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EEC4113 Data Communication

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EEC4113 Data Communication & Multimedia System Chapter 6: Media Access Control of Data Link Sub-Layer by Muhazam Mustapha, October 2011 – PowerPoint PPT presentation

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Title: EEC4113 Data Communication


1
EEC4113 Data Communication Multimedia
System Chapter 6 Media Access Control of Data
Link Sub-Layer by Muhazam Mustapha, October 2011
2
Learning Outcome
  • By the end of this chapter, students are expected
    to understand and able to explain the various
    protocols and technologies in MAC sub-layer

3
Chapter Content
  • MAC Sub-Layer Issues
  • ALOHA Protocols
  • CSMA Protocols
  • Collision-Free Protocols
  • Topology
  • IEEE 802.3 Ethernet
  • IEEE 802.11/15/16 Wireless Ethernet
  • IEEE 802.5 Token Ring

4
Media Access Control Sub-Layer
CO1
5
Media Access Control
  • Media Access Control is a sub-layer of data link
    layer in OSIs 7 layer model
  • Provides access to the shared networking medium
    in LAN or MAN
  • The currently most popular technology that
    provides MAC is the Ethernet technology
  • Others are FDDI (Fiber Distributed Data
    Interface), ARCNET and Token ring

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6
Ethernet
  • A family of frame-based technology defining
    standards for wiring and signaling
  • Standardized in IEEE 802.3 document
  • Combination of twisted wire pair and optical
    fiber
  • Characterized by the used of 8P8C connector

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7
Shared Network Medium
  • In shared environment, packets sent by one sender
    will be received by all nodes, but only the
    packet addressee will process it, the rest will
    discard

packet sent out
sender
recipient
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8
Multiple Access Protocol
  • Since the network medium is shared, there is a
    need to resolve competition between the nodes
  • Two general schemes
  • Static
  • Frequency / Time Division Multiplexing (digital
    communication)
  • Dynamic
  • ALOHA, Carrier Sense Multiple Access (data
    communication)

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9
Channel Allocation Problem
  • In shared medium, a user will first listen to the
    channel for its availability, then sends its
    frame
  • COLLISION occurs when more than one user start
    using the medium at the same time
  • At collision incidence, both user release the
    medium and wait for random time before re-sending

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10
ALOHA Protocols
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11
ALOHA Protocols
  • Created in 1970s in the University of Hawaii by
    Norman Abramson
  • First ingenious method to resolve channel
    allocation problem
  • It was best for wireless communication and the
    concept is still in used by modern protocol like
    Wi-fi

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12
Pure ALOHA
  • Basic ideas
  • Anyone is allowed to transmit their data whenever
    they have something to transmit, without checking
    the channel availability first
  • After sending, the sender will listen to its own
    frequency to tell whether its frame has been
    destroyed due to collision or not
  • This is possible due to feedback property of
    broadcasting channel, or
  • The sender will require an acknowledgement

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13
Pure ALOHA
  • Basic ideas
  • If there is no feedback, then there is collision
  • If collision occurs, the sender will wait for a
    random amount of time, then re-send this called
    backoff

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14
Pure ALOHA
User
A
B
C
D
E
Time
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15
Slotted ALOHA
  • Time is divided into slots, and users can only
    transmit at start of slot
  • Resulting advantage Efficiency is doubled (see
    graph)
  • Disadvantages
  • Requires synchronization clock
  • Still poor at high loads (see graph)

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16
Pure vs Slotted ALOHA
0.40
Slotted ALOHA S Ge-G
S (throughput per frame time)
0.30
0.20
Pure ALOHA S Ge-2G
0.10
0.5
1.0
1.5
2.0
2.5
3.0
G (attempts per packet time)
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17
Carrier Sense Multiple Access Protocols
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18
Carrier Sensing Protocols
  • Network communication can be improved greatly if
    the nodes can sense the existence of any
    transmission signal inside the transmission
    medium
  • Implemented in Carrier Sense Multiple Access
    (CSMA) and a few of its variations
  • Improvement is due to the fact that collisions is
    reduced since hosts will only send data if medium
    is not in use

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19
CSMA
  • A host that needs to transmit data will first
    listen into communication medium and decide
    whether another host is using the medium or not
  • The host will only transmit its data if no one is
    using the medium
  • After finish sending the data frame, there will
    be an interframe gap of 9.6µs idle before any
    host can take the medium

