Sistem Komunikasi

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Sistem Komunikasi

• oleh
• Warsun Najib

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Pengantar

• Telekomunikasi komunikasi yang dipisahkan oleh
  jarak
• Ada 3 komponen utama
• Isyarat Komunikasi
• Kanal Komunikasi
• Jaringan Komunikasi

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Isyarat Komunikasi
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Isyarat Komunikasi

• Sistem Komunikasi harus menyediakan transmisi
  yang bagus dengan frekuensi2 di mana spectrum-nya
  signifikan.
• Kanal komunikasi harus bisa melewatkan isyarat
  dengan range frekuensi 300 Hz s/d 3.4 kHz agar
  isyarat suara (manusia) dapat terkirim
• Sistem telepon menggunakanLPF 4kHz (Low-pass)
• Komponen-komponen frekuensi di dalam gelombang
  isyarat menjadi Lebar Bidang Isyarat

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Isyarat Komunikasi

• Isyarat Musik mengandung komponen-komponen
  frekuensi signifikan dari 20Hz sampai 20kHz.
• Untuk mengirimkan isyarat musik diperlukan LPF
  dengan Bandwidth 15kHz
• Isyarat data biner, interval terkecil antara 0-1,
  T, dinamakan interval pengisyaratan (signaling
  interval)
• 1/T dinamakan kecepatan isyarat (signaling rate)
  dengan satuan bits per second bps.
• Lebar bidang transmisi pada sistem data biner
  secara proporsional mendekati bit rate High data
  rate Wider BW of channel

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Kanal Komunikasi

• Ada 2 kanal komunikasi yang dominan dipakai saat
  ini
• Sistem gelombang terpandu, menggunakan media
  fisik, seperti kable
• Twisted Pair Cable UTP, STP, 10BaseT
• Coaxial Cable thin-coax, thick-coax
• Optical Fibers OC-3, OC-12, OC-48
• Sistem radio, dengan gelombang elektromagnetis

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Kanal Komunikasi
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Kabel Twisted Pair

• Total pelemahan (total attenuation) karena kabel
  tergantung pada panjangnya
• Pelemahan (isyarat) juga tgt pada frekuensi, akan
  meningkat dengan bertambahnya frekuensi. Untuk
  mempertahankan agar pelemahan isyarat tetap
  rendah, maka diperlukan equalizer LPF Low Pass
  Filter
• Untuk single pair cable telephone cable,
  induktor-induktor dipakai untuk meningkatkan
  karakteristik transmisi untuk mendapatkan
  frekuensi respone yang lebih baik
• Pada multi-pair cable, cross-coupling dapat
  menambah kapasitasi antara 2 konduktor. Hal ini
  menyebabkan suatu isyarat dapat terganggu karena
  ada kabel lain.

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Kabel Coaxial

• Dipakai untuk komunikasi yang memerlukan
  Bandwidth yang lebih besar dari padaTwisted pair
• Pelemahan (attenuation) meningkat kira-kira
  mendekati akar kuadrat frekuensi, sehingga untuk
  jarak yang jauh dibutuhkan penapisan
• Efek kulit (Skin Effect), yaitu banyak isyarat
  dgn frekuensi yang tinggi akan hilang di dekat
  permukaan. Dibutuhkan penambahan ukuran kabel
  untuk BW yang lebih besar
• BW Isyarat bisa mencapai 60MHz 140Mbps digital
  transmission
• Crosstalk sangat berkurang pada kabel coaxial

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Fiber Optis
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Fiber Optis 1

• Yang paling sederhana inti fiber diameter
  sekitar 50 micron dengan index bias n1, dilapisi
  fiber dengan index bias yang lebih kecil n2
• Cahaya berjalan sepanjang inti

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Snells Law of Refraction

• Critical angle sin-1 n2/n1

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Total Internal Reflection
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Multimode Step Index

• The pulse spreading places a limit on the
  transmission rate over a fiber of given length
• Hence, the effective bandwidth of MMS fiber
  depends on length and on the index difference
  n1-n2 and it is normally used for a short haul
  systems

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Multimode Graded Index

• The fiber bandwidth is greatly improved with the
  parabolic refractive index profile which
  gradually reduces from a maximum at the center to
  the core-cladding interface
• The pulse spreading is very much reduced compared
  with a step index fiber because all rays paths
  having almost the same propagation delay

