Low Earth Orbit

Low earth orbits (LEO) are satellite systems used in telecommunication, which orbit between 400 and 1,000 miles above the earth's surface. They are used mainly for data communication such as email, video conferencing and paging. They move at extremely high speeds and are not fixed in space in relation to the earth.

LEO-based telecommunication systems provide underdeveloped countries and territories with the ability to acquire satellite telephone service in areas where it otherwise would be too costly or even impossible to lay land lines.

Low earth orbit is defined as an orbit within a locus extending from the earth’s surface up to an altitude of 1,200 miles. Attributing to their high speeds, data transmitted through LEO is handed off from one satellite to another as satellites generally move in and out of the range of earth-bound transmitting stations. Due to low orbits, transmitting stations are not as powerful as those that transmit to satellites orbiting at greater distances from earth’s surface.
Most communication applications use LEO satellites because it takes less less energy to place the satellites into LEO. Moreover, they need less powerful amplifiers for successful transmission. As LEO orbits are not geostationary, a network of satellites are required to provide continuous coverage.
However, as a result of the popularity of this type of satellite, studies reveal that the LEO environment is getting congested with space debris. NASA keeps track of the number of satellites in the orbit, and estimates that there are more than 8,000 objects larger than a softball circling the globe. Not all of these objects are not satellites, but rather pieces of metal from old rockets, frozen sewage and broken satellites.

Most satellites, the International Space Station, the Space Shuttle, and the Hubble Space Telescope are all in Low Earth Orbit (commonly called "LEO"). This orbit is almost identical to our previous baseball orbiting example, except that it is high enough to miss all the mountains and also high enough that atmospheric drag won't bring it right back home again.

Most satellites, the International Space Station, the Space Shuttle, and the Hubble Space Telescope are all in Low Earth Orbit (commonly called "LEO"). This orbit is almost identical to our previous baseball orbiting example, except that it is high enough to miss all the mountains and also high enough that atmospheric drag won't bring it right back home again.

Advantages and Disadvantages of LEO

Low Earth Orbit is used for things that we want to visit often with the Space Shuttle, like the Hubble Space Telescope and the International Space Station. This is convenient for installing new instruments, fixing things that are broken, and inspecting damage. It is also about the only way we can have people go up, do experiments, and return in a relatively short time.

There are two disadvantages to having things so close, however. The first is that there is still some atmospheric drag. Even though the amount of atmosphere is far too little to breath, there is enough to place a small amount of drag on the satellite or other object. As a result, over time these objects slow down and their orbits slowly decay. Simply put, the satellite or spacecraft slows down and this allows the influence of gravity to pull the object towards the Earth.

The second disadvantage has to do with how quickly a satellite in LEO goes around the Earth. As you can imagine, a satellite traveling 18,000 miles per hour or faster does not spend very long over any one part of the Earth at a given time. So what happens if we want a satellite to spend all of its time over just one part of the Earth? For instance, a weather satellite wouldn't be very effective for us in North America if it didn't have a long dwell time over us. (Dwell time = the time a satellite sits over one part of the globe.) Also, a communications satellite wouldn't work very well for us in North American if it spent most of its time over Africa or Asia.

There are two ways to accomplish this. One solution is to put a satellite in a highly elliptical orbit and the other is to place the satellite in a geosynchronous orbit.

http://www.techopedia.com/definition/8044/low-earth-orbit-leo

http://www.polaris.iastate.edu/EveningStar/Unit4/unit4_sub3.htm

Third−Generation (3G) Wireless Systems

A term commonly used to describe the third generation of technology used in a specific application or industry. In cellular telecommunications, third generation systems used wideband digital radio technology as compared to 2nd generation narrowband digital radio. For third generation cordless telephones, products used multiple digital radio channels and new registration processes allowed some 3rd generation cordless phones to roam into other public places.

This diagram shows a 3rd generation broadband wireless system. This system uses two 5 MHz wide radio channels to provide for simultaneous (duplex) transmission between the end-user and other telecommunication networks. There are different channels used for end- user to the system (called the "uplink") and from the system to the end-user (called the "downlink"). This diagram shows that 3G networks interconnect with the public switched telephone network and the Internet. While the radio channel is divided into separate codes, different protocols are used on the radio channels to give high priority for voice information and high-integrity to the transmission of data information.

