LAN & MAC (Medium Access Control) protocolsPeer-to-peer protocols VS. MACWhat are going to be discussedSlide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10MAC and performanceSlide 12Slide 13Examples of efficiencySlide 15Slide 16Slide 17Slide 18Slide 19Slide 20Random AccessALOHASlide 23Slide 24Slide 25Slide 26Slide 27Slide 28CSMA (Carrier sensing multiple access)Slide 30Three different CSMA schemesSlide 32Slide 33Slide 34Slide 35Slide 36CSMA-CDSlide 38Slide 39Slide 401LAN & MAC (Medium Access Control) protocols•Two basic types of networks:–Switched networks: transmission lines, multiplexers, and switches; routing, hierarchical address for scalability.–Broadcast networks: a single shared medium, simpler, no routing, messages received by all stations, flat address; however, when users try to transmit messages into the medium, potential conflict, so MAC is needed to orchestrate the transmission from various users.–LAN is a typical broadcast network.2Peer-to-peer protocols VS. MAC•Both are to transfer user information despite transmission impairments•For peer-to-peer: –Main concern: loss, delay, resequencing–Using control frames to coordinate their actions–Delay-bandwidth is important–Involved only two peer processes•MAC: –Main concern: interference from users–Using some mechanisms to coordinate the access of channel–Delay-bandwidth is important–Need the coordination from all MAC entities, any one does not cooperate, the communication will not take place.3What are going to be discussed•Introduction to broadcast networks•Overview of LANs: frame format & placement in OSI.•Random access: ALOHA & CSMA-CD (Carrier Sensing Multiple Access with Collision Detection ) i.e., Ethernet.•Scheduling: token-ring.•LAN standards (brief view)•LAN bridges: used to connect several LANs.412345MShared MultipleAccess Medium1. Any transmission from any station can be heard by any other stations2. If two or more stations transmit at the same time, collision occurs Figure 6.1Multiple access communications5Satellite Channel= fin= foutFigure 6.3Satellite communication involves sharing of uplink and downlink frequency bands6 AC = authentication centerBSS = base station subsystemEIR = equipment identity registerHLR = home location registerwirelineterminalMSCPSTNBSSBSSSTPSS#7HLRVLREIRACMSC = mobile switching center PSTN = public switched telephone networkSTP = signal transfer pointVLR = visitor location registerFigure 4.52Cellular networks: radio shared by mobile users and require MAC7Multidrop telephone linesInbound lineOutbound lineFigure 6.4Multi-drop telephone line requires access controlHost Terminals8 Ring networks Multitapped BusFigure 6.5Ring networks and multi-tapped buses require MAC9Figure 6.6Wireless LAN: share wireless medium and require MAC10Medium Sharing TechniquesStatic ChannelizationDynamic Medium Access ControlSchedulingRandom AccessFigure 6.2Approaches to sharing transmission mediumPartitioned channelsare dedicated to individual users, sono collision at all.Good for steady trafficand achieve efficient usage of channelsMinimize the incidence of collision to achieve reasonableusage of medium.Good for bursty traffic.Schedule a orderly accessof medium. Good for heaviertraffic.Try and error. if no collision,that is good, otherwise wait a random time, try again. Good for light traffic.11MAC and performance•Shared medium is the only means for stations to communicate•Some kind of MAC technique is needed•Like ARQs, which use ACK frame to coordinate the transmission and consume certain bandwidth, the MAC will need to transfer some coordination information which will consume certain bandwidth of shared medium.•Delay-bandwidth product plays a key role in the performance of MAC (as in ARQs).12A transmits at t = 0Distance d meterstprop = d /  secondsA BB transmits before t = tprop and detectscollision shortlythereafterA BABA detectscollision at t = 2 tpropFigure 6.7Delay-bandwidth product and performance 1.  : the speed of light, 3*108 meters/second 2. Before A begins to transmit, A listens to medium, if busy, wait; otherwise, do it (suppose t=0) 3. If B wants to transmit after t=tprop A’s transmission has reached B, so B waits and A captures mediumsuccessfully and transmits its entire message. (suppose two station A and B want to transmit information)4. If B wants to transmit before t=tprop, it listens and no transmission is going on, so B begins to transmit, then collision occurs. B detects collision shortly, but A detects collision at t=2tprop5. Therefore, 2tprop is required to coordinate the access for each packet transmitted.13MAC efficiencyAnd suppose average length of packets is L.Then efficiency in use of the medium is:Efficiency = L + 2tpropRL=11 + 2tpropRL=11+2aa=tpropR / L i.e., the ratio of (one-way) delay-bandwidth product to the average packet length.Suppose a = 0.01, then efficiency = 1/1.02 = 0.98 a = 0.1, then efficiency = 1 / 2 = 0.50•Suppose bit rate of medium is R, then number of bits “wasted” in access coordination is 2tpropR.14Examples of efficiency•Ethernet (CSMA-CD): –Efficiency = 1/(1+6.44a) where a = tpropR/L.•Token-ring networks:–Efficiency = 1/(1+a’ ) where a’ = ring-latency in bits/L where ring-latency contains:•The sum of bit delays introduced at each ring adapter.•Delay-bandwidth product where delay is the time required for a bit to circulate around the ring.15(a)RAMRAMROMEthernet Processor(b)Figure 6.10Typical LAN structure and network interface card1. NIC is parallel with memory but serial with network2. ROM stores the implementation of MAC3. Unique physical address burn into ROM4. A hardware in NIC recognizes physical, broadcast & multicast addresses.5. NIC can be Set to “promiscuous” mode to catch all transmissions. A LAN connects servers, workstations,Printers, etc., together to achieve sharing16Data LinkLayer802.3CSMA-CD802.5Token Ring802.2 Logical Link ControlPhysicalLayerMACLLC802.11Wireless LANNetwork Layer Network LayerPhysicalLayerOSIIEEE 802Various Physical LayersOtherLANsFigure 6.11IEEE 802 LAN standardsOne LLC and several MACs, each MAC has an associated set of physical layers.MAC provides connectionless transfer. Generally no error control because of relatively error free.MAC protocol is to direct when they should transmit frames into shared medium.17PHYMACPHYMACPHYMACUnreliable Datagram ServiceFigure 6.12The