Synchronous Digital Hierarchy (SDH)

Telecom Revolution

       
   Telecom industries have gone through an exponential transformation since the last century. In the early 19 th century, telephone comes as a surprise addon to the mankind. At that point of time it would have been beyond imagination that the future will be connected more closely by the telecom more than the family relations. The late 19th century saw the boom in the usage of telephone in every day life of common man, then the dot com boom added to the communication revolution.
 
   We are going through the fastest phase of life where anyting and everything is available virtually infront of you. The Internet of Things(IOT) is happening now and will be a legacy soon to be replaced by more advancements in the field. Right from ordinary conversation via wired telephone connection to the latest online shopping via smart phone, this all didn't happen overnight. The journey was piggy backed on top of power full protocols. We call them telecom protocols.
 
   Telecom protocols are the rules and regulations followed by the telecom players/vendors to make the life easier and better for implementation, interoperabilty and usability. There are no of protocols right from the beginning of the telecom journey. Here I will focus on some important protocols which revolutionized the telecom industry. They are Plesiochronous Digital Hierarchy(PDH), Synchronous Digital Hierarchy(SDH)/Synchronous Optical Network(SONET) and Optical Transport Network(OTN).

Introduction To Digital Transmission Hierarchies

 

    In many countries, digital transmission networks were developed based upon the standards collectively known today as Plesiochronous Digital hierarchy. Also, various parts of the world use different hierarchies which lead to problems of international interworking; for example, between those countries using 1.544 Mbits/s systems (U.S.A. and Japan) and those using the 2.048 Mbits/s systems (Europe).

    In order to overcome these difficulties a new standard was formulated. SDH (Synchronous Digital Hierarchy) is formulated by the International Telecommunication Union (ITU) as a standard for telecommunications transport in 1988. It’s deployed at all levels of the network infrastructure, including the access network and the long-distance trunk network. It’s based on overlaying a synchronous multiplexed signal onto a light stream transmitted over fiber-optic cable.SDH satisfies the increasing bandwidth demand and operation, administration, maintenance and protection facilities for the network. Also lower level PDH signals can be carried in the SDH envelope, which makes them useful for supporting already existing PDH rates and demands.

Plesiochronous Digital Hierarchy (PDH).

   Digital transmission systems and hierarchies was based on multiplexing signals which are Plesiochronous (running at almost the same speed) in nature. Also, there were different hierarchies in various parts of the world, this lead to problems of international interworking; for example, specialized interfacing equipments were required to interface those countries(U.S.A. and Japan) using 1.544 Mbit/s systems and those using the 2.048 Mbit/s system(Europe). Also inorder to recover a low speed 64 kbit/s channel from a 140 Mbit/s PDH signal, it’s necessary to demultiplex the signal all the way down to the 2 Mbit/s level before the location of the 64 kbit/s channel can be identified. PDH do step wise multiplexing and demultiplexing (140-34, 34-8, 8-2 demultiplex; 2-8, 8-34, 34-140 multiplex) to drop out or add an individual speech or data channel. This is caused due to the bit-stuffing at each level.
 

Limitations of PDH Network

Although it has numerous advantages over analog, PDH has some shortcomings, The main limitations of PDH are:

• Inability to identify individual channels in a higher-order bit stream
• Insufficient capacity for network management;  
• Most PDH network management is proprietary.  
• There’s no standardized definition of PDH bit rates greater than 140 Mbit/s.
• There are different hierarchies in use around the world, specialized interface equipment is required to interwork the two hierarchies.

 

    
   To overcome these limitations, they started thinking of a new multiplexing technique based on byte interleaving and with optical interface compatible with multiple vendors. The new transmission hierarchy was named Synchronous Optical Network (SONET). The International Telecommunication Union (ITU) established an international standard based on the SONET specifications, known as the Synchronous Digital Hierarchy (SDH), in 1988.

