Rule#2: All links are Dedicated Point to Point Links.
Note: See also FAQ-HTS
, FAQ-Networking On Purpose
Going to faster bit-rates doesn't get rid of collisions caused by Hidden Transmitter Syndrome.
Faster rates just change the traffic volume supported before collisions and network failure occur.
- A TARPN is a saturated network.
That is, traffic volume is both allowed to, and will inevitably, saturate parts of the network.
A network that gets saturated must either control it's transmitters in a systematic way (polling, token ring, additional control channel, time synchronization and slot assignment), or it must limit the number of stations on the channel to 2, or it will fail.
A dedicated point to point link may also be defined as "only 2 stations per channel."
- Many ham radio packet performance problems either go away entirely or are much more trivial to address if there are only two stations on every link.
- These are issues which are hard or impossible to solve on shared channels:
- Collisions leading to retries;
- Stations transmitting too frequently;
- Stations transmitting when another station outside their range is already transmitting, causing loss of data;
- Station is weak to some stations, or station is heard by one or more other stations they
cannot hear, causing asymmetric links to occur, leading to timeouts and link failures;
- Station is in a position where it hears multiple LANs and because the station detects
carrier often, due to overlapping activity in the multiple LANs, the station is unable
to transmit, causing other stations to time out. This is
called Exposed Receiver Syndrome.
- Bad timing adjustment which is only noticeable at some stations on the channel;
- Audio levels or deviation settings which cause loss of data at some stations on the channel but
- Division of network bandwidth in order to counter channel overload, causing
a link to be very slow relative to the bit rate;
- Desire to upgrade a link is thwarted by inability to upgrade all participants on
the channel at the same time.
Any system which has multiple data sources out of simplex range of each other will begin to fail as the traffic load from hidden stations exceeds between 10% and 20% of the available bandwidth.
Even if the stations are all in range of each other, the channel still must remain at under 20% loading when there are multiple initiators.
Once the traffic reaches that level of saturation, collisions will result in retries, which increases the traffic.
Once that chain of events begins, the channel will saturate and links will begin to time-out, resulting in what appears to be a catastrophic network failure to most of the involved users.
The correct way to adapt to hidden transmitter syndrome is to address it head-on by making sure everybody sets PPersistance correctly, taking account of the maximum number of stations on frequency, or to hand off timing to automation at every station.
Everybody has to have delays to permit channel sharing whether the delays are manually (and correctly) set, or whether the delays are automatic, generated by a backoff-and-retry mechanism.
Doing delays correctly could stave off catastrophic increases in retries and eventual disconnection.
This also results in a channel which is operated at a fraction of the apparent bit rate.
Cooperating on parameter setting to make an artificially slow network isn't human nature.
Humans tend to be lasy and to be ignorant of the purpose of the required methods.
This will cause bad blood, bad performance, bad ham radio.
Rule = Design all network links to be point to point.
See FAQ-Networking On Purpose.