IP-based Audio Distribution


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Caveat emptor

The low price of wired IP connections make them tempting replacements for nailed-up data circuits like DDS, satellite channels and T1s. The best advice I can offer here is that you get what you pay for. Don't expect a DSL line at $40/month to come with any form of service level agreement (SLA) or guarantee of up-time. But this is certainly being done in environments where the cost of pro links is prohibitive, and the less-than-100-percent reliability factor is acceptable, like feeding transmitters in sparse rural locations. But even users with reasonable budgets can often manage to convert dedicated expensive synchronous links to IP links with some SLA and QoS specified.

Comrex Access (center) with Wimax modem (left) Inmarsat terminal (right) and a variety of 3G and Wi-fi modems (front)

Comrex Access (center) with Wimax modem (left) Inmarsat terminal (right) and a variety of 3G and Wi-fi modems (front)

The promise of delivering remotes over 3G wireless services has been realized, but with limitations. In many areas, an IP codec coupled with a 3G data card will get you on-air with remarkable quality and low cost. But in other areas, 3G has become a victim of its own success, with network over-subscription and under-deployment being the main culprits.

Those who deploy a 3G remote system as a replacement for ISDN for long-term programming like sports broadcasts are often disappointed by the network's ability to be robust for hours at a time. And the ability of specific networks to carry reliable real-time streams (like IP codecs require) varies dramatically even between cities. Figure 2a shows a snapshot of a statistics display using the AT&T wireless network. The top part of the graph shows a 60-second window of time with low network jitter (light blue). In the bottom window, packet loss is indicated by red bars (2a has no red and therefore 0 percent packet loss). Figure 2b shows a connection from the same location using the Verizon wireless network. The red in the graph indicates packet loss, and this connection will exhibit audible drop-outs and artifacts of this loss. To be fair to Verizon, there are other locations where this situation is reversed. Figure 2c shows an even worse connection, this one from a 3G network utilized on a high-speed train, with multi-second intervals of 100 percent packet loss.

Fixing bad wireless

No technology can fix a channel as corrupted as Figure 2c, but there is hope for enhancing the stability of moderately poor channels. Some attempts have been made to apply forward error correction (FEC) or to switch from the snappy-but-fragile UDP transport (favored by real-time Internet devices) to the bulky-but-robust TCP transport. But both of these methods involve dramatically increasing the amount of data sent and received. About the worst thing you can do to a congested channel is switch to a data transport that requires more data, as that usually increases overall congestion and makes the situation worse.

New firmware for Comrex codecs utilizes an intelligent UDP reliability layer called BRUTE (BRIC UDP Transmission Enhancement). Essentially, while data transmission takes place via UDP in the lowest layer, an additional layer checks whether packets have been received and decides whether it's worth it to request a resend. In addition, this congestion manager has license to reduce codec overhead and step down to modes with lower network utilization dynamically, in essence riding the gain of the data output. Tests show this can allow reasonable operation on networks with moderately high packet loss, without gumming up the works with a lot of overhead data.

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