12 Data link layer Network Layer 4 1 Multiple Access Links and Protocols Two types of links point to point PPP for dial up access broadcast shared wire or medium traditional Ethernet upstream HFC 802 11 wireless LAN 5 DataLink Layer 5 2 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 3 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 4 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 5 DataLink Layer 5 5 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 5 DataLink Layer 5 6 Channel Partitioning MAC protocols FDMA FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go frequency bands idle example 6 station LAN 1 3 4 have pkt time frequency bands 2 5 6 idle 5 DataLink Layer 5 7 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 8 CSMA Carrier Sense Multiple Access CSMA listen before transmit If channel sensed idle transmit entire frame If channel sensed busy defer transmission Human analogy don t interrupt others 5 DataLink Layer 5 9 CSMA collisions spatial layout of nodes collisions can still occur propagation delay means two nodes may not hear each other s transmission collision entire packet transmission time wasted note role of distance propagation delay in determining collision probability 5 DataLink Layer 5 10 CSMA CD Collision Detection CSMA CD carrier sensing deferral as in CSMA collisions detected within short time colliding transmissions aborted reducing channel wastage collision detection easy in wired LANs measure signal strengths compare transmitted received signals difficult in wireless LANs receiver shut off while transmitting human analogy the polite conversationalist 5 DataLink Layer 5 11 CSMA CD collision detection 5 DataLink Layer 5 12 Taking Turns MAC protocols channel partitioning MAC protocols share channel efficiently and fairly at high load inefficient at low load delay in channel access 1 N bandwidth allocated even if only 1 active node Random access MAC protocols efficient at low load single node can fully utilize channel high load collision overhead taking turns protocols look for best of both worlds 5 DataLink Layer 5 13 Taking Turns MAC protocols Polling master node invites slave nodes to transmit in turn concerns polling overhead latency single point of failure master Token passing control token passed from one node to next sequentially token message concerns token overhead latency single point of failure token 5 DataLink Layer 5 14 Summary of MAC protocols What do you do with a shared media Channel Partitioning by time frequency or code Time Division Frequency Division Random partitioning dynamic ALOHA S ALOHA CSMA CSMA CD carrier sensing easy in some technologies wire hard in others wireless CSMA CD used in Ethernet CSMA CA used in 802 11 Taking Turns polling from a central site token passing 5 DataLink Layer 5 15 LAN technologies Data link layer so far services error detection correction multiple access Next LAN technologies addressing Ethernet hubs switches PPP 5 DataLink Layer 5 16 MAC Addresses and ARP 32 bit IP address network layer address used to get datagram to destination IP subnet MAC or LAN or physical or Ethernet address used to get datagram from one interface to another physically connected interface same network 48 bit MAC address for most LANs burned in the adapter ROM 5 DataLink Layer 5 17 LAN Addresses and ARP Each adapter on LAN has unique LAN address 1A 2F BB 76 09 AD Broadcast address FF FF FF FF FF FF LAN wired or wireless 71 65 F7 2B 08 53 adapter 58 23 D7 FA 20 B0 0C C4 11 6F E3 98 5 DataLink Layer 5 18 LAN Address more MAC address allocation administered by IEEE manufacturer buys portion of MAC address space to assure uniqueness MAC flat address portability can move LAN card from one LAN to another IP hierarchical address NOT portable depends on IP subnet to which node is attached 5 DataLink Layer 5 19 ARP Address Resolution Protocol Question how to determine MAC address of B knowing B s IP address 237 196 7 78 1A 2F BB 76 09 AD 237 196 7 23 Each IP node Host Router on LAN has ARP table ARP Table IP MAC address mappings for some LAN nodes 237 196 7 14 LAN 71 65 F7 2B 08 53 58 23 D7 FA 20 B0 0C C4 11 6F E3 98 237 196 7 88 IP address MAC address TTL TTL Time To Live time after which address mapping will be forgotten typically 20 min 5 DataLink Layer 5 20 ARP protocol Same LAN network A wants to send datagram to B and B s MAC address not in A s ARP table A broadcasts ARP query packet containing B s IP address Dest MAC address FFFF FF FF FF FF all machines on LAN receive ARP query B receives ARP packet replies to A with its B s MAC address frame sent to A s MAC address unicast A caches saves IP to MAC address pair in its ARP table until information becomes old times out ARP is plug and play nodes create their ARP tables without intervention from net administrator 5 DataLink Layer 5 21 Routing to another LAN walkthrough send datagram from A to B via R assume A know s B IP address