Introduction to Computer Networks CMPE 150 Fall 2005 Lecture 15 CMPE 150 Introduction to Computer Networks 1 Announcements Midterm moved to 11 04 In class closed books notes Homework 3 is up Due on 11 07 05 CMPE 150 Introduction to Computer Networks 2 Today MAC CMPE 150 Introduction to Computer Networks 3 Example DLL Protocols CMPE 150 Introduction to Computer Networks 4 Example DLL Protocols High Level Data Link Control HDLC Point to Point Protocol PPP CMPE 150 Introduction to Computer Networks 5 HDLC ISO standard Flag Identifies host CMPE 150 Introduction to Computer Networks ACK seq CRC 6 PPP Internet s DLL Router to router Home user to ISP RFC 1661 etc PPP is a multi protocol framing mechanism that can be used over multiple PHYs dial up dedicated point to point connections CMPE 150 Introduction to Computer Networks 7 The Data Link Layer in the Internet CMPE 150 Introduction to Computer Networks 8 PPP Frame Type of protocol in the payload Default Default value value no unumbered frame need for addresses No reXmissions CMPE 150 Introduction to Computer Networks 9 Medium Access Control MAC Tanenbaum Chapter 4 CMPE 150 Introduction to Computer Networks 10 Why MAC Point to point versus shared medium networks Shared medium networks use broadcast channels A k a multi access or random access channels MAC layer protocols regulate access to medium in shared medium networks CMPE 150 Introduction to Computer Networks 11 Where is the MAC Sub Layer MAC CMPE 150 Introduction to Computer Networks 12 Where is the MAC Sub Layer Application Transport Network DLL Link Control MAC PHY CMPE 150 Introduction to Computer Networks 13 MAC and LANs LANs typically use shared medium Examples MAC layer critical BTW in wireless networks also WANs typically use point to point connections CMPE 150 Introduction to Computer Networks 14 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 15 Multiplexing CMPE 150 Introduction to Computer Networks DEMUX MUX Sharing a link channel among multiple source destination pairs Example high capacity long distance trunks fiber microwave links carry multiple connections at the same time 16 Multiplexing Techniques 3 basic types Frequency Division Multiplexing FDM Time Division Multiplexing TDM Statistical Time Division Multiplexing STDM Static Dynamic CMPE 150 Introduction to Computer Networks 17 FDM 1 2 N Frequency Time CMPE 150 Introduction to Computer Networks 18 FDM Simple But What if number of users is large What if number of users changes over time What if traffic is bursty CMPE 150 Introduction to Computer Networks 19 TDM N 1 2 Frequency Time CMPE 150 Introduction to Computer Networks 20 TDM Cont d Time divided into time slots One or more slots assigned to a data source But also inefficient U1 U2 1 2 UN N 1 frame CMPE 150 Introduction to Computer Networks 2 N Time 21 Dynamic Multiplexing Dynamic allocation In particular statistical TDM Dynamically allocates time slots on demand Increased channel utilization But CMPE 150 Introduction to Computer Networks 22 Multiple Access Protocols 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 CMPE 150 Introduction to Computer Networks 23 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 e g token ring 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 CMPE 150 Introduction to Computer Networks 24 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 25 Contention Based MAC No control Stations try to acquire the medium Distributed in nature Perform well for bursty traffic Can get very inefficient under heavy load CMPE 150 Introduction to Computer Networks 26 Contention Based MACs ALOHA family CSMA family CMPE 150 Introduction to Computer Networks 27 Pure ALOHA In pure ALOHA frames are transmitted at completely arbitrary times CMPE 150 Introduction to Computer Networks 28 Pure ALOHA Performance Vulnerable period for the shaded frame CMPE 150 Introduction to Computer Networks 29 Pure ALOHA Throughput versus offered traffic for ALOHA systems CMPE 150 Introduction to Computer Networks 30
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