Chapter 18: MMDS and LMDS

 MMDS

           Multichannel Multipoint Distribution Service (MMDS) is a broadcasting and communications service that operates in the ultra-high-frequency (UHF) portion of the radio spectrum between 2.1 and 2.7 GHz. MMDS is also known as wireless cable. It was conceived as a substitute for conventional cable television. However, it also has applications in telephone/fax and data communications. In MMDS, a medium-power transmitter is located with an omnidirectional broadcast antenna at or near the highest topographical point in the intended coverage area. The workable radius can reach up to 70 miles in flat terrain (significantly less in hilly or mountainous areas). Each subscriber is equipped with a small antenna, along with a converter that can be placed next to, or on top of, a conventional TV set. There is a monthly fee, similar to that for satellite TV service. MMDS frequencies provide precise, clear, and wide−ranging signal coverage. Customers are protected from interference from other users when the provider uses the licensed frequencies.

System Configuration

       The wireless system consists of head−end equipment (satellite signal reception equipment, radio transmitter, other broadcast equipment, and transmission antenna) and reception equipment at each subscriber location

A typical configuration of an MMDS system

Advantages and disadvantages of  MMDS                    
                    • 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

Key Elements of MMDS system

   1. The Headend

                Equipment such as signal processors, demodulators and Satellite Receivers to generate input baseband video and audio signals. It is an optional Encoding System to scramble some channels or an addressable control system to control the decoders at the subscriber’s premises.

   2. The Transmitter

                The Transmitter converts the broadband signal provided by the modulators to the transmit microwave frequency (2500 to 2586 MHz) and amplifies the resulting microwave signal to the power level desired for transmission.

   3. The Transmitting Antenna

                The Transmitting Antenna system includes the cables or waveguide connecting the transmitter to the antenna, as well as the antenna itself and, if required, the pressurization system for the antenna.
   4. The Subscriber Equipment

                The Subscriber Equipment consists of an outdoor unit (an integrated antenna + down-converter), which converts the received microwave signal to frequencies in the 220 to 408 MHz range, which is suitable for feeding standard TV sets. The outdoor unit is connected through a coaxial cable to the subscriber’s home wiring or directly to the TV set.

LMDS

           Local Multipoint Distribution Service (LMDS) is an ideal solution for bringing high-bandwidth services to homes and offices within the last-mile—an area where cable or optical fibre may not be convenient or economical. Having architectural similarities with cellular networks, LMDS is a fixed (non-mobile) point-to-multipoint wireless access technology that typically operates in the 28 GHz band and offers Line-of-Sight (LoS) coverage up to 3-5 km. Depending on the local licensing regulations in a country, such broadband wireless systems may operate anywhere from 2 to 42 GHz. Though data transfer rates for LMDS can reach 1.5 to 2 Gbps, in reality it is designed to deliver data at speeds between 64 Kbps to 155 Mbps a more realistic downstream average being around 38 Mbps.

               At such speeds, LMDS may be the key to bringing multimedia data, supporting voice connections, the Internet, videoconferencing, interactive gaming, video streaming and other high-speed data applications to millions of customers worldwide over the air.

            As with other wireless networks, LMDS technology offers the advantage that it can be deployed quickly and relatively inexpensively. New market entrants who do not have an existing network like incumbent's copper wires or fibres - can rapidly build an advanced wireless network and start competing. LMDS is also attractive to incumbent operators who need to complement or expand existing networks.

How LMDS works and its limitation

             Sending digital signals of the required complexity at 28 GHz is made possible by recent advances in technologies such as Digital Signal Processors, advanced modulation systems and Gallium Arsenide (GaAs) integrated circuits, which are cheaper and function much better than silicon ones at these high frequencies. Unlike a cellular mobile phone network, in which a user can move one cell to another, the transceiver of an LMDS customer has a fixed location and remains within the same cell. Normally the customers' antennas are located on rooftops, to get a good LoS to the hub transceiver.

             Like in other microwave applications, LMDS cell size too is limited by rain fade,Also, walls, hills and even leafy trees block, reflect and distort the signal, creating significant shadow areas for a single transmitter. Some operators may serve a cell with several transmitters to increase coverage; most prefer one transmitter per cell, sited to target as many users as possible.

Advantages and disadvantages of LMDS

                • 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

SOURCES:
http://searchnetworking.techtarget.com/definition/Multichannel-Multipoint-Distribution-Service

http://www.rficsolutions.com/publishedpapers/Broadbandwireless.pdf
http://www.cableaml.com/ing_mmds_system_description.html

http://www.angelfire.com/nd/ramdinchacha/DEC00.html

Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf

Chapter 17: Microwave− and Radio−Based Systems

   

       The technology used for microwave communication was developed in the early 1940's by Western Union. The first microwave message was sent in 1945 from towers located in New York and Philadelphia. Following this successful attempt, microwave communication became the most commonly used data transmission method for telecommunications service providers.

