Cisco Systems Linksys PAP2, RT31P2 1.3. Voice Quality Overview

address/port to the corresponding private source address/port. These characteristics of a NAT can be

exploited by an PHONE ADAPTER to let external entities send SIP messages and RTP packets to it

when it is installed on a private network.

In the case of SIP, the addresses where messages/data should be sent to an PHONE ADAPTER are

embedded in the SIP messages sent by the device. If the PHONE ADAPTER is sitting behind a NAT,

the private IP address assigned to it is not usable for communications with the SIP entities outside the

private network. The PHONE ADAPTER must substitute the private IP address information with the

proper external IP address/port in the mapping chosen by the underlying NAT to communicate with a

particular public peer address/port. For this the PHONE ADAPTER needs to perform the following

tasks:

• Discover the NAT mappings used to communicate with the peer. This could be done with

the help of some external device. For example a server could be deployed on the external

network such that the server will respond to a special NAT-Mapping-Discovery request by

sending back a message to the source IP address/port of the request, where the message

will contain the source IP address/port of the original request. The PHONE ADAPTER can

send such a request when it first attempts to communicate with a SIP entity in the public

network and stores the mapping discovery results returned by the server.

• Communicate the NAT mapping information to the external SIP entities. If the entity is a

SIP Registrar, the information should be carried in the Contact header that overwrites the

private address/port information. If the entity is another SIP UA when establishing a call,

the information should be carried in the Contact header as well as in the SDP embedded in

SIP message bodies. The VIA header in outbound SIP requests might also need to be

substituted with the public address if the UAS relies on it to route back responses.

• Extend the discovered NAT mappings by sending keep-alive packets. Since the mapping is

only alive for short period, the PHONE ADAPTER continues to send periodic keep-alive

packets through the mapping to extend its validity as necessary.

1.3. Voice Quality Overview

Voice Quality perceived by the subscribers of the IP Telephony service should be indistinguishable

from that of the PSTN. Voice Quality can be measured with such methods as Perceptual Speech

Quality Measurement (PSQM) (1-5 – lower is better) and Mean Opinion Score (MOS) (1-5 – higher is

better).

The table below displays speech quality metrics associated with various audio compression

algorithms:

Algorithm Bandwidth Complexity MOS Score

G.711 64 kbps Very Low 4.5

G.726 16, 24, 32, 40 kbps Low 4.1 (32 kbps)

G.729a 8 kbps Low - Medium 4

G.729 8 kbps Medium 4

G.723.1 6.3, 5.3 kbps High 3.8

Please note: The PHONE ADAPTER supports all the above voice coding algorithms.