Low−Earth−Orbit Satellites (LEOs)

  In the year 1989, Motorola announced a proposal to launch a series of 77 satellites in low Earth orbit starting in the year 1994. In the year 1992, a frequency spectrum between 1610 and 1626.5 MHz was allocated for the use by LEO satellites. 

  When we say Low Earth Orbit (LEO, it refers to a satellite which orbits the earth at altitudes between (very roughly) 200 miles and 930 miles. Its satellites must travel very quickly to resist the pull of  gravity approximately 17,000 miles per hour. Because of this, Lowe Earth Orbit satellies can orbit the planet in as little as 90 minutes.

  Low Earth Orbit satellite systems require several dozen satellites to provide coverage of the entire planet. Low Earth Orbit satellites typically operate in polar orbits. Low Earth Orbit satellites are used for applications where a short Round Trip Time (RTT) is very important, such as Mobile Satellite Services (MSS). Low Earth Orbit satellites have a typical service life expectancy of five to seven years.

   Objects in LEO encounter atmospheric drag in the form of gases in the thermosphere (approximately 80–500 km up) or exosphere (approximately 500 km and up), depending on orbit height. LEO is an orbit around Earth between the atmosphere and below the inner Van Allen radiation belt. The altitude is usually not less than 300 km because that would be impractical due to the larger atmospheric drag.

   BillikenSat-II will be operating in Low Earth Orbit, at around 700 km altitude. In this environment, both infrared radiation and albedo (reflected solar energy) must be taken into consideration, along with the direct solar radiation. These forms of radiation, represented graphically below, are the only sources of heat for the satellite since it is operating in the vacuum of space. As you may know, heat is transferred via convection, conduction, or radiation. Radiation is the main mode of heat transfer in space because it requires no medium and is actually more efficient in a vacuum. Because there is no air either in space or the main structure of the satellite, convection plays no part in the thermal environment. Conduction is important when analyzing the heat transfer from the main structure to the internal components.

   Typical Low Earth Orbit (LEO) satellite temperatures can range from -40o Celsius to 85o Celsius, but depend largely on the orbital characteristics as well as the materials used in the spacecraft, in addition to heat generated by internal components (such as micro processors) and active thermal control.

 LEO Satellite Cell Structure

        

Benefits of the LEO concept

• LEOs can offer a communications infrastructure to areas where there is insufficient population to justify a terrestrial based cellular network. This not only includes many developing countries but 80% of the US as well. This explains why most of the initiatives for LEOs has come from the USA.
• Many developing countries are interested in LEO systems as an alternative to investing in an very expensive terrestrial tele-communications infrastructure.
• Communication via LEOs does not suffer from the objectionably long transmission delays associated with geosynchronous systems.
• User equipment does not require high-power transmitters or highly directional antennae that need to be continually pointed to the satellite. In practice, transmit powers can be much lower than 1 watt.
• LEO satellites, are technically much simpler and more robust than geo-synchronous satellites and are less likely to suffer catastrophic failure during deployment or during the satellite lifetime.

LEOS and their advantages

  LEO satellites are small communications transceivers. They are put in continuous motion at heights of 500-1200 miles above the earth. LEOS can provide 100 percent geographic coverage, LEO systems require less power for messages to reach their orbiting satellites , making it possible for specialised portable phones to transmit signals directly to the satellites. This makes phone calls independent of land based servers; allowing people to be reached anywhere in the world. Furthermore, less power requirements translate into lower service and equipment costs for users.

A major problem facing LEOS 

 Over 35 million bits of debris primarily pieces of old satellites, launch vehicles and solid rocket fuel currently orbit the earth, at speeds of up to seven kilometers per second. In comparison, a bullet fired from a high-speed rifle travels at only 0.8 kilometers per second. This forces LEO companies such as Iridium and Globalstar to worry not only about making sure their satellites work but that flying garbage doesn't knock them out of commission. Studies conducted by the National Aeronautics and Space Administration show that a collision with a fragment the size of a marble would do serious damage to a satellite. A 10 centimeter piece would be enough to permanently put the satellite out of order.

Conclusion
 Despite impediments and problems , LEOS still have many advantages over GEOS . Most important of which they are not confined by ground coverage of cellular phone relays , allowing people to communicate anywhere at any time . In the near future most human communication will be made through low earth orbiting satellites . Companies such as Iridium have already constructed a bridge to that future . 

