Chapter 27: Low-Earth-Orbit Satellites (LEOs)


A low Earth orbit (LEO) is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km. Given the rapid orbital decay of objects below approximately 200 km, the commonly accepted definition for LEO is between 160–2,000 km (100–1,240 miles) above the Earth's surface. The sideways speed needed to achieve a stable low earth orbit is about 7.8 km/s, but reduces with altitude.

LEO (Low Earth Orbit) satellite communication systems is a recent development of systems for mobile satellite communications that now exist, such as Inmarsat, AMSC. Mobile Satellite systems (satellites for mobile communications) are in operation today use satellite traveling 36,000 kilometers on the surface of the Earth and have STP 24-hour time. With a career that coincides with the equatorial zone, from a point on the Earth, the satellite appears as if the movement Geostationary Earth Orbit (GEO).

LEO basics

With Low Earth Orbit extending from 200 km to 1200 km it means that it is relatively low in altitude, although well above anything that a conventional aircraft can reach.

However LEO is still very close to the Earth, especially when compared to other forms of satellite orbit including geostationary orbit.

The low orbit altitude of leads to a number of characteristics:

-Orbit times are much less than for many other forms of orbit. The lower altitude means higher velocities are required to balance the earth's gravitational field. Typical velocities are very approximately around 8 km/s, with orbit times sometimes of the order of 90 minutes, although these figures vary considerably with the exact details of the orbit.

-The lower orbit means the satellite and user are closer together and therefore path losses a less than for other orbits such as GEO

-The round trip time, RTT for the radio signals is considerably less than that experienced by geostationary orbit satellites. The actual time will depend upon factors such as the orbit altitude and the position of the user relative to the satellite.

-Radiation levels are lower than experienced at higher altitudes.

-Less energy is expended placing the satellites in LEO than higher orbits.

-Some speed reduction may be experienced as a result of friction from the low, but measurable levels of    gasses, especially at lower altitudes. An altitude of 300 km is normally accepted as the minimum for an orbit as a result of the increasing drag from the presence of gasses at low altitudes.

Applications for LEO satellites

A variety of different types of satellite use the LEO orbit levels. These include different types and applications including:

-Communications satellites - some communications satellites including the Iridium phone system use LEO.

-Earth monitoring satellites use LEO as they are able to see the surface of the Earth more clearly as they are not so far away. They are also able to traverse the surface of the Earth.

-The International Space Station is in an LEO that varies between 320 km (199 miles) and 400 km (249 miles) above the Earth's surface. It can often be seen from the Earth's surface with the naked eye.

Space debris in LEO

Apart from the general congestion experienced in Low Earth Orbit, the situation is made much worse by the general level of space debris that exists.

There is a real and growing risk of collision and major damage - any collisions themselves are likely to create further space debris.

The US Joint Space Operations Center currently tracks over 8 500 objects that have dimensions larger than 10 centimetres. However debris with smaller dimensions can also cause significant damage and could render a satellite unserviceable after a collision.

Human Use

The International Space Station is in a LEO that varies from 320 km (199 mi) to 400 km (249 mi) above the Earth's surface.

While a majority of artificial satellites are placed in LEO, where they travel at about 7.8 km/s (28,080 km/h), making one complete revolution around the Earth in about 90 minutes, many communication satellites require geostationary orbits, and move at the same angular velocity as the Earth. Since it requires less energy to place a satellite into a LEO and the LEO satellite needs less powerful amplifiers for successful transmission, LEO is still used for many communication applications. Because these LEO orbits are not geostationary, a network (or "constellation") of satellites is required to provide continuous coverage. Lower orbits also aid remote sensing satellites because of the added detail that can be gained. Remote sensing satellites can also take advantage of sun-synchronous LEO orbits at an altitude of about 800 km (500 mi) and near polar inclination. ENVISAT is one example of an Earth observation satellite that makes use of this particular type of LEO.

Sources:
( http://en.wikipedia.org/wiki/Low_Earth_orbit)
(http://www.helmigaala.net/low-earth-orbit-satellite-communication-systems.html)

(http://www.helmigaala.net/low-earth-orbit-satellite-communication-systems.html)

Chapter 25: 3G Third Generation Wireless System

3G or 3rd generation mobile telecommunications is a generation of standards for mobile phones and mobile telecommunication services fulfilling the International Mobile Telecommunications-2000 (IMT-2000) specifications by the International Telecommunication Union. Application services include wide-area wireless voice telephone, mobile Internet access, video calls and mobile TV, all in a mobile environment.

