Raman Amplifiers

Amplifiers based on Raman gain, which results from the effect of stimulated Raman scattering.

The Raman-active medium is usually an optical fiber, although it can also be a bulk crystal, or a cell with a gas or liquid medium. An input signal can be amplified while co- or counterpropagating with a pump beam, the wavelength of which is typically a few tens of nanometers shorter. For silica fibers, maximum gain is obtained for a frequency offset of 10–15 THz between pump and signal, depending on the core composition.

Source:www.timbercon.com/Raman-Amplifiers.html

Raman amplifier

A type of amplifier used in long haul, single-mode (SMF) fiber optic transmission systems (FOTS). Raman amplification usually occurs throughout the length of the transmission fiber itself in a process known as distributed amplification, rather than in a discrete amplification, or lumped amplification configuration such as that employed by an erbium-doped fiber amplifier (EDFA). Raman amplification occurs as a high-energy pump wavelength is sent in the reverse direction from the output end of the fiber span, where the incoming signal is weakest. The pump wavelength, which generally is in the 1450 nm range (E-Band), interacts with and excites atoms in the crystalline lattice of the fiber core.The atoms absorb the photons, and quickly release photons with energy equal to the original photon, plus or minus atomic vibration. In other words, a frequency/wavelength shift occurs as the pump wavelength propagates along the fiber in the reverse direction.The energy lost in the pump wavelength shifts to longer-wavelength (within about 100 nm) signals, generally in the 1550 nm window (C-Band), in the forward direction, thereby serving to amplify them. Raman amplifiers offer the advantage of amplifying signals in the broad range extending from 1300 nm to 1700 nm. Further, they perform better than EDFAs in terms of signalto-noise ratio (SNR). Raman amplifiers often are used as preamplifiers to enhance the performance of EDFAs in dense wavelength division multiplexing (DWDM) systems.

Source:www.yourdictionary.com/telecom/raman-amplifier

Raman Amplifier

A Raman optical amplifier is not a "black box" like an EDFA. It consists of little more than a high-power pump laser, usually called a Raman laser, and a WDM or directional coupler. The optical amplification occurs in the transmission fiber itself. The optical amplification is distributed along the transmission line. Optical signals are amplified up to 10 dB in the network optical-fiber. The Raman optical amplifiers have wide gain bandwidth (up to 100 nm). They can use any installed transmission optical-fiber (single-mode optical fiber, TrueWave, etc.). In effect, they reduce the effective span loss and improve noise performance of the transmission line by boosting the optical signal in transit. They can be combined with erbium-doped fiber amplifiers (EDFA) to achieve very wide optical gain flattened bandwidth. Figure 3 shows the topology of a typical Raman optical amplifier. The two key elements of the Raman optical amplifier are the pump laser and the directional coupler. The pump laser, in this case, has a wavelength of 1535 nm. The circulator provides a convenient means of injecting light backwards in to the transmission path with minimal optical loss.
shows the optical spectrum of a forward-pumped Raman optical amplifier. In this case, the pump laser is injected at the transmit end rather than the receive end as shown in Figure 3. The pump laser has a wavelength of 1535 nm. As is usually the case, the amplitude of the pump laser is much greater than the data signals. Figure 5 shows the received signal after the same length of fiber used in the SRS example above. While the amplitude of the pump laser is significantly decreased, the amplitude of the six data signals is now much stronger and they all have roughly equal amplitudes. In this case, a great deal of energy was robbed from the 1535 nm pump laser signal and redistributed to the six data signals.

Source:www.mrfiber.com/Raman_Amplifier.htm

IMS Architecture

The IP-Multimedia Subsystem (IMS) defines the functional architecture for a managed IP-based network. It aims to provide a means for carriers to create an open, standards-based network that delivers integrated multimedia services to increase revenue, while also reducing network CapEx and OpEx.

IMS was originally designed for third-generation mobile phones, but it has already been extended to handle access from WiFi networks, and is continuing to be extended into an access-independent platform for service delivery, including broadband fixed-line access. It promises to provide seamless roaming between mobile, public WiFi and private networks for a wide range of services and devices.

The IMS architecture has been designed to enable operators to provide a wide range of real-time, packet-based services and to track their use in a way that allows both traditional time-based charging as well as packet and service-based charging. It has become increasingly popular both with wireline and wireless service providers as it is designed to increase carrier revenues, deliver integrated multimedia services, and create an open, standards-based network.

IMS provides a wide range of session border control, including call access control, reachability and security. It also provides a framework for the deployment of both basic calling services and enhanced services, including

•multimedia messaging
•web integration
•presence-based services
•push-to-talk.
At the same time, it draws on the traditional telecommunications experience of

•guaranteed QoS
•flexible charging mechanisms (time-based, call-collect, premium rates)
•lawful intercept legislation compliance.
Network operators also hope that IMS will cut their CapEx and OpEx through the use of a converged IP backbone and the open IMS architecture.

