Download Ad-hoc Nets - Buali Sina University

Special Topics in
Computer Engineering
Wireless Networks
By: Mohammad Nassiri
Bu-Ali Sina University, Hamedan
Access method in
Wireless Ad-hoc
Networks
Ad-hoc mode in 802.11
 Ad Hoc
 Simplest
 Rapid deployment
 Peer-to-peer
 No administration
 Basically, ad-hoc mode in 802.11 does not
support multi-hop transmission. However, there
are a lot of mechanisms to provide the multihop transmission with the help of Layer-3,
namely, IP layer.
Multi-hop Ad-hoc Networks
 An Ad-hoc network
 Direct transmission with
neighboring nodes
 Each node can be router and
so it can relay traffic.

B relays packet from A to C
 Self-configuration, Self-
healing
 In this lecture, MAC issues
in Wireless Ad-hoc
Networks
Recall
 Rx = Reception Range
 CS = Carrier Sensing
Range
 A can communicate
to B
 C can only sense a
transmission emitted
from A
 D cannot overhear A
RTS/CTS for hidden problem
 D and C are hidden to
A
 D is within CSR of B
 A sends to B, D sends
to B, collision is
possible.  RTS/CTS
fails to resolve hidden
terminal in this case
RTS/CTS for exposed nodes ?
 RTS/CTS cannot handle exposed node
problem

The left-hand scenario
Masked node
 C cannot decode CTS
from B
 It’s NAV is not up to
date.
 Later it can collide the
transmission of A to B
by sending an RTS.
 C is masked by B and D
Blocked nodes in 3 pairs
 We consider blocked nodes in the
scenario of three parallel pairs

node in the middle has almost no
possibility to access the channel
 Studied by Chaudet et al.
2005
 e.g. each pair in a room
 A, C and E are emitters
 Emitter C is starved by
transmissions of A and E.
9
Three pairs
 How does legacy DCF work in this scenario
when A and E are transmitting ?
EIFS
DIFS
A
Backoff
DATA
DATA
Busy
Channel
DATA
DATA
DATA
DATA
C
E
DATA
DATA
DATA
DATA
C is starved by A and E
10
Three pairs
 How does legacy DCF work in this
scenario when C is transmitting ?
EIFS
DIFS
Backoff
Busy
Channel
E
DATA
DATA
A
C
DATA
DATA
DATA
DATA
DATA
Long term
Unfairness
11
DCF evaluation in a chain
Throughput for chain with different length
Claude Chaudet: IEEE com. Magazine 2005
Next 5 slides from
Does the IEEE 802.11 MAC
Protocol Work Well in Multihop
Wireless Ad Hoc Networks?
Shugong Xu
Tark Saadawi
June, 2001
IEEE Communications Magazine
(Adapted from mnet.cs.nthu.edu.tw/paper/jbb/010704.pps)
Serious Unfairness – (1)
 2 TCP Connections
First session starts at 10.0s ( 6  4 )
 Second session starts 20.0s later ( 2  3 )

1
2
3
4
Source Destination Destination
5
6
Source
Serious Unfairness – (2)
First session start
Second session start
Serious Unfairness – (3)
 The throughput of the first session is zero
in most of its lifetime after the second
session starts.
 There is not even a chance for it to
restart.
 The loser session is completely shutdown
even if it starts much earlier.
Serious Unfairness – (6)
 Discussion:

