Modelling Packet Delay in Ethernet and IP Networks

Transcription

1 Modelling Packet Delay in Ethernet and IP Networks Dominik Schneuwly Dr. André Vallat Slide 1.1

2 Introduction Is it possible to transfer time and frequency over packet switched networks with accuracies commonly required by telecom applications and equipment? If yes, under what conditions? Take as an example the Precise Time Protocol (IEEE 1588) Use simulations to study packet propagation properties and predict performance Study the influence of traffic load Study the influence of protocol support in switching/routing nodes (e.g. Transparent Clocks) Slide 1.2

3 Definition: Packet Delay δ AB (k) Consider two interfaces A and B traversed by a given packet flow. End system Packet switched network End system A B Packets/frames sent over interface A 1 2 k t Packets/frames received over interface B 1 2 k δ AB (k) t Slide 1.3

4 Definition: Packet Delay Asymmetry A(k) Consider two interfaces A and B traversed by bidirectional paired packet flows, where the k-th packet pair experiences the delay δ AB (k) and δ BA (k): A(k) = δ AB (k) - δ BA (k) Slide 1.4

5 PTP IEEE 1588: TWTT Master (M) Slave (S) t M t S = t M + SYNC T 1 FOLLOW_UP (T 1 ) T 2 T 1 T 3 T 4 DELAY_RESP (T 4 ) T 4 Time-critical message Timestamp transfer message Slide 1.5

6 PTP IEEE 1588: Time-stamping Lower Layers of Protocol Stack Slide 1.6

7 PTP IEEE 1588: Transparent Clocks Slide 1.7

8 End-to-end Transparent Clock θ= ( T T) ( T T) δm S δs M A = θ+ = θ 2 2 PTP with End-to-end Transparent Clocks: δ A θ M S = δ NODE, M S = δ NODE, M S NODE, S M ( ) ( ) T T T T A = δ 2 2 Slide 1.8

9 Simulation: network structure Slide 1.9

10 Traffic sources Each source bloc shown in the diagram represents 5 independent smaller sources with the same stochastic properties Packet interarrival times process: «walker held by an elastic leash» Packet sizes: Packet size [octet] Probability Slide 1.10

11 Simulation: node structure Slide 1.11

12 Simulation scenarios Run no. Traffic load λ N N X = non-ptp-capable node O = PTP-capable node O - X - X - O - O - O O - X - X - X - X - O X - X - X - X - X - X O - X - X - O - O - O O - X - X - X - X - O X - X - X - X - X - X - Traffic load λ = (avg. data rate / link capacity) on a longitudinal link N N = number on non-ptp-capable nodes Slide 1.12

13 Simulation parameters and output Simulation parameters: Link capacity C = 100 Mbit/s Input queues = 150,000 octets Output queues = 150,000 octets TWTT interogation rate = 100 s -1 (SYNC & DELAY_REQ) Sampling period T S = 10 µs Simulation length = 30,000 s (=> 400 mio. samples on 68 sources and nodes!) Main simulation output: Residual Packet Delay Residual Packet Delay Asymmetry Slide 1.13

14 Simulator State Display Slide 1.14

15 MinTDEV and Min TDEV Slide 1.15

16 Histogram of 2 x Residual Delay N i / N N i / N N N = N N = N N = N N = δ R [µs] N N = 4 N N = δ R [µs] λ = 6 λ = 0.43 Slide 1.16

17 TDEV of 2 x Residual Delay TDEV (τ) [µs] 1'000 TDEV (τ) [µs] 1' N N = N N = 4 N N = N N = 6 N N = 4 N N = τ [s] τ [s] λ = 6 λ = 0.43 Slide 1.17

18 MinTDEV of Residual Delay MinTDEV (τ) [µs] MinTDEV (τ) [µs] 1'000 1' N N = N N = 6 N N = 4 N N = 4 10 N N = 2 10 N N = τ [s] τ [s] λ = 6 λ = 0.43 Slide 1.18

19 Histogram of Residual Asymmetry N i / N N i / N N N = 6 N N = 4 N N = A R [µs] N N = 6 N N = 4 N N = A R [µs] λ = 6 λ = 0.43 Slide 1.19

20 TDEV of Residual Asymmetry TDEV (τ) [µs] 1' N N = 6 N N = 4 N N = τ [s] λ = 6 λ = 0.43 Slide 1.20

21 Min TDEV (1) of Residual Asymmetry Min TDEV (τ) [µs] Min TDEV (τ) [µs] 1'000 1' N N = N N = 6 10 N N = 4 N N = 2 10 N N = 4 N N = τ [s] τ [s] λ = 6 λ = 0.43 Slide 1.21

22 Filtering Residual Delay TDEV (τ) [s] TDEV (τ) [s] τ c = 300 s f c = 1 mhz TDEV[2 δ R ], λ = 0.43, N N = τ c = 40 s f c = 7.5 mhz τ [s] τ [s] 2 x Network Limit for PDH distribution output, ITU-T Rec. G x Network Limit for PDH distribution output, ITU-T Rec. G.823 Slide 1.22

23 Filtering Residual Asymmetry TDEV (τ) [s] TDEV (τ) [s] τ c = 300 s f c = 1 mhz TDEV[A R ], λ = 0.43, N N = τ c = 4 s f c = 75 mhz τ [s] τ [s] 2 x Network Limit for PDH distribution output, ITU-T Rec. G x Network Limit for PDH distribution output, ITU-T Rec. G.823 Slide 1.23

24 Conclusions With PTP IEEE 1588 telecom performance level is achievable over moderate size networks If traffic load is controlled, performance objectives are attained even in PTP networks without PTP-capable nodes High traffic loads deteriorate performance in PTP networks without PTP-capable nodes PTP suitable for the distribution of time and frequency in network types such as Metro Area Networks, base-station backhaul networks, access aggregation networks, etc. Slide 1.24

25 Thank you Slide 1.25