This document summarizes research on medium access control (MAC) layer protocols for ad-hoc networks. It begins with an introduction to ad-hoc networks and their key properties. It then discusses important issues at the MAC layer for these dynamic networks, including limited bandwidth, errors, and changing topologies. Several MAC protocol classifications and examples are provided, such as power-aware, multiple channel, and quality of service protocols. The document concludes by discussing future research directions for addressing open problems at the MAC layer in ad-hoc networks.

1. Ad-Hoc Network Architecture and
protocols of Medium Access
Control (MAC) Layer
Under the Supervision of Dr. M. P. Singh
Akansha Verma
M.Tech, 2nd Year
Reg. no: 1328014
Computer Science & Engineering

2. Outline
 Introduction
 Key properties
 Applications
 Important Issues
 Mac Layer in Ad-Hoc networks
 Need for Special MAC protocols
 Classification
 QoS
 Conclusion

3. What are ad Hoc Networks?
 A wireless network which does not
have a infrastructure and is self
adaptive and organizing is known as
ad Hoc network.
 In these network any device can
associate with another device within
its link range.

5. Characteristics
 Don’t have any infrastructure.
 Spontaneously Deployable.
 Dynamic.
 Multihop Based.
 Adaptable.
 Usually Mobile.

6. Technical Aspects
 Consists of Multiple nodes and Links.
 Links are influenced by nodes
behavior and link properties.
 As this network is dynamic it should
cope up with issues like restructuring
of links.
 Routing.

7. Applications
 In case of natural disaster.
 Military applications.
 Mobile Computing etc.
 Industrial and Scientific Applications.

8. Important Issues
 Limited Bandwidth
-Wireless Media in tightly controlled and has
limitations.
 Error Prone
- Factors like Multiple access, Signal fading,
Additional Noise and interference cause errors .
 Frequent change in Topology
- Dynamic Nature of network and Mobility cause
unpredictable changes.

9. Continued.
 Routing
- Earlier protocols require a static network for
finding optimal routes but here network in dynamic.
 Security
-The information transferred should be secure.
 Energy Conservation
- Sometimes the node may have limited battery
supply.

10. Mac Layer in Ad-Hoc networks
 Responsibilities of a MAC layer in any
networks are a vast some of which include:
- Error Correction.
- Framing, Physical Addressing, Flow
Control, Error Control.
- Control of conflicts between nodes for
medium accessing.
- Efficiency and reliability.

11. Need for Special MAC
Protocols
 The usual CSMA and its variants
cannot be used.
 Because of fading of signal in wireless
media which remain undetected at
receiver.
 Hidden terminal problem
 Exposed terminal problem

13. Classification
 Mac Schemes can be classified
basically in Contention free and
contention based.
 Contention free scheme used
predefined assignments (ex time in
TDMA) which is good for static
networks.
 Contention Based are preferred for
Ad-Hoc networks.

14. Continued.

15. Continued.
 Classification of MAC protocols can be
done on different grounds like :
- Sender initiated or Receiver initiated.
- Power aware.
- Single channel or multiple channel.
- Quality of Service aware or not.
 The further classification is done on
these major points.

16. General MAC protocols
 These are the basic MAC protocols
from which various other protocols are
derived.
 Some of the examples includes
MACA, MACAW, FAMA etc.

17. Multiple access collision
avoidance (MACA)
 Introduced to solved hidden and
exposed terminal problems.
 Uses RTS-CTS mechanism for data
transfer.
 Key idea is that any neighboring node
that hear the RTS or CTS defers its
transmission.
 Its effective because RTS and CTS
are significantly smaller then the data
packet so their collision causes less

18. Continued
 Not always solves the hidden terminal
problem.
 Lack of acknowledge of data packet.
 Various other variants of MACA were
later introduced like: MACAW, MACA-
BI, FAMA etc.

19. Power Aware MAC protocols
 It is crucial to conserve the energy of
mobile nodes as they are battery
powered.
 Collision is the main cause of power
loss.
 Transceivers should be kept in
standby mode when no transmission
is occurring Ex: PAMAS .
 An optimum power for transmission
should be used instead of full
transmission power Ex: PCM.

20. Power aware MAC signalling
(PAMAS)
 Uses separate channels for data and
control packets.
 While receiving a data on data
channel it sends busy tone on signal
channel.
 Each node decides on its own when to
sleep based on activity of its
neighboring nodes.
 Usually switches off when it has
nothing to transmit or neighbor is

21. Power control medium access
control (PCM)
 Unlike the other approaches which
alternates between sleep and wake
state, it sends RTS and CTS with
maximum power.
 Whereas data is sent with minimum
power.
 Method of determining the power
levels is explained further.