•The IMS architecture defines many common components (for example, call control and configuration storage) so less development work is required to create a new service as this existing infrastructure can be reused.
•The use of standardized interfaces should increase competition between suppliers; preventing operators from being locked into a single supplier's proprietary interfaces.
As a result, IMS should enable new services to be rolled out more quickly and cheaply, compared with the traditional monolithic design of telephony services


Source:www.metaswitch.com/sbc-session-border-controller/ims-architecture.aspx

IMS Architecture

The IP-Multimedia Subsystem (IMS) defines the functional architecture for a managed IP-based network. It aims to provide a means for carriers to create an open, standards-based network that delivers integrated multimedia services to increase revenue, while also reducing network CapEx and OpEx.

IMS was originally designed for third-generation mobile phones, but it has already been extended to handle access from WiFi networks, and is continuing to be extended into an access-independent platform for service delivery, including broadband fixed-line access. It promises to provide seamless roaming between mobile, public WiFi and private networks for a wide range of services and devices.

The IMS architecture has been designed to enable operators to provide a wide range of real-time, packet-based services and to track their use in a way that allows both traditional time-based charging as well as packet and service-based charging. It has become increasingly popular both with wireline and wireless service providers as it is designed to increase carrier revenues, deliver integrated multimedia services, and create an open, standards-based network.

IMS provides a wide range of session border control, including call access control, reachability and security. It also provides a framework for the deployment of both basic calling services and enhanced services, including

•multimedia messaging
•web integration
•presence-based services
•push-to-talk.
At the same time, it draws on the traditional telecommunications experience of

•guaranteed QoS
•flexible charging mechanisms (time-based, call-collect, premium rates)
•lawful intercept legislation compliance.
Network operators also hope that IMS will cut their CapEx and OpEx through the use of a converged IP backbone and the open IMS architecture.

•The IMS architecture defines many common components (for example, call control and configuration storage) so less development work is required to create a new service as this existing infrastructure can be reused.
•The use of standardized interfaces should increase competition between suppliers; preventing operators from being locked into a single supplier's proprietary interfaces.
As a result, IMS should enable new services to be rolled out more quickly and cheaply, compared with the traditional monolithic design of telephony services

IP Multimedia Subsystem (IMS)

IP Multimedia Subsystem (IMS) is a generic architecture for offering multimedia and voice over IP services, defined by 3rd Generation Partnership Project (3GPP). IMS is access independant as it supports multiple access types including GSM, WCDMA, CDMA2000, WLAN, Wireline broadband and other packet data applications. IMS will make Internet technologies, such as web browsing, e-mail, instant messaging and video conferencing available to everyone from any location. It is also intended to allow operators to introduce new services, such as web browsing, WAP and MMS, at the top level of their packet-switched networks

IP Multimedia Subsystem is standardized reference architecture. IMS consists of session control, connection control and an applications services framework along with subscriber and services data. It enables new converged voice and data services, while allowing for the interoperability of these converged services between internet and cellular subscribers. IMS uses open standard IP protocols, defined by the IETF. So users will be able to execute all their services when roaming as well as from their home networks. So, a multimedia session between two IMS users, between an IMS user and a user on the Internet, and between two users on the Internet is established using exactly the same protocol. Moreover, the interfaces for service developers are also based on IP protocols.

Some of the possible applications where IMS can be used are:

•Presence services

•Full Duplex Video Telephony

•Instant messaging

•Unified messaging

•Multimedia advertising

•Multiparty gaming

•Videostreaming

•Web/Audio/Video Conferencing

•Push-to services, such as push-to-talk, push-to-view, push-to-video



Effectively, IMS provides a unified architecture that supports a wide range of IP-based services over both packet- and circuit-switched networks, employing a range of different wireless and fixed access technologies. A user could, for example, pay for and download a video clip to a chosen mobile or fixed device and subsequently use some of this material to create a multimedia message for delivery to friends on many different networks. A single IMS presence-and-availability engine could track a user's presence and availability across mobile, fixed, and broadband networks, or a user could maintain a single integrated contact list for all types of communications.

A key point of IMS is that it is intended as an open-systems architecture: Services are created and delivered by a wide range of highly distributed systems (real-time and non-real-time, possibly owned by different parties) cooperating with each other. It is a different approach to the more traditional telco architecture of a set of specific network elements implemented as a single telco-controlled infrastructure.


Source:www.telecomspace.com/latesttrends-ims.html

Voice Changer Software – Customize the voice as you need

The technology we have today already created many tools that really helps our daily activities, for example, the Internet. When we don’t have an Internet, we often use post office to send all of our letters or documents that will need days, weeks, or even months to arrive.

However, with the help of Internet, we could send the same documents faster and cheaper, even free! This is why technology is really important for our live.

