Expanding a baseline network using OPNET

Thesis

We present a baseline software model of the IEEE 802.11a wireless local area network (WLAN) protocol developed using OPNET Modeler. A complete change to the 802.11a physical (PHY) layer operation compared to previous versions of the 802.11 protocol significantly impacts the operation of the medium access control (MAC) layer. This model addresses both the MAC and PHY layers of the standard. The model offers improved fidelity and flexibility over previous WLAN archetypes. An infrastructure WLAN implementation of the baseline model is simulated under a variety of standard network conditions to verify its operational characteristics and the results are presented.

Introduction

To support the consumer demand for mobility and low latency, high data rate communications, the next generation of WLAN standards has emerged. Foremost among these is the 802.11a addendum [1] to the IEEE 802.11 standard [2]. The 802.11a protocol specifies operation in the 5 GHz band, utilizes orthogonal frequency division multiplexing (OFDM) in the PHY layer and provides for data rates ranging from 6 to 54 Mbps [1]. These data rates are clearly capable of supporting high traffic applications in a mobile, multi-user WLAN environment.

The performance of the 802.11a protocol has not been extensively analyzed as it was only recently approved and there are no exiting commercial WLAN implementations utilizing OFDM in the PHY layer. The goal of this effort is to develop a model of the 802.11a protocol using the OPNET modeling tool that incorporates both the MAC and the PHY layers of the standard. The model can then be used for further research that specifically targets either the MAC layer or PHY layer of the protocol or concerns the performance of 802.11a as a whole.

There are a number of possible approaches to modeling 802.11a in OPNET, especially at the PHY layer. The presented model incorporates features of the 802.11a standard that were developed in OPNET by Dr. Sunghyun Choi of Philips Research Labs [3], and it comprehensively models both the medium access control (MAC) and the physical (PHY) layers of the 802.11a protocol.

The IEEE 802.11a Protocol

The standardization scope of the 802.11 protocol family (i.e., the original 802.11 standard and the 802.11a and 802.11b addendums) includes both the MAC and the PHY layers. With very minor differences the MAC layer of 802.11a is essentially identical to that of the original 802.11 standard. At the PHY layer 802.11a uses a convolutionally coded adaptation of OFDM for encoding and transmission called coded OFDM (COFDM). COFDM is a frequency division multiplexed (FDM) multi-carrier communications scheme that includes the application of convolutional coding to achieve higher raw throughput rates. The 802.11a standard operates in the 5 GHz band thereby avoiding the crowded 2.4 GHz band.

802.11a MAC: The 802.11a MAC supports data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps. Of those, the 6, 12, and 24 Mbps speeds comprise the mandatory rate set. The 802.11 MAC is designed to operate in one of two general network architectures: the infrastructure basic service set (BSS) or ...