Synchronous Digital Hierarchy (SDH)

   

  SDH (Synchronous Digital Hierarchy) is a standard for telecommunications transport formulated by the International Telecommunication Union (ITU).SDH was first introduced into the telecommunications network in 1992. It’s deployed at all levels of the network infrastructure, including the access network and the long-distance trunk network. It’s based on overlaying a synchronous multiplexed signal onto a light stream transmitted over fiber-optic cable. SDH is also defined for use on radio relay links, satellite links, and at electrical interfaces between equipment.The increased configuration flexibility and bandwidth availability of SDH provides significant advantages over the older telecommunications system.

These advantages include: 

• A reduction in the amount of equipment and an increase in network reliability. 
• SDH is synchronous, it allows single stage multiplexing and demultiplexing.  
• The provision of overhead and payload bytes – the overhead bytes permitting management of the payload bytes on an individual basis and facilitating centralized fault sectionalisation.  
• The definition of a synchronous multiplexing format for carrying lower-level digital signals (such as 2 Mbit/s, 34 Mbit/s, 140 Mbit/s) which greatly simplifies the interface to digital switches, digital cross-connects, and add-drop multiplexers.  
• The availability of a set of generic standards, which enable multi-vendor interoperability.  
• The definition of a flexible architecture capable of accommodating future applications, with a variety of transmission rates.

   In brief, SDH defines synchronous transport modules (STMs) for the fiber-optic based transmission hierarchy. The driving force behind these developments is the growing demand for more bandwidth, better quality of service and reliability, coupled with the need to keep costs down in the face of increasing competition.

SDH Multiplexing

    

  SDH multiplexing combines low-speed digital signals such as 2, 34, and 140 Mbit/s signals with required overhead to form a frame called Synchronous Transport Module at level one (STM-1). STM-1 frame is created by 9 segments of 270 bytes each. The first 9 bytes of each segment carry overhead information; the remaining 261 bytes carry payload.

The bit rate of a framed digital signal:

Bit rate = frame rate x frame capacity

The frame rate is 8,000 frames per second.

The frame capacity of a signal is the number of bits contained within a single frame.

Frame capacity = 270 bytes/row x 9 rows/frame x 8 bits/byte = 19,440 bits/frame

The bit rate of the STM-1 signal is calculated as follows:

Bit rate = 8,000 frames/second x 19,440 bits/frame

            = 155.52 Mbit/s

   STM-1 frame is divided into two parts to physically segregate the layers, where each square represents an 8-bit byte. The first nine columns comprise the section overhead (SOH), while the remainder is called the virtual container at level four (VC-4). The SOH dedicates three rows for the regenerator section overhead (RSOH) and six rows for the multiplexer section overhead (MSOH). The VC-4 contains one column for the VC-4 path overhead (VC-4 POH), leaving the remaining 260 columns for payload data (149.76 Mbit/s).




  Virtual Containers can have any phase alignment within the Administrative Unit, and this alignment is indicated by the Pointer in row four. The STM frame is transmitted in a byte-serial fashion, row-by-row, and is scrambled immediately prior to transmission to ensure adequate clock timing content for downstream regenerators.

Virtual Container

  

  SDH supports a concept called virtual containers (VC). Through the use of pointers and offset values, VCs can be carried in the SDH payload as independent data packages. VCs are used to transport lower-speed tributary signals. VC can start (indicated by the J1 path overhead byte) at any point within the STM-1 frame. The start location of the J1 byte is indicated by the pointer byte values. Virtual containers can also be concatenated to provide more capacity in a flexible fashion.

Table below lists the names and some of the parameters of the virtual containers.

SDH Overhead 

   

  The SDH standard was developed using a client/server layer approach. The overhead and transport functions are divided into layers.

They are:

• Regenerator Section 
• Multiplex Section 
• Path

  These layers have a hierarchical relationship, with each layer building on the services provided by all the lower layers.

