Advanced topics in Next- Generation wireless Networks MAC and 802.11 Qian Zhang Department of Computer science HKUST
Advanced Topics in NextGeneration Wireless Networks Qian Zhang Department of Computer Science HKUST MAC and 802.11
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) old-fashioned ethernet upstream HFC 802.11 wireless LAN humans at a shared wire(e.g shared rI shared RF cocktail party cabled Ethernet) (e.g., 802.11 WiFi) (satellite (shared air, acoustical)
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) – old-fashioned Ethernet – upstream HFC – 802.11 wireless LAN shared wire (e.g., cabled Ethernet) shared RF (e.g., 802.11 WiFi) shared RF (satellite) humans at a cocktail party (shared air, acoustical)
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
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
Ideal Multiple access protocol Broadcast channel of rater 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
Ideal Multiple 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 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
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” – nodes take turns, but nodes with more to send can take longer turns