Reference:

• http://www.tech-faq.com/low-earth-orbit.html
• http://www.collegetermpapers.com/TermPapers/Technology/low_earth_orbit_satellites.shtml
• http://www.gare.co.uk/technology_watch/leo.htm

Third−Generation (3G) Wireless Systems

   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 and it also represents the convergence of various 2G wireless telecommunications systems into a single global system that includes both terrestrial and satellite components. 3G technology, which is short for third generation mobile telephone communication systems technology, improves the efficiency of data can be transferred through your cellular phone. The data transfer rates for third generation mobile telecommunications is up to 2 Megabits per second. Aside from this feature, 3G cellular phones also have conventional voice, fax and data services, as well as high-resolution video and multimedia services which can be used while on the move. It also includes mobile office services such as virtual banking and online-billing, video conferencing, online entertainment and access to the Internet.  One of the most important aspects of 3G wireless technology is its ability to unify existing cellular standards, such as CDMA, GSM, and TDMA. 3G wireless networks consist of a Radio Access Network 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.

     3G systems will provide access, by means of one or more radio links, to a wide range of telecommunication services supported by the fixed telecommunication networks and to other services that are specific to mobile users. A range of mobile terminal types will be encompassed, linking to terrestrial and/or satellite-based networks, and the terminals may be designed for mobile or fixed use. Key features of 3G systems are a high degree of commonality of design worldwide, compatibility of services, use of small pocket terminals with worldwide roaming capability, Internet and other multimedia applications, and a wide range of services and terminals. According to the International Telecommunication Union (ITU) International Mobile Telecommunications 2000 initiative ("IMT-2000") third generation mobile ("3G") system services are scheduled to be initiated around the year 2000, subject to market considerations. 

          3G was relatively slow to be adopted globally. In some instances, 3G networks do not use the same radio frequencies as 2G so mobile operators must build entirely new networks and license entirely new frequencies, especially so to achieve high-end data transmission rates. Other delays were due to the expenses of upgrading transmission hardware, especially for UMTS, whose deployment required the replacement of most broadcast towers. Due to these issues and difficulties with deployment, many carriers were not able to or delayed acquisition of these updated capabilities.

The 3G wirless network architecture

    3G wireless solutions allow for the possibility of having an integrated network for circuit-switched and packet-switched services by utilizing ATM technology. The BSC may evolve into an RNC by using add-on cards or additional hardware that is co-located. The carrier frequency (5Mhz) and the bands (2.5 to 5Ghz) are different for 3G wireless technology compared to 2G/2G+ wireless technology. Evolution of BSC to RNC requires support for new protocols such as PDCP, RRC, RANAP, RNSAP and NBAP. Therefore, BTS' evolution into Node B may prove to be difficult and may represent significant capital expenditure on the part of network operators.

Advantages of 3G Services

• you can perform a lot of functions such as sending information and data and acquiring these via wireless access.
• you can use their handsets and let it function as a modem for their computer to mail and send necessary documents. 
• Getting information is one of the best features of 3G technology. You can also watch the latest news and headlines, getting data like the weather, sports and economic details.
• With 3G technology, you get to enjoy data transmission speed leading up to 2Mbps, considering that you have a phone in stationary mode. It also gives you high degree of connectivity and higher networking, plus resistance to noise
• Despite the new speeds and features of 3G technology, the prices of handsets and mobile units are relatively the same. 
• Faster data connectivity
• Uninterrupted video streaming on phones.
• Video calls and big mms.
• Good for data intensive applications.

Reference:

• http://www.dryaseen.pk/wp_3g.pdf
• http://w3.antd.nist.gov/wctg/3G/3G.html
• http://transition.fcc.gov/3G/
• http://www.dciexpo.com/3g/advantages-of-3g.php

General Packet Radio Service (GPRS)

   In the year 1999, cellular networks began incorporating GPRS technology into their infrastructure. The service became available in 2001. Initial data-transmission speed ran to around 28 kilobytes per second, but eventually GPRS phones could surf the Web at 60 kilobytes per second. Data packaging makes GPRS cost-efficient, as phone users only pay for bursts of data rather than a steady stream. It doesn't place an excessive drain on the battery while Web surfing or sending text messages.

