EECS 563 Fall Review Test 2. Review Test 2

Transcription

1 EECS 563 Fall

2 Network traffic Request for resources Rate of requests= λ Average resource hold time = T h Common Assumptions Time between arrivals ~ exponentially Holding time ~ exponentially Voice λ = call/sec T h = average holding time sec/call Load = λ*t h Erlangs 2

3 Network traffic Packet Voice Constraint limit on end-to-end delay Packetization time (ms/packet) Role of the Jitter buffer Too small lose packets Too large increase delay 3

4 Network traffic Packet Voice Components of end-to-end delay (delay budget) & trade-offs Packetization time Clocking time Network delay (random) Jitter buffer VoIP Rate calculation 4

5 Network traffic Video MPEG Frame types Data Burstyness = (Peak rate)/(average rate) Large Burstyness leads to effective statistical multiplexing. 5

6 Performance Anaysis Given Traffic QoS Design system Find C System size Find Number of Servers Assumptions Service time ~ Exponentially distributed Interarrival time ~ Exponentially distributed 6

7 M/M/1 E[ T ] L 1 = = C µ λ 1 ρ Given on Test Where µ = Average Service Rate, λ = Average Arrival Rate, C= Capacity, L = Average Packet Length L λ ρ = λ = = C µ r in C P[ System Busy] = ρ P[ System Empty] = 1 ρ 7

8 M/M/1/N and M/M/N/N M/M/1/N (1 ρ) ρ 1 N P B = N 1 ρ + Given on Test M/M/N/N Erlang B Table Given on Test 8

9 MAC Scaling & trade-offs WRT: rate (b/s), number of users, and size (km) Deterministic (Polling) Operation (why called deterministic) Calculate effective rate & efficiency As ' τ Max ' τ a = L where = Ring Latency C a S 9

10 MAC Random Access Collision process Time vulnerable to collision Detecting Collisions Time Unslotted Slotted Role of backoff 10

11 MAC (Random Access-continued) Types (all can be slotted/unslotted) ALOHA (for unslotted S max =18%, for slotted S max = 36%) CSMA p-persistent (1-persistent) Non-persistent CSMA/CD τ a = where τ = End-to-End Propagation Time L C As a S and as a 1, S ALOHA Max Max Leads to specification of Min/Max Packet size 11

12 MAC Collision Free Protocols Random Access and Reservation Systems In upstream send requests to transmit Use part of frame (contention slots) to send requests Use random access to share contention slots Receive grants in the downstream No contention in downstream If no grant in downstream then assume collision for the request, backoff and resend request in upstream 12

13 MAC- Ethernet IEEE Evolution Bus Hub Switch 10 Mb/s 100 Gb/s Role of CSMA/CD 13

14 Network Elements Repeater Bridge Switch Router Layer 2 Switch Layer 3 Switch Layer 4 Switch Layer Any Switch 14

15 MAC- Wireless Networks Issues Noise Signal Fading Hidden terminal RTS/CTS 15

16 MAC- Cable Networks DOCSIS Access protocol CM, Headend, CMTS 16

17 DLC Goal point-to-point error free link Functions Framing Flags & bit stuffing Error recovery Flow control 17

18 DLC Sliding window flow control n bits/sn in packet header Max window N= 2 n -1 N=1 Stop and Wait When to retransmit? Timeout RNR (NACK) What to retransmit? SN Go-back-N Selective Repeat 18

19 DLC Piggybacking Frame structure HDLC 19

20 DLC Performance η = R eff R eff R # bits = Time to tx # bits Stop&Wait* η stop& wait Sliding window* * Understand assumptions behind these equations η = 1+ SlidingWindow 1 2τ R n f 2τ R 1 if N + 1 n f = N 2τ R if N< + 1 2τ R 1 n + f n f 20

21 DLC Open Loop Control DE bit Methods CIR, B c, B e Token bucket Average rate Maximum burst size 21