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3 Link Layer Introduction and services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet Hubs and switches PPP MPLS CPSC 441: Link Layer 7 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 CPSC 441: Link Layer 8 Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0 CPSC 441: Link Layer 9 3

4 Cyclic Redundancy Check/Polynomial Codes A (n+1)-bit message can be represented as a polynomial of degree n. For example, X = ; M(X) = x + x + x + x So, a sender and receiver can be considered to exchange polyns. Choose k+1 bit pattern (divisor), C(X), a polyn of degree k goal: choose k CRC bits, R, such that <M,R> exactly divisible by C (modulo 2) receiver knows C, divides <M,R> by C. If non-zero remainder: error detected! can detect all burst errors less than k+1 bits CPSC 441: Link Layer 10 CRC continued Goal: design P(X) such that it is exactly divisible by C(X) Multiply M(X) by x k (add k zero s to the end of the message) to get T(X) Divide T(X) by C(X) and find the remainder R(X) Subtract the remainder from T(X) to get P(X). P(X) is now exactly divisible by C(X). Remember all addition/subtract use modulo-2 arithmetic. CPSC 441: Link Layer 11 Link Layer Introduction and services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet Hubs and switches PPP MPLS CPSC 441: Link Layer 12 4

6 Taxonomy of Multiple Access Control Protocols Three broad classes: Channel Partitioning divide channel into smaller pieces (TDM, FDM, Code Division Multiple Access) TDM, FDM covered in chap. 1 allocate piece to node for exclusive use Random Access channel not divided, allow collisions recover from collisions Taking turns Nodes take turns, but nodes with more to send can take longer turns CPSC 441: Link Layer 16 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, CSMA/CD, CSMA/CA CPSC 441: Link Layer 17 Pure ALOHA Let nodes transmit whenever a frame arrives No synchronization among nodes If collision, retransmit after random delay collision probability increases: frame sent at t 0 collides with other frames sent in [t 0-1,t 0 +1] CPSC 441: Link Layer 18 6

7 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... Even worse! = 1/(2e) =.18 CPSC 441: Link Layer 19 Slotted ALOHA Assumptions all frames same size time is divided into equal size slots, time to transmit 1 frame nodes start to transmit frames only at beginning of slots nodes are synchronized if 2 or more nodes transmit in slot, all nodes detect collision Operation when node obtains fresh frame, it transmits in next slot no collision, node can send new frame in next slot if collision, node retransmits frame in each subsequent slot with prob. p until success CPSC 441: Link Layer 20 Slotted ALOHA Pros single active node can continuously transmit at full rate of channel highly decentralized: only slots in nodes need to be in sync simple Cons collisions, wasting slots idle slots nodes may be able to detect collision in less than time to transmit packet clock synchronization CPSC 441: Link Layer 21 7

8 Slotted Aloha efficiency Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send Suppose N nodes with many frames to send, each transmits in slot with probability p prob that node 1 has success in a slot = p(1-p) N-1 prob that any node has a success = Np(1-p) N-1 For max efficiency with N nodes, find p* that maximizes Np(1-p) N-1 For many nodes, take limit of Np*(1-p*) N-1 as N goes to infinity, gives 1/e =.37 At best: channel used for useful transmissions 37% of time! CPSC 441: Link Layer 22 Carrier Sense Multiple Access (CSMA) First listen; transmit only if channel sensed to be idle Can collisions occur in this scheme? One possibility: two nodes might attempt to transmit a frame at the same time Another possibility? CPSC 441: Link Layer 23 CSMA/CD (Collision Detection) CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals difficult in wireless LANs: receiver shut off while transmitting CPSC 441: Link Layer 24 8

9 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! CPSC 441: Link Layer 25 Taking Turns MAC protocols Polling: Token passing: master node control token passed from invites slave nodes one node to next to transmit in turn sequentially. concerns: token message polling overhead concerns: latency token overhead single point of latency failure (master) single point of failure (token) CPSC 441: Link Layer 26 Summary of MAC protocols What do you do with a shared media? Channel Partitioning, by time, frequency or code Time 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 CSMA/CA used in Taking Turns polling from a central site, token passing CPSC 441: Link Layer 27 9