  When we say GPRS (General Packet Radio Service), it is the world's most ubiquitous wireless data service, available now with almost every GSM network. GPRS is a connectivity solution based on Internet Protocols that supports a wide range of enterprise and consumer applications. With throughput rates of up to 40 kbit/s, users have a similar access speed to a dial-up modem, but with the convenience of being able to connect from anywhere. GPRS customers enjoy advanced, feature-rich data services such as colour Internet browsing, e-mail on the move, powerful visual communications such as video streaming, multimedia messages and location-based services. For operators, the adoption of GPRS is a fast and cost-effective strategy that not only supports the real first wave of mobile Internet services, but also represents a big step towards 3GSM (or wideband-CDMA) networks and services.

    GPRS is a network overlay to the existing cellular network. It uses the nature of IP transmissions to its advantage. Because IP traffic is made of “packets”, the network does not need to have continuous data transmission. Thus, IP traffic can easily share the channels. A user may be receiving or transmitting data while another one is reading information. The second user does not need to use the channel during this time, and it makes packet networks more efficient than circuit-switched networks (2G), where the channel would be in use, regardless of the user transmitting or not. Each channel is divided into eight time slots, with a maximum data transmission of 13.4Kbps.


   GPRS is different from the older mik-mac Circuit Switched Data (or CSD) connection included in GSM standards.In CSD, a data connection establishes a circuit, and reserves the full bandwidth of that circuit during the lifetime of the connection. GPRS is packet-switched which means that multiple users share the same transmission channel, only transmitting when they have data to send. This means that the total available bandwidth can be immediately dedicated to those users who are actually sending at any given moment, providing higher utilisation where users only send or receive data intermittently. Web browsing, receiving e-mails as they arrive and instant messaging are examples of uses that require intermittent data transfers, which benefit from sharing the available bandwidth.  

   Usually, GPRS data are billed per kilobytes of information transceived while circuit-switched data connections are billed per second. The latter is to reflect the fact that even during times when no data are being transferred, the bandwidth is unavailable to other potential users. GPRS originally supported (in theory) IP, PPP and X.25 connections. The last has been typically used for applications like wireless payment terminals although it has been removed as a requirement from the standard. X.25 can still be supported over PPP, or even over IP, but doing this requires either a router to do encapsulation or intelligence built into the end terminal.

   GPRS was developed to enable GSM operators to meet the growing demands for wireless packet data service that is a result of the explosive growth of the Internet and corporate intranets. Applications using these networks require relatively high throughput and are characterized by bursty traffic patterns and asymmetrical throughput needs. Applications, such as web browsing, typically result in bursts of network traffic while information is being transmitted or received, followed by long

idle periods while the data is being viewed. In addition, much more information is usually flowing to the client device than is being sent from the client device to the server. GPRS systems are better suited to meet the demand of this bursty data need than the traditional circuit−switched wireless

data systems.

Advantages Of GPRS

• GPRS is cheaper than through the regular GSM network.
• 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.
• GPRS provides wireless access to the Internet from any location where there is a network signal.
• GPRS is still faster than the older WAP (Wireless Application Protocol) and regular GSM services.
• When you access the Internet through GPRS, it does not block incoming calls through the GSM network.

Disadvantages Of GPRS

• GPRS packet loss phenomena occur 
• the actual rate lower than the theoretical value
• , GPRS 
• data transfer rate to achieve the theoretical maximum 172.2kbps.
• users will pay for traffic services
• modulation is not optimal 
• there is switching delay

Applications of GPRS

• information services as text or graphics
• chat
• still images
• moving images
• web browsing
• audio reports
• LAN application
• internet email
• file transfer
• corporate email
• document sharing and remote collaborative working

Reference:

• http://www.mobilein.com/GPRS.pdf
• http://www.ehow.com/list_7628002_advantages-gprs.html
• http://searchmobilecomputing.techtarget.com/definition/GPRS

Multichannel Multipoint Distribution Service (MMDS) & Local Multipoint Distribution Service (LMDS)

