3 Engineering IoT Networks
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Fig. 3.2 Main network topologies: (a) star, (b) tree, (c) mesh
alternatively (half-duplex link) or simultaneously (full-duplex link). The data flow
exiting the user’s interface is named uplink channel, while the opposite one is named
downlink channel.
Nodes and links can be arranged in different ways known as topologies as
depicted in Fig. 3.2. The simplest topology is the point-to-point network. A slightly
more complex (and useful) topology is the star in which a central node is an
intermediate system that connects peripheral nodes. More stars can be joined into
a tree. Stars and tree are very common topologies in the Edge Network of IoT
applications. Stars are usually employed when several sensors collect data and
transmit them to the Internet. In this case, the maximum distance between the sensor
and the gateway is given by the range of every single link. Trees are used to extend
the range of the network by using multi-hop communications. In stars and trees,
there are no multiple paths between nodes, thus simplifying the routing of data but
also sacrificing redundancy. In mesh networks, the interconnection graph contains
cycles which provide more routing opportunities and thus more reliability.
3.2.1.3 Quality of Service
The quality offered by a channel is named quality of service – QoS – and consists
of three main parameters, i.e., capacity, delay, and bit error rate. Capacity is the
maximum number of bits that can be delivered reliably over the channel. The delay
is the difference between the time of leave of a bit from the transmitter node and the
time of its arrival at the receiver node. The error rate is the percentage of bit values
that are erroneously read by the receiver considering all the transmitted bits. It is
worth noting that QoS depends on the characteristics of the physical medium as well
as of all protocols used over it. A physical medium may have a significant bit error
rate (e.g., a radio link), but a retransmission technique provided by a protocol over it
can reduce such rate. The term bandwidth refers to the frequency interval (reported
in Hertz) used to transmit bits at the physical level. This feature is used to allocate
various communications of the electromagnetic spectrum. People may use this
term with the same meaning as capacity, but this practice is formally wrong since
bandwidth is measured in Hertz while capacity is measured in bit/s. Actually, at the
physical level, the relationship between bandwidth and capacity is given by the modulation scheme, and, given it, the higher is the bandwidth, the higher is the capacity.
101
Fig. 3.2 Main network topologies: (a) star, (b) tree, (c) mesh
alternatively (half-duplex link) or simultaneously (full-duplex link). The data flow
exiting the user’s interface is named uplink channel, while the opposite one is named
downlink channel.
Nodes and links can be arranged in different ways known as topologies as
depicted in Fig. 3.2. The simplest topology is the point-to-point network. A slightly
more complex (and useful) topology is the star in which a central node is an
intermediate system that connects peripheral nodes. More stars can be joined into
a tree. Stars and tree are very common topologies in the Edge Network of IoT
applications. Stars are usually employed when several sensors collect data and
transmit them to the Internet. In this case, the maximum distance between the sensor
and the gateway is given by the range of every single link. Trees are used to extend
the range of the network by using multi-hop communications. In stars and trees,
there are no multiple paths between nodes, thus simplifying the routing of data but
also sacrificing redundancy. In mesh networks, the interconnection graph contains
cycles which provide more routing opportunities and thus more reliability.
3.2.1.3 Quality of Service
The quality offered by a channel is named quality of service – QoS – and consists
of three main parameters, i.e., capacity, delay, and bit error rate. Capacity is the
maximum number of bits that can be delivered reliably over the channel. The delay
is the difference between the time of leave of a bit from the transmitter node and the
time of its arrival at the receiver node. The error rate is the percentage of bit values
that are erroneously read by the receiver considering all the transmitted bits. It is
worth noting that QoS depends on the characteristics of the physical medium as well
as of all protocols used over it. A physical medium may have a significant bit error
rate (e.g., a radio link), but a retransmission technique provided by a protocol over it
can reduce such rate. The term bandwidth refers to the frequency interval (reported
in Hertz) used to transmit bits at the physical level. This feature is used to allocate
various communications of the electromagnetic spectrum. People may use this
term with the same meaning as capacity, but this practice is formally wrong since
bandwidth is measured in Hertz while capacity is measured in bit/s. Actually, at the
physical level, the relationship between bandwidth and capacity is given by the modulation scheme, and, given it, the higher is the bandwidth, the higher is the capacity.