Planning Transmission Rates

Planning transmission rates is a juggling act between keeping incoming values relatively up to date while ensuring the radio channel is not crowded such that transmissions continually collide.

A word of warning: Be very aware of the overeager process control engineer, or anyone in the 'food chain' that forces a higher level of traffic than is actually required. This idiotic manoeuvre will only cause pain in the future with delays in the data getting from site to site. Two primary reasons are the units all trying to update at the same time, and the over crowding when the system is updated.

The first happens with the inaccuracies in the timing of updates. Even if all units are set to e.g. 10 minutes the slight variation with some being a little fast and others slow will mean at some time all units will land up trying to update at the simultaneously. Some telemetry makes and types start the update time from the last successful comms. This will have a sort of 'pan out' effect, as long as there is room to pan out! Experience has proved that telemetry traffic beyond approximately 5% radio channel usage will result in collisions, this figure increasing fairly dramatically with a good 'anti-collision' algorithm written into the software although approximately 15% was the highest achieved in a 350 station system without too many collisions.

The second reason for system failure through high update rates is when the system needs to be expanded. From the outset the expected update times need to be established especially with a view to expansion. If a system comprises of only two stations then updates could be almost continuous without affecting the validity but try add just one station and suddenly the update time must be halved (minimum).

Realistic figures: This varies with each and every situation and is relative to surrounding factors. Co-existing with other systems on he same frequency is the worst to cope with. Monitoring of the radio channel for usage before you install your system could prove invaluable and save many hours post-installation, pre-commissioning repair.

Should you be the only system in the area then you are king over your domain but how you rule it will be the test of just how good a king you are. Keeping in mind realistic expectations of the equipment is of paramount importance. The first hurdle is accepting the fact that radio is not a piece of wire - and convincing the process control engineer that radio travelling at the speed of light has nothing to do with how fast the data arrives.

Should the equipment you are using have a very good anti-collision algorithm a good rule of thumb is setting update times to a minimum of 4 times the actual time taken for all transmissions and acknowledgements on a system. E.g. should your system have 60 inputs relayed to a host, each transmission and 'ack' taking 1 second then updates should be set to 4 minutes minimum.

A huge time saving facility is when the system allows 'change of state' transmissions i.e. should an input change from off to on then only does a transmission take place. The only snag being it takes a change for any state to be truly known after a power failure or reset. These inputs, if not critical, can be set at the maximum update times. As long as this time is known a 'settling' time after a reset is given to the system before any data is trusted.

Analogue inputs can also have long update times with the 'change of value' window set relatively small such that should there be a change of value then an update is sent but if things are stable then transmissions are drastically reduced. A word of warning, ensure inputs setup in this manner have 'clean' and stable sources.

Polling systems have a huge advantage over update time based ones. Polling allows the system to be fully regulated from the host with high and low priority signals dynamically configured from one source. The radio channel is also under full control from the host and this regulation of traffic can achieve as high as 80-90% of radio channel use (the odd interference and non-reception being the limiting factors.

There are poor and also very efficient polling schemes, the latter minimising radio traffic dramatically. When choosing a system weigh up the schemes used before taking the plunge.

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