Chapter 5: The data link layer Our goals: Overview: o understand principles o link layer services behind data link layer services: g error detection O error detection correction correction D multiple access protocols o sharing a broadcast channel: multiple access and laNs o link layer addressing o link layer addressing, ARP o reliable data traster O specific link layer flow control: done! g instantiation and technologies implementation of various o Ethernet link layer technologies o hibs bridges, switches 5: DataLink Layer 5a-1
5: DataLink Layer 5a-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 link layer addressing reliable data transfer, flow control: done! instantiation and implementation of various link layer technologies Overview: link layer services error detection, correction multiple access protocols and LANs link layer addressing, ARP specific link layer technologies: Ethernet hibs, bridges, switches
Link Layer: setting the context error actual PDu flow di dI dI Router r1 application app ication transport network net ork data link data link data link physical dIdI d protocol I di d physica Host H1 ph/ph/Ph 回p回 network Host H2 Router R2 Router r3 Router R4 5: DataLink Layer 5a-2
5: DataLink Layer 5a-2 Link Layer: setting the context
Link Layer: setting the context o two physically connected devices: o host-router router-router host-host unit of data: frame APplication HM‖ fransport n ht M network data link network link proToco link Hi HnH+ N hysical phy hys link physical frame adapter card 5: DataLink Layer 5a-3
5: DataLink Layer 5a-3 Link Layer: setting the context two physically connected devices: host-router, router-router, host-host unit of data: frame application transport network link physical network link physical M M M M Ht HnHt Hl HnHt Hl HnHt M frame phys. link data link protocol adapter card
Link Layer Services 口 Framing, link access: encapsulate datagram into frame, adding header, trailer o implement channel access if shared medium o physical addresses' used in frame headers to identify source. dest different from IP address D Reliable delivery between two physically connected devices 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-4
5: DataLink Layer 5a-4 Link Layer Services Framing, link access: encapsulate datagram into frame, adding header, trailer implement channel access if shared medium, ‘physical addresses’ used in frame headers to identify source, dest • different from IP address! Reliable delivery between two physically connected devices: 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 口 Flow Contro|: o pacing between sender and receivers g Error Detection o errors caused by signal attenuation, noise o receiver detects presence of errors: signals Sender for retransmISSion or drops frame g Error Correction o receiver identifies and corrects bit error(s without resorting to retransmission 5: DataLink Layer 5a-5
5: DataLink Layer 5a-5 Link Layer Services (more) Flow Control: pacing between sender and receivers 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