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. The following Table describes some of the key service attributes and capabilities expected of 3G systems:

3G System Capabilities

Capability to support circuit and packet data at high bit rates:

-144 kilobits/second or higher in high mobility (vehicular) traffic

-384 kilobits/second for pedestrian traffic

-2 Megabits/second or higher for indoor traffic

Interoperability and roaming

Common billing/user profiles:

-Sharing of usage/rate information between service providers

-Standardized call detail recording

-Standardized user profiles

Capability to determine geographic position of mobiles and report it to both the network and the mobile terminal

Support of multimedia services/capabilities:

-Fixed and variable rate bit traffic

-Bandwidth on demand

-Asymmetric data rates in the forward and reverse links

-Multimedia mail store and forward

-Broadband access up to 2 Megabits/second

The 3rd Generation Partnership Project (3GPP) was formed in 1998 to foster deployment of 3G networks that descended from GSM. 3GPP technologies evolved as follows.

 -General Packet Radio Service (GPRS) offered speeds up to 114 Kbps.

 -Enhanced Data Rates for Global Evolution (EDGE) reached up to 384 Kbps.

-UMTS Wideband CDMA (WCDMA) offered downlink speeds up to 1.92 Mbps.

-High Speed Downlink Packet Access (HSDPA) boosted the downlink to 14Mbps.

- LTE Evolved UMTS Terrestrial Radio Access (E-UTRA) is aiming for 100 Mbps.

GPRS deployments began in 2000, followed by EDGE in 2003. While these technologies are defined by IMT-2000, they are sometimes called "2.5G" because they did not offer multi-megabit data rates. EDGE has now been superceded by HSDPA (and its uplink partner HSUPA).

A second organization, the 3rd Generation Partnership Project 2 (3GPP2) -- was formed to help North American and Asian operators using CDMA2000 transition to 3G. 3GPP2 technologies evolved as follows.

-One Times Radio Transmission Technology (1xRTT) offered speeds up to 144 Kbps.

-Evolution Data Optimized (EV-DO) increased downlink speeds up to 2.4 Mbps.

-EV-DO Rev. A boosted downlink peak speed to 3.1 Mbps and reduced latency.

-EV-DO Rev. B can use 2 to 15 channels, with each downlink peaking at 4.9 Mbps.

-Ultra Mobile Broadband (UMB) was slated to reach 288 Mbps on the downlink.

The 3GPP2's next-generation technology, UMB, may not catch on, as many CDMA operators are now planning to evolve to LTE instead.

In fact, LTE and UMB are often called 4G (fourth generation) technologies because they increase downlink speeds an order of magnitude. This label is a bit premature because what constitutes "4G" has not yet been standardized. The ITU is currently considering candidate technologies for inclusion in the 4G IMT-Advanced standard, including LTE, UMB, and WiMAX II. Goals for 4G include data rates of least 100 Mbps, use of OFDMA transmission, and packet-switched delivery of IP-based voice, data, and streaming multimedia.

Sources:

http://www.dryaseen.pk/wp_3g.pdf

http://en.wikipedia.org/wiki/3G

http://transition.fcc.gov/3G/

http://searchtelecom.techtarget.com/definition/3G

Chapter 24: GPRS

General packet radio service (GPRS) is a packet oriented mobile data service on the 2G and 3G cellular communication system's global system for mobile communications (GSM). GPRS was originally standardized by European Telecommunications Standards Institute (ETSI) in response to the earlier CDPD and i-mode packet-switched cellular technologies. It is now maintained by the 3rd Generation Partnership Project (3GPP).

In theory, GPRS packet-based services cost users less than circuit-switched services since communication channels are being used on a shared-use, as-packets-are-needed basis rather than dedicated to only one user at a time. It is also easier to make applications available to mobile users because the faster data rate means that middleware currently needed to adapt applications to the slower speed of wireless systems are no longer be needed. As GPRS has become more widely available, along with other 2.5G and 3G services, mobile users of virtual private networks (VPNs) have been able to access the private network continuously over wireless rather than through a rooted dial-up connection.

