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DataLink Layer2-1 2010 session 1 TELE3118: Network Technologies Week 2: Data Link Layer Framing, Error Control, Multiple Access Some slides have been taken.

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Presentation on theme: "DataLink Layer2-1 2010 session 1 TELE3118: Network Technologies Week 2: Data Link Layer Framing, Error Control, Multiple Access Some slides have been taken."— Presentation transcript:

1 DataLink Layer2-1 2010 session 1 TELE3118: Network Technologies Week 2: Data Link Layer Framing, Error Control, Multiple Access Some slides have been taken from: r Computer Networking: A Top Down Approach Featuring the Internet, 3 rd edition. Jim Kurose, Keith Ross. Addison-Wesley, July 2004. All material copyright 1996- 2004. J.F Kurose and K.W. Ross, All Rights Reserved. r Computer Networks, 4 th edition. Andrew S. Tanenbaum. Prentice-Hall, 2003.

2 DataLink Layer2-2 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 “link” data-link layer has responsibility of transferring datagram from one node to adjacent node over a link

3 DataLink Layer2-3 Link Layer Functions r Framing r Error control r Addressing and Multiple-access control r Flow control r Reliable transfer

4 DataLink Layer2-4 Framing A character stream. (a) Without errors. (b) With one error.

5 DataLink Layer2-5 Framing (2) (a) A frame delimited by flag bytes. (b) Four examples of byte sequences before and after stuffing.

6 DataLink Layer2-6 Framing (3) Bit stuffing (a) The original data. (b) The data as they appear on the line. (c) The data as they are stored in receiver’s memory after destuffing.

7 DataLink Layer2-7 Error Detection and Correction 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

8 DataLink Layer2-8 Theory: Hamming Distances r Hamming distance of two codewords: m The number of bit positions in which they differ m Example: 10001001 and 10110001 have Hamming distance 3 r Hamming distance of a code: m Minimum distance between any two valid codewords m E.g. the code: 0000000000, 0000011111, 1111100000, 1111111111 has Hamming distance 5 r message size n = data bits m + redundant bits r m Of 2 n codewords, only 2 m are valid r How many redundant bits needed for 1-bit error correction? m Should satisfy: (n+1)2 m ≤2 n : why?

9 DataLink Layer2-9 Error Detection: Parity Checking Single Bit Parity: Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors 0 0

10 DataLink Layer2-10 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 (ATM, HDCL)

11 DataLink Layer2-11 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

12 DataLink Layer2-12 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 traditional Ethernet m upstream HFC m 802.11 wireless LAN

13 DataLink Layer2-13 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

14 DataLink Layer2-14 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

15 DataLink Layer2-15 MAC Protocols: a taxonomy Three broad classes: r Channel Partitioning m divide channel into “pieces” (TDM, FDM, CDM, WDM) 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

16 DataLink Layer2-16 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

17 DataLink Layer2-17 Slotted ALOHA Assumptions r all frames same size r time is divided into equal size slots, time to transmit 1 frame r nodes start to transmit frames only at beginning of slots r nodes are synchronized r if 2 or more nodes transmit in slot, all nodes detect collision Operation r when node obtains fresh frame, it transmits in next slot r no collision, node can send new frame in next slot r if collision, node retransmits frame in each subsequent slot with prob. p until success

18 DataLink Layer2-18 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

19 DataLink Layer2-19 Slotted Aloha efficiency r Suppose N nodes with many frames to send, each transmits in slot with probability p r prob that node 1 has success in a slot = p(1-p) N-1 r prob that any node has a success = Np(1-p) N-1 r For max efficiency with N nodes, 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 1/e =.37 Efficiency is the long-run fraction of successful slots when there are many nodes, each with many frames to send At best: channel used for useful transmissions 37% of time!

20 DataLink Layer2-20 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]

21 DataLink Layer2-21 Pure Aloha efficiency 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. (1-p) 2(N-1) … choosing optimum p and then letting n -> infty... = 1/(2e) =.18 Even worse !

22 DataLink Layer2-22 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!

23 DataLink Layer2-23 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

24 DataLink Layer2-24 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: receiver shut off while transmitting r human analogy: the polite conversationalist

25 DataLink Layer2-25 CSMA/CD collision detection

26 DataLink Layer2-26 “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!

27 DataLink Layer2-27 “Taking Turns” MAC protocols Polling: r master node “invites” slave nodes to transmit in turn r concerns: m polling overhead m latency m single point of failure (master) 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)

28 DataLink Layer2-28 Summary of MAC protocols r What do you do with a shared media? m Channel Partitioning, by time, frequency or code Time 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 CSMA/CA used in 802.11 m Taking Turns polling from a central site, token passing

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