In Network Processing and Data Aggregation in

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other. There are three general families of protocols that are able to control the access tothe wireless medium. The protocols of each category are presented briefly:• Channel Partitioning Protocols (Contention free)Certain assignments are used to avoid contentions. They are more applicable to staticnetworks and/or networks with centralized control.o Time Division Multiple Access (TDMA) – All users share the same frequencyby dividing it in timeslots. Each one waits for his turn (the time that corresponds to theirtimeslot) and uses all or part of the provided bandwidth.Advantages:a) It naturally avoids collisions so there are no extra overheads.b) It has short duty cycle.c) FairnessDisadvantages:a) Needs synchronization mechanisms.b) It requires the formation of communication clusters.c) No good scalability because the timeslots are difficult to be assigneddynamically.d) Limited transmission rate because even if a node is the only one that hassomething to send it has to wait for its turn.o Frequency Division Multiple Access (FDMA) – The channel is divided intoequal smaller frequency bands that are called subdivisions. Each subdivision is assignedto a node and so every participant can use only a fraction (with equal size for all) of thetotal channel bandwidth. The FDMA has the same advantages as TDMA. The drawbacksare identical too except for the fact that even if a node is the only one who transmitssomething it is not obliged to wait for its turn whereas it may use only a fraction of thebandwidth of the channel.o Hybrid TDMA/FDMA – This approach combines the two previous protocolsand the medium is divided in a time-frequency space basis. It is a very frequent solutionfor wireless communications but it also has similar inefficiencies with TDMA andFDMA.o Code Division Multiple Access (CDMA) – The channel is divided neither intotime nor by frequency but each node is assigned a code (chipping sequence) with which it
encodes the data before transmission. The codes are “mutually orthogonal” to each other,which means that nodes can transmit simultaneously without the possibility of collisions.Of course the selection and the distribution of the codes between the participants is not aneasy task, especially when their total number is high. The receiver listens to a signal thatis spread on the air consisting of multiple encoded sub-signals transmitted from eachsender. By the code of the sender it is an easy process to decode only the signal ofinterest among the rest of them.Advantages:a) Naturally avoids collisions so there are no extra overheads.b) Fairness.c) More efficient use of the channel.d) Theoretically unlimited number of users that CDMA can support, incontrast to TDMA (finite time-slots) and FDMA (finite sub-channels).Disadvantages:Near-far problem: Because all users transmit at the same frequency, internalinterference generated by the system is the most significant factor in determining systemcapacity and call quality. Since one transmission is the other's noise the Signal-to-noiseratio (SNR) can be high in many situations. Thus, it is not always easy to detect andreceive a weak signal among stronger ones.• Random Access Protocols (Contention based, on demand allocation)These protocols are aware of the risk of collisions of transmitted data, and are moresuitable for mobile Ad-Hoc networks.o Pure ALOHA – When a node has data to send it directly transmits the datapacket. It is very possible for a packet to collide with other packets that were transmittedat the same time because all the nodes share the same medium and use the samefrequency bands. When the sender realizes the collision then it performs a retransmissionwith probability p otherwise it waits for a fixed amount of time.Advantages :a) Supports many simultaneous users,b) Ease of management,c) Speed in initial communication.Disadvantages :
Page 1: Athens University of Economics and
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Page 53 and 54: REFERENCES[1] W. Ye, J. Heidemann,

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