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Wireless Local Loop (WLL)
Wireless Local Loop is an ideal application to provide telephone service to a remote rural area. The system is based on a full-duplex radio network that provides local telephone-like service among a group of users in remote areas. These areas could be connected via radio links to the national telephone network, though allowing the WLL subscriber to call or be reached by any telephone in the world. The WLL unit consists of a radio transceiver and the WLL interface assembled in one metal box. Two cables and a telephone connector are the only outlets from the box; one cable connects to a Yagi directional antenna and a phone receptacle to connect to a common telephone set. A fax or modem could also be connected for fax or computer communication. The WLL is an enhancement of the SmarTrunk II system and for more information on the base/repeater station to operate the WLL, please review SmarTrunk under the product listing. WLL solutions are particularly popular in remote or sparsely populated areas of developing countries, where cabled infrastructure is either too expensive to deploy or where speed of deployment is an issue.
We've explored all manner of wireline local loops in this column during the past several years and we've discussed a number of wireless technologies, but it seems that we've never quite put the two together. So, we'll rectify that oversight in the next couple of columns, beginning with RF (Radio Frequency) solutions. We'll discuss FSO (Free Space Optics), or infrared (Ir), solutions in the next column. Local Loops The local loop is the physical link, or circuit, that connects the customer premises to the edge of the carrier, or service provider, network. Traditionally, the local loop was wireline in nature, specifically in the form an electrical circuit (i.e., loop) provisioned over UTP (Unshielded Twisted Pair) in support of voice communications. Although it remains unusual in all but the most demanding, bandwidth-intensive applications, optical fiber local loops are a wireline alternative. The local loop takes the form of a trunk if it connects to a premises-based switching device such as a voice PBX (Private Branch Exchange), or a data switch or router. Technically, a trunk connects switches, and the device at the edge of the service provider network generally is assumed to be a switch of some sort. The local loop takes the form of a line if it connects to a premises-based device other than a switch, with examples being a KTS (Key Telephone System), a single-line or multiline telephone set or a computer modem. At the edge of the carrier network in a traditional PSTN (Public Switched Telephone Network) scenario, the local loop terminates in a circuit switch housed in an ILEC (Incumbent Local Exchange Carrier) CO (Central Office). In another, more contemporary scenario, the local loop may terminate in a circuit switch owned by a CLEC (Competitive LEC) and housed in a POP (Point Of Presence), which typically is either an ILEC CO or a carrier hotel. Increasingly, the local loop may be provisioned in support of data communications applications, or combined voice and data. The Electromagnetic Spectrum The electromagnetic spectrum includes electricity - radio and light - all of which travel in the form of waves, or cycles. The frequency of the signal refers to the frequency of the sinusoidal waveforms, or sine waves. The inverse of frequency is wavelength, which refers to the distance between the peaks or troughs of the waveforms, and which is the inverse of frequency. Frequency generally is used in reference to electrical and radio signals, and is measured in Hz (Hertz). Radio signals are in the range from 3 KHz to 300 GHz. Wavelength generally is used to describe signals in the optical domain, where it is measured in mu (microns). The higher the frequency of the signal, the greater the number of Hz that exist per unit of time (i.e., second), and the more raw bandwidth available to represent data. The specifics of the modulation technique determine the actual amount of data that can be impressed on the signal. So, there are clear advantages to high frequency radio signals. On the downside, however, high frequency signals suffer to a greater extent from signal attenuation (i.e., loss of signal strength). The impact of signal attenuation dictates the extent to which the system will tolerate physical obstructions such as windows, doors and walls. Therefore, line-of-sight is always desirable, and is required at the higher frequencies. Rain attenuation, or rain fade, is the term used to describe the phenomenon by which radio signals are affected by precipitation. Not only rain, but also sleet, snow and hail can impact radio signals to a considerable extent. Fog and even high humidity have an effect, as do smog, agricultural haze and other environmental considerations. For that matter, radio signals suffer from the physical matter in the air, even on the clearest, purest day. Therefore, the distance between the transmitter and the receivers is limited, sensitive to the signal strength (i.e., power level). In the vacuum of space, attenuation is not an issue, so we can receive clear electromagnetic signals from sources that are (or were) billions of light years away. RF Issues RF services generally are provided on the basis of licensed frequencies. This approach of licensing by the FCC (or other national or regional regulatory authority) provides assurances that the signal will be free from interference, as only a single entity is authorized to use a given frequency or frequency range within a given geographical area. Increasingly, however, unlicensed frequencies are used with various access techniques, signal modulation methods and other mechanisms to mitigate issues of interference between competing signals. The unlicensed frequencies, which typically are in the ISM (Industrial, Scientific and Medical) bands, offer the advantages of no licensing cost and no licensing delays, both of which can be quite