Link layer:services framing,link access: encapsulate datagram into frame,adding header,trailer channel access if shared medium "MAC"addresses in frame headers identify source,destination(different from IP address!) reliable delivery between adjacent nodes we already know how to do this! seldom used on low bit-error links wireless links:high error rates Q:why both link-level and end-end reliability? LinkLayer:6-6
Link layer: services ▪ framing, link access: • encapsulate datagram into frame, adding header, trailer • channel access if shared medium • “MAC” addresses in frame headers identify source, destination (different from IP address!) ▪ reliable delivery between adjacent nodes • we already know how to do this! • seldom used on low bit-error links • wireless links: high error rates • Q: why both link-level and end-end reliability? … … Link Layer: 6-6
Link layer:services(more) ■flow control:: pacing between adjacent sending and receiving nodes error detection: errors caused by signal attenuation,noise. receiver detects errors,signals retransmission,or drops frame error correction: receiver identifies and corrects bit error(s) without retransmission half-duplex and full-duplex: with half duplex,nodes at both ends of link can transmit,but not at same time LinkLayer:6-7
Link layer: services (more) ▪ flow control: • pacing between adjacent sending and receiving nodes ▪ error detection: • errors caused by signal attenuation, noise. • receiver detects errors, signals retransmission, or drops frame ▪ error correction: • receiver identifies and corrects bit error(s) without retransmission ▪ half-duplex and full-duplex: • with half duplex, nodes at both ends of link can transmit, but not at same time … … Link Layer: 6-7
Where is the link layer implemented? in each-and-every host -link layer implemented in network interface card (NIC)or on a chip application transport Ethernet,WiFi card or chip network cpu emory link implements link,physical layer host bus (e.g.,PCI) attaches into host's system link physical buses combination of hardware, network interface software,firmware LinkLayer:6-8
Where is the link layer implemented? ▪ in each-and-every host ▪ link layer implemented in network interface card (NIC) or on a chip • Ethernet, WiFi card or chip • implements link, physical layer ▪ attaches into host’s system buses ▪ combination of hardware, software, firmware controller physical cpu memory host bus (e.g., PCI) network interface application transport network link link physical Link Layer: 6-8
Interfaces communicating application application transport transport datagram hetwork cpu memory network link link linkhdatagram link datagram link physical physical physica sending side: receiving side: encapsulates datagram in frame looks for errors,reliable data adds error checking bits,reliable data transfer,flow control,etc. transfer,flow control,etc. extracts datagram,passes to upper layer at receiving side LinkLayer:6-9
controller physical memory CPU Interfaces communicating controller physical cpu memory application transport network link link physical application transport network link link physical sending side: ▪ encapsulates datagram in frame ▪ adds error checking bits, reliable data transfer, flow control, etc. receiving side: ▪ looks for errors, reliable data transfer, flow control, etc. ▪ extracts datagram, passes to upper layer at receiving side linkh link datagram datagram datagram Link Layer: 6-9
Link layer,LANs:roadmap ■introduction -error detection,correction multiple access protocols ■LANS ·addressing,ARP ·Ethernet 。switches ·VLANs a day in the life of a web link virtualization:MPLS request data center networking Link Layer:6-10
Link layer, LANs: roadmap ▪ a day in the life of a web request ▪ introduction ▪ error detection, correction ▪ multiple access protocols ▪ LANs • addressing, ARP • Ethernet • switches • VLANs ▪ link virtualization: MPLS ▪ data center networking Link Layer: 6-10