Advantages:

 -We can enable internet when it is required.

-Internet is access even in remote areas.

-We can down load games, ring tones, images by visiting different web sites.

- Internet application you can enables easily.

- It can connect to laptop or desktop.

- You can enjoy wireless internet connection.

-GPRS has high speed than GSM network.

 -It is flexibility to work internet on your computer.

- Receives the data by sending when user required.

 -GPRS can be work in large distance or remote area.

 -GPRS is the quick and easy to implement.

Disadvantages:

 -The monthly contract cost is high.

 -It is more expensive than sms.

 -It varies between providers and countries.

HSCSD (High Speed Circuit Switched Data) enables data to be transferred more rapidly than the standard GSM (Circuit Switched Data) system by using multiple channels. The maximum number of timeslots that can be used is four, giving a maximum data transfer rate of 57.6 kbps (or 38.4 kbps on a GSM 900 network). HSCSD is more expensive to use than GPRS, because all four slots are used simultaneously - it does not transmit data in packets. Because of this, HSCSD is not as popular as GPRS and is being replaced by EDGE.

EDGE (Enhanced Data rates for GSM Evolution) or EGPRS provides data transfer rates significantly faster than GPRS or HSCSD. EDGE increases the speed of each timeslot to 48 kbps and allows the use of up to 8 timeslots, giving a maximum data transfer rate of 384 kbps. In places where an EDGE network is not available, GPRS will automatically be used instead. EDGE offers the best that can be achieved with a 2.5G network, and will eventually be replaced by 3G.

Sources:
http://en.wikipedia.org/wiki/General_Packet_Radio_Service
http://searchmobilecomputing.techtarget.com/definition/GPRS

http://infotech-milan.blogspot.com/2010/05/strength-feature-advantages-and.html

http://www.mobile-phones-uk.org.uk/gprs.htm

Chapter 18: MMDS and LMDS

MMDS( Multichannel Multipoint Distribution Service)

Broadband Radio Service (BRS) formerly known as Multichannel Multipoint Distribution Service (MMDS), also known as Wireless Cable, is a wireless telecommunications technology, used for general-purpose broadband networking or, more commonly, as an alternative method of cable television programming reception.

MMDS allows two-way voice, data and video streaming. It operates at a lower frequency than LMDS (typically within specified bands in the 2-10GHz range) and therefore has a greater range and requires a less powerful signal than LMDS. MMDS is a less complicated, cheaper system to implement. As a consequence, the CPE is cheaper, thus it has a wider potential addressable market. It is also less vulnerable to rain fade - the interference caused by adverse weather conditions that can undermine the quality of the microwave signal. However, the bandwidth offered by LMDS makes this the more viable option.

Advantage of MMDS

• It has chunks of under-utilized spectrum that will, once completely digital, become increasingly valuable and flexible.
• System Implementation, which is little more than putting an installed transmitter on a high tower and a small receiving antenna on the customer’s balcony or roof, is quick and inexpensive.
• Moreover, since MMDS services have been around for 20 years, there is a wealth of experience--at least in respect to the one-way distribution technology.

Disadvantage of MMDS:

• Large upstream bandwidth in MMDS band requires careful planning, filtering etc.
• Limited capacity without sectorization, cellularization which adds complexity and cost
  
  
  
  

WHAT KIND OF COVERAGE AREA CAN BE EXPECTED FROM AN MMDS SYSTEM?

The term “coverage” should be defined as it relates to the service range of an MMDS system since wireless cable systems transmit at micro wave frequencies (2.0-2.9 GHz), all receive sites must have a clear “line-of-sight” path to the transmit antennas. Therefore, coverage area of an MMDS system is limited to the line-of-sight radius from the transmit site.

LMDS(Local Multipoint Distribution Service)

LMDS 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.

LMDS is a fixed broadband line-of-sight, point-to-multipoint, microwave system, which operates at a high frequency (typically within specified bands in the 24-40GHz range) and can deliver at a very high capacity, depending on the associated technologies. Given the complexity of the equipment required (and the power needed to deliver signals) both of these technologies are regarded as prohibitively expensive for the consumer market. Therefore, LMDS operators will initially be targeting enterprises and network operators, although the consumer market is likely to emerge over time as the cost of CPE comes down (partly driven by the take-up of IP). It should be noted that CPE costs $5,000 for LMDS in the 26GHz range.