  The SDH signal is layered to divide the responsibility for transporting the payload through the network. Each SDH network element (NE) will interpret the incoming signal and generate its overhead layer, and communicates control and status information to the same layer of the other equipment. As the payload traverse through the SDH network, each layer is associated with specific NEs named regenerator section terminating equipment (RSTE), multiplexer section terminating equipment (MSTE), or path terminating equipment (PTE).

   The figure below shows network with the layered functions. The lower-rate non-SDH signal enters the SDH network by a PTE then the POH is generated, PTE may be an Access Multiplexer (AM). The POH is removed when the payload exits the SDH network by a PTE, such as an access demultiplexer (AD). The POH is first-on, last-off, so the alarm and error information contained within this layer Represents end-to-end status.

Regenerator Section Overhead

   

  The Regenerator Section Overhead contains only the information required for the elements located at both ends of a section. This might be two regenerators, a piece of line terminating equipment and a regenerator, or two pieces of line terminating equipment. The Regenerator Section Overhead is found in the first three rows of Columns 1 through 9 of the STM-1 frame .

Regenerator section overhead

Multiplex Section Overhead 

   

  The Multiplex Section Overhead contains the information required between the multiplex section termination equipment at each end of the Multiplex section (that is, between consecutive network elements excluding the regenerators). The Multiplex Section Overhead is found in Rows 5 to 9 of Columns 1 through 9 of the STM-1 frame. 

Multiplex section overhead

Path Overhead (VC-4/VC-3)  

  The Path Overhead is assigned to, and transported with the Virtual Container from the time it’s created by path terminating equipment until the payload is demultiplexed at the termination point in a piece of path terminating equipment. The Path Overhead is found in Rows 1 to 9 of the first column of the VC-4 or VC-3.Byte by byte.

SDH Performance Monitoring

   

  Each layer in the SDH signal provides alarm and error monitoring capabilities between various terminating points in the network. Similar to 2 Mbit/s signals, parity is calculated and stored in the transmitted signal. The parity is recalculated by the receiver and verified against the stored value to determine if an error occurred during transmission.

  Each layer in the SDH signal has its own Bit Interleaved Parity (BIP) calculation. If CRC framing is used at the 2 Mbit/s level, a Remote End Block Error (REBE) may be returned to the sender when a bit error is detected in the framed 2 Mbit/s PDH signal. SDH uses the same algorithm, using a layered approach. If a MSTE receives some number of multiplexer section BIP errors, it transmits the same number of multiplexer section Far End Block Errors (FEBEs) back to the originator. PTEs use the same approach in the path layer of overhead. Like 2, 34, and 140 Mbit/s PDH signals, the SDH signal also contains Alarm Indication Signals (AISs) and Remote Alarms, except that a SDH Remote Alarm is called Remote Defect Indication (RDI), and is layered like all of the other SDH results

SDH Timing Compensation

  

  The SDH signal was designed to be timing-tolerant to support Plesiochronously timed, lower-rate signals and slight timing differences between synchronously timed NEs. Two mechanisms allow for robust timing compensation: variable bit justification of the lower-rate signal, and a technique called pointer adjustments between synchronous elements in the SDH network.

  Pointer adjustments allow the VC-4 to float with respect to the SDH frame. This means that a single VC-4 payload frame typically crosses the STM-1 frame boundary. The pointer is contained in the H1 and H2 bytes of the MSOH, and it is a count of the number of bytes the VC-4 POH J1 byte is away from the H3 bytes, not including the section overhead bytes. A valid VC-4 pointer can range from 0 to 782.

  When timing differences exist, dummy bytes can be inserted into the VC-4 without affecting the data. Since the pointer is adjusted to indicate where the real POH starts, the receiving end can effectively recover the payload (i.e., ignore the dummy bytes). When justified bytes are used, they are always in the same location, regardless of where the POH starts. H3 bytes are called negative justification bytes and carry real payload data for one frame during a pointer decrement. The three bytes following the last H3 byte in the VC-4 are called positive justification bytes and carry three dummy bytes of information for one frame during a pointer increment.

