Packetization And Overhead; Packet Jitter Compensation; Packet/Stream Information Calculation - Harris CM-30 Installation & Operation Manual

NetXpress LX & CM-30 Installation & Operation Manual
Version 1, September 2010
or 16AE module) but not in 4-wire modules (for example, the VF-25E module). When using echo
cancellation, you should not configure streams to encapsulate more than 60 frames per packet.
2.2.6

Packetization and Overhead

Packetization is a process in which frames of data from the TDM bus are collected into IP packets for
transmission through the IP network. A frame on a NetXpress LX TDM bus is divided into 32 partitions
or time slots, each containing a byte of data. Because of the serial nature of the TDM bus, an entire
TDM bus frame encompasses 125 µs in time. The stream packetization interval defines the number of
TDM frames collected by the NetXpress LX system into a single packet for transmission. The higher
the packetization interval, the greater the accumulation delay introduced to the program because each
frame must be accumulated before the packet can be created. Note that the total program delay is a
combination of packetization delay and network delay.
Each packet in a NetXpress LX stream contains 44 bytes of non-program data (overhead)
corresponding to the header information required for the IP, UDP, and RTP protocol layers. The ratio of
non-program data to program data for a packet can be considered the overhead required to get the
packet through the network. The higher the overhead, the more the network bandwidth available to
an application used to transmit non-program data. Program delay can be minimized by using a small
packetization interval at the expense of an increase in overhead. Conversely, increasing the
packetization interval decreases the overhead but increases the program delay (Table 2-1).
Another overhead determination factor is the number of time slots from each TDM frame that get
placed in the packet. As this number 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.
2.2.7

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.
For many network services, 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 LX 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 LX system, you specify the jitter buffer size in number of packets ranging from 8 to 128
(Section 4.3.5.1 – 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.
2.2.8

Packet/Stream Information Calculation

Table 2-1 gives an example of user and computed parameters for a NetXpress LX stream. The user
parameters represent user-defined stream parameters. The computed parameters are for packet
information and network bandwidth based upon the user parameters. This table also gives formulas
for how to derive computed parameters for a stream.
2-6
2 – Functional Design
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