Voice changers are one of the technologies that start rising today. You probably already saw them in the movie, especially the one in Mission Impossible 3. Who could have realized before that most of the technologies we found today starting from an idea? Nobody would really dream of one day that we could be holding realtime conferencing with our counterpart who are sitting in their room , and conducting business meeting via teleconference technology ! The idea that keeps bugging in the head, with a good execution will make it a worthy product.


Source:tnetech.net

3G Tutorial

Original 3G Tutorial – January 2003

By Brough Turner and Marc Orange, NMS Communications

You may download our complete 120+ slide tutorial on 3rd Generation Mobile Wireless Systems, in PDF (2 MB) or PowerPoint *(6 MB) formats. This comprehensive slide set on 3G communications will benefit both the technical and general telecom audience. The tutorial reviews the history and evolution of mobile radio, evolving network architectures, services, applications, and business models. Alternative 3G paths and potential for convergence in the evolving network are reviewed in detail.

*NOTE: The PowerPoint version is roughly three times as large as the PDF version, and requires Microsoft PowerPoint to display. The advantage of the PowerPoint version, however, is that many of the graphical slides "build" on consecutive keystrokes, illustrating message flows or the evolution of network architectures.

Here is an outline of the slide set:

A brief history of cellular mobile telephony
The 3G vision and variants that are included within "3G"
2G and 3G radio technologies and how today's systems evolve to 3G
Core network architectures and their evolution to 3G
The potential for global roaming
New applications and application frameworks
Business models: what's really happening and how is the 3G vision likely to unfold?
Evolving services in 2.5G and 3G: SMS, EMS, MMS Messaging; location and presence; video and IP multimedia


Source:www.nmscommunications.com/DevPlatforms/OpenAccess/Technologies/3G324MandIPVideo/3GTutorial.htm

MS Office applications

Word
Microsoft Word Tutorial
A definitive, step-by-step guide on the basic features of Microsoft Word. This online tutorial is great for those new to the world of computers. It shows you how to do everything from saving and printing documents to adding more stylish and personal touches to your work.

Excel
NO erasers! NO new formulas! NO calculators! Learn how to use Excel with these helpful tutorials, and spreadsheets will never scare you again:

•Basic Excel Tutorial
•Microsoft Excel 2002 Tutorial
•Microsoft Excel Tutorial
PowerPoint
Microsoft PowerPoint 2003
This course of video tutorials shows you how to create presentations, format text, add links, images, animation and media clips to bring your slides to life.

These tutorials covers the basics of creating a PowerPoint presentation, the drawing toolbar, how to apply a design, colour schemes, transitions and much more.

•Technology for Teachers PowerPoint Tutorial
•PowerPoint 2000 Tutorials
•Microsoft PowerPoint Tutorial

Source:www.ltscotland.org.uk/ictineducation/ictadvice/onlinetutorials/index.asp#basic

1.07 Analog Circuits

Telephones transmit information over copper wires using voltage
Voltage is a representation or analog of speaker’s voice
“Analog” circuit


The technique for representing information on an ordinary local loop is called analog. This term is often thrown about with little regard for its actual meaning, so we’ll spend a bit of time understanding of what is meant by “analog”.
The term analog comes from the design of the telephone. A microphone in the telephone handset is placed in the path of the sound pressure waves coming out of the speaker’s throat. As the sound pressure waves hit the microphone, they change its electrical characteristics. We use the fact that the electrical characteristics of the microphone change as the sound pressure waves hit it to make a voltage on the telephone wires change.
This voltage is a representation or analog of the sound pressure waves. This is all we mean by analog: representation. The voltage on the wires is an analog of the sound pressure waves coming out of the speaker’s throat.
People then stretch the terminology to call the two copper wires which form the telephone line an analog circuit, which is not very accurate. The only thing analog in this story is the method for representing information on the copper wires. We can use digital techniques on the same wires.


