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Q.1: What is difference between single-mode & multimode fiber?

A: Singlemode fiber, which provides higher bandwidth over longer distance, supports only single mode transmission of light. While the multimode fiber, which has lower bandwidth, supports multiple modes of light transmission due to its large core diameter. Generally, multimode fibers are used for LAN connectivity or the applications where distances are smaller. Single mode fibers are used for transmission of signal over the longer distances such as trunk links or backbone links

Q.2: What is the relationship between bandwidth and distance?

A: In case of multimode fiber, this relationship is given by the manufacturer as a MHz*km value. As the distance increases, less bandwidth is realized. In this calculation actual laser/LED bandwidth of the source must be used, not just the data rate. Although a link will be limited by the optical budget, but sometimes it can be limited by this bandwidth value. For rough calculations multiplication of bandwidth & distance is usually sufficient. However, for detailed calculations other factors such as chromatic dispersion, modal dispersion, source line width, etc. must be considered.

Q.3: What do we mean by dB/Km loss and what are causes for this loss?

A. As the light is guided through the core of optical fiber, it experiences losses (attenuation) i.e. power loss, which is expressed in dB/km. The attenuation varies with wavelength and typically, it is 0.40 dB/km @ 1310 nm and 0.30 dB/km @ 1550 nm. Following four properties mainly cause attenuation:

1. Absorption: This occurs when light strikes impurities in the core glass and is absorbed.

2. Scattering: This occurs when the light strikes an area where the material density changes.

3. Macrobending: This is large-scale bending of the fiber which exceeds the fiber bend radius and causes light to leave the core and travel in the cladding (usually an installation problem)

4. Microbending : This is microscopic distortion of the fiber which causes light to leave the core and travel in the cladding (created during manufacturing)

Q.4. What do we mean by ITU G 652, G 653, G 655 compliant fiber?

A. There are different types optical fibers. On a broader scale, the optical fibers can be divided in two types i.e. single mode and multimode. ITU, an international standards body for telecommunication has defined the standard specifications for various different types of single mode fiber. The ITU G 652 defines the specification for standard single mode optical fiber, while the G 653 defines the specifications for dispersion shifted single mode optical fiber and G 655 defines the specifications for nonzero dispersion shifted fiber. The basic difference between these three different fiber types is the chromatic dispersion. The chromatic dispersion limits the maximum bit rate transmission through a single mode fiber. The ITU G 652 specification defines the maximum dispersion of 3.5 ps/nm-km in 1310 nm region and generally this type of fibers have maximum dispersion of 20 ps/nm-km in 1550 nm region. The ITU G 653 represents the fibers with zero dispersion wavelength close to 1550 nm. The ITU G 655 represents the fibers with small but non-zero dispersion in 1550 nm region.

Q.5. What is the transmission capacity of single mode fiber?

A. The transmission capacity i.e. bit rate, of a optical fiber is dependent upon dispersion characteristics of an optical fiber. This parameter varies with wavelengths. The dispersion is further divided in two type i.e. chromatic dispersion and polarization mode dispersion. The polarization mode dispersion is more prominent for data rates above 10 Gbps. The chromatic dispersion, which is defined in ps/nm-km, is further divided in two different types i.e. material dispersion and waveguide dispersion. The Chromatic dispersion causes broadening of light pulse as it travels along the core of optical fiber. Because of this Bit Error Rate (BER) is generated in digital transmission systems. With the help of linear imperical formula (with 1 dB power penalty), it is possible to find out the maximum bit rate transmission for a given repeater distance and dispersion. The formula is as follows:

B2 L D < 104,000
B= Bit Rate in Gb/s
L= Distance in Km
D= Dispersion in ps/nm-km

From the above formula, it can be that with a dispersion of 18 ps/nm-km, 2.5 Gbps signal can be transmitted over a distance of 800 Km between the repeater station.