Third Generation - 3G Diagram

3G refers to the third generation of mobile telephony (that is, cellular) technology. The third generation, as the name suggests, follows two earlier generations.
The first generation (1G) began in the early 80's with commercial deployment of Advanced Mobile Phone Service (AMPS) cellular networks. Early AMPS networks used Frequency Division Multiplexing Access (FDMA) to carry analog voice over channels in the 800 MHz frequency band.
The second generation (2G) emerged in the 90's when mobile operators deployed two competing digital voice standards. In North America, some operators adopted IS-95, which used Code Division Multiple Access (CDMA) to multiplex up to 64 calls per channel in the 800 MHz band. Across the world, many operators adopted the Global System for Mobile communication (GSM) standard, which used Time Division Multiple Access (TDMA) to multiplex up to 8 calls per channel in the 900 and 1800 MHz bands.
The International Telecommunications Union (ITU) defined the third generation (3G) of mobile telephony standards IMT-2000 to facilitate growth, increase bandwidth, and support more diverse applications. For example, GSM could deliver not only voice, but also circuit-switched data at speeds up to 14.4 Kbps. But to support mobile multimedia applications,
3G had to deliver packet-switched data with better spectral efficiency, at far greater speeds.
However, to get from 2G to 3G, mobile operators had make "evolutionary" upgrades to existing networks while simultaneously planning their "revolutionary" new mobile broadband networks. This lead to the establishment of two distinct 3G families: 3GPP and 3GPP2.
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). According to the 3GPP, there were 166 HSDPA networks in 75 countries at the end of 2007. The next step for GSM operators: LTE E-UTRA, based on specifications completed in late 2008.
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.


1xRTT became available in 2002, followed by commercial EV-DO Rev. 0 in 2004. Here again, 1xRTT is referred to as "2.5G" because it served as a transitional step to EV-DO. EV-DO standards were extended twice – Revision A services emerged in 2006 and are now being succeeded by products that use Revision B to increase data rates by transmitting over multiple channels. 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.

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

sources:

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

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

http://www.wirelessdictionary.com/wireless_dictionary_3G_definition.html

General Packet Radio Services (GPRS)

GSM was the most successful second generation cellular technology, but the need for higher data rates spawned new developments to enable data to be transferred at much higher rates. The first system to make an impact on the market was GPRS. The letters GPRS stand for General Packet Radio System, GPRS technology enabled much higher data rates to be conveyed over a cellular network when compared to GSM that was voice centric.

GPRS became the first stepping-stone on the path between the second-generation GSM cellular technology and the 3G W-CDMA / UMTS system. With GPRS technology offering data services with data rates up to a maximum of 172 kbps, facilities such as web browsing and other services requiring data transfer became possible. Although some data could be transferred using GSM, the rate was too slow for real data applications.

General Packet Radio Services is a packet-based wireless communication service that promises data rates from 56 up to 114 Kbps and continuous connection to the Internet for mobile phone and computer users. The higher data rates allow users to take part in video conferences and interact with multimedia Web sites and similar applications using mobile handheld devices as well as notebook computers. GPRS is based on Global System for Mobile (GSM) communication and complements  existing services such circuit-switched cellular phone connections and the Short Message Service (SMS). 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.

GPRS benefits

GPRS technology brings a number of benefits for users and network operators alike. It was widely deployed to provide a realistic data capability via cellular telecommunications technology.

GPRS technology offered some significant benefits:

• Speed:   One of the headline benefits of GPRS technology is that it offers a much higher data rate than was possible with GSM. Rates up to 172 kbps are possible, although the maximum data rates realistically achievable under most conditions will be in the range 15 - 40 kbps.
• Packet switched operation:   Unlike GSM which was used circuit switched techniques, GPRS technology uses packet switching in line with the Internet. This makes far more efficient use of the available capacity, and it allows greater commonality with Internet techniques.
• Always on connectivity:   A further advantage of GPRS is that it offers an "Always On" capability. When using circuit switched techniques, charges are based on the time a circuit is used, i.e. how long the call is. For packet switched technology charges are for the amount of data carried as this is what uses the services provider's capacity. Accordingly, always on connectivity is possible.
• More applications:   The packet switched technology including the always on connectivity combined with the higher data rates opens up many more possibilities for new applications. One of the chief growth areas that arose from GPRS was the Blackberry form of mobile or PDA. This provided for remote email applications along with web browsing, etc.
• Capex and opex:   The Capital expenditure (capex) and operational expenditure (opex) are two major concerns for operators. As GPRS was an upgrade to existing GSM networks (often implemented as a software upgrade achieved remotely), the capital expenditure for introducing GPRS technology was not as high as deploying a complete new network. Additionally opex was not greatly affected as the basic basestation infrastructure remained basically the same. It was mainly new core network elements that were required.

The GSM and GPRS elements of the system operate separately. The GSM technology still carries the voice calls, while GPRS technology is sued for the data. As a result voice and data can be sent and received simultaneously. 

GPRS and packet switching

The key element of GPRS technology is that it uses packet switched data rather than circuit switched data, and this technique makes much more efficient use of the available capacity. This is because most data transfer occurs in what is often termed a "bursty" fashion. The transfer occurs in short peaks, followed by breaks when there is little or no activity.

Using a traditional approach a circuit is switched permanently to a particular user. This is known as a circuit switched mode. In view of the bursty nature of data transfer it means that there are periods when it will not be carrying data.