considerable. A definite RF negative is the lack of security. Anyone with an antenna in proximity and tuned to the right frequency can capture the raw signal. Frequency hopping, a spread spectrum technique used in some networks, makes it extremely difficult to capture the signal. However, the only real protection against security breaches involves authentication and encryption, and they're not all created equal. RF-based Solutions Aside from traditional point-to-point microwave, there have evolved a number of RF solutions over the years, with emphasis on the very recent past. Those solutions include LMDS, MMDS, cellular telephony and WLANs. LMDS (Local Multipoint Distribution Services) Originally developed as a technology for distributing TV signals in New York City, LMDS later was used for high-speed Internet access, as well. The licensing rights to the technology subsequently were spun off into a separate company, and the FCC auctioned the first LMDS licenses in 1998. Licenses in the A Block operate in the licensed microwave frequency ranges of 27.5-28.35 GHz, 29.1-29.25 GHz and 31.0-31.15 GHz, for a total width of 1.15 GHz. Licenses in the B Block operate in the range of 31.15-31.3 GHz, providing a width of 150 MHz (0.15 GHz). (Note: Outside the U.S., MLDS operates in the 20 GHz and 45 GHz bands.) Given the high frequencies involved, line of sight is extremely important and distances generally are limited to a cell diameter of 10 miles, although they usually are eamuch smaller in consideration of the advantages of frequency reuse. Each 360° cell can be carved into four quadrants to improve traffic capacity. Error performance generally is excellent, with rain fade compensation made through the use of adaptive power controls. LMDS can support local loops ranging from 1.544 Mbps (T1) to 155.52 Mbps (OC-3).
A common LMDS configuration involves a central hub, or node, antenna positioned on the roof of a high-rise building with good lookdown for achieving line-of-sight to buildings of lesser stature within the radio cell. The hub building can be connected to the service provider's backbone via point-to-point microwave. More commonly and better yet, the building is a node of the backbone fiber optic network. The buildings served by the high-speed LMDS links may be single tenant or multi-tenant and even mixed use in nature. Within the building, of course, the bandwidth can be subdivided to serve individual tenants or user groups. LMDS generated considerable excitement in the 1999-2000 timeframe, but most of the successful bidders at auction subsequently declared bankruptcy (e.g., Teligent, WinStar and XO Communications) or otherwise failed. Only XO remains a significant holder of LMDS licenses. MMDS (Multichannel Multipoint Distribution Services) MMDS initially was developed for one-way TV transmission, but has since been modified in support of two-way communications. Operating in low end of the licensed microwave range at 2.5-2.7 GHz in the U.S. and Canada and the 3.5 GHz range elsewhere, MMDS enjoys excellent signal propagation qualities. Although line-of-sight is not absolutely required, it is highly desirable; distances up to 35 miles are possible. MMDS is a point-to-multipoint technology designed to operate from a centrally located and highly elevated antenna in order to maximize line-of-sight connectivity to large numbers of customer premises for data communications applications such as Internet access. The antenna may be sectorized or may be omnidirectional in nature. As the bandwidth is limited to 200 MHz, MMDS has enjoyed only modest success, primarily in the extension of DSL networks and broadband CATV networks. AT&T, Sprint and Worldcom invested heavily in MMDS licenses. Unfortunately, they all closed these networks to new customers in 2002, and AT&T completely shut its network down. Licensed Microwave The FCC licensed point-to-point microwave in the 24 GHz and 38 GHz bands to a number of service providers, specifically for WLL applications. The licenses provide for aggregate bandwidth of 100 MHz, which can be subdivided into channels of 1.544 Mbps (T1), 12.352 Mbps (8xT1), 44.736 Mbps (T3) and 155.52 Mbps (OC-3). At these frequencies, line-of-sight is critical and distances are limited to approximately five miles, under the best of circumstances. Successful bidders at the spectrum auctions included Advanced Radio Telecom, Teligent and WinStar. These companies subsequently declared bankruptcy, although Teligent and WinStar re-emerged in one form or another. Cellular Telephony Cellular telephony has achieved WLL status, depending on how you want to define it. The statistics suggest that wireline local loops actually decreased in number in the U.S. in 2002. We can't blame it all on cellular growth, since DSL and CATV networks are at least partly to blame, but an increasing number of people have completely forsaken wireline service in favor of cellular. Several people have done this for various reasons including budgetary. I know a number of people in South Africa (I teach a seminar there twice a year) who have done the same thing. And that's quite remarkable, given the current state of cellular networking. There are a number of contemporary digital cellular standards, including GSM, D-AMPS and PCS. While they are incompatible, they all share one thing in common - they are narrowband in nature. The fixed wireless dimension of emerging 2.5G (Generation) and 3G cellular networking is quite another matter, however. All of these grew out of a failed attempt to create a single global standard, and fit under the umbrella of IMT-2000 (International Mobile Telecommunications-2000), an initiative of the ITU-T intended as an international wireless network architecture for the 21st century. The various 3G specifications include the following speeds and intended applications:
Note: The theoretical data rates are best case hyperbole. Actual data rates usually are much lower due to factors such as EMI (ElectroMagnetic Interference), RFI (Radio Frequency Interference), signal attenuation and line-of-sight.