The polarization mode dispersion (PMD) also limits the transmission capacity of fiber but more prominently for data rates above 10 Gbps. The PMD of a fiber is measured in ps/Ökm. As a rule of thumb, the total PMD of a fiber link should be less than one tenth of bit period of a given data rate. For example, a fiber optic link with PMD of 0.5 ps/Ökm would be able to transmit 10 Gbps signal over a distance of 400 Km comfortably.

Q.6. What is splicing and which equipment are used for splicing?

A.: Optical fibers are made of glass and these fibers are protected from external forces by cabling. The process of jointing two finite lengths of optical fibers is called splicing, to be more precise 'fusion splicing'. The optical are provided with first level of protection by the use of acrylic coating, followed by coloring for identification and then by either placing in single or multiple tubes. The splicing (more simply jointing) of fibers consists of following set of activities:

1. Accessing fibers

2. Removing coating of fibers

3. Cleaning of fibers

4. Cleaving of fibers

5. And finally splicing of fibers with the help of splicing machine. Various different technologies adopted by splicing machine are such as Local Injection and Detection (LID) and profile alignment system (PAS). Various equipment and tools used in splicing process are Fiber Stripper, Fiber Cleaver and Fusion Splicing machine.

Q.7. What do we mean transmission at 1310 nm and 1550 nm ?

A.: The optical fibers transport the light (lasers) modulated with the signal. 1310 and 1550 nm refers to the specific wavelength used for transmission. The single mode fibers were initially optimized for transmission at 1310 nm and 1550 nm, however with the advances in the fiber manufacturing technology, today's' fibers can transmit wavelengths from 1530 to 1565 nm (called as C band), from 1565 to 1620 nm (called as L band) and even wavelengths in 1440 nm region.

Q.8. What is the difference between unarmoured, armoured, ADSS,and OPGW cables?

A. The optical fiber cables are installed in various different ways in the Outside Plant (OSP). Based on the way, the cables are installed different cable constructions have been devised. The unarmoured cable or alternatively called metal free cables do not have any metallic components used in the cable construction. These cables are installed in ducts.

The armoured cables have metallic components such as corrugated metal tape and metal strength members (generally used in central tube type cable). The presence of corrugated metal tape provides strength to the construction and has superior mechanical characteristics than unarmoured cable. These types of cables are directly buried.

ADSS stands for All Dielectric Self-Supporting cable. These types of cables have superior tensile properties such that they can be installed directly over the supporting poles with necessary fixing hardware. These type of cables do not have any metallic components instead they use aramid yarns for extra tensile strength.

OPGW stands for Optical Ground Wire cable. The fibers in this type of cable are placed in the slots provided and the complete cable is made of metal. These type of cable serve dual purpose of housing and protecting the fibers and act as a ground wire for high power transmission system.

Q.9. What does PDH & SDH stand for?

A. PDH stands Plesiosynchronous Digital Hierarchy and SDH stands for Synchronous Digital Hierarchy. Both PDH & SDH use Time Division Multiplexing technique. Further, there are different levels in each hierarchy.

PDH has 4 levels viz. 2 Mbps, 8 Mbps, 34 Mbps and 140 Mbps. The basic multiplexing frame in PDH is 2 Mbps (also called as E1 in European market) which consists of 30 x 64Kb time slots (channels). Each 64Kb channel is required for transmission one voice channel. 8 Mbps system is realized by using four (4) 2 Mbps, 34 Mbps is realized by using four (4) 8 Mbps system and 140 Mbps system is realized by using four (4) 34 Mbps system.

SDH has four levels viz. STM-1 (155 Mbps), STM-4 (625 Mbps), STM-16 (2.5 Gbps) and STM-64 (10 Gbps). The basic multiplexing rate in this hierarchy is STM-1, which is 155 Mbps, and this is equivalent to 63 E1s. Similar to PDH, higher levels of SDH are realized by using multiple immediate lower levels. In SDH, it possible to extract an E1, unlike in PDH which requires down conversion of higher level signal to lower level.