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20
Persistent and Non-persistent CSMA
  • CSMA is called persistent if
  • when sensing that a medium is being used, the
    host waits and will definitely transmit once the
    current transmission ends
  • may cause collision if more than one host was
    waiting
  • And non-persistent if
  • the host waits for a random duration and re-sends
    only if no one using it
  • results in less collision

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21
CSMA/CD (Collision Detection)
  • The system will be having 3 states transmission,
    contention and idle
  • Transmission state is the state where one host
    sends data.
  • After that host finishes, more than one of other
    hosts might be sending at the same time a
    collision

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22
CSMA/CD (Collision Detection)
  • On sensing a collision, all hosts involve would
    release the medium and they send a jamming signal
    to tell others that there is collision happened
  • so that everyone releases the medium
  • Then they will wait for a random duration and
    re-try
  • The above two steps is the contention state

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23
CSMA/CD (Collision Detection)
  • Once one of the competing host gains control the
    system is in transmission state again
  • Idle state is just the state that no one is using
    the medium

collisions
transmission
transmission
transmission
transmission
contention
idle
contention
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24
CSMA/CA (Collision Avoidance)
  • CSMA/CD is a persistence variation of CSMA it
    handles collision when it happens
  • CSMA/CA is a non-persistence variation CSMA
  • CSMA/CA avoids collision by
  • not sending jamming signal
  • instead, just wait for a random duration then
    re-sends if no one is using

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25
Collision-Free Protocol
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26
Collision-Free Protocols
  • In collision free protocols, instead of sensing
    the medium, the hosts will tell if they want to
    transmit
  • There is a special frame called contention frame
    whose content is contributed by all hosts

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27
Collision-Free Protocols
  • Contention frame is slotted and the hosts will
    take turns at a very precise timing to write
    information into the frame
  • A host sets a binary 1 at bit location reserved
    for it in contention frame if it wants to use the
    medium

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28
Collision-Free Protocols
  • Once all hosts write the binary bits according to
    its intention, the actual transmission will be
    granted to the requesting hosts in sequence.
  • Once all transmissions finish, the hosts will
    then re-fill the contention frame
  • This protocol is called basic bit-map protocol

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29
Collision-Free Protocols
8 contention slots
8 contention slots
8 contention slots
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
0
1
2
3
4
5
6
7
1
1
1
1
1
1
1
7
3
5
1
2
frames
frames
frames
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30
Ethernet Topology
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31
Topology
Bus
Linear Bus 2 ends
Distributed Bus more than 2 ends
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32
Topology
Star
Ring
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33
Topology
Mesh
Tree
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34
Bus Topology
  • Use of multipoint medium
  • All stations attach directly to transmission
    medium (bus) through appropriate hardware
    interfacing known as tap

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35
Bus Topology
  • A transmission from any station propagates the
    length of the medium in both directions can be
    received by all other stations
  • At each end of the bus is a terminator, which
    absorbs any signal, removing it from the bus

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36
Tree Topology
  • Use of multipoint medium
  • Transmission medium is a branching cable with no
    closed loops
  • Tree layout begins at a point known as the headend

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37
Tree Topology
  • One or more cables start at the headend, and each
    of these may have branches
  • The branches in turn may have additional branches
    to allow quite complex layouts
  • A transmission from any station propagates
    throughout the medium can be received by all
    other stations

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38
Ring Topology
  • Repeaters joined by point-to-point links in
    closed loop
  • Receive data on one link and retransmit on
    another
  • Links are unidirectional
  • Stations attached to repeaters

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39
Ring Topology
  • Data in frames
  • Circulate past all stations
  • Destination recognizes address and copies frame
  • Frame circulates back to source where it is
    removed
  • Medium access control determines when station can
    insert frame

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40
Star Topology
  • Each station connected directly to central node
  • Usually via two point-to-point links
  • Two alternatives operation of central node
  • Broadcast Physical star, logical bus
  • Frame-switching device Only one station can
    transmit at a time

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41
Star Topology
  • Broadcast
  • A transmission of a frame from one station to the
    central node is retransmitted on all of the
    outgoing links
  • Central node is referred as hub
  • Frame-switching device
  • Incoming frame is buffered in the node
    retransmitted on an outgoing link to the
    destination station

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IEEE 802.3 Standard of Ethernet
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43
IEEE 802.3 Standard
  • Defines Ethernet as CSMA/CD protocol on bus or
    ring topology
  • Also defines the minimum frame length
  • Also defines the cabling hardware
  • Frame format

