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1
Collision-Free Asynchronous Multi-Channel Access in Ad Hoc
Networks IEEE Globecom 2009, Hawaii
University of California Santa Cruz*Palo Alto Research Center^
Duy Nguyen*, J.J. Garcia-Luna-Aceves*^, and Katia Obraczka*
2
Motivation for Multi-Channel MAC
• 3 non-overlapping 20MHz channels available in 2.4 GHz 802.11b/g/n
• 12 non-overlapping 20MHz channel available in 5Ghz of 802.11a
• 9 non-overlapping 40MHz channel in 5GHz of 802.11n
• Good bandwidth utilization
3
Challenges
• Hidden terminal problems• Using only a single transceiver: can only
transmit or receive but not both• How to make sure all neighbors aware of the
channel selection• Perception of available channel status is
different among nodes: – my neighbors’ views of available channel status is
different from mine (multi-hop networks)
4
Approaches
– Dedicated Control Channel (DCA[S.Wu and et])
• Dedicated control radio or channel for all control messages
– Split Phase (MMAC[J.So and N. Vaidya])
• Fixed periods divided into (i) channel negotiation phase on default channel & (ii) data transfer phase on negotiated channels
– Common Hopping (CHMA[A. Tzamaloukas and J.J Garcia-Luna-Aceves])
• All non-busy nodes follow a common, well-known channel hopping sequence -- the control channel changes.
– Parallel Rendezvous (SSCH[P. Bahl et] and McMAC[J. So et])
• Each node publishes its own channel hopping schedule
5
Dedicated Control Channel
Ch3
Ch2
Ch1
Time
Channel
Rts(2,3)
Cts(2)
Rsv(2)
Rts(3)
Cts(3)
Rsv(3)
Data . . . Ack
Data Ack
Rendezvous & contention occur on the control channel.
Legend: Node 1 Node 2 Note 3 Node 4
Node 1+2
Slide courtesy of H. Wilson So
6
Split-Phase
Ch2
Ch1
Ch0
Time
Channel
Hello(1,2,3)
Ack(1)
Rsv(1)
Channel negotiation on a common channel
Data AckRts Cts
Control Phase Data Transfer Phase
Data AckRtsCts
Hello(2,3) ...
Legend: Node 1 Node 2 Note 3 Node 4
Slide courtesy of H. Wilson So
7
Common Hopping
Ch2
Ch1
Ch0
Time
ChannelIdle nodes hop together in “common channel”
Ch3
1 2 3 4 5 6 7 8 9 10 11
Cts, Data, Ack
Enough for one RTS
RTS (c to d)
Legend: Node a Node b Note c Node d
RTS (b to a)
Slide courtesy of H. Wilson So
8
Parallel Rendezvous
t=1 2 3 4 5 6 ...
Ch 1
Ch 2
Ch 3
Ch 4
• Sender needs to know the home channel of the receiver
? ?
Slide courtesy of H. Wilson So
10
Parallel Rendezvous
t=1 2 3 4 5 6 7 8 9
Ch1
Ch2
1. Data arrives 4. Hopping
resumes3. Hopping stopped during data transfer
2. RTS/ CTS/ Data
Original schedule
Slide courtesy of H. Wilson So
12
Yet another MAC?
• Current MACs are not sufficient: – CSMA of current IEEE 802.11 MAC
performance can be seriously degraded by the hidden terminal problems.
• Many current multi-channel MACs rely on synchronization
• Goal: To design a simple, asynchronous, and collision-free MAC with very minimal modifications to 802.11
13
Asynchronous Multi-Channel MAC (AM-MAC or “I’m MAC!”)
• Asynchronous Split Phase Approach
• Allows nodes to switch to rendezvous channel immediately once arrangement is made
• New and unique handshake is introduced to eliminate hidden terminal problems and guarantee collision freedom
14
AM-MAC: Assumptions
• N available orthogonal channels are of the same bandwidth.
• A single transceiver, can either transmit or receive but not both.
• Transmission time of RTS, CTS, ATS is • Maximum end-to-end propagation delay is • Switching delay is
'',',
15
AM-MAC: Basic mechanisms
• Borrow RTS/CTS and carrier sensing mechanism from 802.11
• Introduce ATS packet (Announce to Send)
• Additional fields:– RTS: available channel list, data time– CTS: selected channel, data time– ATS: selected channel, data time
16
Conditions for collision-free
MAXO TT
'','2 AM-MAC provides correct data channel acquisition provides that and
Let be the maximum channel observation time be the maximum data transmission time
AM-MAC is collision-free if
OTMAXT
17
RTS/CTS/ATS Based Access
• Duration field in RTS, CTS, ATS frames distribute Medium Reservation information which is stored in a Net Allocation Vector Net Allocation Vector (NAV)(NAV).