22. Continued

23. Continued.
 Node D sends the RTS to node E at a
transmit power level Pmax, and also
includes this value in the packet.
 E measures the actual signal strength,
say Pr, of the received RTS packet.
 Based on Pmax, Pr and the noise level at
its location, E then computes the
minimum necessary power level (say,
Psuff) that would actually be sufficient for
use by D.
 Now, when E responds with the CTS
packet using the maximum power it has
available, it includes the value of Psuff
that D subsequently uses for data

24. Drawbacks
 Accurate estimation of signal strength
required.
 dynamics of wireless signal
propagation due to fading and
shadowing effect may degrade its
performance.
 Difficulty in implementing frequent
changes in the transmit power levels.

25. Multiple channel protocols
 A major problem of single shared
channel schemes is that the
probability of collision increases with
the number of nodes.
 It has two approaches first using
separate channel for data and signal
messages.
 Second is using multiple channel for
data transmissions Ex : multichannel
CSMA, MMAC etc.

26. Multi channel CSMA MAC
protocol
 Divides the total available bandwidth (W)
into N distinct channels of W/N
bandwidth each.
 A transmitter would use carrier sensing
to see if the channel it last used is free.
 It uses the last used channel if found
free. Otherwise, another free channel is
chosen at random.
 If no free channel is found, the node
should back off and retry later.

27. Multichannel medium access
control
(MMAC) Utilizes multiple channels by switching
among them dynamically unlike the
earlier protocol.
 MMAC is an adaptation to the DCF in
order to use multiple channels.
 Time is divided into multiple fixed time
beacon intervals with small ATIM
window in beginning.
 Every node maintains a preferred
channel list (PCL) that stores channel
priorities.

28. Continued
 Suppose anode wants to transmit a
data packet it sends out an ATIM
packet along with its PCL.
 Receiver compares sender’s PCL with
its own and selects appropriate
channel for use and responds for the
following in ATIM ACK packet.
 If accepted sender replies with ATIM-
RES packet otherwise wait till the next
beacon frame.

29. Quality of Services
 QoS are the specific services like low
delay, high throughput etc. expected
from a network by an application.
 Issues Affecting QoS.
 QoS aware MAC protocols.
 An example of each type.

30. Issues affecting QoS
 Several issues need to be considered
in context of providing QoS in Ad-hoc
networks.
 All the layers should take part in
providing QoS.
 Some of the issue affecting are listed
next.

31. Continued
 Signaling
- in order to reserve, maintain resources.
- Should be performed reliably.
- Minimum overhead.
 Routing
- if application needs guaranteed bandwidth and low
delay then they should be QoS aware.
- nodes in the nodes should have sufficient resources to
provide QoS.
 Service model
- Specifies the framework within which services are
provided in a network..

32. QoS aware MAC protocols.
 The MAC layer protocol should
support resource reservation and real
time (rt) traffic.
 It needs to be related with above layer.
 These protocols can be further
classified as: synchronous and
asynchronous.
 Discussion of one protocol for each
category is done next.

33. Cluster TDMA
 Organization of nodes is in clusters.
 A cluster head coordinates the
activities of the nodes.
 A common, globally synchronous
slotted TDMA frame is defined among
clusters.
 Slots can be reserved by rt traffic and
free slots are used by non-real-time
(nrt) data.
 However, time synchronization is a

34. Real-time MAC (RT-MAC)
 In earlier protocols, packets that have
missed their deadlines are still
retransmitted, even though they are
not useful any more.
 This scheme uses scheme uses a
packet transmission deadline and an
enhanced collision avoidance scheme.
 For every rt packet a time stamp is
associated.

35. Continued.
 Sending nodes checks the expiration
of packet at three point, expired
packets are dropped.
 Sender chooses the back off value
from contention window and sends it
in packet header.
 Collision window (CW) is a function of
number of nodes.

36. Future directions
 Hidden and exposed terminal issues.
 Energy conservation issues.
 Antenna related issues.

37. Conclusion
 In this presentation a broad overview of
research work conducted in the field of
ad hoc wireless networks with respect to
MAC protocols is given.
 Discussed many schemes and identified
their salient features and also discussed
issues of collision resolution, power
conservation, multiple channels,
advantages of using directional antennas
and QoS.

38. Thank
You.