Raytheon SNV-12, Инструкция по установке и эксплуатации

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SNV-12 Operations Manual
3-17
The addition of VoIP devices carries many additional timing problems. Again remember that
an analog voter needs near-identical timing on all paths between voting receivers & the voter.
Typical VoIP communications do not share this concern and hence do not have methods to
maintain the relative timing between the various paths. The timing issues include:
Jitter buffer variations: If the jitter buffers are not identical in each path, audio begins
streaming at different times to different sites (and as will be explained later, there are several
ways that the timing of the buffers can be made to vary).
Lack of coincidence of control signals: Some VoIP devices allow a COR and or PTT signal to
be embedded in the data packets. Unfortunately, the operating systems of the VoIP devices
introduce an inherent granularity in how perfectly these signals are synchronized with the
audio. These delay variations can be so great that one voting site may send a COR signal, and
have the voter act on that COR indication and vote this site, before another site has even sent
the active COR indication to the voter.
Variations in VoIP equipment clocks: All VoIP timing is dependant on internal clocks. These
clocks are not precisely identical (there‟s no need for synchronization in typical VoIP
communications). This means that that overall timing of packets varies between the sending
unit (at the voting receiver) and the receiving unit (at the voter location), and eventually
(typically within hours) this will add up to a sufficient total delay to cause the receiving unit‟s
jitter buffer to either empty or overflow. This will occur, for example, if continually streaming
audio is used to send pilot tone. At this time, the voter will experience either a gap in audio (if
the jitter buffer had emptied and is being refilled) or a sudden jump ahead in the audio content
(as the jitter buffer “dumps” excess packets). Both of these are problematic to throughput
audio at the moment they occur, but this process also creates, during and afterwards, a
constantly changing throughput delay variation that builds up to the duration of the jitter buffer
before recycling. These potential delay variations are far too long to allow proper voting
operation.
There are four main voting performance issues related to improper relative timing between the
receipt of audio & control signals (COR) from the remote RX sites:
The voter can only vote sites that are declared unsquelched – if COR or pilot tone
arrives out of sync between the various sites, the voter may have already selected a poor
quality site before a better site has declared itself unsquelched and eligible for voting.
An unsquelched site with no audio (due to a gap in packet reception or during the filling
of an empty jitter buffer) looks like clean, fully quieted signal and will be voted.
Variations in audio timing between sites results in gaps and echoes if the voting process
calls for a transition between sites during a field transmission.
Variations in audio timing can result in improper voting because the voter is not
comparing receiver signals at the same relative times. Analog voters use the quantity of
total high frequency spectral content as a measurement of noise. There is some speech
content included in these high frequency content measurements. This doesn‟t affect
non-IP linked voting systems because the same speech content is found in all sites at the
same time.
The site-to-site audio delay variations can be no greater than approximately 30 ms to insure
proper voting performance. This degree of time-synchronization is extremely difficult to
achieve between multiple IP/VoIP paths.