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The HD Radio Network
The advent of HD Radio is requiring the broadcast industry to take a closer look at its station's networking system and infrastructure. Minimizing network-related issues that can induce dropouts must be a prime consideration in any successful HD Radio implementation. A network that performs quite well for day-to-day data traffic may be significantly challenged by the near-real-time, isochronous demands of the HD Radio stream. The key issues that appear to be causing station engineers and IT personnel the most difficulties are traffic management, bandwidth provisioning and reference timing synchronization between the various HD Radio components across the network.
There are two distinct physical configurations that the station may implement for deployment of AAS for multicasting on the HD Radio system:
- Importer to exciter (I2E)
- Exporter to exgine (E2X)
Figure 1 shows the studio importer connection over bi-directional (duplex) STL.
The I2E configuration connects an importer to an exporter/exgine via a bidirectional Ethernet connection. Only the Advanced Applications Services (AAS), such as multicast programming and data services, are transported by this link, which is not concerned with the main program digital service. A bidirectional link is required to accommodate the command and response nature of the I2E configuration.
In this configuration, even with moderately bad network conditions (up to one percent packet loss and 100 millisecond latency) the system continues to perform well. The key is to provide adequate bandwidth overhead to allow the system to recover lost packets through TCP packet retransmission.
For a station running MP1 mode with 48kb/s of AAS, the average utilized bandwidth will be 54kb/s, requiring at least 90kb/s to be available through the STL. A 128kb/s LAN/WAN extender or two DS0s should provide sufficient bandwidth for any MP1 configuration. For the maximum MP3 extended hybrid configuration of one SPS at 48kb/s and a second SPS at 24kb/s, the minimum bandwidth required of the STL/WAN link is 156kb/s, requiring three DS0s for 196kb/s.
Exporter to exgine (E2X)
Figure 2 shows the exporter to exciter configuration.
The importer-to-exporter-to-exgine (E2X) configuration is the most bandwidth-efficient method of deploying an HD Radio multicasting data network. With this implementation, a single data stream may be conveyed to the transmitter site over the STL/WAN link, which contains all of the MPS information as well as the Advanced Applications Services from the importer, such as SPS and associated data.
Studio-to-transmitter transport of the E2X data stream is currently supported only as simplex (one-way) UDP and can operate over most unidirectional STL systems of sufficient bandwidth and robustness. With UDP transmission, the loss of a single packet results in the loss of the entire audio frame of which it is a part. The resulting outage will last for the duration of that single audio frame: 1.48 seconds.
A 128kb/s LAN/WAN extender or two DS0s will provide sufficient bandwidth for any MP1 configuration. For MP3, 256kb/s or four DS0s should be considered. Packet loss across the link becomes a critical factor and must be kept below 10
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