Medium Access Control (MAC)
(forts.)
Foreleser:
Kjell Åge Bringsrud
E-mail: kjellb
2/28/2003
1
Persistent og ikkepersistent CSMA
Sammenlikning av kanal-utnyttelse versus belastning for
noen tilfeldig aksess protokoller.
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CSMA med
kollisjonsdeteksjon
CSMA/CD kan være i én av tre tilstander: “contention”,
overføring eller uvirksom.
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1
Kollisjonsfrie protokoller
Den basale bit-map protokollen
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Collision-Free Protocols (2)
The binary countdown protocol. A dash indicates silence.
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Trådløs
Mobiltelefoni
Satelitter
GPRS
UMTS
Geostasjonære
Lavbane
Trådløs “Ethernet” - IEEE 802.11
. . . Bare fantasien (og noen få fysiske lover)
setter grenser:
Ad-hoc eller spontane nettverk
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2
Growth of Wireless and
Broadband Internet
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Utstyr
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Roaming/Mobility Models
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3
Explosion of New Internet Appliances
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Example: MIT’s “Expressive
Footwear”
Dance shoes with wireless link & a suite of sensors
measure dynamic parameters at a dancer's foot
⌧differential pressure at 3 points and bend in the sole, 2-axis tilt,
3-axis shock, height off the stage, orientation, angular rate and
translational position)
example use: generate accompanying music
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Computing View of the
Future Information Systems
The most profound technologies are those that disappear.
They weave themselves into the fabric of everyday life until they
are indistinguishable from it... the idea of a “personal computer”
itself is misplaced... the vision of laptop machines, dynabooks and
“knowledge navigators” is only a transitional step... a new way of
thinking about computers, one that takes into account the human
world and allows the computers themselves to vanish into the
background.
Mark Weiser (Chief Technologist, Xerox PARC)
Scientific American, September1991
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4
Alternate Models of Mobile
Computing Systems
Ubiquitous Information Access
information distributed everywhere by the “net”
terminal centric
⌧users carry wireless terminals
⌧terminal is the universal service access device
⌧terminal adapts to location and services
Ubiquitous Computing
cheap computers of different scales and types embedded
everywhere
⌧100s of computer in every room in the form of common, day-today objects
user centric
⌧computers swapped among users
⌧computers dedicated to service
⌧computers adapt to location and users
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Novel Attributes of Mobile
and Wireless Systems
Wireless
⌧limited bandwidth
⌧high latency
– < 3 ms indoor
– > 100 ms outdoor (cellular, satellite)
⌧variable link quality
– noise, disconnections, interference
⌧link asymmetry
⌧heterogeneous air interfaces
⌧easier snooping
More
Signal
Processing
Mobility
Portability
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Novel Attributes of Mobile
and Wireless Systems (2)
Wireless
Mobility
⌧user and terminal location
– are system variables of interest
– change dynamically
⌧speed of terminal mobility impacts
wireless bandwidth
⌧constants become variable
– location, environment, connectivity,
b/w,
I/O devices, security domain
More
Protocol
Processing
⌧easier spoofing
Portability
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5
Novel Attributes of Mobile
and Wireless Systems (3)
Wireless
Mobility
Portability
⌧limited battery capacity
⌧limited computing
⌧limited storage
⌧small dimensions
⌧risk to data (easily lost)
More
Energy
Efficiency
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Heterogeneous Networks
Satellite
Regional Area
Low-tier
High-tier
Local Area
Wide Area
Low Mobility
High Mobility
Seamless mobility across diverse overlay networks
⌧“vertical” hand-offs
⌧software “agents” for heterogeneity management
⌧IP as the common denominator?
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Ad Hoc Networks
•
•
•
•
Disaster recovery
Battlefield
‘Smart’ office
Etc.
Rapidly deployable
infrastructure
Wireless: cabling
impractical
Ad-Hoc: no advance
planning
Backbone network:
wireless IP routers
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•
Network of access devices
•
•
•
Wireless: untethered
Ad-hoc: random deployment
Edge network: Sensor networks,
Personal Area Networks (PANs), etc.
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6
Location-dependent
Information
Location affects configuration parameters
DNS, timezone, printer etc.
Location affects answer to user queries
e.g. where is the nearest printer
More complex location-dependent queries
e.g. where is the nearest taxi
Privacy concerns due to location tracking
Changing context
small movements may cause large changes
caching may become ineffective
dynamic transfer to nearest server for a service
Localization
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Portability
Power is key
long mean-time-to-recharge, small weight, volume
Risk to data due to easier privacy breach
network integrated terminals with no local storage
Small user interfaces
small displays, analog inputs (speech, handwriting)
instead of buttons and keyboards
Small storage capacity
data compression, network storage, compressed
virtual memory, compact scripts vs. compiled code
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Low Power & Energy-awareness
Battery technology is a hurdle… no Moore’s Law to help
out
Typical laptop: 30% display, 30% CPU, 30% rest
wireless communication and multimedia processing incur
significant power overhead
Low power
circuits, architectures, protocols
Power management
Right power at the right place at the right time
Battery model
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7
Challenges in Mobile and
Wireless Computing
Portable, energy-efficient devices
End-to-end quality of service
Seamless operation under context
changes
Context-aware operation
Secure operation
Sophisticated services for simple clients
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Trådløs “Ethernet”
IEEE 802.11
Tre overføringsmetoder
Spredt spektrum (to typer)
Diffus infrarød
Avstand: 5 cm til 5 km
Spredt spektrum kan sendes i alle retninger eller
konsentreres i en bestemt retning
Maksimum tillatt sendestyrke, derfor går bølgene lengst
når de er rettet (vi har målt opp til 5 km.)
