MAC Protocols and Security in
Ad hoc and Sensor Networks
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
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DCF in IEEE 802.11 is based on CSMA/CS (Carrier Sense Multiple Access with
Collision Avoidance)
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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
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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
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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
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Power control can reduce energy consumption
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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
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In Figure 4, suppose nodes A and B use lower power level than nodes C and D
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When A is transmitting to B, C and D may not sense the transmission
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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
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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
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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
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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
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Since node A is in carrier sensing zone of node D, it sets its NAV for EIFS duration
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Similarly node H sets its NAV for EIFS duration when it senses transmission from E
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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
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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
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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]
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
–
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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
Security in Wireless Ad hoc Networks
[Buttyan+ 2002]
–
Security in wireless ad hoc networks is difficult for many reasons:

Vulnerability of channels

Vulnerability of nodes

Absence of infrastructure

Dynamically changing topology
–
The problem is broad and there is no general solution
–
Different applications will have different security requirements
–
Security aspects can be categorized into four groups:
1.
Trust and key management
2.
Secure routing and intrusion detection
3.
Availability
4.
Cryptographic protocols
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.
[Buttyan+ 2002] L. Buttyan and J.-P. Hubaux, Report on a Working Session on Security in Wireless Ad
Hoc Networks, Mobile Computing and Communications Review, Volume 6, Number 4.