Chapter 5 5A Link Layer A note on the use of these ppt slides We re making these slides freely available to all faculty students readers They re in PowerPoint form so you see the animations and can add modify and delete slides including this one and slide content to suit your needs They obviously represent a lot of work on our part In return for use we only ask the following If you use these slides e g in a class that you mention their source after all we d like people to use our book If you post any slides on a www site that you note that they are adapted from or perhaps identical to our slides and note our copyright of this material Thanks and enjoy JFK KWR Computer Networking A Top Down Approach 6th edition Jim Kurose Keith Ross Addison Wesley March 2012 All material copyright 1996 2012 J F Kurose and K W Ross All Rights Reserved Link Layer 5 1 Chapter 5 Link layer our goals understand principles behind link layer services error detection correction sharing a broadcast channel multiple access link layer addressing local area networks Ethernet VLANs instantiation implementation of various link layer technologies Link Layer 5 2 Link layer LANs outline 5 1 introduction services 5 2 error detection correction 5 3 multiple access protocols 5 4 LANs 5 5 link virtualization MPLS 5 6 data center networking 5 7 a day in the life of a web request addressing ARP Ethernet switches VLANS Link Layer 5 3 Link layer introduction terminology hosts and routers nodes communication channels that connect adjacent nodes along communication path links wired links wireless links LANs layer 2 packet frame encapsulates datagram global ISP data link layer has responsibility of transferring datagram from one node to physically adjacent node over a link Link Layer 5 4 Link layer context datagram transferred by different link protocols over different links e g Ethernet on first link frame relay on intermediate links 802 11 on last link each link protocol provides different services e g may or may not provide rdt over link transportation analogy trip from Princeton to Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne tourist datagram transport segment communication link transportation mode link layer protocol travel agent routing algorithm Link Layer 5 5 Link layer services framing link access encapsulate datagram into frame adding header trailer channel access if shared medium MAC addresses used in frame headers to identify source dest different from IP address reliable delivery between adjacent nodes we learned how to do this already chapter 3 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 Link Layer 5 6 Link layer services more flow control pacing between adjacent sending and receiving nodes 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 nodes at both ends of link can transmit but not at same time Link Layer 5 7 Where is the link layer implemented in each and every host link layer implemented in adaptor aka network interface card NIC or on a chip Ethernet card 802 11 card Ethernet chipset implements link physical layer attaches into host s system buses combination of hardware software firmware application transport network link cpu memory controller link physical host bus e g PCI physical transmission network adapter card Link Layer 5 8 Adaptors communicating datagram datagram controller controller receiving host sending host datagram frame sending side encapsulates datagram in frame adds error checking bits rdt flow control etc receiving side looks for errors rdt flow control etc extracts datagram passes to upper layer at receiving side Link Layer 5 9 Link layer LANs outline 5 1 introduction services 5 2 error detection correction 5 3 multiple access protocols 5 4 LANs 5 5 link virtualization MPLS 5 6 data center networking 5 7 a day in the life of a web request addressing ARP Ethernet switches VLANS Link Layer 5 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 Link Layer 5 11 Parity checking single bit parity detect single bit errors two dimensional bit parity detect and correct single bit errors 0 0 Link Layer 5 12 Internet checksum review goal detect errors e g flipped bits in transmitted packet note used at transport layer only sender receiver treat segment compute checksum of contents as sequence received segment of 16 bit integers check if computed checksum addition 1 checksum equals s complement sum checksum field value of segment contents NO error detected sender puts YES no error checksum value into detected But maybe UDP checksum field errors nonetheless Link Layer 5 13 Cyclic redundancy check more powerful error detection coding view data bits D as a binary number choose r 1 bit pattern generator G 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 Ethernet 802 11 WiFi ATM Link Layer 5 14 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 to satisfy R remainder D 2r G G D r 3 101000 1001 101110000 1001 101 000 1010 1001 010 000 100 000 R 1000 0000 1000 Link Layer 5 15 Link layer LANs outline 5 1 introduction services 5 2 error detection correction 5 3 multiple access protocols 5 4 LANs 5 5 link virtualization MPLS 5 6 data center networking 5 7 a day in the life of a web request addressing ARP Ethernet switches VLANS Link Layer 5 16 Multiple access links protocols two types of links point to point PPP for dial up access point to point link between Ethernet switch host broadcast shared wire or medium old fashioned Ethernet upstream HFC 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 Link Layer 5 17 Multiple access protocols single shared broadcast channel two or more simultaneous transmissions by nodes
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