    Multichannel Multipoint Distribution Service (MMDS), also known wireless cable, is a broadcasting and communications service that operates in the ultra−high−frequency (UHF) portion of the radio spectrum between 2.1 and 2.7 GHz. MMDS is also known as wireless cable. It was conceived as a substitute for conventional cable television (TV). The MMDS wireless spectrum originally consisted of 33 analog video channels, which were 6 MHz wide. The evolution of video technology into digital capacities enables the carriers to convert these 33 analog MMDS channels into 99 digital, 10 Mbps data streams, enabling full Ethernet connectivity. Therefore, a carrier with a normal operation can have as much as 1 Gbps of capacity at a single transmitter providing adequate capacities for most applications. This capacity is also readily expandable by using a sector cell concept (see the analog cellular chapter to get a handle on sectors), which reuses the same frequency many times. The combination of super cells and sectors enable the carrier to reuse the same frequency many times by building multiple cell sites. When enough customers sign on and as their bandwidth demands grow, the growth in traffic can be handled expeditiously through a new cell or a new sector. The MMDS frequency band has room for several dozen analog or digital video channels, along with narrowband channels that can be used by subscribers to transmit signals to the network. The narrowband channels were originally intended for use in an educational setting (so-called wireless classrooms). The educational application has enjoyed some success, but conventional TV viewers prefer satellite TV services, which have more channels. Because of recent deregulation that allows cable TV companies to provide telephone and Internet services, along with the development of digital technologies that make efficient use of available bandwidth, MMDS has considerable future potential. An MMDS network can provide high-speed Internet access, telephone/fax, and TV together, without the constraints of cable connections.

     Local Multipoint Distribution Service (LMDS), is a broadband wireless point to multipoint communication system that provides reliable digital two-way voice, data and Internet services. The term "Local" indicates that the signals range limit. "Multipoint" indicates a broadcast signal from the subscribers, the term "distribution" defines the wide range of data that can be transmitted, data ranging anywhere from voice, or video to Internet and video traffic(Later on in the 3rd section, the emergence of LMDS shows why it is good at transmitting such a wide variety of data.)It provides high capacity point to multipoint data access that is less investment intensive. LMDS with its wireless delivery combined with a significant amount of spectrum allocated, promises to allow for a very high quality communication services. It transmits milliwave signals with-in small cells. As it has been tested by the US milatary and the corporate pioneers like the SpeedUs, it is undoubtedly a proven technology.
       Originally designed for wireless digital telivision transmission LMDS and MMDS (Microwave Multipoint Distribution System) were predicted to serve the wireless Subscription Television needs. MMDS is also a broadband wireless communication service which operates at lower frequencies. Usually, LMDS operates at frequencies above the 10Ghz range and MMDS at frequencies below the 10GHz range. Later on they were extended to offer other interactice services.

 The MMDS frequency band has room for several dozen analog or digital video channels, along with narrowband channels that can be used by subscribers to transmit signals to the network. The narrowband channels were originally intended for use in an educational setting (so-called wireless classrooms). The educational application has enjoyed some success, but conventional TV viewers prefer satellite TV services, which have more channels. Because of recent deregulation that allows cable TV companies to provide telephone and Internet services, along with the development of digital technologies that make efficient use of available bandwidth, MMDS has considerable future potential. An MMDS network can provide high-speed Internet access, telephone/fax, and TV together, without the constraints of cable connections.

   Local Multipoint Distribution Services (LMDS), is one solution for bringing high bandwidth services to homes and offices within the “last mile” of connectivity, an area where cable or optical fiber may not be convenient or economical. LMDS operates in 28GHz band to provide digital two-way voice, data, internet, and video services via high-speed dedicated links between high-density nodes in a network. The majority of system operators will be using Point-to-Multipoint PMP wireless design.

LMS was considered a broadband fixed service and it is a broadband wireless access technology originally designed for digital television transmission (DTV). It was conceived as a fixed wireless, point-to-multipoint technology for utilization in the last mile. LMDS commonly operates on microwave frequencies across the 26 GHz and 29 GHz bands. In the United States, frequencies from 31.0 through 31.3 GHz are also considered LMDS frequencies. Although LMDS may render any kind of communications service consistent with Commission rules, one current use is in competitive local exchange carrier (CLEC) service providing voice and data connectivity to business customers. LMDS uses a cellular infrastructure, with multiple base stations and small customer transceivers able to return communications.
 LMDS, as its name implies, is a broadband wireless technology that is used to deliver the multiple service offerings in a localized area. The services possible with LMDS include the following:
·         Voice dial−up services
·         Data
·         Internet access
·         Video

     LMDS may be the key to bringing multimedia data, supporting voice connections, the Internet, videoconferencing, interactive gaming, video streaming and other high-speed data applications to millions of customers worldwide over the air. As with other wireless networks, LMDS technology offers the advantage that it can be deployed quickly and relatively inexpensively. New market entrants who do not have an existing network like incumbent's copper wires or fibres - can rapidly build an advanced wireless network and start competing. LMDS is also attractive to incumbent operators who need to complement or expand existing networks.
     Like in other microwave applications, LMDS cell size too is limited by rain fade,Also, walls, hills and even leafy trees block, reflect and distort the signal, creating significant shadow areas for a single transmitter. Some operators may serve a cell with several transmitters to increase coverage; most prefer one transmitter per cell, sited to target as many users as possible.

