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- Introduction
- Data in Wireless Cellular Systems
- Data in Wireless Local Area Networks
- Internet Protocols
- TCP over Wireless Link
- Ad-Hoc Networks, Sensor Networks
- Services and Service Discovery
- System Support for Mobile Applications
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- “Traditional” LANs: WaveLan, Proxim, IEEE 802.11
- More specific “personal” LANs, also called “Personal Area Networks”:
Bluetooth, IEEE 802.15
- High-speed wireless LANs (approaching ATM data rates): HiperLAN
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- Advantages
- very flexible within the reception area
- Ad-hoc networks without previous planning possible
- (almost) no wiring difficulties (e.g. historic buildings, firewalls)
- more robust against disasters like, e.g., earthquakes, fire - or users
pulling a plug...
- Disadvantages
- typically very low bandwidth compared to wired networks
(1-10 Mbit/s)
- many proprietary solutions, especially for higher bit-rates, standards
take their time (e.g. IEEE 802.11)
- products have to follow many national restrictions if working wireless,
it takes a vary long time to establish global solutions like, e.g.,
IMT-2000
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- global, seamless operation
- low power for battery use
- no special permissions or licenses needed to use the LAN
- robust transmission technology
- simplified spontaneous cooperation at meetings
- easy to use for everyone, simple management
- protection of investment in wired networks
- security (no one should be able to read my data), privacy (no one should
be able to collect user profiles), safety (low radiation)
- transparency concerning applications and higher layer protocols, but
also location awareness if necessary
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- Infrared
- uses IR diodes, diffuse light, multiple reflections (walls, furniture
etc.)
- Advantages
- simple, cheap, available in many mobile devices
- no licenses needed
- simple shielding possible
- Disadvantages
- interference by sunlight, heat sources etc.
- many things shield or absorb IR light
- low bandwidth
- Example
- IrDA (Infrared Data Association) interface available everywhere
- Radio
- typically using the license free ISM band at 2.4 GHz
- Advantages
- experience from wireless WAN and mobile phones can be used
- coverage of larger areas possible (radio can penetrate walls, furniture
etc.)
- Disadvantages
- very limited license free frequency bands
- shielding more difficult, interference with other electrical devices
- Example
- WaveLAN, HIPERLAN, Bluetooth
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- Standard for wireless local area networks, approved by IEEE in 1997
- Scope: physical layer (PHY) and media access control sublayer (MAC) for
wireless connectivity for fixed, portable, and moving stations with a
local area
- Supports data rates of 1 or 2 Mbps, using infrared or radio
- Supports two basic architectures: independent basic support set (IBSS)
and infrastructure networks
- Most recent commercial products (including the new WaveLAN generation)
are compatible with 802.11
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- Station (STA)
- terminal with access mechanisms to the wireless medium and radio
contact to the access point
- Basic Service Set (BSS)
- group of stations using the same radio frequency
- Access Point
- station integrated into the wireless LAN and the distribution system
- Portal
- bridge to other (wired) networks
- Distribution System
- interconnection network to form one logical network (EES: Extended
Service Set) based
on several BSS
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- Direct communication within a limited range
- Station (STA):
terminal with access mechanisms to the wireless medium
- Basic Service Set (BSS):
group of stations using the same radio frequency
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- PLCP Physical Layer Convergence Protocol
- clear channel assessment signal (carrier sense)
- PMD Physical Medium Dependent
- PHY Management
- Station Management
- coordination of all management functions
- MAC
- access mechanisms, fragmentation, encryption
- MAC Management
- synchronization, roaming, MIB, power management
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- 3 versions: 2 radio (typ. 2.4 GHz), 1 IR
- FHSS (Frequency Hopping Spread Spectrum)
- spreading, despreading, signal strength, typ. 1 Mbit/s
- min. 2.5 frequency hops/s (USA), two-level GFSK modulation
- DSSS (Direct Sequence Spread Spectrum)
- DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying),
DQPSK for 2 Mbit/s (Differential Quadrature PSK)
- preamble and header of a frame is always transmitted with 1 Mbit/s,
rest of transmission 1 or 2 Mbit/s
- chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker
code)
- max. radiated power 1 W (USA), 100 mW (EU), min. 1mW
- Infrared
- 850-950 nm, diffuse light, typ. 10 m range
- carrier detection, energy detection, synchonization
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- Traffic services
- Asynchronous Data Service (mandatory)
- exchange of data packets based on “best-effort”
- support of broadcast and multicast
- Time-Bounded Service (optional)
- implemented using PCF (Point Coordination Function)
- Access methods
- DCF CSMA/CA (mandatory)
- collision avoidance via randomized „back-off“ mechanism
- minimum distance between consecutive packets
- ACK packet for acknowledgements (not for broadcasts)
- DCF w/ RTS/CTS (optional)
- Distributed Foundation Wireless MAC (DFWMAC)
- avoids hidden terminal problem
- PCF (optional)
- access point polls terminals according to a list
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- Priorities
- defined through different inter frame spaces
- no guaranteed, hard priorities
- SIFS (Short Inter Frame Spacing)
- highest priority, for ACK, CTS, polling response
- PIFS (PCF IFS)
- medium priority, for time-bounded service using PCF
- DIFS (DCF, Distributed Coordination Function IFS)
- lowest priority, for asynchronous data service
