KitPlus - The TV-Bay Magazine

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WHAT CAN BE THE IMPACT OF EXCESSIVE JITTER? The rate of packets flowing out of the de-jitter buffer is known as the “drain rate”. The rate at which the buffer receives data is known as the “fill rate”. If the buffer size is too small then if the drain rate exceeds the fill rate, then it will eventually underflow, resulting in stalled packet flow. If the sink rate exceeds the drain rate, then eventually the buffer will overflow, resulting in packet loss. However, if the buffer size is too large, then the network element will introduce excessive latency. A series of packets with long inter-arrival intervals, will inevitably result in a corresponding burst of packets with short inter-arrival intervals. It is this burst of traffic, that can result in buffer overflow conditions and lost packets. This occurs if the sink rate exceeds the drain rate for a period of time that exceeds the length of the remaining buffer size, when represented in microseconds. sink rate > drain rate for ∆T > remaining temporal buffer size HOW DO YOU ESTABLISH THE DE-JITTER BUFFER SIZE? HOW DO YOU MEASURE IP PACKET JITTER? Jitter is measured by plotting the time-stamps of the packet inter- arrival times versus time. To establish the necessary de-jitter buffer size, an alternative form of jitter measurement known as Delay Factor (DF) is used. This is a temporal measurement indicating the temporal buffer size necessary to de-jitter the traffic. In IP Video networks, the media payload is transported over RTP (Real Time Protocol). One form of DF measurement takes advantage of the fact that the RTP header carries timestamp information which reflects the sampling instant of the RTP data packet. This is known as Time-Stamped Delay Factor or TS-DF (as defined by EBU Tech 3337). Packet inter-arrival intervals plotted versus time. This is useful to identify variances in jitter over time, but it is also useful to be able to plot the distribution of inter-arrival intervals versus frequency of occurrence as a histogram. If the jitter value is so large that it causes packets to be received out of the range of the de-jitter buffer, then the out-of-range packets are dropped. Being able to identify outliers is an aid in identifying if the network jitter performance is either likely to or already the cause of packet loss. TS-DF represents temporal buffer size in microseconds. The TS-DF measurement is based on the relative transit time, which is the difference between a packet’s RTP timestamp and the receiver’s clock at the time of arrival, measured in microseconds. The measurement period is 1 second, with the first packet at the start of the measurement period being considered to have no jitter and is used as a reference packet. For each subsequent packet, the relative transit time between this packet and the reference packet is calculated and at the end of the measurement period, the maximum and minimum values are extracted and the Time-Stamped Delay Factor is calculated as: TS-DF = D(Max) – D(Min) Packet inter-arrival intervals plotted versus frequency of occurrence. TThe maximum value of TS-DF over a given period is indicative of the de-jitter buffer size required during that period, for a receiving device at that network node. KITPLUS - THE TV-BAY MAGAZINE: ISSUE 119 NOVEMBER 2016 | 51