Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC) [Ye+ 2002](S-MAC)A Power Control MAC (PCM) Protocol for Ad hoc Networks [Jung+ 2002]Power Control MAC (PCM)Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Medium Access Control in Ad hoc and Sensor NetworksMAC allows multiple users to share a common channel. MAC allows multiple users to share a common channel. Conflict-free protocolsConflict-free protocols ensure successful transmission. Channel can be allocated to ensure successful transmission. Channel can be allocated to users statically or dynamically.users statically or dynamically.Only static conflict-free protocols are used in cellular mobile communicationsOnly static conflict-free protocols are used in cellular mobile communications- - Frequency Division Multiple Access (FDMA): provides a fraction of the frequency (FDMA): provides a fraction of the frequency range to each user for all the timerange to each user for all the time- - Time Division Multiple Access (TDMA)(TDMA) : : The entire frequency band is allocated to a The entire frequency band is allocated to a single user for a fraction of timesingle user for a fraction of time- - Code Division Multiple Access (CDMA) : provides every user a portion of bandwidth (CDMA) : provides every user a portion of bandwidth for a fraction of timefor a fraction of timeContention based protocolsContention based protocols must prescribe ways to resolve conflicts must prescribe ways to resolve conflicts- Static Conflict Resolution: Carrier Sense Multiple Access (CSMA) - Static Conflict Resolution: Carrier Sense Multiple Access (CSMA) - Dynamic Conflict Resolution: the Ethernet, which keeps track of various system - Dynamic Conflict Resolution: the Ethernet, which keeps track of various system parameters, ordering the users accordinglyparameters, ordering the users accordinglyMultiple Access Control (MAC) ProtocolsChannels are assigned to the user for the duration of a call. No other user can 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 access the channel during that time. When call terminates, the same channel can be re-assigned to another usercan be re-assigned to another userFDMA is used in nearly all first generation mobile communication systems, like FDMA is used in nearly all first generation mobile communication systems, like AMPS (30 KHz channels)AMPS (30 KHz channels)Number of channels required to support a user population depends on the 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 average number of calls generated, average duration of a call and the required quality of service (e.g. percentage of blocked calls)quality of service (e.g. percentage of blocked calls)Channel 1Channel 1Channel 2Channel 2Channel 3Channel 3Channel 4Channel 4BandwidthBandwidthTimeTimeFrequency Division Multiple Access (FDMA)The whole channel is assigned to each user, but the users are multiplexed in 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 during communication. Each communicating user is assigned a particular time slot, during which it communicates using the entire frequency spectrumtime slot, during which it communicates using the entire frequency spectrumThe data rate of the channel is the sum of the data rates of all the multiplexed The data rate of the channel is the sum of the data rates of all the multiplexed transmissionstransmissionsThere is always channel interference between transmission in two adjacent There is always channel interference between transmission in two adjacent slots because transmissions tend to overlap in time. This interference limits slots because transmissions tend to overlap in time. This interference limits the number of users that can share the channelthe number of users that can share the channelTimeTimeChannel 1Channel 1BandwidthBandwidthChannel 2Channel 2Channel 3Channel 3Channel 4Channel 4Channel 1Channel 1Channel 2Channel 2Channel 3Channel 3Time Division Multiple Access (TDMA)CDMA, a type of a CDMA, a type of a spread-spectrumspread-spectrum technique, allows multiple users to share the technique, allows multiple users to share the same channel by multiplexing their transmissions in code space. Different signals 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 from different users are encoded by different codes (keys) and coexist both in time and frequency domainsand frequency domainsA code is represented by a wideband pseudo noise (PN) signalA code is represented by a wideband pseudo noise (PN) signalWhen decoding a transmitted signal at the receiver, because of low cross-When decoding a transmitted signal at the receiver, because of low cross-correlation of different codes, other transmissions appear as noise. This property correlation of different codes, other transmissions appear as noise. This property enables the multiplexing of a number of transmissions on the same channel with enables the multiplexing of a number of transmissions on the same channel with minimal interferenceminimal interferenceThe maximum allowable interference (from other transmissions) limits the number The maximum allowable interference (from other transmissions) limits the number of simultaneous transmissions on the same channelof simultaneous transmissions on the same channelAll channels share bandwidthAll channels share bandwidthBandwidthBandwidthTimeTimeCode Division Multiple Access (CDMA)Spreading of the signal bandwidth can be performed usingSpreading of the signal bandwidth can be performed usingDirect SequenceDirect Sequence (DS): (DS): The narrow band signal representing digital data is multiplied by a wideband The narrow band signal representing digital data is multiplied by a wideband pseudo noise (PN) signal representing the code. Multiplication in the time domain pseudo noise (PN)
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