  If there is no timing difference between two nodes, the incoming STM-1 payload bit rate is identical to the transmitting source that drives the outgoing STM-1 frame rate, so no pointer adjustments are needed. Figure below shows a SDH node that has an incoming frequency f1 and an outgoing frequency f2. If f1 is less than f2, there is a constant lack of payload data to place into the outgoing SDH signal. To compensate, three dummy bytes are placed into the positive stuff bytes and the data is moved to the right by three bytes, so the VC-4 pointer is incremented by one. On the other hand, if f1 is greater than f2 then three extra VC-4 payload bytes are stored into the negative stuff bytes, H3, in the MSOH for one frame, while the entire payload data is moved to the left by three bytes and the pointer is decreased by one. The only equipment that can perform path pointer adjustments is MSTE, since the pointer value is contained in the MSOH. Also, path pointer adjustments are not performed by PTE (where the payload data enters the SDH network) even though there are potential timing differences at these locations as well. The timing differences at PTEs are due to Plesiochronously-timed tributary signals and are corrected by traditional bit justification techniques.

SDH Multiplexing

 

The multiplexing principles of SDH follow, using these terms and definitions:

• Mapping – A process used when tributaries are adapted into Virtual Containers (VCs) by adding justification bits and Path Overhead (POH) information. 
• Aligning – This process takes place when a pointer is included in a Tributary Unit (TU) or an Administrative Unit (AU), to allow the first byte of the Virtual Container to be located.  
• Multiplexing – This process is used when multiple lower-order path layer signals are adapted into a higher-order path signal, or when the higher-order path signals are adapted into a Multiplex Section.  
• Stuffing – As the tributary signals are multiplexed and aligned, some spare capacity has been designed into the SDH frame to provide enough space for all the various tributary rates. Therefore, at certain points in the multiplexing hierarchy, this spare capacity is filled with “fixed stuffing” bits that carry no information, but are required to fill up the particular frame.

The ITU-T recommended SDH multiplexing structure is shown below in the figure.






   At the lowest level, containers (C) are input to virtual containers (VC). The purpose of this function is to create a uniform VC payload by using bit-stuffing to bring all inputs to a common bit-rate ready for synchronous multiplexing. Various containers (ranging from VC-11 at 1.728 Mbit/s to VC-4 at 150.336 Mbit/s) are covered by the SDH hierarchy. 

   Next, VCs are aligned into tributary units (TUs), where pointer processing operations are implemented. These initial functions allow the payload to be multiplexed into TU groups (TUGs). As Figure 4 illustrates, the xN label indicates the multiplexing integer used to multiplex the TUs to the TUGs. The next step is the multiplexing of the TUGs to higher level VCs, and TUG-2 and TUG-3 are multiplexed into VC-3 (ANSI mappings) and VC-4. These VCs are multiplexed with fixed byte-stuffing to form administration units (AUs) which are finally multiplexed into the AU group (AUG) as shown in the figure below. This payload then is multiplexed into the Synchronous Transport Module (STM).

Types of Single Mode Fibers

Types of Single Mode Optical Fiber

   We’ve already discussed how single-mode fiber is used for essentially all long-reach fiber applications.But there are also several different types of SMF.

The most common types are:
• “Standard” SMF (ITU-T G.652) A.K.A. SMF-28
• Low Water Peak Fiber (ITU-T G.652.C/D)
• Dispersion Shifted Fiber (ITU-T G.653)
• Low-Loss Fiber (ITU-T G.654)
• Non-Zero Dispersion Shifted Fiber (ITU-T G.655)
• Bend Insensitive Fiber (ITU-T G.657)

these are the different fibers defined theoretically, however the below mentioned fibers are the one you see often in the field

“Standard” Single-Mode Fiber (G.652)  

 The original and mode widely deployed fiber is Frequently called “SMF-28” or SMF. SMF-28 is actually a product name from Corning.this fiber is Optimized for the 1310/1550nm band.the Lowest rate of dispersion occurs in this 1310 band but Attenuation is lower at 1550nm, but more dispersion there.
 

