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Introduction and History |
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Data in Wireless Cellular Systems |
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Data in Wireless Local Area Networks |
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Internet Protocols |
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Routing and Ad-Hoc Networks |
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TCP over Wireless Link |
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Services and Service Discovery |
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System Support for Mobile Applications |
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“Traditional” LANs: WaveLan, Proxim, IEEE 802.11 |
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More specific “personal” LANs, also called
“Personal Area Networks”: Bluetooth, IEEE 802.15 |
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High-speed wireless LANs (approaching ATM data
rates): HiperLAN |
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Commercial product, developed by Lucent
Technologies (de-facto market leader), available since early 1990s |
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Development closely aligned with early IEEE
802.11 standard effort, first product generation differs in key aspects
from final IEEE 802.11 standard |
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Will discuss WaveLAN version 1, newer version 2
follows finally approved 802.11 standard (plus enhancements to allow for
data rates up to 10 Mbps) |
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Physical layer: |
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915 MHz (902-928 MHz) or 2.4 GHz (2.412-2.475
GHz) ISM band |
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Bands are divided into frequency channels of 26
MHz each |
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Modulation scheme: DSSS, raw data rate of 2 Mbps |
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Basically CSMA/CA |
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Sense media before transmission, if free,
transmit |
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Defer: |
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wait until end of current transmission, plus
fixed delay (WaveLAN InterFrame Space, WIFS) of 60 msec |
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apply random backoff procedure: pick number
between 0 and 31 (antenna slot number S) |
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each slot corresponds to 23 ms |
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wait S*23 ms, sense media again, if busy, double backoff
range until we reach range 0-255 |
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drop packet after 15 attempts |
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If station has more than one packet to send:
wait WIFS plus backoff-period in range 0-15 (avoid monopolization) |
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Standard for wireless local area networks,
approved by IEEE in 1997 |
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Scope: physical layer (PHY) and media access
control sublayer (MAC) for wireless connectivity for fixed, portable, and
moving stations with a local area |
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Supports data rates of 1 or 2 Mbps, using
infrared or radio |
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Supports two basic architectures: independent
basic support set (IBSS) and infrastructure networks |
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Most recent commercial products (including the
new WaveLAN generation) are compatible with 802.11 |
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Station (STA) |
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terminal with access mechanisms to the wireless
medium and radio contact to the access point |
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Basic Service Set (BSS) |
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group of stations using the same radio frequency |
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Access Point |
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station integrated into the wireless LAN and the
distribution system |
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Portal |
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bridge to other (wired) networks |
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Distribution System |
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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 |
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Station (STA):
terminal with access mechanisms to the wireless medium |
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Basic Service Set (BSS):
group of stations using the same radio frequency |
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PLCP Physical Layer Convergence Protocol |
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clear channel assessment signal (carrier sense) |
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PMD Physical Medium Dependent |
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modulation, coding |
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PHY Management |
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channel selection, MIB |
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Station Management |
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coordination of all management functions |
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MAC |
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access mechanisms, fragmentation, encryption |
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MAC Management |
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synchronization, roaming, MIB, power management |
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3 versions: 2 radio (typ. 2.4 GHz), 1 IR |
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data rates 1 or 2 Mbit/s |
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FHSS (Frequency Hopping Spread Spectrum) |
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spreading, despreading, signal strength, typ. 1
Mbit/s |
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min. 2.5 frequency hops/s (USA), two-level GFSK
modulation |
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DSSS (Direct Sequence Spread Spectrum) |
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DBPSK modulation for 1 Mbit/s (Differential
Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature
PSK) |
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preamble and header of a frame is always
transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s |
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chipping sequence: +1, -1, +1, +1, -1, +1, +1,
+1, -1, -1, -1 (Barker code) |
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max. radiated power 1 W (USA), 100 mW (EU), min.