Source:www.telecommunications-tutorials.com/tutorial-analog-circuits.htm

1.05 The Public Switched Telephone Network

Many communication technologies are based on those used in the Public Switched Telephone Network (PSTN), so regardless of whether you're interested in voice, data or networking, it is important to have an understanding of the structure and operation of the telephone network.
We begin with a basic model for the telephone network and will build on it in subsequent discussions. At the top of the diagram, we have a telephone and a telephone switch. The telephone is located in a building called a Customer Premise (CP), and the telephone switch is located in a building called a Central Office (CO). One could refer to the telephone as Customer Premise Equipment (CPE).
The telephone is connected to the telephone switch with two copper wires, often called a local loop or a subscriber loop, or simply a loop. This a dedicated access circuit from the customer premise into the network. We usually have the same arrangement at the other end, with the far-end telephone in a different customer premise and the far-end telephone switch usually in a different central office.
Copper is a good conductor of electricity - but not perfect: it has some resistance to the flow of electricity through it. Because of this, the signals on the loop diminish in intensity or attenuate with distance, and if the loop were too long, you wouldn't be able to hear the other person. The maximum resistance allowed is usually 1300 ohms, which works out to about 18,000 feet or 18 kft, which is 3 miles or 5 km on standard-thickness 26-gauge cable, but could be as long as 14 miles or 22 km on thicker 19-gauge cable. Thus, COs traditionally had a serving area of three miles radius around them, about 27 square miles or 75 km2. With suburban sprawl, we can't build COs every five miles, so in practice, new subdivisions are served from remote switches, which are low-capacity switches in small huts or underground controlled environment vaults. The remote provides telephone service locally on the loops in the subdivision. The remote and the loops are connected back to the nearest CO via a loop carrier system that uses fiber or radio.
Telephone switches are connected with trunks. While subscriber loops are dedicated access circuits, trunks are shared connections between COs. To establish a connection between one customer premise and another, the desired network address (telephone number) is signaled to the network (to the CO switch or remote) over the loop, then the switch seizes an unused trunk circuit going in the correct direction and the connects the loop to that trunk - for the duration of the call. When one end or the other hangs up, the trunk is released for someone else to connect between those two COs. This method for sharing the trunks is known as circuit switching. It was called dial-up when telephones had rotary dials. It is important to note that even though today there may be digital switching and digital transmission, the last 3 mi. / 5 km of the network, the subscriber loop, most often still has its original characteristics, which date back to the late 1800s (!).
Voice and data equipment which connects to the PSTN over regular telephone lines must work within the characteristics of the local loop, so an understanding of the characteristics and limitations of the local loop is essential.


Source:www.telecommunications-tutorials.com/tutorial-PSTN.htm

Optical Networking Tutorial Reference

1. ATM Fundamentals ýLearn the basics of ATM

2. Communication Tutorials from the University of New Hampshire Interoperability Lab ýThis site delivers a host of communication design tutorials. Topics include MPLS, Gigabit Ethernet/10 Gigabit Ethernet, WLANs, IP, ATM, ADSL, DOCSIS, and more

3. DWDM Overview ýA look at how DWDM works and why itýs important

4. DWDM Tutorial ýCienaýs look at the inner workings of DWDM

5. Gigabit Ethernet Overview -10 Gigabit Ethernet Allianceýs look at Gigabit Ethernet technology

6. Next-Gen Backplanes Overview ýSite providing technical overviews of some hot new backplane architectures

7. Optical Switching Tutorial ýTechnical look at optical switching

8. Resilient Packet Ring RPR White Paper ýTake a close look at the emerging RPR spec

9. SONET Tutorial ýTechnical overview of the Sonet specifications

Source:www.commsdesign.com/centers/optical_tutorials.html

Optical Networking

What Is Optical Networking?
As the name suggests, optical networks form a class of networks where optical, rather than electronic, components are the building blocks of the network. Compared to metallic cable, fiber optic systems offer greater bandwidths, lower attenuation, and no crosstalk or electrical interference. Those advantages have led to the dramatic growth of fiber optic systems worldwide. Today, nearly all long-haul telecommunications depend on the use of optical networks for their large capacity and robust performance.

Standards
Standards for fiber optic cable and other optical components have been developed over the last 20 years primarily by the American National Standards Institute (ANSI) and the International Telecommunications Union (ITU). Standards for fiber optic transmission have been developed initially in North America under the name Synchronous Optical Network (SONET) and later by the ITU using the name Synchronous Digital Hierarchy (SDH).

Historical Milestones

* 1958: Discovery of laser
* Mid-60s: Demonstration of guided wave optics
* 1970: Production of low-loss fibers, which made long-distance optical transmission possible
* 1970: Invention of semiconductor laser diode, which made highly refined optical transceivers possible
* 70s-80s: Use of fiber in telephony: SONET/SDH standards from ITU
* Mid-80s: LANs/MANs: broadcast-and-select architectures
* 1988: First trans-Atlantic optical fiber laid
* Late-80s: Development of EDFA (optical amplifier), which greatly alleviated distance limitations
* Mid/late-90s: DWDM systems explode
* Late-90s: Intelligent Optical networks
* 20?? Soliton transmission with optical TDM

Optical Networking: Why?
The “traditional" networks consist, for the most part, of a collection of electronic switches interconnected by point-to-point optical fiber links, which can span local, metropolitan, or wide area networks. To accommodate continually increasing demand for bandwidth and flexibility, such networks are being enhanced by adding more fibers and switches, increasing the bit rate per fiber, and upgrading the switches' size, throughput and functionality. Such enhancements eventually lead to very large and complex networks that are difficult and expensive to construct, operate and maintain. Recent and emerging advances in optical technology promise revolutionary all-optical networks capable of providing improved economy, flexibility and robustness while still capable of making use of the large existing fiber base.

Principles and Operation
An optical fiber is a cylindrical waveguide made of two transparent materials each with a different index of refraction. The two materials, usually high-quality glass, are arranged concentrically to form an inner core and an outer cladding. Different entry angles of the light source result in multiple modes of wave propagation. Propagation can be restricted to a single mode by using a small-diameter core.

