Chapter 2

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Chapter 2 Topics in Data Communications

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Data Communications ConceptsIntroduction

• Essential definitions for Data Communications
• Data, Signaling, Transmission Systems
• Analog Digital
• Data are entities that convey meaning, while
  signaling is the transfer of encoded data thru a
  transmission system
• Analog versus digital signaling
• Digital signaling usually less expensive than
  analog but care must be taken to properly
  engineer system (e.g. - attenuation)
• Combinations of analog digital data and signals
• Analog data -gt Analog signals
• Digital data -gt Analog signals (Key equipment is
  a modem)
• Analog data -gt Digital signals (Key equipment is
  a codec)
• Digital data -gt Digital signals

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Data Communications ConceptsAnalog versus
Digital Transmission Systems

• Analog systems transmit analog signals without
  regard for the content of the signal
• Amplifiers are used to boost the energy of the
  signal
• Amplifiers also boost the strength of any noise
  on the line, introducing the possibility that the
  signal could be lost
• Digital Transmission Systems are concerned with
  the content of the signal
• Repeaters used to regenerate the signal,
  overcoming attenuation
• Repeaters output a new copy cleansed of any
  noise, so noise is not cumulative (however, bit
  errors can still occur if the signal is not
  regenerated before it degrades too much)

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Data Encoding TechniquesIntroduction

• Encoding is the process of mapping digital data
  into the appropriate signal elements for
  transmission
• Encoding may be very complex or as simple as
  using binary signal elements (0s and 1s)
• Encoding schemes are chosen to assist the
  receiver in its two key tasks
• Determining when the signal element begins and
  ends (so sampling is done at the proper time)
• Determining the value of the signal element (Is
  it a one? A zero?)
• Attenuation, data rate, noise all play a role
  at receiver
• With analog data the encoding scheme also plays a
  key role in system performance but the details
  are a little different

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Data Encoding TechniquesAnalog encoding of
digital data

• The basis for analog encoding is a base signal
  called the carrier signal
• Digital data is encoded (and decoded at the other
  end) by a device called a modem
• Three basic schemes for analog encoding of
  digital data
• Amplitude Shift-keying (ASK)
• Frequency Shift-keying (FSK)
• Phase Shift-keying (PSK)
• These schemes can be combined for more
  sophisticated digital transmission systems that
  carry more data per signal element

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Data Encoding TechniquesAnalog encoding of
digital data

• Amplitude-shift Keying
• Data represented by different amplitude levels of
  the carrier signal
• Simplest scheme, but inefficient and prone to
  noise
• Most valuable use is in optical systems
• Frequency-shift Keying
• Data represented by different frequency values
  near the carrier signal frequency
• Less prone to errors but requires more complex
  circuitry
• Phase-shift Keying
• Data represented by different phase shifts to the
  carrier frequency
• More efficient and noise resistant than ASK or
  FSK but requires more complex circuitry

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Data Encoding TechniquesDigital Encoding of
Digital Data

• The most common way to encode digital data is to
  use a binary signaling scheme consisting of two
  voltage levels
• NRZ-L (Non-Return to Zero Level)
• Each voltage level defines the value of the
  digital data
• Used only in very short connections
• NRZ-I (Non-Return to Zero Inverted)
• A transition at the beginning of a signal unit
  denotes a binary one
• This type of signaling is known as differential
  signaling it is usually easier to detect a
  transition out of the background noise and the
  signals are polarity insensitive
• Clocking and DC current are usually problems

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Data Encoding TechniquesDigital Encoding of
Digital Data

• Manchester Encoding
• Example of a bi-phase coding up to two signaling
  transitions per signal element (needs more
  bandwidth to transmit a given data rate)
• The mid-signal transition provides clocking as
  well as the data value (a zero data element is a
  high-to-low transition and a one is a low-to-high
  transition)
• Used in Ethernet LANs (IEEE 802.3)
• Differential Manchester Encoding
• Another bi-phase code
• The mid-signal transition provides clocking the
  transition at the beginning of the signal element
  represents data (a zero data element has no
  transition at the beginning of a bit time while a
  one does)
• Used in Token Ring LANs (IEEE 802.5)

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Data Encoding TechniquesDigital Encoding of
Analog Data

