A frame handler module for a side-channel in mobile ad hoc networks

In this paper, we establish a hidden 802.11 wireless channel, with the masking of the channel achieved by inserting intentional errors in the frame check sequence (FCS). We design a frame handler module to provide a proof-of-concept model of the side-channel using MATLAB and Simulink with communication toolbox. We justify using MATLAB over the other simulation tools because of its existing functions: physical layer IEEE 802.11 wireless local area networking (WLAN) standard, existing modular channel fading models, the MAC layer cyclic redundancy checksum (CRC) generator, the CRC Syndrome detector, and the capability of modifying fields in a frame. These existing functions allow for the creation of a frame handler which generates frames, according to our design, to be inserted as erroneous frames and recovers frames from normal 802.11 traffic. Herein we provide the design and details of the implementation of the channel. Our design offers the ability to introduce error detection and correction capabilities, and protection against passive monitoring defences. This simulation framework is a step towards the development of more sophisticated environments including multi-node simulations that maintain robust and reliable side-channel communication.

I. INTRODUCTION

WIRELESS networks are designed to tolerate errors, measured by bit or frame error rates: BER (10⁻³ − 10⁻⁷) and FER (1 % − 3%). Errors are generally caused by fluctuations of the signal strength through the medium, known as fading and shadowing. These types of variations are inherent in a Mobile Ad hoc Network (MANET) environment, meaning that MANETs can be expected to have measurable FERs even when operating in seemingly ideal conditions. The Physical (PHY) and Medium Access Control (MAC) layers are designed, in almost all wireless protocols, to detect corrupted frames. Using a cyclic redundancy check (CRC) on the frame payload and appending information derived from this CRC into a Frame Check Sequence (FCS) field is a common technique for checking its integrity at the receiver. The CRC functions like a hash value of the payload contents so the receiver should be able to take the payload and verify that performing the same hashing operation yields the same value in the FCS field. ¹

In general, a CRC is not strictly a hash function but many can be implemented as such and the mapping principles are analogous.

If the values do not match, the frame is dropped. The handling of error differs among different wireless protocols. For example, military grade wireless protocols may be capable of a correcting certain level of error, whereas commercial wireless protocols, such as the IEEE 802.11 Wireless Local Area Networks (WLAN), discard the frame and ask/wait for retransmissions.