Multiple access links and Protocols Two types of "links 口 point-to-point o PPP for dial-up access o point-to-point link between Ethernet switch and host O broadcast (shared wire or medium) o traditional Ethernet o upstream HFc o 802.11 wireless laN Blah. blah, blah shared wire shared wireless satellite cocktail party (e. g Ethernet)(e.g. Wavelan) 5: Datalink layer 5a-16
5: DataLink Layer 5a-16 Multiple Access Links and Protocols Two types of “links”: point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) traditional Ethernet upstream HFC 802.11 wireless LAN
Multiple access protocols o single shared broadcast channel o two or more simultaneous transmissions by nodes interference o only one node can send successfully at a time multiple access protocol o distributed algorithm that determines how nodes share channel, i.e. determine when node can transmit D communication about channel sharing must use channel tself o what to look for in multiple access protocols 5: DataLink Layer 5a-17
5: DataLink Layer 5a-17 Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference only one node can send successfully at a 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! what to look for in multiple access protocols:
Ideal Mulitple Access protocol Broadcast channel of rate r bps 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 o no special node to coordinate transmissions o no synchronization of clocks, slots 4.Simple 5: Datalink layer 5a-18
5: DataLink Layer 5a-18 Ideal Mulitple Access Protocol Broadcast channel of rate R bps 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: a taxonomy Three broad classes: o Channel Partitioning o divide channel into smaller "pieces"(time slots requency, code o allocate piece to node for exclusive use g Random Access o channel not divided allow collisions o recover" from collIsions 口" Taking turns" o tightly coordinate shared access to avoid collisions 5: Datalink layer 5a-19
5: DataLink Layer 5a-19 MAC Protocols: a 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” tightly coordinate shared access to avoid collisions
Channel Partitioning Mac protocols: TDMA TDMA: time division multiple access access to channel in rounds O each station gets fixed length slot (length= pkt trans time) in each round 口 unused slots go idle D example: 6-station LAN, 1, 3, 4 have pkt, slots 2, 5, 6 idle frame 134 3 5: Datalink layer 5a-20
5: DataLink Layer 5a-20 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle