Medium Access Control in
Ad hoc and 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)
–
–
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 Power Control MAC (PCM) Protocol for Ad
hoc Networks [Jung+ 2002]
–
A power control MAC protocol allows nodes to vary transmit power level on a
per-packet basis
–
Earlier work has used different power levels for RTS-CTS and DATA-ACK,
specifically, maximum transmit power is used for RTS-CTS and minimum
required transmit power is used for DATA-ACK transmissions
–
These protocols may increase collisions, degrade network throughput and result
in higher energy consumption than when using IEEE 802.11 without power
control
–
Power saving mechanisms allow nodes to enter a doze state by powering off
its wireless network interface whenever possible
–
Power control schemes vary transmit power to reduce energy consumption
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
–
Specifies two MAC protocols:

Point Coordination Function (PCF)  centralized

Distributed Coordination Function (DCF) distributed
Transmission range:
When a node is in transmission range of a sender node, it can receive and
correctly decode packets from sender node.
Carrier Sensing Range:
Nodes in carrier sensing range can sense the sender’s transmission. It is generally
larger than transmission range. Both carrier sensing range and transmission range
Depends on the transmit power level.
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
Carrier Sensing Zone:
Nodes can sense the signal, but cannot decode it correctly. The carrier sensing zone
does not include transmission range
[Figure adapted from Jung+ 2002]
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
–
DCF in IEEE 802.11 is based on CSMA/CS (Carrier Sense Multiple Access with
Collision Avoidance)
–
Each node in IEEE 802.11 maintains a NAV (Network Allocation Vector) that
indicates the remaining time of the on-going transmission sessions
–
Carrier sensing is performed using physical carrier sensing (by air interface) and
virtual carrier sensing (uses the duration of the packet transmission that is
included in the header of RTS, CTS and DATA frames)
–
Using the duration information in RTS, CTS and DATA packets, nodes update
their NAVs whenever they receive a packet
–
The channel is considered busy if either physical or virtual carrier sensing
indicates that channel is busy
–
Figure 2 shows how nodes in transmission range and the carrier sensing zone
adjust their NAVs during RTS-CTS-DATA-ACK transmission
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
[Figure adapted from Jung+ 2002]
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
–
IFS is the time interval between frames and IEEE 802.11 defines four IFSs which
provide priority levels for accessing the channel

SIFS (short interframe space)

PIFS (Point Coordination Function interframe space)

DIFS (Distributed Coordination Function interframe space)

EIFS (extended interframe space)
–
SIFS is the shortest and is used after RTS, CTS, and DATA frames to give the
highest priority to CTS, DATA and ACK respectively
–
In DCF, when the channel is idle, a node waits for DIFS duration before transmitting
–
Nodes in the transmission range correctly set their NAVs when receiving RTS/CTS
–
Since nodes in carrier sensing zone cannot decode the packet, they do not know
the duration of the packet transmission. So, they set their NAVs for the EIFS
duration to avoid collision with the ACK reception at the source node
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
–
The intuition behind EIFS is to provide enough time for a source node to receive the
ACK frame, meaning that duration of EIFS is longer than that of ACK transmission
–
In PCM, nodes in the carrier sensing zone use EIFS whenever they can sense the
signal but cannot decode it
–
IEEE 802.11 does not completely prevent collisions due to the hidden terminal
problem (nodes in the receiver’s carrier sensing zone, but not in the sender’s carrier
sensing zone or transmission range, can cause a collision with the reception of a
DATA packet at the receiver
–
In Figure 3, suppose node C transmits packet to node D
–
When C and D transmit an RTS and CTS respectively, A and F set their NAVs for
EIFS duration
–
During C’s data transmission, A defers its transmission due to sensing C’s
transmission. However, since node F does not sense any signal during C’s
transmission, it considers channel to be idle (F is in D’s carrier sensing zone, but
not in D’s)
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
C’s carrier sensing range
D’s carrier sensing range
[Figure adapted from Jung+ 2002]
Power Control MAC (PCM)
IEEE 802.11 MAC Protocol
–
When F starts a new transmission, it can cause a collision with the reception of
DATA at D
–
Since F is outside of D’s transmission range, D may be outside of F’s transmission
range; however, since F is in D’s carrier sensing zone, F can provide interference at
node D to cause collision with DATA being received at D
Power Control MAC (PCM)
BASIC Power Control Protocol
–
Power control can reduce energy consumption
–
Power control may bring different transmit power levels at different hosts, creating
an asymmetric scenarios where a node A can reach node B, but node B cannot
reach node A and collisions may also increase a result
–
In Figure 4, suppose nodes A and B use lower power level than nodes C and D
–
When A is transmitting to B, C and D may not sense the transmission
–
When C and D transmit to each other using higher power, their transmission may
collide with the on-going transmission from A to B
[Figure adapted from Jung+ 2002]
Power Control MAC (PCM)
BASIC Power Control Protocol
–
As a solution to this problem, RTS-CTS are transmitted at the highest possible
power level but DATA and ACK at the minimum power level necessary to
communicate
–
In Figure 5, nodes A and B send RTS and CTS respectively with highest power level
such that node C receives the CTS and defers its transmission
–
By using a lower power level for DATA and ACK packets, nodes can save energy
[Figure adapted from Jung+ 2002]
Power Control MAC (PCM)
BASIC Power Control Protocol
–
In the BASIC scheme, RTS-CTS handshake is used to decide the transmission
power for subsequent DATA and ACK packets which can be achieved in two
different ways