Low Water Peak Fiber (G.652.C/D)

   Modified G.652 is designed to reduce water peak.Water peak is a high rate of attenuation at certain frequencies due to OH- hydroxyl molecule within the fiber.these type of fiber give very good raman gain.

Dispersion Shifted Fiber (ITU-T G.653)

    This type of fiber is a n attempt to improve dispersion at 1550nm.The rate at which chromatic dispersion occurs changes across different frequencies of light.The natural point of zero dispersion occurs at 1300nm.But this is not the point of lowest attenuation. DSF shifts the point of zero dispersion to 1550nm.But this turned out to cause big problems.Running DWDM over DSF causes non-linear interactions.The notable example is called Four Wave Mixing where 3 equally spaced wavelengths interact to produce a 4th wavelength.As a result, this fiber is rarely used today.
This type of fiber is hugely deployed in Japan.

Non-Zero Dispersion Shifted Fiber

    They have similar concept to Dispersion Shifted Fiber but the zero point is moved outside of the 1550nm band. This leaves a small amount of dispersion, but avoids the non-linear cross-channel interactions cause by DSF. To manage dispersion, NZDSF comes in 2 types, they are NZD+ and NZD-, with opposite dispersion “slopes”.One spreads the 1550nm band out and the other compresses it in the opposite direction.By switching between the two slopes, the original signal can be maintained even over extremely long distances.this is what is done in subsea cable

Other Single-Mode Fiber Types

the other type of single mode fibers are 

ELEAF, TWRS, TWS, Teralight etc

 

Dispersion of different Fiber types

Optical Power in dB

What is Optical Power? 

• The brightness (or “intensity”) of the light. 

• As light travels through fiber, some of the energy is lost. 

• Either absorbed by the glass particles, and converted to heat;

• Or scattered by microscopic imperfections in the fiber. 

• This loss of intensity is called “attenuation”. 

• We typically measure optical power in “Decibels” 

• A decibel (dB, 1/10th of a Bel) is a logarithmic-scale unit expressing the relationship between two values.
• The decibel is a “dimensionless-unit”, meaning it does NOT express an actual physical measurement on its own. 

Optical Power and the Decibel

A decibel itself is simply a ratio between values! 

• 0 dB is no change, +3 dB is double, -3 dB is half, etc.
• To express an absolute value (i.e. an actual light level), it must be compared to a known reference value. 

In optical networking, this is typically a “dBm”.

• That is, a decibel relative to 1 milliwatt (mW) of power. 
• 0 dBm is 1 mW, 3 dBm is 2 mW, -3 dBm is 0.5mW, etc.
• So what does this make 0mW? Negative Infinity dBm.
• Confusion between dB and dBm is one of the most common mistakes when working with optical networks! 19 Optical Power and the Decibel 

So why do we measure light with Decibels? • Light, like sound, follows the “inverse square” law.

• The signal is inversely proportional to the distance squared.
• After a signal travels distance X, and loses half of its intensity.
• After it travels another distance X, it loses half again.
• Thus after 2X only 25% remains, after 3X only 12.5% remains, etc.

Using a logarithmic scale simplifies the calculations.

• A 3dB change is approximately half/double the original signal. 
• In the example above, there is a 3dB loss per distance X. 
• At distance 2X there is 6dB of loss, at distance 3X it is 9dB, etc.
• Using a logarithmic scale “cancels out” the exponential loss, giving us a linear system that lets us use simple math (like addition and subtraction) when calculating losses. 20 Decibel to Power Conversion Table 

Frequently asked questions about Optical networks

Many students and engineers asks this below questions

1.What is Coherent Transmission?

