Overview of Wireless Networking

Overview

• Fundamental issues and impact– wireless– mobility

• For each layer in the protocol stack– A subset of design requirements– Design challenges/constraints– Possible design options

Physical Properties of Wireless

• Wireless = Waves, electromagnetic radiation emitted by sinusoidal current running through a transmitting antenna– Signal will be received by everyone

nearby.– Makes wireless network different

from wired networks

A

BC

D

E F

Typical Radio System(Sender)

• A radio system transmits information to the transmitter.

• The information is transmitted through an antenna which converts the RF signal into an electromagnetic wave.

• The transmission medium for electromagnetic wave propagation is free space.

Typical Radio System(Receiver)

• The electromagnetic wave is intercepted by the receiving antenna which converts it back to an RF signal.

• Ideally, this RF signal is the same as that originally generated by the transmitter.

• The original information is then demodulated back to its original form.

6

Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a

medium reflection at large obstacles scattering at small obstacles diffraction at edges

reflection scattering

diffraction

shadow fadingrefraction

Signal Propagation

The signal takes many paths to the destination. The propagation delay along each path is different. How many meters difference gives you

0.00001 seconds of delay difference?

Multipath Fading

301

2The shorter

path

The longer path

Received signal – a combination of the two signals

301

2

Wireless Channel Characteristics• Radio propagation

– Multipath, fade, attenuation, interference & capture

– Received power is inversely proportional to the distance: distance-power gradient•Free space: factor 2•In building corridors or large open indoor areas: <2

•Metal buildings: factor 6•Recommended simulation factors: 2~3 for residential areas, offices and manufacturing floors; 4 for urban radio communications

Free-Space Isotropic( 各向同性的 ) Signal Propagation

• In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver)

• Suppose transmitted signal is x,received signal y = h x, where h is proportional to 1/d²

• Reduction also depends on wavelength– Long wave length (low frequency) h

as less loss– Short wave length (high frequency)

has more loss

2

4

dGG

P

Ptr

t

r

Pr: received power Pt: transmitted power Gr, Gt: receiver and

transmitter antenna gain (=c/f): wave length

d

Pt Pr

Wireless Channel• Wireless transmission is error

prone• Wireless error and contention

are location dependent• Wireless channel capacity is also

location dependent

Channel

信道容量 :香农定理

(b/s) )1(log2 NSBC C ：信道容量，信道可能传输的最大信息速率，

即信道所能达到的最大传输能力B ：信道带宽S ：信号平均功率N ：噪声平均功率

S/N: 信噪比

Link-Level Measurements• Measurements taken from 802.11b-ba

sed MIT Roofnet• Focus:

– Explore reasons for loss– mainly on long outdoor links

D. Aguayo, etc., "Link-level measurements from an 802.11b mesh network," ACM Sigcomm 2004.

Roofnet: multihop wireless mesh

1 kilometer

Using omni-direction antenna

+ Easy to deploy

+ Provide high connectivity

- Don’t allow engineered link quality

Lossy radio links are common

1 kilometer

1-30%

30-70%

70-100%

Broadcast packet delivery probability

Delivery prob. uniformly distributed

Node Pair

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ad

cast

Packet

Delivery

Pro

bab

ilit

y

> two-thirds of linksdeliver less than 90%

ImplicationsProtocols should exploit intermediate-

quality links– 802.11 transmit bit-rate selection– Link-quality-aware routing (ETX, LQSR)– Opportunistic protocols (OMAC, ExOR)

An emerging research direction

Hypotheses for intermediate delivery probability1. Marginal signal-to-noise ratios2. Interference: Long bursts3. Interference: Short bursts

(802.11)4. Multi-path interference

Methodology: Link-level measurements of packet loss• Goal: all-pairs loss rates• Each node broadcasts for 90 seconds• All other nodes listen• Raw link-level measurements:

– No ACKs, retransmissions, RTS/CTS– No other Roofnet traffic– No 802.11 management frames– No carrier sense

Hypothesis 1: Marginal S/N• Simplified model for packet loss:

– P(delivery) = f(signal/noise)– Signal strength reflects

attenuation – Noise reflects interference

• Perhaps marginal S/N explains intermediate delivery probabilities

Delivery vs. S/N with a cable and attenuator

Signal-to-noise ratio (dB)

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adca

st p

acke

td

eliv

ery

pro

bab

ility

Laboratory

Delivery vs. S/N on Roofnet

S/N does not predict delivery probability for intermediate-quality links

Signal-to-noise ratio (dB)

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adca

st p

acke

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eliv

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pro

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ility

Roofnet

Laboratory

Hypothesis 2: long bursts of interference

Bursty noise might corrupt packets without affecting S/N measurements

A B

Loss over time on two different Roofnet links

The top graph is consistent with bursty interference. The bottom graph is not.

avg: 0.5stddev: 0.28

avg: 0.5stddev: 0.03

Del

iver

y p

rob

abili

ty

Time (seconds)

Hypothesis 3: short bursts of interference (802.11)

AB

• MAC doesn’t prevent all concurrent xmits• Outcome depends on relative signal levels• Hypothesis: When a nearby AP sends a packet, we lose a

packet.

