HP Survivable Branch Communication zl Module powered by Microsoft Lync Planning and Design Guide 2011-02

2-13
Design Considerations
Planning Call Capacity
■ The overhead added by headers—The larger the headers, the greater the
bandwidth requirements. The maximum requirements are calculated for
Ethernet headers, which are larger than, for example, PPP headers.
■ The sample size (in milliseconds) used by the codec—The smaller the
sample size, the better the call quality but the higher the bandwidth
requirement (because more VoIP frames are sent per second, the over-
head added by headers has a greater effect).
■ Whether forward error correction (FEC) is used—FEC adds redundant
data from previous samples to the current sample in case a packet is lost.
Because FEC adds several bytes to the VoIP packet, it increases the
bandwidth requirements.
Table 2-10 displays Microsoft’s recommendations for bandwidth required on
the WAN link (from the Microsoft guide for planning Lync Enterprise Voice).
Table 2-10. WAN Bandwidth Requirements
You must plan enough bandwidth for each call; on the other hand, unless you
want to ensure that calls always use FEC and have the highest quality, you do
not need to plan for the absolute maximum bandwidth for each call. For
example, you might choose to provision your lines for RTAudio narrowband
peer-to-peer calls rather than wideband calls. Looking at the requirements for
that codec, you might plan for the typical requirements plus 25 percent for
safety, or you might plan for the maximum requirements for narrowband.
Consider how important the absolute highest quality is for your company and
decide the appropriate bandwidth between the typical and maximum band-
width requirements.
Call Type Encoding Typic al
Bandwidth
Maximum
Bandwidth
without FEC
Maximum
Bandwidth
with FEC
Remote peer-
to-peer
RTAudio
narrowband
(8 kHz)
25.9 Kbps 39.8 Kbps 51.6
RTAudio
wideband
(16 kHz)
34.8 Kbps 57 Kbps 86
Remote PSTN RTAudio
narrowband
(8 kHz)
25.9 Kbps 39.8 Kbps 51.6
G.711, A-law or
mu-law (20 ms
frame sample)
59.8 Kbps 92 Kbps 156 Kbps