GatesAir Intraplex NetXpress, Installation And Operation Manual

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Intraplex NetXpress IP Multiplexer 2—Functional Design
Version 3.05
2-8 GatesAir, Inc.
Intraplex Products
Another factor in determining the overhead associated with a stream is the number of time slots from
each TDM frame that get placed in the packet. As the number of time slots from each frame increases,
the stream overhead decreases because the ratio of overhead data to program data decreases.
However, the number of time slots consumed from each frame does not affect the program delay.
Therefore, it is desirable to combine time slots from a TDM bus headed for the same destination into a
single stream to reduce overhead and to make the best use of available network bandwidth.
2.2.10 Packet Jitter Compensation
In an IP network, the time required for a packet to travel through the network from sender to receiver
is not guaranteed to be maintained. As a result, the receiver can see the interval between the arrival
of packets vary throughout the reception of the packet stream. For a given packet, the difference
between the packet’s delay and the average of the delay values for all packets in the stream is known
as
jitter . The jitter can be positive or negative depending on whether the packet delay is less than or
greater than the average delay.
Many times packet jitter is not an issue, and no technique is needed to compensate. However, the
continuous playout nature of audio and video require that there must be some means of guaranteeing
that the receiver always has the next packet of data when the previous packet is consumed. The
receiver must also hold onto packets that arrive early so that they are available for playout at the
proper time.
The NetXpress system compensates for stream packet jitter through the use of a jitter buffer . The
system maintains a jitter buffer for each stream on the receive side. The buffer is sized so that the
buffering delay is greater than the maximum packet delay expected through the network. In the
NetXpress system, you specify the jitter buffer size in number of packets ranging from 8 to 128
(Section 4.4.5.1 –
Stream Creation ). The packetization interval of the stream controls the amount of
packet delay for which the jitter buffer can compensate. Increasing the number of TDM frames
contained in a packet increases the size of the jitter buffer as measured in time.
2.2.11 Packet/Stream Information Calculation
Table 2-1 represents an example of user and computed parameters for a NetXpress stream. The user
parameters represent user defined stream parameters. The computed parameters are for packet
information and network bandwidth based upon the user parameters. The table gives formulas for how
to derive computed parameters for a stream.