The choice between single-mode and multimode fiber depends on the desired repeater spacing or transmission rate; single mode is the preferred choice for long-haul or high data-rate systems. The earliest form of multimode fiber was the step-index, where the core has a uniform index of refraction and the concentric cladding also has a uniform but lower index. In this case the propagation velocity within the core is constant, so that rays traveling a longer path arrive behind rays traveling a shorter path, thus producing pulse spreading, or dispersion. These dispersive effects may be remedied by constructing a fiber whose refractive index increases toward the axis, with a resulting refractive index profile that is parabolic. With a graded-index fiber, rays that travel longer paths have greater velocity than rays traveling the shorter paths due to decreasing refractive index with radial distance. The various modes then tend to have the same arrival time, such that dispersion is minimized and greater bandwidths become possible for multimode fibers.

Within the spectrum available in a fiber optic system, there are three low-loss windows, at wavelengths of approximately 850, 1300, and 1550 nm. Early applications of fiber optics for communications applications were based on the short-wavelength band of roughly 800 to 860 nm. Operation in the longer-wavelength bands, particularly at 1300 and 1550 nm, is attractive because of improved attenuation and dispersion characteristics at these wavelengths. Typically today the shorter-wavelength band is used for short-haul, low data rate systems, and the longer-wavelength bands are applied to long-haul, high data rate systems. Special fibers have been developed that shift the minimum dispersion to about 1550 nm to take advantage of lower attenuation as well as minimum dispersion. These fibers are called dispersion-shifted fibers, and are important to single-mode fiber applications.

Low-data rate, short-haul fiber optic systems tend toward multimode cable, LED transmitters, and PIN diode receivers. High-data rate, long-haul systems tend toward single-mode cable, laser diode transmitters, and avalanche photodiode receivers. Latest generation fiber optic systems have introduced innovations that have significantly improved the bandwidth and repeater spacing possible. Coherent detection via either homodyne or heterodyne techniques allows much greater bandwidths to be realized. Several wavelengths can be transmitted simultaneously in wavelength-division multiplexing, analogous to frequency-division multiplexing used in telephony. Optical amplifiers are now available that eliminate electronics and instead use specially doped fiber or semiconductor laser devices. The use of optical amplifiers will allow a fiber optic system to be upgraded in bit rate without replacement of the repeaters. Optical amplifiers have also been used to achieve ultra-long distances via soliton transmission, which is the transmission of an idealized pulse without loss of pulse shape.

Types of Optical Networks
Optical networks may be classified in several ways. Opaque optical networks include optical-electronic-optical (OEO) conversion, while in all-optical networks each connection is totally optical (or transparent) except at the end nodes.

Optical networks may be single wavelength or multiple-wavelengths (WDM). The use of SONET/SDH with a single carrier is a typical example of an opaque, single-wavelength optical network.

Finally, optical networks may be passive or active. A passive optical network (PON) is an all-optical network that utilizes only passive optical components, e.g., fibers, directional couplers, star couplers, wavelength routers, wavelength multiplexers, and filters. The intended applications are fiber-in-the-loop (local loop) and fiber-to-the-home (FTTH). The optical signaling formats in PONs can employ wavelength-division multiplexing (WDM), subcarrier multiplexing, time-division multiplexing (TDM) or any combination of these. An active all-optical network (AON) enables each of a large number of optical WDM channels (wavelengths) to propagate from source to destination over long distances and high bit rates without optical-to-electronic format conversion within the network.

Optical Network Architecture
There are two standard optical architectures, linear and ring, both of which can provide network protection and restoration of services. SONET rings are the most widely deployed architecture. They can be thought of linear networks folded back to create a loop or ring. But unlike linear architectures, rings are designed to guarantee automatic restoration of services when cable or nodes fails, by use of loops around the failed component. Because of this automatic protection against failures, these rings are called self-healing. There are several SONET ring architectures that depend on the number of fibers, transmission direction, and level of switching protection.

Originally developed in the United States, the SONET standard was adopted by the ITU-T but renamed as the Synchronous Digital Hierarchy (SDH). These standards provide a complete set of specifications to allow national and international connections at various levels. Optical interfaces are defined that provide a universal fiber interface and permit mid-span interconnection of different vendor equipment. A standardized signal structure allows any existing hierarchical rates (for example, DS-1, DS-3, E-1, and E-3) to be accommodated. Overhead within the SONET signals facilitate synchronization, add and drop multiplexing, electronic switching, performance monitoring and network management of the composite and tributary signals. The SONET hierarchy is built on synchronous multiplexing of a basic SONET rate of 51.84 Mb/s, so that higher SONET rates are simply N x 51.84 Mb/s. The basic signal structure provides sufficient flexibility to carry a variety of lower-level rates within the 51.84 Mb/s signal.

