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IP remotes: Instantly Practical
Inevitably, some packets will still be lost though, and there are other mechanisms designed to further minimize the negative effects. One such method is known as forward error correction (FEC). FEC is basically the addition of redundant packets to the data stream — the idea being that these redundant bits will effectively take the place of the packets that somehow end up missing at the far end. One can easily see that the addition of too many redundant packets could possible create a problem in and of itself with respect to network congestion. Therefore, like packet size, the amount of FEC should be adjustable by the user, to best meet network conditions.
The nature of the Internet also results in packets sometimes arriving late at the receive end late, or even out of order. For an audio stream, this is obviously a problem — one addressed by way of a packet jitter buffer. This buffer stores received packets for a certain amount of time, allowing late packets to catch up; out of order packets can also be re-sequenced prior to being sent to the audio decoder. The obvious problem here is that the buffer adds delay time-generally considered to be something that isn't good when doing remotes. Therefore, once again, one must strike a compromise between problems in the audio caused by late or out of sequence packets, and the amount of delay one can deal with at the remote site.
To review, let's 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 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.
So the problems I've just discussed are ones taken on and solved by the equipment designers themselves. Now let's go in to some of the issues you'll experience as a user of this type of equipment. They're all related to network security.
At your studio headquarters, likely you'll have a rack-mounted version of the IP codec you've chosen. Obviously it will need a network connection and access to the public Internet. Assuming you originate your remote broadcasts from the field, your HQ device will need to be able to be contacted from the public Internet. This presents a big problem to most network administrators because it is often against company IT policy to allow uncommon ports to be open on the router or firewall. (More on the topic of ports below.) For this reason, the HQ unit will not be able to work on your company LAN.
One way around this (and there is more than one) is to put the HQ unit on its own network. As I wrote earlier, broadband access is common now: you can look into the installation of a DSL line in to your facility, or alternatively, if cable TV is available, a cable modem. Hang the HQ codec on this mini-LAN, and it will be easily accessible from the outside.
Another way to get around this security issue is by way of a proxy server. This proxy server is located outside of your network, and a connection is made to it from your HQ codec via the public Internet. A session is initiated by your HQ codec, and the proxy server records the IP address of your codec and actually maintains a connection with it thereafter. From the field, then, you connect to the proxy server, and it in turn redirects the packets to your studio codec through the connection that it has maintained.
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