1. MAC Protocols In Sensor Networks

2.  MAC allows multiple users to share a common channel.  Conflict-free protocols ensure successful transmission. Channel can be allocated to users statically or dynamically.  Only static conflict-free protocols are used in cellular mobile communications - (FDMA): provides a fraction of the frequency range to each user for all the time - (TDMA) : The entire frequency band is allocated to a single user for a fraction of time - (CDMA) : provides every user a portion of bandwidth for a fraction of time  Only static conflict-free protocols are used in cellular mobile communications - Frequency Division Multiple Access (FDMA): provides a fraction of the frequency range to each user for all the time - Time Division Multiple Access (TDMA) : The entire frequency band is allocated to a single user for a fraction of time - Code Division Multiple Access (CDMA) : provides every user a portion of bandwidth for a fraction of time  Contention based protocols must prescribe ways to resolve conflicts - Static Conflict Resolution: Carrier Sense Multiple Access (CSMA) - Dynamic Conflict Resolution: the Ethernet, which keeps track of various system parameters, ordering the users accordingly Multiple Access Control (MAC) Protocols

3.  Channels are assigned to the user for the duration of a call. No other user can access the channel during that time. When call terminates, the same channel can be re-assigned to another user  FDMA is used in nearly all first generation mobile communication systems, like AMPS (30 KHz channels  Number of channels required to support a user population depends on the average number of calls generated, average duration of a call and the required quality of service (e.g. percentage of blocked calls) Channel 1 Channel 2 Channel 3 Channel 4 Bandwidth Time Frequency Division Multiple Access (FDMA)

4.  The whole channel is assigned to each user, but the users are multiplexed in time during communication. Each communicating user is assigned a particular time slot, during which it communicates using the entire frequency spectrum  The data rate of the channel is the sum of the data rates of all the multiplexed transmissions  There is always channel interference between transmission in two adjacent slots because transmissions tend to overlap in time. This interference limits the number of users that can share the channel Time Channel 1 Bandwidth Channel 2 Channel 3 Channel 4 Channel 1 Channel 2 Channel 3 Time Division Multiple Access (TDMA)

5.  CDMA, a type of a spread-spectrum technique, allows multiple users to share the same channel by multiplexing their transmissions in code space. Different signals from different users are encoded by different codes (keys) and coexist both in time and frequency domains  A code is represented by a wideband pseudo noise (PN) signal  When decoding a transmitted signal at the receiver, because of low cross- correlation of different codes, other transmissions appear as noise. This property enables the multiplexing of a number of transmissions on the same channel with minimal interference  The maximum allowable interference (from other transmissions) limits the number of simultaneous transmissions on the same channel All channels share bandwidth Bandwidth Time Code Division Multiple Access (CDMA)

6.  Spreading of the signal bandwidth can be performed using - Direct Sequence (DS): the narrow band signal representing digital data is multiplied by a wideband pseudo noise (PN) signal representing the code. Multiplication in the time domain translates to convolution in the spectral domain. Thus the resulting signal is wideband - Frequency Hopping (FH): carrier frequency rapidly hops among a large set of possible frequencies according to some pseudo random sequence (the code). The set of frequencies spans a large bandwidth. Thus the bandwidth of the transmitted signal appears as largely spread Code Division Multiple Access (CDMA)

7. –S- MAC protocol designed specifically for sensor networks to reduce energy consumption while achieving good scalability and collision avoidance by utilizing a combined scheduling and contention scheme –The major sources of energy waste are: 1.collision 2.overhearing 3.control packet overhead 4.idle listening –S-MAC reduce the waste of energy from all the sources mentioned in exchange of some reduction in both per-hop fairness and latency An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC) [Ye+ 2002]

8. –S- MAC protocol consist of three major components: 1.periodic listen and sleep 2.collision and overhearing avoidance 3.Message passing –Contributions of S-MAC are:  The scheme of periodic listen and sleep helps in reducing energy consumption by avoiding idle listening. The use of synchronization to form virtual clusters of nodes on the same sleep schedule  In-channel signaling puts each node to sleep when its neighbor is transmitting to another node (solves the overhearing problem and does not require additional channel)  Message passing technique to reduce application-perceived latency and control overhead (per-node fragment level fairness is reduced)  Evaluating an implementation of S-MAC over sensor-net specific hardware (S-MAC) [Ye+ 2002]

9. [Ye+ 2002] W. Yei, J. Heidemann and D. Estrin, Energy-Efficient MAC Protocol for Wireless Sensor Networks, Proceedings of the Twenty First International Annual Joint Conference of the IEEE Computer and Communications Societies (INFOCOM 2002), New York, NY, USA, June 23-27 2002. References