Reference:

•  http://en.wikipedia.org/wiki/Local_Multipoint_Distribution_Service
• Broadband Telecommunications Handbook(VPN 3GW GPRS MPLS VoIP SIP).pdf
• http://searchnetworking.techtarget.com/definition/Multichannel-Multipoint-Distribution-Service

Microwave− and Radio−Based Systems

Microwave System  
     A  microwave system is a system of equipment used for microwave data transmission. The typical microwave system includes radios located high atop microwave towers, which are used for the transmission of microwave communications using line of sight microwave radio technology.
     Microwaves are radio waves with wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz.[1] This broad definition includes both UHF and EHF (millimeter waves), and various sources use different boundaries.[2] In all cases, microwave includes the entire SHF band (3 to 30 GHz, or 10 to 1 cm) at minimum, with RF engineering often putting the lower boundary at 1 GHz (30 cm), and the upper around 100 GHz (3 mm).
     A microwave system is composed of at least two microwave towers. At the top of these towers are microwave antennas. These antennas are what allow the transmitter hardware of the microwave system to transmit data from site to site. The area between the microwave system components must be clear of any major structures, such as tall buildings, mountains, or other objects that could potentially obstruct microwave transmission. Only when this has been achieved can data travel through the microwave system.
     This is why microwave communication is categorized as a “line of sight” technology. When planning a microwave radio system, one must remember the requirements of microwave equipment. Microwave antennas must be placed at the top of tall radio towers to provide a clear line communication path. This allows the microwave system data to travel the long distances required by telecommunications service providers.
     The myths run rampant with radio−based systems. Despite the rumors about the various risks and perils for the radio signal, microwave usually operates 99.99−plus percent of the time. Microwave is normally impervious to the following:
• Snow
• Sleet
• Fog
• Birds
• Pollution
• Sandstorms
• Sunspot activity

    Analog microwave communication may be most economical for use at your tower sites simply because it is already paid for and in service. If you are already operating microwave equipment, it is most likely analog. To avoid having to retrain your operators, you may want to stick with the analog microwave communication equipment you already have. Because you've already gotten comfortable with this equipment, you've probably also learned its capabilities, so you're unlikely to overburden your transport system with new digital equipment.
    Digital microwave communication utilizes more advanced, more reliable technology. It is much easier to find equipment to support this transmission method because it is the newer form of microwave communication. Because it has a higher bandwidth, it also allows you to transmit more data using more verbose protocols. The increased speeds will also decrease the time it takes to poll your microwave site equipment. This more reliable format provides for more reliable reporting with advanced communication equipment, while also allowing you to bring in your LAN connection when it becomes available at the site.

 Uses of Microwave  System
-Communocation                      -Spectroscopy
-Radar
-Radio Astronomy
-Power
-Navigation

Radio-Based System

       The first system of radio navigation was the Radio Direction Finder. By tuning in a radio station and then using a directional antenna to find the direction to the broadcasting antenna, radio sources replaced the stars and planets of celestial navigation with a system that could be used in all weather and times of day. By using triangulation, two such measurements can be plotted on a map where their intersection is the position. Commercial AM radio stations can be used for this task due to their long range and high power, but strings of low-power radio beacons were also set up specifically for this task. Early systems used a loop antenna that was rotated by hand to find the angle to the signal, while modern systems use a much more directional solenoid that is rotated rapidly by a motor, with electronics calculating the angle.

    Today's radio based systems are sophisticated, reliable and economical and provide the same technical benefits as analogue and addressable systems.

