Some Basic Concepts of Fiber Optic Access Network
Multiplexing Concept
Multiplexing is the merging of several information signal channels into one information channel with the aim that the information signals can be sent simultaneously in 1 channel. Several types of multiplexing methods, are as follows:

FDM (Frequency Division Multiplexing)
The technique of merging information signal channels by using different frequency channels. See figure 1. The principle is that n channels with different frequencies are transmitted simultaneously on 1 transmission channel. This technique is used for both analog and digital systems.
fiber optic access network

TDM (Time Division Multiplexing)
The technique of merging information channels by using the same frequency bandwidth, but alternately. See figure 2. TDM is a multiplexing process by dividing time into time slots that represent information from each channel. This technique is only possible for digital signals.

WDM (Wavelength Division Multiplexing)
This technique is similar to FDM, only using the wavelength domain as the variable. WDM is commonly used in fiber optic communication systems. See figure 3.

PCM (Pulse Code Modulation)
PCM (Pulse Code Modulation), which is the process of converting analog signals into digital signals. There are 3 processes, namely: sampling, quantizing, and coding. The type of PCM that is widely used is PCM-30, which functions as an analog to digital converter, multiplexing, and as line coding. Here is the PCM-30 process:

Bandpass Filter

Sampling (Sampling)

The sampling process is the process of taking samples from voice signals with a frequency band width of between 300-3400 Hz; where this process is carried out by an amplitude modulator. The working principle of this sampler is the same as a door / gate or switch, which opens and closes with a certain and continuous period of time; which opens and closes the door / gate or switch is done by a frequency, which is known as the sampling frequency. For this sampling frequency, a French expert named Harry Nyquist has conducted the following experiments. :
a) The Sampling Frequency (Fs) used is smaller than 2
x width of sound frequency (2 x BW Finf):

The Sampling Frequency (Fs) used is = 2 x width
sound frequency (2 x BW Finf):

Pricing Process for PAM signals; whose size is in accordance with the value of the closest comparison voltage
Each pulse will be placed into a positive polarity or negative polarity.

Each polarity is divided into several segments / sub segment (interval). There are 2 kinds of quantization:

Uniform (uniform) (Linear)

Non-uniform (non-uniform) (Non-linear)

Coding is the process of converting the PAM signal into a digital signal (A – D Converter). In PCM-30 the Companding-A Law applies:
a. Each PAM pulse is placed on a positive or negative polarity; and
marked with the letter “S”
• For Positive Polarity S = 1
• For Negative Polarity S = 0
b. Each polarity is divided into 8 segments; segment to -0 to 7, and
marked with the letters “ABC”.

Each segment is divided into 16 sub-segments (intervals); 0 to 15 intervals, and marked with the letter “WXYZ”

So that the PAM signal will turn into a signal with the following bits arrangement:

In connection with this quantization and coding process, it is known that there is a companding law; and in PCM-30 applies Companding Law “A”, which has the following rules. :

  1. Put the signal into 2 polarities; i.e. positive polarity, which is indicated by a single digit “1”; or negative polarity indicated by a single digit “0”.
  2. Each Polarity is divided into 8 segments; marked with three digits “0” and/or “1”, with numbers ranging from “0” to “7”.
  3. Each segment is further divided into 16 subsegments, or intervals; and marked with four digits “0” and/or “1”, with numbers ranging from “0” to “15”.

The function of PCM 30 after A/D Converter is multiplexing:
a. Principle: Time Division Multiplexing
b. Method: “Word-by-Word Interleaving” or “Byte-by-byte Interleaving”; or “Cyclic Word Interleaving” or “Cyclic Byte Interleaving”.
c. Combine:

  • 30 telephone channels 64 kbps,
  • 1 signaling channel 64 kbps
  • 1 channel FAS 64 kbps.
    Into a single line of 2048 Kbps serial signal.
    d. Each channel occupies one “Time Slot” (TS):
  • TS-0 for FAS/Alarm
  • TS-1 to TS-15 for telephone lines 1 to 15
  • TS-16 for Signaling
  • TS-17 to TS-31 for telephone lines 16 to 30.
    And the next function is: line coding, which is the conversion of the unipolar NRZ 2048 Kbps signal into an HDB-3 signal:
    • The digit “1” is encoded to be an alternating positive or negative voltage, the polarity of which is always opposite to the previous digit “1”
    • Digit-0 is encoded as 0 volts.
    • A maximum of 3 consecutive “0” digits.