Node5 cannot reach node4 when
• Node2 is sending (collision)
• Node3 is sending ACK (defer)
1
2
3
4
Source Destination Destination
5
6
Source
Conclusion
 The hidden terminal problem still exists in
multihop networks.
 The exposed terminal problem will be more
harmful in a multihop network and there is
no scheme in IEEE 802.11 standard to deal
with this problem.
 The binary exponential backoff scheme
always favors the latest successful node.
It will cause unfairness.
Multiple Channels for Wireless
Networks
Traditional Ad Hoc Network:
Single Channel
 Each device has 1 radio. All radios are tuned to
the same channel.
20
Motivation
Exploit multiple channels to improve network
throughput’ … why ?
1
1
defer
2
Greater parallel communication is possible
Typical Wireless Networks
Power
Density
t=0
t=1
t=2
Each network uses 1 channel only.
Sender 1
frequency
Can we do better?
Sender 2
frequency
Sender 3
frequency
:
:
Channel 1
Channel 2 Channel 3
22
Can we do better?
Power
Density
t=0
t=1
t=2
Simultaneous sending on different channels?
Sender 1
Sender 4
Sender 3
frequency
Sender 2
Sender 1
Sender 4
frequency
Sender 3
Sender 4
Sender 2
frequency
:
:
Channel 1
Channel 2 Channel 3
23
Goal
 Given a wireless network where:
 M (>1) channels are available
 each node has 1 tunable radio
 each node has many neighbors
 Design a Multi-Channel MAC protocol:
 increases total network throughput
 achieves low average delay
 robust, practical
24
Why Multi-Channel MAC?
Multi-Channel MAC
t=0
t=1
Sender 1
Sender 4
Sender 3
frequency
Sender 2
Sender 1
Sender 4
frequency
Single “Super” Channel
t=0
t=1
Sender 1
frequency
Sender 2
frequency
25
M-Channel Schedule example
26
M-Channel Schedule example
27
Core Design Issues
Q1: Which channel is receiver Y listening on?
time=t
?
?
?
frequency
receiver Y
Q2: Is channel i free?
time=t
Free ?
Chan i
frequency
28
Multi-channel Hidden Terminals
Multi-channel Hidden Terminals
Observations
1. Nodes may listen to different channels
2. Virtual Carrier Sensing becomes difficult
3. The problem was absent for single channel
Multi-Channel MAC Protocols
 (1) Dedicated Control Channel (2 radios)
 Dedicated control radio & channel for all control messages

DCA [Wu2000], DCA-PC [Tseng2001], DPC [Hung2002].
 (2) Split Phase
 Time divided into alternate (i) channel negotiation phase on
default channel & (ii) data transfer phase on all channels

MMAC [J.So2003], MAP [Chen et al.]
 (3) Common Hopping Sequence
 All idle nodes follow the same channel hopping sequence

HRMA [Tang98], CHMA, CHAT [Tzamaloukas2000]
 (4) Parallel Rendezvous
 Each node follows its own channel hopping sequence

