FAIRNESS CONSIDERATIONS FOR PLCA EXAMPLE MICROPHONE USE CASE. Dr. Kirsten Matheus

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1 FAIRNESS CONSIDERATIONS FOR PLCA EXAMPLE MICROPHONE USE CASE Dr. Kirsten Matheus

2 CONTENT Problem description Data rates Worst case delay Solution space Summary Fairness in PLCA, Kirsten Matheus, BMW Page 2

3 PROBLEM DESCRIPTION On the multidrop link segment multiple units share the bandwidth available on the channel. The channel access is organized via PLCA, i.e. on a fairness per packet basis. If packet sizes on the channel vary, the channel access might lead to unfavourable delays, even if the overall bandwidth required is well under the limit. This presentation investigates various possibilities to create better delays without needing to change the existing solution. Fairness in PLCA, Kirsten Matheus, BMW Page 3

4 Data rate [Mbps] MICROPHONE USE CASE SET UP (1). Audio stream Sampling interval No. bits No. bits No. bits Data rate per stereo audio channel sampled with 16 Class A Class B Class 64b , ,5 50 minimum maximum Sampling Interval [us] Header (Eth ) Payload Length of audio packet in bytes is independent of Ethernet speed grade Data rate needed depends on sampling rate and sampling interval only Data rate [MHz] = payload data rate [MHz] + header data rate [MHz] = 2*sample bits*sample frequency [MHz] + (42+24)*8 bits/ Sampling interval [ms] Fairness in PLCA, Kirsten Matheus, BMW Page 4

5 MICROPHONE USE CASE SET UP (2). HN TCP Sampling interval TCP/IP interval 3 Audio Class A Class B Class 64b The overall data rate needs to fit the channel Data rate [MHz] = No. microphones*audio data rate [MHz] + 2*No. microphones * TCP/IP data rate [MHz] Fairness in PLCA, Kirsten Matheus, BMW Page 5

6 WORST CASE DELAY SCENARIO. HN 0 1 TCP (0, 1, 2, 3) TCP/IP interval 2 3 Audio (1, 2, 3) Sampling interval Channel output Delay 1. audio packet microphone 3 Delay 2. audio packet microphone 3 Senario description: Three microphones are connected to the head node In the microphones audio data has priority over TCP/IP data All units want to transmit their TCP/IP data at the same time All units want to transmit their audio at the same time The first audio just miss their slots Delay 3. audio packet microphone 3 Delay 4. audio packet microphone 3 Delay increases until HN has sent all its TCP Microphones send their first TCP once there is no delay of the audio Fairness in PLCA, Kirsten Matheus, BMW Page 6

7 SOLUTION 1: SHORTENED TCP/IP PACKETS. *) Results The delay decreases with decreasing payload size (even if this increases the number of TCP/IP ) Once the length of the TCP/IP is such that No. Microphones * length audio packet + length TCP packet < sampling interval The delay decreases significantly and its maximum delay value converges to ~ 2 * (length TCP packet + length audio packet) *) 5* 1460 bytes TCP/IP every 0,5s, audio sampling with 16bits@48kHz every 1,333ms Fairness in PLCA, Kirsten Matheus, BMW Page 7

8 SOLUTION 2: INCREASED AUDIO SAMPLING INTERVAL. (1) HN 0 1 TCP (0, 1, 2, 3) TCP/IP interval 2 3 Audio (1, 2, 3) Sampling interval Channel output Delay 1. audio packet microphone 3 Delay 2. audio packet microphone 3 Delay 3. audio packet microphone 3 Fairness in PLCA, Kirsten Matheus, BMW Page 8

9 SOLUTION 2: INCREASED AUDIO SAMPLING INTERVAL. (2) *) *) Results The delay decreases with increasing sampling interval up to the point where the maximum delay is determined by the first packet Then the delay increases and is = (No. Microphones-1) * length audio packet + 2*length TCP packet *) 5* 1460 bytes TCP/IP every 0,5s, audio sampling with 16bits@48kHz Fairness in PLCA, Kirsten Matheus, BMW Page 9

10 SOLUTION 3: ASSIGNING MULTIPLE PLCA SLOTS. (1) HN 0 1 TCP (0, 1, 2, 3) TCP/IP interval 2 3 Audio (1, 2, 3) Sampling interval Channel output Delay 1 Delay 2 Delay 3 Slots Max. delay [ms] 12,41 4,56 2,98 2,98 Fairness in PLCA, Kirsten Matheus, BMW Page 10 Delay 4 Delay 5 Results Smallest max. delay is limited by 2*length TCP packet + (No. Microphones -1) * length audio packet

11 SUMMARY Especially in set ups, in which units with short and long share a multidrop link, unfavourable delays can occur with PLCA that regulates fairness on a per packet basis. Without adding any other mechanisms there are three possibilities to reduce that delay Decreasing the size of the (TCP/IP) control Increasing the sampling interval Assigning multiple PLCA slots With all three mechanisms the maximum delay can be reduced significantly. However, the results achievable are directly linked to the size of the control. A combination of mechanisms has not been investigated but should achieve good results (e.g. Assigning multiple PLCA slots and limiting the size of from the end nodes). Tbd: Automated/flexible configuration Fairness in PLCA, Kirsten Matheus, BMW Page 11

2.993: Principles of Internet Computing Quiz 1. Network

2.993: Principles of Internet Computing Quiz 1 2 3:30 pm, March 18 Spring 1999 Host A Host B Network 1. TCP Flow Control Hosts A, at MIT, and B, at Stanford are communicating to each other via links connected