Now, an architectural umbrella is a fine thing, but it doesn't do much to resolve issues of incompatibility, which plague the contemporary wireless world. It has been said that the nice thing about wireless standards is that there are so many from which to choose. And so it seems there will be into the foreseeable future, as well. Here's a short list of nextgen wireless standards, along with brief descriptions of them. (Note: The data rates quoted are best case rates, and are not realistic in terms of actual throughputs in the real world.) HCSD (High-Speed Circuit Switched Data) is a 2+G approach that is considered to be an interim step toward 2.5G and 3G networks. HCSD improves on the current cellular data transmission rates of 9.6 Kbps by supporting packet data transmission over the circuit-switched GSM network through the linking of up to four GSM time slots at 14.4 Kbps for a total transmission rate of 57.6 Kbps. A small number of carriers have deployed HCSD. GPRS (General Packet Radio Service) is the 2.5G enhancement to GSM. It is a packet-switched service that supports TCP/IP and X.25 packet protocols, with QoS (Quality of Service) differentiation. GPRS has been demonstrated to run at speeds up to 115 Kbps and has a theoretical transmission rate as high as 171.2 Kbps, although the actual data rate is generally much less. The applications include e-mail and mobile Internet browsing. In the U.S. AT&T Wireless, Cingular and T-Mobile are deploying GPRS networks. EDGE (Enhanced Data rates for Global Evolution) is a 2.5G standard touted as the final stage in the GSM evolution in Europe. EDGE also is capable of running over IS-136 D-AMPS (Digital-Advanced Mobile Phone System) networks in the U.S. EDGE is an intermediate step in the evolution to 3G WCDMA (Wideband CDMA), although some carriers are expected to stop short of that final step. EDGE is planned to support data transmission at rates up to 384 Kbps. AT&T Wireless, Cingular and T-Mobile have made commitments to EDGE at various levels. UMTS (Universal Mobile Telecommunications System), also known as WCDMA (Wideband Code Division Multiple Access), is a 3G technology intended to support data transmission rates of 128 Kbps for high-mobility applications, 384 Kbps at pedestrian mobility speeds and 2 Mbps for fixed wireless applications. AT&T Wireless, Cingular and T-Mobile all have announced plans to deploy UMTS. CDMA2000 (Code Division Multiple Access 2000), also known as IS-856, is a 3G technology based on earlier versions of CDMA. The initial version, known as 1xRTT (1 times Radio Transmission Technology), is a 2.5G technology initially offering data speeds up to 153 Kbps, with throughput in the range of as much as 90 Kbps. The enhanced version, known as 1xEV-DO (1 times EVolution-Data Optimized), is an asymmetric technology offering peak data rates of up to 2.4 Mbps on the forward link and 153 Kbps on the reverse link. GSM1x is a version intended as a transition specification for GSM operators. Nextel, Sprint PCS and Verizon Wireless all have made commitments to CDMA2000. As should now be abundantly clear, the confusion over cellular standards is not likely to abate anytime soon. Multimode terminal equipment resolves these issues of incompatibility in some cases. Cingular Wireless, for example, has introduced a handset that allows its subscribers to roam freely between its GSM and TDMA networks. The AT&T multi-band phone will operate on GSM/GPRS networks in countries where AT&T has roaming agreements in place. 802.11 WLANs We've discussed WLAN (Wireless Local Area Network) technologies in quite some detail in previous columns, having focused on 802.11b (Wi-Fi), 802.11a (Wi-Fi5) and Bluetooth. All of these technologies run in unlicensed frequency bands. Specifically, Wi-Fi and Bluetooth both run in the 2.4-GHz band, and Wi-Fi5 runs in the 5-GHz band. The use of unlicensed frequencies eliminates licensing issues (Duh!) and, thereby speeds deployment and lowers cost. While we don't normally think of LANs in the context of local loops, 802.11b is an unusual case. The low cost and ease of deployment have made Wi-Fi a real winner, not only in enterprise applications, but also in public venues such as airports and coffee shops. As Ethernet LANs, including Wi-Fi, will support VoIP (Voice over Internet Protocol) as well as conventional data applications, Wi-Fi can be characterized as a WLL technology, although some of you might consider that a bit of a stretch at the moment. I