Evaluation of WAVE and 802.11p Models in

Network Simulator ns3 Pavel Vasilyev, Senior Software Engineer, Sreda Software Solutions

Vasilii Aleksandrov, Senior Software Engineer, Sreda Software Solutions

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This presentation shows the results of WAVE and 802.11p models evaluation in Network Simulator (ns3). The goal of the evaluation is to understand how both models work in ns3 in purpose of using it for simulating V2X scenarios.

The evaluation consist of two types of scenarios: static and dynamic using variable data rates, distances and speeds. The purpose of researching both models is to compare results in order to understand which differences WAVE model introduces to plain 802.11p since IEEE 802.11p standard is used as a physical layer for WAVE protocol stack.

WAVE model in ns3 simulator has only IEEE 1609.4 (multi-channel operation) standard implementation. On top of this implementation can be used WSMP protocol with both control and service channels access and IPv4/IPv6 applications with service channel access.

WAVE model in ns3 has configuration that allows to provide continuous (full time) access for service channel in wireless medium. It means that both channels’ intervals: SCH interval and CCH interval will be accessible for service channel. In order to compare both models in similar way it has been decided to use this configuration.

Introduction

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Common parametersIn simulation scenarios we use two cars equipped with WAVE and 802.11p devices. All scenarios simulated for all supported data rates (3, 4.5, 6, 9, 12, 18, 24 and 27 Mbit/s) defined in 802.11p standard with 10 MHz bandwidth. For traffic generation we use UDP application which generates data-flow with throughput higher than selected data rate in order to fill up channel bandwidth. Results were measured on a receiving side.

Static scenariosIn static scenarios cars don’t move. We change distance from 200 to 1000 meters with 200 meters step. As the result, we executed 40 tests for the static scenarios.

Dynamic scenariosIn dynamic scenarios cars are starting at distance 1000 meters between each other and are moving towards each other, so they meet at 500 meters after the start. We change car speeds from 20 to 180 km/h with 20 km/h step. As the result, we executed 72 tests for the dynamic scenarios.

Scenarios

1000m

20 km/h ... 180 km/h 20 km/h ... 180 km/h

200m … 1000m0 km/h 0 km/h

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NS3 Setup for WAVE and 802.11p

Devices configuration● We use Yans model (YansWavePhyHelper for WAVE, YansWifiPhyHelper for 802.11p) to set up channel and phy layer.● We set Tx Power Level according to US regulatory domain to - 44.8 dBm.● We use mobility models with constant distance for static scenarios and with constant speed for dynamic scenarios.● We set WAVE specific configuration as:

○ SCH3 (channel number 176) with continuous access used for applications.

Applications configuration● UDP over IPv4 is used for throughput measurements.● Modified BulkSendApplication is used on transmitting side

○ UDP support has been added● PacketSink application is used on receiving side.● FlowMonitor is used for throughput logging.

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Static Scenarios Results for WAVE and 802.11p

The following graphics show measured throughput of static scenarios for each data rate at different distances for WAVE and 802.11p simulation.

The figures show a couple of differences in comparison between WAVE and 802.11p simulation results:

1. The first four data rates: 3, 4.5, 6 and 9 Mbit/s show absolutely the same result for WAVE model. Moreover, data rate 9 Mbit/s works in WAVE model at distance 1000 meters, but doesn’t work in 802.11p model.

2. Throughput in 802.11p a bit higher than in WAVE (As continuous access was used to simulate WAVE the results are perfectly matches theory).

There are also some similarities for both models:1. As expected that the highest data rates stop working at higher distances.2. The lowest data rates show results more close to theoretical values than

the highest data rates.

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The figures show throughput results of different speed for the same data rate. First three data rates are omitted, because as was observed in the static tests, they show the same throughput results for WAVE.Data rate 24 Mbit/s shows results close to 27 Mbit/s.

Dynamic scenarios showed that WAVE model in ns3 mostly has no dependency on speed factor. All tests with different speeds for the same data rate have close average throughput.

For the 802.11p these figures show that there are more bandwidth available for data when using 802.11p comparing to WAVE. But at low data rates (9 Mbit/s) the distance is less than while using WAVE for data transfer. Also the distance between connection established and maximum throughput achieved is higher for 802.11p.

Dynamic Scenarios Results for WAVE and 802.11p

WAVE

802.11p

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These figures also indicate at what exact distance particular data rate starts working.

A spread in first results in the figures is linked with ARP Request delay since both nodes don’t know about each other.

Data rates 12 Mbit/s, 18 Mbit/s and 27 Mbit/s have comparable throughput results while low data rate 9 Mbit/s shows that 802.11p could provide more throughput versus WAVE.

Dynamic Scenarios Results for WAVE and 802.11p (Cont)

WAVE

802.11p

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Dynamic Scenarios Results for WAVE and 802.11p

Separated figures have been created to understand how close the results of different speed factor are lying.

These figures show throughput measurements of data rate at 27 Mbit/s with 3 different speed factors: 20 km/h, 80 km/h and 180 km/h for WAVE and 802.11p models

It could be observed that higher speed the nodes have, higher spread of throughput results will be. The lowest speed shows results close to average throughput, but the highest speed periodically shows throughput results much higher than average throughput.

Results for WAVE and 802.11p are look very similar

WAVE 802.11p

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Taking into account that WAVE model is build on top of 802.11p model in ns3, there are couple of major and minor issues that WAVE model has introduced:

● Data rates 3, 4.5, 6 and 9 Mbit/s have an issue since they all show the same result and 9 Mbit/s data rate in WAVE model works at the highest distance. The other data rates (12, 18, 24 and 27 Mbit/s) have no such issue.

● Throughput in WAVE a bit lower than in 802.11p that is probably related to a guard interval time between channel access that is introduced in standard IEEE 1609.4.

● Data rates 12, 18, 24 and 27 Mbit/s could be used to simulate V2X environment with continuous access to the service channel. In order to enable all possible modes of operations ns3 should be adjusted.

Summary

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The issue with the first four data rates (3, 4.5, 6, 9Mbps) described in the previous slide is in TxProfile class implementation of ns3 WAVE module. According to IEEE 1609.4 section 7.3.4.2 MLMEX-REGISTERTXPROFILE.request tx profile registration primitive shall provide a data rate parameter. In ns3 version 3.26 the TxProfile class implementation doesn’t provide data rate as a parameter so it doesn't allow user to specify transmission data rate according to IEEE 1609.4 standard.

Additionally it forces to use 6Mbps data rate for lowest data rate. That is why throughput results were the same and 9Mbps data rate worked at distance 1000 meters, but it shouldn’t since in 802.11p model it doesn’t work so.

The following graphics show measurements of WAVE model with modified TxProfile class in comparison with 802.11p model:

WAVE data rate issue investigation