3rd Generation Wireless Systems (3G), a simple discussion

Third Generation (3G) mobile devices and services will transform wireless communications into on-line, real-time connectivity. 3G wireless technology will allow an individual to have immediate access to location-specific services that offer information on demand. The first generation of mobile phones consisted of the analog models that emerged in the early 1980s. The second generation of digital mobile phones appeared about ten years later along with the first digital mobile networks. During the second generation, the mobile telecommunications industry experienced exponential growth both in terms of subscribers as well as new types of value-added services. Mobile phones are rapidly becoming the preferred means of personal communication, creating the world's largest consumer electronics industry.

The rapid and efficient deployment of new wireless data and Internet services has emerged as a critical priority for communications equipment manufacturers. Network components that enable wireless data services are fundamental to the next-generation network infrastructure. Wireless data services are expected to see the same explosive growth in demand that Internet services and wireless voice services have seen in recent years.

Third Generation (3G) Wireless Networks 3G wireless technology represents the convergence of various 2G wireless telecommunications systems into a single global system that includes both terrestrial and satellite components. One ofthe most important aspects of 3G wireless technology is its ability to unify existing cellular standards, such as CDMA, GSM, and TDMA, under one umbrella. The following three air interface modes accomplish this result: wideband CDMA, CDMA2000 and the Universal Wireless Communication (UWC-136) interfaces. Wideband CDMA (W-CDMA) is compatible with the current 2G GSM networks prevalent in Europe and parts of Asia. W-CDMA will require bandwidth of between 5Mhz and 10  Mhz, making it a suitable platform for higher capacity applications. It can be overlaid onto existing GSM, TDMA (IS-36) and IS95 networks. Subscribers are likely to access 3G wireless services initially via dual band terminal devices. W-CDMA networks will be used for high-capacity applications and 2G digital wireless systems will be used for voice calls.

The second radio interface is CDMA2000 which is backward compatible with the second generation CDMA IS-95 standard predominantly used in US. The third radio interface, Universal Wireless Communications – UWC-136, also called IS-136HS, was proposed by the TIA and designed to comply with ANSI-136, the North American TDMA standard. 3G wireless networks consist of a Radio Access Network (RAN) and a core network. The core network consists of a packet-switched domain, which includes 3G  SGSNs and  GGSNs, which provide the same functionality that they provide in a GPRS system, and a circuit-switched domain, which includes 3G MSC for switching of voice calls. Charging for services and access is done through the Charging Gateway Function (CGF), which is also part of the core network. RAN functionality is independent from the core network functionality.  The access network provides a core network technology independent access for mobile terminals to different types of core networks and network services. Either core network domain can access any appropriate RAN service; e.g. it should be possible to access a “speech” radio access bearer from the packetswitched domain. Below is the achitecture of third generation  wireless systems.

 Evolution to 3G Wireless Technology

Initial coverage

Initially, 3G wireless technology will be deployed as "islands" in business areas where more capacity and advanced services are demanded. A complete evolution to 3G wireless technology is mandated by the end  of  2000 in Japan (mostly due to capacity requirements) and by the end of 2001 in Europe. NTT DoCoMo is deploying 3G wireless services in Japan in the third quarter  of 2000. In contrast, there is no similar mandate in North America and it is more likely that competition will drive the deployment of 3G wireless technology in that region. For example,

Nextel Communications has announced that it will be deploying 3G wireless services in North America during the fourth quarter of 2000. The implementation of 3G wireless systems raises several critical issues, such as the successful backward compatibility to air interfaces as well as to deployed infrastructure.

Interworking with 2G and 2G+ Wireless Networks

The existence of legacy networks in most regions of the world highlights the challenge that communications equipment manufacturers face when implementing next-generation wireless technology.  Compatibility and interworking between the new 3G wireless systems and the old legacy networks must be achieved in order to ensure the acceptance of new 3G wireless technology by service providers and end-users.