Optical Networking vis-à-vis Other Technologies

* Size and Weight: Since individual optic fibers are typically only 125 μm in diameter, a multiple fiber cable can be made that is much smaller than corresponding metallic cables.

* Bandwidth: Fiber optic cables have bandwidths that can be orders of magnitude greater than metallic cable. Low data rate systems can be eas¬ily upgraded to higher rate systems without the need to replace the fibers. Upgrading can be achieved by changing light sources (LED to laser), improving the modulation technique, improving the receiver, or using wavelength division multiplexing.

* Repeater spacing: With low-loss fiber optic cable, the distance between repeaters can be significantly greater than in metallic cable systems. More¬over, losses in optical fibers are independent of bandwidth, whereas with coaxial or twisted pair cable the losses increase with bandwidth. Thus this advantage in repeater spacing increases with the system’s bandwidth.

* Electrical isolation: Fiber optic cable is electrically nonconducting, which eliminates all electrical problems that now beset metallic cable. Fiber optic systems are immune to power surges, lightning induced currents, ground loops, and short circuits. Fibers are not susceptible to electro¬magnetic interference from power lines, radio signals, adjacent cable sys¬tems, or other electromagnetic sources.

* Crosstalk: Because there is no optical coupling from one fiber to another within a cable, fiber optic systems are free from crosstalk. In metallic cable systems, by contrast, crosstalk is a common problem and is often the limiting factor in performance.

* Environment: Properly designed fiber optic systems are relatively unaf¬fected by adverse temperature and moisture conditions and therefore have application to underwater cable. For metallic cable, however, mois¬ture is a constant problem particularly in underground (buried) applica¬tions, resulting in short circuits, increased attenuation, corrosion, and increased crosstalk.

* Reliability: The reliability of optical fibers, optical drivers, and optical receivers has reached the point where the limiting factor is usually the associated electronics circuitry.

* Cost: The numerous advantages listed here for fiber optic systems have resulted in dramatic growth in their application with attendant reductions in cost due to technological improvements and sales volume.

* Frequency allocations: Fiber (and metallic) cable systems do not require frequency allocations from an already crowded frequency spectrum. Moreover, cable systems do not have the terrain clearance, multipath fading, and interference problems common to radio systems.

Business Implications and Applications
Today fiber optic systems are much more cost effective than metallic cable, satellite, and radio for long haul, high bit rate applications. Fiber optic cable is also expected eventually to overtake metallic cable in short haul applications, includ¬ing metro facilities and local networks. One final cost factor in favor of fiber optics is the choice of material, namly silicon, which of course is one of the earth's most abundant elements, versus copper, which may someday be in short supply, or the radio spectrum, which is already in short supply.

Source: www.eogogics.com/talkgogics/tutorials/optical-networking

packet switching

Refers to protocols in which messages are divided into packets before they are sent. Each packet is then transmitted individually and can even follow different routes to its destination. Once all the packets forming a message arrive at the destination, they are recompiled into the original message.

Most modern Wide Area Network (WAN) protocols, including TCP/IP, X.25, and Frame Relay, are based on packet-switching technologies. In contrast, normal telephone service is based on a circuit-switching technology, in which a dedicated line is allocated for transmission between two parties. Circuit-switching is ideal when data must be transmitted quickly and must arrive in the same order in which it's sent. This is the case with most real-time data, such as live audio and video. Packet switching is more efficient and robust for data that can withstand some delays in transmission, such as e-mail messages and Web pages.

A new technology, ATM, attempts to combine the best of both worlds -- the guaranteed delivery of circuit-switched networks and the robustness and efficiency of packet-switching networks.

Source:www.webopedia.com/TERM/p/packet_switching.html

Global System for Mobile Communication (GSM)

1. Introduction: The Evolution of Mobile Telephone Systems

Cellular is one of the fastest growing and most demanding telecommunications applications. Today, it represents a continuously increasing percentage of all new telephone subscriptions around the world. Currently there are more than 45 million cellular subscribers worldwide, and nearly 50 percent of those subscribers are located in the United States. It is forecasted that cellular systems using a digital technology will become the universal method of telecommunications. By the year 2005, forecasters predict that there will be more than 100 million cellular subscribers worldwide. It has even been estimated that some countries may have more mobile phones than fixed phones by the year 2000 (see Figure 1).

Figure 1. Cellular Subscriber Growth Worldwide

The concept of cellular service is the use of low-power transmitters where frequencies can be reused within a geographic area. The idea of cell-based mobile radio service was formulated in the United States at Bell Labs in the early 1970s. However, the Nordic countries were the first to introduce cellular services for commercial use with the introduction of the Nordic Mobile Telephone (NMT) in 1981.

Cellular systems began in the United States with the release of the advanced mobile phone service (AMPS) system in 1983. The AMPS standard was adopted by Asia, Latin America, and Oceanic countries, creating the largest potential market in the world for cellular.

