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1
Exploiting Antenna Capabilities in Wireless Networks
Nitin Vaidya
Electrical and Computer Engineering, and
Coordinated Science Lab (CSL)
University of Illinois at Urbana-Champaign
www.crhc.uiuc.edu/wireless/
2
Wireless Capacity
Wireless capacity limited
In dense environments, performance suffers
How to improve performance?
3
Improving Per-Flow Capacity
4
Add Spectrum
Multi-channel versions of IEEE 802.11
Practical limits on how much spectrum may be used
5
Power Controlto Improve Spatial Reuse
A B C D
A B C D
6
Improving Communication Locality
Local communication (among nearby nodes) uses less “space”
Allows spatial reuse among different flows
Improves per-flow capacity
Not always feasible: Application-dependent
7
Exploit Infrastructure
Infrastructure provides a “tunnel” through which packets can be forwarded
Can effectively improve locality of communication Infrastructure access can become a bottleneck
EA
BS1 BS2
X
Z
infrastructure
Ad hoc connectivity
8
Improving Per-Flow Capacity
Previous techniques are all useful,but have limitations
Dense networks likely to require further improvements in capacity
Exploit other forms of diversity Mobility Antennas
9
Exploiting Antennas
10
Antennas: Many Possibilities
Directional antennas
Diversity antennas
Reconfigurable antennas
…
11
Exploiting Antennas
Need protocol adaptations to exploit available antenna capabilities
Not sufficient to modify physical layer alone
Higher layer adaptation often necessary:medium access control (MAC) and routing
12
This TalkProtocols for Ad Hoc Networks using
Directional Antennas
Issues of interest Medium access control Neighbor discovery Routing
Longer links, shorter routes Longer times to failure Broadcast-based discovery harder
This talk Deafness problem MAC-Layer Anycasting
13
Outline
Preliminaries
A simple MAC protocol and the “deafness” problem
MAC-layer anycasting
14
Ad Hoc Networks
Formed by wireless hosts which may be mobile
Without necessarily using a pre-existing infrastructure
Routes between nodes may potentially contain multiple hops Hidden terminals
15
Antenna Model
2 Operation Modes: Omni & Directional
Directional mode typically has sidelobes
Not all antennas represented by this model
16
Antenna Model
Omni Mode: Omni Gain = Go
Directional Mode: Capable of beamforming in specified direction Directional Gain = Gd (Gd > Go)
Received power Transmit power * Gtx * Grx
17
Benefits of Directional AntennasGreater Received Power
Longer links may be formed
B
A
C
D
May lower Tx power, reducing interference to others
18
Benefits of Directional Antennas
Low gain in unwanted directions
Reduces interference to others
Example ….
19
Using Omni-directional Antennas
When C receives from D, B cannot transmit
CB
A
D
20
Using Directional Antennas
C may receive from D, and simultaneously B may transmit to A
CB
A
D
21
A detour …
22
A B C
Hidden Terminal Problem
Node B can communicate with A and C both A and C cannot hear each other
When A transmits to B, C cannot detect the transmission using the carrier sense mechanism
If C transmits, collision may occur at node B
23
RTS/CTS Handshake in 802.11
Sender sends Ready-to-Send (RTS) Receiver responds with Clear-to-Send (CTS) RTS and CTS announce the duration of the transfer Nodes overhearing RTS/CTS keep quiet for that
duration
D
C
BACTS (10)
RTS (10)
10
10
24
Outline
Preliminaries
A simple MAC protocol and the “deafness” problem
MAC-layer anycasting
25
Directional MAC(DMAC)
Idle node listens in omni-directional mode
Sender sends a directional RTS towards intended receiver
Receiver responds with directional CTS
26
Directional MAC(802.11 Variant)
