Presentation on theme: "The Medium Access Control Sublayer"— Presentation transcript:
1The Medium Access Control Sublayer Chapter 4The Medium Access Control Sublayer
2The Channel Allocation Problem Static Channel Allocation in LANs and MANsDynamic Channel Allocation in LANs and MANs
3Dynamic Channel Allocation in LANs and MANs Station Model.N independent stations sending messages according Poisson distribution.Single Channel Assumption.A single channel is available to all stations to transmit to and receive from.Collision Assumption.If stations transmit at the same time frames will collide and garbled. Allstations can detect collision and retransmit frame later.(a) Continuous Time: Frame can start transmission at any time.(b) Slotted Time: Frame can start transmission at given time instance.(a) Carrier Sense: Stations can sense if the channel is in use and wait. (b) No Carrier Sense: Stations cant tell if the channel is free.
4Multiple Access Protocols ALOHACarrier Sense Multiple Access ProtocolsCollision-Free ProtocolsLimited-Contention ProtocolsWavelength Division Multiple Access ProtocolsWireless LAN Protocols
5In pure ALOHA, frames are transmitted at completely arbitrary times.
6Collision conditions in pure Aloha If another frame startshere we will have collisionperiod is 2tAll frames are of the same size.Frame transmission can start at any time instant.
7Throughput versus offered traffic for ALOHA systems. (throughput per packet time)Max throughputs:18% at G = 0.5 for pure37% at G = 1 for slotted.
9CSMA with Collision Detection CSMA/CD can be in one of three states: contention, transmission, oridle. Minimum contention slot is 2t where t is the propagation delaybetween the two most remote stations.
10Collision free protocols: the basic bit-map protocol .
11The binary countdown protocol . A dash indicates silence.
12Wireless LAN Protocols A wireless LAN. (a) A transmitting. (b) B transmitting.
13Wireless MACA (Multiple Access with Collision Avoidance) C hears RTS to B 30 bytes frame with the length of the frame to follow.D hears B responding with a CTS to A (copying the length of the next frame).A starts transmitting.
14Ethernet Ethernet Cabling Manchester Encoding The Ethernet MAC Sublayer ProtocolThe Binary Exponential Backoff AlgorithmEthernet PerformanceSwitched EthernetFast EthernetGigabit EthernetIEEE 802.2: Logical Link ControlRetrospective on Ethernet
15Ethernet 802.3 Base band Segment length in hundredths meters 10 MHz Vampire tapsT conn.
16Ethernet Cabling Three kinds of Ethernet cabling. (a) 10Base5, (b) 10Base2, (c) 10Base-T.
17Ethernet coding(a) Binary encoding, (b) Manchester encoding, (c) Differential Manchester encoding.
18Ethernet MAC Sublayer Protocol Address bit 46 determines local or global address.Min frame is 64 bytes from dest. address to checksum.Frame formats. (a) DIX Ethernet, (b) IEEE
19Collision detection can take as long as 2t This is to sense a collision before end of the frame reach far end.In 10 Mbps LAN 1 bit is 100 nsec, and max segment 2500 m roundtrip delay is 2t = 50 mksec = 500 bits = 64 bytes.
20Binary exponential backoff ContentionperiodContentionperiodIdleperiodFrameFrameFrameFramep = 1/2p = 1/4p = 1/8If k stations contend for a channel probability that any of k gets a channel is:A = kp(1 – p)k p = 1/k gives AmaxAverage number of contention slots = S000 jA(1 – A)j-1 = 1/A.Therefore,channel efficiency = P/(P + 2t/A)
21Efficiency of Ethernet at 10 Mbps with 512-bit slot times. Ethernet PerformanceEfficiency of Ethernet at 10 Mbps with 512-bit slot times.