Managing HD Radio network traffic

Fig 3 - Recommended network deployment.
Click image to enlarge.

Because the STL system is usually the tightest bandwidth bottleneck in the HD Radio network, it is imperative that broadcast, multicast and other extraneous traffic be kept off the network path to the transmitter site. All HD Radio devices — importer, exporter and exciter — should use statically assigned IP addresses within their own subnet. This subnet must be separate from the rest of the facility through the use of VLANs or physically separated networks. The only way to be sure that no extraneous traffic is traversing the STL link is to place the entire HD Radio system on its own IP subnet. Figure 3 shows a recommended network deployment of subnetting using VLANs.

The exciter should always be on the WAN subnet, which it may share with the exporter and importer, or the importer may be placed on program automation subnet. Except for equipment that may be necessary to build the infrastructure — that is, routers and switches — no other station equipment should be on the WAN link subnet.

The implementation of VLANs or connection of devices through a dedicated physical network will substantially reduce packet loss and data collisions. Monitoring the traffic across the WAN with a network protocol analyzer or packet sniffer such as Ethereal is essential if a problem is suspected.

Provisioning the STL/WAN link

For a TCP data stream to function properly under adverse conditions, the link that carries it must have reserve bandwidth above and beyond the data rate of the stream. For TCP, the STL/WAN link must have a minimum of 40 percent reserve bandwidth. This is necessary to accommodate the temporarily higher data rates that occur when the stream recovers from packet loss. If a TCP WAN link is provisioned such that the aggregate data stream, including VNC, utilities and other extraneous traffic, occupies no more than 60 percent of the WAN link's available bandwidth, then the installation should be successful under all but the most adverse network conditions. For UDP, the total traffic across the link should be no more than 75 percent of the provisioned bandwidth to allow for network contention.

Additional bandwidth beyond these guidelines allows operation under poorer conditions, but with diminishing returns. In general, bandwidth should not be used to adjust for a poor network.

If other traffic is going through the WAN, the link should have class of service, QOS or other prioritization techniques employed to ensure that the HD Radio traffic has the necessary bandwidth.

Reference timing synchronization

While not specifically a networking issue, reference timing between the importer, exporter and exgine are not maintained across the network infrastructure. The use of GPS as a timing reference for the importer, exporter and exgine to precisely lock their respective clocks in step eliminates the phase and frequency issues and is highly recommended. Without GPS, or some other method of providing absolute frequency lock between the exporter and exgine, buffer underflow in the exgine or data overflow of the exporter's audio cards will eventually occur resulting in data frame misalignment, eventual audio dropout and significant diversity delay slippage on the main HD program channel. Without the use of GPS as a 44.1kHz timing reference for the station's AES audio chain or at least to the importer's audio cards, any difference in the importer's audio clock frequency and the exporter's 10MHz reference will result in the eventual underflow or overflow of the importer audio cards, which will result in occasional audio dropout of the SPS channels. The frequency of these dropouts will be directly proportional to the frequency disparity of the two references.

For more information on HD Radio and networking implementation, several white papers are available on the Ibiquity website at www.ibiquity.com/broadcasters/quality_implementation/iboc_white_papers.

Anderson is president of TBA Communications and a contract engineer for Ibiquity Digital.