In the early 1980s, most mobile telephone systems were analog rather than digital, like today's newer systems. One challenge facing analog systems was the inability to handle the growing capacity needs in a cost-efficient manner. As a result, digital technology was welcomed. The advantages of digital systems over analog systems include ease of signaling, lower levels of interference, integration of transmission and switching, and increased ability to meet capacity demands. Table 1 charts the worldwide development of mobile telephone systems.

Year	Mobile System
1981	Nordic Mobile Telephone (NMT) 450
1983	American Mobile Phone System (AMPS)
1985	Total Access Communication System (TACS)
1986	Nordic Mobile Telephony (NMT) 900
1991	American Digital Cellular (ADC)
1991	Global System for Mobile Communication (GSM)
1992	Digital Cellular System (DCS) 1800
1994	Personal Digital Cellular (PDC)
1995	PCS 1900—Canada
1996	PCS—United States

Table 1. The Development of Mobile Telephone Systems

2. GSM

Throughout the evolution of cellular telecommunications, various systems have been developed without the benefit of standardized specifications. This presented many problems directly related to compatibility, especially with the development of digital radio technology. The GSM standard is intended to address these problems.

From 1982 to 1985 discussions were held to decide between building an analog or digital system. After multiple field tests, a digital system was adopted for GSM. The next task was to decide between a narrow or broadband solution. In May 1987, the narrowband time division multiple access (TDMA) solution was chosen. A summary of GSM milestones is given in Table 2.

Year	Milestone
1982	GSM formed
1986	field test
1987	TDMA chosen as access method
1988	memorandum of understanding signed
1989	validation of GSM system
1990	preoperation system
1991	commercial system start-up
1992	coverage of larger cities/airports
1993	coverage of main roads
1995	coverage of rural areas

Table 2. GSM Milestones

3. The GSM Network

GSM provides recommendations, not requirements. The GSM specifications define the functions and interface requirements in detail but do not address the hardware. The reason for this is to limit the designers as little as possible but still to make it possible for the operators to buy equipment from different suppliers. The GSM network is divided into three major systems: the switching system (SS), the base station system (BSS), and the operation and support system (OSS). The basic GSM network elements are shown in Figure 2.

Figure 2. GSM Network Elements

The Switching System

The switching system (SS) is responsible for performing call processing and subscriber-related functions. The switching system includes the following functional units.

• home location register (HLR)—The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.
• mobile services switching center (MSC)—The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.
• visitor location register (VLR)—The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.
• authentication center (AUC)—A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.
• equipment identity register (EIR)—The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

The Base Station System (BSS)

All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).

• BSC—The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.
• BTS—The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.

The Operation and Support System

The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.

Additional Functional Elements

Other functional elements shown in Figure 2 are as follows:

• message center (MXE)—The MXE is a node that provides integrated voice, fax, and data messaging. Specifically, the MXE handles short message service, cell broadcast, voice mail, fax mail, e-mail, and notification.
• mobile service node (MSN)—The MSN is the node that handles the mobile intelligent network (IN) services.
• gateway mobile services switching center (GMSC)—A gateway is a node used to interconnect two networks. The gateway is often implemented in an MSC. The MSC is then referred to as the GMSC.
• GSM interworking unit (GIWU)—The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GIWU, users can alternate between speech and data during the same call. The GIWU hardware equipment is physically located at the MSC/VLR.

4. GSM Network Areas

The GSM network is made up of geographic areas. As shown in Figure 3, these areas include cells, location areas (LAs), MSC/VLR service areas, and public land mobile network (PLMN) areas.

Figure 3. Network Areas

The cell is the area given radio coverage by one base transceiver station. The GSM network identifies each cell via the cell global identity (CGI) number assigned to each cell. The location area is a group of cells. It is the area in which the subscriber is paged. Each LA is served by one or more base station controllers, yet only by a single MSC (see Figure 4). Each LA is assigned a location area identity (LAI) number.

Figure 4. Location Areas

An MSC/VLR service area represents the part of the GSM network that is covered by one MSC and which is reachable, as it is registered in the VLR of the MSC (see Figure 5).

Figure 5. MSC/VLR Service Areas

The PLMN service area is an area served by one network operator (see Figure 6).

Figure 6. PLMN Network Areas

5. GSM Specifications

Before looking at the GSM specifications, it is important to understand the following basic terms:

• bandwidth—the range of a channel's limits; the broader the bandwidth, the faster data can be sent
• bits per second (bps)—a single on-off pulse of data; eight bits are equivalent to one byte
• frequency—the number of cycles per unit of time; frequency is measured in hertz (Hz)
• kilo (k)—kilo is the designation for 1,000; the abbreviation kbps represents 1,000 bits per second
• megahertz (MHz)—1,000,000 hertz (cycles per second)
• milliseconds (ms)—one-thousandth of a second
• watt (W)—a measure of power of a transmitter

Specifications for different personal communication services (PCS) systems vary among the different PCS networks. Listed below is a description of the specifications and characteristics for GSM.