DATA and ACK transmitted and received directionally
Nodes overhearing RTS or CTS remember not to transmit in corresponding directions
Overhearing nodes may transmit in other directions
27
Directional MAC
C remembers not to transmit in A’s direction C may transmit towards D
D
A
C
BRTS
28
Issues with DMAC
Hidden terminals due to asymmetry in gain A does not get RTS/CTS from C/B
C
A B
DataRTS
A’s RTS may interfere with C’s reception of DATA
29
Issues with DMAC: Deafness
Deafness: C does not know why no response from A
Cannot differentiate between collision, and busy node A
Conservative response is to “backoff” and try later
D
A B
CRTS
30
Illustration
B initiates communication to A
While A is busy, C transmits RTS to A
No response from A
C waits a while, tries again
No response, C waits longer …
When A becomes free, C in wait mode
A become busy again, …. Repeat
A B
CRTS
31
RTS
RTS
Backoff
Data
RTS
CTS
ACK
Data
CTS
RTS
B initiates communication to A
While A is busy, C transmits RTS to A
No response from A
C waits a while, tries again
No response, C waits longer …
When A becomes free, C in wait mode
A become busy again, …. Repeat
Illustration
Packetdrop
A B
C
32
Impact of Deafness
Unnecessary transmissions of RTS
Increased packet drops
Increased delay and variance
Unfairness among flows
33
Solutions to Deafness
Deafness since C does not know A is busy
Make C aware that A is busy Require A to transmit a
busy signal while receiving
Alternative: A transmits a “free” signal after it become idle
RTS
RTS
Backoff
Data
RTS
CTS
ACK
Data
CTS
RTS
Packetdrop
A B
C
34
Solution: Tone DMAC
Nodes unable to communicate with A adapt backoff based on the “tone” from A Think of it as “free-tone”
as opposed to a “busy-tone”
A node need only use tone or data channel at any time, not both
RTS
RTS
Backoff
Data
RTS
CTS
ACK
A B
C
Tone
RTSRTS
CTS
Data
Backoff
35
Tone DMAC
Why a narrow-band tone? Save bandwidth
Trade-off Narrow-band signal prone to fading: Use long enough tone
duration Aliasing, since C cannot tell who transmitted a tone
– Use multiple tones
– One tone per node too expensive
– Share tones
36
Tone DMAC
Node i transmit tone fi for duration ti
fi and ti functions of the node identifier i
fi = i mod F
ti = i mod T
37
Tone DMAC
When a node, such as C in our example, hears a tone f for duration t, node C determines whether the tone could have been sent by its intended traget (node A in our example)
If C determines that A is the tone sender, C reduces its waiting time before next RTS
Aliasing can occur since multiple nodes can hash to the same tuple { f, t }
38
Tone DMAC Example
39
Backoff: Two flows to common receiver
Another possible improvement:
Backoff Counter for DMAC flows
Backoff Counter for ToneDMAC flows
time
Ba
cko
ff V
alu
es
40
Packet Drops: Three flows, common receiver
DMAC
ToneDMAC
time
41
UDP Throughput: Multiple multihop flows
ToneDMAC outperforms DMAC, ZeroToneDMACZeroToneDMAC = DMAC with only omnidirectional Backoff
42
Delay Performance: 2 flows, common Rx
Large fluctuation in DMAC packet delay Higher variance
43
TCP Throughput: Multiple multihop flows
RTT estimation of TCP better with ToneDMAC due to low delay variance
44
DMAC Summary
Deafness aggrevated by directional communication
“Free” tones, or other alternative mechanisms, appear useful to reduce degradation caused by deafness
Practicality issue: Tone assignment Fading
Topic of ongoing research
45
MAC-Layer Anycasting
46
Observation
Network layer typically selects one “optimal” route
MAC layer required to forward packet to next hop neighbor on this route
“Optimal” route selection based on a long-term view of the network Independent of instantaneous channel conditions at each
hop
47
Improvement ?
