Netxpress Lx System And Fec - Harris CM-30 Installation & Operation Manual

2 –Functional Design
Internal Timing
NetXpress LX
A
2.7.2.2 Timing Summary
NetXpress LX units that generate streams into the network should share a common timing reference
to guarantee that any NetXpress LX unit can receive or monitor any stream in the network. You can
use stream timing, external (Stratum-1 or GPS) timing, or a combination of the two. In cases where
audio streams are already present and can be used as timing sources, stream timing is essentially
"free." If a dedicated timing stream must be added, the recurring cost of the stream bandwidth can be
traded off against the one-time cost of implementing GPS receivers.
You should also consider timing failure scenarios. Ideally, a fallback timing source should be available
at each site that keeps that site in synchronization with the other sites even if its primary timing
source fails. Once again, the backup source can be either a stream or an external input (Stratum-1 or
GPS) with the corresponding cost trade-off.

2.8 NetXpress LX System and FEC

The NetXpress LX system maintains uninterrupted audio transport by supporting Forward Error
Correction (FEC). Specifically, the NetXpress LX system supports RFC 2733. Forward error correction
is the process obtaining error control in data transmission in which the source (transmitter) sends
redundant data and the destination (receiver) recognizes only the portion of the data that contains no
apparent errors.
An FEC packet is a special type of RTP packet. It is constructed by placing an FEC header and FEC
payload in the RTP payload.
In the NetXpress LX system, up to 32 streams (16 in each direction) can be designated as FEC
streams. For each of the FEC streams, you can specify either of two techniques for forward error
correction: FEC LOW or FEC HIGH (Section 4.3.5.1 – Stream Creation). This specification affects the
sender only. The receiver processes all FEC packets.
FEC LOW (appropriate for most low packet loss conditions)
Single error correction is engaged. This scheme introduces a 50% overhead (increased bandwidth
required) and can correct all single packet losses – consecutive packets are not lost.
Harris Corporation
Intraplex Products
Stream
Timing
Audio Stream
Monitor
Stream
Audio Stream
Monitor
Stream
Figure 2-9. Timing with Off-Air Monitor Streams
NetXpress LX & CM-30 Installation & Operation Manual
Audio
NetXpress LX
Audio
B
Stream Timing
NetXpress LX
C
Version 1, September 2010
Off-air monitor
audio
Audio
Off-air monitor
audio
Audio
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