Midterm Review EECS 122. University of California Berkeley

Transcription

1 Midterm Review EECS 122 University of California Berkeley

2 Topics Network Architecture Network hierarchy Layering Performance Link Layer Ethernet Wi-Fi 2

3 Review: Network WAN MAN 3

4 Review: Network WAN MAN LAN 4

5 Review: Network WAN LAN 5

6 Review: Layers & Protocols Application Transport TH Data Data HTTP, FTP, UDP - TCP Application Transport Network Asynchronous routed path Asynchronous routed path Network PH Data PH Data IP Network Data Link Control Asynchronous reliable bit pipe FH Data Data Link Control Asynchronous reliable bit pipe FH Data Data Link Control Physical Interface Synchronous unreliable bit pipe Physical Interface Synchronous unreliable bit pipe Physical Interface End Node Physical Link Router Physical Link End Node 6

7 Timing: Queuing Link: P bits Q R bps T seconds Q/R Q/R = queuing delay (load-dependent) T P/R Time 7

8 Timing: Store & Forward - Multiple System: 10Mbps 5Mbps 100Mbps 10Mbps 8

9 Little s Result N S 1 2 D X(t) T(N) T(N - 1) S = area S = T(1) + + T(N) = integral of X(t) 1 T(1) + + T(N) N X(t)dt = S =. T T N T T Average occupancy = (average delay)x(average arrival rate) 9

10 The basic queuing system 10

11 Performance Metrics Throughput Delay Jitter 11

12 Connection Throughput Connection: Send W bits (window size) Wait for ACKs Repeat Assume that the round-trip time is RTT seconds Throughput = W/RTT bps Source RTT K Destination RTT K Time 12

13 Stop-and-Wait Scenarios Transmit one packet, wait for ack, and repeat Set timeout for retransmission [Peterson & Davie] 13

14 Sliding Window Start with a window size of W packets Decrement available window size for each packet transmitted Increment available window size for each ack received [Peterson & Davie] 14

15 Random Multiple Access How to share a channel? Multiple Access Multiplexing ALOHA: First random multiple access system Efficient for many users, each with low utilization Try; If collide, wait random time then repeat Analysis: Slotted Aloha efficiency 1/e = 36% p, indpdt. Slot N nodes P(success) = Np(1 p) N-1 1/e if p = 1/N 15

16 Random Multiple Access Ethernet: First version CSMA/CD Wait until channel is idle; try; if collide, stop, wait, repeat Idea: CD should improve efficiency if fast enough If CD, wait random multiple of 512 bit times (exponential back off) Analysis: Worst-case Efficiency 1/(1 + 5a), a = PROP/TRANS A B 16

17 Switching Ethernet: Later versions Switched Larger aggregate throughput VLANs: partition in disjoint logical LANs Link Aggregation Each port is in its own collision domain as opposed to a hub where all ports are in the same collision domain Fast, GE, 10GE Improved modulation schemes 17

18 Ethernet Service? Operations: Addresses, MAC, Hub, Switch, Learning, Spanning Tree MAC: Why not Aloha? Why Switch? Why Loops? 18

19 a - 5GHz, up to 54Mbps b - 2.5GHz, up to 11Mbps g - 2.5GHz, up to 54Mbps MAC: CSMA/CA with or without RTS/CTS Distributed (DCF): CSMA/CA using different Interframe Gaps maintain network allocation vector Centralized (PCF): access point polls nodes 19

20 MAC If medium is idle for DIFS interval after a correctly received frame and backoff time has expired, transmission can begin immediately If previous frame contained errors, medium must be free for EIFS If medium is busy, access is deferred until medium is idle for DIFS and exponential backoff Backoff counter is decremented by one if a time slot is determined to be idle Unicast data must be acknowledged as part of an atomic exchange 20

21 Virtual Carrier Sensing Virtual Carrier Sensing using Network Allocation Vector (NAV) 21

22 Efficiency Calculation Example 22

23 What are hidden/exposed terminal problems? Why not CSMA/CD? Objectives of new MAC? Why RTS/CTS? How does NAV work? Why different IFS? Why more than 2 addresses? Why different PHYs? Why multiple channels? Efficiency calculation 23

24 Check List Network hierarchy Layering Performance: Timing & Metrics Layer 2 Ethernet MAC Wi-Fi MAC Repeaters, hubs, bridges, switches 24