5: DataLink Layer5-71 MAC Algorithm from the Receiver Side r Senders handle all access control r Receivers simply read frames with acceptable address m Address to host m Address to broadcast m Address to multicast to which host belongs m All frames if host is in promiscuous mode

72 5: DataLink Layer5-72 CSMA/CD efficiency r T prop = max prop delay between 2 nodes in LAN r t trans = time to transmit max-size frame r efficiency goes to 1 m as t prop goes to 0 m as t trans goes to infinity r better performance than ALOHA: and simple, cheap, decentralized !

73 5: DataLink Layer5-73 Ethernet Performance Efficiency of Ethernet at 10 Mbps with 512- bit slot times.

74 5: DataLink Layer5-74 802.3 Ethernet Standards: Link & Physical Layers r many different Ethernet standards m common MAC protocol and frame format m different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps m different physical layer media: fiber, cable application transport network link physical MAC protocol and frame format 100BASE-TX 100BASE-T4 100BASE-FX 100BASE-T2 100BASE-SX 100BASE-BX fiber physical layer copper (twister pair) physical layer

75 5: DataLink Layer5-75 Ethernet Technologies: 10Base2 r 10: 10Mbps; 2: under 185 (~200) meters cable length r Thin coaxial cable in a bus topology r Repeaters used to connect multiple segments m Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only!

76 5: DataLink Layer5-76 Ethernet Cabling The most common kinds of Ethernet cabling.

77 5: DataLink Layer5-77 Ethernet Cabling (2) Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.

78 5: DataLink Layer5-78 Ethernet Cabling (3) Cable topologies. (a) Linear, (b) Spine, (c) Tree, (d) Segmented.

79 5: DataLink Layer5-79 Ethernet Cabling (4) (a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.

80 5: DataLink Layer5-80 Manchester encoding r used in 10BaseT r each bit has a transition r allows clocks in sending and receiving nodes to synchronize to each other m no need for a centralized, global clock among nodes! r Hey, this is physical-layer stuff!

81 5: DataLink Layer5-81 Physical Layer Configurations for 802.3 r Physical layer configurations are specified in three parts r Data rate (10, 100, 1,000) m 10, 100, 1,000Mbps r Signaling method (base, broad) m Baseband Digital signaling m Broadband Analog signaling r Cabling (2, 5, T, F, S, L) m 5 - Thick coax (original Ethernet cabling)  F – Optical fiber  S – Short wave laser over multimode fiber  L – Long wave laser over single mode fiber

82 5: DataLink Layer5-82 Ethernet Overview r Most popular packet-switched LAN technology r Bandwidths: 10Mbps, 100Mbps, 1Gbps r Max bus length: 2500m m 500m segments with 4 repeaters r Bus and Star topologies are used to connect hosts m Hosts attach to network via Ethernet transceiver or hub or switch Detects line state and sends/receives signals m Hubs are used to facilitate shared connections m All hosts on an Ethernet are competing for access to the medium Switches break this model r Problem: Distributed algorithm that provides fair access

83 5: DataLink Layer5-83 Switched Ethernet A simple example of switched Ethernet.

84 5: DataLink Layer5-84 Switched Ethernet r Switches forward and filter frames based on LAN addresses  It ’ s not a bus or a router (although simple forwarding tables are maintained) r Very scalable m Options for many interfaces m Full duplex operation (send/receive frames simultaneously)  Connect two or more “ segments ” by copying data frames between them m Switches only copy data when needed key difference from repeaters r Higher link bandwidth m Collisions are completely avoided r Much greater aggregate bandwidth m Separate segments can send at once

85 5: DataLink Layer5-85 Fast Ethernet The original fast Ethernet cabling.

86 5: DataLink Layer5-86 Gigabit Ethernet (a) A two-station Ethernet. (b) A multistation Ethernet.

87 5: DataLink Layer5-87 Gigabit Ethernet (2) Gigabit Ethernet cabling.

88 5: DataLink Layer5-88 Fast and Gigabit Ethernet r Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet m Uses different physical layer encoding (4B5B)  Many NIC ’ s are 10/100 capable Can be used at either speed r Gigabit Ethernet (1,000Mbps) m Compatible with lower speeds m Uses standard framing and CSMA/CD algorithm m Distances are severely limited m Typically used for backbones and inter-router connectivity m Becoming cost competitive m How much of this bandwidth is realizable?

89 5: DataLink Layer5-89 Experiences with Ethernet r Ethernets work best under light loads m Utilization over 30% is considered heavy Network capacity is wasted by collisions r Most networks are limited to about 200 hosts m Specification allows for up to 1024 r Most networks are much shorter m 5 to 10 microsecond RTT r Transport level flow control helps reduce load (number of back to back packets) r Ethernet is inexpensive, fast and easy to administer!

90 5: DataLink Layer5-90 Ethernet Problems  Ethernet ’ s peak utilization is pretty low (like Aloha) r Peak throughput worst with m More hosts More collisions needed to identify single sender m Smaller packet sizes More frequent arbitration m Longer links Collisions take longer to observe, more wasted bandwidth m Efficiency is improved by avoiding these conditions

91 5: DataLink Layer5-91 Physical and Data Link Features of Ethernet r Standards and Implementation

92 5: DataLink Layer5-92 IEEE 802.2: Logical Link Control (a) Position of LLC. (b) Protocol formats.