          With the development of satellite and cellular technologies, microwave has become less widely used in the telecommunications industry. Fiber-optic communication is now the dominant data transmission method. However, microwave communication equipment is still in use at many remote sites where fiber-optic cabling cannot be economically installed.      

           In terms of business industry, the technology of microwaves have been taken for granted for years. It is quietly grown into a $4.6 billion global business annually with the expectation that will reach up to $10 billion by 2006. Microwaves also becomes a vital link in the overall backbone networks over the years. Achieving the new acclaim in the wireless revolution, microwaves are now relaying thousands of telephone conversations from place to place, bypassing the local landlines. Microwaves are between 1 mm and 30 cm long, and operate in a frequency range from 300 MHz to 300 GHz.

Possible market share for microwave products

          Today's microwave radios can be installed quickly and relocated easily.Several installations have taken over a year to be approved, only to have the radio system installed and running within a day or two. Compared to landlines which are vulnerable to everything including flooding, rodent damage, backhoe cuts, and vandalism, microwave systems provide more reliable service. Using a radio system, a developing country without a wired communications infrastructure can install a leading−edge telecommunications system within a matter of months. For these reasons, regions with rugged terrain or without any copper landline backbone in place find it easier to leap into the wireless age and provide the infrastructure at a fraction of the cost of installing wires.

         The cellular and Personal Communications Service (PCS) industries invested heavily in microwave radios to interconnect the components of their networks.In addition, a new use of microwave radio, called micro/millimeter wave radio, is bringing transmission directly into buildings through a new generation of tiny receiver dishes.Tens of thousands of new cell sites and PCS sites have been constructed and will continue to be constructed over the next few years, further expanding the use of microwave radio systems in each of these sites. As third−generation, handheld devices make their way into the industry, more wireless inter-connectivity will be used.

         Microwave also played a very crucial part of the PCS industry as the PCS systems use the 1.9 to

2.3 GHz frequency band.One study indicated that the PCS industry would spend over $3 billion in microwave equipment and services by 2005.

    The newer micro/millimeter−wave radios, which are smaller and usually less expensive than other microwaves, are also popular with these CAPs and PCS suppliers. They are used in urban areas to extend the fiber networks. These radio units use the high−frequency and width that hadn't been used before.An advantage of these systems is the small antennas that can be hidden on rooftops without interfering with zoning ordinances or creating aesthetic controversy.

Advantage and Disadvantage of Microwave

Advantage:                                                        
            1. No cables needed                                        
            2. Multiple channels available
            3.Wide bandwidth
 Disadvantage:
            1. Line-of-sight will be disrupted if any obstacle, such as new buildings, are in the way

       2.Signal absorption by the atmosphere. Microwaves suffer from attenuation due to atmospheric conditions.

            3.Towers are expensive to build

Analog microwave into Digital microwave

         Analog microwave communication may be most economical for use at your tower sites simply because it is already paid for and in service. If you are already operating microwave equipment, it is most likely analog. To avoid having to retrain your operators, you may want to stick with the analog microwave communication equipment you already have. Because you've already gotten comfortable with this equipment, you've probably also learned its capabilities, so you're unlikely to overburden your transport system with new digital equipment.

        Digital microwave communication utilizes more advanced, more reliable technology. It is much easier to find equipment to support this transmission method because it is the newer form of microwave communication. Because it has a higher bandwidth, it also allows you to transmit more data using more verbose protocols. The increased speeds will also decrease the time it takes to poll your microwave site equipment. This more reliable format provides for more reliable reporting with advanced communication equipment, while also allowing you to bring in your LAN connection when it becomes available at the site.

For more information about microwaves,kindly watch this video:

SOURCE: 
http://www.dpstele.com/dpsnews/techinfo/microwave_knowledge_base/microwave_communication.php
http://www.sqa.org.uk/e-learning/NetTechDC01CCD/page_44.htm
Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf

Chapter 16: xDSL

Overview             

                     One of the major problems facing the incumbent local exchange carriers (ILEC) is the ability to maintain and preserve their installed base. Ever since the Telecommunications Act of 1996, there has been mounting pressure on the ILECs to provide faster and more correct Internet access. In order to provide the higher−speed communications abilities, these carriers have continually looked for new means of providing the service.

                  However, the ILECs have an installed base of unshielded twisted pair in the local loop that cannot be ignored or abandoned. Therefore, a new form of communications was needed to work over the existing copper cable plant. One of the technologies selected was the use of xDSL. The DSL family includes several variations of what is known as digital subscriber line.

a.  ADSL

                   ADSL is a modem technology used to transmit speeds of between 1.5 Mbps and 6 Mbps under current technology.ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services. Most people consider ADSL as a transmission system instead of a modification to the existing transmission facilities.