The existing core technology used in mobile networks is based on traditional circuit-switched technology for delivery of voice services. However, this traditional technology is inefficient for the delivery of multimedia services. The core switches for next-generation of mobile networks will be based on packet-switched technology which is better suited for data and multimedia servicesThird Generation (3G) mobile devices and services will transform wireless communications into on-line, real-time connectivity. 3G wireless technology will allow an individual to have immediate access to location-specific services that offer information on demand. The first generation of mobile phones consisted of the analog models that emerged in the early 1980s. The second generation of digital mobile phones appeared about ten years later along with the first digital mobile networks. During the second generation, the mobile telecommunications industry experienced exponential growth both in terms of subscribers as well as new types of value-added services. Mobile phones are rapidly becoming the preferred means of personal communication, creating the world's largest consumer electronics industry.

The rapid and efficient deployment of new wireless data and Internet services has emerged as a critical priority for communications equipment manufacturers. Network components that enable wireless data services are fundamental to the next-generation network infrastructure. Wireless data services are expected to see the same explosive growth in demand that Internet services and wireless voice services have seen in recent years.

Third Generation (3G) Wireless Networks 3G wireless technology represents the convergence of various 2G wireless telecommunications systems into a single global system that includes both terrestrial and satellite components. One ofthe most important aspects of 3G wireless technology is its ability to unify existing cellular standards, such as CDMA, GSM, and TDMA, under one umbrella. The following three air interface modes accomplish this result: wideband CDMA, CDMA2000 and the Universal Wireless Communication (UWC-136) interfaces. Wideband CDMA (W-CDMA) is compatible with the current 2G GSM networks prevalent in Europe and parts of Asia. W-CDMA will require bandwidth of between 5Mhz and 10  Mhz, making it a suitable platform for higher capacity applications. It can be overlaid onto existing GSM, TDMA (IS-36) and IS95 networks. Subscribers are likely to access 3G wireless services initially via dual band terminal devices. W-CDMA networks will be used for high-capacity applications and 2G digital wireless systems will be used for voice calls.

The second radio interface is CDMA2000 which is backward compatible with the second generation CDMA IS-95 standard predominantly used in US. The third radio interface, Universal Wireless Communications – UWC-136, also called IS-136HS, was proposed by the TIA and designed to comply with ANSI-136, the North American TDMA standard. 3G wireless networks consist of a Radio Access Network (RAN) and a core network. The core network consists of a packet-switched domain, which includes 3G  SGSNs and  GGSNs, which provide the same functionality that they provide in a GPRS system, and a circuit-switched domain, which includes 3G MSC for switching of voice calls. Charging for services and access is done through the Charging Gateway Function (CGF), which is also part of the core network. RAN functionality is independent from the core network functionality.  The access network provides a core network technology independent access for mobile terminals to different types of core networks and network services. Either core network domain can access any appropriate RAN service; e.g. it should be possible to access a “speech” radio access bearer from the packetswitched domain.

What Is Special about EDGE?

EDGE is a new modulation scheme that is more bandwidth efficient than the GMSK modulation

scheme used in the GSM standard. It provides a promising migration strategy for HSCSD and

GPRS. The technology defines a new physical layer: 8−PSK modulation, instead of GMSK. 8−PSK enables each pulse to carry 3 bits of information versus the GMSK 1−bit−per−pulse rate. Therefore,EDGE has the potential to increase the data rate of existing GSM systems by a factor of three.

UMTS is a part of the ITU's IMT−2000 vision of a global family of 3G mobile communications

systems. UMTS will play a key role in creating the future mass market for high−quality wireless

multimedia communications that will approach 2 billion users worldwide by the year 2010.

UMTS is a modular concept that takes full advantage of the trend of converging existing and future information networks, devices, and services, and the potential synergies that can be derived from such convergence. UMTS will move mobile communications forward from where we are today into the 3G services and will deliver speech, data, pictures, graphics, video communication, and other wideband information direct to people on the move. UMTS is one of the major new 3G mobile communications systems being developed within the framework, which has been defined by the ITU and is known as IMT−2000.