To improve the situation the overall capacity can be shared between several users. To achieve this, the data is split into packets and tags inserted into the packet to provide the destination address. Packets from several sources can then be transmitted over the link. As it is unlikely that the data burst for different users will occur all at the same time, by sharing the overall resource in this fashion, the channel, or combined channels can be used far more efficiently. This approach is known as packet switching, and it is at the core of many cellular data systems, and in this case GPRS.

GPRS network

GPRS and GSM are able to operate alongside one another on the same network, and using the same base stations. However upgrades are needed. The network upgrades reflect many of those needed for 3G, and in this way the investment in converting a network for GPRS prepares the core infrastructure for later evolution to a 3G W-CDMA / UMTS.

The upgraded network, as described in later pages of this tutorial, has both the elements used for GSM as well as new entities that are used for the GPRS packet data service.

The upgrades that were required for GPRS also formed the basis of the network required for the 3G deployments (UMTS Rel 99). In this way the investment required for GPRS would not be a one off investment used only on GPRS, it also formed the basis of the network for further developments. In this way GPRS became a stepping stone used for the migration from 2G to 3G.

GPRS mobiles

Not only does the network need to be upgraded for GPRS, but new GPRS mobiles were also required. It is not possible to upgrade an existing GSM mobile for use as a GPRS mobile, although GSM mobiles can be used for GSM speech on a network that also carries GPRS. To utilise GPRS new modes are required to enable it to transmit the data in the required format.

With the incorporation of packet data into the network, this allowed far greater levels of functionality to be accessed by mobiles. As a result a new bread of started to appear. These PDAs were able to provide email and Internet browsing, and they were widely used especially by businesses as they allowed their key people to remain in touch with the office at all times.

Key GPRS parameters

The key parameters for the GPRS, General Packet Radio System, are tabulated below:

Parameter	Specification
Channel Bandwidth	200 kHz
Modulation type	GMSK
Data handling	Packet data
Max data rate	172 kbps

GPRS technology offered a significant improvement in the data transfer capacity over existing cellular systems. It enabled many of the first email and web browsing phones such as PDAs, Blackberrys, etc to be launched. Accordingly GPRS technology heralded the beginning of a new era in cellular communications where the mobile phone capabilities allowed significantly more than voice calls and simple texts. GPRS enabled real data applications to be used and the new phones to become mobile computers on the move allowing businessmen to be always in touch with the office and domestic users to be able to use many more data applications.

Advantages:

• Allows users use the Internet anywhere at any time
• Allows the user to communicate on a world wide scale
• Can be used on both mobiles and laptops

Disadvantages:

• When GPRS is in use, other network related functions cannot be used.
• Expensive to buy a mobile or laptop that has this feature
• GPRS is billed per kilobyte or megabyte depending on the service provider

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. 

 

GPRS also complements Bluetooth, a standard for replacing wired connections between devices with wireless radio connections. In addition to the Internet Protocol (IP), GPRS supports X.25, a packet-based protocol that is used mainly in Europe. GPRS is an evolutionary step toward Enhanced Data GSM Environment (EDGE) and Universal Mobile Telephone Service (UMTS).

sources:

http://searchmobilecomputing.techtarget.com/definition/GPRS

http://www.radio-electronics.com/info/cellulartelecomms/gprs/gprs_tutorial.php

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

MMDS and LMDS

The local multipoint distribution service (LMDS) and multichannel multipoint distribution service (MMDS) have their historical roots in television. MMDS's pre-cursor, the multipoint distribution service (MDS), was established by the Federal Communications Commission (FCC) in 1972. The Commission originally thought MDS would be used primarily to transmit business data. However, the service became increasingly popular for transmitting entertainment programming. Unlike conventional broadcast stations, whose transmissions are received universally, MDS programming is designed to reach only a subscriber-based audience. 

LMDS

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. 

Local multipoint distribution services (LMDS), a line-of-sight technology running in the 28 GHz band. LMDS is most suited for densely populated urban areas where it is difficult and expensive to deploy additional or new wired infrastructures. Typical speeds are 45M bit/sec downstream in a point-to-multipoint configuration. However, LMDS has the potential to exceed OC-3 (155M bit/sec) speeds. Distances between sites are limited to 4 kilometers. 

MMDS

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.

Multichannel multipoint distribution service (MMDS) operates in the 2 GHz to 3 GHz band, is less susceptible to interference than LMDS, and has no line-of-sight requirements. MMDS can support greater distances than LMDS - up to 30 miles between sites. The tradeoff is that MMDS is slower, delivering downstream speeds in the neighborhood of 10M bit/sec.  

TECHNOLOGY

LMDS and MMDS share a number of common architectural features although they vary from one manufacturer to another according to features and capabilities. The core components are a base-station transceiver (transmitter and receiver), a customer-premise transceiver and some kind of CPE network interface unit (NIU) or card.