SSCH [Bahl04], McMAC ()
31
Protocol (1):
Dedicated Control Channel
Channel
Keys: 2 Radios/Node; Rendezvous on 1
channel; No time sync
Ch3
Data
(data)
Ch2
Data
(data)
Ack ...
Data
Ack
Ack
Ch1
(Ctrl)
RTS
(2,3)
CTS
(2)
RTS
(3)
CTS
(3)
Legend: Node 1 Node 2 Node 3 Node 4
Time
32
Protocol (2):
Split-Phase
Channel
Ch3
Keys: 1 Radio; Rendezvous on a common
channel; Coarse time sync
...
Unused
Ch2
Rts Cts Data Ack ...
Ch1
...
...
Rts Cts Data
Hello Ack
(1,2,3) (1)
Ack
Hello Ack
(2,3) (2)
Control Phase
Time
Data Transfer
Phase
33
Protocol (3):
Common Hopping
Channel
Key: 1 radio; Non-busy nodes hop together;
Tight time sync
Ch4
Ch3
Ch2
Data/Ack ...
RTS+CTS
Ch1
1
2
3
4
Enough for RTS/CTS
5
6
7
8
9
10
11
Time
34
A MAC protocol based on Split
Phase
802.11 PSM (Power Saving
Mode)
Doze mode – less energy consumption but no
communication
ATIM – Ad hoc Traffic Indication Message
Beacon
Time
A
B
C
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
Beacon
A
Time
ATIM
B
C
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
Beacon
A
Time
ATIM
B
ATIM-ACK
C
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
Beacon
A
ATIM ATIM-RES
B
ATIM-ACK
C
ATIM Window
Beacon Interval
Time
802.11 PSM (Power Saving Mode)
Beacon
A
ATIM ATIM-RES
Time
DATA
B
ATIM-ACK
Doze Mode
C
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
Beacon
A
ATIM ATIM-RES
Time
DATA
B
ATIM-ACK
ACK
Doze Mode
C
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
Summary
 All nodes wake up at the beginning of a beacon
interval for a fixed duration of time (ATIM window)
 Exchange ATIM during ATIM window
 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
MMAC : Assumptions
 All channels have same BW and none of them are
overlapping channels
 Nodes have only one transceiver
 Transceivers are capable of switching channels but
they are half-duplex
 Channel switching delay is approx 250 us, avoid per
packet switching
MMAC : Steps
 Divide time into beacon intervals
 At the beginning, nodes listen to a pre-defined
channel for ATIM window duration
 Channel negotiation starts using ATIM messages
 Nodes switch to the agreed upon channel after the
ATIM window duration
MMAC
 Preferred Channel List (PCL)
 For a node, PCL records usage of channels inside Tx range
 HIGH preference – always selected
 MID preference – others in the vicinity did not select the
channel
 LOW preference – others in the vicinity selected the
channel
MMAC
Channel Negotiation
 Sender transmits ATIM to the receiver and includes its
PCL in the ATIM packet
 Receiver selects a channel based on sender’s PCL and its
own PCL
 Receiver sends ATIM-ACK to sender including the
selected channel
 Sender sends ATIM-RES to notify its neighbors of the
selected channel
MMAC
Common Channel
Selected Channel
A
Beacon
B
C
D
Time
ATIM Window
Beacon Interval
MMAC
Common Channel
ATIMATIM RES(1)
A
B
Selected Channel
Beacon
ATIMACK(1)
C
D
Time
ATIM Window
MMAC
Common Channel
A
B
C
D
Selected Channel
ATIM ATIMRES(1)
Beacon
ATIMACK(1)
ATIMACK(2)
ATIM ATIMRES(2)
ATIM Window
Time
MMAC
Common Channel
Selected Channel
ATIM ATIMRES(1)
RTS
DATA
Channel 1
A
Beacon
Channel 1
B
ATIMACK(1)
CTS
ATIMACK(2)
CTS
ACK
ACK
Channel 2
C
Channel 2
D
ATIM ATIMRES(2)
RTS DATA
ATIM Window
Beacon Interval
Time
Experimental Parameters
Transmission rate: 2Mbps
Transmission range: 250m
Traffic type: Constant Bit Rate (CBR)
Beacon interval: 100ms
Packet size: 512 bytes
ATIM window size: 20ms
Default number of channels: 3 channels
Compared protocols
802.11: IEEE 802.11 single channel protocol
DCA: Wu’s protocol
MMAC: Proposed protocol
WLAN - Throughput
Multi-hop Network - Throughput
Analysis
 For MMAC:
 ATIM window size significantly affects performance
 ATIM/ATIM-ACK/ATIM-RES exchanged once per flow
per beacon interval – reduced overhead
 ATIM window size can be adapted to traffic load
Discussions
 MMAC requires a single transceiver per host to
work in multi-channel ad hoc networks
 MMAC
achieves
throughput
performance
comparable to a protocol that requires multiple
transceivers per host
 Beaconing mechanism may fail to synchronize in a
multi-hop network – probabilistic beaconing may
help
 Starvation can occur with common source and
multiple destinations
Multi-interface Multi-channel
 Each node has multiple interfaces
References
 J. So, N. Vaidya; ``Multi-Channel MAC for Ad Hoc Networks:
Handling Multi-Channel Hidden Terminals Using A Single
Transceiver''; Proc. ACM MobiHoc 2004
 S.-L.Wu, C.-Y. Lin, Y.-C. Tseng, and J.-P. Sheu. "A new multichannel
MAC protocol with on-demand channel assignment for multi-hop
mobile ad hoc networks."; In Int’l Symp. on Parallel Architectures,
Algorithms and Networks (I-SPAN), 2000.
 C. Chaudet, D. Dhoutaut, I. G. Lassous, Performance issues with
IEEE 802.11 in ad hoc networking , IEEE Communications
magazine, Volume 43, Number 7; Pages: 110-116, July 2005
 S. Xu, T. Saadawi, Does the IEEE 802.11 MAC Protocol Work Well in
Multihop Wireless Ad Hoc Networks?, IEEE Communications
magazine, June 2001
Review
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