1mW |
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Infrared |
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850-950 nm, diffuse light, typ. 10 m range |
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carrier detection, energy detection,
synchonization |
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Traffic services |
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Asynchronous Data Service (mandatory) |
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exchange of data packets based on “best-effort” |
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support of broadcast and multicast |
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Time-Bounded Service (optional) |
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implemented using PCF (Point Coordination
Function) |
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Access methods |
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DCF CSMA/CA (mandatory) |
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collision avoidance via randomized „back-off“
mechanism |
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minimum distance between consecutive packets |
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ACK packet for acknowledgements (not for
broadcasts) |
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DCF w/ RTS/CTS (optional) |
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Distributed Foundation Wireless MAC (DFWMAC) |
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avoids hidden terminal problem |
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PCF (optional) |
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access point polls terminals according to a list |
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Priorities |
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defined through different inter frame spaces |
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no guaranteed, hard priorities |
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SIFS (Short Inter Frame Spacing) |
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highest priority, for ACK, CTS, polling response |
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PIFS (PCF IFS) |
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medium priority, for time-bounded service using
PCF |
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DIFS (DCF, Distributed Coordination Function
IFS) |
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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) |
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if the medium is free for the duration of an
Inter-Frame Space (IFS), the station can start sending (IFS depends on
service type) |
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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) |
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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 |
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If media is free, transmit if media stays idle
for a fixed amount of time (DCF Interframe Space, DIFS) |
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Defer: |
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wait until end of current transmission, plus
DIFS |
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apply random backoff procedure: pick number
between 0 and 7, check whether medium is idle during each backoff slot |
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if media is busy, suspend backoff process at
beginning of current slot |
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after media was idle for selected number of
slots, transmit immediately |
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if this transmission results in collision,
backoff again, doubling the backoff |
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Upon receipt of packet: |
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receiver waits short interval (Short Interframe
Space, SIFS) |
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transmits acknowledgement frame (ACK) back to
sender |
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If sender receives no ACK within ACKTimeout
interval, assume collision |
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Sending unicast packets |
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station has to wait for DIFS before sending data |
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receivers acknowledge at once (after waiting for
SIFS) if the packet was received correctly (CRC) |
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automatic retransmission of data packets in case
of transmission errors |
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Sending unicast packets |
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station can send RTS with reservation parameter
after waiting for DIFS (reservation determines amount of time the data
packet needs the medium) |
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acknowledgement via CTS after SIFS by receiver
(if ready to receive) |
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sender can now send data at once,
acknowledgement via ACK |
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other stations store medium reservations
distributed via RTS and CTS |
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Types |
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control frames, management frames, data frames |
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Sequence numbers |
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important against duplicated frames due to lost
ACKs |
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Addresses |
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receiver, transmitter (physical), BSS
identifier, sender (logical) |
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Miscellaneous |
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sending time, checksum, frame control, data |
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Synchronization |
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try to find a LAN, try to stay within a LAN |
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timer etc. |
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Power management |
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sleep-mode without missing a message |
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periodic sleep, frame buffering, traffic
measurements |
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Association/Reassociation |
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integration into a LAN |
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roaming, i.e. change networks by changing access
points |
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scanning, i.e. active search for a network |
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MIB - Management Information Base |
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managing, read, write |
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Idea: switch the transceiver off if not needed |
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States of a station: sleep and awake |
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Timing Synchronization Function (TSF) |
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stations wake up at the same time |
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Infrastructure |
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Traffic Indication Map (TIM) |
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list of unicast receivers transmitted by AP |
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Delivery Traffic Indication Map (DTIM) |
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list of broadcast/multicast receivers
transmitted by AP |
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Ad-hoc |
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Ad-hoc Traffic Indication Map (ATIM) |
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announcement of receivers by stations buffering
frames |
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more complicated - no central AP |
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collision of ATIMs possible (scalability?) |
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No or bad connection? Then perform: |
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Scanning |
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scan the environment, i.e., listen into the
medium for beacon signals or send probes into the medium and wait for an
answer |
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Reassociation Request |
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station sends a request to one or several AP(s) |
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Reassociation Response |
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success: AP has answered, station can now
participate |
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failure: continue scanning |
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AP accepts Reassociation Request |
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signal the new station to the distribution
system |
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the distribution system updates its data base