PCM-30 . Frame Structure

  1. One Multi Frame, with a time length of 1 Multi Frame 2 mS
  2. Sixteen Frames, with a time length of 1 Frame 125 S
  3. 32 TS/Frame, with a time length of 1 TS 3.9 S
  4. 8 Bit/TS, with a time length of 1 bit 488 nS
  5. Number of bits/Frame 256 bit
  6. Number of bits/Multi Frame 4096 bit
  7. FAS bits are 7 bits ( 0011011); bit-2 to 8 TS-0, Even frames (frames- 0, 2, 4, etc.)
  8. MFAS bits are 4 bits, with the arrangement 0000; located on bit-1 to 4 TS-16, Frame-0.
  9. Bit Signaling (4 bits/channel); on bit-1 to 4, and bit-5 to 8 TS-16, Frame-1 to Frame-15
  10. Alarm bit (A1) 2 Mbit/s signal located on bit-3 TS-0, Odd frames (1, 3.5 etc.)
  11. Alarm bit (A2) signal 64 Kbit/s (Signalling) is located on bit-6 TS-16, Frame- 0.

The following figure (15) shows the PCM-30 . Frame Structure

Multiplex PDH; divided into 2 groups, namely:

  1. Low Order (Low Order); often also referred to as First Order,
    or most popularly called “PCM-30”
  2. High Order (High Order); consists of Order-2, Order-3 and Order-4

How PDH works:

  1. HDB-3/Unipolar NRZ converter.
    Function to convert HDB-3 Bipolar signal into Unipolar NRZ Signal (Binary Signal). The signal received from the previous device is the signal with the channel code HDB-3 Bipolar; by HDB-3/Unipolar NRZ Converter circuit, this HDB-3 Bipolar signal is converted into Unipolar NRZ signal, or Binary signal.

Buffer Memory.
Serves to equalize the speed of the Unipolar NRZ signal (Binary Signal), with the speed of other Unipolar NRZ signals. Buffer Memory will store Unipolar NRZ signal output from HDB-3/Unipolar NRZ converter;
and then this Buffer Memory will issue a stored signal based on the read clock coming from the Clock Frequency Generator. In this case the 4 Buffer Memory will receive a read clock that comes from the same source, so that the output from the Buffer Memory will be a signal that is already in sync with one another. See Figure (18).

Functions to combine 4 digital signals that have been synchronized by the Buffer Memory into 1 series of serial signals; for which address is channel 1, 2, 3, and which channel is 4, in row 4 of this serial signal, FAS bits will be added. The Multiplexing process in High Order digital Multiplex runs bit-by-bit interleaving, where every 4 bits of 4 channels will form 1 word (1 TS).
See Figure 19

Unipolar NRZ/Bipolar Converter.
The multiplexed signal is a Unipolar Non Return to Zero (NRZ) signal. This signal must be converted into a bipolar signal before being transmitted:
a. HDB-3, for PDH Order-2 and Order-3
b. CMI, for PDH Order-4
See Figure (20).

  1. Generating Clock Frequency.
    Serves to generate the clock frequency needed for all processes in the sending direction.
  2. Frame Patterns.
    Function generates the Frame Alignment Signal bits, where:
    ! For Order – II and Order – III FAS bits are 10 bits, with the arrangement is 1111010000

For Order – IV the FAS bits are 12 bits, the order is 111110100000.

Serves as the main generator of the clock frequency, which is usually an X-tall Oscillator.

  1. Frame Structure.
    The arrangement of the PDH multiplex frames can be divided into 3 (three), namely:
    1) 8.448 Mbit/s . PDH Multiplex Frame Structure
    2) PDH Multiplex Frame Structure 34.368 Mbit/s
    3) 139.264 Mbit/s . PDH Multiplex Frame Structure


SDH (Synchronous Digital Hierarchy), is a digital multiplex that functions to combine:

Digital signal 2 Mbit/s, 34 Mbit/s, 140 Mbit/s becomes:

  • STM-1 signal (155.52 Mbit/s) or
  • STM-4 signal (622.08 Mbit/s).

