Download ppt - Advanced Topics in Next-Generation Wireless Networks

Qian ZhangDepartment of Computer Science

HKUST

Advanced Topics in Next-Generation Wireless Networks

Basic, PHY and MAC of Ad Hoc Network

• Definitions:

– An ad-hoc network is one that comes together as needed, not necessarily with any assistance from the existing Internet infrastructure

– Instant infrastructure

– A MANET is a collection of mobile platforms or nodes where each node is free to move about arbitrarily

– A MANET: distributed, possibly mobile, wireless, multihop network that operates without the benefit of any existing infrastructure (infrastructure-less), except the nodes themselves

What is an Ad Hoc Network?

Mobile Ad Hoc Networks

• May need to traverse multiple links to reach a destination

Mobile Ad Hoc Networks (MANET)

• Mobility causes route changes

Why Ad Hoc Networks ?

• Ease of deployment

• Speed of deployment

• Decreased dependence on infrastructure

History

• In 1972, DoD-sponsored Packet Radio Network (PRNet)– Initial network used centralized control station

– Evolved into a distributed architecture

• A network of broadcast radios

• Minimal central control

• Use multi-hop store-and-forward routing

• Use ALOHA/CSMA, spread-spectrum

– Demonstrated the technologies needed to create a MANET

History (Cont.)

• In 1983, Survivable Radio Network (SURAN) program Motivations:– Move towards smaller size, low-cost, low power radio

– Develop and demonstrate scalable algorithms, up to 10ks of nodes.

– Develop and demonstrate robustness and survivability against sophisticated attacks

– Technology: spread spectrum improvements, hierarchical network topology, dynamic clustering, and even cross-layer design

History (Cont.)

• Army (Army Research Office-ARO)– The army did not embrace the new MANET technology until it

was demonstrated experimentally in mid-1980s

– Used it for primarily land-based applications

– Utilized mainly as overlays to existing networks

• Navy (Office of Naval Research-ONR)– Primarily for use by ships at sea

– Network is not as dense as a ground network

– Required integration with satellite links

• Air Force– Explored utilizing aircrafts to provide communications between

ground stations

Commercialization

• (Arguably) two papers, both addressing the routing problem, helped spur the ad-hoc networks research– [Perkins-Bhagwat94, Johnson94]

• The community started to adopt the term “Ad-Hoc Networks”– Commercial applications started to appear

• A number of standards activities evolved in the mid90s—– Notably, the MANET working group (within the IETF) to

standardize routing protocols for ad hoc networks• Reactive and proactive routing

– The 802.11 subcommittee standardized an ad-hoc mode MAC layer

• Made it possible to build ad-hoc networks using laptops

Fundamental Challenges

It is better to know some of the questions than all of the answers. — James Thurber (1835-1910)

1. Energy Efficiency

• No infrastructure means must rely on batteries (or, in general, limited energy resources)

• Possible solutions– Selectively sending nodes into a sleep mode– Using transmitters with variable power (the Power

Control problem)– Using energy-efficient paths– Using co-operative techniques (still relatively new)

2. Mobility

• Mobility-induced route changes• Mobility-induced packet losses

• Mobility patterns may be different– Controlled e.g. robots

• Offers opportunities for improving the network functions e.g. connectivity

– Uncontrolled e.g. nomadic users• Offers challenges to network design

• But also offers opportunities for improvement, e.g.

– Users “carry” delay-tolerant data closer to destination

– Delay Tolerant Network (Challenge Networks)

3. QoS

• Providing QoS even in wired networks (e.g. the Internet) is a challenging problem

• Wireless RF channels further complicate the problem– Unpredictability

– Medium access: broadcast medium with hidden terminal problem

• Possible solutions:– New MAC design

– Cross-layer integration: allow different layers to adapt depending on available information at other layers

4. Scalability

• Limited wireless transmission range• Whether the network is able to maintain an

acceptable level of service even as the number of nodes is increased– How fast the network protocol control overhead

increases as N increases

• Possible solutions:– Introducing hierarchy– Utilizing location information– Limiting reactions to changes– Fixing things (e.g. paths) locally

5. Utilizing New Technologies

• What are the gains that could be achieved by using newly available technologies such as– Smart directional (beamforming) antennas

• Increases the spatial reuse in cellular, but how about ad-hoc?

• Can several nodes together act as an antenna array? Practical issues?

– Software Radio• The ability to quickly switch the operating frequency may

provide opportunities, but also challenging

– GPS• Location information may help

6. Security

• Ease of snooping on wireless transmissions

• From crypto point of view, lack of a trusted authority is one of the main challenges– How to generate/share keys reliably

• Harder to track or even detect attackers in a wireless environment, given that:– Network relies on in-situ connections to other nodes

which may be malicious

• Malicious nodes may be especially harmful by injecting bogus control packets

• DoS attacks that deplete a node’s battery

7. Lack of Reference

• Lack of sufficient experimental data to confirm models– What does a multi-hop path really mean?

– What is a link?

• Simplistic models that do not capture the complexities, or complex models that do not lead to insights?

• Are the protocols good enough, have they reached closed to the best possible?

