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New Contributor II

908e SIP to PRI out of order packets

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on sunday we had one way audio problems on calls.

we use a TA to convert a CenturyLink SIP to a PRI for our Shoretel phone system

the TA 908e had number other than 0 in the out of order column on the calls with the problems.

what does "out of order" mean?

log copied from  call quality stats in the 908e (phone numbers obfuscated):       

Start timefromtolengthcodecout of orderdiscardsavg delaymax delay
10/12/2014 11:02541224****503228****0:35G.729508071295.75100
10/12/2014 11:02503267****503228****0:52G.72928655117190
10/12/2014 11:04541224****503228****0:28G.729307511297.69100

the problem was resolved when CenturyLink reset the SIP on their end.

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Re: 908e SIP to PRI out of order packets

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When voice is digitized for VoIP, the continuous analog waveform is analyzed and broken up into chunks that become  data packets. Each chunk is typically 20 milliseconds long (50 packets per second). These packets are referred to as RTP (real-time protocol). 

When a person speaks, these packets are sent out in a continuous stream one after the other, one packet per 20 milliseconds.

On the receiving end, the packets are expected to arrive in order, where they are converted back into a continuous analog stream and delivered to the earpiece in the receiver's handset.

Packets traversing the Internet can be delayed randomly for various reasons, or lost completely.  There is a "jitter buffer" in the receiving software that inserts a bit of delay so that the stream to the receiver is at the 20-millisecond rate even if the inbound pacing is slightly off after traversing the Internet. Likewise, if a packet is lost, the jitter buffer substitutes a similar sound based on the previous and following packets so that the person doesn't notice the dropout.

If the jitter (difference in transport delay between packets) is extreme, it is possible for a packet to be delayed for more than the 20-millisecond inter-packet delay and arrive after the next "chunk" of speech. It would be very confusing to the person listening to the voice if a missing chunk of speech is re-inserted after subsequent chunks have been played.

The Adtran device keeps track of these packets arriving out of sequence, discards them due to the confusion they would cause if played, and counts them as errors. "Out of order" in this case means "out of sequence" as opposed to "not functioning".

The cause of out-of-order RTP packets is typically improperly configured load-balancing between two or more different bandwidth links somewhere along the Internet transport path.

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Honored Contributor
Honored Contributor

Re: 908e SIP to PRI out of order packets

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When voice is digitized for VoIP, the continuous analog waveform is analyzed and broken up into chunks that become  data packets. Each chunk is typically 20 milliseconds long (50 packets per second). These packets are referred to as RTP (real-time protocol). 

When a person speaks, these packets are sent out in a continuous stream one after the other, one packet per 20 milliseconds.

On the receiving end, the packets are expected to arrive in order, where they are converted back into a continuous analog stream and delivered to the earpiece in the receiver's handset.

Packets traversing the Internet can be delayed randomly for various reasons, or lost completely.  There is a "jitter buffer" in the receiving software that inserts a bit of delay so that the stream to the receiver is at the 20-millisecond rate even if the inbound pacing is slightly off after traversing the Internet. Likewise, if a packet is lost, the jitter buffer substitutes a similar sound based on the previous and following packets so that the person doesn't notice the dropout.

If the jitter (difference in transport delay between packets) is extreme, it is possible for a packet to be delayed for more than the 20-millisecond inter-packet delay and arrive after the next "chunk" of speech. It would be very confusing to the person listening to the voice if a missing chunk of speech is re-inserted after subsequent chunks have been played.

The Adtran device keeps track of these packets arriving out of sequence, discards them due to the confusion they would cause if played, and counts them as errors. "Out of order" in this case means "out of sequence" as opposed to "not functioning".

The cause of out-of-order RTP packets is typically improperly configured load-balancing between two or more different bandwidth links somewhere along the Internet transport path.

View solution in original post

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New Contributor II

Re: 908e SIP to PRI out of order packets

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do you know of a way i can monitor for large numbers of out of order packets? so i can setup an alert with cacti or a SNMP trap...

i need to know when this happens so i can get the carrier to bounce the SIP circuit.

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