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.