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Single Mode Step Index

• Single mode step index is known as Monomode
• By making a core diameter and index difference
  sufficiently small, there will be only one
  possible mode travel in the fiber.
• Thus, pulse spreading due to mode velocity
  difference cannot exist
• Single mode step index is the fastest mode in
  optical communications
• Due to the complexity of manufacturing, a single
  mode fiber optic cable is therefore the most
  expensive one

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Radio Systems

• An Electromagnetic wave is launched from an
  antenna at the transmitter, propagates through
  the atmosphere an on arrival at the receiver is
  picked up by a receiving antenna
• There are five major of Radio Propagation
• 1. Line of Sight
• 2. Surface wave or ground wave
• 3. Ionospheric propagation
• 4. Tropospheric scattering
• 5. Free space propagation

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Radio Propagation

• Line of Sight
• Omidirectional is used for radio broadcasting
• Directional is used for point-to-point radio
  communication link
• Surface wave or ground wave
• A radio wave travels along the curvature of the
  earth as a result of currents flowing in the
  ground
• Ionospheric propagation
• Radio wave can refracted by the ionosphere and
  returned to the surface some considerable
  distance from the transmitter
• This is because the gradual reduction with height
  of the refractive index in the ionosphere

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Radio Propagation1

• Tropospheric scattering
• Radio wave can be scattered by small particles in
  the lower atmosphere to provide over-the-horizon
  radio communication
• Free space propagation
• Wave can propagate without the aid of a physical
  medium and this is how light from the Sun and
  other stars reaches the Earth

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Communication Network

• The communication cannot takes place in isolation
• Each source and destination needs to have at
  least one connection to make communication happen
• The classic example of communication network are
  Switched Network
• Data Network
• Integrated Services Digital Network

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Switched Network

• Switching center can produce all possible
  connections required but not all at once
• The statistical properties of the communication
  requirements of large user groups are relied upon
  to effect an economy
• The idea of a star connection can be extended to
  form a star of star hierarchical network
• the central switching center must provide a
  sufficient communication capacity to allow
  several message to be transferred simultaneously
• This is achieved by combining several message
  waveforms to form a single composite message and
  it is known as multiplexing TDM,FDM

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Data Network

• A protocol is a well-defined procedure to enable
  terminals/nodes to communicate effectively over a
  network
• Ring topology allows any nodes to communicate to
  any other nodes by simply applying signals to the
  ring
• This is essence of a form of Local Area Network
  LAN used for computer/data communications
• Data circulate around the ring in packets, a
  packet being a sequence of binary digits
• Protocol layering of OSI reference model provide
  the standard for data network

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Protocol Layering (OSI layer)

• Layer1 Physical Please
• Layer2 Data link Do
• Layer3 Network Not
• Layer4 Transport Throw
• Layer5 Session Sausage
• Layer6 Presentation Pizza
• Layer7 Application Away

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ISDN

• Increasingly, both signal transmission and
  switching are implemented using digital data
  technique and the telephone systems are rapidly
  moving to a digital network
• In addition, there is an increasing need for the
  wide range of telecommunication services data
  transmission, E-mail, Video conference, etc.
• In combining a various services and make these
  available on a single network with the continued
  expansion of digital transmission and switching
• It gives rise to form an Integrated Services
  Digital Network ISDN

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Satuan ukuran

• Common Unit of measurement Gain Loss
• Current Gain Ai Io/Ii
• Voltage Gain Av Vo/Vi
• Power Gain G Po/Pi
• Power Loss L Pi/Po
• Note G 1/L or L 1/G

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Satuan Decibels dB

• Power Gain expressed in dB GdB 10 log G

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Satuan dBw dan dBm

• dBw dan dBm digunakan untuk menyatakan daya
  absolute dengan referensi 1 Watt (untuk dBw) dan
  1milliwatt (untuk dBm)
• Contoh, daya P100W
• Dinyatakan dlm dBw 10 log P/Pref
• 10 log 100W/1W
• 20 dBw
• Dinyatakan dlm dBm 10 log P/Pref
• 10 log 100W/1mW
• 10 log 105mW/1mW
• 50 dBm
• Catatan dBm dBw 30