Advantage of LMDS:

• Very large bandwidth available for data, IP telephony, video conferencing services
• Large Capacity
• Small Cell size, 2-8Km
• Network management and maintenance is vary cost effect
  
  
  
  

Disadvantage of LMDS:

• Does not cover entire metropolitan area of a large city without adding  many cells at high cost
• Signal strength is greatly reduced by the presence of heavy rainfall

Benefits using MMDS, LMDS and unlicensed band together:

• Complete coverage of a large city  right in the beginning
• Lower infrastructure and deployment costs
• More type of services can be offered ie. lower cost services with MMDS network and high bandwidth services with LMDS network
• Can grow subscribers and services by adding additional LMDS cells
• More options to address interference with other ITFS and MMDS licensees
• Lower back haul costs - LMDS and MMDS cells can be used for back haul

Example of combined combined MMDS / LMDS / Unlicensed band network:

• Overlay MMDS and LMDS network on top of each other
• MMDS network used for less dense rural areas and to increase overall coverage area
• LMDS network used in densely populated down town areas  and business parks
• Add additional LMDS cells as needed to increase capacity and to offer higher bandwidth services 
• Minor modification to Hub equipment

Difference between the LMDS AND MMDS

LMDS and MMDS use different areas of the spectrum. Their location in the spectrum offers specific characteristics which make them different. One example is that LMDS is typically used between 3 - 5 miles line of site distance from the main hub/office. However, MMDS can be used around 30 miles from the main hub/office and also requires line of site. The big difference between the two is bandwidth.

Sources:

http://en.wikipedia.org/wiki/Local_Multipoint_Distribution_Service

http://www.mobilecomms-technology.com/projects/mmds/

http://www.lmds.vt.edu/LMDS_about_fluid.html

http://www.rficsolutions.com/publishedpapers/Broadbandwireless.pdf

Chapter 17: Microwave and Radio-Based System

Microwave transmission refers to the technology of transmitting information or energy by the use of radio waves whose wavelengths are conveniently measured in small numbers of centimeters; these are called microwaves. This part of the radio spectrum ranges across frequencies of roughly 1.0 gigahertz (GHz) to 30 GHz. These correspond to wavelengths from 30 centimeters down to 1.0 cm.

Microwaves are widely used for point-to-point communications because their small wavelength allows conveniently-sized antennas to direct them in narrow beams, which can be pointed directly at the receiving antenna. This allows nearby microwave equipment to use the same frequencies without interfering with each other, as lower frequency radio waves do. Another advantage is that the high frequency of microwaves gives the microwave band a very large information-carrying capacity; the microwave band has a bandwidth 30 times that of all the rest of the radio spectrum below it. A disadvantage is that microwaves are limited to line of sight propagation; they cannot pass around hills or mountains as lower frequency radio waves can.

Properties:

• Suitable over line-of-sight transmission links without obstacles

• Provides large useful bandwidth when compared to lower frequencies (HF, VHF, UHF)

•     Affected by the refractive index (temperature, pressure and humidity) of the atmosphere, rain (see rain fade), snow and hail, sand storms, clouds, mist and fog, strongly depending on the frequency.

Uses:

Wireless transmission of information

• One-way (e.g. television broadcasting) and two-way telecommunication using communications satellite
• Terrestrial microwave radio broadcasting relay links in telecommunications networks including e.g. backbone or backhaul carriers in cellular networks linking BTS-BSC and BSC-MSC.

Wireless transmission of power

• Proposed systems e.g. for connecting solar power collecting satellites to terrestrial power grids

Advantage:

• No cables needed
• Multiple Channels available
• Wide Bandwidth
• Able to transmit Large quantities of data
• Relatively low costs

Disadvantage:

• Line of sight will be disrupted if any obstacle, such as new building are in the way
• Signal absorption by atmosphere. Microwaves are suffer from attenuation due to atmospheric condition
• Towers are expensive to build
• Subject to electromagnetic and other interference

A microwave link is a communications system that uses a beam of radio waves in the microwave frequency range to transmit video, audio, or data between two locations, which can be from just a few feet or meters to several miles or kilometers apart. Microwave links are commonly used by television broadcasters to transmit programmes across a country, for instance, or from an outside broadcast back to a studio.