WCDMA is an ITU standard derived from CDMA and is officially known as IMT−2000 direct spread. WCDMA is a 3G mobile wireless technology offering much higher data speeds to mobile and portable wireless devices than commonly offered in today's market. WCDMA can support mobile/portable voice, images, data, and video communications at up to 2 Mbps (local area access) or 384 Kbps (wide area access). The input signals are digitized and transmitted in coded, spread−spectrum mode over a broad range of frequencies. A 5 MHz wide carrier is used, compared with a 200 kHz wide carrier for narrowband CDMA.

Mobile Internet — A Way of Life

The mobile Internet is about to enter our daily lives in a big way. It will change the way we keep in touch with our friends and family, the way we do business, the way we shop, the way we access entertainment, and the way we conduct our personal finances.

The Internet is already a part of daily life for most of us, giving us access to a vast range of

information and online services from our desktop computers. As a way of conducting business, it is also of growing importance to the global economy. Unlike today's fixed Internet, the mobile Internet will give us access to these services and applications wherever we are, whenever it suits us, from personal mobile devices.

Sources:

http://www.satmagazine.com

http://www.dryaseen.pk

Broadband Telecommunications Handbook

General Packet Radio Service (GPRS)

Life in 3G (or 2G or 4G)

Mobile phone/internet technology is evolving so rapidly that it would be nearly impossible to produce a piece that was completely up-to-date. The evolution of this technology is actually quite a bit ahead of what is readily available to the common user. So, while I write to you about GPRS, this is a technology that is already somewhat outdated even though it is still new enough not to have been adopted by many countries.

Mobile phones are categorized by generations. Older mobile phones that were made primarily for voice and sometimes photo transfer were considered to be “2nd Generation (2G)”. GPRS was a technology that evolved to allow 2G users to access the internet and also to increase interest among users in gaining more use of this option for the future. However, third generation mobile phones have now evolved, allowing for internet access at much higher speeds than their predecessors. In fact, 4G phones are already in existence. So is GPRS obsolete? No – not yet. At the moment, because of expense and licensing issues, many countries have not built 3G networks and for many that have begun to build, the coverage is still quite limited. Therefore, the international traveler should not expect to be able to access 3G features while away making GPRS a continued necessity.

GPRS is a service commonly associated with 2.5G technology. It has data transmission rates of 28 kbps or higher. GPRS came after the development of the Global System for Mobile (GSM) service, which is classified as 2G technology, and it was succeeded by the development of the Universal Mobile Telecommunication Service (UMTS), which is classified as 3G technology.

A 2.5G system may make use of 2G system infrastructure, but it implements a packet-switched network domain in addition to a circuit-switched domain. This does not necessarily give 2.5G an advantage over 2G in terms of network speed, because bundling of timeslots is also used for circuit-switched data services (HSCSD).

Cost

• Communication via GPRS is cheaper than through the regular GSM network. Instant-messenger services and mobile email facilities allow you to send longer messages for cheaper rates through the GPRS connection, as opposed to transmitting messages in SMS or short message service. Customers only pay for the amount of data transported, and not for the duration of the Internet connection.

Constant Connection

• Through GPRS technology, users are constantly connected to the Internet. As GPRS services are available wherever there is GSM coverage, it allows you to connect to the Internet even when other services such as 3G or HSDPA are not available.

Mobility

• GPRS provides wireless access to the Internet from any location where there is a network signal. This enables you to surf the Internet on your laptop or phone, even in remote areas.

Speed

• Although new, faster technology exists today, GPRS is still faster than the older WAP (Wireless Application Protocol) and regular GSM services. GPRS data is transferred at speeds ranging from 9.6 kilobytes per second up to 114kbps.

Simultaneous Use

• When you access the Internet through GPRS, it does not block incoming calls through the GSM network. This enables you to make or receive voice calls while you are browsing the Internet or downloading data.

