Energy Efficient and Reliable Networking
6.1.12 Differences with WASP
In CICADA all traffic reaches the sink in 1 cycle, which results in a lower delay.
Each CICADA-cycle consists of 2 different subcycles: the control subcycle and the data subcycle. This results in enhanced mobility support: it is possible to leave a parent, to detect loss of a parent or a child and to join a node with much lower delays. CICADA does not rely on broadcasting to pass information from children to their parents, instead additional information is put in the data packets or HELLO packets are used. Moreover, generating CICADA schemes is easier because of the simple computation of the waiting period and the receive period. This is important as CICADA is meant to run on sensors where computational resources are scarce.
The performance differences will be shown in the next session.
6.2 Performance Analysis
In this section, we present the performance analysis of CICADA. We will address performance issues such as the energy efficiency, reliability and mobility. Further, it is assumed that the nodes are loosely synchronized. The SCHEME-messages can be extended with a time stamp that can be used to adjust the clock of the child nodes.
6.2.1 Energy Efficiency
The most important causes of wasting energy in radio communication are idle listening, overhearing and collisions. CICADA takes care of all those causes by assigning slots in the control cycle and using them in the data cycle. Nodes only need to operate their radio during used slots, i.e. the slots where the node is re-ceiving or sending data and during the control subcycle. All slots are allocated so a node perfectly knows when it is allowed to sleep, when it has to send or when it has to switch on his radio to receive data. Idle listening and overhearing can occur in the control subcycle as the nodes have to wait for the control scheme of their parent and consequently have to switch on their radios. However, the slots of the control cycle are shorter than the ones of the data cycle and nodes can sleep when a scheme from their parent has been received. In the data subcycle, nodes only have to wake up when transmitting or receiving data. Using these mechanisms, the dissipation of energy is minimized.
A diagram of the different states and their transitions can be found in Fig-ure 6.5.
The sleep ratio ρiof node i for a full duty cycle can be written as:
ρi = Tcycle− Ton,i
Tcycle (6.4)
142 CICADA: ENERGYEFFICIENT ANDRELIABLENETWORKING End contention slot: start sending immediately
End contentio
Figure 6.5: State diagram in CICADA. The striped line pattern indicates two alternative paths, depending on the fact if the node directly starts sending after its con-tention slot or not. The radio is powered down in the sleep state.
Ton,i is the time in slots a node i has its radio on. This corresponds to the exterior states in Figure 6.5.
Ton,i = Tcc + 2 · X
j∈Chi
αj + δi + 1 (6.5)
In this equation, TCC represents the length of the control subcycle, the factor 2 in the second term indicates the time spent for receiving and sending the data, the third term covers the transmission of its own data and the fourth term the con-tention slot.
The cycle length Tcyclecan be calculated using the length of the control sub-cycle, the waiting period and receiving period of the sink and a contention slot at the end: func-tion γ(n) gives the number of total waiting slots for a node n. In a data subcycle, the lowest level only has a contention slot and no waiting period, thus in the last but one level, γ(n) should be 1.
γ(n) =
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The network topology clearly plays an important role in the energy efficiency of the protocol: it determines the cycle length as it effects the waiting period and the length of the control subcycle. If the tree structure is rather flat, the waiting period will be short and vice versa for a line topology. As in Section 5.3.2, we assume a ζ-balanced tree and that every node just needs one time slot per cycle (δi
= 1 for all nodes). Doing so γ(n) is given as
where L is the number of levels in the network and y the level node n resides in (the sink has level 0). This can be proved as follows. The summation in (6.7) can be seen as the number of nodes below node i as every packet needs only one time slot. In a ζ-balanced tree, we thus can writePl
j=1ζjwhere l depends on the level of node i. Together with the contention slot, which equals one time slot, we get the second summation of (6.8). Each level, except the last one, adds to the waiting period. Thus the first summation counts till L − 2 − y as the first level has number 0.
In a ζ balanced tree, the length of the control subcycle TCC can be written as (L − 1) · ζ + 1. The sink uses one slot and in each level below the sink all the children send their schemes one after the other before the next level starts sending.
Of course, this is only the case when the tree is fully populated, i.e. when all the nodes on the last but one level have ζ children. In order to account for this overestimation, the number of nodes in the lowest level can be calculated with N −PL−2
i=0 ζi. Further, we assume that all the nodes are equally divided in the lowest level: the number of nodes is thus divided by ζ and rounded to the nearest greater integer. The slots in the control subcycle can be smaller than the slots in the data subcycle, the ratio of the slot length in the data subcycle and control subcycle is called . Hence, TCCbecomes
TCC =
The number of slots needed for receiving and forwarding the data for a node on the level below the sink is given by
X
The only thing that we need now is the number of levels in the network. When we have a fully ζ-balanced tree where also the nodes of the last but one level have ζ children, we can write N = PL−1
144 CICADA: ENERGYEFFICIENT ANDRELIABLENETWORKING
where the ceiling function is needed when the tree is not fully balanced, i.e. when there are not enough nodes to completely fill the lowest level. Now the sleep ratio for a node on the level below the sink for a ζ balanced tree can be formulated as
ρ =
Figure 6.6: Sleep ratio in CICADA for varying ζ, varying network size N and full duty cycle. The slots in the control cycle are a factor 5 smaller.
Figure 6.6 compares the sleep ratio ρ for varying ζ for the nodes in level 1, below the sink. The sleep ratio is lower when ζ is smaller, similar to the sleep ratio in WASP. Further, the sleep ratio is almost similar for the varying network sizes.
When ζ is higher than N /2, the sleep ratio remains stable as both the numerator and denominator change at the same rate. Every node forwards at most data from 1 child node and the length of the control cycle remains the same. For a network of 50 nodes, a sleep ratio of almost 93% is obtained. When ζ = N, a maximum is reached as all the nodes are directly connected to the personal device. Figure 6.7 shows the influence of shortening the timeslots in the control cycle. If the timeslots are five times shorter, the sleep ratio drops about 5 to 10%, especially for larger ζ. So, it is good practice to use a slot ratio of 10 or more in order to have an adequate energy efficiency. The figure also compares the sleep ratio of CICADA with WASP’s. When ζ is small, CICADA outperforms WASP and for larger ζ, WASP performs better when the time slot ratio is low.
Overall, the tree should not have too many levels. That way, the nodes can sleep more and the upper bound for the delay will be lower, see [5] for more information.
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Figure 6.7: Sleep ratio in CICADA and WASP for varying ζ, N = 50 and full duty cycle.
For CICADA, the ratio between the slot length in the data subcycle and control subcycle is varied ().
Figure 6.8: Influence of duty cycle on the sleep ratio in CICADA for varying ζ and N = 50.
The analysis above has been done with the assumption that we have a full duty cycle (∆ = 100%). For lower duty cycles, longer inactive periods will be inserted after a data subcycle. This will of course affect the sleep ratio in a positive way.
The sleep ratio defined in (6.4) can than be written as:
ρi = TDutycycle− Ton,i
TDutycycle
= Tcycle− (∆ · Ton,i) Tcycle
(6.13) Figure 6.8 shows the sleep ratio for different duty cycles. It can be seen that
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changing the duty cycle seriously affects the lifetime of the network. By setting a duty cycle of 50%, the sleep ratio is increased with 10%. When the duty cycle is 10%, the sleep ratio is higher than 99%. Of course, the value that can be used for the duty cycle depends on the amount of traffic in the WBAN. For low loads, a low duty cycle can be chosen, allowing a more energy efficient use of the network resources.