Medium Access Control (MAC) ProtocolsMAC Protocols: a taxonomyThree 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 turnsChannel Partitioning MAC protocols: TDMATDMA: 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 Channel Partitioning MAC protocols: FDMAFDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle  example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle  TDM (Time Division Multiplexing): channel divided into N time slots, one per user; inefficient with low duty cycle users and at light load. FDM (Frequency Division Multiplexing): frequency subdivided.frequency bandstimeRandom 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/CAMAC 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 ROMLAN Addresses and ARPEach adapter on LAN has unique LAN addressBroadcast address =FF-FF-FF-FF-FF-FF= adapter1A-2F-BB-76-09-AD58-23-D7-FA-20-B00C-C4-11-6F-E3-9871-65-F7-2B-08-53LAN(wired orwireless)LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to assure uniqueness) Analogy:(a) MAC address: like Social Security Number(b) IP address: like postal address 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 attachedARP: Address Resolution Protocol Each IP node (Host, Router) on LAN has ARP table ARP Table: IP/MAC address mappings for some LAN nodes< IP address; MAC address; TTL> TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)Question: how to determineMAC address of Bknowing B’s IP address?1A-2F-BB-76-09-AD58-23-D7-FA-20-B00C-C4-11-6F-E3-9871-65-F7-2B-08-53LAN237.196.7.23237.196.7.78237.196.7.14237.196.7.88ARP 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 = FF-FF-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)  soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: nodes create their ARP tables without intervention from net administratorRouting to another LANwalkthrough: send datagram from A to B via Rassume A know’s B IP address Two ARP tables in router R, one for each IP network (LAN) In routing table at source Host, find router 111.111.111.110 In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etcARB A creates datagram with source A, destination B  A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram A’s adapter sends frame  R’s adapter receives frame  R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address  R creates frame containing A-to-B IP datagram sends to BARBEthernet Frame StructureSending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet framePreamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock ratesEthernet Frame Structure (more) Addresses: 6 bytes if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol otherwise, adapter discards frame Type: indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply droppedEthernet uses CSMA/CD No slots adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense transmitting adapter aborts when it senses that another adapter is transmitting, that is, collision detection Before attempting a retransmission, adapter waits a random time, that is, random accessEthernet CSMA/CD algorithm1. Adaptor receives datagram from net layer & creates frame2. If adapter senses channel idle, it starts to transmit frame. If it senses channel busy, waits until channel idle and then transmits3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame !4. If adapter detects another transmission while transmitting, aborts and sends jam signal5. After aborting, adapter enters exponential backoff: after the mthcollision, adapter chooses a K at random from {0,1,2,…,2m-1}. Adapter waits K·512 bit times and returns to Step 2Self learning A switch has a switch table entry in switch table:  (MAC Address, Interface, Time Stamp) stale entries in table dropped (TTL can be 60 min)  switch learns which hosts can be reached through which interfaces when frame received, switch “learns” location of sender: incoming LAN segment records sender/location pair in switch tableFiltering/ForwardingWhen switch receives a frame:index switch table using MAC dest