Sistem Komunikasi

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Telekomunikasi = komunikasi yang dipisahkan oleh jarak. Ada 3 komponen utama :

  • Isyarat Komunikasi
  • Kanal Komunikasi
  • Jaringan Komunikasi

Isyarat Komunikasi

Isyarat Komunikasi
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 dapat isyarat suara (manusia) dapat terkirim. Sistem telepon menggunakanLPF 4kHz (Low-pass). Komponen-komponen frekuensi di dalam gelombang isyarat menjadi Lebar Bidang Isyarat.

Bacaan Lainnya

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].

Kanal Komunikasi

Ada 2 kanal komunikasi yang dominan dipakai saat ini

  1. 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]
  2. Sistem radio, dengan gelombang elektromagnetis
VLFVery Low FrequencyLess than 30kHz
LFLow Frequency300kHz
MFMedium Frequency3MHz
HFHigh Frequency30MHz
VHFVery High Frequency300MHz
UHFUltra High Frequency3GHz
SHFSuper High Frequency30GHz
EHFExtra High Frequency300GHz

Kabel Twisted Pair

Kabel Twisted Pair
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].

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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.

Kabel Coaxial

Kabel Coaxial
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.

Fiber Optic

Advantages

  1. Economic : high BW, low transmission loss
  2. Dimension : small size, low weight
  3. Chemical/Physical properties : resist chemical corrosion, high temperature capacity
  4. Reliability : insensitive to electromagnetic
  5. Raw Material : abundant and cheap [sand]

Disadvantages

  1. Optical sources detectors, connector, couplers, and test equipment are relatively complex and expensive at present

Yang paling sederhana : inti fiber [sekitar 50 micron diameter] dengan index bias n1, dilapisi fiber dengan index bias yang lebih kecil n2.

Cahaya berjalan sepanjang inti

Rumus
Rumus

Snell’s Law of Refraction

Critical angle = sin-1 n2/n1

Snell’s Law of Refraction
Snell’s Law of Refraction

Total Internal Reflection

Total Internal Reflection
Total Internal Reflection

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.

Multimode Step Index
Multimode Step Index

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.

Multimode Graded Index
Multimode Graded Index

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

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
  • 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

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

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.

Protocol Layering

Layer1Physical[Please]
Layer2Data link[Do]
Layer3Network[Not]
Layer4Transport[Throw]
Layer5Session[Sausage]
Layer6Presentation[Pizza]
Layer7Application [Away]

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].

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

Satuan Decibels [dB]

Power Gain expressed in dB : G[dB] = 10 log G

Gain G = Po/PiG [dB]
100+20 dB
10 dB
0.1-10 dB

Satuan dBw dan dBm

dBw dan dBm digunakan untuk menyatakan daya absolute dengan referensi 1 Watt (untuk dBw) dan 1milliwatt (untuk dBm).

Contoh, daya P=100W

Dinyatakan dlm dBw = 10 log P/P[ref]
= 10 log 100W/1W
= 20 dBw

Dinyatakan dlm dBm = 10 log P/P[ref]
= 10 log 100W/1mW
= 10 log 105mW/1mW
= 50 dBm

Catatan : dBm = dBw + 30

Akhir Kata

Itulah informasi “Sistem Komunikasi” yang dapat admin infosolution.biz sampaikan. Semoga bermanfaat.

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