Introduction to Computer Networks CMPE 150 Fall 2005 Lecture 17 CMPE 150 Introduction to Computer Networks 1 Announcements Midterm on 11 04 In class closed books notes Homework 3 is up Due on 11 07 05 Lab this week discussion review sessions for midterm CMPE 150 Introduction to Computer Networks 2 Today MAC cont d CMPE 150 Introduction to Computer Networks 3 Types of MAC Control Distributed Centralized How they coordinate medium access Round robin Scheduled access Contention based CMPE 150 Introduction to Computer Networks 4 Round Robin MAC Centralized polling Distributed token passing CMPE 150 Introduction to Computer Networks 5 Scheduled Access MAC Time divided into slots Station reserves slots in the future Multiple slots for extended transmissions Suited to stream traffic CMPE 150 Introduction to Computer Networks 6 Scheduled Access MAC Example The basic bit map protocol CMPE 150 Introduction to Computer Networks 7 Contention Based MAC No control Stations try to acquire the medium Distributed in nature CMPE 150 Introduction to Computer Networks 8 MAC Protocols Contention based ALOHA and Slotted ALOHA CSMA CSMA CD Round robin token based protocols Token bus Token ring CMPE 150 Introduction to Computer Networks 9 Contention Based MACs ALOHA family CSMA family CMPE 150 Introduction to Computer Networks 10 ALOHA Protocols Performance Throughput versus offered traffic for ALOHA systems CMPE 150 Introduction to Computer Networks 11 ALOHA Protocols Summary Simple But poor utilization When CMPE 150 Introduction to Computer Networks 12 CSMA Protocols CMPE 150 Introduction to Computer Networks 13 Carrier Sense Multiple Access The capacity of ALOHA or slotted ALOHA is limited by the large vulnerability period of a packet By listening before transmitting stations try to reduce the vulnerability period to one propagation delay This is the basis of CSMA Kleinrock and Tobagi UCLA 1975 CMPE 150 Introduction to Computer Networks 14 CSMA Station that wants to transmit first listens to check if another transmission is in progress carrier sense If medium is in use station waits else it transmits Collisions can still occur Transmitter waits for ACK if no ACKs retransmits CMPE 150 Introduction to Computer Networks 15 Collisions Can collisions still occur CMPE 150 Introduction to Computer Networks 16 CSMA Flavors 1 persistent CSMA IEEE 802 3 If medium idle transmit if medium busy wait until idle then transmit with p 1 If collision waits random period and starts again Non persistent CSMA if medium idle transmit otherwise wait a random time before re trying Thus station does not continuously sense channel when it is in use P persistent when channel idle detected transmits packet in the first slot with p Slotted channel i e with probability q p 1 defers to next slot CMPE 150 Introduction to Computer Networks 17 CSMA vesrsus Aloha Comparison of the channel utilization versus load for various random access protocols CMPE 150 Introduction to Computer Networks 18 CSMA CD CSMA with collision detection Problem when frames collide medium is unusable for duration of both damaged frames For long frames when compared to propagation time considerable waste What if station listens while transmitting CMPE 150 Introduction to Computer Networks 19 CSMA CD Protocol 1 If medium idle transmit otherwise 2 2 If medium busy wait until idle then transmit with p 1 3 If collision detected transmit brief jamming signal and abort transmission 4 After aborting wait random time try again CMPE 150 Introduction to Computer Networks 20 CSMA CD Performance Wasted capacity restricted to time to detect collision Time to detect collision 2 maximum propagation delay Rule in CSMA CD protocols frames long enough to allow collision detection prior to end of transmission Thus frames need to be at least 2 RTT long CMPE 150 Introduction to Computer Networks 21 CSMA with Collision Detection CSMA CD can be in one of three states contention transmission or idle CMPE 150 Introduction to Computer Networks 22 Ethernet CMPE 150 Introduction to Computer Networks 23 Ethernet IEEE 802 family Standards for LANs and MANs Ethernet defined in the IEEE 802 3 standard PHY MAC and LLC CMPE 150 Introduction to Computer Networks 24 Where in the Stack a Position of LLC b Protocol formats CMPE 150 Introduction to Computer Networks 25 Ethernet MAC CSMA CD Binary exponential back off CMPE 150 Introduction to Computer Networks 26 Ethernet MAC Cont d Collision detection can take as long as 2t CMPE 150 Introduction to Computer Networks 27 Ethernet Frame Length At 10Mbps with 2 500 m maximum distance RTT 50 microsec Thus at least 500 bit frames It is actually 512 bits If fewer bits than that add padding CMPE 150 Introduction to Computer Networks 28 Ethernet Frame Frame formats a DIX Ethernet b IEEE 802 3 Destination address 1 for group addresses Type mux demux of network layer protocols Data max of 1500 bytes CMPE 150 Introduction to Computer Networks 29 Binary Exponential Backof Randomization after collision CMPE 150 Introduction to Computer Networks 30 BEB Cont d After first collision each station waits for 0 or 1 slot before trying again After second collision they pick either 0 1 2 or 3 slots at random to wait After 3rd collision number of slots to wait is between 0 and 23 1 In general after I collisions wait is between 0 and 2i 1 After 10 collisions randomization interval frozen at 1023 slots After 16 collisions error CMPE 150 Introduction to Computer Networks 31 Ethernet Performance CMPE 150 Introduction to Computer Networks 32 Ethernet Cabling The most common kinds of Ethernet cabling CMPE 150 Introduction to Computer Networks 33 Ethernet Cabling Cont d Three kinds of Ethernet cabling a 10Base5 b 10Base2 c 10Base T CMPE 150 Introduction to Computer Networks 34 Ethernet Topologies Cable topologies a Linear b Spine c Tree d Segmented CMPE 150 Introduction to Computer Networks 35 Switched Ethernet A simple example of switched Ethernet CMPE 150 Introduction to Computer Networks 36 Fast Ethernet The original fast Ethernet cabling CMPE 150 Introduction to Computer Networks 37 Gigabit Ethernet a A two station Ethernet b A multistation Ethernet CMPE 150 Introduction to Computer Networks 38 Gigabit Ethernet Cont d Gigabit Ethernet cabling CMPE 150 Introduction to Computer Networks 39
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