Link Layer Services 口 Framing, link access: encapsulate datagram into frame adding header trailer channel access if shared medium o physical addresses' used in frame headers to identify source. dest different from IP address D Reliable delivery between adjacent nodes o we learned how to do this already (chapter 3) o seldom used on low bit error link(fiber some twisted pair) o wireless links: high error rates Q: why both link-level and end-end reliability? 5: DataLink Layer 5a-6
5: DataLink Layer 5a-6 Link Layer Services Framing, link access: encapsulate datagram into frame, adding header, trailer channel access if shared medium ‘physical 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 Services(more g Flow Control: o pacing between ad jacent sending and receiving nodes 口 Error Detection: o errors caused by signal attenuation, noise o receiver detects presence of errors: signals sender for retransmission or drops frame 口 Error Correction o receiver identifies and corrects bit error(s without resorting to retransmission o Half-duplex and full-duplex o with half duplex, nodes at both ends of link can transmit but not at same time 5: DataLink Layer 5a-7
5: DataLink Layer 5a-7 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
Adaptors Communicating datagram roving sending link layer protocol node node frame frame adapter adapter o link layer implemented in o receiving side adaptor"(aka NIC) looks for errors, rdt flow o Ethernet card, PCmcI control etc card. 802.11 card o extracts datagram, passes 口 sending side to roving node o encapsulates datagram in D adapter is sem a frame autonomous o adds error checking bits, o link& physical layers rdt, flow control, etc 5: DataLink Layer 5a-8
5: DataLink Layer 5a-8 Adaptors Communicating link layer implemented in “adaptor” (aka NIC) Ethernet card, PCMCI card, 802.11 card sending side: encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc. receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to rcving node adapter is semiautonomous link & physical layers sending node frame rcving node datagram frame adapter adapter link layer protocol
Chapter 5 outline g 5.1 Introduction and 口56Hubs, bridges,and services switches 0 5.2 Error detection g 5.7 Wireless links and and correction LANS 0 5. 3 Multiple access 05. 8 PPP protocols 059ATM 0 5.4 LAN addresses o 5.10 Frame Relay and Arp 0 5.5 Ethernet 5: DataLink Layer 5a-9
5: DataLink Layer 5a-9 Chapter 5 outline 5.1 Introduction and services 5.2 Error detection and correction 5.3Multiple access protocols 5.4 LAN addresses and ARP 5.5 Ethernet 5.6 Hubs, bridges, and switches 5.7 Wireless links and LANs 5.8 PPP 5.9 ATM 5.10 Frame Relay
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 datagram datagram a bits in D N ○K ? error ← d data bits D EDC D EDC o bit-error prone link 0 5: DataLink Lo 5a-10
5: DataLink Layer 5a-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