Digital Signal Processing Reference
In-Depth Information
Several challenges, however, need to be addressed to make WLAN telephony
as successful as cellular and wired telephony. Not only the actually available
bandwidth for WLAN's can be significantly below that of their wired coun-
terparts, but wireless links are also strongly time-varying and may have high
error rates. Other issues are specific of 802.11 WLAN's, including the MAC
layer effects on performance, the consequences of interfering data traffic, and
determining the optimal configurations for both Access-Point-based and ad-hoc
802.11 networks for a given application scenario.
Previous research evaluated the performance of interactive voice traffic over
Wireless LAN's [7][19], mainly by means of statistical analysis of throughput
and packet losses to assess the number of supportable voice conversations.
In this chapter we present a new technique for improving the quality of in-
teractive voice communications over 802.11 wireless packet networks. The op-
erating rate of a network-driven variable-bitrate speech coder is chosen on a
frame-by-frame basis according to the instantaneous channel conditions: higher
rates (i.e., longer packets) when the channel is good, lower rates (i.e., shorter
packets) when the channel is poor. Performance is measured in terms of aver-
age packet losses and average delay, with and without interfering traffic, using
a network simulator. The disadvantage of temporarily lowering the source cod-
ing quality is clearly offset by the advantages deriving from lower packet losses
and delays. The proposed system thus consistently outperforms constant-bitrate
speech transmission at the same average bit-rate.
The chapter is organized as follows. In Section 2, we introduce the wireless
Voice over IP scenario and the adaptive multi-rate speech coder. In Section 3,
we describe the proposed speech transmission scheme. Results and conclusions
are presented in Section 4 and 5, respectively.
2. VOICE OVER 802.11 WLAN'S
Voice over IP over wireless packet networks is becoming increasingly attrac-
tive. In particular, the widespread adoption of WLAN technology is creating the
basis for the introduction of a new form of cordless telephony in offices, homes,
hospitals, etc.
Two-way conversational applications, however, are characterized by stringent
requirements on the end-to-end delay. The upper limit for one-way delay is set
to only 150 ms, according to the guidelines of ITU-T Recommendation G.114.
Moreover, packet losses should be kept below 1% to prevent significant percep-
tual degradation.