• Pulse Code Modulation (PCM) is an example used
  in the phone system to transmit analog data
  across digital networks
• Sampling rate based on the Nyquist theorem
• Digitized into 8 bit samples based on a nonlinear
  scale that provides good reproduction of the
  human voice
• Other digital-to-analog encoding schemes
• Adaptive Differential Pulse Code Modulation
  (ADPCM) - used with voice transmission
• Delta Modulation - used rarely but also for voice
  transmission systems
• Code Excited Linear Prediction (CELP) - used in
  very low-bandwidth voice and multimedia
  communication systems

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Multiplexing Introduction

• Allows a transmission system to carry multiple
  independent signals simultaneously for higher
  efficiency
• Two general schemes are in use FDM and TDM
• Frequency Division Multiplexing (FDM)
• Takes advantage of the fact that the useful
  bandwith of the transmission system exceeds the
  required bandwidth of a given signal
• Allows frequency spectrum to be divided
  allocated to different signal sources
• Most commonly used with analog signaling and
  transmission
• Time Division Multiplexing (TDM)

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MultiplexingTechniques

• Allows a transmission system to carry multiple
  independent signals simultaneously for higher
  efficiency
• Two general schemes are in use FDM and TDM
• Time Division Multiplexing (TDM)
• Takes advantage of the fact that the maximum bit
  rate of the system exceeds the required bit rate
  of the digital signal
• Each source is allocated a time slot in the
  multiplexer
• Analog signals can be time division multiplexed,
  but it is very uncommon
• Two varieties of TDM statistical and fixed
  time-slot
• Both FDM and TDM can be used in a synchronous or
  asynchronous manner

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Transmission Media Introduction

• The transmission media is the physical signal
  path between the transmitter and the receiver
• Can be guided (cables, waveguides, etc.) or
  unguided (open air)
• Our key concerns for transmission systems are
  data rate and distance
• Influencing factors
• Bandwidth of the media
• Transmission impairments
• Interference
• Number of Receivers

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Transmission Media (2)Twisted Pair Cable

• Consists of a minimum of two copper wires twisted
  together and enclosed within a protective sheath
• Advantages inexpensive, easy to work with, may
  already be installed where needed
• Disadvantages limited in distance, data rate,
  and bandwidth susceptible to interference
• Comes in two general varieties shielded twisted
  pair (STP) and unshielded twisted pair (UTP)
• Shielding provides more noise immunity,
  especially at lower data rates
• STP costs more and is more difficult to work with

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Transmission Media (3)Twisted Pair Cable

• Category 3 and Category 5 UTP
• Rating standards devised by the Electronic
  Industries Association (EIA)
• The higher the category the better the cable Cat
  3 designed to support 10Mbps Ethernet while Cat 5
  will support 100Mbps Ethernet
• The key difference between the two categories is
  the number of twists per unit length of cable
• Near-end Crosstalk (NEXT) is a key transmission
  impairment to minimize in any twisted pair
  cabling system
• While these are regarded as the most commonly
  found UTP installations, there are higher
  performance UTP choices

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Transmission Media Twisted Pair Cable

• High-performance Twisted Pair
• Category 5e (or enhanced Category 5) supports
  125-MHz bandwidth on all four pairs, allowing
  Gigabit Ethernet to run over UTP up to 100 meters
• Attenuation (db/100m)12.3 for 100 MHz,
  12.3-10Log(Vin/Vout) gives Log(Vin/Vout)-1.23
  or Vin/Vout is about 1/10 (tenth of signal
  magnitude exits from the UTP.
• Crosstalk32db-10Log(Vneighbor/Vsignal), about
  1/1000 crosstalk.
• Cat 5 UTP provides 100 Mbps over 100m.
• Category 6 supports over 200-MHz bandwidth on
  all four pairs could potentially run high data
  rate ATM connections
• Category 7 will require special shielding and
  will likely support up to 700-MHz bandwidth on
  each pair

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Transmission Media Coaxial Cable

• Provides a two conductor transmission system
  where one conductor is situated inside the outer
  hollow conductor with an insulating dielectric in
  between
• Because of its structural characteristics coaxial
  cable is more resistant to noise than twisted
  pair
• Harder to work with and more expensive than
  twisted pair
• Coax systems can be grouped in three categories
  based on the type of signaling used
• Baseband digital signaling occupies the entire
  spectrum of the cable
• Broadband carrier-band analog signaling is
  used, allowing multiple channels on the cable
• Carrierband carrier-band analog signaling with
  low-end components signal occupies entire
  spectrum of cable
• Coaxial cable provides 100 Mbps over 1Km.

