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EECS 325/425, Fall 2005 November 11 Quiz #2 Discussions Introduction to The Link Layer Error detection and correction 1 Chapter 5: The Data Link Layer Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access protocols link layer addressing reliable data transfer, flow control: also supported by link layer. instantiation and implementation of various link layer technologies 2 Chapter 5 outline 5.1 Introduction and 5.6 Hubs and switches services 5.7 PPP 5.2 Error detection 5.8 Link Virtualization and correction 5.9 Summary 5.3Multiple access protocols Also wireless links 5.4 LAN addresses and ARP 5.5 Ethernet 3 Link Layer: Introduction “link” Some terminology: hosts and routers are nodes (bridges and switches too) communication channels that connect adjacent nodes along communication path are links wired links wireless links LANs 2-PDU is a frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link 4 Link layer: context transportation analogy Datagram transferred by trip from Princeton to Lausanne different link protocols over limo: Princeton to JFK different links: plane: JFK to Geneva e.g., Ethernet on first link, train: Geneva to Lausanne frame relay on intermediate tourist = datagram links, 802.11 on last link transport segment = Each link protocol provides communication link different services transportation mode = link layer e.g., may or may not provide protocol reliable data transfer over travel agent = routing algorithm link 5 Link Layer Services Framing encapsulate datagram into frame, adding header, trailer Link access: channel access if shared medium „physical addresses‟ used in frame headers to identify source, dest • different from IP address! Reliable delivery we learned how to do this already (chapter 3, between two end-hosts)! seldom used on low bit error link (fiber, some twisted pair) wireless links: high error rates • Q: why both link-level and end-end reliability? 6 Link Layer Services (more) Flow Control: pacing between adjacent sending and receiving nodes. Similar to transport-layer flow control? when do we need it? Error Detection: errors caused by signal attenuation, noise. receiver detects presence of errors: • signals sender for retransmission or drops frame Error Correction: receiver identifies and corrects bit error(s) without resorting to retransmission Half-duplex and full-duplex with half duplex, both ends of link can transmit, but not at same time 7 Adaptors Communicating datagram link layer protocol receiving sending node node frame frame bus physical link adapter adapter link layer implemented in receiving side “adaptor” (a.k.a. NIC) looks for errors, reliable Ethernet card, PCMCI transfer, flow control, etc card, 802.11 card extracts datagram, passes to receiving node sending side: encapsulates datagram in adapter is semi- a frame autonomous adds error checking bits, link & physical layers reliable data transfer, flow control, etc. 8 Chapter 5 outline 5.1 Introduction and 5.6 Hubs, bridges, and services switches 5.2 Error detection 5.7 Wireless links and and correction LANs 5.3Multiple access 5.8 PPP protocols 5.9 ATM 5.4 LAN addresses 5.10 Frame Relay and ARP 5.5 Ethernet 9 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 10 Parity Checking Single Bit Parity: Two Dimensional Bit Parity: Detect single bit errors Detect and correct single bit errors 0 0 FEC (Forward Error Correction) commonly used multimedia transmission. Advantages? 11 Recall: Internet checksum Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only) Receiver: Sender: compute checksum of received treat segment contents segment as sequence of 16-bit check if computed checksum integers equals checksum field value: checksum: addition (1‟s NO - error detected complement sum) of YES - no error detected. But segment contents maybe errors nonetheless? sender puts checksum value into UDP checksum field 12 Checksumming: Cyclic Redundancy Check sender/receiver choose a well-known r+1 bit pattern (generator), G. It makes sense to have a highest bit “1”. view data bits, D, as a binary number goal: choose r CRC bits, R, such that <D,R> exactly divisible by G (modulo 2) receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! can detect all burst errors less than r+1 bits widely used in practice 13 CRC Example Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R equivalently: if we divide D.2r by G, want remainder R D.2r R = remainder[ ] G 14 Food for thought: CRC claims Last paragraph in the section “Each of the CRC standards can detect burst errors of fewer than r+1 bits” “Under appropriate assumption, a burst of length greater than r+1 bits is detected with probability 1-0.5r ” “Each of the CRC standards can detect any odd number of errors.” 15 Next week Multiple access protocols Ethernet 16
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