METRO AREA VARA
Updated: Jun 17, 2023
The AL0R-12 gateway/digi is now operational, running on 145.07.
The coverage map has been updated. Note that just because coverage isn't shown for an area, it doesn't mean you can't connect. There is a user near Prague connecting to AL0R-12, AL0R-10 and AL0R-8! Coverage is good!
VARA is a recently developed mode utilizing OFDM and commodity sound interfaces in Windows. There are two versions - VARA (HF) and VARA FM. The FM designator is straightforward, meaning this version is intended for VHF/UHF use on FM channels. The HF version is considered a weak signal mode and connections are often maintained below -20snr! Huffman compression is used which reduces typical text payloads by 30%.
VARA uses orthogonal frequency division multiplexing with up to 59 carriers that run at 42bps, complying with the FCC limitation of 300 baud. If and when this limitation is ever removed, additional speed will be possible. There are three bandwidths available to use, 500, 2300, and 2750hz. Running 2750 requires a good radio with wide filters. Note that VARA FM has a narrow and wide mode, roughly related to 1200bd and 9600bd packet.
No! Packet (AX.25) uses CSMA (carrier sense multiple access) and "shares" the channel by having the sender ask the receiver if all the frames were received. If they weren't, then missing frames are retransmitted. Frequently, as you get more than a couple of stations on frequency, you'll have collisions, and frames will get resent. As stations spread out and reach a point where not everyone can hear the other, collision frequency escalates significantly, reducing the overall throughput of the channel.
VARA FM more or less takes over the channel between two stations until the traffic is passed. There are exceptions to this, depending on what application is being used.
VARA (HF) typically uses a 500Hz channel and can support multiple QSOs in a standard pass band, even supporting multiple in the same channel in many instances.