For downstream traffic to the customers' premises, the base station converts the digital bitstream containing voice, data and video information into microwaves that are transmitted to a small antenna on the customer's premises. The microwaves are then reconverted back into a digital bitstream by the NIU and delivered to the end-user. The process is reversed for upstream traffic. When the base station receives the microwave signal and has converted it into a digital bitstream, this is routed through, or 'backhauled' to, the wider network, through which the data or call is delivered to its destination.

Unlike the lower frequency cellular systems, LMDS and MMDS both require a line-of-sight between the base station and customer premise transceivers. This is a prerequisite for any system operating above approximately 2-3.5GHz. The base station is connected to the wide-area network switch or internet POP via either a high-capacity wireline (usually fibre optic) or wireless. Similarly, at the customer's premises, the signal can be delivered to the end-user terminals via either of these.

BENEFITS

Wireless systems are being deployed to fulfil a number of functions. On a network level they are suitable for both access and backbone infrastructure. It is generally agreed, however, that it is in the access market where the key advantages are held over wireline alternatives. The principal strengths of LMDS/MMDS are:

• Speed of network deployment is much quicker with wireless systems enabling rapid, early market entry
• Entry, deployment and upgrading costs are much lower than for wireline alternatives, for which engineering (cabling and trenching) costs are significantly higher
• The maintenance, management and operation expenditure is lower. Wireless systems can be rolled out much faster, enabling an earlier return on investment
• Scalable architectures enable expanded coverage and services in direct relation to the level of demand
• Only one network architecture is required to provide a full suite of interactive voice, video and data services that can be expanded as and when desired

Advantages and disadvantages of MMDS spectrum

• Propagation over long distances up to 100 km. with single tower
• Less attenuation due to rain, foliage
• RF component costs lower at 2.5 GHz
• Equipment readily available today
• Limited capacity without sectorization, cellularization which adds
complexity and cost
• Interference issues with other MMDS and ITFS licensees
• Large upstream bandwidth in MMDS band requires careful
planning, filtering etc.
• Cellularization later on may require retuning the entire network
(every CPE )

Advantages and disadvantages of
LMDS spectrum

 
• Very large bandwidth available for data, IP telephony,
   video conferencing services
• Large capacity
• Higher RF component costs
• Small cell size, 2-8 Km.
• Does not cover entire metropolitan area of a large city
without adding many cells at high cost.

Difference between MMDS and LMDS technology:

 

MMDS uses the lower frequency range compared to LMDS, hence MMDS covers larger areas than LMDS but provides lower access speed. 

LMDS, MMDS race for low-cost implementation

 LMDS, MMDS race for low-cost implementation
Advocates for wireless residential broadband access had a busy November, as the millimeter-wave camp examining 28- and 38-GHz local multipoint distribution service (LMDS) networks raced to make consumer and business systems cost-effective for single-user Internet access. They faced an onslaught of new efforts from competing radio system specialists aiming to make the microwave networks known as multichannel multipoint distribution service (MMDS) at 2.5 GHz and 5 GHz easier to deploy. Cost-reduction efforts in both markets may result in making wireless "last-mile" access technologies viable alternatives to always-on Internet access technologies such as cable modems and digital subscriber lines.

The IEEE's new 802.16 working group for broadband wireless access networks is trying to hammer out common principles for both MMDS and LMDS systems. At a meeting in Koloa, Hawaii, in mid-November, the panel worked on physical-transceiver, Medium Access Control and frequency-coexistence standards for both camps. On Nov. 11, the working group approved an official study group extension to examine MMDS and similar networks that operate at less than 10 GHz. The existing 802.16.1 air interface project is for networks ranging in frequency from 10 GHz to 66 GHz.

With both technologies, an antenna and radio are installed on the roof of a business' site and are connected by coaxial cable to customer premises equipment in the LAN wiring closet. Commercial service availability is imminent. WorldCom has been conducting MMDS service trials with schools, residential and business customers in Boston; Dallas; Jackson, Miss.; Baton Rouge, La.; and Memphis, Tenn., using equipment from Cisco and Motorola. Meanwhile, Cisco has said it plans to begin commercially shipping LMDS and MMDS interfaces for its routers by midyear - which sounds like any day now. The move could prove to be a major stepping stone for service provider deployments. 


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


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


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


http://www.networkworld.com/newsletters/wireless/2000/0626wire1.html

Microwave Radio and based systems:

Microwave frequencies range from 300 MHz to 30 GHz, corresponding to wavelengths of 1 meter to 1 cm.  These frequencies are useful for terrestrial and satellite communication systems, both fixed and mobile.  In the case of point-to-point radio links, antennas are placed on a tower or other tall structure at sufficient height to provide a direct, unobstructed line-of-sight (LOS) path between the transmitter and receiver sites. In the case of mobile radio systems, a single tower provides point-to-multipoint coverage, which may include both LOS and non-LOS paths.  LOS microwave is used for both short- and long-haul telecommunications to complement wired media such as optical transmission systems.  Applications include local loop, cellular back haul, remote and rugged areas, utility companies, and private carriers.   Early applications of LOS microwave were based on analog modulation techniques, but today’s microwave systems used digital modulation for increased capacity and performance. 