Data rates for ADSL, based on installed wiring at varying gauges

The Analog Modem History

                   In the early days of modem communications, the Bell telephone companies provided all services across North America.Leased lines were used when specific speeds or volumes were anticipated, but not guaranteed by the dial−up services. Regardless of the modem and lines used, the main provider was the key ingredient.

               In 1968, things began to change. With court decisions allowing the introduction of competitive devices and the connection of these devices on the regulated carrier's network, demands began to escalate. Restrictions on power output and energy levels were in place to prevent any interference from the modems on the voice network. Also, the customer−provided modems were interconnected through a data coupler provided by the local regulated carriers. This, of course, involved a fee for the connection through the telephone company that provided protection equipment.

           Later, the Federal Communications Commission (FCC) in the United States and the Communications Radio and Television Commission (CRTC) in Canada allowed changes in the way the interconnection was handled. Modem manufacturers were allowed to produce their products according to a set of specifications and registrations, eliminating the need for the telephone company protection equipment and the fee associated with the monthly rentals.

2. IDSL

                    IDSL is a system in which digital data is transmitted at 128Kbps on a regular copper telephone line from a user to a destination using digital transmission, bypassing the telephone company's central office equipment that handles analog signals.IDSL uses ISDN-based technology to provide a data communication channel across existing copper telephone  lines at a rate of 144 kbit/s, slightly higher than a bonded dual channel ISDN connection a 128 kbit/s.

The IDSL line connection enables 128 Kbps in total simultaneously.

3. HDSL

          

                     HDSL delivers 1.544 Mbps of bandwidth each way over two copper twisted pairs. Because HDSL provides T1 speed, telephone companies have been using HDSL to provision local access to T1 services whenever possible. The operating range of HDSL is limited to 12,000 feet (3658.5 meters), so signal repeaters are installed to extend the service. HDSL requires two twisted pairs, so it is deployed primarily for PBX network connections, digital loop carrier systems, interexchange POPs, Internet servers, and private data networks.

4. SDSL

                 SDSL was developed to provide high-speed communications on that single cable pair but at distances no greater than 10k. Despite the distance limitation, SDSL was designed to deliver 1.544Mbps on the single cable pair. Typically, the providers offer SDSL at 768 Kbps. This creates a dilemma for the carriers because HDSL dan do the same things as SDSL.

5. RADSL

                

                      This is a popular variation of ADSL that allows the modem to adjust the speed of the connection depending on the length and quality of the line. This gives the flexibility to adapt to the changing conditions and adjust the speeds in each direction to potentially maximize the throughput on each line. Additionally, as line conditions change, you can see the speeds changing in each direction during the transmission. Many of the ILEC's have installed RADSL as their choice, given the local loop conditions. Speeds of up to 768 Kbps are the preferred rates offered by the incumbent providers.

6. CDSL

                     CDSL does not use, nor need, a splitter on the line. Moreover, speeds of up to 1 Mbps in the download direction and 160 Kbps in the upward direction are provided. It is expected that the speeds and DSL will meet the needs of the average consumer for some time to come.

7. SHDSL

                SHDSL supports repeaters, which further increase the reach capability of this technology. Another critical advantage of SHDSL is its increase in symmetric bandwidth. SHDSL is also rate adaptive, enabling flexible revenue−generation models and enabling service providers to offer service−level agreements that ensure businesses get the service they want, when they want it.

8. VDSL

                      VDSL provides an incredible amount of bandwidth, with speeds up to about 52 megabits per second (Mbps). Compare that with a maximum speed of 8 to 10 Mbps for ADSL or cable modem and it's clear that the move from current broadband technology to VDSL could be as significant as the migration from a 56K modem to broadband. As VDSL becomes more common, you can expect that integrated packages will be cheaper than the total amount for current separate services.

Summary of DSL speeds and operations using current methods

Source: http://en.wikipedia.org/wiki/ISDN_digital_subscriber_line
                    http://searchnetworking.techtarget.com/definition/IDSL
             http://www.cisco.com/en/US/tech/tk175/tk318/tsd_technology_support_protocol_home.html
                   http://computer.howstuffworks.com/vdsl3.htm
                   Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf

Chaper 12: Asynchronous Transfer Mode

What is ATM?

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

                       ATM is a member of the fast packet−switching family called cell relay. As part of its heritage, it is an evolution from many other sets of protocols. In fact, ATM is a statistical time−division multiplexed

(TDMed) form of traffic that is designed to carry any form of traffic and enables the traffic to be delivered asynchronously to the network. When traffic in the form of cells arrives, these cells are mapped onto the network and are transported to their next destination. When traffic is not available, the network will carry empty (idle) cells because the network is synchronous.

