Putting it all together

Access, from Comrex has optional AAC.

So let me summarize the problems with Internet transmission of audio and the techniques used to minimize them. First, there is network congestion, or plain lack of bandwidth. That issue is tackled by minimizing the necessary bandwidth, by using a lossy codec (or in the case of APT, making use of ADPCM) and striking a correct balance between bandwidth and packet size. Loss of packets is addressed to the extent practicable by FEC. Packet jitter is addressed with a jitter buffer.

All that said, network security is yet another issue. Your LAN is likely attached to a router that allows users on your network to access the Internet. The connections made through this router originate behind it - on the LAN side. The router will allow access to the Internet, and in turn it expects a response from the far end. But think about it: If you are in the field, and trying to connect to an IP codec connected to your network, and the router serves as a firewall, the connection will be refused. As far as that router is concerned, an intrusion is being attempted.

There are several ways around this. The first is to inform the network administrator that for the new IP codec to work, certain ports need to be open on the firewall, so the IP codec in the field can set up communication between itself and the studio codec. If your network architecture includes a DMZ, your network admin may allow you to place the IP codec on that subnet instead.

The Tieline IP codec is just one IP product from the company, which features Tieline Voice and Tieline Music algorithms.

The second way is a bit more complicated; consider this if your network administer doesn't want to play ball with you. A proxy server can be used as an intermediary. This proxy server is located outside the firewall. A session can be initiated by the studio codec to this proxy server; the proxy records the IP address (among other things) of the studio codec and actually maintains the connection thereafter. From the field, you connect to the proxy server, and it redirects the packet data to the studio codec, through the same connection it has kept open.

And there is a final way to do this, which may be the easiest way. Have an Internet connection put into the studio (like DSL or cable) and reserve its use for just the IP codec. Leave it completely isolated from the LAN, so you can forsake those network security issues. One problem with this method is that your Internet provider may not provide you with a static IP address. Either pick one that does, or make sure you know if and when the IP address has changed before you head out into the field.

Making use of the Internet for remotes can be looked at as a double-edged sword; while one has to take the time to learn about a whole new technology (and undoubtedly be tripped up a few times along the way), the universe of locations from which remotes can be done opens up dramatically. I for one believe that good remotes can make for good radio; and I'm quite sure that, 5 to 10 years out, the trepidation experienced in going out to do an IP remote for the first time will have long since evaporated.

Resource Guide

Irwin is the chief engineer of WKTU-FM, New York City.