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.

Microwave radio transmission is commonly used in point-to-point communication systems on the surface of the Earth, in satellite communications, and in deep space radio communications. Other parts of the microwave radio band are used for radars, radio navigation systems, sensor systems, and radio astronomy.

Standards:

In the United States, radio channel assignments are controlled by the Federal Communications Commission (FCC) for commercial carriers and by the National Telecommunications and Information Administration (NTIA) for government systems.

The FCC's regulations for use of spectrum establish eligibility rules, permissible use rules, and technical specifications. FCC regulatory specifications are intended to protect against interference and to promote spectral efficiency. Equipment type acceptance regulations include transmitter power limits, frequency stability, out-of-channel emission limits, and antenna directivity.

The International Telecommunications Union Radio Committee (ITU-R) issues recommendations on radio channel assignments for use by national frequency allocation agencies. Although the ITU-R itself has no regulatory power, it is important to realize that ITU-R recommendations are usually adopted on a worldwide basis.

The Digital Microwave System supports and interconnects the following two way-radio services:  

• Local Base - Provides instantaneous two-way radio communications between a dispatch center and employees working in the immediate area or close proximity. As an example a State Parks Dispatcher at Park headquarters could contact a Park Ranger working on the other side of the Park.
• Remote Base - Provides instantaneous two-way radio communications between a dispatch center and employees working long distances from the center. As an example a State Police Dispatcher could contact an officer on patrol possibly hundreds of miles from the dispatch center.
• Single Channel Console - Provides a convenient, functional, and inexpensive interface between a dispatcher and one local or remote base station.
• Multi Channel Console - Allows a dispatcher to control multiple local and/or remote base stations from a single dispatch center.
• Mobile Radio - Usually installed in a service vehicle, allows employees in the field to communicate to a dispatch center through a local or remote base station.
• Mobile Radio with Repeater - Allows an employee, typically police officers, to operate their mobile radio when they are out of the vehicle. An office on foot can instantaneously communicate from a hand held radio to a dispatch center hundreds of mile away.
• Radio Pager - A radio pager functions as a hand held radio and can also receive pages from any telephone.
• Portable Radio - Provides communication between a small handheld device and another portable radio in the area or through a base station to a dispatch center. 

Advantage: Able to Transmit Large Quantities of Data

According to "Microwave Communication," microwave radio systems have the capacity to broadcast great quantities of information because of their higher frequencies. They use repeaters (a device that receives the transmitting signal through one antenna, converts it into an electrical signal and retransmits it) to transmit large volumes of data over great distances. Microwave radio communication systems propagate signals through the earth's atmosphere. These signals are sent between transmitters and receivers that lie on top of towers. This allows microwave radio systems to transmit thousands of data channels between two points without relying on a physical transmitting medium (optical fibers or metallic cables).

Advantage: Relatively Low Costs

Microwave communication systems have relatively low construction costs compared with other forms of data transmission, such as wire-line technologies. A microwave communication system does not require physical cables or expensive attenuation equipment (devices that maintain signal strength during transmission). Mountains, hills and rooftops provide inexpensive and accessible bases for microwave transmission towers.

Disadvantage: Line of Sight Technology

Microwave radio systems are a line of sight technology, meaning the signals will not pass through objects (e.g., mountains, buildings and airplanes). This drawback limits microwave communication systems to line of sight operating distances. Signals flow between one fixed point to another, provided no solid obstacle disrupts the flow.

Disadvantage: Subject to Electromagnetic and Other Interference

 According to "Rural America at the Crossroads: Networking for the Future," microwave radio signals are affected by electromagnetic interference (EMI). EMI is any disturbance that degrades, obstructs or interrupts the performance of microwave signals. Microwave signal disruption EMI is caused by electric motors, electric power transmission lines, wind turbines, television/radio stations and cell phone transmission towers. Wind turbines, for instance, scatter and diffract TV, radio and microwave signals when placed between signal transmitters and receivers. Microwave radio communication is also affected by heavy moisture, snow, vapor, rain and fog due to rain fade (the absorption of microwave signals by ice, snow or rain, causing signal degradation and distortion).

The Statewide Digital Microwave System also supports broadband data services and telephone to agency field offices: 

• T-1 or greater bandwidth connecting isolated field offices to headquarter data networks and Internet services.
• Telephone service to remote stations lacking commercial providers.
• Collocation agreements with cell providers to expand cellular service to rural areas.
• Security alarming and video monitoring of critical State communications facilities. 

