Wireless Broadband Access Media

Wireless Broadband Access Media

Broadband wireless access is increasingly an exciting option, especially because cable modems and DSL aren't available as easily or as widely as we would like. Broadband wireless access provides the opportunity to make use of Direct Broadcast Satellite (DBS), Multichannel Multipoint Distribution Services (MMDS), Local Multipoint Distribution Service (LMDS), Free Space Optics, and various unlicensed bands, so wireless increasingly offers more options that can operate in a wider range of footprints.

More than 50 million wireless local loops were deployed globally by the end of 1999, and it is likely that within the next couple years, more than half of the new fixed phone lines installed worldwide each year will be wireless. Fixed wireless is a strong contender to fill the gap where existing wiring is not up to the job or where there's no wiring at all. For example, about one in five people in the United States lives in an area that is too remote to receive any type of fast wireline access to the Internet, and this is also the situation in vast parts of Asia-Pacific, Latin America, Europe, and Africa. Furthermore, millions of people work in modest industrial parks that do not have fast access and that are unable to receive DSL connections.

The cost of radio links has been halving every seven years, and the data capacity of these links has been doubling every three years. These factors combined mean that the cost-to-capacity ratio in wireless communications has been dropping by 50% about every two years. As mentioned earlier in the chapter, for wireless links, the construction costs account for approximately 20% of the total installation cost, and equipment accounts for the other 80%.

Wireless systems (see Figure 13.9) often operate in a point-to-multipoint mode. The antenna communicates with several different clients' antennas, usually installed within a well-defined region. Because the air is a shared medium, like cable, the maximum transmission rate that can be provided to any one client decreases as more clients are served. Clients that need the greatest bit rate obtainable from a system (for example, an ISP) may find it advisable to arrange for a point-to-point system.

Figure 13.9. A broadband wireless configuration inside a home

DBS

One of the first approaches to broadband wireless for Internet access includes the use of DBS, also called Direct to Home (DTH), which uses very-small-aperture terminal (VSAT) technology. (VSATs are discussed in more detail in Chapter 3.) VSATs provide more than 150 digital channels when used in this DBS environment, and DBS was actually the first DTV system to be introduced. As discussed earlier in this chapter, there's a great benefit to offering a large number of channels; besides offering the consumer variety in programming, large numbers of channels means that more channels can be used to deliver advertising content. A great number of advertising dollars were diverted away from cable TV providers when DBS companies came along, because advertisers could reach a wider audience over a larger number of channels to distribute their message.

DBS requires a set-top box that is digital and relies on MPEG-2 as the video compression decompression scheme (see Figure 13.10). The DBS satellites currently in use are Ku-band systems that operate one-way, providing high-speed downstream service. This means the user must rely on the telephone line or cable connection as the return channel. The new DBS systems that are being planned will include Ka-band satellites and will support two-way high-speed data flows. (The frequency allocations for satellite are described in Chapter 3.)

Figure 13.10. DBS

There are approximately 20 million DBS subscribers worldwide today, but they subscribe predominantly for television, and watching two different channels on different devices requires two receivers. So the costs increase steeply as you enable multiparty viewing across different platforms in the home. A number of companies are planning to offer Internet access over two-way satellite connections, including AOL (via its US$1.5 billion stake in Hughes Electronics), Wild Blue, Globalstar, and Star Band (which is a combined effort of Gilat Satellite Networks, Microsoft, Echostar, and ING Furman Selz). More and more people are looking forward to enjoying high-speed Internet access through satellite facilities, particularly in situations where DSL or cable modems are a far cry from current reality.

MMDS

MMDS was first licensed in the 1970s, when it was called Multipoint Distribution Services and was licensed to broadcast one-way 6MHz television channels (see Figure 13.11). In 1996 the U.S. Federal Communications Commission (FCC) expanded the band to cover its present range and to allow for multichannel services. Licensees of these channels can compete directly with cable TV providers, and for this reason, MMDS is sometimes referred to as wireless cable. In 1998 the FCC permitted MMDS providers to offer interactive two-way services for the Internet, requiring upgrades to bidirectional systems.

Figure 13.11. MMDS

Today, there are some five million MMDS customers in 90 nations, but as with DBS, they mostly receive just TV service. About one million of these customers receive services from 250 providers in the United States alone. More than three million subscribers to the service in Budapest are also using it for access to the Internet.

