Adaptive Codec Selection for VoIP in Multi-Rate WLANs

Voip Over Wlans

Voice over IP (VoIP) over wireless LANs (WLANs) has been a hot research topic during the past years due to the widespread deployment and ease-of-use of both technologies. The capacity of a wireless cell in terms of number of supported calls, as well as the quality of the voice transmission over the wireless link under different channel conditions, are crucial for deciding whether this technology can be widely deployed and accepted for voice service. In spite of all the research efforts in this area, there are still unsolved issues concerning the quality of VoIP calls, most commonly caused by the specific wireless network characteristics listed below (note that the purely physical effects, such as meteorological impact on channel conditions, are beyond the scope of this chapter):

• 1.

  Unfairness between uplink and downlink streams

• 2.

  High protocol layer overheads

• 3.

  Fast VoIP degradation in presence of TCP flows

• 4.

  Variable capacity due to multi-rate transmissions

Although the first three problems have been widely analyzed in previous works, there is scarce bibliography considering the last problem. Multi-rate transmission is one of the key features of the IEEE 802.11 PHY/MAC specifications, which allows each mobile node to select its physical layer parameters (modulation and channel coding) to optimize the bit transmission over the noise/fading-prone channel. These sporadic rate changes occurring on the mobile nodes as they move around the cell due to the link adaptation algorithm of the 802.11 specification have an impact on the transmissions of all active calls and produce a general degradation of the network performance.

This chapter intends to provide an overview of the impairments observed on voice flows due to the multi-rate characteristic of 802.11, and discuss some of the solutions that have been proposed so far, with particular focus on a codec adaptation solution. Main goals are:

• 1.

  To give some background on VoIP over WLAN systems and analyze the source of the various problems and how they affect the voice transmission in terms of total cell capacity and quality of service

• 2.

  To describe what is a multi-rate wireless LAN and its impact on VoIP calls

• 3.

  To review the literature on how to best cope with the multi-rate problem and introduce the benefits of a codec adaptation solution

• 4.

  To present a cross-layer codec adaptation algorithm and demonstrate how it is able to satisfy the QoS needs of VoIP traffic

• 5.

  To examine the suitability of proposing some future enhancements to the SIP architecture to better cope with this issue