Dispatch operations include monitoring, gathering, processing and disseminating all radio transmissions from field units of all agencies as well as all related incoming and outgoing telephone communications for each agency served. 

 

http://www.eogogics.com/talkgogics/tutorials/microwave-line-of-sight-systems

 

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

 

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

 

http://www.doit.state.nm.us/service_catalog/radio.html

 

ADSL

ADSL (Asymmetric Digital Subscriber Line) is a technology for transmitting digital information at a high bandwidth on existing phone lines to homes and businesses. Unlike regular dialup phone service, ADSL provides continuously-available, "always on" connection. ADSL is asymmetric in that it uses most of the channel to transmit downstream to the user and only a small part to receive information from the user. ADSL simultaneously accommodates analog (voice) information on the same line. ADSL is generally offered at downstream data rates from 512 Kbps to about 6 Mbps. A form of ADSL, known as Universal ADSL or G.Lite, has been approved as a standard by the ITU-TS. 

It is a type of DSL broadband communications technology used for connecting to the Internet. ADSL allows more data to be sent over existing copper telephone lines on plain old telephone services (POTS), when compared to traditional modem lines. A special filter, called a microfilter, is installed on a subscriber's telephone line to allow both ADSL and regular voice (telephone) services to be used at the same time. ADSL requires a special ADSL modem and subscribers must be in close geographical locations to the provider's central office to receive ADSL service. Typically this distance is within a radius of 2 to 2.5 miles. ADSL supports data rates of from 1.5 to 9 Mbps when receiving data (known as the downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate). ADSL is designed to support the typical home user who frequently downloads large amounts of data from Web sites and P2P networks but upload relatively less often. ADSL works by allocating a majority of the available phone line frequencies for communication of downstream traffic. ADSL was specifically designed to exploit the one-way nature of most multimedia communication in which large amounts of information flow toward the user and only a small amount of interactive control information is returned. Several experiments with ADSL to real users began in 1996. In 1998, wide-scale installations began in several parts of the U.S. In 2000 and beyond, ADSL and other forms of DSL are expected to become generally available in urban areas. With ADSL (and other forms of DSL), telephone companies are competing with cable companies and their cable modem services. 

Benefits that ASDL can provide:

• Provides the ability to talk on the phone while surfing through the Internet, because, as noted above, voice and data work in separate bands, which implies a separate channel.
•  Use existing infrastructure (the basic telephone network). This is advantageous both for the operators who do not face large costs for the implementation of this technology to users, since the cost and time it takes to keep available the service is less than if the operator had to undertaking works to build new infrastructure.
• ADSL users have access to the Internet, not having to establish this connection by dialing or signaling to the network.
• Provides connection speeds much higher than the one made by dial-up Internet. This is the most interesting to users.

This is possible because they have point to point, so that the line between the PBX and the user is not shared, which also ensures dedicated bandwidth to each user, and increases the quality of service. 

Advantages of ADSL:

• 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.

Disadvantages of ADSL:

• 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.

In other respects, ADSL possesses all of the characteristics one associates with DSL, including "high-speed" service, an "always on" combination of voice and data support, and availability and performance that is limited by physical distance. ADSL is technically capable of up to 6 Mbps (roughly 6000 Kbps), but ADSL customers in practice obtain 2 Mbps or lower for downloads and up to 512 Kbps for uploads. 

Sources:

http://searchnetworking.techtarget.com/definition/ADSL

http://compnetworking.about.com/od/dsldigitalsubscriberline/g/bldef_adsl.htm

http://www.webopedia.com/TERM/A/ADSL.html

http://computer.howstuffworks.com/dsl.htm

http://freepressreleases.eu/advantages-and-disadvantages-of-adsl-technology/

ASYNCHRONOUS TRANSFER MODE

ATM (asynchronous transfer mode) a high-speed networking standard designed to support both voice and data communications. ATM is normally utilized by Internet service providers on their private long-distance networks. ATM operates at the data link layer (Layer 2 in the OSI model) over either fiber or twisted-pair cable. ATM is a dedicated-connection switching technology that organizes digital data into 53-byte cell units and transmits them over a physical medium using digital signal technology. Individually, a cell is processed asynchronously relative to other related cells and is queued before being multiplexed over the transmission path. Because ATM is designed to be easily implemented by hardware (rather than software), faster processing and switch  speeds are possible. The prespecified 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 (BISDN).

ATM provides the opportunity for both end users and communications carriers to transport virtually to any type of information using a common format. It allows variety of different applications and services (video, data, voice etc) to be supported on a single network. It has been adapted as the transmission mechanism for B-ISDN which is a digital network standard which will replace many existing network standards. 