Uses of microwave links:

• In communications between satellites and base stations

• As backbone carriers for cellular system
• In-short range indoor communications
• Telecommunications, in linking remote and regional telephone exchanges to larger(main) 
• Exchanges without the need for copper/optical fiber lines

Bandwidth:

Bandwidth is always a touchy subject. It can become a "never satisfied drain" on the corporate

bottom line if due diligence is not practiced. There is a direct relationship to cost and total

bandwidth. The more bandwidth needed, the greater the cost.

It's wiser to buy bandwidth as you need it and not before (there will be a small amount of

incremental add−on, but limited). In the future, there will be the following:

• More choices
• Increase providers
• Greater availability
• Lower Cost

The risks associated with buying bandwidth fall into the two categories pointed out earlier:

• buying too little bandwidth will increase incremental growth costs that can add up to more than buying a larger quantity at the onset would.
• buying more bandwidth  than immediately needed means paying for bandwidth that may not be required for some time, or that will be less expensive in the future.

Having too much bandwidth is possible. Having too much reliability is just the opposite.

Organizations lose significant amounts of money when the network connection is too slow, but far

more when the link is down completely.

Microwave radio is a flexible and cost-effective alternative for transmission of voice, data, and video services in all parts of a fixed or wireless mobile network, including applications for the backhaul or direct access services. With the scale and flexibility of today’s new radio technology, implementing a microwave network is more economical and easier than ever.Microwave Networks Incorporated provides fixed wireless products for next generation “converged” networks.

Sources:

http://www.sqa.org.uk/e-learning/NetTechDC01CCD/page_44.htm

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

http://www.microwavenetworks.com/solutions/white-papers/ethernet-applications-and-how-microwave-radios-can-play-a-part/

http://en.wikipedia.org/wiki/Microwave_transmission

Broadband Telecommunications Handbook, Second Edition

Topic:Chapter 16 xDSL

Digital subscriber line (DSL, originally digital subscriber loop) is a family of technologies that provide internet access by transmitting digital data over the wires of a local telephone network. In telecommunications marketing, the term DSL is widely understood to mean Asymmetric Digital Subscriber Line (ADSL), the most commonly installed DSL technology. DSL service is delivered simultaneously with wired telephone service on the same telephone line. This is possible because DSL uses higher frequency bands for data separated by filtering. On the customer premises, a DSL filter on each outlet removes the high frequency interference, to enable simultaneous use of the telephone and data.

                                                                        A DSL modem

How It Works

Traditional phone service (sometimes called POTS for "plain old telephone service") connects your home or small business to a telephone company office over copper wires that are wound around each other and called twisted pair . Traditional phone service was created to let you exchange voice information with other phone users and the type of signal used for this kind of transmission is called an analog signal. An input device such as a phone set takes an acoustic signal (which is a natural analog signal) and converts it into an electrical equivalent in terms of volume (signal amplitude) and pitch (frequency of wave change). Since the telephone company's signalling is already set up for this analog wave transmission, it's easier for it to use that as the way to get information back and forth between your telephone and the telephone company. That's why your computer has to have a modem - so that it can demodulate the analog signal and turn its values into the string of 0 and 1 values that is called digital information. 

Types of  DSL:

ADSL -The variation called ADSL (Asymmetric Digital Subscriber Line) is the form of DSL that will become most familiar to home and small business users. ADSL is called "asymmetric" because most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages.

CDSL -CDSL (Consumer DSL) is a version of DSL, trademarked by Rockwell Corp., that is somewhat slower than ADSL (1 Mbps downstream, probably less upstream) and has the advantage that a "splitter" does not need to be installed at the user's end. 

G.Lite or DSL Lite - G.Lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially a slower ADSL that doesn't require splitting of the line at the user end but manages to split it for the user remotely at the telephone company.

HDSL - HDSL (High bit-rate Digital Subscriber Line), one of the earliest forms of DSL, is used for wide band digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of HDSL is that it is symmetrical: an equal amount of bandwidth is available in both directions.

IDSL - IDSL (ISDN DSL) is somewhat of a misnomer since it's really closer to ISDN data rates and service at 128 Kbps than to the much higher rates of ADSL.