7
1
6
6
0-46
0-1500
4
Bytes
S O F
Destination address
Source address
Preamble
Length
Data
Pad
Checksum
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44
Frame Fields
  • Preamble 7 bytes of alternating 1-s and 0-s for
    synchronization
  • Start of Frame (SOF) Sequence of 10101011
  • Destination Address 6 bytes of MAC address
  • Source Address 6 bytes of address
  • Length Total size of data and pad

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45
Frame Fields
  • Data Packet from upper layer
  • Pad Series of 0-s to make up a minimum total
    size of 46 bytes of data and pad so that the
    min frame size is 64 bits
  • Checksum 32 bit CRC

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46
MAC Address
  • Identifying each individual network card uniquely
  • 46 bits address in 48 bits string
  • Binary 0 in MSB indicates ordinary address
  • Binary 1 in MSB indicates the 46 bits address is
    a group address (for multicast)

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47
MAC Address
  • If all address bit are 1-s then it is a broadcast
    (all nodes are getting the message)
  • If two MSB are 0-s then the 46 bits address is a
    combination of source and destination MAC address

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48
MAC Address
  • Examples of possible MAC addresses include
  • 00-0C-F1-56-98-AD
  • 00-11-F5-4B-20-56
  • The first three bytes of this address identify
    the manufacture of this network device
  • 00-0C-F1 for Intel
  • Assigned by the IEEE and the database is
    available online at IEEE OUI and Company_id
    Assignments website

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49
Need for Frame Minimum Size
  • In CSMA/CD, if there is a collision, the first
    node to detect it will send a jamming signal
  • We need to calculate the maximum delay after a
    node sends a message until the first jamming
    signal is heard by all nodes
  • Then from there we can calculate what is the
    minimum frame size so that NO nodes will finish
    transmitting before it hears the jamming signal

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50
Need for Frame Minimum Size
  • The diagram below shows that there is a maximum
    of 2td delay before the first jamming signal is
    heard by every node (td propagation delay)

A
B
A
B
Frame sent at t 0s
At t td s, the frame almost reach the receiver
collision
A
B
A
B
At t td s, suddenly the receiver sends out frame
Jamming signal finishes propagating at t 2td s
Jamming signal sent out
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51
Need for Frame Minimum Size
  • (compare this calculation to link utilization
    calculation)
  • Hence max delay is a function of bit rate, max
    distance allowed and velocity of propagation
  • Given
  • Ethernet bit rate 10 Mbps (802.3 Standard for
    10Base5 and 10Base-T)
  • Max distance 500m (802.3 Standard)
  • Velocity of propagation 2 108 ms-1

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52
Need for Frame Minimum Size
  • (compare this calculation to link utilization
    calculation)
  • Hence
  • td 2.5µs, hence 2td 5µs
  • Bit duration 0.1µs
  • No. bits traveling in 2td time 50
  • Adding some gap for error, the best min frame
    size chosen is 64 bits
  • 802.3 Std sets 512 bits as min, because it allows
    max distance of 2.5km with 4 passive repeaters

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Ethernet Physical Standard
  • 10Base5 10 Mbps, Baseband transmission, 500m
    cable length
  • 10Base2 10 Mbps, Baseband transmission, 200m
    cable length
  • 10Base-T 10 Mbps, Baseband transmission, 500m
    UTP cable
  • 100Base-TX 100 Mbps, Baseband transmission, 200m
    UTP cable

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54
Ethernet Physical Standard
  • Wiring
  • Unshielded Twisted Pair (UTP)
  • Bundle of eight wires (only uses four)
  • Terminates in RJ-45 connector

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55
Ethernet Physical Standard
  • Hubs (10Base-T)
  • A kind of passive repeater
  • Used to connects nodes in bus topology
  • Max length of UTP 100m
  • Max no. hubs in series 4
  • Hence, max distance between farthest nodes 500m

100m
10Base-T hubs
100m
100m
500m, 4 hubs
100m
100m
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Repeaters
  • Regenerates signal
  • Used to extend the network coverage
  • Hubs are repeaters
  • There will be a limit to the length of the
    farthest node due to physical signal limitation

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Bridges
  • Used to join LANs
  • Results in local internet
  • May filter the data traffic

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58
Switches
  • More intelligent kind of bridges
  • Must be arranged in hierarchical arrangement
    only one path from one switch to another
  • Due to its intelligent close to a small node,
    there is no limit in number of switches in a LAN
    as opposed to hubs

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Hubs vs Bridges vs Switches
  • Hub
  • Has many ports
  • Redistributes data to all nodes
  • It depends on the receiver to process the data
  • Almost no intelligence
  • Used to extend connection within standard limit