    Radio-based systems utilize a receiver with an omni-directional antenna to detect a digitally coded spread spectrum or narrow band radio transmission from an EV. In these systems, the direction of preemption is selected in the vehicle and direction-unique signal is transmitted to the intersection. Radio-based systems avoid the line-of-sight limitations associated with light- and infrared-based systems. Once a Radio frequency pulse is detected and the proper direction of travel is determined, the preemption request is processed by the phase selector and the signal controller. Today, technology has arisen to improved microwave systems. They are being used for Personal Communication Service and Cellular systems. Installations of microwaves systems are of easy nowadays than installing cables a mile long. PCS systems and cellular systems have installed microwave radio systems around the globe to further increase their range and performance. Some operators like police, fire department and emergency units use these microwave systems. The Bandwidth of microwave is costly and in need as consumers grows fast. Bandwidth may be bought but for a higher price but this would satisfy the users. Buying bandwidth would be used for the future and an increase on your capacity which means lesser traffic. Microwave systems are complex and require maintenance so it might also be a cut to the budget of buying bandwidth for users. Reliability is also a factor. Buying bandwidth is not just for capacity but also for reliability. But equipment should also be secured and reliable not just the bandwidth.
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
 

Reference:

• http://www.ebooksx.com/search.php?key=Microwave+and+Radio+Based+Systems&type=title&page=5

• http://ntl.bts.gov/lib/jpodocs/repts_te/14097_files/section_3.htm

• http://www.smecc.org/arnold_acker.htm

• http://bigdesignevents.com/2011/09/innovations-from-world-war-ii/

xDSL

   The motivation of digital subscriber line technology was the Integrated Services Digital Network (ISDN) specification proposed in 1984 by the CCITT (now ITU-T) as part of Recommendation I.120, l. xDSL is similar to ISDN in as much as both operate over existing copper telephone lines and both require the short runs to a central telephone office usually less than 20,000 feet. However, xDSL offers much higher speeds - up to 32 Mbps for upstream traffic, and from 32 Kbps to over 1 Mbps for downstream traffic. DSL technologies use sophisticated modulation schemes to pack data onto copper wires. They are sometimes referred to as last-mile technologies because they are used only for connections from a telephone switching station to a home or office, not between switching stations. A new form of communications was needed to work over the existing copper cable plant. One of the technologies selected was the use of xDSL. The DSL family includes several variations of what is known as digital subscriber line. The lower case x in front of the DSL stands for the many variations. These will include: 

      First we have, Asymmetrical digital subscriber line (ADSL), it is used in a data communications technology that enables faster data transmission over copper telephone lines than a conventional voiceband modem can provide. It does this by utilizing frequencies that are not used by a voice telephone call. Some of its advantages was no expensive modification is required to central office switch, simple splitter splits ADSL signal from the existing analog line, and it had a high bandwidth. Then we have, ISDN Digital Subscriber Line (IDSL),  also called the last mile, a system in which digital data is transmitted at 128 Kbps on a regular copper telephone line (twisted pair) from a user to a destination using digital or analog transmission, by passing the telephone company's central office equipment that handles analog signals. We have also  what we called High bit−rate Digital Subscriber Line (HDSL), it uses two-pair copper cable for full-duplex transmission, using echo cancellation with each pair carrying 784 kb/s. The lower bit rate allows for a lower frequency range of operation that reduces channel loss and next. Then we have, Consumer Digital Subscriber Line (CDSL), a version of Digital Subscriber Line (DSL) service that is somewhat slower than Asymmetric DSL up to 1 Mbps downstream, probably less upstream and has the advantage that a splitter does not need to be installed at the user's end. It allows more data to be sent over existing copper telephone lines and it supports data rates up to 3 Mbps.

Digital Subscriber Line Advantages

• Independent services: Loss of high speed data does not mean you lose your telephone service. Imagine your telephone, television, and Internet access going out when a cable company amplifier/repeater dies.
• Security: Unlike cable modems, each subscriber can be configured so that it will not be on the same network. In some cable modem networks, other computers on the cable modem network are left visibly vulnerable and are easily susceptible  to break ins as well as data destruction.
• Integration: DSL will easily interface with ATM, Nx64, and WAN technology. Telecommuting may get even easier.
• High bandwidth
• Cheap line charges from the phone company.
• Good for "bursty" traffic patterns

Digital Subscriber Line Disadvantages

• No current standardization
• Expensive
• Distance dependence: The farther you live from the DSL Access Multiplexer, the lower the data rate. The longest run lengths are 18,000 feet, or a little over 3 miles.
• Limited availability
• Low or no Committed Information Rate.  This means that as traffic across the telco switch increases your data could in effect, be locked out, until call volumes and other traffic subsides.
• Reliability and potential down time issues makes DSL a very risky choice for mission critical systems unless backup / fail over links are put in place.
• Sometimes the process of getting DSL may be an adventure.