RADSL - RADSL (Rate-Adaptive DSL) is an ADSL technology from Westell in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2 system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272 Kbps to 1.088 Mbps upstream over an existing line.

SDSL - SDSL (Symmetric DSL) is similar to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada) or 2.048 Mbps (Europe) each direction on a duplex line. It's symmetric because the data rate is the same in both directions.

UDSL - UDSL (Unidirectional DSL) is a proposal from a European company. It's a unidirectional version of HDSL

VDSL- VDSL (Very high data rate DSL) is a developing technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 1,000 feet or 300 meters in length). 

x2/DSL - x2/DSL is a modem from 3Com that supports 56 Kbps modem communication but is upgradeable through new software installation to ADSL when it becomes available in the user's area. 3Com calls it "the last modem you will ever need."

Here are some advantages of DSL:

-  You can leave your Internet connection open and still use the phone line for voice calls.

-  The speed is much higher than a regular modem

- DSL doesn't necessarily require new wiring; it can use the phone line you already have.

- The company that offers DSL will usually provide the modem as part of the installation.

But there are disadvantages:

-   A DSL connection works better when you are closer to the provider's central office. The farther away you

        get from the central office, the weaker the signal becomes.

- The connection is faster for receiving data than it is for sending data over the Internet.

- The service is not available everywhere.  

Difference Between ISDN and DSL

Speed - In terms of speed, DSL is faster than ISDN. DSL sends data packets with speeds ranging from 128Kbps – 1.5Mbps. On the other hand, ISDN comes in two different speeds i.e., 64Kbps and 128Kbps.

Price - In terms of price, ISDN is somewhat more expensive than DSL. The main reason is that DSL utilizes wires that are already installed into homes or businesses, and there is no special line installation needed

Technology - ISDN is a dial-up service and transmits voice and data through a single line. There are two types of ISDN: Basic Rate Interface (BRI) and Primary Rate Interface (PRI). BRI is used mostly for residential homes and comes with three channels.

DSL connections are often referred to as "always on" connections, so don't do not need to dial up a number. In DSL, there is only a single route for carrying voice, data and video. Two types of DSL connections are widely: Symmetric DSL (SDSL) and Asymmetric DSL (ADSL).These two types of DSL connections differ in their data carrying capacities i.e., upload and download. For more downloading, ADSL is a better choice.

ISDN and DSL are both distance sensitive. To get either service, your place should not be more than 18,000 feet away from the central office.

Sources:

http://en.wikipedia.org/wiki/Digital_subscriber_line

http://www.ehow.com/facts_5472956_dsl-benefits.html

http://whatis.techtarget.com/definition/0,,sid9_gci213915,00.html

http://www.tech-faq.com/difference-between-isdn-and-dsl.html

Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing,and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that use variable sized packets or frames. ATM provides data-link layer services that run over a wide range of OSI physical layer links. ATM has functional similarity with both circuit switch networking and small packet switched networking. It was designed for a network that must handle both traditional high-throughput data traffic (e.g., file transfers), and real time, low latency content such as voice and video. ATM uses a connection oriented model in which a virtual circuit must be established between two endpoints before the actual data exchange begins.ATM is a core protocol used over the SONET/SDH backbone of the Public Switch Telephone Network (PSTN) and Intergrated Service Digital Network (ISDN), but its use is declining in favour of All IP. 

The Rationale and Underlying Technology

 ATM can be considered to represent a unifying technology because it was designed to transport voice, data, and video (including graphics images) on both local and wide area networks. Until the development of ATM, networks were normally developed based on the type of data to be transported. Thus, circuit-switched networks, which included the public switched telephone network and high-speed digital transmission facilities, were primarily used to transport delay-sensitive information, such as voice and video. In comparison, on packet-based networks, such as X.25 and Frame Relay, information can tolerate a degree of delay. Network users can select a networking technology to satisfy a specific communications application, but most organizations support a mixture of applications. Thus, most organizations are forced to operate multiple networks, resulting in a degree of inefficiency and escalating communications costs. By combining the features from both technologies, ATM enables a single network to support voice, data, and video.

