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Product Series SDK 6.7.0 Release Notes 32

Appendix A - Fax Pass-through (G711 RTP) Network Design

Considerations

In spite of being similar in nature, voice and fax pass-through calls are affected differently by IP network impairments.

Because fax pass-through calls’ data cannot be altered during its transport, these calls are more susceptible to IP

problems than voice calls. Voice calls may experience some degradation from certain network impairments, and the

parties involved on the call might not even realize the degradation is occurring. In addition, there are mechanisms in

place for most compressed audio codecs such as predictive algorithms and packet loss concealment techniques that can

assist in masking many network problems. These techniques, however, do not protect fax pass-through transmissions.

Pass-through and T.38 fax calls may also respond differently to certain IP network impairments. The following table lists

specific impairments and descriptions of how each one may impact T.38 and pass-through calls.

Impairment

Definition

Description

Packet Loss

A relative measure of the

number of packets that

were not received compared

to the total number of

packets transmitted.

Fax pass-through calls are very sensitive to packet loss,

especially when carrying high-speed modem modulations.

Lab testing shows that as little as 0.02 percent packet loss can

cause pass-through calls to fail.

T.38 fax calls may use the protocol’s redundancy mechanism

to handle substantially more packet loss than pass-through. It

has been shown that T.38 calls can succeed with up to 10

percent random packet loss.

Delay

The ﬁnite amount of time it

takes a packet to reach the

receiving endpoint after

being transmitted from the

sending endpoint.

The recommendation for voice is to keep the one-way latency

(mouth-to-ear) to less than 150 ms. In the case of fax pass-

through and T.38 calls, delay is not typically as much of an

issue as it can be for voice.

Jitter

The delay variation

between packets or the

difference in the end-to-end

delay between packets.

Average one-way jitter of less than 30 ms is the

recommendation to ensure voice QoS. With T.38 and fax

pass-through, average jitter less than 30 ms is not quite as

critical.

Clock Skew

The running sum of the differences

between when packets actually

arrive at a destination and when

they were expected.

Synchronization issues between a voice gateway and an IP

endpoint are more critical for fax pass-through than for T.38

and voice.

When using the pass-through transport method for long fax

calls, there can be issues because of the lack of clock

synchronization between the DSPs on the voice gateway and

an IP endpoint. The gateway and endpoint use different clocks

therefore, a clocking discrepancy, ever so slight in some

cases, will always exist between the rates that packets are

generated and consumed. This slight clocking discrepancy can

cause playout buffer underrun/overrun on the voice gateway,

which can result in bad image lines or PPRs in Error

Correction Mode (ECM).

It should also be noted that Voice Activity Detection (VAD) and silence suppression should be disabled for fax pass-

through calls on gateways that do not already perform this action upon detection of fax signals. This is needed in order to

avoid fax signal clipping that can be caused by VAD algorithms that are used to suppress silence in voice calls.

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