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Hubs vs Bridges vs Switches
  • Bridge
  • Only two ports
  • Transfers data from one end to the other only if
    the receiver address is at the other end
  • Have intelligence to interpret MAC addresses
  • Used to join two separate LANs

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Hubs vs Bridges vs Switches
  • Switch
  • More than two ports
  • Have intelligence to interpret MAC addresses
  • Transfers data from one end to another only if
    the receiver address is at that end
  • Extends LAN unlimitedly, but must conform to
    hierarchical (tree) structure
  • Router
  • Switch that works on IP address instead of MAC
  • For internet instead of LAN
  • Smart enough to do protocol conversion

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Hubs vs Bridges vs Switches
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63
IEEE 802.11/15/16 Standards of Wireless Ethernet
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IEEE 802.11
  • Uses CSMA/CA instead of CSMA/CD
  • Could not detect collision due to hidden nodes
    (target nodes beyond signal range)
  • Sender listen to the medium (air) to see whether
    it is busy or not
  • After the medium is free for a period of DIFS
    (Distributed Inter-Frame Space 128µs), the
    sender sends RTS (request to send) signal to tell
    its intention, and others will make way

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IEEE 802.11a
  • Frequency 5 GHz
  • Maximum Speed 54 Mbps
  • Range about 35 meters (varies)
  • Encoding Scheme Orthogonal FDM (closely located
    frequencies but far enough not to interfere each
    other)

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IEEE 802.11b
  • Frequency 2.4 GHz
  • Maximum Speed 11 Mbps
  • Range about 38 meters (varies)
  • Encoding Scheme DSSS
  • Modulation Technique BPSK(1 Mbps), QPSK(2
    Mbps), CCK(5.5 Mbps,11Mbps)

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IEEE 802.11g
  • Frequency 2.4 GHz
  • Maximum Speed 54 Mbps
  • Range about 38 meters (varies)
  • Encoding Scheme OFDM

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IEEE 802.11n
  • Frequency 5 GHz, 2.4 GHz
  • Modulation OFDM
  • Maximum Speed 150 Mbps
  • Range about 70 meters (varies)
  • Encoding Scheme OFDM
  • Addition of MIMO (Multiple Input Multiple Output)
  • sender and receiver have 2 antennas to send and
    receive 2 signals (one is modified redundancy) to
    improve performance

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IEEE 802.16 WiMAX
  • WiMAX is 802.11/Wi-Fi networks with coverage and
    cellular networks quality of service
  • Stands for "Worldwide Interoperability for
    Microwave Access"
  • Most of WiMAX physical layer definitions and
    topology follows those of 802.11
  • Provider in Malaysia P1 Yes

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IEEE 802.16 WiMAX
  • Consists of two standards Fixed Mobile
  • Fixed WiMAX (IEEE 802.16d)
  • Speed up to 70 Mbps
  • Range up to 50 km
  • Mobile WiMAX (IEEE 802.16e)
  • Speed up to 30 Mbps
  • Range up to 15 km

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IEEE 802.15 Bluetooth
  • Open proprietary standard created by Ericsson
  • Not a direct descendent if 802.11
  • Designed for communication between electronics
    devices as alternative to cabled RS-232
  • Consisting of 1 master devices and up to 8 slaves
  • Logo

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IEEE 802.15 Bluetooth
  • Frequency 2.4-2.8 GHz
  • Speed 1 Mbps
  • Range 10 meters
  • Encoding Scheme FHSS with 79 channels at 1600
    hops per second
  • Most common uses Mobile phone headset, wireless
    mouse keyboard

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Token Based Protocol
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IEEE 802.5 Token Ring
  • The network is arranged in ring topology
  • There is a special frame to be passed around the
    nodes named TOKEN
  • Whoever is having the token can transmit data
    into transmission medium, otherwise it passes the
    token to the next node

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IEEE 802.5 Token Ring
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IEEE 802.4 Token Bus
  • The network is arranged in bus topology
  • Just as token ring, there is a special frame
    TOKEN used
  • Whoever is having the token can transmit data
    into transmission medium, otherwise it passes the
    token to the next node
  • The use of this type of protocol is shown by the
    presence of coaxial cable connector on the
    network card instead of 8P8C

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FDDI
  • Fiber Distributed Data Interface
  • Data rate 100Mbps
  • Used as a backbone
  • With multi-mode fiber any given ring segment can
    be up to 200 km in length
  • A total of 500 stations can be connected with a
    maximum separation of 2 km
  • Two complete rings to overcome failures

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FDDI
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FDDI Interface in High Speed LANs
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