Overview of Wireless Networking. Overview Fundamental issues and impact –wireless –mobility For...
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Transcript of Overview of Wireless Networking. Overview Fundamental issues and impact –wireless –mobility For...
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.
Using omni-direction antenna
+ Easy to deploy
+ Provide high connectivity
- Don’t allow engineered link quality
Delivery prob. uniformly distributed
Node Pair
Bro
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)
Bro
adca
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acke
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pro
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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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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
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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
Reflection causes intermediate packet loss
Del
iver
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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
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