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Application Notes for Wireless 802.11 Networks
The Toshiba SoftIPT™ works on wireless 802.11 networks. However, current access point technology
does not provide for the management of quality of service over the wireless network, which can result in a
few observable degradations in voice quality.
Our objective here is to describe these issues for the network engineer, and offer some recommendations.
As the technology evolves, and the knowledge base grows, other problems and solutions will emerge,
hence the network engineer is encouraged to search out other sources of information, from the specific
access point manufacturers or from experts in the 802.11 wireless industry.
Possible problems and their implications are:
1. Dropped packets may occur due to RF signal strength, especially as you get further away from the
access point. When small amounts of packet loss occur, you will notice a popping or clipping sound.
With larger amounts of packet loss, gaps in the conversation will occur. This is very similar to the
behavior of digital cell phones in the same situations. The best way to deal with this is to design the
wireless network for good coverage where it is needed.
2. Dropped packets may also occur due to excessive traffic on the wireless network. With multiple
devices accessing data through the same wireless network, current access points do not have a method
for prioritizing voice packets over data packets so when data applications create more traffic than the
wireless bandwidth can sustain, voice packets may get dropped. In addition, access points do not have
a mechanism for limiting the number of simultaneous sessions. In our lab we find that up to seven calls
can take place simultaneously on a single access point. This number will vary depending on the access
point, the RF environment, and any other activity in the wireless environment.
3. When one terminal is in a poor signal area for the access point that it is associated with, the access
point may automatically reduce its data rate in order to achieve better communications with that
terminal. This new data rate affects communications with all terminals, which will reduce the number
of simultaneous calls the access point can support. Network engineers should consider whether they
want to disable the Auto TX Data Rate Feature.
4. Disconnected calls will result if you move too far away from the access point, or generally are in a
poor signal strength area. With current 802.11 wireless technology, when a terminal can't
communicate with an access point, it will disassociate with it. This appears to the phone application as
a loss of the network, and the phone application disconnects. Even if you quickly move back into range
of an access point, it will take several seconds for the 802.11 terminal to detect and re-associate with
the access point.
5. Wireless networks can only get so large, before you have to break them up into multiple subnets.
Somewhere between 256 and 1000 nodes is where networks begin to need to be broken up into
multiple subnets. When multiple subnets are employed, the call will get dropped as the terminal gets
out of range of its subnet. But the terminal can reestablish its call after associating with the new subnet.
FYI: At Toshiba we are working on technology to address problems 2~4 for the SoftIPT. Check back with
us.
Installing and Operating the SoftIPT
Application Notes for Wireless 802.11 Networks
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