(i.e., location information) |
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typically, the distribution system now informs
the old AP so it can release resources |
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IEEE 802.11a |
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compatible MAC, but now 5 GHz band |
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transmission rates up to 20 Mbit/s |
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close cooperation with BRAN (ETSI Broadband
Radio Access Network) |
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IEEE 802.11b |
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higher data rates at 2.4 GHz |
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proprietary solutions already offer 10 Mbit/s |
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IEEE WPAN (Wireless Personal Area Networks) |
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market potential |
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compatibility |
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low cost/power, small form factor |
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technical/economic feasibility
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Bluetooth |
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open specification for wireless communication of
data and voice |
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based on a low-cost short-range radio link,
built into a 9 x 9 mm microchip (design goal: cost of US$ 5/device) |
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facilitates protected ad hoc connections for
stationary and mobile communication environments |
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Bluetooth is a cooperation between computer and
telecommunication industries (Ericsson, IBM, Toshiba, Intel, Nokia, …) |
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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 |
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nominal link range: 10 cm to 10 m, can be
increased to 100 m (transmitting with more power) |
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uses Frequence Hop (FH) spread spectrum |
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supports up to 8 devices in a piconet (two or
more Bluetooth units sharing a channel) |
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built-in security |
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non line-of-sight transmission through walls and
briefcases (distinguishes it from IrDA) |
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omni-directional |
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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 |
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delivers opportunities for rapid, ad hoc
connections, and in the future, possibly for automatic, unconscious,
connections between devices |
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power-efficient radio technology can be used in
many of the same devices that use IR: |
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Phones and pagers |
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Modems |
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LAN access devices |
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Headsets |
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Notebook, desktop, and handheld computers |
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frequency hopping in 79 hops displaced by 1 MHz,
starting at 2.402 GHz and stopping at 2.480 GHz |
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to function on a worldwide basis, Bluetooth
requires a radio frequency that is license-free and open to any radio |
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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. |
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due to local regulations the bandwidth is
reduced in Japan, France and Spain. This is handled by an internal software
switch |
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the maximum frequency hopping rate is 1600
hops/s. |
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Time Division Duplex (TDD) scheme for
full-duplex transmissions |
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master device establishes connection, slave
devices synchronize their clock with master clock for duration of
connection |
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Synchronous Connection Oriented (SCO) type (used
primarily for voice) |
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channel symmetric, only data packets
retransmitted |
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Asynchronous Connectionless (ACL) type (used
primarily for packet data) |
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master unit controls the link bandwidth and
decides how much piconet bandwidth is given to each slave, and the symmetry
of the traffic |
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slaves must be polled before they can transmit
data. |
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The ACL link also supports broadcast messages
from the master to all slaves in the piconet |
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Error correction: |
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1/3 rate forward error correction code (FEC) |
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for SCO only |
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2/3 rate forward error correction code FEC |
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Automatic repeat request (ARQ) scheme for data |
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data transmitted in one slot is directly
acknowledged by the recipient in the next slot. |
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Authentication and Privacy |
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one-way, two-way, or no authentication possible |
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use stream cipher based on secret keys (0, 40,
64 bits) |
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key management left to higher layer software |
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if stronger protection (longer key is needed),
use better encryption at network and/or application level |
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Multiple overlapping piconets (sets of
communicating devices) with own hopping sequence, max of one master and 8
slaves |
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Collisions do occur when two piconets use same
frequency at the same time |
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as more piconets overlap, performance degrades |
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degradation gradual: 10 overlapping piconets
reduce aggregate bandwidth by 10% |
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Single device can participate in multiple
piconets, though only one at a time |
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need to re-adjust clock to re-sync with master
when entering a piconet |
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inform master when device leaves piconet, will
suppress data being sent/device being polled |
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ETSI standard |
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European standard, cf. GSM, DECT, ... |
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Enhancement of local Networks and interworking
with fixed networks |
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integration of time-sensitive services from the
early beginning |
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HIPERLAN family |
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one standard cannot satisfy all requirements |
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range, bandwidth, QoS support |
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commercial constraints |
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HIPERLAN 1 standardized since 1996 |
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Data transmission |
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point-to-point, point-to-multipoint,
connectionless |
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23.5 Mbit/s, 1 W power, 2383 byte max. packet
size |
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Services |
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asynchronous and time-bounded services with
hierarchical priorities |
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compatible with ISO MAC |
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Topology |
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infrastructure or ad-hoc networks |
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transmission range can be larger then coverage
of a single node („forwarding“
integrated in mobile terminals) |
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Further mechanisms |
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power saving, encryption, checksums |
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CAC service |
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definition of communication services over a
