Find out the Phenomenon that is typical for IP-telephony – Jitter

Find out the Phenomenon that is typical for IP-telephony – Jitter

Jitter Buffer

To compensate for the uneven packet rate, a temporary packet store is created on the receiving side, or a so-called jitter buffer. Its task is to collect the incoming packets in the correct order in accordance with the timestamps and issue them to the codec at the correct intervals and in the correct order.

The receiving VOIP device calculates the size of the buffer during operation, or forcedly set in the settings. On the one hand, it can not be too large not to increase the transport delay. On the other hand, a small buffer size causes packet loss when the delay time in the IP network changes.

Hence, one of the main contradictions occurs between Internet providers and users of IP telephony. From the provider’s point of view, all packets are delivered to the subscriber, so there are no losses. But from the point of view of the VoIP device, the time difference between the arrival of packets significantly exceeds the jitter buffer. Therefore, in fact, there is a loss. In practice, a loss of more than 1% causes certain unpleasant sensations. At 2%, the conversation is difficult. At values greater than 4%, the conversation is almost impossible.

Size of the jitter buffer

The random propagation delay Ji for the i-th packet can be determined by the formula:

Where:

Di – deviation from the expected arrival time of the i-th package.

The deviation from the expected arrival time of the i-th package Di is determined by the formula:

Where:

R is the arrival time of the packet in the RTP time stamps,

S – time stamp RTP, taken from the packet.

Let’s give an example of calculating the expected size of the random delay of propagation of the 5th package, based on the two previous ones.

Let J4 = 10 ms; R4 = 10, R3 = 11, S4 = 6, S3 = 5, then D5 will be equal to (10-11) – (6-5) = – 2.

On average, the random propagation time delay for one packet in the current example is 10 ms (more precisely, you can calculate it using the formula given above). Then, in order no packets be discarded, the size of the jitter buffer should be 10 ms.

To determine the required jitter buffer size in megabytes, one should multiply the obtained value by 100 mb / s – the average network capacity: 10 • 10 ^ -3 • 100 = 128 kb.

The size of jitter buffer should be greater than the fluctuation of transit time in the network. For example, if for 10 packets the transit time varies from 5 to 10 ms, then the buffer must be at least 8 ms, so that no packet is lost. Better, if the buffer is even larger, for example, 12 ms, then the mechanism for re-querying the lost packets will work.

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Posted on: May 11, 2017Ana Nichols