• frequency band—The frequency range specified for GSM is 1,850 to 1,990 MHz (mobile station to base station).
• duplex distance—The duplex distance is 80 MHz. Duplex distance is the distance between the uplink and downlink frequencies. A channel has two frequencies, 80 MHz apart.
• channel separation—The separation between adjacent carrier frequencies. In GSM, this is 200 kHz.
• modulation—Modulation is the process of sending a signal by changing the characteristics of a carrier frequency. This is done in GSM via Gaussian minimum shift keying (GMSK).
• transmission rate—GSM is a digital system with an over-the-air bit rate of 270 kbps.
• access method—GSM utilizes the time division multiple access (TDMA) concept. TDMA is a technique in which several different calls may share the same carrier. Each call is assigned a particular time slot.
• speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC is to reduce the bit rate. The LPC provides parameters for a filter that mimics the vocal tract. The signal passes through this filter, leaving behind a residual signal. Speech is encoded at 13 kbps.

6. GSM Subscriber Services

There are two basic types of services offered through GSM: telephony (also referred to as teleservices) and data (also referred to as bearer services). Telephony services are mainly voice services that provide subscribers with the complete capability (including necessary terminal equipment) to communicate with other subscribers. Data services provide the capacity necessary to transmit appropriate data signals between two access points creating an interface to the network. In addition to normal telephony and emergency calling, the following subscriber services are supported by GSM:

• dual-tone multifrequency (DTMF)—DTMF is a tone signaling scheme often used for various control purposes via the telephone network, such as remote control of an answering machine. GSM supports full-originating DTMF.
• facsimile group III—GSM supports CCITT Group 3 facsimile. As standard fax machines are designed to be connected to a telephone using analog signals, a special fax converter connected to the exchange is used in the GSM system. This enables a GSM–connected fax to communicate with any analog fax in the network.
• short message services—A convenient facility of the GSM network is the short message service. A message consisting of a maximum of 160 alphanumeric characters can be sent to or from a mobile station. This service can be viewed as an advanced form of alphanumeric paging with a number of advantages. If the subscriber's mobile unit is powered off or has left the coverage area, the message is stored and offered back to the subscriber when the mobile is powered on or has reentered the coverage area of the network. This function ensures that the message will be received.
• cell broadcast—A variation of the short message service is the cell broadcast facility. A message of a maximum of 93 characters can be broadcast to all mobile subscribers in a certain geographic area. Typical applications include traffic congestion warnings and reports on accidents.
• voice mail—This service is actually an answering machine within the network, which is controlled by the subscriber. Calls can be forwarded to the subscriber's voice-mail box and the subscriber checks for messages via a personal security code.
• fax mail—With this service, the subscriber can receive fax messages at any fax machine. The messages are stored in a service center from which they can be retrieved by the subscriber via a personal security code to the desired fax number.

Supplementary Services

GSM supports a comprehensive set of supplementary services that can complement and support both telephony and data services. Supplementary services are defined by GSM and are characterized as revenue-generating features. A partial listing of supplementary services follows.

• call forwarding—This service gives the subscriber the ability to forward incoming calls to another number if the called mobile unit is not reachable, if it is busy, if there is no reply, or if call forwarding is allowed unconditionally.
• barring of outgoing calls—This service makes it possible for a mobile subscriber to prevent all outgoing calls.
• barring of incoming calls—This function allows the subscriber to prevent incoming calls. The following two conditions for incoming call barring exist: baring of all incoming calls and barring of incoming calls when roaming outside the home PLMN.
• advice of charge (AoC)—The AoC service provides the mobile subscriber with an estimate of the call charges. There are two types of AoC information: one that provides the subscriber with an estimate of the bill and one that can be used for immediate charging purposes. AoC for data calls is provided on the basis of time measurements.
• call hold—This service enables the subscriber to interrupt an ongoing call and then subsequently reestablish the call. The call hold service is only applicable to normal telephony.
• call waiting—This service enables the mobile subscriber to be notified of an incoming call during a conversation. The subscriber can answer, reject, or ignore the incoming call. Call waiting is applicable to all GSM telecommunications services using a circuit-switched connection.
• multiparty service—The multiparty service enables a mobile subscriber to establish a multiparty conversation—that is, a simultaneous conversation between three and six subscribers. This service is only applicable to normal telephony.
• calling line identification presentation/restriction—These services supply the called party with the integrated services digital network (ISDN) number of the calling party. The restriction service enables the calling party to restrict the presentation. The restriction overrides the presentation.
• closed user groups (CUGs)—CUGs are generally comparable to a PBX. They are a group of subscribers who are capable of only calling themselves and certain numbers.

Source:/www.iec.org/online/tutorials/gsm/index.asp

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