special topics in computer engineering wireless networks
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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. - PowerPoint PPT PresentationTRANSCRIPT
Special Topics in Computer
Engineering
Wireless NetworksBy: Mohammad Nassiri
Bu-Ali Sina University, Hamedan
Access method in Wireless Ad-hoc
Networks
Ad-hoc mode in 802.11
Basically, ad-hoc mode in 802.11 does not support multi-hop transmission. However, there are a lot of mechanisms to provide the multi-hop transmission with the help of Layer-3, namely, IP layer.
Ad HocSimplestRapid
deploymentPeer-to-peerNo
administration
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
9
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.
10
A
C
E
DATA
DATA DATA
DATA
DATA
DATA
C is starved by A and E
DATA
DATA
How does legacy DCF work in this scenario when A and E are transmitting ?
DIFS EIFS Backoff Busy Channel
DATADATA
Three pairs
11
Three pairs
A
C
E
DIFS EIFS Backoff Busy Channel
Long term Unfairness
DATA DATA DATA
DATA
DATA
How does legacy DCF work in this scenario when C is transmitting ?
DATADATA
DCF evaluation in a chain
Throughput for chain with different length
Claude Chaudet: IEEE com. Magazine 2005
Does the IEEE 802.11 MAC Protocol Work Well in Multihop Wireless Ad Hoc Networks?
Shugong XuTark SaadawiJune, 2001
IEEE Communications Magazine(Adapted from
mnet.cs.nthu.edu.tw/paper/jbb/010704.pps)
Next 5 slides from
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 5 6
Source
Destination
Destination
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 5 6
Source
Destination
Destination
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
20
Traditional Ad Hoc Network: Single Channel
Each device has 1 radio. All radios are tuned to the same channel.
Motivation
Exploit multiple channels to improve network throughput’ … why ?
Greater parallel communication is possible
1
defer
1
2
22
Typical Wireless Networks
t=0frequencySender 1
t=1frequencySender 2
t=2frequencySender 3
Each network uses 1 channel only.
Channel 1 Channel 2 Channel 3
: :
Can we do better?
PowerDensity
23
Can we do better?
t=0frequency
PowerDensity
Sender 1
Simultaneous sending on different channels?
Channel 1 Channel 2 Channel 3
Sender 3
t=1frequencySender 2 Sender 1 Sender 4
Sender 4
t=2frequencySender 3
: :
Sender 2Sender 4
24
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
25
Why Multi-Channel MAC?
t=0
t=1
frequency
frequency
Sender 1
Sender 2 Sender 1
Sender 3
Sender 4
Sender 4
t=0
t=1
frequency
frequency
Sender 1
Sender 2
Single “Super” Channel
Multi-Channel MAC
26
M-Channel Schedule example
27
M-Channel Schedule example
28
Core Design Issues
Q1: Which channel is receiver Y listening on?
Q2: Is channel i free?
time=t
time=t
frequency
frequencyFree ?
receiver Y
? ? ?
Chan i
Multi-channel Hidden Terminals
Multi-channel Hidden Terminals
Observations
1. Nodes may listen to different channels2. Virtual Carrier Sensing becomes difficult3. The problem was absent for single channel
31
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 ()
32
Protocol (1): Dedicated Control Channel
Ch3(data)
Ch2(data)
Ch1(Ctrl)
Time
Channel
RTS(2,3)
CTS(2)
RTS(3)
CTS(3)
Data Ack
Keys: 2 Radios/Node; Rendezvous on 1 channel; No time sync
Legend: Node 1 Node 2 Node 3 Node 4
Data AckData Ack ...
33
Protocol (2): Split-Phase
Ch3
Ch2
Ch1
Time
Channel
Hello(1,2,3)
Ack (1)
Keys: 1 Radio; Rendezvous on a common channel; Coarse time sync
Control Phase Data TransferPhase
...
...
...Data AckRts Cts
DataRts Cts Ack ...
Hello(2,3)
Ack (2)
Unused
34
Protocol (3): Common Hopping
Ch3
Ch2
Ch1
Time
Channel Key: 1 radio; Non-busy nodes hop together; Tight time sync
Ch4
1 2 3 4 5 6 7 8 9 10 11
Data/Ack ...
Enough for RTS/CTS
RTS+CTS
A MAC protocol based on Split Phase
802.11 PSM (Power Saving Mode)
Doze mode – less energy consumption but no communicationATIM – Ad hoc Traffic Indication Message
A
B
C
Time
Beacon
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
A
B
C
Time
Beacon
ATIM
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
A
B
C
Time
Beacon
ATIM
ATIM-ACK
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
A
B
C
Time
Beacon
ATIM
ATIM-ACK
ATIM-RES
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
A
B
C
Time
Beacon
ATIM
ATIM-ACK
DATAATIM-RES
Doze Mode
ATIM Window
Beacon Interval
802.11 PSM (Power Saving Mode)
A
B
C
Time
Beacon
ATIM
ATIM-ACK
DATA
ACK
ATIM-RES
Doze Mode
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
A
B
C
DTime
ATIM Window
Beacon Interval
Common Channel Selected Channel
Beacon
MMAC
A
B
C
D
ATIM
ATIM-ACK(1)
ATIM-RES(1)
Time
ATIM Window
Common Channel Selected Channel
Beacon
MMAC
A
B
C
D
ATIM
ATIM-ACK(1)
ATIM-RES(1)
ATIM-ACK(2)
ATIM ATIM-RES(2)
Time
ATIM Window
Common Channel Selected Channel
Beacon
MMAC
ATIM
ATIM-ACK(1)
ATIM-RES(1)
ATIM-ACK(2)
ATIM ATIM-RES(2)
Time
ATIM Window
Beacon Interval
Common Channel Selected Channel
Beacon
RTS
CTS
RTS
CTS
DATA
ACK
ACK
DATA
Channel 1
Channel 1
Channel 2
Channel 2
A
B
C
D
Experimental Parameters
Transmission rate: 2MbpsTransmission range: 250mTraffic type: Constant Bit Rate (CBR)Beacon interval: 100ms
Packet size: 512 bytesATIM window size: 20msDefault number of channels: 3 channels
Compared protocols802.11: IEEE 802.11 single channel protocolDCA: Wu’s protocolMMAC: 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 1-60