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
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]
–
Why STUDY MAC protocols in sensor networks?

Application behavior in sensor networks leads to very different traffic
characteristics from that found in conventional computer networks

Highly constrained resources and functionality

Small packet size

Deep multi-hop dynamic topologies

The network tends to operate as a collective structure, rather than
supporting many independent point-to-point flows

Traffic tends to be variable and highly correlated

Little or no activity/traffic for longer periods and intense traffic over
shorter periods
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]
–
Major factors to be considered in the design of MAC:


Communication efficiency in terms of energy consumed per each packet
o
Communication by radio channel consumes the highest energy
o
Transmit , receive and idle consume roughly the same amount of energy
Fairness of the bandwidth allocated to each node for end to end data
delivery to sink
o
Each node acts as a router as well as data originator resulting in two
kinds of traffic
o
The traffics compete for the same upstream bandwidth
o
Hidden nodes

Contention at the upstream node may not be detected and results
in significant loss rate

Efficient channel utilization
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]
–
–
Major factors to be considered in the design of MAC:

The routing distance and degree of intermediate competition varies
widely across the network

The cost of dropping a packet varies with place and the packet
Contribution of this paper are as follows:

Listening mechanism:
o
Listening is effective when there are no hidden nodes
o
It comes at an expense of energy cost as the radio must be on to listen
o
Many protocols such as IEEE 802.11 require sensing the channel even
during backoff
o
Shorten the length of carrier sensing and power off the node during
backoff
o
Highly synchronized nature of the traffic causes no packet transfer at all
in the absence of collision detection hardware
o
Introduce random delay for transmission to unsynchronized the nodes
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]


Backoff Mechanism:
o
Used to reduce the contention among the nodes
o
In the sensor networks, traffic is a superposition of different periodic
streams
o
Apply back off as a phase shift to the periodicity of the application so
that the synchronization among periodic streams of traffic can be broken
Contention-based Mechanism
o
Explicit control packets like RTS and CTS are used to avoid contention
o
ACKS indicate lack of collision
o
Use of lot of control packets reduces bandwidth efficiency
o
ACKS can be eliminated by hearing the packet transmission from its
parent to its upstream which serves as an ACK for the downstream node
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]

Rate Control Mechanism
o
The competition between originating traffic and route-thru traffic has a
direct impact in achieving the fairness goal.
o
MAC should control the rate of originating data of a node in order to
allow route-thru traffic to access the channel and reach the base
station and some kind of progressive signaling for route-thru traffic
such the rate is controlled at the origin.
o
A passive implicit mechanism is used to control the rate of
transmission of both traffics
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]

Multi-hop Hidden Node problem:
o
It avoid the hidden node problem by constantly tuning the
transmission rate and performing phase changes so that the
aggregate traffic will not repeatedly collide with each other.
o
A child can reduce a potential hidden node problem with its grand
parent by not sending packets for t+ x+ packet time at the end of
packet transmission t by its parent
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]
Advantages:
–
The amount of computation for this scheme is small and within
networked sensor’s computation capability
–
The scheme is totally computational which is much cheaper in energy
cost than on the radio
–
The control packet overhead is reduced
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]
Disadvantages:
–
The MAC protocol developed here takes into consideration the
periodicity of the originating traffic which doesn’t help for non periodic
traffic
A Transmission Control Scheme for Media Access in
Sensor Networks [Woo+, 2003]
Suggestions/Improvements/Future Work:
An Adaptive Energy-Efficient MAC Protocol for Wireless
Sensor Networks [Van dam+, 2003]
–
T-MAC is a contention based Medium Access Control Protocol
–
Energy consumption is reduced by introducing an active/sleep duty cycle
–
Handles the load variations in time and location by introducing an
adaptive duty cycle

It reduces the amount of energy wasted on idle listening by dynamically
ending the active part of it
–
In T-MAC, nodes communicate using RTS, CTS, Data and ACK pkts
which provides collision avoidance and reliable transmission
–
When a node senses the medium idle for TA amount of time it
immediately switches to sleep
–
TA determines the minimal amount of idle listening time per frame
–
The incoming messages between two active states are buffered
An Adaptive Energy-Efficient MAC Protocol for Wireless
Sensor Networks [Van dam+, 2003]
–
The buffer capacity determines an upper bound on the maximum frame
time
–
Frame synchronization in T-MAC follows the scheme of virtual clustering
as in S-MAC
–
The RTS transmission in T-MAC starts by waiting and listening for a
random time within a fixed contention interval at the beginning of the each
active state
–
The TA time is obtained using TA > C + R + T
–
T-MAC suffers from early sleeping problem
–
Its overcome by sending Future request to send or taking priority on full
buffers
An Adaptive Energy-Efficient MAC Protocol for Wireless
Sensor Networks [Van dam+, 2003]
Advantages:
–
The T-MAC protocol is designed particularly for wireless sensor
networks and hence energy consumption constraints are taken into
account
–
The T-MAC protocol tries to reduce idle listening by transmitting all
messages in bursts of variable lengths and sleeping between burst
–
T-MAC facilitates collision avoidance and overhearing -- nodes transmit
their data in a single burst and thus do not require additional RTS/CTS
control packets.
–
By stressing on RTS retries, T-MAC gives the receiving nodes enough
chance to listen and reply before it actually goes to sleep -- this
increases the throughput in the long run
An Adaptive Energy-Efficient MAC Protocol for Wireless
Sensor Networks [Van dam+, 2003]
Disadvantages:
–
The authors do not outline how a sender node would sense a FRTS packet
and enable it to send a DS packet
–
Also sending a DS packet increases the overhead.
–
The network topology in the simulation considers that the locations of the
nodes are known
–
T-MAC has been observed to have a high message loss phenomenon
–
T-MAC suffers from early sleeping problem for event based local unicast
An Adaptive Energy-Efficient MAC Protocol for Wireless
Sensor Networks [Van dam+, 2003]
Suggestions/Improvements/Future Work:
–
If a buffer is full there would be a lot of dropped packets decreasing the
throughput. A method to overcome this drawback is that we could have
the node with its buffer 75% full broadcast a special packet Buffer Full
Packet
–
MAC Virtual Clustering technique needs to be further investigated
–
An adaptive election algorithm can be incorporated where the schedule
and neighborhood information is used to select the transmitter and
receivers for the current time slot, hence avoiding collision and
increasing energy conservation
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.
[Woo+ 2003] A. Woo and D. Culler, A Transmission Control Scheme for Media Access in Sensor
Networks, Proceedings of the ACM/IEEE International Conference on Mobile Computing and
Networking, Rome, Italy, July 2001, pp. 221-235.
[Van Dam+ 2003] T. V. Dam and K. Langendoen, An Adaptive Energy-Efficient MAC Protocol for
Wireless Sensor Networks, ACM SenSys, Los Angeles, CA, November, 2003.