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Method for a voice over packet jitter buffer Disclosure Number: IPCOM000008124D
Publication Date: 2002-May-20
Document File: 6 page(s) / 133K

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Disclosed is a method for a voice over packet (VoP) jitter buffer. Benefits include improved functionality and improved voice quality.

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Method for a voice over packet jitter buffer


Disclosed is a method for a voice over packet (VoP) jitter buffer. Benefits include improved functionality and improved voice quality.


An application of the IXP225 implementation of voice-over AAL2 (VoAAL2) in a network can include several components (see Figure 1):

·        IXP225 customer premises equipment (CPE) reference platform

·        Digital subscriber line access multiplexer (DSLAM)

·        Asynchronous transfer mode (ATM) switch

·        Voice gateway

·        Public switched telecommunications/telephone network (PSTN)

Connections for an application can include SONET or SDH, AAL2 over DSL, and time division multiplexing (TDM) trunk lines.

Three phenomena that occur in packet networks are delay, bursts and jitter. Delay is one characteristic where multimedia applications, such as voice, differ from data applications. Human perception is well adapted to extrapolate missing portions of a pattern, such as a voice conversation. As a result, data loss can be acceptable for multimedia. However, excessive delay interferes with the flow of conversation and becomes an annoyance over 250-300 ms (depending on the listener). Industry standards specify that no single device can inject more than 25 ms of delay into the chain.

Bursts are caused when a previously congested network node becomes decongested. This enables the backlog of traffic to flow again. However, the packets become bunched together, so the inter-arrival time is below expectations at the terminating equipment. A burst period is usually followed by an equivalent quiet period, during which no packets arrive.

Jitter is a variation in the inter-arrival time with a frequency of greater than 10 Hz. Jitter is an artifact of bursts. Over long periods, the bit rate of the connection approaches the ideal. Therefore, if there are short periods when the bit rate is higher (such as during a burst), comparable periods also occur when the bit rate is lower. Such cycles in rapid succession are called jitter and are a normal characteristic of VoP networks. The primary cause of jitter is packet buffering at switching or routing points within the packet network.

Jitter is conventionally handled with a jitter buffer (see Figure 2). It is a FIFO data structure. A jitter buffer for each voice channel is typically used when converting from packet to time division multiplexing (TDM) transmission. It pulls a voice packet from the head of the queue (P1) every n ms, where n is the duration of the voice sample contained in a voice packet. Subsequent packets are enqueued at the tail of the queue (P4). Packet overruns and underrunsshould be managed by a jitter buffer.

An overrun occurs when so many extra packets arrive that the jitter buffer fills. The IXP225 software is a real-time system, so fixed-length jitter buffers are used. They are susceptible to overrun, which results in some data being discarded. An objective of a jitter buffer mechanism is to drop as few packets as possible. Dr...