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Chapter 3. Concept 33

The assignment in diﬀerent structures makes certain assumptions: On the one hand, transmitters

do not have a state, which corresponds to IT and HE. Instead, they store the date and time of

the last action, which are continuously updated. On the other hand, receivers do not have any

on/oﬀ-codes and are therefore associated with transmitters that have these codes. This linkage

grants both sides access to the data of the other side. For example, the time when a receiver

changed can be obtained by reading out the time of the transmitter with the changed_by_ID. It

is also possible to tell whether the last message of a transmitter was an oﬀ or on-code by reading

out the current state of the associated receiver. A disadvantage of this separation is that these

indirect data requests require two accesses to the database and are therefore a bit slower than one

single access. However, this makes both structure types smaller and there will be less data traﬃc.

Frequently used information such as the protocol type is therefore saved in both structures.

3.5. Requirement Analysis

The requirements of the overall system are greatly dependent of the used HA system, in this case

of IT and HE. The following points play an important role for the software performance and are

therefore examined more closely:

• Latency: Are there noticeable delays between diﬀerent actions? Does this make the use of

the system unattractive for users or even unstable?

• Computing power: Are there worst case scenarios where the computer may be overloaded?

Do data packages accumulate in the message buﬀers and can these buﬀer overﬂow?

• Bandwidth requirement: How much traﬃc is caused by the synchronization between the

nodes? Can it be so large that slow internet connection cannot be used or they would cause

a great delay?

In order to assess these points, this section will consider the characteristics of IT and HE systems.

Both IT and HE devices form either very small networks or highly spatially distributed networks.

This is due the fact that the single 433 MHz transmission channel, the low transmission rate

and a missing error correction lead to a growing number of package collisions, which makes

large, spatially conﬁned networks very unreliable. Theoretically, IT supports a network size of 2

and HE up to 2

members, but these high numbers are not primarily intended to allow large

networks. Instead, they serve to prevent the coexistence of devices with the same addresses

in neighbouring systems to minimize the interference between them. Small premises like rooms

typically do not have more than 20 diﬀerent devices installed. These small networks are favourable

for the performance of the processing software, because they lead to small databases and little data

traﬃc. In larger distributed networks, it may be necessary to use multiple gateways at diﬀerent

locations to be connected to all devices of the system, as the IT and HE devices have a relatively

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RFXCOM RFXtrx433 Spezifikationen Seite 40