The STM-1 signal becomes:

  • STM-4 signal, or
  • STM-16 signal (2.48832 Gbit/s).

The STM-4 signal becomes:

  • STM-16 signal,
  • STM-64 signal (9.95328 Gbit/s)

The PDH and STM-n signals become SDH signals with a level

SDH function
Converts the input bipolar PDH signal on the tributary port to a unipolar NRZ signal.

  1. Placing unipolar NRZ signals in their respective containers:
    a. C-12 for 2048 Kbps signal.
    b. C-3 for 34368 Kbps signal
    c. C-4 for 139264 Kbps signal
  2. Complete the C-12, C-3 and C-4 signals with the bytes:
    a. Over Head (POH), and
    b. pointer
  3. Combining signals that have been equipped with Over Head and Pointer bytes into a series of serial signals.
  4. Change the multiplexed signal to:
    a. Bipolar CMI Signal, for STM-1 transmitted via SDH Digital Microwave Radio, or via higher SDH levels.
    b. Signal with optical power for STM-1 transmitted via optical cable.

How SDH Works:

  1. Mapping Process
    a. Mapping PDH Signals Into Container (C).
    Because the capacity of the container is made larger than the capacity of the PDH signals, the mapping of PDH signals into the container is always done by adding the required bits, to equalize the capacity of PDH signals with the capacity of the container (figure 22).

Mapping Container Signals Into Virtual Containers (VC). Mapping container (C) signals into a Virtual Container (VC) is done by adding Path Over Head (POH) bits to the C signal. See Figure 23.

This POH works for:

  • Sending error checking bits
  • Sends signal indication, normal or fault
  • Sending signal labels
  1. Aligning Process.
    a. Aligning VC Into Tributary Units (TU).
    The process of aligning the virtual container (VC) signals into the Tribuatry Unit (TU) is done by adding Pointer bits (bytes) to the VC signal. This process applies to both VC-12 and VC-3. See Figure (24) below.

Pointers are used for:

  • Indicates the start of a VC
  • Equalize VC bit rate with TU bit rate
  • Indicates the condition of the signal sent/received
    b. Aligning VC Into Administrative Unit (AU)
    The process of aligning the virtual container (VC) signal into the Administrative Unit (AU) is carried out by adding Pointer bits (bytes) to the VC signal. This process applies to VC-4. See Figure 25 below.

Multiplexing Process.
a. Multiplexing TU to become a Tributary Unit Group (TUG).
i) Multiplexing 3 x TU-12 Into TUG-2
ii) Multiplexing 1 x TU-3 Into TUG-3
iii) Multiplexing 7 x TUG-2 Into TUG-3
iv) Multiplexing 3 x TUG-3 Into VC-4
v) Multiplexing 1 x AU-4 Into AUG
vi) Multiplexing 1 x AUG Into STM-1
vii) Multiplexing 4 x STM-1 To STM-4
viii) Multiplexing 16 x STM-1 To STM-16
ix) Multiplexing 4 x STM-4 To STM-16

STM Frame Structure – 1
STM-1 frames:

  1. Capacity of 9 rows x 270 columns = 2,430 bytes.
  2. Bit Rate STM-1 is 2430 bytes x 64 kbit/s = 155.520 Mbit/s
  3. The time interval for each Frame is 125 ms or the repetition frequency for each Frame is 8,000 Hz.
  4. The sending principle is byte-per-byte, starting from the byte (column)
    first first line; up to the last byte (column) of the last row.

Section Over Head (SOH)
The SOH byte added to AU-4 works:

  1. Contains STM-1 frame information.
  2. Information on monitoring the performance of the relevant section.
  3. Maintenance
  4. Operation functions (such as intermediate regenerator monitoring and protection switching controller).
  5. Lines 1 to 3 of SOH are used for RSOH bytes, lines 4 for Pointer AU-4, and lines 5 to 9 are used for MSOH bytes.
    4.3.2. Path Over Head (POH)
    POH bytes added to VCn works:
  6. Carry the required information in accordance with the VC-4 payload sent.
  7. Marks the corresponding payload, and will remain until the payload is demultiplexed.
  8. POH consists of 9 bytes, which are marked with J1, B3, C2, G1, F2, H4,
    Z3, Z4 and Z5.