What Kind of GPRS Phone?

If you read our article on Cellphone Basics , you will already know the importance of getting a GSM phone. This is no different when wanting GPRS capability. GSM systems are the only systems where GPRS is currently in use.

There are 3 classes of capability you may find when searching GSM/GPRS phones:

Class C phones cannot transfer voice (GSM) and data (GPRS) at the same time. With a class C device the user must use only one service at a time and switch the phone manually to change over.

Class B phones connect to both GSM and GPRS simultaneously but only one service at a time can be used. The phone automatically resets after the call or connection is finished and there is no need for the user to switch the phone manually.

Class A phones can use both GSM and GPRS simultaneously allowing the user to speak and transfer data all at the same time.

Most phones on the market right now are Class B phones.

Making it Work

So you have your GPRS enabled phone and a GPRS enabled SIM card. The next step will be to learn how to use the service. There are 3 methods for connecting your mobile phone to your laptop.

1. Data-cable – yes, the good old fashioned wire method. This is reliable and not too inconvenient when traveling.
2. Infrared – requires the alignment of the IR port on the laptop with the IR port on the phone. However, if you are on a jostling train or in a limited space this might be difficult. For instance, some laptops have their IR port in front of the keyboard making typing impractical. You also need a phone that includes IR connecting.
3. Bluetooth – this can be an ideal option as it allows connection just through proximity but will require configuration of a Bluetooth enabled phone and laptop with Bluetooth or Bluetooth card. In addition, this option may slow your connection and run your phone battery down fast.

Once the phone and laptop are communicating, you will need to access the GPRS network. Accessing GPRS networks usually involves dialing in access codes and passwords. These codes and passwords will vary depending on your service provider and country in which you are traveling. Instructions for accessing the network should be provided by this service.

And, voila! You’re in business, nomad.

Sources:

www.telecomspace.com

www.webopedia.com

www.nuntius.com

www.lteworld.org

Broadband Telecommunications Handbook

MMDS and LMDS

The demand for affordable, fast data connections is increasing both in the United States and around the globe. There are several reasons why faster connections are not readily available and affordable. They are a complex mix of entrenched interests of the incumbent connection providers, the high costs of wireline upgrades and the associated slow pace, cumbersome regulations, and tariffs; and the difficulty of forcing more data through already crowded data pipes.

A new wireless broadband point-to-multipoint microwave technology called local multipoint distribution service (LMDS) stands ready to bypass those barriers to readily available broadband connections. In the United States, incumbent connection providers were prevented from owning or controlling the large block of LMDS microwave spectrum in their territory for a period of 36 months (from the auction); consequently, the chances of entrenched interests limiting bandwidth availability are small. In Canada, local multipoint communication service (LMCS) applications from entrenched landline providers were not accepted (see sidebar, "More Communication Choices for Canadians"). The 1 GHz of LMCS spectrum was awarded to newly established companies and consortiums.

LMDS is a wireless broadband service and consequently does not require landline wire upgrades, which makes it affordable when compared with landline technologies. And LMDS is lightly regulated and can be used for two-way transmission of voice, video, and data. Finally, the LMDS spectrum is immense. This large amount of radiofrequency (rf) spectrum allows operators to realize data rates above 1 billion bits per second (bps).

LMDS also provides high-capacity point-to-multipoint data access that is less investment-intensive. LMDS, with its wireless broadband delivery, combined with the significant amount of spectrum allocated, allows for a very high quality communications services. It transmits milliwave signals within small cells. Since it has been tested by the U.S. military and corporate pioneers like SpeedUs.com, Inc., it is undoubtedly a proven technology.

The key to business and consumer acceptance of LMDS as an attractive solution is the affordability and availability of the systems. Because of its point-to-multipoint nature, one LMDS cell with a single hub transceiver can serve hundreds or thousands of simultaneous customers. The affordability of the overall LMDS solution is therefore largely dependent on the cost of the customer premises equipment.