shared medium |
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specification of access priorities |
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abstraction of media characteristics |
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MAC protocol |
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MAC service, compatible with ISO MAC and ISO MAC
bridges |
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uses HIPERLAN CAC |
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CAC protocol |
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provides a CAC service, uses the PHY layer,
specifies hierarchical access mechanisms for one or several channels |
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Physical protocol |
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send and receive mechanisms, synchronization,
FEC, modulation, signal strength |
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Scope |
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modulation, demodulation, bit and frame
synchronization |
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forward error correction mechanisms |
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measurements of signal strength |
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channel sensing |
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Channels |
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3 mandatory and 2 optional channels (with their
carrier frequencies) |
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mandatory |
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channel 0: 5.1764680 GHz |
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channel 1: 5.1999974 GHz |
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channel 2: 5.2235268 GHz |
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optional (not allowed in all countries) |
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channel 3: 5.2470562 GHz |
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channel 4: 5.2705856 GHz |
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Maintaining a high data-rate (23.5 Mbit/s) is
power consuming - problematic for mobile terminals |
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packet header with low bit-rate comprising
receiver information |
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only receiver(s) address by a packet continue
receiving |
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Frame structure |
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LBR (Low Bit-Rate) header with 1.4 Mbit/s |
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450 bit synchronization |
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minimum 1, maximum 47 frames with 496 bit each |
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for higher velocities of the mobile terminal
(> 1.4 m/s) the maximum number of frames has to be reduced |
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Modulation |
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GMSK for high bit-rate, FSK for LBR header |
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Channel Access Control (CAC) |
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assure that terminal does not access forbidden
channels |
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priority scheme, access with EY-NPMA |
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Priorities |
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5 priority levels for QoS support |
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QoS is mapped onto a priority level with the
help of the packet lifetime (set by an application) |
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if packet lifetime = 0 it makes no sense to
forward the packet to the receiver any longer |
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standard start value 500ms, maximum 16000ms |
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if a terminal cannot send the packet due to its
current priority, waiting time is permanently subtracted from lifetime |
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based on packet lifetime, waiting time in a
sender and number of hops to the receiver, the packet is assigned to one
out of five priorities |
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the priority of waiting packets, therefore,
rises automatically |
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EY-NPMA (Elimination Yield Non-preemptive
Priority Multiple Access) |
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3 phases: priority resolution, contention
resolution, transmission |
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finding the highest priority |
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every priority corresponds to a time-slot to
send in the first phase, the higher the priority the earlier the time-slot
to send |
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higher priorities can not be preempted |
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if an earlier time-slot for a higher priority
remains empty, stations with the next lower priority might send |
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after this first phase the highest current
priority has been determined |
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Several terminals can now have the same priority
and wish to send |
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contention phase |
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Elimination Burst: all remaining terminals send
a burst to eliminate contenders (11111010100010011100000110010110, high
bit- rate) |
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Elimination Survival Verification: contenders
now sense the channel, if the channel is free they can continue, otherwise
they have been eliminated |
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Yield Listening: contenders again listen in
slots with a nonzero probability, if the terminal senses its slot idle it
is free to transmit at the end of the contention phase |
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the important part is now to set the parameters
for burst duration and channel sensing (slot-based, exponentially
distributed) |
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data transmission |
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the winner can now send its data (however, a
small chance of collision remains) |
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if the channel was idle for a longer time (min.
for a duration of 1700 bit) a terminal can send at once without using
EY-NPMA |
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synchronization using the last data transmission |
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Compatible to ISO MAC |
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Supports time-bounded services via a priority
scheme |
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Packet forwarding |
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support of directed (point-to-point) forwarding
and broadcast forwarding (if no path information is available) |
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support of QoS while forwarding |
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Encryption mechanisms |
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mechanisms integrated, but without key
management |
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Power conservation mechanisms |
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mobile terminals can agree upon awake patterns
(e.g., periodic wake-ups to receive
data) |
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additionally, some nodes in the networks must be
able to buffer data for sleeping terminals and to forward them at the right
time (so called stores) |
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Route Information Base (RIB) - how to reach a
destination |
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[destination, next hop, distance] |
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Neighbor Information Base (NIB) - status of
direct neighbors |
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[neighbor, status] |
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Hello Information Base (HIB) - status of
destination (via next hop) |
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[destination, status, next hop] |
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Alias Information Base (AIB) - address of nodes
outside the net |
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[original MSAP address, alias MSAP address] |
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Source Multipoint Relay Information Base (SMRIB)
- current MP status |
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[local multipoint forwarder, multipoint relay
set] |
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Topology Information Base (TIB) - current
HIPERLAN topology |
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[destination, forwarder, sequence] |
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Duplicate Detection Information Base (DDIB) -
remove duplicates |
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[source, sequence] |
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