We have also Single High Speed DSL (SHDSL), was developed to provide high−speed communications on that single cable pair but at distances no greater than 10K. Despite this distance limitation, SDSL was designed to deliver 1.544 Mbps on the single cable pair. Typically, however, the providers offer SDSL at 768 Kbps.Then we have Rate−adaptive digital subscriber line (RADSL), was designed to increase range and noise tolerance by sacrificing up stream speed. We have also Very high−bit rate digital subscriber line (VDSL),a digital subscriber line technology providing faster data transmission over a single flat untwisted or twisted pair of copper wires up to 52 Mbit/s downstream and 16 Mbit/s upstream. And lastly, we have what we called  Single or symmetric digital subscriber line (SDSL), it works by sending digital pulses in the high-frequency area of telephone wires and can not operate simultaneously with voice connections over the same wires.

Benefits & Applications

Benefits

• High-speed data service

• DSL typically >10x faster than 56-kbps analog modem

• Always on connection

• No need to “dial-up”

• Uses existing copper wires

• Co-exists w/ POTS service

• Reasonably priced today and getting cheaper

Applications

• High speed Internet access
• SOHO
• Multimedia, Long distance learning, gaming
• Video on Demand
• VPN
• VoDS

Reference:

•  http://www.webopedia.com/TERM/X/xDSL.html
•  Richard D. Gitlin, Sailesh K. Rao, Jean-Jacques Werner, Nicholas Zervos. "Method and apparatus for        wideband transmission of digital signals between, for example, a telephone central office and customer    premises". US Patent 4,924,492. Filed March 22, 1988. Issued May 8, 1990
• http://www.state.net/dsl/DSLadvantages.html
• IDSL Frequently Asked Questions" (Press release). Cisco. February 26, 2008.

(ATM) Asynchronous Transfer Mode

      The first ATM Assembly standardized was released in 1992. Different ATM Facility working groups are work defining additional standards required to enable ATM to cater a communications potentiality for the comprehensive potentiality of LAN and WAN transmission schemes it is organized to hold. This normalisation labor will likely remain in validity for a extensive punctuation due to the umbrella organisation content of the technol-ogy, which was formed to reinforcement expression, aggregation, and video on both local and wide area networks. The concept of ATM is that its connection oriented network technology. The connections are not tangible end-to-end connections similar with  telephone networks, but they are virtual connections. This agency that, equal with telephony connections, a connectedness must be falsification before any collection can be transferred between the end-points of the transportation. After the connexion has been used and is no someone requisite, it is terminated. ATM connections have properties which describe the kind of traffic carried over the connection and the way it is treated by the network. These properties like Traffic Descriptors are negotiated at connection setup time between the calling party, the ATM network and the called party.

  Because ATM is designed to be easily implemented by hardware rather than software, faster processing and switch speeds are possible. The specified bit rates are either 155.520 Mbps or 622.080 Mbps. Speeds on ATM networks can reach 10 Gbps. Along with Synchronous Optical Network (SONET) and several other technologies, ATM is a key component of broadband ISDN .

   When we say Asynchronous Transfer Mode, it is a technology that has the potential of revolutionizing data communications and telecommunications. Based on the emerging standards for Broadband Integrated Services Digital Networks, ATM offers the economically sound "bandwidth on demand" features of packet-switching technology at the high speeds required for today's LAN and WAN networks -- and tomorrow's.

     ATM is a cell-relay technology that divides upper-level data units into 53-byte cells for transmission over the physical medium. It operates independently of the type of transmission being generated at the upper layers AND of the type and speed of the physical-layer medium below it. This allows the ATM technology to transport all kinds of transmissions like:

Data

                 

Video

Voice

  Recognizing the differences between voice and data transportation requirements, ATM is designed to adapt to the time sensitivity of different applications.
ATM Protocol

   In the ATM protocol domain the socket interface provides an application with direct access to an ATM virtual connection. The ATM protocol is offered as a new address family, AF_ATM. Each Protocol Data Unit to be transmitted over the connection is handed directly to the ATM adaptation layer for segmentation into ATM cells and subsequent transmission. Similarly, received PDUs are handed up to the socket layer by the adaptation layer on completion of reassembly. Draft standards for ATM UNI signalling 3 provide a mechanism for the selection of the ATM adaptation layer and Quality of Service to be used for an ATM connection. However, there is no mechanism in the socket interface to permit an application to specify these parameters to the connect system call in a clean manner. Consequently, for this implementation all ATM connections make use of AAL5, and a default ``best-effort'' QoS specification is used. Additionally the ATM code provides logical interfaces to the IP code within the system and will set up tunnels over the ATM network to carry IP traffic to its destination. In this case the upper layer for the ATM protocol is a logical interface engine rather than the socket code.