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- station ready to send starts sensing the medium (Carrier Sense based on
CCA, Clear Channel Assessment)
- if the medium is free for the duration of an Inter-Frame Space (IFS),
the station can start sending (IFS depends on service type)
- if the medium is busy, the station has to wait for a free IFS, then the
station must additionally wait a random back-off time (collision
avoidance, multiple of slot-time)
- if another station occupies the medium during the back-off time of the
station, the back-off timer stops (fairness)
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- Sense media before transmission
- If media is free, transmit if media stays idle for a fixed amount of
time (DCF Interframe Space, DIFS)
- Defer:
- wait until end of current transmission, plus DIFS
- apply random backoff procedure: pick number between 0 and 7, check
whether medium is idle during each backoff slot
- if media is busy, suspend backoff process at beginning of current slot
- after media was idle for selected number of slots, transmit immediately
- if this transmission results in collision, backoff again, doubling the
backoff
- Upon receipt of packet:
- receiver waits short interval (Short Interframe Space, SIFS)
- transmits acknowledgement frame (ACK) back to sender
- If sender receives no ACK within ACKTimeout interval, assume collision
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- Sending unicast packets
- station has to wait for DIFS before sending data
- receivers acknowledge at once (after waiting for SIFS) if the packet
was received correctly (CRC)
- automatic retransmission of data packets in case of transmission errors
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- Sending unicast packets
- station can send RTS with reservation parameter after waiting for DIFS
(reservation determines amount of time the data packet needs the
medium)
- acknowledgement via CTS after SIFS by receiver (if ready to receive)
- sender can now send data at once, acknowledgement via ACK
- other stations store medium reservations distributed via RTS and CTS
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- Types
- control frames, management frames, data frames
- Sequence numbers
- important against duplicated frames due to lost ACKs
- Addresses
- receiver, transmitter (physical), BSS identifier, sender (logical)
- Miscellaneous
- sending time, checksum, frame control, data
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- Synchronization
- try to find a LAN, try to stay within a LAN
- timer etc.
- Power management
- sleep-mode without missing a message
- periodic sleep, frame buffering, traffic measurements
- Association/Reassociation
- integration into a LAN
- roaming, i.e. change networks by changing access points
- scanning, i.e. active search for a network
- MIB - Management Information Base
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- Idea: switch the transceiver off if not needed
- States of a station: sleep and awake
- Timing Synchronization Function (TSF)
- stations wake up at the same time
- Infrastructure
- Traffic Indication Map (TIM)
- list of unicast receivers transmitted by AP
- Delivery Traffic Indication Map (DTIM)
- list of broadcast/multicast receivers transmitted by AP
- Ad-hoc
- Ad-hoc Traffic Indication Map (ATIM)
- announcement of receivers by stations buffering frames
- more complicated - no central AP
- collision of ATIMs possible (scalability?)
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- No or bad connection? Then perform:
- Scanning
- scan the environment, i.e., listen into the medium for beacon signals
or send probes into the medium and wait for an answer
- Reassociation Request
- station sends a request to one or several AP(s)
- Reassociation Response
- success: AP has answered, station can now participate
- failure: continue scanning
- AP accepts Reassociation Request
- signal the new station to the distribution system
- the distribution system updates its data base (i.e., location
information)
- typically, the distribution system now informs the old AP so it can
release resources
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- Data rate
- 1, 2, 5.5, 11 Mbit/s, depending on SNR
- User data rate max. approx. 6 Mbit/s
- Transmission range
- 300m outdoor, 30m indoor
- Max. data rate ~10m indoor
- Frequency
- Security
- Limited, WEP insecure, SSID
- Cost
- $100 adapter, $250 base station, dropping
- Availability
- Many products, many vendors
- Connection set-up time
- Quality of Service
- Typ. Best effort, no guarantees (unless polling is used, limited
support in products)
- Manageability
- Limited (no automated key distribution, sym. Encryption)
- Special Advantages/Disadvantages
- Advantage: many installed systems, lot of experience, available
worldwide, free ISM-band, many vendors, integrated in laptops, simple
system
- Disadvantage: heavy interference on ISM-band, no service guarantees,
slow relative speed only
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- Data rate
- 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR
- User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54)
- 6, 12, 24 Mbit/s mandatory
- Transmission range
- 100m outdoor, 10m indoor
- E.g., 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m,
18 up to 40 m, 12 up to 60 m
- Frequency
- Free 5.15-5.25, 5.25-5.35, 5.725-5.825 GHz ISM-band
- Security
- Limited, WEP insecure, SSID
- Cost
- $280 adapter, $500 base station
- Availability
- Some products, some vendors
- Connection set-up time
- Quality of Service
- Typ. best effort, no guarantees (same as all 802.11 products)
- Manageability
- Limited (no automated key distribution, sym. Encryption)
- Special Advantages/Disadvantages
- Advantage: fits into 802.x standards, free ISM-band, available, simple
system, uses less crowded 5 GHz band
- Disadvantage: stronger shading due to higher frequency, no QoS
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- 802.11d: Regulatory Domain Update
- 802.11e: MAC Enhancements – QoS
- Enhance the current 802.11 MAC to expand support for applications with
Quality of Service requirements, and in the capabilities and efficiency
of the protocol.