Architecture

ATM is based on the switching of 53-byte cells, in which each cell consists of a 5-byte header and a payload of 48 bytes of information. Figure 14.1 illustrates the format of the ATM cell, including the explosion of its 5-byte header to indicate the fields carried in the header.

 

         Advantage of ATM:

         

         - Universal Switching Standard

         - Full support of Multimedia

         - Single Network Access

         - Reduction in network delay

         - True bandwith-on-demand

         - Optimization of network resources

         - Technical Long Life

          Disadvantage of ATM:

         

         - Cost, although it will decrease with time.

         - New customer premises hardware and software are 
           required:

           Competition on other technologies - 100Mbps FDDI,
           100Mbps Ethernet and fast Ethernet.

         - Presently the appilications that can benefit from 
           ATM as multimedia  are rare. 

• Vci/Vpi Connections   The entire ATM network is based on virtual connections set up by the switches upon initialization of a call. Virtual Channel Identifiers (VCI) and Virtual Path Identifiers (VPI) are used to identify these virtual connections. They are used to route information from one switch to another. VCI and VPI are not addresses; they are explicitly assigned to each segment within a network. A Virtual Channel Connection (VCC) is set up between two end users through the network and used for full-duplex flow of cells. Virtual channels having the same endpoints are often grouped together to form a Virtual Path Connection (VPC). This grouping of channels makes the task of network management easier without losing flexibility.
  
  
  ATM consists of three layers:
  
  
  1.Physical Layer - The physical layer of ATM is similar to layer 1 of the Open Systems Interconnections (OSI) model and performs bit level functions. It defines electrical characteristics and network interfaces. It is further divided into two layers: Physical Medium (PM) and Transmission Convergence (TC) sub-layer. 
  
  
  
  
  2.Atm Layer - The ATM layer is next above the physical layer. The ATM layer takes the data to be sent and adds the 5-byte header information. It performs the following four actions:  
  
  
  -Cell header generation/extraction.
  -Cell multiplex and demultiplex function.
  -VPI and VCI translation.
  -Generic Flow Control (GFC). 
  
  
  
  
  3.Atm Adaptation Layer - The AAL performs the adaptation of OSI higher layer protocols, as most applications cannot deal directly with cells. The Adaptation Layer assures the appropriate service characteristics, and divides all types of data into the 48-byte payload that will make up the ATM cell. AAL is further divided into two sublayers: Segmentation and Reassembly (SAR) and Convergence Sublayer (CS).
  
  Key issues in ATM:
   
  - Multiple logical connections over single physical  
    interface.
  - Flow on each logical connections is in fixed sized packets
    called cells.
  - Minimal error and flow control.
  - Data rates (physical layer)25.6Mbps to 622.08Mbps
  
  
  ATM is designed to support:
  
  
  1.Bussiness and institutions who:
  - connect LANS with fiber optic facilities to support  
    specific applications.
  - often send high volumes of data between several of their 
    locations.
  - have linked sites using applications such as CAD/CAM or  
    image processing.
   
  How does ATM differ from Frame Relay?
  
  
  - Atm makes use of a 53byte fixed length cell while the  
    frame in frame relay is much longer, and may vary in  
    length.
  - Error cheking is only done on the header in ATM rather  
    than on the whole cell or frame.
  - Virtual channels of ATM that follow the same route  
    through the network are bundled into paths. A similar  
    mechanism is not used in frame relay.
  
  
  ATM offers significant benefits to users and those who design and maintain communications networks. Because network transport functions can be separated into those related to an individual logical connection (virtual connection) and those related to a group of logical connections (virtual path), ATM simplifies network management. ATM also allows for the integration of networks, improving efficiency and manageability and providing a single network for carrying voice, data, and video.
  
  
  ATM increases network performance and reliability because the network is required to deal with fewer aggregated entities. There is also less processing needed and it takes less time to add new virtual channels because capacity is reserved beforehand on a virtual path connection. Finally, ATM offers a high degree of infrastructure compatibility. Because ATM is not based on a specific type of physical transport, it can be transported over twisted pair, coaxial, and fiber optic cables.
  
