Specifications

6 General
General
Introduction to Wireless Planning
Adding Voice to a Wireless LAN
Data and voice traffic has different characteristics and thus put different requirements on the de-
sign of the WLAN network.
Data clients, like a laptop set up to use its wireless card for browsing the Internet, tries to use the
max packet size that is allowed to transport the relative high amount of data that modern web pag
-
es contain. It also uses TCP as its transport protocol and therefore the connection to the web serv-
er can withstand delays and loss of packets since the protocol is defined to overcome any glitches
in the transfer of data.
Voice clients, on the other hand, use a relative small packet size, but instead require regular ac-
cess to the radio channels because packets are generated in a steady stream. Since the voice
data packet is small, it is important that the overhead created by the protocols is as small as pos
-
sible. Using UDP instead of TCP reduces the overhead. The acknowledgements that are used in
the TCP protocol for every packet sent are also eliminated in the UDP protocol. Since UDP also
lacks other features that TCP has, an additional protocol is used, so packets can be sorted in the
right order and the voice recorded will be played back at the correct time. This protocol is RTP.
The following table illustrates the differences:
In short, the behaviour of the two traffic types - data and voice - make it difficult to design a WLAN
for mixed traffic. The demand they put on the WLANs design is nearly diametrical on every point.
Many current WLAN networks are used for data only and seem to be working just fine. Most users
do not notice that the WLAN may suffer of congestion, packet loss, and retransmissions etc. The
applications are tolerant against such errors and there is no information visible on a laptop about
the performance of the network. Slow loading of web pages are accepted and is blamed either on
the software or on the Internet and not on the WLAN. When adding VoWiFi to such a network those
problems will raise to the surface and be experienced as bad voice quality and will be blamed on
the handset.
Furthermore, the design problems gets even more complex if Wi-Fi RFID tagging and location traf-
fic is also using the WLAN, because it requires a completely different design.
The best solution to avoid these design problems is to use separation of traffic types, either phys-
ical or logical, so they do not interfere with each other.
Physical separation
Data transport Voice transport
Protocol: FTP, HTTP over TCP. RTP over UDP.
Packet size: Varies from small to large up to
max size depending on applica
-
tion.
Small
All the same size < 300 Bytes.
Sensible to lost packets: No. Uses built in recovery pro-
cess in TCP.
Yes. Will result in bad voice
quality.
Sensible for delays: No. Can stand delays of several
minutes.
Yes. Requires steady access to
the channel.
Sensible for disconnection: Not always. Session may be re-
stored where interrupted.
Call will be dropped.