• Good balance between mathematical and experimental work

Multiple-Layer Problem

• PHY– Adapt to rapid changes in link characteristics

• MAC– Minimize collision, allow fair access, and semi-reliably

transport under rapid change and hidden/exposed terminals

• Network– Determine efficient transmission paths when links change

often and bandwidth is at a premium

• Transport– Handle delay and packet loss statistics that are very different

than wired networks

• Application– handle frequent disconnection and reconnection as well as

varying delay and packet loss characteristics

Topics in PHY and MAC

• Fairness• Energy efficiency

– Power save– Power control

• Adaptive modulation (multi-rate)

Fairness Issue

• Assume that initially, A and B both choose a backoff interval in range [0,31] but their RTSs collide

• Nodes A and B then choose from range [0,63]– Node A chooses 4 slots and B choose 60 slots– After A transmits a packet, it next chooses from range

[0,31]– It is possible that A may transmit several packets before

B transmits its first packet

A B

C D

Two flowsUnfairness occurs when one node has backed off much more than some other nodes

Fairness in Multi-Hop Networks

• Several definitions of fairness [Ozugur98,Vaidya99MSR,Luo00Mobicom, Nandagopal00Mobicom]

• Hidden terminals make it difficult to achieve a desired notion of fairness


• Fairness• Energy conservation



Energy Conservation

• Since many mobile hosts are operated by batteries, MAC protocols which conserve energy are of interest

• Two approaches to reduce energy consumption– Power save: turn off wireless interface when desirable– Power control: reduce transmit power

Power Characteristics for a Mica2 Mote Sensor

Power Save in IEEE 802.11 Ad Hoc Mode

• Time is divided into beacon intervals

• Each beacon interval begins with an ATIM window– ATIM =

Beacon interval

ATIMwindow

Power Save in IEEE 802.11 Ad Hoc Mode (Cont.)

ATIMReq

ATIMAck AckData

Sleep

Node A

Node C

Node B

• If host B has a packet to transmit to A, B must send an ATIM Request to A during an ATIM Window

• On receipt of ATIM Request from A, B will reply by sending an ATIM Ack, and stay up during the rest of the beacon interval

• If a host does not receive an ATIM Request during an ATIM window, and has no pending packets to transmit, it may sleep during rest of the beacon interval


• Size of ATIM window and beacon interval affects performance– If ATIM window is too large, reduction in energy

consumption reduced• Energy consumed during ATIM window

– If ATIM window is too small, not enough time to send ATIM request

• How to choose ATIM window dynamically?– Based on observed load [Jung02infocom]


• How to synchronize hosts?– If two hosts’ ATIM windows do not overlap in time,

they cannot exchange ATIM requests

– Coordination requires that each host stay awake long enough (at least periodically) to discover out-of-sync neighbors [Tseng02infocom]

ATIM

ATIM

Impact on Upper Layers

• If each node uses the 802.11 power-save mechanism, each hop will require one beacon interval– This delay could be intolerable for some

applications

• Allow upper layers to dictate whether a node should enter the power save mode or not [Chen01mobicom]

Power Control

Recall:Power control has two potential benefit

• Reduced interference & increased spatial reuse

• Energy saving

Power Control with 802.11

• Transmit RTS/CTS/DATA/ACK at least power level needed to communicate with the receiver

• A/B do not receive RTS/CTS from C/D. Also do not sense D’s data transmission

• B’s transmission to A at high power interferes with reception of ACK at C

B C DA

Data sensed

A Plausible Solution

• RTS/CTS at highest power, and DATA/ACK at smallest necessary power level

• A cannot sense C’s data transmission, and may transmit DATA to some other host

• This DATA will interfere at C• This situation unlikely if DATA transmitted at highest

power level– Interference range ~ sensing range

B C DA

RTS DataInterference range

Ack

Other Cons

• Transmitting RTS at the highest power level also reduces spatial reuse

• Nodes receiving RTS/CTS have to defer transmissions

Modification to Avoid Interference

• Transmit RTS/CTS at highest power level, DATA/ACK at least required power level

• Increase DATA power periodically so distant hosts can sense transmission [Jung02tech]

• Need to be able to change power level rapidly

Powerlevel


• Fairness• Energy efficiency



Adaptive Modulation

• Channel conditions are time-varying– Received signal-to-noise ratio changes with time

• Multi-rate radios are capable of transmitting at several rates, using different modulation schemes

• Choose modulation scheme as a function of channel conditions

Distance

ThroughputModulation schemes providea trade-off betweenthroughput and range

Adaptive Modulation (Cont.)

• If physical layer chooses the modulation scheme transparent to MAC– MAC cannot know the time duration required for

the transfer

• Must involve MAC protocol in deciding the modulation scheme– Some implementations use a sender-based scheme

for this purpose– Receiver-based schemes can perform better

Sender-Based “Autorate Fallback” [Kamerman97]

• Probing mechanisms

• Sender decreases bit rate after X consecutive transmission attempts fail

• Sender increases bit rate after Y consecutive transmission attempt succeed

Autorate Fallback

• Advantage– Can be implemented at the sender, without making

any changes to the 802.11 standard specification

• Disadvantage– Probing mechanism does not accurately detect

channel state– Channel state detected more accurately at the

receiver– Performance can suffer

• The sender will periodically try to send at a rate higher than optimal

• When channel conditions improve, the rate is not increased immediately

Receiver-Based Autorate MAC [Holland01mobicom]

• Sender sends RTS containing its best rate estimate

• Receiver chooses best rate for the conditions and sends it in the CTS

• Sender transmits DATA packet at new rate

• Information in data packet header implicitly updates nodes that heard old rate

D

C

BACTS (1 Mbps)

RTS (2 Mbps)

Data (1 Mbps)NAV updated using rate specified in the

data packet