1. MAC Protocols and Security in Ad hoc and Sensor Networks

2. 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 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

3. Power Control MAC (PCM)
IEEE 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.

4. Power Control MAC (PCM)
IEEE 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]

5. Power Control MAC (PCM)
IEEE MAC Protocol
DCF in IEEE is based on CSMA/CS (Carrier Sense Multiple Access with Collision Avoidance)
Each node in IEEE 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

6. Power Control MAC (PCM)
IEEE MAC Protocol
[Figure adapted from Jung+ 2002]

7. Power Control MAC (PCM)
IEEE MAC Protocol
IFS is the time interval between frames and IEEE 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

8. Power Control MAC (PCM)
IEEE 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 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)

9. Power Control MAC (PCM)
IEEE MAC Protocol
C’s carrier sensing range
D’s carrier sensing range
[Figure adapted from Jung+ 2002]

10. Power Control MAC (PCM)
IEEE 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

11. 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]

12. 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]

13. 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

14. 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

15. 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]

16. 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:
Source and destination nodes transmit the RTS and CTS using pmax. Nodes in the carrier sensing zone set their NAVs for EIFS duration
The source may transmit DATA using a lower power level
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
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

17. 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 by using less energy
PCM, like , does not prevent collisions completely
[Figure adapted from Jung+ 2002]

18. An Energy-Efficient MAC Protocol for Wireless Sensor Networks (S-MAC)
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:
collision
overhearing
control packet overhead
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

19. S-MAC S- MAC protocol consist of three major components:
periodic listen and sleep
collision and overhearing avoidance
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

20. 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:
Trust and key management
Secure routing and intrusion detection
Availability
Cryptographic protocols

21. 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
[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.