• Defer on either NAV or "CCA" indicating Medium BusyMedium Busy.
RTS
CTS Ack on channel n
Data on channel n
NAV
Src
Dest
Other
Defer Access RTS/CTS/ATS exchange continues
NAV
DIFS
ATS
ATS
18
AM-MAC Summary
• A sends RTS with available channels to B, assumes A had already met the observation time requirement.
• B replies with a CTS with the selected data channel to A, starts a timer for CTS so that upon expiration sends ATS
• On receiving CTS, A prepares to send ATS• Both A and B broadcast ATS with their intention
on data channel concurrently• A begins sending data to B on selected channels
31
B
XSA
YR
CTS arrives at X in error (X is aware of it because CTS is slightly longer than RTS). X backs off
CTS RTS
''
32
B
XSA
YR
X stays on the control channel.Y later switches to the data channel and, simply, times out and returns to the control channel.
ATS CTS
ATS
33
Simulation Models
• Simulation parameters from MMAC• Wireless LAN and Multi-hop scenarios• Channel bit-rate 3mb with CBR traffic• Transmission range approximately 250m• 3 or 4 channels where stated• Packet size of 512 bytes; Drop tail queue
length 50• 400x400, 1000x1000 topology
34
Aggregate Throughput 18 Flows
0
1000
2000
3000
4000
5000
6000
7000
1 10 100 1000
Packet Arrival Rate per Flow (packets/sec)
Ag
gre
gat
e T
hro
ug
hp
ut
(Kb
ps)
802.11
mmac
am-mac
Wireless LAN 36 nodes in 400x400 Throughput
35
Average Packet Delay, 18 Flows
0
2
4
6
1 10 100 1000
Packet Arrival Rate per Flow (pkts/sec)
Ave
rag
e P
acke
t D
elay
(se
c)
802.11
mmac
am-mac
Wireless LAN 36 nodes in 400x400
Delay
36
Aggregate Throughput 32 Flows
0
1000
2000
3000
4000
5000
6000
7000
1 10 100 1000
Packet Arrival Rate per Flow (packets/sec)
Ag
gre
gat
e T
hro
ug
hp
ut
(Kb
ps)
802.11
mmac
am-mac
Wireless LAN 64 nodes in 400x400Throughput
37
Average Packet Delay, 32 Flows
0
2
4
6
1 10 100 1000
Packet Arrival Rate per Flow (pkts/sec)
Ave
rag
e P
acke
t D
elay
(se
c)
802.11
mmac
am-mac
Wireless LAN 64 nodes in 400x400
Delay
38
Aggregate Throughput 42 Flows, 3 channels, Multihop
0
1000
2000
3000
4000
5000
6000
7000
1 10 100 1000
Packet Arrival Rate per Flow (packets/sec)
Ag
gre
gat
e T
hro
ug
hp
ut
(Kb
ps)
802.11
mmac
am-mac
Multi-hop 121 nodes in 1000x1000 Throughput
39
Average Packet Delay, 42 Flows, 3 channels, Multihop
0
1
2
3
4
5
6
1 10 100 1000
Packet Arrival Rate per Flow (pkts/sec)
Avera
ge P
acket D
ela
y (sec)
mmac
am-mac
802.11
Multi-hop 121 nodes in 1000x1000 Delay
40
Multi-hop 121 nodes in 1000x1000 Throughput
Aggregate Throughput 42 Flows, 4 channels, Multihop
0
1000
2000
3000
4000
5000
6000
7000
1 10 100 1000
Packet Arrival Rate per Flow (packets/sec)
Aggre
gat
e Thro
ughput (K
bps)
802.11
mmac
am-mac
41
Average Packet Delay, 42 Flows, 4 channels, Multihop
0
1
2
3
4
5
6
1 10 100 1000
Packet Arrival Rate per Flow (pkts/sec)
Ave
rag
e P
acke
t D
elay
(se
c)
mmac
am-mac
802.11
Multi-hop 121 nodes in 1000x1000 Delay
42
Analytical Analysis Assumptions
• A finite population of N nodes
• Arrival of RTS is Poisson distributed
• Network is fully connected with the same number of neighbors
• Successful RTS and DATA occurs as a single event
• Packet length are independent and geometrically distributed