Internationally, governments are working quickly to enable the use of high-gigahertz microwave spectrum for wireless broadband data, voice, and video transport. In many ways, the opportunities internationally are greater than those domestically because of the poor state of the communications infrastructure internationally and the desire of the ministries of telecommunications to move rapidly to make their systems competitive with those in the United States.

LMDS Advantages

• Ease and speed of deployment

• Fast realization of revenue

• Easy network management

• Large bandwidth

• Small cell size

LMDS Disadvantages

• Requires Line of Sight

• Affected by rain, foliage and reflections

• Many cell sites are required

• Multiple cell sites cause interference

• Security concerns

MMDS stands for Mutlichannel Multipoint Distribution Service and this is most commonly termed as Wireless Cable. MMDS is really a wireless communications technology which can be used in high speed networking within the telecoms market. It is frequently utilized rather than a programming reception involving cable connection and is also most favored in India, Brazil, Australia, Pakistan, america, Barbados, Mexico, Russia, Belarus, Lebanon and various areas. Throughout these locations it is primarily used in non-urban parts which happen to have sparse population. This really is caused by a defieicency of regular use cables. Thats usually where laying wires is most popular subsequently MMDS assistance are usually accessible.

MMDS is specified by means of UHF or ultra-high-frequency communications. It operates inside the given FCC accredited frequency. In the usa the FCC is split up into Basic Trading Areas or BTA's which inturn sell the right to transfer MMDS in areas where service providers can be obtained.

MMDS uses BRS bands of 2.1 GHz and also from 2.5 GHz to 2.7 GHz microwave frequencies. Rooftop microwave antennas are widely used to pass on data indicators and also BRS-delivered television reception. Antennas are attached to a transceiver or down-converter which in turn obtains and transmits microwaves signals. They're afterward converted to the frequencies which can be compatible with TV tuners. This is similar to the way indicators are transformed into the frequencies for satellite dishes but instead these are suitable for TV coaxial cables.

The MMDS band is afterward segregated into channels of 33 6 MHz. Because of this these kinds of people can own a number of other channels, radio, multiplex televisions along with Internet data. Digital cable channels are then capable of regulate 64QAM as well as 30.34 Mbps together with 256QAM modulation having 42.88 Mbps.

MMDS Disadvantages

• Limited two way capabilities (upstream bandwidth is limited)

• Shadowing and interference prevents ubiquitous coverage

• Normal radio security concerns

MMDS vs. LMDS

• Supports high bandwidths but gets affected by
  rain, foliage and reflections
• Requires Line of Sight conditions to serve
• Operates in smaller radius and hence more
  number of cell sites required for a city coverage
• Supports relatively lower bandwidths but does
  not get affected by the rain or foliage conditions
• Also works in Near Line of Sight (NLoS)
  conditions
• Supports larger coverage area hence few cell
  sites required to cover a city

Originally designed for wireless digital television transmission, LMDS and multipoint microwave distribution system (MMDS) were predicted to serve wireless broadband subscription television needs. MMDS is also a broadband wireless communications service that operates at lower frequencies. Usually, LMDS operates at frequencies above the 10-GHz range and MMDS at frequencies below the 10-GHz range. Later on they were extended to offer other interactive services.

Sources:

http://www.networkcomputing.com/netdesign/bb3.html

http://www.eetimes.com/electronics-news/4039196/LMDS-MMDS-race-for-low-cost-implementation

http://www.networkcomputing.com/netdesign/1223wireless1.html

http://quantumwimax.com/index.php?page=History-of-Wimax

broadband telecommunications handbook

Microwave and Radio Based Systems, a simple discussion

Microwave system is always a Line-of-Sight system which is based on the visibility between the Transmit End & Receive End.

•It is mandatory to avoid any physical obstruction in the line of sight design.

•The obstructions would include High rise buildings, Hillocks & Vegetations like tall trees.

•To overcome these obstructions we may have to increase the number of towers or increase

the height of the towers.