Suppose node A wants to send a packet to node B. Node A transmit RTS at
power level pmax (maximum possible). When B receives the RTS from A with
signal level pr, B calculates the minimum necessary transmission power level,
pdesired. For the DATA packet based on received power level, pr, transmitted
power level, pmax, and noise level at the receiver B. Node B specifies pdesired in
its CTS to node A. After receiving CTS, node A sends DATA using power level
pdesired.

When a destination node receives an RTS, it responds by sending a CTS (at
power level pmax). When source node receives CTS, it calculates pdesired based
on received power level, pr, and transmitted power level (pmax) as
Pdesired = (pmax / pr) x Rxthresh x c
where Rxthresh is minimum necessary received signal strength and c is constant
Power Control MAC (PCM)
BASIC Power Control Protocol
–
The second alternative makes two assumptions:

Signal attenuation between source and destination nodes is assumed to be the
same in both directions

Noise level at the receiver is assumed to be below some predefined threshold
Deficiency of the BASIC Protocol
–
In Figure 6, suppose node D wants to transmit to node E
–
When nodes D and E transmits RTS and CTS respectively, B and C receives RTS
and F and G receives CTS, therefore, these nodes defer their transmissions
–
Since node A is in carrier sensing zone of node D, it sets its NAV for EIFS duration
–
Similarly node H sets its NAV for EIFS duration when it senses transmission from E
–
When source and destination decide to reduce the transmit power for DATA-ACK,
not only transmission range for DATA-ACK but also carrier sensing zone is also
smaller than RTS-CTS
Power Control MAC (PCM)
Deficiency of the BASIC Protocol
–
Thus, only C and F correctly
receives DATA and ACK packets
–
Since nodes A and H cannot sense
the transmissions, they consider
channel is idle and start
transmitting at high power level
which will cause collision with the
ACK packet at D and DATA packet
at E
–
This results in throughput
degradation and higher energy
consumption (due to
retransmissions)
[Figure adapted from Jung+ 2002]
Power Control MAC (PCM)
Proposed Power Control MAC Protocol
–
Proposed Power Control MAC (PCM) is similar to BASIC scheme such that it uses
power level, pmax, for RTS-CTS and the minimum necessary transmit power for
DATA-ACK transmissions
–
Procedure of PCM is as follows:
–
1.
Source and destination nodes transmit the RTS and CTS using pmax. Nodes in
the carrier sensing zone set their NAVs for EIFS duration
2.
The source may transmit DATA using a lower power level
3.
Source transmits DATA at level of pmax, periodically, for enough time so that
nodes in the carrier sensing zone can sense it and this would avoid collision
with the ACK packets
4.
The destination node transmits an ACK using the minimum required power to
reach the source node
Figure 7 presents how the transmit power level changes during the sequence of
RTS-CTS-DATA-ACK transmission
Power Control MAC (PCM)
Proposed Power Control MAC Protocol
–
The difference between PCM and BASIC scheme is that PCM periodically increases
the transmit power to pmax during the DATA packet transmission. Nodes that can
interfere with the reception of ACK at the sender will periodically sense the channel
is busy and defer their own transmission. Since nodes reside in the carrier sensing
zone defer for EIFS duration, the transmit power for DATA is increased once every
EIFS duration
–
PCM solves the problem posed with BASIC scheme and can achieve throughput
comparable to 802.11 by using less energy
–
PCM, like 802.11, does not prevent collisions completely
[Figure adapted from Jung+ 2002]
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
[Jung+ 2002] E.-S. Jung and N.H. Vaidya, A Power Control MAC Protocol for Ad hoc Networks,
Proceedings of ACM MOBICOM 2002, Atlanta, Georgia, September 23-28, 2002.
[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.