Introduction to Computer Networks CMPE 150 Fall 2005 Lecture 14 CMPE 150 Introduction to Computer Networks 1 Announcements Midterm moved to 11 04 In class closed books notes We will have lab this week Homework 1 is due today Homework 2 will be up soon CMPE 150 Introduction to Computer Networks 2 Today Finish Layer 2 CMPE 150 Introduction to Computer Networks 3 Flow Error Control Stop and Wait ACKs ARQ Noisy channels ACKs Sequence numbers Timers CMPE 150 Introduction to Computer Networks 4 Can we do better Can we do better Piggybacking Bi directional transmission Wait for data packet and use that to piggyback the ACK Use ACK field only a few additional bits in the header But how long should Layer 2 wait to send an ACK ACK timers CMPE 150 Introduction to Computer Networks 5 Can we do even better In Stop and Wait only 1 frame outstanding at any given point in time What s the problem with that Loooong pipes S Fat pipes R S CMPE 150 Introduction to Computer Networks R 6 Sliding Window Window number of outstanding frames at any given point in time So what s the window size of Stop and Wait Every ACK received window slides CMPE 150 Introduction to Computer Networks 7 Sliding Window Example A sliding window of size 1 with a 3 bit sequence number a Initially b After the first frame has been sent c After the first frame has been received d After the first acknowledgement has been received CMPE 150 Introduction to Computer Networks 8 Sliding Window Basics Allows multiple frames to be in transit at the same time Receiver allocates buffer space for n frames Transmitter is allowed to send n window size frames without receiving ACK Sequence number CMPE 150 Introduction to Computer Networks 9 Sliding Window Receiver Receiver maintains window corresponding with frames it can receive Receiver ack s frame by including sequence number of next expected frame Cumulative ACK ack s multiple frames Example if receiver receives frames 2 3 and 4 it sends an ACK with sequence number 5 which ack s receipt of 2 3 and 4 CMPE 150 Introduction to Computer Networks 10 Sliding Window Sender Sender maintains window corresponding to frames sequence numbers it s allowed to send Sequence numbers are bounded if frame reserves k bit field for sequence numbers then they can range from 0 2k 1 Transmission window shrinks each time frame is sent and grows each time an ACK is received CMPE 150 Introduction to Computer Networks 11 Example 3 bit sequence number and window size 7 A Sender B Receiver 0 1 2 3 4 5 6 7 0 1 2 3 4 0123456701234 0 2 ACK 3 0123456701234 0123456701234 0123456701234 1 ACK 4 0123456701234 CMPE 150 Introduction to Computer Networks 0123456701234 0123456701234 3 45 0 1 2 3 4 5 6 7 0 1 2 3 4 6 0123456701234 12 One Bit Sliding Window Protocol Two scenarios a Normal case b Abnormal case Notation is seq ack packet number An indicates where a network layer accepts packet ACK indicates last sequence number received CMPE 150 Introduction to Computer Networks 13 Bandwidth Delay Product How large should the sender s window be Function of how fat and long the pipe is BW S R RTT W BW RTT data size CMPE 150 Introduction to Computer Networks 14 Pipelining Receiver s window size is 1 discard frames after error with no ACK Go Back N Receiver s window size is large buffers all frames until error Selective Repeat recovered Pipelining and error recovery Effect on error when a Receiver s window size is 1 b Receiver s window size is large CMPE 150 Introduction to Computer Networks 15 Medium Access Control MAC Tanenbaum Chapter 4 CMPE 150 Introduction to Computer Networks 16 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 17 Where is the MAC Sub Layer MAC CMPE 150 Introduction to Computer Networks 18 Where is the MAC Sub Layer Application Transport Network DLL Link Control MAC PHY CMPE 150 Introduction to Computer Networks 19 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 20
View Full Document
Unlocking...