•The microwave systems can operate in LOS mode for around a few hundred meters to over 60 Km.

•So it is imperative to determine the LOS over the complete hop length between

 he Transmit end & Receive end

When a microwave signal is sent it travels from the transmit end to the receive end the signals take the form of an ellipsoid. The size of ellipsoid depends on the frequency of operation. The higher the operating frequency the smaller is the size of the ellipsoid. The size of the ellipsoid is biggest at the center of the LOS. If any obstruction is allowed into the fresnel zone, the obstruction will reflect the signal. This reflected signal will cancel out and distort the main signal thus reducing the strength of the main signal.

The height of the LOS should be high enough to not permit any obstructions to enter the fresnel zone.

The microwave signals do not travel in a straight line but tend to travel in a curved path following the curvature of the earth. The reason for the effect is the refractive index of the atmosphere reduces with the increase in altitude. This is known as the earth bulge factor ( k).

The antenna is a device that converts the electrical signals into the electromagnetic waves that propagate through free space.

 In Microwave systems the Standard Antenna used are Parabolic Antennas.

Antenna gain is a measure of the antenna’s ability to transmit the waves in a specific direction instead of in all direction.

Whats with Microwave Radio Based System?

Series 875 LAN+T1 Microwave Radio System is a low cost, high bandwidth, radio system capable of transporting full bandwidth LAN traffic and T1 telephony up to 3 miles. The radio system offers the following features:

The Ethernet interface of the radio system supports all 802.3 protocols and includes an AUI connection to network devices and BNC connections to the microwave unit. The T1 data interface of the radio system supports Bell Standard 100 ohm, two-wire twisted pair designed to connect to telephony and multimedia interfaces. No on-site programming is required and no additional test equipment is needed.

The Series 875 LAN+T1 system provides point-to-point connections for hubs, bridges, routers and repeaters allowing full 10 Mbps Ethernet connectivity. The 875 LAN+T1 interface has been designed to connect directly to standard T1 PBX's, Channel Banks or Telecom Multiplexers without the need for additional equipment. 

The Series 875 LAN+T1 is designed for rapid installation and alignment without special tools or test equipment. Rugged, modular radio design for ease of service and field support for years of trouble free operation in harsh weather conditions.

Why Microwave Radio Based Sytem?

Microwave Radio System offers the following features Standard IEEE 802.3 LAN Interface Standard Bell T1 1.544 Mbps Interface Standard 10 Mbps or Full Duplex Ethernet Lightweight package 7 Ibs (3.2kg) Interference free operation -30C to +55C temperature range EMI/RFI protection Compact Size 9" antenna Easy to Install Easy to Maintain

Sources:

http://www.arcelect.com/Wireless_875_LAN+T1_23GHz.htm

http://www.ehow.com/list_6137210_microwave-radio-communications-advantages-disadvantages.html

nystec.com/files/Microwave.pdf

http://www.dpstele.com/dpsnews/techinfo/microwave_knowledge_base/microwave_system.php

Broadband Telecommunications Handbook

Digital Subscriber Line (xDSL)

 DSL or xDSL is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, but as of 2009 the term digital subscriber line has been widely adopted as a more marketing-friendly term for ADSL, the most popular version of consumer-ready DSL. DSL can be used at the same time and on the same telephone line with regular telephone, as it uses high frequency bands, while regular telephone uses low frequency.

The DSL family includes several variations of what is known as digital subscriber line. The lower case x in front of the xDSL stands for the many variations.

ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services. Most people consider ADSL as a transmission system instead of a modification to the existing transmission facilities. the IDSL technique is all digital operating at two channels of 64 Kbps for voice or non voice operation and a 16 Kbps data channel for signaling, control, and data packets. ISDN was very slow to catch on, but the movement to the Internet created a whole new set of demands for the carriers to deal with. HDSL was developed as a more efficient way of transmitting over the existing copper wires. HDSL does not require the repeaters on a local loop of up to 12K. Bridge taps will not bother the service, and the splices are left in place. This means that the provider can offer HDSL as a more efficient delivery of 1.544 Mbps. The modulation rate on the

HDSL service is more advanced. The goal of the DSL family was to continue to support and use the local copper cable plant. It was developed to provide high−speed communications on that single cable pair but at distances no greater than 10K. SDSL uses only one pair of wires, but is limited in its distance to provide duplex, high−speed communications. Not all users require symmetrical speeds at the same time. ADSL was, therefore, designed to support differing speeds in both directions over a single cable pair at distances of up to 18K. Because the speeds requested are typically for access to the Internet, most users look for higher speeds in a download direction and the lower speed for an upward direction. If the line conditions vary, the speed will be dependent on the sensitivity of the equipment. In order to achieve variations in the throughput and be sensitive to the line conditions, RADSL was developed. This gives the flexibility to adapt to the changing conditions and adjust the speeds in each direction to potentially maximize the throughput on each line.

CDSL does not use, nor need, a splitter on the line. Moreover, speeds of up to 1 Mbps in the download direction and 160 Kbps in the upward direction are provided. One of the most significant improvements SHDSL brings to the business market is increased reach — at least 30 percent greater than any earlier symmetric DSL technology. Furthermore, SHDSL supports repeaters, which further increase the reach capability of this technology. Clearly, changes will always occur as we demand faster and more reliable communications capabilities. It was only a matter of time until some users demanded higher−speed communications than was offered by the current DSL technologies. As a result, VDSL was introduced to achieve the higher speeds.

The download speed of consumer DSL services typically ranges from 256 kilobits per second (kbit/s) t o24,000 kbit/s, depending on DSL technology. Line conditions and service-level implementation, typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line and equal to download speed for the rarer Symmetric Digital Subscriber Line

Voice and data

Comparing DSL & Dial-Up

DSL (VDSL) typically works by dividing the frequencies used in a single phone-line into two primary "bands". The ISP data uses the high-frequency band (25 kHz and above) whereas the voice utilizes the lower-frequency band (4 kHz and below). (See the ADSL article for information on the subdivision of the high-frequency band.) The user typically installs a DSL filter on each phone outlet. This filters out the high frequencies from the phone line, so that the phone only sends or receives the lower frequencies and the user hears only the human voice. The DSL modem and the normal telephone equipment can be used simultaneously on the line without interference from each other provided filters are used for all voice devices.

History and science

DSL, like many other forms of communication, stems directly from Claude Shannon's seminal 1948 scientific paper: A Mathematical Theory of Communication. Employees at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone-company central offices and customers on conventional twisted pair cabling.

U.S. telephone companies promote DSL to compete with cable Internet. the first DSL service ran over a dedicated "dry loop", but when the FCC required the incumbent local exchange carriers (ILECs) to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over Unbundled Network Element, this allows a single pair to carry data (via a digital subscriber line access multiplexer [DSLAM]) and analog voice (via a circuit switched telephone switch) at the same time.

Operation

Regular DSL

Telephone engineers initially developed the local loop of the public switched telephone network (PSTN) to carry POTS voice communication and signaling: no requirement for data communication as we know it today existed. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as voice band or commercial bandwidth.

The local telephone exchange or central office generally digitizes speech signals into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore, according to the Nyquist theorem, any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).

Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a properly installed and working load coil, while voice service cannot be maintained past a certain distance without such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this, phone companies are endeavoring to remove load coils on copper loops that can operate without them, and conditioning lines to avoid them through the use of fiber to the neighborhood or node FTTN.

The commercial success of DSL and similar technologies largely reflects the advances made in electronics that, over the past few decades, have been getting faster and cheaper even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive.

Naked DSL

Dry-loop DSL or "naked DSL," which does not require the subscriber to have traditional land-line telephone service, started making a comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC, and Verizon's merger with MCI, those telephone companies have an obligation to offer naked DSL to consumers.

