IEEE 802.11p Tutorial: WAVE and DSRC Protocol Stack

This tutorial provides an overview of IEEE 802.11p, also known as WAVE (Wireless Access for Vehicular Environments) or DSRC (Dedicated Short Range Communication). It covers the key features, frequency spectrum, and protocol stack (physical and MAC layers) of 802.11p.

Introduction

802.11p is an amendment to the IEEE 802.11 standard designed to facilitate ad-hoc communication between vehicles and between vehicles and roadside infrastructure. This standard includes modifications to both the Physical (PHY) and Media Access Control (MAC) layers to better suit the unique challenges of vehicular environments.

The primary goal of 802.11p compliant devices is to enhance traffic efficiency and, most importantly, improve safety by helping to prevent accidents. The network formed by these devices is commonly referred to as a VANET (Vehicular Ad-hoc Network).

802.11p/WAVE/DSRC Frequency Spectrum

Fig-1: 802.11p WAVE frequency spectrum

The FCC has allocated a 75 MHz bandwidth spectrum, ranging from 5850 to 5925 MHz, specifically for vehicle-to-vehicle and vehicle-to-infrastructure communication. This is illustrated in Figure 1.

Key Features of IEEE 802.11p WAVE Devices

Here are some notable features of IEEE 802.11p WAVE devices:

• Designed for low-latency communication in highly mobile environments.
• Supports data rates suitable for safety applications.
• Operates in the 5.9 GHz band.
• Enhances road safety and traffic efficiency through vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication.

Physical Layer (PHY) Modifications in IEEE 802.11p

The following changes were implemented in the Physical layer, building upon the 802.11a standard:

• Bandwidth: Supports a 10 MHz bandwidth, compared to the 20 MHz used in 802.11a.
• Bit Rates: Supports half the bit rates of 802.11a: 3, 4.5, 6, 9, 12, 18, 24, and 27 Mbps.
• Modulation: Retains the same modulation schemes as 802.11a: BPSK, QPSK, 16QAM, and 64QAM.
• Data Carriers: Uses 52 data carriers per symbol, the same as 802.11a.
• Symbol Duration: Doubles the symbol duration compared to 802.11a, using 8 µs instead of 4 µs.
• Guard Interval: Employs a guard time interval of 1.6 µs, while 802.11a uses 0.8 µs.

802.11p Protocol Stack (PHY, MAC)

Fig-2: 802.11p protocol stack

Figure 2 illustrates the complete DSRC/WAVE protocol stack. As depicted, it includes the IEEE 1609 standard and the IEEE 802.11p standard. IEEE 802.11p defines the PHY and MAC layers, while the upper layers are defined by IEEE 1609.

The layers are divided into a management plane and a data plane.

• Management Plane: Contains the WME (WAVE Management Entity), MLME (MAC Layer Management Entity), and PLME (Physical Layer Management Entity).
• Data Plane: Consists of WSMP, TCP, UDP, IPv6, LLC, MAC, and PHY layers.

Functions of Each Layer in the IEEE 802.11p Protocol Stack

• IEEE 1609.2: Defines security services for application messages and management messages in WAVE.
• IEEE 1609.3: Defines connection setup and management for WAVE-compliant devices.
• IEEE 1609.4: Sits atop the 802.11p layers and enables upper-layer operational aspects across multiple channels without requiring knowledge of Physical layer parameters.
• IEEE 802.11p PHY Layer: Handles modulation/demodulation, error correction techniques, and improvements to the power transmission mask. Supports 10 MHz bandwidth and enhanced receiver performance.
• IEEE 802.11p MAC Layer: Manages messages to establish and maintain connections in harsh vehicular environments. Defines signaling techniques and interface functions. Stations communicate directly without needing to communicate or join a BSS.

The frame structure, Physical layer modules, and MAC layer messages in 802.11p are similar to those in IEEE 802.11a.