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Transmission MediaFiber Optic Cabling

• A transmission system composed of a guided medium
  that allows the propagation of optical rays
• A range of fiber optic cabling exists for various
  needs, from ultra-pure fused silica (expensive
  but high data rate) to plastic (cheap with lower
  data rate for short runs)
• Advantages
• Huge bandwidth capacity
• Smaller size and lightweight
• Lower attenuation
• Electromagnetic isolation (high security
  minimal interference)
• 100 Gbps over 10 Km (multimode fiber)
• Common transmitters used are LEDs for (low-cost
  low-speed systems) or Injection Laser Diodes
  (long-haul high-speed systems)

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Transmission MediaFiber Optic Cabling

• Basic fiber types
• Step-index multimode cheapest to manufacture
  but allows light to travel different paths down
  the fiber, causing signal distortion lowering
  the maximum data rate.
• Graded-index multimode Higher grade of fiber
  with a varying refractive index that limits
  distortion of the signal.
• Singlemode contains a core with a diameter
  close to the wavelength to be transmitted allows
  only a single transmission path down the fiber
  which practically eliminates distortion
• Three wavelength windows provide the best light
  propagation 850, 1300, 1550 nm
• Most multimode systems use the 850 nm window
• Long-haul transmission systems use the 1550 nm
  window because loss is lower at higher wavelengths

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Transmission Media Unguided Media

• Microwave
• Occupies the frequency spectrum from 1GHz to
  30GHz can provide either a highly directional or
  omni-directional system
• There are 3 main challenges to using microwave
  for data transmission
• Frequency Allocation and licensing
• Interference
• Security
• Infrared
• Uses light in the infrared spectrum for data
  transmission
• Must be used line-of-sight or in an environment
  that allows infrared waves to be reflected
• Less issues associated with microwave but only
  for specialized uses

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Data Communication NetworksIntroduction

• For most WAN and MANs, transmission of data
  usually involves a number of intermediate
  switching nodes that move the data between source
  and destination
• The complete set of end nodes, data links,
  intermediate switches is known as a
  communications network
• There is a spectrum of communication switching
  techniques the two main variations are circuit
  and packet switching

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Data Communication NetworksCircuit Switching

• Communication between end nodes is via a
  dedicated communications channel
• Communications via circuit switching involves
  three phases
• Circuit establishment the path is established
  before any data is transferred. The path is
  digital or analog and may include internal links
  operated using TDM or FDM.
• Data transfer
• Circuit disconnect release of resource
  dedicated to the connection
• The fixed capacity of the channel is allocated
  for the duration of the connection can be very
  inefficient with bursty traffic (repeatedly ON
  during T and OFF later)
• Circuit switching is best suited for synchronous
  data such as voice or real-time video

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Data Communication NetworksPacket Switching

• Packet switching breaks data up into a series of
  packets, each appended with enough control
  information to ensure the packet transits the
  network successfully from source to destination
• Developed to address problems certain data
  sources have with circuit switching
• Bursty data transmission
• Source and destination must operate at the same
  data rate
• Inefficient resource allocation
• Connection setup can be too slow for certain
  applications
• (set up Virtual Circuit along logical connection
  path links, send packets without routing decision
  over VC)
• In addition to addressing the above problems,
  packet switching also has other benefits for data
  transmission
• Under heavy load the network will accept packets
  but delay increases
• Priorities for transmission of the packets can be
  set
• Data rate conversion along links with short store
  and forward

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Data Communication NetworksPacket Switching

• Two main varieties datagrams or virtual
  circuits
• Datagram Approach Each packet is routed
  independently of all others, leading to the
  following consequences
• Packets dont take the same routes, may arrive
  out of sequence
• Routing based on neighboring info on traffic and
  failure
• Possible packet discard (overflow in queues) and
  no control-flow
• No circuit setup time, so data flow begins
  without delay
• Data can easily flow around problems in the
  network
• Virtual Circuit Approach a preplanned route
  through the network is established before any
  data is sent
• Requires logical circuit setup and teardown but
  routes along the connection are shared
  (identical) with other packets
• A routing decision does not have to be made for
  every packet
• May provide enhanced services such as error
  flow control, and packet sequencing not available
  in a datagram environment

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• Problem
• M bit to be transmitted using UDP and Virtual
  Circuit (VC). t(UDP) and t(VC) is routing
  overhead for UDP and VC for each link. There are
  K links on the path. The largest packet has only
  N bits and data rate is R.
• Under what condition UDP and VC have the same
  transmission time.
• Using UDP T(UDP) kM/N(1/Rt(UDP))
• Using VC T(UDP) k(t(VC) M/NR)
• The same transmission time when kMt(UDP)/N
  kt(VC).
• However, UDP produces shorter time when M/N is
  small.

