MAC Protocols In Sensor Networks
Multiple Access Control (MAC) Protocols
 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
- 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
Frequency Division Multiple Access (FDMA)
 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)
Bandwidth
Channel 1
Channel 2
Channel 3
Channel 4
Time
Time Division Multiple Access (TDMA)
 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
Channel 3
Channel 2
Time
Channel 1
Channel 4
Channel 3
Channel 2
Channel 1
Bandwidth
 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
Code Division Multiple Access (CDMA)
 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 crosscorrelation 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
Bandwidth
 The maximum allowable interference (from other transmissions) limits the
number of simultaneous transmissions on the same channel
All channels share bandwidth
Time
Code Division Multiple Access (CDMA)
 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
An Energy-Efficient MAC Protocol for Wireless
Sensor Networks (S-MAC) [Ye+ 2002]
–
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
(S-MAC) [Ye+ 2002]
–
–
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
References
[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.