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Q: explain the problem of energy consumption in sensor networks
As sensor run on batteries, energy consumption is a serious problem as we want sensors' batteries to last as long as possible. It's often challenging to replace or recharge the batteries. Energy consumption is caused by a several things:
- overhearing
- overemitting
- idle listening
- collisions
- overhead caused by control packets
- continuous operation
- transmission distance
To achieve a low energy consumption is very important to define good MAC and routing strategies. For MAC we can use protocols such as S-MAC, allows sensor to sleep most of the time when they are not communicating. S-MAC works by letting sensors do the carrier sense only for a small fraction of the time while idle. To make this work, neighbor nodes needs to be synchronized to each-other, to be able to do carrier sensing at the same time. ecc.
Q: Challenges of routing in wireless sensor networks
routing protocols must be:
- scalable to support networks with very different sizes, and performance should not degrade increasing the size
- wide range of node density
- limited resources for each node
- low computation capability
- can not use too much energy
- nodes may even not have a global ID
- fault tollerant
- a node failure should not destroy the entire network
- support mobility as some nodes may be mobile A good routing protocol should also guarantee that the network will have a long lifetime, as long as possible. Energy consumption is very important as we seen before, for this reason, based on the needs, we can have different kind of routing protocols:
- proactive
- reactive
- geo-based
Q: Explain the difference between Framed Slotted Aloha and Tree Slotted Aloha protocols in RFID system
Both protocols are based on Slotted ALOHA: a frame is divided in time slots, and a tag randomly choses a slot to answer, in a way to reduce collisions. In Frame Slotted Aloha, the number of slots in a frame is always the same. If two (or more) nodes decide to take the same slot, they create a collision. To try to address the collision, a new query is issued by the reader. In TSA instead, for each collision slot s, a new child frame with a smaller slot number is issued. And only the tag that decided to transmit in slot s will transmit in the same frame. The TSA protocol improves the system efficiency as the probability of having a collision is lower. But for both protocols, to have good performance is important to have an estimate of the number of tags to identify as we need to chose the number of slots based on it. If we have too many slots, we will have a lot of time wasted in idle slots, if we have too few slots, we will have a lot of collisions.
Q: in a slotted aloha protocol for RFID system how is the estimated tag population participating into intermediate frames?
Main issues:
- total number of tags to identify is not known
- initial frame size is set to a predefined value (e.g. 128)
- the size of the following frames is estimated by:
tags\_per\_collision\_slot=\frac{(estimated\_total\_num\_of\_tags) - (identified\_tags)}{collision\_slots}The key issue is that we don't know the total number of tags! We can estimate it with the Chebyshev inequality. The problem is that for very large tag number, it can be inaccurate.
Q: explain the binary splitting protocol for RFID systems (discuss its performance)
All the tags have a counter set initially to 0. When the reader sends the first query every tag responds. Every time a collision is generated, rags randomly increments their counter. The process repeat until a single tag or no tag responds. In this case all tags will decrement the counter. As each time the tags are split into two sets, we can "see" it as a binary tree, so we can count the node of the tree to get an estimation.
BS_{tot}(n)=\begin{cases}1,n\le1\\ 1+\sum_{k=0}^{n}\binom{n}{k}\left(\frac12\right)^{k}\left(1-\frac12\right)^{n-k}\left(BS_{tot}\left(k\right)+BS_{tot}\left(n-k\right)\right),n>1\end{cases}