know people who have set up ad hoc neighborhood Wi-Fi networks connected to either ADSL or a cable modem for high-speed Internet access. Add VoIP to that, and you've got a full-fledged ad hoc neighborhood WLL network that may not be legal, but it works and it's a lot faster and a lot less expensive than the alternatives. Distance limitations are an issue, but the reach can be extended well beyond the specified 100 feet or so with a high-gain antenna, and with good link quality. In fact, wireless bridges can be used to extend the reach up to five or six miles, assuming that line-of-sight is not an issue. There also are at least a few CLECs that have established themselves as WLL providers, making use of Wi-Fi and Wi-Fi5 to do so. A handful of companies have rolled out public Wi-Fi networks in the U.S., providing Internet access based on agreements with local ISPs. While those networks are highly limited in geographic scope today, the plan is to extend their reach to many thousands of hotspots, nationwide. T-Mobile, the cellular service provider, even acquired the assets of Wi-Fi provider MobileStar in 2002. That deal gave T-Mobile a number of active hotspots and a highly attractive access deal with Starbucks. AT&T Wireless also is involved in Wi-Fi networking, although currently to a much more limited extent. News Flash: the ITU-R's World Radiocommunication Conference 2003 (WRC-03) just announced the addition of 455 MHz of spectrum in the 5-GHz band for WLANs, with a few restrictions. This globally coordinated set of frequencies will be used by 802.11a (Wi-Fi5) and emerging 802.11g systems. Outdoor operation of WLANs in the 5250-5350 MHz band will be allowed, although member countries are requested to take appropriate measures to ensure that a predominant number of stations are operated indoors. These measures are requested in order to minimize issues of interference with Earth-sensing satellites in use by the EU, and U.S. Department of Defense radar systems. And there's more from the IEEE The IEEE (Institute of Electrical and Electronics Engineers) has gotten into the WLL act formally with 802.16, which specifies the WirelessMAN air interface for wireless metropolitan area networks. This group of standards addresses a range of applications, defined much like those in the 3G cellular world. Here's the lowdown on the 802.16 project: 802.16 is a specification for systems operating in the 10-66 GHz range and addressing fixed, line-of-sight applications. 802.16a is a developing specification for systems operating in the 2-11 GHz band and addressing portable, non line-of-sight applications. 802.16e is a developing 3G cellular specification operating in the band at 6 GHz and below and addressing mobile applications supporting handoffs at speeds up to 250 kph (kilometers per hour) And that's not all We've focused on standards-based solutions, but that's not the end of the story. There are a large number of custom (i.e., non-standard) systems, many of which operate in the 2.4-GHz and 5-GHz bands. A number of these systems appear to offer potential, and are being trailed by carriers and service providers in the U.S. and abroad. Note: WLL solutions are particularly popular in remote or sparsely populated areas of developing countries, where cabled infrastructure is either too expensive to deploy or where speed of deployment is an issue. And the winner is... The winner is (Drum roll, please.) wireless. I don't mean to be facetious, but there undoubtedly will continue to be explosive growth in wireless, in general. Nobody is laying new cables and wires under streets these days and recent FCC rulings make it unlikely that CLECs will be able to continue to share ILEC local loops once improvements are made. So, the wireline competition for your broadband access business will likely be nil. That makes WLL all the more attractive. In specific, each of the areas we have discussed will enjoy continued and considerable success. And therein lies a bit of a problem. Spectrum is limited, and always will be. The Good Lord, Mother Nature and the laws of physics just aren't cranking out any more spectrum these days. As the popularity of unlicensed systems in the 2.4-GHz and 5-GHz bands continues to grow, there will surface major issues of interference with which we'll have to contend. I don't doubt that we'll find a way to manage those issues, but they will, indeed, continue to be issues.
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