1 exploiting antenna capabilities in wireless networks nitin vaidya electrical and computer...

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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/

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Wireless Capacity

Wireless capacity limited

In dense environments, performance suffers

How to improve performance?

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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

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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

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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

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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

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Exploiting Antennas

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Antennas: Many Possibilities

Directional antennas

Diversity antennas

Reconfigurable antennas

…

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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

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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

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Outline

Preliminaries

A simple MAC protocol and the “deafness” problem

MAC-layer anycasting

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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

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Antenna Model

2 Operation Modes: Omni & Directional

Directional mode typically has sidelobes

Not all antennas represented by this model

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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

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Benefits of Directional AntennasGreater Received Power

Longer links may be formed

B

A

C

D

May lower Tx power, reducing interference to others

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Benefits of Directional Antennas

Low gain in unwanted directions

Reduces interference to others

Example ….

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Using Omni-directional Antennas

When C receives from D, B cannot transmit

CB

A

D

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Using Directional Antennas

C may receive from D, and simultaneously B may transmit to A

CB

A

D

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A detour …

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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

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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

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Outline

Preliminaries

A simple MAC protocol and the “deafness” problem

MAC-layer anycasting

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Directional MAC(DMAC)

Idle node listens in omni-directional mode

Sender sends a directional RTS towards intended receiver

Receiver responds with directional CTS

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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

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Directional MAC

C remembers not to transmit in A’s direction C may transmit towards D

D

A

C

BRTS

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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

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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

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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

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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

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Impact of Deafness

Unnecessary transmissions of RTS

Increased packet drops

Increased delay and variance

Unfairness among flows

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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

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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

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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

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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

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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 }

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Tone DMAC Example

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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

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Packet Drops: Three flows, common receiver

DMAC

ToneDMAC

time

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UDP Throughput: Multiple multihop flows

ToneDMAC outperforms DMAC, ZeroToneDMACZeroToneDMAC = DMAC with only omnidirectional Backoff

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Delay Performance: 2 flows, common Rx

Large fluctuation in DMAC packet delay Higher variance

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TCP Throughput: Multiple multihop flows

RTT estimation of TCP better with ToneDMAC due to low delay variance

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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

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MAC-Layer Anycasting

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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

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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

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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

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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

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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

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Directional MAC

X

DSDRTS

Y

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Directional MAC

X

DSDCTS

Remember to not transmit towards D

Y

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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

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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

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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

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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

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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}

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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}

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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}

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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}

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Design Issues and

Tradeoffs

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“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

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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}

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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

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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

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Preliminary Evaluation(Anycasting)

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Grid topology, 5 flows, 3 hops

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Large Grid topology, 10 flows, 5 hops

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Anycast: Summary

MAC-Layer anycasting can improve performance

Several tradeoffs arise

On-going work

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Conclusion

Directional antennas can benefit performance

But need suitable protocols

On-going work: Cheaper antennas that can improve performance Testbed deployment

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Thanks!

www.crhc.uiuc.edu/wireless

Acknowledgements

Romit Roy Choudhury, UIUC

Ram Ramanathan, BBN

Xue Yang, UIUC

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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

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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