IMA Wireless Workshop 6/05 – 1

Wireless Local Area Network Simulation

Abstract: We describe a software system that simulates wireless local area networks supporting heterogeneous services and multiple protocols. We present applications of this system in three areas: analysis of voice capacity, maximization of data throughput while protecting voice quality of service, and the design and evaluation of scheduling algorithms in a polling-based system.

Martin Eiger, Moncef Elaoud, David Famolari, Raquel Morera

Telcordia Technologies

Piscataway, NJ

IMA Wireless Workshop 6/05 – 2

Overview

Multi-year collaborationModel and analyze many configurations

–Existing standards–Possible future standards

Several IEEE 802.11 variations–One queue/separate queues–Contention-based/contention-free

C/UNIX with CSIM simulation package

IMA Wireless Workshop 6/05 – 3

Wireless Local Area Network

Heterogeneous services Shared transmission medium

access point

IMA Wireless Workshop 6/05 – 4

Listen-Before-Talk Protocol

Transmit only after idle medium for appropriate IFS (interframe space) interval

Random backoff after every transmission–Pick random integer, decrement periodically, transmit when counter reaches 0

–Suspend countdown on busy medium, resume after idle IFS

–Promotes fairness Increase random backoff range after collision; restore on success

IMA Wireless Workshop 6/05 – 5

Simulator “Knobs”

Time to simulate802.11 standardNumber of nodesMobilityTransmission ratesService mixPacket sizesPacket arrival ratesDelay thresholds

Interframe spacingContention windows

Queue limitsScheduling policyDesired output

●

●

●

IMA Wireless Workshop 6/05 – 6

Voice Capacity (IEEE 802.11b)

Conclusions: QoS degrades

for all users concurrently

Packetization interval is key determinant of max capacity (reason: fixed overhead per packet)

0

10

20

30

40

50

60

1 6 11 16 21 26 31 36 41 46 51 56

Active Voice Users

Supp

orte

d V

oice

Use

rs

10ms 20ms 30ms 40ms

50ms 60ms 70ms 80ms

packet-ization

intervals:

IMA Wireless Workshop 6/05 – 7

Multiple Queues

Several queues at each node

Each queue senses medium, backs off independently

Motivation: set each queue’s interframe space and contention window parameters separately to assure quality of service

IMA Wireless Workshop 6/05 – 8

voiceAIFS CW

voice UDP voice UDP

1 50 70 1 8

2 50 90 1 8

3 30 90 1 8

4 30 30 1 16

5 30 30 1 16

6 30 50 1 16

7 30 110 1 8

8 30 70 1 16

9 30 110 1 16

10 30 70 1 32

Voice and Data

Conclusion: Failing to adapt AIFS and contention window values can cost ~20% of data throughput

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

1 2 3 4 5 6 7 8 9 10

voice supported

UD

P t

hrou

ghpu

t (M

b/s)

max throughputfixed AIFS and CW

IMA Wireless Workshop 6/05 – 9

Contention-Free Scheduling

Access point grants Transmission Opportunities

Uplink Downlinkpoll

pkt1 … pktn

block ack req

block ack

pkt1 … pktn

block ack req

block ack

The big question: Scheduling

IMA Wireless Workshop 6/05 – 10

Scheduling Problem

Inputs–Traffic stream specifications (packet size, average packet arrival rate, …)

Output–AP schedules traffic streams–Polling-based packets meet delay bounds

–Maximize time left over for contention-based traffic

IMA Wireless Workshop 6/05 – 11

A Work in Progress

We have examined several scheduling algorithms

Algorithms with best performance–Account for delays (be sure to grant TXOPs often enough)

–Try to aggregate (don’t grant TXOPs too often)

Patent work begun; results to be published

IMA Wireless Workshop 6/05 – 12

Summary

Wireless LAN simulation– Many environments– “What if” scenarios

Results– Voice capacity– Maximize data while protecting voice– Scheduling protocol

Ultimate goal– Wireless LAN design and operations maximize capacity, assure quality of service