93 5: DataLink Layer5-93 Physical and Data Link Features of Ethernet  Logic Link Control – Connecting the Upper Layers

94 5: DataLink Layer5-94 Physical and Data Link Features of Ethernet r Media Access Control (MAC)

95 5: DataLink Layer5-95 Physical and Data Link Features of Ethernet r Physical Implementations of the Ethernet

96 5: DataLink Layer5-96 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3 Multiple access protocols r 5.4 Link-layer Addressing r 5.5 Ethernet r 5.6 Link-layer switches r 5.7 PPP r 5.8 Link Virtualization: ATM, MPLS

97 5: DataLink Layer5-97 Inter - Networking r Hubs r Bridges r Switches r Routers

98 5: DataLink Layer5-98 Hubs … physical-layer (“dumb”) repeaters: m bits coming in one link go out all other links at same rate m all nodes connected to hub can collide with one another m no frame buffering m no CSMA/CD at hub: host NICs detect collisions twisted pair hub

99 5: DataLink Layer5-99 Switch r link-layer device: smarter than hubs, take active role m store, forward Ethernet frames m examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment r transparent m hosts are unaware of presence of switches r plug-and-play, self-learning m switches do not need to be configured

100 5: DataLink Layer5-100 Switch: allows multiple simultaneous transmissions r hosts have dedicated, direct connection to switch r switches buffer packets r Ethernet protocol used on each incoming link, but no collisions; full duplex m each link is its own collision domain r switching: A-to-A’ and B- to-B’ simultaneously, without collisions m not possible with dumb hub A A’ B B’ C C’ switch with six interfaces (1,2,3,4,5,6) 1 2 3 4 5 6

101 5: DataLink Layer5-101 Switch Table r Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? r A: each switch has a switch table, each entry: m (MAC address of host, interface to reach host, time stamp) r looks like a routing table! r Q: how are entries created, maintained in switch table? m something like a routing protocol? A A’ B B’ C C’ switch with six interfaces (1,2,3,4,5,6) 1 2 3 4 5 6

102 5: DataLink Layer5-102 Switch: self-learning r switch learns which hosts can be reached through which interfaces m when frame received, switch “learns” location of sender: incoming LAN segment m records sender/location pair in switch table A A’ B B’ C C’ 1 2 3 4 5 6 A A’ Source: A Dest: A’ MAC addr interface TTL Switch table (initially empty) A 1 60

103 5: DataLink Layer5-103 Switch: frame filtering/forwarding When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

104 5: DataLink Layer5-104 Self-learning, forwarding: example A A’ B B’ C C’ 1 2 3 4 5 6 A A’ Source: A Dest: A’ MAC addr interface TTL Switch table (initially empty) A 1 60 A A’ r frame destination unknown: flood A’ A r destination A location known: A’ 4 60 selective send

105 5: DataLink Layer5-105 Interconnecting switches r switches can be connected together A B r Q: sending from A to G - how does S 1 know to forward frame destined to F via S 4 and S 3 ? r A: self learning! (works exactly the same as in single-switch case!) S1S1 C D E F S2S2 S4S4 S3S3 H I G

106 5: DataLink Layer5-106 Self-learning multi-switch example Suppose C sends frame to I, I responds to C r Q: show switch tables and packet forwarding in S 1, S 2, S 3, S 4 A B S1S1 C D E F S2S2 S4S4 S3S3 H I G 1 2

107 5: DataLink Layer5-107 Switch: traffic isolation r switch installation breaks subnet into LAN segments r switch filters packets: m same-LAN-segment frames not usually forwarded onto other LAN segments m segments become separate collision domains hub switch collision domain

108 5: DataLink Layer5-108 Switches: dedicated access r Switch with many interfaces r Hosts have direct connection to switch r No collisions; full duplex Switching: A-to-A’ and B-to-B’ simultaneously, no collisions switch A A’ B B’ C C’

109 5: DataLink Layer5-109 More on Switches r cut-through switching: frame forwarded from input to output port without first collecting entire frame m slight reduction in latency r combinations of shared/dedicated, 10/100/1000 Mbps interfaces

110 5: DataLink Layer5-110 Institutional network to external network router IP subnet mail server web server

111 5: DataLink Layer5-111 Switches vs. Routers r both store-and-forward devices m routers: network layer devices (examine network layer headers) m switches are link layer devices r routers maintain routing tables, implement routing algorithms r switches maintain switch tables, implement filtering, learning algorithms

112 5: DataLink Layer5-112 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (a) Which device is in which layer. (b) Frames, packets, and headers.

113 5: DataLink Layer5-113 Repeaters, Hubs, Bridges, Switches, Routers and Gateways (2) (a) A hub. (b) A bridge. (c) a switch.

114 5: DataLink Layer5-114 Comparison

115 5: DataLink Layer5-115 Link Layer r 5.1 Introduction and services r 5.2 Error detection and correction r 5.3Multiple access protocols r 5.4 Link-Layer Addressing r 5.5 Ethernet r 5.6 Hubs and switches r 5.7 PPP r 5.8 Link Virtualization: ATM

116 5: DataLink Layer5-116 Chapter 5: Summary r principles behind data link layer services: m error detection, correction m sharing a broadcast channel: multiple access m link layer addressing r instantiation and implementation of various link layer technologies m Ethernet m switched LANS

117 5: DataLink Layer5-117 Chapter 5: let’s take a breath r journey down protocol stack complete (except PHY) r solid understanding of networking principles, practice r ….. could stop here …. but lots of interesting topics! m wireless m multimedia m security m network management

118 Review Questions See the textbook (P.527-)  R2, R7, R8, R9, R12  P5, P14, P17, P20 5: DataLink Layer5-118


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