ATM NETWORK

The technology allows both public (i.e., RBOC or carrier) and private (i.e., LAN or LAN-to-internal switch) ATM networks. This capability gives a seamless and transparent (to the user) connection from one end user to another end user, whether in the same building or across two continents. The basic network structure is as shown on the following page.

                  .^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^.
.-----------.     | .--------.   2   .--------.   |
|End User 1 |-----|-|  ATM   |-------|  ATM   |   |
`-----------'   1 | | Switch |       | Switch |---|-------+
                  | `--------'       `--------'   |       |
                  |        ATM Network  1         |       |
                  `vvvvvvvvvvvvvvvvvvvvvvvvvvvvvvv'       |
                                                        3 |
                                                          |
                  .^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^.        |
.-------------.   | .--------.   2   .--------.  |        |
| Private ATM |---|-|  ATM   |-------|  ATM   |  |        |
|    Switch   | 1 | | Switch |       | Switch |--|--------+
`------+------'   | `--------'       `--------'  |
     1 |          |         ATM Network 2        |
 .-----+------.   `vvvvvvvvvvvvvvvvvvvvvvvvvvvvvv'
 | End User 2 |
 `------------'

Three types of interfaces exist in this diagram:

1. User-to-Network Interface (UNI)
2. Network-to-Network Interface (NNI)
3. Inter-Carrier Interface (ICI)

The UNI exists between a single end user and a public ATM network, between a single end user and a private ATM switch, or between a private ATM switch and the public ATM network of an RBOC.

The NNI exists between switches in a single public ATM network. NNIs may also exist between two private ATM switches.

The ICI is located between two public ATM networks (an RBOC and an interexchange carrier).

All of these interfaces are very similar. The major differences between these types of interfaces are administrative and signalling related. The only type of signalling exchanged across the UNI is that required to set up a VIRTUAL CHANNEL for the transmission. 

ADVANTAGES OF ATM

• ATM 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 

DISADVANTAGES OF ATM

• 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
  The wait, with all the promise of ATM’s capabilities many details are still in the standards process

 ATM differs from more common data link technologies like Ethernet in several ways. For example, ATM utilizes no routing. Hardware devices known as ATM switches establish point-to-point connections between endpoints and data flows directly from source to destination. Additionally, instead of using variable-length packets as Ethernet does, ATM utilizes fixed-sized cells. ATM technology is designed to improve utilization and Quality of service (QoS) on high-traffic networks. Without routing and with fixed-size cells, networks can much more easily manage bandwidth under ATM than under Ethernet, for example. The high cost of ATM relative to Ethernet is one factor that has limited its adoption to "backbone" and other high-performance, specialized networks. 

In a data communications environment, the network can range in scope from a token-ring LAN to an X.25 or Frame Relay WAN. Thus, although some features are common to both LAN and WAN environments, there is also some variability. In general, a data communications network transports data by using variable-length packets. Although many WAN protocols are connection-oriented, some are connectionless. Similarly, many LAN protocols are connectionless, whereas others are connection-oriented. Because data communications networks were designed to transport files, records, and screens of data, transmission delay or latency, if small, does not adversely affect users. In comparison, in a telecommunications network, a similar amount of latency that is acceptable on a data network could wreak havoc with a telephone conversation. Recognizing the differences among voice, video, and data transportation, ATM was designed to adapt to the time sensitivity of different applications. It includes different classes of service that enable the technology to match delivery to the time sensitivity of the information it transports.


Sources:

http://searchnetworking.techtarget.com/definition/ATM

http://www.techfest.com/networking/atm/atm.htm

http://compnetworking.about.com/od/networkprotocols/g/bldef_atm.htm

http://homepages.uel.ac.uk/u0124452/MyPage/Advantages%20and%20Disadvantages%20of%20ATM.htm

FRAME RELAY

 Frame relay is a protocol standard for LAN internetworking which provides a fast and efficient method of transmitting information  from a user device LAN ti a bridge and routers. Data is sent in HLDC packets which refers to "frames". It also uses a packet-switching technology similar to X.25 which can make your networking quicker,simpler and less costly. Like X.25, Frame Relay is a packet-switched protocol. But the Frame-Relay process is streamlined. There are significant differences that make Frame Relay a faster, more efficient form of networking. A Frame-Relay network doesn't perform error detection, which results in a considerably smaller amount of overhead and faster processing than X.25. Frame Relay is also protocol independent-it accepts data from many different protocols. This data is encapsulated by the Frame-Relay equipment, not the network.

  X.25 circuits can be initiated and ended from the users terminals. Frame relay circuits are set up at the time of installation and are maintained 24 hours per day, 7 days per week. Frame relay circuits are not created and ended by user at their terminals or PC's. However, the user may have an application running over a frame relay circuit where computer to terminal sessions are initiated and ended by the user. These sessions are related to the application, not to the underlying frame relay network.