Pointer Function:

  1. To suppress transmission delays, VCs are placed anywhere in the payload; This process is called “floating”.
  2. to indicate the beginning of the VC in the payload, after the floating process will then be added a “pointer”; so the pointer serves to indicate the address of the first byte of the VC.
    There are 2 types of pointers; that is :
  3. Pointer AU (pointer Administration Unit), which is a pointer located on the fourth line of the “Section Over Head (SOH)” frame STM-N, which functions to indicate the initial location of VC-4.
  4. The TU pointer (Tributary Unit pointer), which is a pointer located in the “payload section” of the STM-N frame, is used to indicate the starting location of VC-12/ VC-3.
    4.4. SDH Network Architecture
    There are 2 levels of use of SDH network elements in the transmission network:
  5. Access Network to combine and distribute services using all types of bit rates (64 kbps, VC-12, VC-3, VC-4) and with transmission bit rates of STM-1, STM-4, STM-16 and STM-64.
  6. Transport Level for the transmission of STM-1 STM-4, STM-16 and STM-64 signals as well as network nodes with Cross-Connect systems that use all types of bit rates (VC-12, VC-3 and VC- 4).

The Network Element is an interface that is placed on the SDH Node and functions for communication between the SDH Node and the Supervision network (Telecomunication Management Network).
Types of network elements:

  1. Terminal Multiplexer (MUX)
  1. Digital Cross Connect (DXC)

An indispensable support tool to handle the management of the entire SDH network which offers wider settings in the management of device functions at any time. One of the advantages is that the SDH network will serve as a centralized operation and maintenance system. The TMN configuration can consist of Network Elements, Mediation Devices, Operations Systems and Work Stations.

TMN configuration

The function of each part in the TMN is:
1) Operating System (OS); Serves to process all the information needed for network monitoring and control.
2) Data Communication (DC). Serves as a basis for communication between elements of the TMN.
3) Mediation Device (MD). Serves as the person responsible for controlling the exchange of information between OS and NE.
4) Network Element (NE). The part that becomes the object for TMN.
5) Q and F adapters (Qn and F). Liaison between sections within TMN

PDH (Plesiochronous Digital Hierarchy) – 3
! Plesiochronous network (almost synchronous) (Internally free running oscillator) Asynchronous multiplex If a tributary is multiplexed to a tributary with a higher bit rate, bit stufing is used and a memory buffer is used to synchronize it with the higher bit rate.
! Higher order tributary bit rate > than the sum of the multiplexed bit rates: for synchronization, signaling and bit stufing Each multiplex level has its own frame format
! Bit by bit multiplexing
! Timing alignment using bit-by-bit justification/stuffing
! Access to individual channels is only possible after demultiplexing
! Bit rate standardized to 140 Mbps

SDH (Synchronous Digital Hierarchy SDH (Synchronous Digital Hierarchy)
ITU-T G-707

! Synchronous network(internal oscillator synchronized with external reference clock)
! Synchronous multiplex technique
! All multiplex signals have identical frame structures
! Byte by byte multiplexing
! Access to individual channels can be done using pointers, without having to demultiplex everything first.
! Bit rate is standardized based on 155 Mbps

Advantages of SDH
! Internationally standardized bit rate above 140 Mbps
! Standardized transmitted optical signal/Compatibility between vendors
! Modular structure

  • Bit rate multiplex is a multiple of the base bit rate (155.52 Mbps)
  • The multiplex signal frame structure is identical to the basic signal frame structure
    ! Access to an individual channel can be done without having to demultiplex the entire signal, only the channels that are needed are demultiplexed. This method is very useful for cross connect and branch systems (add and drop multiplexer)
    ! Accommodates PDH signal
    ! Broadband signal transmission

Presence of protection (Self Healing Ring, Path protection, Multiplex section protection)
! Software configuration (add, drop, crossconnect)
! Centralized management

