Internet of Things – Architecture © - 93 -
Figure 23 : Schematic diagram of a general communication system.Figure 23 depicts end-to-end abstraction of a packet delivery between distant
Devices. The information source can be abstracted as a resource in the IoT
Domain Model, and the transmitter as a Device; while the receiver and
destination pair can be mapped as a Device-Service pair.
Following this abstraction, and pushing it forward, we focus on the channel
modelling. In the IoT context, the channel can assume a multiplicity of forms.
Please notice that the following abstraction is useful in order to have an abstract
description but when it comes to apply the Shannon-Hartley theorem it is crucial
to remember this theorem has to be applied independently to each link
composing the path between the sender and the receiver: CI=BI log(1+SI/NI),
where CI is the channel capacity, BI is the channel bandwidth, SI/NI is the
signal-to-noise ratio (or the carrier-to-noise ratio in case of modulated signals),
each of them related to the I-th link. This channel capacity metric is concave
and it can be aggregated according the following rule: Ci,k=min(Ci,j,Cj,k), where
Ci,k is the aggregated capacity from i to k, while Ci,j is capacity of the link from i
to j and Cj,k is the capacity of the link between j and k.
Given two adjacent channels, which require to be connected by the means of a
gateway, their aggregated capacity is extremely useful in order to dimension the
gateway itself. Nonetheless, assuming you cannot control the routing on the
Internet the scope is limited to the portion of links of which you know the
characteristics, or for a link of which you can suppose to know the lower bound.
A valid assumption will be anyway that the aggregated capacity cannot be
bigger than the capacity of the known links, providing a strong tool to avoid
over-dimensioning the gateways. Indeed, this is extremely useful when
designing a constrained network and its ingress and its egress.
It is important to point out that there is a distinction between the channel model
in the current Internet and that of the IoT. The former is depicted in Figure 24
below, where the Internet block acts as a black-box summarising every channel
transformation that may happen between the two gateways.