Multiple access links and protocols Two types of " links ● Point-to-point PPP for dial-up access e Point-to-point link between Ethernet switch and host o Broadcast (shared wire or medium) e Old-fashioned ethernet ● Upstream hfo e 802.11 wireleSs lan x、&√A blah 旦d shared wire shared wireless satellite cocktail party (e.g. Ethernet)(e.g. Wavelan 5: Datat
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 5: DataLink Layer 5-11
Multiple Access protocols o Single shared broadcast channel o two or more simultaneous transmissions by nodes interference Collision if node receives two or more signals at the same time Multiple access protocol o Distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit o Communication about channel sharing must use channel itself e no out-of-band channel for coordination 5: Dat
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 5: DataLink Layer 5-12
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 o No synchronization of clocks, slots 4. Simple 5: Dat
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 5: DataLink Layer 5-13
MAC Protocols: a Taxonomy Three broad classes Channel Partitioning TDMA, FDMA, CDMA(in wireless) e Divide channel into smaller"pieces"(time slots, frequency, code) e Allocate piece to node for exclusive use o Random Access ALoHA, CSMA, CSMA/CD, CSMA/CA e Channel not divided, allow collisions e Recover"from collisions ●“ Taking turns" Polling, Token passing o Nodes take turns, but nodes with more to send can take longer turns
MAC Protocols: a Taxonomy Three broad classes: Channel Partitioning TDMA, FDMA, CDMA (in wireless) Divide channel into smaller “pieces” (time slots, frequency, code) Allocate piece to node for exclusive use Random Access ALOHA, CSMA, CSMA/CD, CSMA/CA Channel not divided, allow collisions “Recover” from collisions “Taking turns” Polling, Token passing Nodes take turns, but nodes with more to send can take longer turns 5: DataLink Layer 5-14
Random Access protocols o When node has packet to send e Transmitat full channel data rate R o No a priori coordination among nodes o Two or more transmitting nodes "collision o Random access MAC protocol specifies e How to detect collisions e How to recover from collisions(e.g, via delayed retransmissions) o Examples of random access mac protocols e Slotted aloha e ALOHA e CSMA, CSMA/CD, CSMA/CA 5: Dat
Random Access Protocols When node has packet to send Transmit at full channel data rate R No a priori coordination among nodes Two or more transmitting nodes ➜ “collision” Random access MAC protocol specifies: How to detect collisions How to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: Slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA 5: DataLink Layer 5-15