  • remote alarm
  • remote reconfiguration/ rerouting (2 Mbps lines)
  • remote service activation and configuration of interfaces
  • S/W download to card level

SDH Frame Format

Sonet (Synchronous Optical Network): Bellcore America
! Signal base bit rate : 50,688 Mbps (STS-1 = Synchronous
Transport Signals)

Interface V5.x

ETSI interface standard
Connecting access network (AN) with local central (LE)
Open interface (multivendor interface, allowing AN from any vendor to connect with any LE) .
Interface V5.1 based on the principle of static multiplex and interface
V5.2 is based on the principle of dynamic multiplex and concentrator.
Advantages of Using Interface V5.x
Does not depend on one vendor for the provision of an access network.
Support the development of technology and access network structures that are more cost effective.
Supports a standard interface for network management.

Works on the principle of static multiplex
Each link between LE and AN uses 2Mb/s, connecting LE to AN via copper cable,
optical and radio media.
Supports POTS, ISDN BRA applications.
Signaling time slots 15, 16 and 31 are used as TS
signaling, under normal conditions using TS 16 (TS 16 mandatory).

Interface V5.2
Works on the principle of dynamic multiplex
Using multilink up to 16 links 2048 kb/s (ETSI)
Supported concentrator function on AN, so that more customers can be connected.
Supports POTS, ISDN BRA applications.
Has a protection system against failures that may occur in the signaling channel.

Main function of OLTE
• Converts an electrically powered signal to an optically powered signal and vice versa.
• Combining service signals (service bits) with the main signal.
• Transmits and receives signals with optical power.
• Provide security for officers equipped with a laser diode shut-off circuit.
• Provide an order wire channel for coordination between officers.

Unit B/U Converter

  • Receives bipolar electrical signal (CMI/HDB-3) from multiplex.
  • Improve signal characteristics caused by cable attenuation (Equalization).
  • Change the code of the electrical signal channel from bipolar to unipolar (NRZ).
  • Sends electrical signals from the multiplex to the coder unit.
  • Sends alarm indication to Alarm controller unit.

Coder Unit
• Receives a unipolar electrical signal from the B/U converter unit and from the service channel/auxilary unit.
• Combines the main signal with the service channel signal.
• Encodes the combined signal according to the optical channel code used.
• Replaces the main interrupted signal with an AIS signal.
• Sends an alarm signal in case of interference with the main signal.

Optical Sender Unit

  • Set the pulse width and shape of the unipolar electrical pulses received from the coder unit.
  • Controlling the electric current flowing in the optical source.
  • Converts conditioned unipolar electrical pulse signals into optical pulse signals.
  • Transmits optical pulse signal to opposing terminal via optical fiber.
  • If there is interference it will send an alarm signal.
  • Performs optical source beam disconnection if it receives a shut-off signal.

Optical Sender
There are 2 types of Optical Sources:

  1. LED (Light Emitting Diode).
  2. LASER Diode (Light Amplification by Stimulated Emission of Radiation).

Optical Detector Unit

  • Receives optical signals from opponents via optical fiber.
  • Converts optical signals into unipolar electrical signals.
  • Amplifies the unipolar electrical signal.
  • Sends a unipolar electrical signal to the decoder unit.
  • Sends an alarm signal to the alarm control unit.

Optical Receiver
There are 2 types of Optical Photodiode, namely:

  1. Diode pin (Positive Intrinsic Negative)
  2. PPE (Avalanche Photo Diode)

• Receives a unipolar electrical signal sent by an optical detector unit.
• Redecodes the combined signal (main signal and service channel).
• Separate the main signal from the service channel signal.
• Replaces the main interrupted signal with an AIS signal.
• Sends an alarm signal in case of interference with the main signal.

U/B Converter
• Receives a unipolar electrical signal from the decoder unit.
• Converts a unipolar electrical signal into a bipolar electrical signal.
• Improve signal characteristics due to cable attenuation.
• Sends a bipolar electrical signal to a demultiplex device.
• If it does not receive a signal from the decoder unit, it will send an alarm signal to the alarm controller unit.

Source: TELKOM Knowledge 2007

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