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Data Communication NetworksHybrids

• Multi-rate Circuit Switching (to improve circuit
  switching)
• Extends circuit switching to allow one or more
  fundamental channels to be bundled together to
  provide a range of data connection rates
• Examples of multi-rate switching are ISDN (2x64
  Kbps and 1x16Kbps channels) and inverse
  multiplexing
• Frame Relay (FR)
• Packet switching was operating under high error
  rate and overhead added to enhance redundancy and
  reduce errors which produced the FR scheme (from
  64 Kbps to 2 Mbps by removing overhead).
• WAN service based on a connection-oriented packet
  data protocol
• Frame Relay evolved from X.25 the new protocol
  was streamlined by eliminating features necessary
  on earlier, less reliable X.25 data
  communications networks
• Cell Relay (ATM)
• A further evolution of connection-oriented packet
  data services
• Unlike frame relay fixed length data units
  (cells) are used which allow high-speed hardware
  based switching
• Connection oriented, fixed cell size, fast
  switching devices.

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Comparison

• Packet Switching vs. Circuit Switching
• Is packet switching a clear winner?
• Great for bursty data
• Resource sharing
• No call setup
• Excessive congestion packet delay and loss
• Protocols needed for reliable data transfer,
  congestion control
• Q How to provide circuit-like behavior?
• Bandwidth guarantees needed for audio/video apps
• Still an unsolved problem

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Routing

• Routing in Packet-Switched Networks
• Goal move packets among routers from source to
  destination
• Well study several path selection algorithms
  (chapter 5)
• Datagram network
• Destination address determines next hop
• Routes may change during session
• Analogy postal service
• Virtual circuit network
• Each packet carries tag (virtual circuit ID),
  tag determines next hop
• Fixed path determined at call setup time, remains
  fixed through call
• Routers maintain per-call state

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Taxonomy
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Network Access

• End hosts are connected to edge routers through
  access networks
• Types of access networks
• Residential access
• Company access
• Mobile access
• Types of physical media technologies for access
  networks
• Fiber
• Coaxial cable
• Twisted-pair telephone wire
• Radio spectrum

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Access Network

• Access Network Residential Access
• Connects home end systems to the network edge
• Typically, through an ISP
• End hosts are PCs
• AKA last mile
• Means of residential access dialup, DSL, Cable,
  etc.
• Dial-up modem
• Uses POTS line ? twisted pair copper wire
• Calls ISPs number
• Max. data rate 56 Kbps
• Phone line is tied up when connected to ISP
• Digital Subscriber Line (DSL)
• Does not tie up the phone line
• Uses existing twisted-pair line
• Asymmetric upstream and downstream data rates
• Downstream 384 Kb/s1.5 Mb/s
• Upstream 128256 Kb/s
• Hybrid Fiber Coaxial (HFC) Cable

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LAN access

• LAN access
• Company/university local area network (LAN)
  connects end system to edge router
• Ethernet
• Shared or dedicated cable connects end system and
  router
• 10 Mbs, 100Mbps, Gigabit Ethernet
• Deployment institutions, home LANs soon
• LANs Link layer (chapter 5)
• Wireless Access Networks
• Shared wireless access network connects mobile
  end system to router at a base station
• Laptops, PDAs, etc.
• Wireless LANs
• Radio spectrum replaces wire
• Wireless LANs are based on IEEE 802.11 b standard
  (11 Mbps)
• Wider-area wireless access
• CDPD (Cellular Digital Packet Data) wireless
  access to ISP router via cellular network
• Third Generation (3G) wireless packet-switched
  wide-area Internet access at 384 Kbps

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Example 1

• How long will it take to send a file of 640,000
  bits from host A to host B over a
  circuit-switched network.
• Suppose all links in the network are TDM with
• 24 slots and
• have a bit rate of 1.536Mbps
• It takes 500 msec to establish an end-to-end
  circuit before host A begins transmitting to B
• How long will it take to send file?

