Digital Signal Processing Reference
In-Depth Information
The WLAN environment is quite challenging on two counts: the wireless link
is inherently noisy, due to fading and interference; the contention-based medium
access control (MAC) layer and the retransmission-based error-control scheme
may introduce strong delays as well as packet losses.
Efficient WLAN-based telephony systems must thus be designed carefully
to overcome the difficulties of the environment if toll quality service is to be
delivered.
2.1 IEEE 802.11 Wireless LAN's
Users may conveniently access the Internet via Wireless LAN technology.
Bridging functionality is provided by access points that interconnect wireless
nodes to the wired infrastructure, i.e. IEEE 802.11 networks in infrastructure
mode. The IEEE 802.11b physical layer —which operates in the license free
2.4 GHz ISM (Industrial, Scientific, and Medical) band— implements a Direct
Sequence Spread Spectrum (DSSS) system with an 11 Mbps top bit-rate. The
MAC sublayer is responsible for the channel allocation procedures, frame for-
matting, error checking, fragmentation and reassembly. The fundamental trans-
mission medium defined to support asynchronous data transfer on a best effort
basis is called Distributed Coordination Function (DCF). It operates in a con-
tention mode requiring all stations to contend for access to the channel for each
packet transmitted. Contention services promote fair access to the channel for
all stations.
In the IEEE 802.11 MAC, each data-type frame consists of the following ba-
sic components: a MAC header, a variable length information frame body, and
a frame check sequence. All fields except the frame body (28 bytes in total)
contribute to the MAC protocol data unit (MPDU) overhead for a data frame.
Upon packet transmission the destination station positively acknowledges each
successfully received packet by sending an ACK frame back to the source sta-
tion. When an ACK is not received, the source station contends again for the
channel to transmit the unacknowledged packet and, in case of further errors,
retries until a maximum retry limit is reached.
2.2 The GSM AMR Speech Coding Standard
The GSM Adaptive Multi-Rate (AMR) standard [10] is a state-of-the-art
network-driven variable-bitrate speech coder. Its operating bit-rate can be cho-
sen on a frame-by-frame basis to match the instantaneous channel conditions. In
the case of cellular telephony, the objective is to change the ratio between band-
width devoted to speech and bandwidth devoted to forward error correction. For
