2.2 Programming

Required Bandwidth for Voice Communication via WAN (PPP: Point-to-Point Protocol)

CODEC

 

Packet Sending Interval

 

 

 

 

 

 

20 ms

30 ms

40 ms

60 ms

90 ms

 

 

 

 

 

 

 

G.711

84 kbps

77.3 kbps

74 kbps

70.7 kbps

—

 

 

 

 

 

 

G.729A

28 kbps

21 kbps

18 kbps

14.7 kbps

—

 

 

 

 

 

 

G.723.1 5.3 kbps

—

18.7 kbps

—

12 kbps

9.8 kbps

 

 

 

 

 

 

G.723.1 6.3 kbps

—

19.7 kbps

—

13.1 kbps

10.8 kbps

 

 

 

 

 

 

When assessing your bandwidth requirements, keep in mind that the longer the packet sending interval, the smaller the amount of required bandwidth, and vice versa.

However, also consider that the shorter the packet sending interval, the clearer the expected speech quality, because delays in packet transmissions will be small. When the packet sending interval is long, delays are more likely to occur, resulting in overall degradation in speech quality with more pauses and loss in voice communications.

Therefore, it is recommended that you select the shortest packet sending interval that network bandwidth can accommodate.

DTMF Detection

A VoIP network does not guarantee accurate end-to-end transmission of DTMF signals because the DTMF signals are coded/decoded during VoIP communications, in the same way as voice signals. In addition, packets can get lost during transmission.

To compensate for this problem, it is possible to enable DTMF detection for the VoIP Gateway Card to carry out accurate end-to-end DTMF relay over the network. Upon detecting DTMF signals from the PBX, the card encodes the signals and then sends them to the destination, instead of as voice signals. Then at the destination, the card regenerates the DTMF signals from the received encoded signals, and then sends them to the PBX.

Note that when this feature is enabled, the sending of packets is delayed by approximately 30 ms. Therefore, it is recommended that you disable this feature unless DTMF detection is necessary.

FAX Signal Detection

When sending fax signals using a CODEC other than G.711, the signals cannot be received accurately at the destination because they are coded/decoded over the VoIP network, in the same way as voice signals.

To compensate for this problem, it is possible to enable fax detection for the card. Upon detecting fax signals (CED tones) from the PBX, the card automatically switches the CODEC to G.711 to communicate with the card at the destination. With the G.711 CODEC, it is possible to assure error- free fax communications to a certain extent.

To further assure fax communications, it is strongly recommended that the communicating fax machines be equipped with the ECM (Error Correction Model) feature, an automatic error correction feature. When, for example, the receiving fax machine detects errors in transmission, it can have the sending fax machine resend the relevant data.

When using the fax detection feature, the communicating cards must share the same value (either "G.711Mu" or "G.711A") for the parameter Voice CODEC Priority (see "Voice CODEC Settings" in "2.2.2 H.323 Parameters").

Notes

•To carry out fax communications between the KX-TDA0490 and KX-TDA5480/KX-TDA0484 VoIP Gateway Cards, it is necessary to disable the "FAX High Reliable Method" for the KX-

24 Programming Guide