MAC layer aware of local link conditions Congestion, channel fluctuations at smaller time scale Power constraints for transmission Virtual carrier sensing information (NAV in 802.11)
Exploit MAC layer awareness Especially when using directional antennas
Forward packets based on combination of Long-term directives of routing layer, and Short-term knowledge at MAC layer
48
Our Proposal
Make forwarding decisions at the MAC layer
Utilize information already available to the MAC layer (as opposed to explicitly gathering feedback)
With DMAC, a node already knows that it cannot transmit in certain directions
Our approach can be combined with mechanisms that gather information explicitly
49
MAC-Layer Anycasting
Source often has multiple “good” routes to sink Typically, one random downstream neighbor chosen
Supply multiple downstream neighbors to MAC layer
MAC layer chooses any one of the neighbors based on available information, and unicasts the packet
50
MAC-Layer Anycast Framework
Anycast module receives group of downstream neighbors
Anycast group = {A, B, X}
Anycast module forms anycast sequence (based on chosen policy)
Seq. = {X, X, B, A, X, B, A}
MAC layer attempts to transmit to “available” neighbors
Network Layer
MAC Layer
Physical Layer
AnycastModule
51
Directional MAC
X
DSDRTS
Y
52
Directional MAC
X
DSDCTS
Remember to not transmit towards D
Y
53
MAC Constraints
Route from S to D: {S,A,B,D}
Assume A communicating with B
S cannot send packet to A
Multiple retransmissions can be avoided by forwarding packet to X instead
Specify anycast group specifiedas {A, X}
A
S
Y
DB
X
Directional Beam Patterns
54
DNAV Constraints
Communication between E and F requires S to set DNAV in direction of E
Communication between S and A not possible until E completes transmission
Communication between S and X may be possible
Anycasting with group {A,X} canimprove performance
F
E
X
A
S
55
Not Allowed
DNAV Constraints
F
E
XS
A
Communication between E and F requires S to set DNAV in direction of E
Communication between S and A not possible until E completes transmission
Communication between S and X may be possible
Anycasting with group {A,X} canimprove performance
56
DNAV Constraints
F
E
XS
A
Allowed
Communication between E and F requires S to set DNAV in direction of E
Communication between S and A not possible until E completes transmission
Communication between S and X may be possible
Anycasting with group {A,X} canimprove performance
57
MAC Constraints – Omni Antennas
Route from S to D: {S,A,B,D}
While F communicating to E, A is silenced by CTS from E
S transmits RTS to A, receives no reply, retransmits
Multiple retransmission can be avoided by forwarding packet to X
Anycast group specified to Scan be {A, X}
58
Power Constraints
R T
P
N
With PCMA, node R announces additional interference that it can tolerate
To initiate communication to N, T must choose power level according to this tolerance
Power level to transmit to N is too high. However, transmission to P is feasible
MAC-Layer anycasting canforward packets with PCMA.
Anycast group {P, N}
59
Power Constraints
R T
P
N
With PCMA, node R announces additional interference that it can tolerate
To initiate communication to N, T must choose power level according to this tolerance
Power level to transmit to N is too high. However, transmission to P is feasible
MAC-Layer anycasting canforward packets with PCMA.
Anycast group {P, N}
60
Power Constraints
R T
P
N
With PCMA, node R announces additional interference that it can tolerate
To initiate communication to N, T must choose power level according to this tolerance
Power level to transmit to N is too high. However, transmission to P is feasible
MAC-Layer anycasting canforward packets with PCMA.
Anycast group {P, N}
61
Design Issues and
Tradeoffs
62
“Digression”
Anycasting can bypass unavailable links
Each intermediate node locally performs anycasting
Local (greedy) decisions can cause Route to digress significantly from global optimal
Need to restrict digression below tolerance
63
Digression
Say, Anycast group = Neighbors on the minimum and
(minimum+1)-hop routes {S,X,J,P,K,Z,D} digresses 3 hops more that {S,A,B,D}
64
Out-of-Order Delivery
MAC-Layer anycasting performed on per-packet basis Delay on the different routes can be different Out of order packet delivery possible TCP-like transport protocols may encounter problems
65
Source Routing
Source routing – source specifies all possible routes
To perform anycasting with source routing Source includes enough information for intermediate nodes
to form anycast group Possible implementation – include a directed acyclic graph
(DAG)
Including DAG in packet – larger control overhead
66
Preliminary Evaluation(Anycasting)
67
Grid topology, 5 flows, 3 hops
68
Large Grid topology, 10 flows, 5 hops
69
Anycast: Summary
MAC-Layer anycasting can improve performance
Several tradeoffs arise
On-going work
70
Conclusion
Directional antennas can benefit performance
But need suitable protocols
On-going work: Cheaper antennas that can improve performance Testbed deployment
71
Thanks!
www.crhc.uiuc.edu/wireless
Acknowledgements
Romit Roy Choudhury, UIUC
Ram Ramanathan, BBN
Xue Yang, UIUC
72
Another Problem
Performing directional carrier sensing when in wait mode leads to another instance of deafness
While C waits to transmit to A, it beamforms and performs carrier sensing
C cannot hear RTS from D
A B
CRTS
DRTS
73
Solutions to Deafness
Nodes required to switch to omni mode during back-off
C can hear D while waiting for A
Trade-off: C may receive transmission from E to F, and not be able to receive from D, or transmit to A
A B
CRTS
DRTS
E