1MACMedia Access Control#7Chapter 62MACOutline Why use MAC protocols General classes of MAC protocols Deterministic Random Access Standard LAN protocols Broadband Access Cable DSL Other.. Satellite Networks3MACMedia Access Control: Protocols provide:  Direct access to the media Distributed control over resource allocation Typically broadcast (real or virtual)4MACMedia Access Control: Advantages High data rates (open new applications) Low cost Local organizational control Wireless is a broadcast media and efficient use of resources is important Enable sharing of resources Mobility via Wireless5MACMedia Access Control MAC protocols establish a set of rules that govern who gets to use the shared transmission media in an efficient manner. Obstacle to perfect channel utilization Finite propagation delay means that each users’ knowledge of the state of the system is imperfect and thus they can not perfectly schedule transmissions, i.e., some time will be wasted attempting to learn the state of the system and/or learning the fate of transmissions. Lost messages6MACMedia Access Control Perfect Knowledge would lead to FIFO performance. Performance of MAC protocols will be compared to FIFO performance.Ideal MACPerformance7MACImpact of MAC OverheadTransfer DelayLoadE[T]/E[X]ρSmax-211Smax-1MAC Protocol 1MAC Protocol 2Adapted from: Leon-Garcia & Widjaja: Communication Networks8MACAlternative Media Access Control Strategies Static Allocation FDM TDM Problems Management; not easy to add users– Requires signaling Wasteful in resources for bursty traffic Example A transmission media has a rate of 10 Mb/s and supports 50 users. The system uses static allocation. A user has a 1 Mbyte file to transmit. The file transfer time is:9MACAlternative Media Access Control Strategies Suppose you send a message to all the other 49 users saying, ‘I need the whole channel for about 1sec, do not use it, please’ As long as the overhead incurred in sending the message is less than 39 sec. the user will get better performance.10MACAlternative Media Access Control Strategies Deterministic Polling Token networks Random Access ALOHA Carrier sense multiple access (CSMA) CSMA with collision detection (CSMA/CD)11MACAlternative Media Access Control Strategies: Dynamic allocation of resources Deterministic; Polling, Token Ring &Token Bus Advantage: the maximum time between users chances to transmit is bounded. (assuming a limit on the token holding time) Disadvantage: Time is wasted polling other users if they have no data to send. The technology does not scale12MACDeterministic Protocols Roll Call PollingMaster/slave arrangementMaster polls each node; Do you have data to send?If the polled node has data it is sent otherwise next node is polled.13MACDeterministic ProtocolsMasterNodeNodeNodeNodeMaximum token holding time= Maximum time a station is allowed to transmit before passing on the permission to transmit, the token.14MACDeterministic Protocols Hub Polling No master station Each nodes polls the next node in turnNodeNodeNodeNodeNode15MACDeterministic Protocols Example: # nodes = 10 Link rate = 1 Mb/s Packet Size = 1000 bits Assume Low load no queueing 0.1 ms between nodes (30 km/(3x108m/s) = 0.1ms) Find the effective transmission rate and efficiency.– On average destination is 5 nodes away .5 ms– Time to transmit 1000 bits = 0.5 ms + 1 ms = 1.5 ms– Effective transmission rate = 1000 bits/ 1.5 ms = 666Kb/s– Efficiency = (666 Kb/s)/(1000 Kb/s) = 0.66 Repeat for link rate = 10 Mb/s– On average destination is 5 nodes away .5 ms– Time to transmit 1000 bits = 0.5 ms + .1 ms = .6 ms– Effective transmission rate = 1000 bits/ .6 ms = 1.67 Mb/s– Efficiency = (1.67 Mb/s)/(10 Mb/s) = 16.7% Conclusion Polling does not scale with link rate16MACAlternative Media Access Control Strategies: Random Access Each node sends data with limitedcoordination between users: No explicit permission to transmit Total chaos: Send data as soon as ready Limited chaos: Listen before sending data, if the channel is busy do not send. Further Limiting chaos: Listen before sending data, continue listening after sending and if collision with another transmission stop sending.[Carrier Sense Multiple Access with Collision Detection CSMA/CD]17MACAlternative Media Access Control Strategies: Random Access Advantage: Simple Disadvantage: No guarantee that you will ever get to send.The MAC protocol technology does not scale18MACRandom Access Protocols: Assumptions Overlap in time and space of two or more transmissions causes a collision and the destruction of all packets involved.[ No capture effects] One channel For analysis no station buffering19MACRandom Access Protocols: Assumptions Time-Alternatives Synchronization between users (Slotted time) No synchronization between users (unslotted time) Knowledge of the channel state-Alternatives Carrier sensing (Listen before talk-LBT) Collision detection20MACRandom Access ProtocolsStrategies ALOHA No coordination between users Send a PDU, wait for acknowledgment,if no acknowledgment ASSUME collision thenbackoff and try again Backoff Select “random” time to attempt another transmission Slotted ALOHA Same as ALOHA only time is slotted21MACRandom Access ProtocolsStrategies p-persistent CSMA Listen to channel, on transition from busy to idle transmit with probability p After sending the PDU, wait for acknowledgment,if no acknowledgment then backoff and retransmit Non-persistent, if channel busy then reschedule transmission 1-persistent, Transmit as soon as idle22MACRandom Access ProtocolsStrategies CSMA/CD1-persistent but continue to sense the channel, if collision detected then stop transmission.CSMA/CD is used in 10, 100 Mb/s, and 1 Gb/s Ethernet23MACLimitations on Random Access Protocols Distance Time to learn channel state Propagation time Speed Time to learn channel state Clocking speed24MACRandom Access ProtocolsAnalysis of ALOHA: Goal: Find Smax Protocol Operation Packet of length L (sec) arrives at station i– Station i transmits immediately– Station i starts an acknowledgment timer If no other station transmits while i is transmitting then success Else a