Let's Not Forget Layer One!
What's "Layer One," and why should we remember it? The seven-layer model used to describe digital networking systems defines Layer One as the physical layer -- in packet radio's case, the analog and RF path between two TNCs.
I don't think that packet radio users have paid nearly enough attention to Layer One issues and, as a result, our networks don't perform nearly as well as they should. This is especially true for 1200 baud operation, because most of us think that it's a "plug and play" system when it isn't (at least, not quite).
Nearly every time I've worked with someone who complains that they aren't being copied, or are being disconnected from the BBS, properly setting the analog parts of the system cured the problem. And, channel monitoring shows that many, many stations are far enough out of adjustment to significantly affect their performance.
There's been remarkably little published about these radio-related issues in the 18 years that 1200 baud packet has been around; this is my attempt to shed some light on the subject. These pages show you how to optimize your packet station, and hopefully explain why these issues are important.
Setting Your TNC's Audio Drive Level explains the single biggest problem in packet radio performance, and how you can easily correct it.
Optimizing Packet Receive Audio Performance, which is still under construction, will describe the receive side of the TNC, and how to make it work better.
Packet Radio Timing Issues (still under construction) will show the impact of timing parameters such as TXDelay.
Packet DCD Performance (alas, under construction as well) won't contain much new content, but will summarize issues about the importance of having a good DCD circuit in your TNC, and will describe modification kits avaiable to improve DCD performance at a low cost.
In the course of putting this material together, I've tested the audio characteristics of several FM transceivers. I've graphed the transmit and receive frequency response, and noted some quirks that affect how some of these radios work. Here are pages describing each tested radio:
- Kenwood TM-V7A
- Alinco DR-1200T
- GE Century II
- Icom W32A
- Kenwood TW-4100A
- Motorola Mitrek
- HP8920 Service Monitor (This was the basis for much of the testing, so it made sense to characterize its performance as well.)
Here is information about the test setup that was used to characterize radio performance.