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Example 1

• How long will it take to send a file of 640,000
  bits from host A to host B over a
  circuit-switched network.
• Suppose all links in the network are TDM with
• 24 slots and
• have a bit rate of 1.536Mbps
• It takes 500 msec to establish an end-to-end
  circuit before host A begins transmitting to B
• How long will it take to send file?

• Transmission rate for each circuit 1.536 Mbps /
  24 64 Kbps
• Time to send 640 Kbits file (640000 bits)/(64
  Kbits/sec) 10 seconds
• Including circuit setup overhead, time to send
  file is 10.5 seconds
• This calculation is independent of the of
  end-to-end links and does not include propagation
  delays

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Example 2

• Packet Switching
• Two forwarding mechanisms
• No segmentation ? message switching
• With segmentation?pipelining
• Example 7.5 million bits message sent over 3
  links, each of 1.5 Mbps
• Time required without segmentation
  (7.5/1.5)x315 sec
• Now segment packet into 5000 chunks each of 1500
  bits
• Time for whole packet 5.002 sec
• Pipelining results in reduction of delays as all
  links are being utilized simultaneously

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Delay in Packet-Switched networks

• Transmission delay
• Rlink bandwidth (bps)
• Lpacket length (bits)
• Time to send bits into link L/R
• Propagation delay
• d length of physical link
• s propagation speed in medium (2x108 m/sec)
• Propagation delay d/s

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Example 3

• Packet Switching Calculation of delay
• A packet of L bits
• Q links between source and destination hosts
• Each link has a data rate of R bits/sec
• Assume
• No queuing delays
• No end-to-end propagation delays
• No connection establishment is required
• How long it takes to send this L bit packet from
  source to destination?
• Time to traverse the first link from source host
  L/R seconds
• Q-1 more such links are traversed before reaching
  destination
• Thus, total delay QL/R seconds ? more delay for
  larger packets

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Example LAN Hierarchy

• N stations to be arranged in a LAN hierarchy
• Each ST transfers on the average M bits per
  second, of which qM are to STs that are local to
  its segment and (1-q)M to STs in other segments
• How to arrange the LAN Hierarchy so that traffic
  in one LAN segment does not exceed the segment
  capacity B, where B is 0.8xData.Rate of a LAN
  segment, i.e. a reference to a congestion point.

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Solution

• Suppose there are K segments each consists of N/K
  stations, where k rages from 1 (1 seg) to N (N
  segs).
• In a segment, the locally destined traffic by is
  qMN/K
• In a segment, the exported traffic by is
  (1-q)MN/K
• In a segment, the imported traffic by is
  (1-q)MN/K, because
• there are (K-1) segments each generates (1-q)MN/K
  external traffic,
• Total external traffic for K-1 segments is
  (K-1)(1-q)MN/K destined to (k-1) segments.
• Thus, a segment imports (1-q)MN/K data.
• Total traffic for a segment is Q qMN/K (local)
  (1-q)MN/K (exported) (1-q)MN/K (imported)MN/K
  (1-q)MN/K MN(2-q)/K
• Lets derivate Q w.r.t. K to find its minimum
  value dQ/dK-MN(2-q)/K2 which is minimum for K
  maximum or KN.
• We may incease the number of STs in a segment
  until Q(K)0.8xData.Rate or MN(2-q)/K lt
  0.8xData.Rate or
• MN(2-q)/(0.8xData.Rate) lt K.

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• wireless transmission
• Wireless local area networks (e.g IEEE 802.11)
• 2.4 GHz (microwave band) 1-2 Mbps, 150 m
• Infrared (IR) link 1-10 Mbps, 10 m
• Earth based cellular data
• Basic individual connection, 13 Kbps, 3 km
• Higher rate PCS (e.g. EDGE), 384 Kbps, 3 km
• Satellite links (GHz frequencies)
• geosynchronous, 600-1000 Mbps, continent
• Low earth orbit (LEO) 13 kbps - 400 Mbps, 800
  km
• (e.g., Motorola Iridium, 66 satellite
  constellation)
• services available from carriers
• Integrated Services Digital Network (ISDN) 144
  Kbps
• Asymmetric Digital Subscriber Lines 1.5-8
  Mbps/16-640 Kbps
• Cable modems 0.5-2 Mbps
• T1 (old electronic telephony standard) 1.544 Mbps
• T2 6.312 Mbps
• T3 44.736 Mbps

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