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Overview of Wireless Networking
• Wireless Link Characteristics
• Services and Applications
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
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•Inbuilding 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
Wireless Channel
• Wireless transmission is error prone
• Wireless error and contention are location dependent
• Wireless channel capacity is also location dependent
Link-Level Measurements
• Measurements taken from 802.11b-based MIT Roofnet
• Focus:– Explore reasons for loss– mainly on long outdoor links
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
Bro
ad
cast
Packet
Delivery
Pro
bab
ilit
y
> two-thirds of linksdeliver less than 90%
ImplicationsProtocols should exploit intermediate-
quality links• Link-quality-aware routing (ETX, LQSR)• 802.11 transmit bit-rate selection• Multicast data distribution• Opportunistic protocols (OMAC, ExOR)
• An emerging research direction
Hypotheses for intermediate delivery probability
1. 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
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)
Bro
adca
st p
acke
td
eliv
ery
pro
bab
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)
Most links aren’t burstyC
um
ula
tive
fra
ctio
n
of
no
de
pai
rs
Std dev of one-second delivery averages
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)
Roofnet links are long
Link distance (feet or nanoseconds)
Cu
mu
lati
ve f
ract
ion
o
f lin
ks
It’s reasonable to expect delays >500 ns
Summary
• Most Roofnet links have intermediate loss rates
• S/N does not predict delivery probability
• Loss is not consistent with bursty interference
• Multi-path is likely to be a major cause
Satellites• Geostationary Earth Orbit (GEO)
Satellites– example: Inmarsat
SAT
ground stations
Satellites• Low-Earth Orbit (LEO) Satellites
– example: Iridium (66 satellites, 2.4 Kbps)
SAT
ground stations
SAT
SAT
constellation
Satellites
• GEO– long delay: 250-300 ms propagation
delay
• LEO– relatively low delay: 40 - 200 ms– large variations in delay - multiple
hops/route changes, relative motion of satellites, queueing
Wireless Connectivity - Characteristics
• Transmission errors– Wireless LANs - 802.11, Hyperlan– Cellular wireless– Multi-hop wireless– Satellites
• Low bandwidth– Cellular wireless– Packet radio (e.g., Metricom)
• Long or variable latency– GEO, LEO satellites– Packet radio - high variability
• Asymmetry in bandwidth, error characteristics– Satellites (example: DirectPC)
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?
Possible scenarios of mobility
• Scenario 1: user logs out from computer 1, moves to computer 2 and logs in– Should the user see the same workspace?
• Scenario 2: different devices for different networks
• Scenario 3: user docks a laptop, works in a networked mode for a while, then disconnects and works in the standalone mode for a while, and then docks back– In stand-alone mode
• What kind of activities can the user do?• What cannot be done?• Can we provide an illusion of connectivity in this
case?• Can we automatically re-integrate the work he has
done while disconnected when he finally reconnects to the network server?
Impact of Mobility• Scenario 4: a user has a notebook with a
wireless connection, connects to a remote host via network 1, shuts down connections, connects to the remote host via network 2, continues to work– Is the disconnection between network
migration necessary?– When can we make the disconnection
transparent to users? When we cannot?– What are the key issues to ensure seamless
network migration?– Is it really important or users do not care
about the automatic process? For what applications? What to change for the applications?
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 files as if connected– availability and reliability as if connected– ACID (atomic/recoverability, consistent, isolated/
serializablity, 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,
etc.)
Link Layer• Requirements:
– Error sensitive application•A reliable link abstraction on top of
error-prone physical channels– Delay sensitive application
•A bounded delay link abstraction on top of error-prone channels
• Constraints:– Errors in the channel– Spatial congestion– Link capacity is changing (PHY multi-
rate option)
Link Layer• Possible options at the link layer
– Windowing to provide error and flow control– Combating error:
•Proactive: error correction via e.g. FEC•Reactive: error detection+retransmission,
ARQ•Channel-state prediction+channel swapping
– Fairness options: long term vs short term, deterministic vs probabilistic, temporal vs throughput•All links are treated equal•Users in error prone or congested location
suffer
Network Layer• Requirements:
– Maintain connectivity while user roams– Allow IP to integrate transparently with
roaming 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
migrate between failure-prone networks– Ability to provide quality of service: what
is QoS 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
addressing <-> location dependent addressing)
• A smart forwarding agent which encapsulates packets from unware host to forward them to MH
• Location directory for managing location updates
– 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 packet losses– Handling different losses (mobility-induced
disconnection, 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