10 Next Stop LAN Technologies Data link layer so far: services, error detection/correction, multiple access Next: LAN technologies Ethernet s, switches PPP CPSC 441: Link Layer 28 Link Layer Introduction and services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet Hubs and switches PPP MPLS CPSC 441: Link Layer 29 Ethernet dominant wired LAN technology: cheap \$20 for 100Mbs! first widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10 Mbps 10 Gbps CPSC 441: Link Layer 30 10

13 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 See/interact with Java applet on AWL Web site: highly recommended! 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 512 bit transmission times after second collision: choose K from {0,1,2,3} after ten collisions, choose K from {0,1,2,3,4,,1023} CPSC 441: Link Layer 37 CSMA/CD efficiency t prop = max prop between 2 nodes in LAN t trans = time to transmit max-size frame 1 efficiency = 1+ 5t / prop t trans Efficiency goes to 1 as t prop goes to 0 Goes to 1 as t trans goes to infinity Much better than ALOHA, but still decentralized, simple, and cheap CPSC 441: Link Layer 38 10BaseT and 100BaseT 10/100 Mbps rate; latter called fast ethernet T stands for Twisted Pair Nodes connect to a : star topology ; 100 m max distance between nodes and twisted pair CPSC 441: Link Layer 39 13

14 Hubs Hubs are essentially physical-layer repeaters: bits coming from one link go out all other links at the same rate no frame buffering no CSMA/CD at : adapters detect collisions provides net management functionality twisted pair CPSC 441: Link Layer 40 Link Layer Introduction and services Error detection and correction Multiple access protocols Link-Layer Addressing Ethernet Interconnections: Hubs and switches PPP MPLS CPSC 441: Link Layer 41 Interconnecting with s Backbone interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large collision domain Can t interconnect 10BaseT & 100BaseT CPSC 441: Link Layer 42 14

15 Switch Link layer device stores and forwards Ethernet frames examines frame header and selectively forwards frame based on MAC dest address when frame is to be forwarded on segment, uses CSMA/CD to access segment transparent hosts are unaware of presence of switches plug-and-play, self-learning switches do not need to be configured CPSC 441: Link Layer 43 Forwarding switch How do determine onto which LAN segment to forward frame? Looks like a routing problem... CPSC 441: Link Layer 44 Self learning A switch has a switch table entry in switch table: (MAC Address, Interface, Time Stamp) stale entries in table dropped (TTL can be 60 min) switch learns which hosts can be reached through which interfaces when frame received, switch learns location of sender: incoming LAN segment records sender/location pair in switch table CPSC 441: Link Layer 45 15

16 Filtering/Forwarding When switch receives a frame: index switch table using MAC dest address 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 CPSC 441: Link Layer 46 Switch: traffic isolation switch installation breaks subnet into LAN segments switch filters packets: same-lan-segment frames not usually forwarded onto other LAN segments segments become separate collision domains switch collision domain collision domain collision domain CPSC 441: Link Layer 47 Switches: dedicated access Switch with many interfaces Hosts have direct connection to switch No collisions; full duplex C switch A B Switching: A-to-A and B-to-B simultaneously, no collisions B A C CPSC 441: Link Layer 48 16

17 Switching types cut-through switching: frame forwarded from input to output port without first collecting entire frame slight reduction in latency Disadvantage? Store-and-forward combinations of shared/dedicated, 10/100/1000 Mbps interfaces CPSC 441: Link Layer 49 Institutional network to external network router mail server web server switch IP subnet CPSC 441: Link Layer 50 Switches vs. Routers both store-and-forward devices routers: network layer devices (examine network layer headers) switches are link layer devices routers maintain routing tables, implement routing algorithms switches maintain switch tables, implement filtering, learning algorithms CPSC 441: Link Layer 51 17

20 Link Layer and LANS: Summary 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 Ethernet switched LANS PPP MPLS Next Stop: Wireless and Mobile Networking CPSC 441: Link Layer 58 20

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