CMPE 150 Winter 2009 Lecture 8 Januaryy 29 2009 PE P E Mantey CMPE 150 Introduction to Computer Networks Instructor Patrick Mantey mantey soe ucsc edu htt http www soe ucsc edu mantey d t Office Engr 2 Room 595J Office hours Tues 3 5 PM Mon 5 6 PM TA Anselm Kia akia soe ucsc edu Web site http www soe ucsc edu classes cmpe150 Winter09 Text Tannenbaum Computer p Networks 4th edition available in bookstore etc Syllabus Assignment 4 Available on the web site http www soe ucsc edu classes cmpe150 Winter09 Due Tuesday Tuesday February 3 3 2009 Today s Agenda LAN topologies Wireless 802 11 802 16 802 16 Bluetooth Wi l Wireless USB Repeaters Hubs Switches Routers VLANS http www javvin com protocolDQDB html LAN Topologies Star Ring Tree C t l node Central d Bus Bus Topology Use of multipoint medium medium Stations attach to bus through tap Full duplex Full duplex communication allows data to be sent to received from bus Transmission from any station propagates in both directions and is received by all At each end end terminator absorbs and removes signal from bus Tree Topology Tree is generalization of bus Headend start of 1 or more cables branches Transmission from one station propagates to all others Issues Inherently broadcast Frames to transmit data Need for specifying the destination Addresses Multi access Need for controlling access to medium Avoid collisions MAC protocol Ring Topology Stations attach to repeaters Repeaters are linked to each other by point to point links forming a closed loop Links are unidirectional Repeaters receive data from one link and repeat it on the other with no buffering Ring Stations transmit receive via repeater p Frames circulate past all stations destination copies frame as it goes by source removes frame Ring shared by multiple stations Need MAC protocol p Determine when each station may insert frame Star Topology Each station directly connected to central node via point to point link Central node node ss modes of operation Broadcast mode node broadcasts received frame on all other links logically works like bus Switching mode node sends frame out only on the link to the destination Central node as single point of failure Medium Access Control Control access to shared medium Where and how Where centralized versus decentralized How synchronous versus asynchronous Centralized versus Distributed MAC Centralized approaches Controller grants access to medium Simple greater control priorities qos But single point of failure and performance bottleneck Decentralized schemes All stations collectively run MAC to decide when to transmit Synchronous versus Asynchronous Synchronous approaches Static channel allocation Examples FDM TDM Simple but inefficient Asynchronous or dynamic Example STDM 3 categories round robin reservation and contention Round Robin MAC Each station is allowed to transmit station may decline or transmit bounded by some maximum transmit time Centralized e g polling or distributed control of who is next to transmit When done station relinquishes and right to transmit goes to next station Efficient when many stations have data to transmit over extended period stream Reservation Time divided into slots Station reserves slots in the future p slots for extended transmissions Multiple Suited to stream traffic Contention No control control Stations try to acquire the medium Distributed in nature nature Perform well for bursty traffic C gett very inefficient Can i ffi i t under d heavy h load l d NOTE round robin d bi andd contention i are the h most common Standardized MACs Topologies Techniques Round robin Reservation Contention Bus Token bus 802 4 Polling 802 11 802 11 DQDB 802 6 CSMA CD 802 3 CSMA 802 11 Ring Token T k ring i 802 5 FDDI LLC for LANs Similar functions as general LLCs But it has to interface with MAC sublayer LLC functions Addressing source and destination LLC address versus MAC address Control data exchange between 2 users higher layer layer protocol in the station station User as higher Encapsulation Application pp data TCP header IP header MAC header LLC header MAC trailer LLC PDU MAC frame TCP segment IP datagram LLC Services 3 different services 1 Unacknowledged 1 U k l d d connectionless ti l type t 1 1 No error or flow control No N ddelivery li guarantees 2 Connection mode type 2 Logical connection established Flow and congestion control provided 3 Acknowledged connectionless type 3 No logical connection Flow and error control IEEE 802 Standards The 802 workingg ggroups p The important p ones are marked with The ones marked with are hibernating The one marked with gave up LLC 802 2 Protocol Similar to HDLC ISO standard LLC PDU protocol data unit 1 byte 1 byte 1 or 2 bytes DSAP SSAP LLC control variable Information Wireless LANs Use wireless transmission media Infrared IR limited to indoors and single room IR is light doesn t penetrate walls R Radio di Narrowband microwave Spread Spectrum LANs Wireless LAN Applications Nomadic access e g users roaming around campus campus LAN interconnection e g across buildings Ad Hoc Networks e g disaster relief crew Sensor Networks MAC Protocols Contention based ALOHA and d Sl Slotted tt d ALOHA ALOHA CSMA CSMA CD CSMA CD Round robin token based protocols Token bus Token ring CSMA Maximum utilization is function of frame size and propagation time time Longer frames or shorter propagation time higher utilization CSMA Flavors 1 persistent CSMA IEEE 802 3 If medium di idl idle ttransmit it if medium di busy b wait it until til idle then transmit with p 1 If collision collision waits random period and starts again again Non persistent CSMA if medium idle transmit otherwise wait a random time before re trying re trying P persistent when channel idle detected transmits packet in the first slot with probability p p CSMA CD CSMA with collision detection Problem when frames collide medium is unusable for duration of both damaged frames F llong frames For f when h comparedd to propagation i time i considerable waste Wh nott li Why listen t while hil ttransmitting itti CSMA CD Protocol 1 If medium idle transmit otherwise 2 2 If medium busy wait until idle then transmit with p 1 g and 3 If collision detected transmit brief jjammingg signal abort transmission 4 After aborting wait random time try again CSMA with Collision Detection If collision detected immediately abort transmission CSMA CD can be in one of three states contention transmission or idle CSMA CD Performance Wasted capacity restricted
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