What challenges does mobile radio communication pose?
A mobile unit's radio based communication to a fixed control room (server) poses special challenges for the network protocol. Contrary to all allegations, the current radio networks (GSM, wireless LAN) are always an unstable communication platform. There are no assurances about band width, package terms, error rates or quality of service. There is merely an average expectation based on experiences.
Any radio coverage would significantly deteriorate with necessary work between two trains (e.g. train access control) or under a pipe bridge. Blanket statements such as, users can always be “online” nowadays, are unrealistic.
Mobile applications have to make do with minimum data traffic, so that short response times can also be achieved with “worst case” radio conditions. They also have to work offline independently of the communication medium to not disturb the “fluid” work process.
This also includes the data transmission status being easy to recognise. A “green signal” indicates that online data communication is possible.
The influence of radio coverage
- from shielding solid objects (other wagons, trains, buildings, ...)
- the use of the radio cell by other users,
- a change of location of the mobile unit within a radio cell,
- roaming when transferring between radio cells and
- the degree of the respective provider network's development
are examples for the fact that the quality of service for the transfer of data cannot be presumed. In fact the worst case, i.e. disconnection whilst transferring data, is rather to be seen as the norm. What's typical are constant changes between narrowband connections with few kBit/s, transfer rates, sometimes connection rates of over 100kBit/s net and short and/or permanent interruptions to data transfer.
A network protocol for the planned purpose of use must therefore respond stalwartly to disconnections and have a status at all time that guarantees a reliable automatic restart of an interrupted transfer.
At the sending end, the non-availability of a radio connection, e.g. in a “dead spot” can lead to accrued user data not being able to be transferred. It has to be temporarily stored until the connection to the server is restored or predefined criteria prevent a buffer overflow by deleting the data.