Q.10 What does WDM/DWDM & OADM stand for?

A. WDM or DWDM stands for Wavelength Division Multiplexing or Dense Wavelength Division Multiplexing. In WDM or DWDM each wavelength is modulated with a flexible bit rate, thus multiple modulated wavelengths can be transmitted through a optical fiber. WDM systems have 2 or more up to 8 wavelengths while DWDM has 8 or more wavelengths. At present, commercial DWDM systems with 128 wavelengths are available. WDM/DWDM systems consists of multiplexer and demultiplexers. OADM stands for Optical Add/Drop Multiplexer.

OADM either add or drop a single or multiple wavelengths. OADM are analogous to Digital Add/Drop Multiplexer.

Q.11. What is an optical amplifier and where it is used?

A.: As the light travels along the optical fiber link, it experiences some losses. The optical signal is converted to electrical at the receiving end, however the receiver sensitive to power of the signal being received. Traditionally, in order to boost the optical signal level, the optical signal is regenerated at regenerator station, however this option is very costly which requires space, power and time. Optical amplifiers are analogous to audio amplifiers. Optical Amplifiers boost the optical signal level so as to extend the link length. Optical amplifier consists of Erbium Doped fiber, laser pump (980 nm or 1480 nm) and associated components. Optical Amplifiers are also called as Erbium Doped Fiber Amplifier or EDFA in short. The present commercially available Optical Amplifiers boost the optical levels of wavelengths in C band.

Q.12. What does ATM stand for?

A. ATM stands for Asynchronous Transfer Mode (not to be confused with Any Time Money) and it does not have mux hierarchy. ATM is used for variety of applications i.e. voice, video and data transmission and unlike SDH or PDH systems which uses Time Division Multiplexing technique, ATM uses statistical multiplexing technique. Further, the service or data rate is adaptive unlike fixed data rate in PDH & SDH systems.

Q.13. What is DSL?

A. DSL stands for Digital Subscriber Line. This is a broadband communication technology designed for use on regular phone lines. It has the ability to move data over the phone lines at speeds up to 140 times faster than the fastest analog modems available today. The technology delivers broadcast quality video, video conferencing, and high-speed data transmission to the home or business, all without interrupting normal telephone service.

Q.14 What is xDSL?

A. xDSL is the generic term for the various class of DSL technologies. The "x" stands for any one of a number of implementations :

H : High-bit-rate, S : Symmetric or Single-line high-bit-rate, A : Asymmetric, V : Very-high-bit-rate

Q.15 What is ADSL? What are the benefits of this technology?

A. ADSL stands for Asymmetric Digital Subscriber Line. With the help of ADSL, it is possible to convert the existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. Thus, traditional "twisted pair" network provided the access to the "information highway". ADSL can literally transform the existing public information network from one limited to voice, text and low-resolution graphics to a powerful, ever-present system capable of bringing multimedia, including full motion video, to everyone's home. Furthermore, ADSL empowers service providers to provide either a guaranteed sustained rate or alternatively a rate adaptive or best effort service similar to analog modems. With ADSL, users can obtain speeds of up to:

* 300 times faster than 24.4 Kbps modems
* 100 times faster than 56 Kbps modems
* 70 times faster than 128 Kbps ISDN

Q.16 What is ADSL Lite ?

A. ADSL Lite is a variant of ADSL technology that enables high-speed Internet access over existing phone networks. ADSL Lite was designed for easy, low-cost deployment. Using existing phone lines and ordinary phone jacks, ADSL Lite-enabled modems deliver transmission speeds up to 1.5Mbps - over 25 times faster than today's top-of-the-line 56 kbps modems. ADSL Lite has been standardized by the International Telecommunications Union (ITU). The new standard, which is referred to as G.992.2, was approved by the ITU in June 1999.

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