Digital Signal Processing Reference
In-Depth Information
mechanisms, and the performance they obtained. In [9] and [11] the mesh struc-
ture of an ad hoc network is exploited to allow multiple paths between a source
and a destination, thus improving reliability of video transmissions. To further
enhance error recovery, an ARQ technique is applied. Also, in [9] the authors
explore the possibility to employ layered coding as well as multiple description
coding. However, none of these works considers an 802.11b ad hoc scenario
supporting both real-time video and data traffic, and investigates the effects of
interfering traffic.
The objective of our work is to study the transfer of video sequences over
wireless ad hoc networks using the 802.11b technology, and globally optimize
the parameters involved in a real-time video transmission, ranging from video
encoding and packetization to the MAC interface parameters. Moreover, we
evaluate the possibility to provide good quality real-time video under multi-hop
network scenarios.
We consider the state-of-the-art ITU-T H.264 [8] video encoder and config-
ure it to optimally match the ad hoc network scenario, as well as to adapt to
varying channel conditions. Standard video test sequences are packetized ac-
cording to the H.264 Network Adaptation Layer (NAL) for transmission using
the RTP/UDP/IP protocol stack. Error resilience tools provided by the H.264
standard are also configured and adapted to the characteristics of the 802.11b
wireless medium. We consider the presence of interfering data traffic carried by
TCP connections. The quality perceived by the video user at the receiver is ob-
jectively evaluated, using the PSNR as a distortion measure. By means of the
ns
[15] network simulator, we simulate several network conditions, which include
various different channel conditions and different numbers of hops in the path
between source and destination.
2. IEEE 802.11b AD HOC NETWORKS
We consider an ad hoc network composed of stationary wireless stations,
using the IEEE 802.11b technology.
The 802.11b standard operates in the 2.4 GHz frequency bands, enabling
transmission rates ranging between 1 and 11 Mbps. IEEE 802.11 cards transmit
at a constant power, achieving a transmission range of up to hundreds of meters.
Two wireless stations are said to be within range and said to be neighbors of
each other if they can receive the each other's transmission. Every station can
employ multihop transmissions to transfer information toward its final destina-
tion; also, it always behaves in a cooperative fashion accepting to act as a router
and relay traffic destined to other stations. For instance, if station
needs to