- 802.11f: Inter-Access Point Protocol
- Establish an Inter-Access Point Protocol for data exchange via the
distribution system.
- 802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM –802.11h:
Spectrum Managed 802.11a (DCS, TPC)
- 802.11i: Enhanced Security Mechanisms
- Enhance the current 802.11 MAC to provide improvements in security.
- Study Groups
- 5 GHz (harmonization ETSI/IEEE)
- Radio Resource Measurements
- High Throughput
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- open specification for wireless communication of data and voice
- based on a low-cost short-range radio link, built into a 9 x 9 mm
microchip (design goal: cost of US$ 5/device)
- facilitates protected ad hoc connections for stationary and mobile
communication environments
- Bluetooth is a cooperation between computer and telecommunication
industries (Ericsson, IBM, Toshiba, Intel, Nokia, …)
- SIG started in February 1998 with above five members, has grown since
(64 companies joined in January 1999 alone)
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- operates in the 2.4 GHz Industrial-Scientific-Medical (ISM) band
- nominal link range: 10 cm to 10 m, can be increased to 100 m
(transmitting with more power)
- uses Frequence Hop (FH) spread spectrum
- supports up to 8 devices in a piconet (two or more Bluetooth units
sharing a channel)
- built-in security
- non line-of-sight transmission through walls and briefcases
(distinguishes it from IrDA)
- omni-directional
- supports both isochronous and asynchronous services; easy integration of
TCP/IP for networking
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- connect a wide range of computing and telecommunications devices without
the need to buy, carry, or connect cables
- delivers opportunities for rapid, ad hoc connections, and in the future,
possibly for automatic, unconscious, connections between devices
- power-efficient radio technology can be used in many of the same devices
that use IR:
- Phones and pagers
- Modems
- LAN access devices
- Headsets
- Notebook, desktop, and handheld computers
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- History
- 1994: Ericsson (Mattison/Haartsen), “MC-link” project
- Renaming of the project: Bluetooth according to Harald “Blĺtand” Gormsen
[son of Gorm], King of Denmark in the 10th century
- 1998: foundation of Bluetooth SIG, www.bluetooth.org
- 1999: erection of a rune stone at Ericsson/Lund ;-)
- 2001: first consumer products for mass market, spec. version 1.1
released
- Special Interest Group
- Original founding members: Ericsson, Intel, IBM, Nokia, Toshiba
- Added promoters: 3Com, Agere (was: Lucent), Microsoft, Motorola
- > 2500 members
- Common specification and certification of products
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- frequency hopping in 79 hops displaced by 1 MHz, starting at 2.402 GHz
and stopping at 2.480 GHz
- to function on a worldwide basis, Bluetooth requires a radio frequency
that is license-free and open to any radio
- 2.45 GHz ISM band satisfies these requirements, although it must cope
with interference from baby monitors, garage door openers, cordless
phones and microwave ovens, which also use this frequency.
- due to local regulations the bandwidth is reduced in Japan, France and
Spain. This is handled by an internal software switch
- the maximum frequency hopping rate is 1600 hops/s.
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- Time Division Duplex (TDD) scheme for full-duplex transmissions
- master device establishes connection, slave devices synchronize their
clock with master clock for duration of connection
- Synchronous Connection Oriented (SCO) type (used primarily for voice)
- channel symmetric, only data packets retransmitted
- Asynchronous Connectionless (ACL) type (used primarily for packet data)
- master unit controls the link bandwidth and decides how much piconet
bandwidth is given to each slave, and the symmetry of the traffic
- slaves must be polled before they can transmit data.