MMDS is a digital system that involves terrestrial line-of-sight microwave. It operates in the 2GHz to 3GHz range, and it has a wide coverage area of about 35 miles (55 kilometers). Data throughput ranges from 128Kbps to 10Mbps. Because MMDS is digital, it can support more channels than the analog system, and MMDS supports upward of 150 digital channels. It requires a digital set-top that incorporates MPEG-2 for video compression and decompression. Its long reach and throughput rates make it a good match for residential and rural applications.

A key issue regarding MMDS is regulation. Wide deployment of MMDS service cannot begin until the licensing process is complete. Another issue is that MMDS parts are expensive it's still a challenge to get price points low enough to compete. It costs around US$1,000 to US$2,000 to install a two-way radio. (In the United States, WorldCom, Sprint, and Pacific Bell have bought rights to offer MMDS.) The biggest problem with MMDS, though, is maintaining line of sight, which is required between the base stations and the subscriber or remote units. Extra transmitters and antennas are needed to overcome obstructions, such as trees and precipitation (remember from Chapter 3 that microwave is very susceptible to distortions caused by moisture), and multipath fading can also cause problems.

Nonlinear deployment is an important concept with MMDS. One emerging technique, Orthogonal Frequency Division Multiplexing (OFDM), promises to improve capacity and performance of wireless systems, including MMDS. OFDM enables more data to be transmitted at higher speeds than over traditional networks, and the signal is strong enough to transmit through some obstacles. By using OFDM, transmitted data is spread over a set of individual frequencies that span a very broad range and is therefore impervious to impairments such as multipath fading and interference. Cisco uses its own version of ODFM that supports up to 22Mbps downstream over a 6MHz channel and up to 18Mbps upstream. As nonlinear deployment options begin to appear, MMDS will be capable of attaining better performance, even in challenging locations and conditions. Another aspect of nonlinear deployment is adaptive antenna techniques. By combining the outputs from multiple antennas, by using sophisticated algorithms, individual beams can be formed at the base station for each user. The algorithms also form nulls in the direction of interferers, further limiting system interference. Because the energy is concentrated into beams, the signal propagates far more effectively than it would if it were radiated in an omnidirectional manner. The end result is that the link budget is increased, further improving cell radius, signal robustness, and the capability to support nonline-of-sight wireless links.

LMDS

LMDS is also known as millimeter, or microwave, technology, and it involves line-of-sight microwave (see Figure 13.12). (LMDS is referred to as Multipoint Video Distribution Service [MVDS] and broadband wireless access in Europe.) It operates in a much higher frequency band than does MMDS. LMDS operates in a 1.3GHz allocation that falls somewhere in the 10GHz to 45GHz range, depending on where you are in the world. The United States uses 24GHz, 28GHz, 31GHz, 38GHz, and 39GHz. The United Kingdom uses 10GHz, and the rest of Europe uses 25GHz. Carriers in Asia-Pacific are conducting trials in the 24GHz to 26GHz and 38GHz ranges.

Figure 13.12. LMDS

A typical LMDS installation has a central base station with an omnidirectional antenna that serves many residences, each of which has a directional dish aimed at the base station. The throughput is about 1.5Gbps downstream and 200Mbps upstream, over shared media. The architecture involves dividing the area surrounding the central base station into sectors. Each sector gets a particular amount of bandwidth that is shared across the subscribers. LMDS tends to operate over microcells of 0.5 to 3 miles (1 to 5 kilometers), serving 5,000 to 10,000 homes. Systems work best if users are within 2 miles (3.5 kilometers) of the base station. LMDS supports two-way symmetrical switched broadband networking, and an average system can support around 150 video channels and more than 7,000 64Kbps voice channels.

Some of the key issues with LMDS have to do with licensing and opening up more frequencies for its use, manufacturing issues (radio parts for the higher frequencies are more exacting and more expensive than those for the lower frequencies), rain fade, and a lack of agreement on standards, especially whether LMDS should use Frequency Division Duplexing (FDD) or Time Division Duplexing (TDD). (LMDS, FDD, and TDD are discussed further in Chapter 14, "Wireless Communications.")

LMDS is expected to experience great growth over the next several years because it allows CLECs to offer broadband services to the small- and medium-sized business market much more cost-effectively than can competitors that are banking on fiber networks.