As mobile networks advance towards data networks, intelligent terminals are widely used, and new services, such as IPTV,eCommerce and cloud computing,online gaming are increasing rapidly, the transmission capacities of legacy networks have to be improved. The system supports high-speed transmission with the use of advanced modulation formats like ePDM-16QAM, ePDM-QPSK, ePDM-BPSK, and coherent detection technologies to meet the high-speed transmission requirements on OSNR, CD, PMD, and nonlinear effects. It offers ultra-large bandwidth (400G, 200G, 100G and 40G).

2.What is ROADM?

It’s the abbreviation of reconfigurable optical add/drop multiplexer. With ROADM technology, flexible optical-layer grooming is available. The ROADM reconfigure wavelengths by blocking or cross-connecting the wavelengths. It changes the static allocation of the resource to flexible and dynamic allocation.

The main technology of RAODM is WSS. The WSS module on a WSS board splits a colored light signal into multiple parallel monochromatic signals and adjusts the optical power of each monochromatic signal. The WSS module directs each monochromatic signal into the corresponding multiplexer using its 1xN optical switch. The WSS module then multiplexes the monochromatic signals into one signal for further transmission. In this way, a monochromatic signal can be transmitted out of the board through any port.

3.What is ASON?

It’s the abbreviation of Automatically Switched Optical Network. ASON introduces Generalized Multiprotocol Label Switching (GMPLS) control plane to achieve dynamic connection management, automatic discovery, protection & restoration, and CAPEX & OPEX reduction. The control plane of ASON complies with Link Management Protocol (LMP), Link Management Protocol (OSPF), and Resource Reservation Protocol-Traffic Engineering (RSVP-TE) protocols.

ASON can transport services of different Service Level Agreements (SLAs) based on customers’ requirements. The SLA divides services into various levels according to the service protection capability.

There two kinds of ASON, optical-layer ASON and electrical-layer ASON. The optical-layer ASON, which is also known as WSON, is based on flexible ROADM with the using of WSS technologies to implement colorless, directionless and contentionless applications. As to electrical-layer ASON, there two types of electrical-layer ASON: OTN ASON and SDH ASON. Electrical-layer ASON is based on optical-layer server trails. The OTUk link or VC link may be inconsistent with the physical topology. OTN ASON can be easily deployed on standard OTN networks to improve the network reliability.


NB:I will describe each of the above terminology in another post

Polarization Mode Dispersion

Introduction

      There are three fundamentally different dispersive phenomena in optical fiber, of which polarization mode dispersion (PMD) is the most complex. In digital multimode fiber systems, a light pulse separates into multiple spatial paths or modes. Each component reaches the receiver at a slightly different time as shown in the figure broadening the received pulse. Single-mode fiber solves the differential mode delay problem, allowing data rates to be increased until chromatic dispersion — the variation of propagation speed with wavelength — produces unacceptable pulse spreading. The amount of chromatic dispersion that a system can tolerate is inversely proportional to the square of the bit rate because an increased data rate means not only a wider spectrum and increased spreading, but also narrower bit slots that are more sensitive to the spreading of neighboring pulses. 

Three Different types of dispersion in fiber

     When chromatic dispersion is compensated — typically to a small but nonzero value in dense wavelength division multiplexed (DWDM) systems — the bit rate can be increased until it is limited by the third dispersive effect, PMD. Every network exhibits two slightly different propagation delays that correspond to different input polarization's. Some of the pulse energy experiences the longer delay and the rest of the energy experiences the shorter delay. As with the other dispersive effects, the result is a broadening of the received pulse. 

      PMD is considerably more subtle and interesting than this, however, and the topic accounts for a rapidly growing body of technical literature. This article will explore the origins, statistical character, measurement and mitigation of first-order polarization mode dispersion. 