The configuration of Ethernet and ATM combined

• Ethernet−attached end systems may communicate with other Ethernet−attached end systems through bridges across the ATM network in a backbone−type configuration.
• ATM−attached end systems may communicate with ATM−attached servers and both may communicate with Ethernet−attached end systems via the bridges.

Asynchronous Transfer Mode Advantages:
-ATM supports voice, video and data allowing multimedia and mixed services over a
single network.
-High evolution potential, works with existing, legacy technologies
-Provides the best multiple service support
-Supports delay close to that of dedicated services
-Supports the broadest range of burstiness, delay tolerance and loss performance through the implementation of multiple QoS classes
-Provides the capability to support both connection-oriented and connectionless traffic using AALs
-Able to use all common physical transmission paths like SONET.
-Cable can be twisted-pair, coaxial or fiber-optic
-Ability to connect LAN to WAN
-Legacy LAN emulation
-Efficient bandwidth use by statistical multiplexing
-Scalability
-Higher aggregate bandwidth
-High speed Mbps and possibly Gbps

Asynchronous Transfer Mode Disadvantages:
-Flexible to efficiency’s expense, at present, for any one application it is usually possible to find a more optimized technology
-Cost, although it will decrease with time
-New customer premises hardware and software are required
-Competition from other technologies -100 Mbps FDDI, 100 Mbps Ethernet and fast Ethernet
-Presently the applications that can benefit from ATM such as multimedia are rare

Multiprotocol on ATM

    Multiprotocol on ATM or MPOA is the efficient transfer of inter−subnet unicast data in a

LAN Emulation LANE environment. MPOA integrates LANE and Next Hop Resolution Protocols, also known as Next Hop Routing Protocols, to preserve the benefits of LANE while enabling intersubnet, internetwork layer protocol communication over ATM Virtual Circuit Connections without requiring routers in the data path. MPOA provides a framework for effectively synthesizing bridging

and routing with ATM in an environment of diverse protocols, network technologies, and IEEE 802.1

virtual LANs. This is designed to provide a unified paradigm for overlaying internetwork

layer protocols on ATM. MPOA is capable of using both routing and bridging information to locate

the optimal exit from the ATM cloud.

     Multiprotocol on ATM  enables the physical separation of internetwork layer route calculation and forwarding, a technique known as virtual routing. This separation provides a number of key benefits:
- It enables efficient intersubnet communication.
-It increases manageability by decreasing the number of devices that must be configured to
perform internetwork layer route calculation.
-It increases scalability by reducing the number of devices participating in internetwork layer
route calculation.
-It reduces the complexity of edge devices by eliminating the need to perform internetwork
layer route calculation.
    Multiprotocol on ATM  provides MPOA Clients and MPOA Servers, and defines the protocols that are required for MPCs and MPSs to communicate. MPCs issue queries for shortcut ATM addresses and receive replies from the MPS using these protocols. MPOA also ensures interoperability with
the existing infrastructure of routers. MPOs also make use of routers that run standard internetwork
layer routing protocols, such as Open Shortest Path First, providing a smooth integration
with existing networks.

Combining the ATM and DSL at the local loop

  These devices make it possible for the local exchange provider or the competitive DSL provider to

use the existing facilities and still satisfy the needs of voice and data over the existing local loop.

Using voice over the DSL circuit enables up to 16 simultaneous VoIP calls and Internet access to

simultaneously run over the bandwidth on the single cable pair.

Reference:

            http://technet.microsoft.com/en-us/library/bb726929.aspx

            http://margo.student.utwente.nl/simon/finished/thesis/thesis2/node26.html

            Daniel Minoli and Mark Raymond, “Technology Overview: Asynchronous Transfer Mode                                                                          

            (ATM)”,   http://www.be.datapro.com, 1997.