Even without the regulatory mandate, however, many ILECs offer naked DSL to consumers. The number of telephone landlines in the US dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has grown to 195 million.. This lack of demand for landline service has resulted in the expansion of naked DSL availability.

Typical setup and connection procedures

Physical connection must come first. On the customer side, the DSL Transceiver, or ATU-R, or more commonly known as a DSL modem, is hooked up to a phone line. The telephone company connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single box. The location of the DSLAM depends on the telephone company but it cannot be located too far from the user because of attenuation, the loss of data due to the large amount of electrical resistance encountered as the data moves between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.

When the DSL modem powers up it goes through a sync procedure. The actual process varies from modem to modem but generally involves the following steps:

1.     The DSL Transceiver does a self-test.
2.   The DSL Transceiver checks the connection between the DSL Transceiver and the computer.
3.    The DSL Transceiver then attempts to synchronize with the DSLAM. Data can only come into the computer when the DSLAM and the modem are synchronized.

Modern DSL gateways have more functionality and usually go through an initialization procedure very similar to a PC boot up.

Equipment

The customer end of the connection consists of a Terminal Adaptor or in layman's terms “DSL Modem” This converts data from the digital signals used by computers into a voltage signal of a suitable frequency range which is then applied to the phone line.

In some DSL variations (for example, HDSL), the terminal adapter connects directly to the computer via a serial interface, using protocols such as RS-232 or V.35. In other cases (particularly ADSL), it is common for the customer equipment to be integrated with higher level functionality, such as routing, firewalling, or other application-specific hardware and software. In this case, the entire equipment is usually referred to as a DSL router or DSL gateway.

Some kinds of DSL technology require installation of appropriate filters to separate, or "split", the DSL signal from the low frequency voice signal. The separation can take place either at the demarcation point, or with filters installed at the telephone outlets inside the customer premises.

At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. In the case of ADSL, the voice component is also separated at this step, either by a filter integrated in the DSLAM or by a specialized filtering equipment installed before it. The DSLAM terminates all connections and recovers the original digital information.

Protocols and configurations

Many DSL technologies implement an ATM layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over Ethernet or ATM (PPPoE or PPPoA), while authenticating with a userid and password and using PPP mechanisms to provide network details.

DSL technologies

The line-length limitations from telephone exchange to subscriber impose more restrictions on higher data-transmission rates. Technologies such as VDSL provide very high speed, short-range links as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures). Technologies likes GDSL can further increase the data rate of DSL. Fiber Optic technologies exist today that allow the conversion of copper based IDSN, ADSL and DSL over fiber optics.

Example DSL technologies (sometimes called xDSL) include:

1.    ISDN Digital Subscriber Line (IDSL), uses ISDN based technology to provide data flow that is slightly higher than dual channel ISDN.

2.    High Data Rate Digital Subscriber Line (HDSL / HDSL2), was the first DSL technology that uses a higher frequency spectrum of copper, twisted pair cables.

3.    Symmetric Digital Subscriber Line (SDSL / SHDSL), the volume of data flow is equal in both directions..

4.    Asymmetric Digital Subscriber Line (ADSL), the volume of data flow is greater in one direction than the other.

5.    Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and noise tolerance by sacrificing up stream speed

6.    Very High Speed Digital Subscriber Line (VDSL)

7.    Etherloop Ethernet Local Loop

8.    Gigabit Digital Subscriber Line (GDSL), based on binder MIMO technologies.

Transmission methods

Transmission methods vary by market, region, carrier, and equipment.

1.      2B1Q: Two-binary, one-quaternary, used for IDSL and HDSL
2.     CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996 for ADSL, used for HDSL
3.    DMT: Discrete multitone modulation, the most numerous kind, also known as OFDM (Orthogonal frequency-division multiplexing)

 Sources:

Broadband Communications Handbook

www.aaxnet.com/topics/cblmdm.html

www.jawin.com/protocolxDSL.html

www.business.com

www.dsl-direct.com/