Introduction to Computer Networks CMPE 150 Fall 2005 Lecture 16 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 next week discussion review sessions for the midterm CMPE 150 Introduction to Computer Networks 2 Today MAC cont d CMPE 150 Introduction to Computer Networks 3 Channel Allocation Problem How to allocate single shared broadcast channel among several stations users If no arbitration several stations users may transmit at the same time COLLISIONS A k a floor control CMPE 150 Introduction to Computer Networks 4 Multiplexing Static FDM TDM Dynamic Statistical TDM CMPE 150 Introduction to Computer Networks 5 Dynamic Multiplexing Dynamic allocation In particular statistical TDM Dynamically allocates time slots on demand Increased channel utilization But CMPE 150 Introduction to Computer Networks 6 Types of MAC Control Distributed Centralized How they coordinate medium access Round robin Scheduled access Contention based CMPE 150 Introduction to Computer Networks 7 Round Robin MAC Centralized polling Distributed token passing CMPE 150 Introduction to Computer Networks 8 Scheduled Access MAC Time divided into slots CMPE 150 Introduction to Computer Networks 9 Contention Based MAC No control Stations try to acquire the medium Distributed in nature CMPE 150 Introduction to Computer Networks 10 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 11 Contention Based MACs ALOHA family CSMA family CMPE 150 Introduction to Computer Networks 12 The ALOHA Protocols Developed U of Hawaii in early 70 s Packet radio networks Free for all whenever station has a frame to send it does so Station listens for maximum RTT for an ACK If no ACK re sends frame for a number of times and then gives up Receivers check FCS and destination address to ACK CMPE 150 Introduction to Computer Networks 13 Pure ALOHA In pure ALOHA frames are transmitted at completely arbitrary times CMPE 150 Introduction to Computer Networks 14 Collisions Invalid frames may be caused by channel noise or Because other station s transmitted at the same time collision Collision happens even when the last bit of a frame overlaps with the first bit of the next frame CMPE 150 Introduction to Computer Networks 15 Pure ALOHA Performance Vulnerable period for the shaded frame CMPE 150 Introduction to Computer Networks 16 ALOHA s Performance Cont d S G e 2G where S is the throughput rate of successful transmissions and G is the offered load S Smax 1 2e 0 184 for G 0 5 CMPE 150 Introduction to Computer Networks 17 Slotted Aloha Doubles performance of ALOHA Frames can only be transmitted at beginning of slot discrete ALOHA Vulnerable period is halved S G e G S Smax 1 e 0 368 for G 1 CMPE 150 Introduction to Computer Networks 18 ALOHA Protocols Performance Throughput versus offered traffic for ALOHA systems CMPE 150 Introduction to Computer Networks 19 ALOHA Protocols Summary Simple But poor utilization When CMPE 150 Introduction to Computer Networks 20 CSMA Protocols CMPE 150 Introduction to Computer Networks 21 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 22 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 23 CSMA Protocol Packet ready Channel Busy no yes transmit delay packet transmission k times wait for a round trip time yes positive ack CMPE 150 Introduction to Computer Networks no compute random backoff integer k 24 CSMA cont d Collisions can occur only when 2 or more stations begin transmitting within short time If station transmits and no collisions during the time leading edge of frame propagates to farthest station then NO collisions CMPE 150 Introduction to Computer Networks 25 CSMA Flavors After detecting carrier a station can persist trying to transmit after the channel is idle again 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 26 CSMA vesrsu Aloha Comparison of the channel utilization versus load for various random access protocols CMPE 150 Introduction to Computer Networks 27 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 28 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 29 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 CMPE 150 Introduction to Computer Networks 30 CSMA with Collision Detection CSMA CD can be in one of three states contention transmission or idle CMPE 150 Introduction to Computer Networks 31 Ethernet CMPE 150 Introduction to Computer Networks 32 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 33 Where in the Stack a Position of LLC b Protocol formats CMPE 150 Introduction to Computer Networks 34 Ethernet MAC CSMA CD Binary exponential back off CMPE 150 Introduction to Computer Networks 35 Ethernet Frame Frame
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