Digital Signal Processing Reference
In-Depth Information
Chapter 15
LINK-ADAPTIVE VARIABLE BIT-RATE SPEECH
TRANSMISSION OVER 802.11 WIRELESS LANS
*
Antonio Servetti
1
and Juan Carlos De Martin
2
1
2
Dipartimento di Automatica e Informatica;
IEIIT-CNR Politecnico di Torino, Corso Duca degli
Abruzzi, 24
—
I-10129 Torino, Italy
Email: demartin@polito.it
Abstract
We present an adaptive technique to transmit speech over 802.11 wireless packet
networks. According to the proposed scheme, the speech coding rate of a network-
driven variable bit-rate coder is selected to match the instantaneous wireless chan-
nel conditions: higher rates (i.e., larger packets) for low error rates, lower rates
(i.e., smaller packets) when the channel is noisy. Packet size is, in fact, directly
related to the probability of retransmission, one of the major sources of delay and
losses in contention-based medium access control. Network simulations using
the 3GPP GSM-AMR speech coding standard show that the adaptive approach
can address the stringent quality of service requirements for two-way interactive
speech applications over wireless packet networks, reducing packet loss rates and
end-to-end delays.
Keywords:
wireless network, 802.11, voice over IP, adaptive speech, GSM AMR, link adap-
tation, adaptive packet size
1. INTRODUCTION
Wireless technology keeps changing the communications scenario. Wireless
local area networks (WLAN's), in particular, are being enthusiastically adopted
by users worldwide, shaping a new world where tetherless access will be pos-
sible not only in homes and offices, but also in an increasing number of previ-
*
This work was supported in part by CERCOM, the Center for Wireless Multimedia Communications,
Torino, Italy, http://www.cercom.polito.it.