  Frame relay relies on the customer equipment to perform end to end error correction. Each switch inside a frame relay network just relays the data (frame) to the next switch. X.25, in contrast, performs error correction from switch to switch.

 Frame Relay offers an attractive alternative to both dedicated lines and X.25 networks for connecting LANs to bridges and routers. The success of the Frame Relay protocol is based on the following two underlying factors Because virtual circuits consume bandwidth only when they transport data, many virtual circuits can exist simultaneously across a given transmission line. In addition, each device can use more of the bandwidth as necessary, and thus operate at higher speeds.The improved reliability of communication lines and increased error-handling sophistication at end stations allows the Frame Relay protocol to discard erroneous frames and thus eliminate time-consuming error-handling processing. 

 These two factors make Frame Relay a desirable choice for data transmission; however, they also necessitate testing to determine that the system works properly and that data is not lost.

 Frame routers translate existing data communications protocols for transmission over a Frame-Relay network, then route the data across the network to another frame router or other Frame-Relay compatible device. Frame routers can handle many types of protocols, including LAN protocols. They're used in environments that require T1 or slower network access speeds. Each router supports one of many physical data interfaces and can provide several user ports. While Frame Relay offers many benefits, a host of problems have to be overcome before it can be used effectively as a carrier for voice, fax, or video traffic. Until recently, the advancements were vendor-specific solutions that offered no interoperability. Recently ratified industry standards have addressed such issues as compression, packetization, and prioritization. This move towards standardization has been led by the Frame-Relay Forum (FRF) and the International Telegraphic Union (ITU). 

 Most frame relay WANs are hosted by commercial network operators that charge flat rates based on the speed of service or volume of data required. Supported by relatively inexpensive networking hardware, frame relay is based on establishing a logical or virtual circuit across a network with another computer. In frame relay, the packets, or frames, of data may vary in size, and no attempt is made to correct errors. This latter feature is based on the assumption that frame relay is run over relatively high quality, digital networks and the data is less susceptible to errors. This also improves speed since the network protocol isn't trying to correct the data. The stability of this connection allows frame relay service providers to guarantee a certain minimum level of service. The comparative low cost and high quality of service made frame relay one of the most popular WAN technologies in the 1990s. 

sources:

http://www.referenceforbusiness.com/encyclopedia/Val-Z/Wide-Area-Networks-WANs.html

http://www.protocols.com/pbook/frame.htm

http://www.dcbnet.com/notes/framerly.html

http://www.arcelect.com/frame_relay-56kbps_ft1-t1.htm

This would be my understanding about the Broadband telecommunications particularly about the ISDN or the Integrated Services Digital Network. Base on my research that it is a system of digital phone connections which allows voice and data to be transmitted simultaneously across the world by using end-to-end digital connectivity. All central and end office switching is performed by digital switches, and signaling system or the call establishment occurs through digital protocols. ISDN in concept is the integration of both analog or voice data together with digital data over the same network. So definitely, ISDN is a digital transmission from the customer-premises equipment such as telephones, data terminals and fax machines over the other media at the same network. The voice data and the digital data has been integrated for fast transmission and provides a raw data rate of 144 kbps for a single telephone company. With that data rate, it would sufficiently enough teleconferencing and would be enough for a a data to be transmitted. ISDN has two levels of services, the Basic Rate Interface (BRI), which is intended for individual users and small enterprises while the Primary Rate Interface (PRI), provides high capacity service for larger users. In BRI, the initial concept had this as being everything from analog telephone calls which is been digitized for teleconferencing data and this would be switched channels. The BRI has two switched channels of 64 kbps B-channel and one 16 kbps D-channel. PRI is intended for users with greater capacity requirements. PRI  has 23 B-channels and one 64 kbps D-channel. B-channel can carry any type of digital information such as voice, data, or video with no restrictions on format or protocol imposed by the ISDN carrier. D-channel carries the information needed to connect or disconnect calls and to negotiate special calling parameters such as automatic number ID, call waiting, data protocol. ISDN also provide internet service that works over telephone lines. Generally, internet service supports data rates of 128 kbps. ISDN emerged as an alternative to traditional dial-up networking. Advantages of ISDN is that it allows multiple digital channels to be operated simultaneously through the same regular phone wiring used for analog lines. For multiple device, ISDN allows multiple devices to share a single line. It is possible to combine many different digital data sources and the information routed to the proper destination. ISDN integrates all services by providing a small set of standard interfaces and access protocols that apply to all services because ISDN is an international standard, the same interfaces and access protocols should be available anywhere in the world, across international boundaries, and among equipment from any set of vendors.


sources:
http://www.techfest.com/networking/wan/isdn.htm

http://www.jet.net/isdn/isdnintro.html

http://compnetworking.about.com/od/internetaccessbestuses/g/bldef_isdn.htm

http://www.ralphb.net/ISDN/defs.html

http://searchenterprisewan.techtarget.com/definition/ISDN