- The ACL link also supports broadcast messages from the master to all
slaves in the piconet
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- Error correction:
- 1/3 rate forward error correction code (FEC)
- 2/3 rate forward error correction code FEC
- Automatic repeat request (ARQ) scheme for data
- data transmitted in one slot is directly acknowledged by the recipient
in the next slot.
- Authentication and Privacy
- one-way, two-way, or no authentication possible
- use stream cipher based on secret keys (0, 40, 64 bits)
- key management left to higher layer software
- if stronger protection (longer key is needed), use better encryption at
network and/or application level
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- Collection of devices connected in an ad hoc fashion
- One unit acts as master and the others as slaves for the lifetime of the
piconet
- Master determines hopping pattern, slaves have to synchronize
- Each piconet has a unique hopping pattern
- Participation in a piconet = synchronization to hopping sequence
- Each piconet has one master and up to 7 simultaneous slaves (> 200
could be parked)
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- All devices in a piconet hop together
- Master gives slaves its clock and device ID
- Hopping pattern: determined by device ID (48 bit, unique worldwide)
- Phase in hopping pattern determined by clock
- Addressing
- Active Member Address (AMA, 3 bit)
- Parked Member Address (PMA, 8 bit)
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- Linking of multiple co-located piconets through the sharing of common
master or slave devices
- Devices can be slave in one piconet and master of another
- Communication between piconets
- Devices jumping back and forth between the piconets
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- Multiple overlapping piconets (sets of communicating devices) with own
hopping sequence, max of one master and 8 slaves
- Collisions do occur when two piconets use same frequency at the same
time
- as more piconets overlap, performance degrades
- degradation gradual: 10 overlapping piconets reduce aggregate bandwidth
by 10%
- Single device can participate in multiple piconets, though only one at a
time
- need to re-adjust clock to re-sync with master when entering a piconet
- inform master when device leaves piconet, will suppress data being
sent/device being polled
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- Inquiry/response protocol for discovering services
- Searching for and browsing services in radio proximity
- Adapted to the highly dynamic environment
- Can be complemented by others like SLP, Jini, Salutation, …
- Defines discovery only, not the usage of services
- Caching of discovered services
- Gradual discovery
- Service record format
- Information about services provided by attributes
- Attributes are composed of an 16 bit ID (name) and a value
- values may be derived from 128 bit Universally Unique Identifiers
(UUID)
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- Data rate
- Synchronous, connection-oriented: 64 kbit/s
- Asynchronous, connectionless
- 433.9 kbit/s symmetric
- 723.2 / 57.6 kbit/s asymmetric
- Transmission range
- POS (Personal Operating Space) up to 10 m
- with special transceivers up to 100 m
- Frequency
- Security
- Challenge/response (SAFER+), hopping sequence
- Cost
- $50 adapter, drop to $5 if integrated
- Availability
- Integrated into some products, several vendors
- Connection set-up time
- Depends on power-mode
- Max. 2.56s, avg. 0.64s
- Quality of Service
- Manageability
- Public/private keys needed, key management not specified, simple system
integration
- Special Advantages/Disadvantages
- Advantage: already integrated into several products, available
worldwide, free ISM-band, several vendors, simple system, simple ad-hoc
networking, peer to peer, scatternets
- Disadvantage: interference on ISM-band, limited range, max. 8
devices/network&master, high set-up latency
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- 802.15-2: Coexistance
- Coexistence of Wireless Personal Area Networks (802.15) and Wireless
Local Area Networks (802.11), quantify the mutual interference
- 802.15-3: High-Rate
- Standard for high-rate (20Mbit/s or greater) WPANs, while still
low-power/low-cost
- Data Rates: 11, 22, 33, 44, 55 Mbit/s
- Quality of Service isochronous protocol
- Ad hoc peer-to-peer networking
- Security
- Low power consumption
- Low cost
- Designed to meet the demanding requirements of portable consumer
imaging and multimedia applications
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- 802.15-4: Low-Rate, Very Low-Power
- Low data rate solution with multi-month to multi-year battery life and
very low complexity
- Potential applications are sensors, interactive toys, smart badges,
remote controls, and home automation
- Data rates of 20-250 kbit/s, latency down to 15 ms
- Master-Slave or Peer-to-Peer operation
- Support for critical latency devices, such as joysticks
- CSMA/CA channel access (data centric), slotted (beacon) or unslotted
- Automatic network establishment by the PAN coordinator
- Dynamic device addressing, flexible addressing format
- Fully handshaked protocol for transfer reliability
- Power management to ensure low power consumption
- 16 channels in the 2.4 GHz ISM band, 10 channels in the 915 MHz US ISM
band and one channel in the European 868 MHz band
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Notes