  
  Sources: 
  
  
  

  http://www.bookrags.com/research/asynchronous-transfer-mode-atm-csci-04/

  http://en.wikipedia.org/wiki/Asynchronous_Transfer_Mode
  http://technet.microsoft.com/en-us/library/bb726929.aspx
  http://www.scribd.com/doc/12468863/Asynchronous-Transfer-Mode-ATM-wwwstudentcenterin
  
  
  
  
  

  
  
  
  
  
  
  
  
  
  

 

Frame Relay is a standardized wide area network technology that specifies the physical and logical link layers of digital telecommunications channels using a packet switching methodology. Originally designed for transport across Integrated Services Digital Network (ISDN) infrastructure, it may be used today in the context of many other network interfaces.

Advantage and Disadvantage of Frame Relay
 Advantage:
*Reasonable wan speed (64Kbps-1.5Mbps)
*Telco is responsible for insuring connectivity
*One serial part at the central site can suppport multiple incoming PVC's
Disadvantage:
*More difficult to configure and manage properly
*No educational tariff is currently available in Alabama.

Where People Use Frame Relay

Frame Relay is designed as a WAN technology primarily for data. When the deployment began, end

users and carriers alike all felt that digital voice (data) could ride on Frame services. However, that

aside, the network and protocols were designed to carry data traffic across the WAN.

A typical Frame Relay connection

The Frame

The frame is a High−level Data Link Control (HDLC)−framed

format, as shown in this figure. The beginning of the frame (as with most HDLC formats) starts with

an opening flag. Next, a two−byte sequence defines the addressing of the frame. This is called the

Data Link Connection Identifier (DLCI). By very nature of the title (DLCI), we can assume that

Frame Relay works at the data link layer.

Frame Relay Speeds

Frame Relay was designed for speeds up to T−1/E−1 (1.544—2.048 Mbps); it later evolved to

speeds of up to 50 Mbps.Actually, few end users have ever implemented Frame Relay at the

higher speeds; this is more of a speed for the carrier community, but the need for stepped

increments has always been a requirement for data transmission.

Frame Relay Access

A link is installed between the end−user location and the network carrier's node. The normal link

speed is T−1, although many locations can and do use Integrated Services Digital Network (ISDN)

or leased lines at lower rates. Some customers may choose to install a local loop at speeds up to

T−3 (45 Mbps approximately) to support higher−speed access and faster data throughput. (In most

implementations, when a customer exceeds 256 Kbps access, the normal installed link for access is

a T−1 in North America at 1.544 Mbps.

A basic frame relay network

From a functional production environment point of view, there are two types of Frame Relays that can be implemented:

1. Permanent Virtual Circuits (PVC) – It is in the formation of logical end-to-end links mapped over a physical network that PVCs are used.
2. Switched Virtual Circuits (SVC) – SVCs are much harder to implement and maintain. Not surprisingly, they are not very common.

Short for permanent virtual circuit, a virtual circuit that is permanently available. The only difference between a PVC and a switched virtual circuit (SVC) is that an SVC must be reestablished each time data is to be sent. Once the data has been sent, the SVC disappears. PVCs are more efficient for connections between hosts that communicate frequently.

Benefits of SVC is Some public Frame Relay services' pricing structures are forcing end users to build star networkseven when the underlying traffic patterns warrant more meshing. Some pricing structures incant

high subscription rates on ports, which result in star and hub−and−spoke configurations.

Frame Relay Selected for Wireless Data on GPRS - General Packet Radio Services (GPRSs) are designed

around the movement of IP datagrams (always−on Internet access) from a cell phone or personal

digital assistant (PDA) to the public Internet or a VPN connection to an intranet.Once this message

gets to the base station, it is then encapsulated into a Frame Relay frame to be carried across the

wireless carrier's network to a router.Our packets are sent to the PCU(Pocket Control Unit) where it is slotted into the Frame Relay service and carried

through the cellular network.

Today: the biggest factors influencing the “useful” life expectancy of Frame Relay as a “live” production environment protocol are the massive increases in data transmission speeds that we are currently seeing in native IP-based networking. Much of these performance gains are in fact directly attributable to the improvements in transmission technologies such as fiber-optic cable and high-speed Ethernet over copper wire over longer distances (MANs & WANs).

Sources:

http://en.wikipedia.org/wiki/Frame_Relay

http://wiki.answers.com/Q/What_are_the_advantages_and_disadvantages_of_frame_relay

http://www.bukisa.com/articles/136_frame-relay

Broadband Telecommunications Handbook