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:6-16
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: 6-16
Multiple access links,protocols two types of“'links": point-to-point point-to-point link between Ethernet switch,host PPP for dial-up access broadcast(shared wire or medium) old-fashioned Ethernet upstream HFC in cable-based access network .802.11 wireless LAN,4G/4G.satellite shared wire (e.g.. cabled Ethernet) shared radio:4G/5G shared radio:WiFi shared radio:satellite humans at a cocktail party (shared air,acoustical) LinkLayer:6-17
Multiple access links, protocols Link Layer: 6-17 two types of “links”: ▪ point-to-point • point-to-point link between Ethernet switch, host • PPP for dial-up access ▪ broadcast (shared wire or medium) • old-fashioned Ethernet • upstream HFC in cable-based access network • 802.11 wireless LAN, 4G/4G. satellite shared wire (e.g., cabled Ethernet) shared radio: WiFi shared radio: satellite humans at a cocktail party (shared air, acoustical) shared radio: 4G/5G
Multiple access protocols single shared broadcast channel two or more simultaneous transmissions by nodes:interference collision if node receives two or more signals at the same time multiple access protocol distributed algorithm that determines how nodes share channel, i.e.,determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination Link Layer:6-18
Multiple access protocols Link Layer: 6-18 ▪ single shared broadcast channel ▪ two or more simultaneous transmissions by nodes: interference • collision if node receives two or more signals at the same time ▪ distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit ▪ communication about channel sharing must use channel itself! • no out-of-band channel for coordination multiple access protocol
An ideal multiple access protocol given:multiple access channel (MAC)of rate R bps desiderata: 1.when one node wants to transmit,it can send at rate R. 2.when M nodes want to transmit,each can send at average rate R/M 3.fully decentralized: no special node to coordinate transmissions no synchronization of clocks,slots 4.simple Link Layer:6-19
An ideal multiple access protocol Link Layer: 6-19 given: multiple access channel (MAC) of rate R bps desiderata: 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: • no special node to coordinate transmissions • no synchronization of clocks, slots 4. simple
MAC protocols:taxonomy three broad classes: -channel partitioning ·divide channel into smaller“pieces”(time slots,,frequency,.code) allocate piece to node for exclusive use ■random access channel not divided,allow collisions ·“recover"from collisions ■“taking turns" nodes take turns,but nodes with more to send can take longer turns Link Layer:6-20
MAC protocols: taxonomy Link Layer: 6-20 three broad classes: ▪ channel partitioning • divide channel into smaller “pieces” (time slots, frequency, code) • allocate piece to node for exclusive use ▪ random access • channel not divided, allow collisions • “recover” from collisions ▪ “taking turns” • nodes take turns, but nodes with more to send can take longer turns