RACOM RipEX, Руководство пользователя

Страница: 32 / 234

3. Network planning
The significance of planning for even a small radio network is often neglected. A typical scenario in
such cases goes as follows – there's not enough time (sometimes money) to do proper planning, so
the network construction is started right away while decisions on antennas etc. are based mainly on
budget restrictions. When the deadline comes, the network is ready but its performance does not meet
the expectations. Finally the (expensive) experts are invited to fix the problem and that fix costs ten
times more than a proper design process done beforehand would have.
The following paragraphs are not a guide to network planning – that is a topic far beyond the scope of
a product manual. What is provided is the essential RipEX data needed plus some comments on
common problems which should be addressed during the planning process.
3.1. Data throughput, response time
A UHF radio network provides very limited bandwidth for principal reasons. Hence the first and very
important step to be taken is estimating/calculating the capacity of the planned network. The goal is to
meet the application bandwidth and time-related requirements. Often this step determines the layout
of the network, for example when high speed is necessary, only near-LOS (Line-of-sight) radio hops
can be used.
RipEX offers an unprecedented range of data rates. The channel width available and signal levels ex-
pected/measured on individual hops limit the maximum rate which can be used. The data rate defines
the total capacity of one radio channel in one area of coverage, which is shared by all the radio modems
within the area. Then several overhead factors, which reduce the total capacity to 25-90% of the "raw"
value, have to be considered. They are e.g. RF protocol headers, FEC, channel access procedures
and number of store-and-forward repeaters. There is one positive factor left – an optimum compression
(e.g. IP optimization) can increase the capacity by 20-200%.
All these factors are heavily influenced by the way the application loads the network. For example, a
simple polling-type application results in very long alarm delivery times – an event at a remote is reported
only when the respective unit is polled. However the total channel capacity available can be 60-95%
of the raw value, since there are no collisions. A report-by-exception type of load yields much better
application performance, yet the total channel capacity is reduced to 25-35% because of the protocol
overhead needed to avoid and solve collisions.
The basic calculations of network throughput and response times for different RipEX settings can be
done at www.racom.eu
1
.
Let us add one comment based on experience. Before committing to the actual network design, it is
very wise to do a thorough bench-test with real application equipment and carefully monitor the load
generated. A difference against the datasheets, which may be negligible in a LAN environment, may
have fundamental consequences for the radio network design. To face that "small" difference when
the network is about to be commissioned may be a very expensive experience. The bench test layout
should include the application centre, two remotes (at least) and the use of a repeater. See the following
picture for an example.
1
http://www.racom.eu/eng/products/radio-modem-ripex.html#calculation
RipEX Radio modem & Router – © RACOM s.r.o. 32
Network planning