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Voice Feature Guide 01

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Fax/Modem Quality Enhancement

Fax/Modem Quality Enhancement


After the IP network takes the place of the PSTN network, the use of fax and modem on the VoIP network becomes more and more popular. Therefore, the AG is required to provide applications similar to those of the PSTN network. Currently, the Voice Band Data (VBD) transparent transmission is adopted by the medium gateway (MG) in the application of the fax and modem. Transparent transmission, however, relies heavily on the bearer network and deterioration of the network quality may lead to service failures.

The fax/modem quality enhancement feature consists of the RFC2198 intelligent startup function and the packetization at the interval of 10 ms. After the fax/modem quality enhancement feature is enabled, the RFC2198 function and the packetization at the interval of 10 ms are automatically started. The following table provides more details.




Packet loss

RFC 2198

The RFC 2198 standard uses the data stream redundancy mechanism to prevent the packet loss of the network from degrading the service quality. When the average consecutive packet loss ratio is low, the receiver can reassemble and restore the lost packet based on the redundant packets in the later received packets. The audio redundancy mechanism described in RF2198 can be used to restore the events lost in the packets, while the mode described in RFC2833 can be used to process the DTMF signals transmitted through the RTP packets.

Network delay

10-ms packetization

Information carried by packets assembled every 10 ms is less the information carried by packets assembled every 20 ms. Therefore, in case of packet loss, the packetization at the interval of 10 ms causes less impact on services that the packetization at the interval of 20 ms. Using this technology, the device automatically detects fax and modem signals and switches the 20-ms packetization interval (intended for voice services) to the 10-ms packetization interval, thereby reducing the network delay of fax and modem data transmission.

The enhanced quality feature of the fax and modem is mainly used to improve the put-through rate and online duration of the fax and modem services. For example, if POS terminals are connected to a modem in a shopping mall or a bank, the fax/modem quality enhancement feature can be used to improve the stability and online duration of the Modem, thus preventing the disconnection caused by the poor network quality, as shown in Figure 1-135.
Figure 1-135 Enhanced Modem

Table 1-16 lists the test data recorded before and after the quality enhancement feature is enabled. The data shows that stability of services improves substantially after the quality enhancement feature is enabled.

Table 1-16 Test data before and after the quality enhancement



Network Packet Loss Ratio (Random)

Online Duration


20 ms


22 hours


1.5 hours



1.00% (rfc2198)

12.5 hours

10 ms


> 24 hours


22.5 hours


5.5 hours

1.00% (rfc2198)

24 hours


The fax/modem quality enhancement is supported.

Updated: 2019-02-22

Document ID: EDOC1100067358

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