NetXpress LX & CM-30R Installation & Operation Manual
2
–
Functional Design
Version 1.1
2-6
GatesAir, Inc.
Intraplex Products
or 16AE module) but not in 4-wire modules (for example, the VF-25E module) excepting when a 2-
wire module adapter is also being used.
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 (E1) or 24
(T1) 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 stream
content because each frame must be accumulated before the packet can be created. Note that the
total stream delay is a combination of packetization delay and network delay.
Each packet in a NetXpress LX stream contains 44 bytes of non-consumer data (overhead)
corresponding to the header information required for the IP, UDP, and RTP protocol layers. The ratio of
non-consumer data to consumer data for a packet can be considered the overhead required to get the
packet through the network. High overhead means that the greater portion of the network bandwidth
available to an application is being used to transmit non-consumer data. Stream 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 transport
delay (Table 2-1).
Another overhead determination factor is the number of time slots from each TDM frame that get
placed into each 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 transport 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 synchronous data applications requires 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 by utilizing 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
