wireless - Ad Hoc

1. Multichannel Medium Access
Control (MMAC)
Eng.Ahmed Ali El-Kosairy
eng.aelkosairy@gmail.com

2. Multichannel Medium Access
Control (MMAC)
Need to know MAC :
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IEEE- 802.11
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It only utilizes one channel at present
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The primary reason is that hosts with a single
half-duplex transceiver can only transmit or listen
to one channel at a time.

3. MAC -Con
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has 2 modes
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DCF (Distributed Coordination Function)
–
PCF (Point Coordination Function)
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For centrally coordinated infrastructure-based networks

4. MAC-Con
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DCF (Distributed Coordination Function)
–
for ad hoc networks
–
based on CSMA with Collision Avoidance
(CSMA/CA) protocol which:
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combination of the CSMA and MACA schemes.

5. MMAC
●
●
MMAC is an adaptation to the DCF to use
multiple channels.
How does it work :
–
Similar to the Dynamic Power-Saving Mechanism
(DPSM) scheme

6. Dynamic Power-Saving Mechanism
(DPSM)
●
●
Time is divided into beacon intervals
All nodes wake up at the beginning of a beacon interval
for a fixed duration of time (ATIM window)
–
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<Ad Hoc Traffic Indication Message (ATIM)>
Exchange ATIM (Ad-hoc Traffic Indication Message)
during ATIM window

7. DPSM-Con
●
●
Nodes that receive ATIM message stay up
during for the whole beacon interval
Nodes that do not receive ATIM message may
go into doze mode after ATIM window

8. How does it work (MMAC)
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●
●
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Divide time into beacon intervals
At the beginning of each beacon interval, all nodes must listen to a
predefined common channel for a fixed duration of time (ATIM
window)
Nodes negotiate channels using ATIM messages
Nodes switch to selected channels after ATIM window for the rest
of the beacon interval
Use PCL

9. Preferred Channel List (PCL)
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Each node maintains PCL
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Records usage of channels inside the transmission range
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High preference (HIGH)
–
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Medium preference (MID)
–
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Already selected for the current beacon interval
No other vicinity node has selected this channel
Low preference (LOW)
–
This channel has been chosen by vicinity nodes
–
Count number of nodes that selected this channel to break ties

10. Channel Negotiation
Common Channel
Selected Channel
A
Beacon
B
C
D
Time
ATIM Window
Beacon Interval
Key Ideas

11. Channel Negotiation
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In ATIM window, sender transmits ATIM to the receiver
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Sender includes its PCL in the ATIM packet
●
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Receiver selects a channel based on sender’s PCL and its
own PCL
Order of preference: HIGH > MID > LOW

12. Channel Negotiation-con
●
●
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Tie breaker: Receiver’s PCL has higher priority
Receiver sends ATIM-ACK to sender including
the selected channel
Sender sends ATIM-RES to notify its neighbors
of the selected channel

13. Channel Negotiation
Common Channel
A
B
Selected Channel
ATIMATIM RES(1)
Beacon
ATIMACK(1)
C
D
Time
ATIM Window
Beacon Interval
Key Ideas

14. Channel Negotiation
Common Channel
A
B
C
D
Selected Channel
ATIMATIM RES(1)
Beacon
ATIMACK(1)
ATIMACK(2)
ATIM
ATIMRES(2)
Time
ATIM Window
Beacon Interval
Key Ideas

15. Channel Negotiation
Common Channel
A
B
C
D
ATIMATIM RES(1)
Selected Channel
RTS
DATA
Channel 1
Beacon
Channel 1
CTS
ATIMACK(1)
ATIMACK(2)
ACK
CTS
ACK
Channel 2
Channel 2
ATIM
ATIMRES(2)
RTS
DATA
Time
ATIM Window
Beacon Interval
Key Ideas

16. Advantages of MAC
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The performance of MMAC is better than that of
IEEE 802.11 and Dynamic Channel Assignment
(DCA) in terms of throughput.
It can be easily integrated with IEEE 802.11
Power-Saving Mechanism (PSM) mode while
using a simple hardware.

17. Aggregate Throughput (Kbps)
Performance Test
2500
2500
MMAC
2000
DCA
1500
1000
500
MMAC
2000
1500
DCA
1000
802.11
1
10
100
1000
Packet arrival rate per flow (packets/sec)
30 nodes
500
1
802.11
10
100
1000
Packet arrival rate per flow (packets/sec)
64 nodes
MMAC shows higher throughput than DCA and 802.11

18. Disadvantages of MAC
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It has a longer packet delay than DCA. More
over, it is not suitable for multi hop ad hoc
networks as it assumes that the nodes are
synchronized.

19. Looking for
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A better scheme for clock synchronization

20. Dynamic Channel Assignment with
Power Control (DCA-PC)

21. DCA-PC
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DCA-PC is an extension of their DCA protocol
that did not consider the issue of power control.
It combines concepts of power control and
multiple-channel medium access in the context
of MANETs.

22. How Does It work
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The hosts are assigned channels dynamically
Every node is equipped with two half-duplex
transceivers, and the bandwidth is divided into
–
Control channel ,which:
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operate to exchange control pkt
Multiple data channels
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Exchanging data and acknowledgments (with power control).

23. How Does It work-2
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When a host needs a channel to talk to another,
it engages in an RTS–CTS–RES exchange
where RES is a special reservation packet
–
indicating the appropriate data channel to be used.

24. How Does It work-3
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very node keeps a table of power levels to be:
–
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These power levels are calculated based on:
–
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used when communicating with any other node.
the RTS–CTS exchanges on the control channel
Because:
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every node is always listening to the control channel.
–
it can even dynamically update the power values based on the
other control exchanges happening around it

25. Advantages of DCA-PC
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DCA-PC has been shown to achieve higher
throughput than DCA.
DCA-PC is a novel attempt at solving dynamic
channel assignment and power control issues
in an integrated fashion.