SpectraLink NetLink Wireless Telephones Best Practices White Paper Wireless Telephone - Manual (Page 6)

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Page 5
2.3 Capacity The network capacity requirements also factor into the number of APs
required, although in most cases the coverage area is the primary factor.
Data traffic is very bursty and sporadic, but data applications can tolerate
network congestion with reduced throughput and slower response times.
On the other hand, voice traffic cannot tolerate unpredictable delays, but
at least the bandwidth requirements are constant and consistent for
every phone call. Also, telephone traffic can be predicted using
probabilistic usage models, allowing a network to be designed with high
confidence in meeting anticipated voice capacity requirements. Beyond
the normal IP telephony design guidelines, there are several additional
considerations that need to be addressed for Wi-Fi telephony with
NetLink Wireless Telephones.
2.3.1 Access Point
Bandwidth
Considerations
There are several factors that determine the AP bandwidth utilization of
a telephone call. The first is the VoIP protocol used and its
characteristics. The type of codec utilized combined with the packet rate
will determine the size of the voice packets, along with any additional
overhead information required for the protocol. The payload information
makes up a little more than half of a typical voice packet, with 802.11
and IP protocol overhead filling the rest. The 802.11 protocols include
timing gaps for collision avoidance, which means bandwidth utilization is
more accurately quantified as a percentage rather than actual data
throughput. The percentage of bandwidth used increases for lower data
rates, but it is not a linear function because of the bandwidth consumed
by the timing gaps and overhead. For example, a call using standard 64
kb/s voice encoding (G.711) utilizes about 4.5% of the AP bandwidth at
11 Mb/s, and about 12% at 2 Mb/s. In this example, four simultaneous
calls on an AP would consume about 18% of the available bandwidth at
11 Mb/s or about 48% at 2 Mb/s.
The following table lists the theoretical percentage of available
bandwidth used per telephone call for each 802.11b data rate:
1 Mb/s 2 Mb/s 5.5 Mb/s 11 Mb/s
NetLink Telephony Gateway (24
kb/s), 20 ms sample rate
15.7% 10.0% 6.4% 5.4%
G.711 (64 kb/s), 30 ms sample rate 20.5% 11.7% 6.1% 4.5%
G.729 (8 kb/s), 30 ms sample rate 9.3% 6.1% 4.1% 3.5%
Theoretical Call Bandwidth Utilization of 802.11b Access Points