Digital Signal Processing Reference
In-Depth Information
Figure 15-3.
Protocol stack for IP-based real-time multimedia transmission over wireless
LAN's.
Figure 15-3 presents the protocol stack used for the wireless real-time mul-
timedia transmission. Every 20 ms the GSM AMR encoder produces a com-
pressed speech payload of 14 bytes for the 4.75 kb/s mode and of 32 bytes for the
12.2 kb/s mode. The payload is then encapsulated by the RTP protocol, UDP is
used for multiplexing different flows, and IP takes care of addressing and deliv-
ering the packets to their destination. For the RTP/UDP/IP headers, a compres-
sion scheme has been assumed that allows the 40-byte header to be compressed
in 4 bytes as defined by RFC 3095 for Robust Header Compression [17]. Each
data-type MPDU (MAC Protocol Data Unit) has a 24-byte header plus a 4-byte
frame check sequence. Finally the physical level adds additional 24 bytes for
the PLCP preamble, PLCP header, tail and pad bits.
4.1 Results
Simulations were performed for an 11 Mb/s IEEE 802.11 wireless LAN sce-
nario where an infrastructure network is populated by three mobile terminals
placed at the same distance from an access point (AP). The AP is then connected
to another host through a wired link. All the communications are directed from
the mobile stations to the wired node, and they are forwarded by the access
point. Because the wireless path represents only the first transmission hop, we
consider 20 ms as the maximum acceptable value for the one-way transfer delay
over the wireless link: the percentage of voice packets that are lost or received
with a delay greater than 20 ms is monitored on the access point.
Firstly, we tested the proposed adaptive solution against plain transmission of
a single VoIP source without interfering traffic. A wireless node sends a speech
