Medium Access Control Protocols Local Area Networks and

Medium Access Control Protocols, Local Area Networks, and Wireless Local Area Networks Lecture Note 8

Medium Access Control Protocols, Local Area Networks, and Wireless Local Area Networks Part I: Medium Access Control Part II: Local Area Networks Part III: Wireless Local Area Networks

Overview Ø Ø Broadcast Networks Ø Ø All information sent to all users No routing Shared media Radio Ø Ø Ø To coordinate access to shared medium Ø Data link layer since direct transfer of frames Local Area Networks Ø High-speed, low-cost communications between co-located computers Ø Typically based on broadcast networks Ø Simple & cheap Ø Limited number of users Cellular telephony Wireless LANs Copper & Optical Ø Ø Medium Access Control Ethernet LANs Cable Modem Access Prof. Xi Zhang

Medium Access Control Protocols for Local Area Networks and Wireless Local Area Networks Part I: Medium Access Control Multiple Access Communications Random Access Scheduling Channelization Delay Performance

Medium Access Control Protocols for Local Area Networks and Wireless Local Area Networks Part II: Local Area Networks Overview of LANs Ethernet Token Ring and FDDI 802. 11 Wireless LAN Bridges

Medium Access Control Protocols and Local Area Networks Multiple Access Communications

Multiple Access Communications Ø Ø Shared media basis for broadcast networks Ø Inexpensive: radio over air; copper or coaxial cable Ø M users communicate by broadcasting into medium Key issue: How to share the medium? 3 1 2 4 Shared multiple access medium M 5 Prof. Xi Zhang

Approaches to Media Sharing Medium sharing techniques Static channelization Ø Partition medium Ø Dedicated allocation to users Ø Satellite transmission Ø Cellular Telephone Dynamic medium access control Scheduling Random access Ø Polling: take turns Ø Request for slot in transmission schedule Ø Token ring Ø Wireless LANs Ø Loose coordination Ø Send, wait, retry if necessary Ø Aloha Ø Ethernet Prof. Xi Zhang

Channelization: Satellite Channel uplink fin downlink fout Prof. Xi Zhang

Channelization: Cellular uplink f 1 ; downlink f 2 uplink f 3 ; downlink f 4 Prof. Xi Zhang

Scheduling: Polling Data from 1 from 2 Data Poll 1 Host computer Inbound line Data to M Poll 2 Outbound line 1 2 M 3 Stations Prof. Xi Zhang

Scheduling: Token-Passing Ring networks token Data to M token Station that holds token transmits into ring Prof. Xi Zhang

Random Access Multitapped Bus Crash!! Transmit when ready Transmissions can occur; need retransmission strategy Prof. Xi Zhang

Wireless LAN Ad. Hoc: station-to-station Infrastructure: stations to base station Random access & polling Prof. Xi Zhang

Selecting a Medium Access Control Ø Applications Ø Ø Ø Scale Ø Ø Ø What type of traffic? Voice streams? Steady traffic, low delay/jitter Data? Short messages? Web page downloads? Enterprise or Consumer market? Reliability, cost How much traffic can be carried? How many users can be supported? Current Examples: Ø Ø Design MAC to provide wireless DSL-equivalent access to rural communities Design MAC to provide Wireless-LAN-equivalent access to mobile users (user in car travelling at 130 km/hr) Prof. Xi Zhang

Delay-Bandwidth Product Ø Ø Delay-bandwidth product key parameter Ø Coordination in sharing medium involves using bandwidth (explicitly or implicitly) Ø Difficulty of coordination commensurate with delay -bandwidth product Simple two-station example Ø Station with frame to send listens to medium and transmits if medium found idle Ø Station monitors medium to detect collision Ø If collision occurs, station that begin transmitting earlier retransmits (propagation time is known) Prof. Xi Zhang

Two-Station MAC Example Two stations are trying to share a common medium A transmits at t = 0 Distance d meters tprop = d / seconds A B Part II: Local Area Networks Case 1 A B Case 2 A detects collision at t = 2 tprop A B B does not transmit before t = tprop & A captures channel B transmits before t = tprop and detects collision soon thereafter Prof. Xi Zhang

Efficiency of Two-Station Example Ø Each frame transmission requires 2 tprop of quiet time Ø Station B needs to be quiet tprop before and after time when Station A transmits Ø R transmission bit rate Ø L bits/frame Normalized Delay. Bandwidth Product Propagation delay Time to transmit a frame Prof. Xi Zhang

Typical MAC Efficiencies Two-Station Example: CSMA-CD (Ethernet) protocol: Token-ring network Ø If a<<1, then efficiency close to 100% Ø As a approaches 1, the efficiency becomes low (But, a can be larger than 1) a΄= latency of the ring (bits)/average frame length Prof. Xi Zhang

Typical Delay-Bandwidth Products Distance 10 Mbps 100 Mbps 1 m 3. 33 x 10 -02 3. 33 x 10 -01 1 Gbps Network Type 3. 33 x 100 Desk area network 100 m 3. 33 x 1001 3. 33 x 1002 3. 33 x 1003 Local area network 10 km 3. 33 x 1002 3. 33 x 1003 3. 33 x 1004 Metropolitan area network 1000 km 3. 33 x 1004 3. 33 x 1005 3. 33 x 1006 Wide area network 100000 3. 33 x 1006 km 3. 33 x 1007 3. 33 x 1008 Global area network Ø Max size Ethernet frame: 1500 bytes = 12000 bits Ø Long and/or fat pipes give large a Prof. Xi Zhang

MAC protocol features Ø Delay-bandwidth product Ø Efficiency Ø Transfer delay Ø Fairness Ø Reliability Ø Capability to carry different types of traffic Ø Quality of service Ø Cost Prof. Xi Zhang