              ATM is a cell-relay technology that divides upper-level data units into 53-byte cells for transmission over the physical medium. It operates independently of the type of transmission being generated at the upper layers AND of the type and speed of the physical-layer medium below it.The technology was designed for the high-speed transmission of all forms of media from basic graphics to full-motion video. Because the cells are so small, ATM equipment can transmit large amounts of data over a single connection while ensuring that no single transmission takes up all the bandwidth. It also allows Internet Service Providers (ISPs) to assign limited bandwidth to each customer. While this may seem like a downside for the customer, it actually improves the efficiency of the ISP's Internet connection, causing the overall speed of the connection to be faster for everybody.

                    This allows the ATM technology to transport all kinds of transmissions (e.g, data, voice, video, etc.) in a single integrated data stream over any medium, ranging from existing T1/E1 lines, to SONET OC-3 at speeds of 155 Mbps, and beyond.

ATM Standards

                  The following are some of the basic ATM standards documents available from the International Telecommunications Union (ITU).

ITU-T I.361 - Defines the ATM Layer functions.

ITU-T I.363 - Defines the ATM Adaptation Layer protocols.

ITU-T I.610 - Defines the ATM Operation and Maintenance (OAM) functions.

Why the Interest in ATM?

                                                           Summary of speeds for various technologies

             When one considers the disappointing capacities of past technologies, we can see why there is the hype for ATM. ATM will be the basis of many of our future broadband communications systems; as such, it starts where other technologies stop. Many organizations have escalated their demands and needs for raw bandwidth, yet no single entity has emerged as a clear−cut winner to deliver the services necessary to support the demands of today's multimedia applications. Table 12−2 compares the capacities of ATM to the other techniques we used in the past. This will give the reader a chance to see what the excitement is all about.

ATM Protocols

                It takes many protocols to support an ATM network, which is one of the issues that continually

comes up as a negative from the supporters of the gigabit Ethernet crowd. To develop the necessary interfaces in support of the various points within a network, different protocols are necessary. The actual protocols needed depend on where the traffic originates, what transport mechanisms must be traversed, and where the traffic will terminate.

Graphic representation of the ATM protocol interfaces

The ATM Cell

                      Each individual ATM cell consists of a 5-byte cell header and 48 bytes of information encapsulated within its payload. The ATM network uses the header to support the virtual path and the virtual channel routing, and to perform a quick error check for corrupted cells.

The 5-byte header is structured as shown below:

Generic Flow Control (GFC)

               The GFC field of the header is only defined across the UNI. It is intended to control the traffic flow across the UNI and to alleviate short-term overload conditions. It is currently undefined and these 4 bits must be set to 0's.

Virtual Path Identifier (VPI)

                The VPI, an 8-bit field for the UNI and a 12-bit field for the NNI, is used to identify virtual paths. In an idle cell, the VPI is set to all 0's. (Together with the Virtual Channel Identifier, the VPI provides a unique local identification for the transmission.)

Virtual Channel Identifier (VCI)

             This 16-bit field is used to identify a virtual channel. For idle cells, the VCI is set to all 0's. (Together with the Virtual Path Identifier, the VCI provides a unique local identification for the transmission.)

Payload Type Identifier (PTI)

              The three bits of the PTI are used for different purposes. Bit 4 is set to 1 to identify operation, administration, or maintenance cells (i.e., anything other than data cells). Bit 3 is set to 1 to indicate that congestion was experienced by a data cell in transmission and is only valid when bit 4 is set to 0. Bit 2 is used by AAL 5 to identify the data as Type 0 (beginning of message, continuation of message; bit = 0) or Type 1 (end of message, single-cell message; bit = 1) when bit 4 is set to 0. It may also be used for management functions when bit 4 is set to 1. This bit is currently carried transparently through the network and has no meaning to the end user when AAL 5 is NOT in use.

Cell Loss Priority (CLP)

              The 1-bit CLP field is used for explicit indication of the priority of the cell. It may be set by the AAL Layer to indicate cells to discard in cases of congestion, or by the network as part of the traffic management on commercial subscriber networks.

Header Error Control (HEC)

              This is an 8-bit cyclical redundancy check computed for all fields of the first 4 bytes of the ATM cell header ONLY. It is capable of detecting all single-bit errors and some multiple-bit errors. The HEC is compared by each switch as the ATM cell is received and all cells with HEC discrepancies (errors) are discarded. Cells with single-bit errors may be subject to error correction (if supported or discarded. When a cell is passed through the switch and the VPI/VCI values are altered, the HEC is recalculated for the cell prior to being passed out the port.

Source: http://www.techfest.com/networking/atm/atm.htm
            http://www.techterms.com/definition/atm

            Broadband Telecommunications Handbook (VPN 3GW GPRS MPLS VoIP SIP).pdf