Properties of polarized light

     The electric and magnetic fields of a lightwave fluctuate at right angles to one another in the plane perpendicular to the direction of propagation like in the figure below. PMD in single-mode optical fiber originates with noncircularity of the core  Fiber birefringence has two components. Form birefringence is a basic characteristic of any oval waveguide. Stress birefringence — generally dominant — is induced by the mechanical stress field that is set up when the fiber is drawn to other than a perfectly circular shape. Over short lengths, fiber birefringence splits the input pulse into linear slow and fast polarization modes, behaving like a linearly birefringent crystal. The corresponding difference in propagation time is called the differential group delay (DGD), expressed in picoseconds (1 ps = 10-12 s). Together, the differential group delay and the orthogonal polarization modes are the fundamental manifestations of first-order PMD. 

Lightwave Fluctuating between electric and magnetic field

   Given the extremely weak birefringence of telecom fiber, mode coupling is easily induced in the fiber by the mechanical forces arising from spooling, cabling or installation. 

 Core noncircularity is the root of PMD in single-mode fiber.

     The differential group delay at a given wavelength and time is called the instantaneous differential group delay. The average value of the DGD over wavelength is called the PMD delay. The average DGD divided by the square root of fiber length is called the PMD coefficient. 

How much can be tolerated?

    Digital transmission systems are designed to tolerate 10 to 15 percent of a bit period of average differential group delay, or 10 to 15 ps for a 10 Gb/s system. The average differential group delay of long routes of legacy fiber is often greater than this limit and in particularly severe cases can exceed 100 ps. 

     New optical fiber generally exhibits an average differential group delay in the range of 0.05 to 0.10 ps/km1/2. 

PMD mitigation

      The development of PMD mitigation techniques is driven by the upgrade of legacy fiber links to 10+ Gb/s. Any mitigation scheme must account for random changes in the differential group delay and principal states of polarization. One approach is to eliminate pulse spreading by coupling the transmitter output to a single input principal state of polarization of the link. Drawbacks are the need for specialized hardware at both transmitter and receiver and the delay of the feedback loop, which is twice the length of the link. 

    PMD can also be electrically mitigated by means of an equalizer circuit installed following the receiver photodetector. The detected signal is split into several paths to be differentially delayed and scaled, then recombined to squeeze the pulse back to a narrower shape. This technique has a history in microwave communications. PMD mitigation is receiving wide research attention and field trials have been run on several methods. 

Summary 

   When chromatic dispersion is compensated, PMD becomes a bit-rate limiting factor in digital fiber optic communications systems. The high PMD of many legacy fibers calls for measurement of the installed fibers and motivates the development of PMD mitigation. Specifications for components and fibers are tightening, and an understanding of polarized light and its interaction with hardware has become a key success factor for component manufacturers

Simple Optical Fiber Calculations

Introduction

Here i will try to describe about the simple Optical Fiber calculations done in the field.

Attenuation:
1) For an SMF fiber with attenuation coefficient 0.25 dB/Km, what is the attenuation for 100 Kms of SMF fiber?

attenuation coefficient= 0.25 dB/Km

length of the Fiber= 100 Km

Attenuation for 100 Km SMF fiber = Attenuation Coeffiteint x Length of the Fier
                                                          = 0.25 x 100
                                                          = 25 dB

Attenuation is usually represented in dB

Dispersion:

The Chromatic Dispersion of a fiber is expressed in ps/(nm*km), representing the differential delay, or time spreading (in ps), for a source with a spectral width of 1 nm traveling on 1 km of the fiber. It depends on the fiber type, and it limits the bit rate or the transmission distance for a good quality of service.
For a standard SMF fiber, the dispersion coeffitient is  17 ps/(nm*km)
Here also calculations can be done by direct multiplication,

ie for a 100Km standard SMF, the Residual Dispersion will be = Length of the Fiber x Dispersion Coeffitient
                                                                                                     = 100 x 17 = 1700 ps