Linksys SPA942, SPA962 Bandwidth Requirements, Setting System Features, Ensuring Voice Quality

Setting System Features

Ensuring Voice Quality

Codec (Voice	Complexity and Description	MOS Score
Compression
Algorithm)
G.726	Low complexity. Supports compressed 16, 24, 32, and 40 kbps	4.1 (32 kbps)
	digitized voice transmission at one through ten 10 ms voice
	frames per packet. When no static payload value is assigned per
	RFC 1890, SPA9x2 phones can support dynamic payloads for
	G.726
G.729 and	G.729A low-medium complexity. G.729 medium complexity.	4
G.729A	G.729A requires about half the processing power of G.729.
	The G.729 and G.729A bit streams are compatible and
	interoperable, but not identical.
G.723.1	High complexity. SPA9x2 phones support the use of ITU G.723.1	3.8
	audio codec at 6.4 kbps. Up to two channels of G.723.1 can be
	used simultaneously. For example, Line 1 and Line 2 can be using
	G.723.1 simultaneously, or Line 1 or Line 2 can initiate a three-
	way conference with both call legs using G.723.1.

Bandwidth Requirements

Depending on how you have your IP phones configured, each call requires 55 to 110 kbps in each direction. Therefore, using G.729 as the voice codec setting, and with an average business- grade broadband Internet connection supporting 1.5 Mbps downstream and 384 kbps upstream, a total of seven (7) simultaneous conversations can be reliably supported with adequate bandwidth available for file downloads.

Linksys recommends using the SPA9x2 with QoS-capable networking equipment that can prioritize the VoIP application traffic. QoS features are available on many Linksys data networking switches and routers. A QoS-enabled router prioritizes the packets going upstream to the ISP.

The following table approximates the bandwidth budget for each side of the conversation (in each direction) using different codecs and number of calls. This table is based on the following assumptions:

•Bandwidth calculated with no silence suppression

•20 millisecond of payload per RTP packet

Codec	Est. Bandwidth	2 Calls	4 Calls	6 Calls	8 Calls
	Budget
G.711	110 kbps	220 kbps	440 kbps	660 kbps	880 kbps
G.726-40	87 kbps	174 kbps	348 kbps	522 kbps	696 kbps
G.726-32	79 kbps	158 kbps	316 kbps	474 kbps	632 kbps
G.726-24	71 kbps	142 kbps	284 kbps	426 kbps	568 kbps

Linksys SPA9x2 Phone Administration Guide 38