Page 26 - ITU Journal, Future and evolving technologies - Volume 1 (2020), Issue 1, Inaugural issue

ITU Journal on Future and Evolving Technologies, Volume 1 (2020), Issue 1

tions [21]. In this section, we discuss these challenges preserve connectivity and to maintain optimal routes.
and propose some solutions that have been considered Furthermore, the fact that these changes occur with lit-
in this field. tle delay, and thus little advanced warning, and the fact
that these changes may occur frequently, even more ex-
First, in BTTNs there is strong dependence of the range acerbate the problem.
over which a tag can communicate on the distance be-
tween the tag and the source of its backscattered RF Third, the backscattering tags experience high level
energy, whether an ambient source or an RF exciter is of interference at various protocol layers. Interference
used. In other words, a tag which is located close to is generally not a problem in sparse networks or net-
the RF energy source will be able to backscatter over a works with infrequent communications among the net-
larger distance, than the same tag if it is placed further work nodes. But in the envisioned applications of BTTN
away from the RF energy source. This is, of course, dif- ([21]), such as those for densely deployed IoT systems,
ferent than a typical sensor/ad hoc networks, where the even a simple query might cause at least some mes-
length of a link depends on the node itself and does not sage flooding among the tags, significantly affecting the
change with the location of the node. This has a num- throughput of big portions of the network. This problem
ber of implications in the design of the routing and MAC is further intensified in real-time IoT applications.
protocols, including the fact that the set of destination Fourth, in some configurations where the RF exciter is
of a node in a BTTN depends on the location of the tasked with at least part of the computational functions,
RF energy source relative to the node. Because of this some part of the routing and MAC processing could be
phenomenon, there is also a larger likelihood of unidirec- done by the exciter [20], offloading some of the complex-
tional links being present between two communicating ity from the tags. On the other hand, in the case of a
tags, when the two tags are at different distances from zero-intelligence exciter or when an ambient RF source
the RF source. (In general, this likelihood depends on is utilized, all the computations need to be performed
the tags and the distribution of RF sources.) As BTTNs distributively by the tags themselves. Thus, to adjust
tend to be distributed in their operation (i.e., there is to different operational scenarios, the routing and MAC
no central element that coordinates the MAC access or protocols may need to adapt to the division of processing
the routing discovery operations) the need to perform between the tags and the exciters. Furthermore, in the
these operations over unidirectional links is a more dif- case of a zero-intelligence exciter, distributed processing
ficult problem than in undirected networks ([22]), often is especially a challenge, since the BTTN tags operate at
leading to network partitions in the unidirectional graph extremely low energy levels, significantly limiting their
type. Of course, preservation of network integrity is crit- processing capabilities. Depending on the limited pro-
ical for most networking environments. This is unlike cessing capabilities of the tags and their extreme low-
other typical wireless networks where each node is pow- power operation, there is a need for new approaches to
ered by its own battery, thus creating links with similar design very simple MAC and routing protocols [20, 21],
capabilities in the two directions, a fact that is quite of- such as by trading the protocols’ performance for pro-
ten relied upon in the design of the protocols. (E.g., if cessing complexity.
node A sends a message to node B, it is given that node
B expects to be able to reply to node A on the link in In applications such as IoT, the network of tags should
the reverse direction.) facilitate interactions among smart objects, each tagged
with a passive tag that stores information about the
Second, as multiple tags are usually powered by a single object, such as the object’s identity, its capabilities, at-
RF source, any increase or decrease in the source’s RF tributes, and past history of interactions with other ob-
power is likely to drastically affect large portions of the jects. As an example, if the BTTN is designed to track
network topology. Thus, movements of nodes (of the RF infectious contacts among individuals, as to alert them
power sources) or changes in the RF propagation impair- of possible infection ([21]), the lists of contacts need to
ments of the RF sources could significantly, and more be stored and maintained in the tags. Routing among
problematically nearly instantaneously, affect large por- such passive tags, each being associated with a particu-
tions of the network topology. Similarly, a movement of lar object, requires creation of a suitable routing infras-
another RF source into the network coverage area would tructure and appropriate protocols. More specifically,
increase the lengths of at least some of the network links the routing functions consist mainly of: finding paths
– and typically of all the links in a particular area – between specific tags or among related tags; ensuring
thus, creating a topology with richer connectivity. This that the communication among the tags is expedited
is unlike other wireless networks (e.g., typical ad hoc or and takes information priority into consideration; maxi-
sensor networks), where the changes of topology caused mizing the network throughput, i.e., concurrent commu-
by a movement of a single node are mostly limited to in- nications among the tags; and reducing the interference
dividual nodes in the neighborhood of the moving node among the selected paths. Although the basic operation
only. Such sizable changes of BTTN topology require a of the routing protocol is to facilitate communication
much more robust and adaptable routing approach to among the tags, i.e., finding multi-hop routes among

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