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Spredt spektrum
Sprer signalet over et (stort) frekvensområde
Signalet er “skjult” i dette området
Tåler mye støy (interferens fra annet utstyr)
To teknikker:
Frekvens hopping:
⌧Sender (og mottaker) hopper rundt i
frekvensområdet og sender noen bit hvert sted
Direkte sekvens:
⌧hvert bit i rammen representert ved flere bit i det fysiske
signalet (fig. 2.37 side 137)
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8
Diffus infrarød
( i IEEE 802.11)
Millimeter (og kortere) bølger
Som lys trenger de ikke gjennom vegger
Diffus: Reflekteres av vegger mm.
Behøver derfor ikke direkte synslinje
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Trådløs Ethernet
Ethernet, Men 1:
Når både A og C sender til B blir det kollisjon
som ingen av dem oppdager
A
B
A og C
ser ikke
hverandre
(skjulte
noder)
C
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Trådløs Ethernet
Men 2:
Hvis B hører at C sender, må B da ti stille ?
Kan B sende (til A) samtidig som C sender (til D)
?
A
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B
C
D
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9
Collison Avoidance
Multiple Access with Collison Avoidance
(MA/CA)
1. En avsender spør en mottaker om å få lov å sende en
viss stund (sendelengden)
2. Mottaker svarer tilbake til avsenderen: “Klart til å sende”
Alle som hører en “Klart til å sende” vet de er nær
mottakeren, og må ikke sende selv i dette tidsrommet
Alle som bare hørte forespørselen om å sende (men ikke
svaret) kan gjerne sende selv
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Kollisjon og retransmisjon
Når to sendeforespørsler kolliderer vil mottaker
ikke skjønne meldingen og gir ikke noe svar
Nå det ikke kommer noe svar venter sender en
stund før den sender en ny forespørsel
(på samme måte som i vanlig Ethernet)
Alle datapakker kvitteres (ved ACK-melding)
Kommer det ikke noen ACK retransmitteres
pakken
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Aksesspunkter og celler
A, B og C er aksesspunkter
M (mobil) sender “Probe”-pakke (til alle)
A svarer at den eksisterer med å sende “Probe Response” pakke til M
M godtar A som aksesspunkt og svarer til A: “Association Request”
A besegler forbindelsen ved å svare “Association Response” til M
M
Distribusjonsnett
A
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B
C
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10
Aksesspunkter og celler
Hva skjer nå M flytter seg ?
M sender ut nye “Probe” pakker når signalet fra A
blir svakt (kalles “active scanning”)
Ny forbindelse opprettes så med B
M
M
Distribusjonsnett
A
B
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C
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IEEE 802
The LAN/MAN Standards Committee
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P802.16 Scope
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11
Point-to-Multipoint Wireless
MAN: not a LAN
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PHY Considerations
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Adaptive PHY
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12
Definitions
• BS
– Base Station
• SS
– Subscriber Station
• Downlink (DL)
– BS to SS
• Uplink (UL)
– SS to BS
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Adaptive Burst Profiles
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Duplex Scheme Support
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13
TDD Frame (10-66 GHz)
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Burst FDD Framing
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Allows scheduling flexibility
41
Baud Rates & Channel Size
(10-66 GHz)
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MAC Requirements
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802.16 MAC: Overview
•
•
•
•
Point-to-Multipoint
Metropolitan Area Network
Connection-oriented
Supports difficult user environments
– High bandwidth, hundreds of users per channel
– Continuous and burst traffic
– Very efficient use of spectrum
• Protocol-Independent core (ATM, IP, Ethernet, …)
• Flexible QoS offerings
• Supports multiple 802.16 PHYs
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MAC PDU Transmission
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MAC PDU format
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Downlink transmissions
• Two kinds of bursts: TDM and TDMA
• All bursts are identified by a DIUC
– Downlink Interval Usage
• Each terminal listens to all bursts
except when told to transmit
• Each burst may contain data for
several terminals
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Uplink Transmissions
• Invited transmissions Invited transmissions
• Transmissions in contention slots
– Bandwidth requests
– Contention resolved using truncated exponential backoff
• Transmissions in initial ranging slots
– Ranging Requests (RNG-REQ)
– Contention resolved using truncated exponential backoff
• All transmissions have synchronization preamble
• Ideally, all data from a single SS is concatenated into a
single PHY burst
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16
IP connectivity and
configuration file download
• IP connectivity established via DHCP
• Configuration file downloaded via
• contains provisioned information
– operational parameters
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17