Đang chuẩn bị liên kết để tải về tài liệu:
Scalable voip mobility intedration and deployment- P8

Scalable voip mobility intedration and deployment- P8: The term voice mobility can mean a number of different things to different people. Two words that can be quite trendy by themselves, but stuck together as if forgotten at a bus station long past the last ride of the night, the phrase rings a number of different, and at times discordant, bells. | Elements of Voice Quality 69 3.2.4 Jitter Jitter is the variation in delays that the receiver experiences. Jitter is a nuisance that the user does not hear directly because the phones employ a jitter buffer to correct for any delays. Jitter can be defined in a number of ways. One way is to use the standard deviation or maximum deviation around the mean delay per packet. Another way is to use the known arrival intervals such as 20ms and subtract consecutive delays of packets that were not lost from the known arrival time then take the standard deviation or the maximum deviation. Either way the jitter measured in times or percentages against the mean tells how variable the network is. Jitter is introduced by variable queuing delays within network equipment. Phones and PBXs are well known for having very regular transmission intervals. However the intervening network may have variable traffic. As the queue depths change and the network loads fluctuate and as contention-based media such as Wi-Fi links clog with density packets are forced to wait. Wireless networks are the biggest culprit for introducing delay into an enterprise private network. This is because wireless packets can be lost and retransmitted and the time it takes to retransmit a packet can usually be measured in units of a millisecond. A jitter buffer s job is to sit on the receiver and prevent the jitter from causing an underrun of the voice decoder. An underrun is an awkward period of silence that happens when the phone has finished playing the previous packet and needs another packet to play but one has not yet arrived. These underruns count as a form of error or loss even if every packet does make it to the receiver and loss concealment will work to disguise them. The problem with jitter becomes that an underrun must be followed by an increase in delay of the same amount assuming no packets are lost. This can be seen by realizing that the delayed packet will hold up the line for packets behind it. .