Free Space Optics

Optical wireless, known as Free Space Optics, uses low-powered infrared lasers (see Figure 13.13). There are two categories of Free Space Optics: point-to-point products, which provide high-speed connection between two buildings, and multiple high-speed connections through the air that operate over much shorter distances, either in a point-to-multipoint or meshed architecture. The point-to-point architectures operate over a range of 1.2 to 2.5 miles (2 to 4 kilometers) and provide throughput of 155Mbps to 10Gbps. The point-to-multipoint architectures operate over a range of 0.5 to 1.2 miles (1 to 2 kilometers) and offer throughput of 155Mbps to 10Gbps. The meshed architectures operate over shorter distances, 650 to 1,500 feet (200 to 450 meters), and offer throughput of around 622Mbps.

Figure 13.13. Free Space Optics

The key problem with Free Space Optics is weather, especially fog. Recall from Chapter 3 that as we go up in the electromagnetic spectrum, the wave form becomes smaller, so droplets of moisture can cause interference. It is possible to reduce the impact of bad weather on the link by reducing the range of the link or by deploying redundant (possibly wired) infrastructure. Another problem with Free Space Optics is that buildings actually sway a little bit, so autotracking mechanisms are required, to ensure that the beams stay highly focused on one another.

Another problem can be flying objects, such as birds, which can cause distortion; meshed architecture should be deployed to get better reliability in areas where such flying objects are numerous. However, it has been reported (www.cablefree.co.uk) that birds can in fact see in the infrared section of the EM spectrum and will typically avoid the beam. Nonetheless, it is still possible that they could fly through the beam, and this is likely to disrupt communication for possibly a second or more. But if a reliable protocol such as TCP/IP has been implemented, any lost packets will be retransmitted, making it likely that the overall impact will be negligible from the user's perspective.

Free Space Optics also requires people to take safety precautions. Although the beams involved emit less power than laser pointers, it's wise to be aware that, depending on which frequencies you're operating at, damage can be caused by looking at optical equipment. All persons allowed access should be made aware of the hazardous nature of the laser communication system.

From a market development perspective, there is a strong incentive to get this and other new last-mile broadband access technologies deployed: They potentially decrease the dependence that private network operators and alternative public service providers have on using the incumbents' infrastructure.

Unlicensed Bands

Unlicensed bands can be used to provide broadband access. These bands are known as Industrial, Scientific, and Medical (ISM) radio bands. ISM operates at 900MHz, 2.4GHz, 5.8GHz, and 24GHz in the United States, but spectrum allocations vary around the world. The ITU World Radio Conference defines the world into three regions, and frequency assignments vary between regions (for a complete list see www.itu.int/brterr/faq/ISM.htm). ISM has a range of about 35 miles (55 kilometers) and offers throughput from 128Kbps to 10Mbps over shared media. Traditionally, the lower frequencies have been used for the deployment of wireless LANs.

One benefit of unlicensed bands is that no licenses are required, so there are no up-front costs to obtain a license to use the spectrum in certain bands. But remember that it still may be necessary to obtain a license if that spectrum were to be used to deliver public services. Also, because unlicensed bands operate in the lower frequencies, there is less environmental distortion than with other wireless options. One disadvantage of unlicensed bands is that because the spectrum is unlicensed, interference can occur between providers; this can be a big problem, especially at the lower frequencies.

Several groups are working on standards for unlicensed bands. The IEEE 802.16 Working Group on Broadband Wireless Access Standards is looking into developing standards for unlicensed bands; three of its task groups are working on standards for the bands 10GHz to 66GHz, 2GHz to 11GHz, and 5GHz to 6GHz. The Wireless DSL Consortium wants to develop an open standard that meets the requirements of the marketplace today and that offers a migration path from existing technology to a next-generation standard, perhaps trying to avoid the sorts of problems we're experiencing in planning the move from second-generation digital cellular to third-generation systems, which are discussed in Chapter 14. Another standards group is the OFDM Forum, which was formed by Phillips and Wi-LAN and whose members include Ericsson, Nokia, Samsung, and Sony. This forum wants to foster a single, compatible, global OFDM standard for wireless networks. The forum claims to be vendor-neutral, and it is planning to submit a physical-layer proposal to the 802.16 2 11GHz task group. The Broadband Wireless Internet Forum is another group that's active in this arena, pursuing development of a standard based on Cisco's OFDM technology approach.