Methodology: record non-Roofnet 802.11 traffic

• Goal: measure non-Roofnet traffic

• Before the broadcast experiments

• Each node records all 802.11 traffic

No correlation between foreign traffic observed and packets lost

Non-Roofnet packets observed per second (before the experiment)

Exp

erim

ent

pac

kets

lo

stp

er s

eco

nd

Hypothesis 4: Multi-path interference

Reflection is a delayed andattenuated copy of the signal

A

BB

A channel emulator to investigate multi-path effects

Sender Receiver

delay attenuation

Reflection causes intermediate packet loss

Del

iver

y p

rob

abili

ty

Delay of second ray(nanoseconds or feet)

Summary• Most Roofnet links have intermediate

loss rates• SNR is related to packet loss rate, but doe

s not predict delivery probability • Loss is not consistent with bursty interfer

ence• Multi-path is likely to be a major cause

wireless environment is really different from wired, and is almost unpredictable.

Mobility

• Why mobility?– 30~40% of the US workforce is

mobile (Yankee group)– Hundreds of millions of users are

already using portable computing devices and more than 60% of them are prepared to pay for wireless access to the backbone information

Mobility• Four types of activities for a typical office

work during a workday:– Communication (fax, email)– Data manipulation (word processing,

directory services, document access & retrieval)

– Information access (database access and update, internet access and search)

– Information share (groupware, shared file space)

• Question: how does mobility affect each of the above activities?

Protocol StackLook at:

1. Applications/Services

2. OS issues

3. Middleware (skip):1. Transcoding

Application Layer

Middleware and OS

Transport Layer

Network Layer

Link Layer & Below

Issues in building services in mobile networking environments

• Mobility induced issues:– Seamless services: service migration– Location services: location itself is a service

• Heterogeneity induced issues:– Hardware diversity

• Client devices & different networks– Software diversity

• System software: OS, networking protocols• Application software

• Wireless induced issues:– Time-varying network connectivity: disconnection, partial

connection, full connection

Possible services for mobile environments

• Location service• Location transparent services

– Hide locations from users• Location dependent services

– Services “local” to a geographic location– Not available globally

• Location aware services– Services are globally available, but multiple

instantiations of the same service are a function of locations

– Service adapts to a location

Issues in Operating Systems• Energy-efficient scheduler• File systems for disconnected operation due to

mobility and disconnected wireless links– access the same file as if connected– retain the same consistency semantics for shared file

s as if connected– availability and reliability as if connected– ACID (atomic/recoverability, consistent, isolated/ seri

alizablity, durable) properties for transactions• Constraints:

– disconnection and/or partial connection– low bandwidth connection– variable bandwidth and latency connection– connection cost

Next Step:

Networking Issues

Physical/MAC Layer• Requirements:

– Continuous access to the channel to transmit a frame without error

– Fair access to the channel: how is fairness quantified?

– Low power consumption– Increase channel throughput within the given

frequency band

• Constraints:– Channel is error prone– Channel contention and error are location

dependent– Transmission range is limited (but also enables

channel reuse)– Shared channel (hidden/exposed station problem)

Physical/MAC Layer

• Possible options:– Physical layer:

• Narrow band vs wide band: direct sequence, frequency hopping, OFDM

• Antenna technology: smart antenna, directional antenna, MIMO

• Adaptive modulation– MAC layer

• Multiple access protocols (CSMA/CA, MACAW, etc.)• Frame reservation protocols (TDMA, DQRUMA, et

c.)

Network Layer• Requirements:

– Maintain connectivity while user roams– Allow IP to integrate transparently with roami

ng hosts• Address translation to map location-independent

addressing to location dependent addressing• Packet forwarding• Location directory

– Support multicast, anycast– Ability to switch interfaces on the fly to migrat

e between failure-prone networks– Ability to provide quality of service: what is Qo

S in this environment?

Network Layer

• Constraint:– Unaware hosts running IP– Route management for mobile hosts

needs to be dynamic– A backbone may not exist (ad-hoc

network)

Network Layer• Possible options:

– Mobile IP and its variants• Two-tier addressing (location independent addre

ssing <-> location dependent addressing)• A smart forwarding agent which encapsulates pa

ckets from unware host to forward them to MH• Location directory for managing location update

s– Ad hoc routing

•Shortest path, source routing, multipath routing

Transport Layer• Requirements:

– Congestion control and rate adaptation• Doing the right thing in the presence of different p

acket losses– Handling different losses (mobility-induced disconne

ction, channel, reroute)– Improve transient performance

• Constraints: – Typically unware of mobility, yet affected by mobility– Packet may be lost due to congestion, channel error,

handoffs, change of interfaces, rerouting failures– Link-layer and transport layer retransmit interactions

Transport Layer

• Options:– Provide indirection– Make transport layer at the end hosts

ware of mobility– Provide smarts in intermediate nodes

(e.g. BS) to make lower-layer transport aware

– Provide error-free link layers