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(chapter3)！ 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 5-6 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 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
Link Layer 5-7 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 services (more)
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 application Ethernet card,802.11 transport network cpu memory card;Ethernet chipset link implements link, host physical layer bus link (e.g.,PCI) attaches into host's physical physical system buses L transrission combination of hardware, software,firmware network adapter card Link Layer 5-8
Link Layer 5-8 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 controller physical transmission cpu memory host bus (e.g., PCI) network adapter card application transport network link link physical
Adaptors communicating datagram datagram ontrolle controller sending host receiving host datagram frame sending side: ÷receiving side ■encapsulates looks for errors,rdt, datagram in frame flow control,etc ■adds error checking extracts datagram, bits,rdt,flow control, passes to upper layer etc. at receiving side Link Layer 5-9
Link Layer 5-9 Adaptors communicating 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 controller controller sending host receiving host datagram datagram datagram frame
Link layer,LANS:outline 5.1 introduction, 5.5 link virtualization: services MPLS 5.2 error detection, 5.6 data center correction networking 5.3 multiple access 5.7 a day in the life of a protocols web request 5.4 LANs ·addressing,ARP a Ethernet ■switches VLANS Link Layer 5-10
Link Layer 5-10 Link layer, LANs: outline 5.1 introduction, services 5.2 error detection, correction 5.3 multiple access protocols 5.4 LANs addressing, ARP Ethernet switches VLANS 5.5 link virtualization: MPLS 5.6 data center networking 5.7 a day in the life of a web request