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Digital Services

Today, all of the international telephone network is digital, with the exception of one wire葉he one running from the telephone company central office to your home. This local loop, sometimes called the last mile, is the same stuff that Alexander Graham Bell experimented with and uses the same technology he developed預 pair of copper wires designed to carry analog voice signals. Eventually, this local loop will be replaced with a newer connection technology, such as coaxial cable or fiber optics carrying only digital data.

Businesses already have this option available to them. Modern cable television systems offer digital data communications capabilities as well, although dial-up voice circuits remain the province of the telephone company (mostly for legal reasons). Even so, there's little doubt you'll eventually shift from analog to digital services for all of your telecommunications needs.

You can already get all-digital circuits for your data. The only question is who will provide the connection. Three technologies can provide you with a high-speed all-digital link葉elephone, cable, and satellite.

All three work. All three deliver speeds that make modem connections seem like they are antiquated. In fact, the most important limiting factor is availability. Only satellite services can promise a link in any part of the United States (and most of the world). The others depend on your local telephone company or cable provider upgrading its facilities to handle digital subscriber services.

Telephone Services

One key player in the supply of digital telecommunications services is quite familiar葉he telephone company. Beyond traditional POTS, telephone companies have developed a number of all-digital communication services. Some of these have been around for a while, aimed at business users with heavy data needs. Several new, all-digital services are aimed directly at you as an individual consumer.

The range of digital services supplied by telephone companies is wide and spans a range of data rates. Table 13.3 lists many of these and their maximum data rates.

Table 13.3. Maximum Data Rates of Digital Telecommunications Standards
Standard Connection Type Downstream Rate Upstream Rate
V.34 Analog 33.6Kbps 33.6Kbps
V.90 Digital/analog 56Kbps 33.6Kbps
V.92 Digital 56Kbps 48Kbps
SDS 56 Digital 56Kbps 56Kbps
ISDN Digital 128Kbps 128Kbps
SDSL Digital 1.544Mbps 1.544MBps
T1 Digital 1.544Mbps 1.544MBps
E1 Digital 2.048Mbps. 2.048Mbps
ADSL Digital 9Mbps 640Kbps
VDSL Digital 52Mbps 2Mbps

You will still talk on the telephone for ages to come (if your other family members give you a chance, of course), but the nature of the connection may finally change. Eventually, digital technology will take over your local telephone connection. In fact, in many parts of America and the rest of the world, you can already order a special digital line from your local telephone company and access all-digital switched systems. You get the equivalent of a telephone line, one that allows you to choose any conversation mate who's connected to the telephone network (with the capability of handling your digital data, of course), as easily as dialing a telephone.

T1

The basic high-speed service provided by the telephone company is called T1, and its roots go back to the first days of digital telephony in the early 1960s. The first systems developed by Bell Labs selected the now-familiar 8KHz rate to sample analog signals and translate them into eight-bit digital values. The result was a 64Kbps digital data stream. To multiplex these digital signals on a single connection, Bell's engineers combined 24 of these voice channels together to create a data frame 193 bits long, the extra bit length defining the beginning of the frame. The result was a data stream with a bit rate of 1.544Mbps. Bell engineers called the resulting 24-line structure DS1. AT&T used this basic structure throughout its system to multiply the voice capacity of its telephone system, primarily as trunk lines between exchanges.

As telephone demand and private business exchanges (PBXs) became popular with larger businesses, the telephone company began to offer T1 service directly to businesses. As digital applications grew, T1 became the standard digital business interconnect. Many Web servers tie into the network with a T1 line.

A key feature of the DS1 format was that it was compatible with standard copper telephone lines, although requiring repeaters (booster amplifiers) about every mile. The signal itself is quite unlike normal analog telephone connections, however, and that creates a problem. Its signal transmission method is called Alternate Mark Inversion (AMI), a formatting code for T1 transmissions over twisted-pair copper cable. T1 transmissions are in bipolar form. AMI represents a zero (or space) by the absence of a voltage; a one (or mark) is represented by a positive or negative pulse, depending on whether the preceding one was negative or positive (that is, marks are inverted on an alternating basis). This encoding system generates a signal with a bandwidth about equivalent to its data rate, 1.5MHz. This high-speed signal creates a great deal of interference, so much that two T1 lines cannot safely cohabitate in the 50-pair cables used to route normal telephone services to homes.

Outside of the United States, the equivalent of T1 services is called E1. Although based on the same technology as T1, E1 combines 30 voice channels with 64Kbps bandwidth to create a 2.048Mbps digital channel.

Serious Web surfers dream of having a dedicated T1 line. The cost, however, is prohibitive. Installation is often thousands of dollars and monthly charges may be a thousand dollars, sometime more. Typically, your Internet Service Provider has an T1 (or better) connection and divides it up, giving each customer a single modem-slice.

T3 is a synonym for DS3 service, which is approximately 45Mbps, and OC-3 is approximately 155Mbps fiber interface.

The primary problem with T1 is the interference-causing modulation system it uses, one based on 1960s technology. Using the latest modulation techniques, the telecommunications industry has developed a service called High data rate Digital Subscriber Line (HDSL) that features the same data rate as T1 or E1 but requires a much narrower bandwidth, from 80 to 240KHz. One basic trick to the bandwidth-reduction technique is splitting the signal across multiple phone lines. For T1 data rate, the service uses two lines; for E1, three. Besides reducing interference, the lower data rate allows longer links without repeaters, as much as 12,000 feet.

HDSL delivers high-speed data networking, up to 1.544Mbps over two copper pairs and up to 2.048Mbps over three pairs, at a maximum range of 20,000 feet (about 3.8 miles or 6.1 km) from a central office. It is similar to Symmetrical Digital Subscriber Line (discussed later) and has symmetrical transmission capabilities. Most T1 lines installed today utilize this technology.

Unfortunately the "subscriber" in the name of the standard was not meant to correspond to you as an individual. It fits into the phone company scheme of things in the same place as T1様inking businesses and telephone company facilities.

DSL

The service you're most likely to buy from your telephone company goes under the name Digital Subscriber Line. It uses your ordinary telephone wires to carry high-speed digital signals. In fact, today's technologies let you use both the analog phone service and the digital capacity of the same wires at the same time. The high frequencies of the digital signal are easily split from the low frequencies of the analog signal, so one wire can do double duty. When telephone companies first introduced this piggyback service, they called it G.Lite, but it has become the standard for residential DSL.

G.Lite is a special case of a more generalized service called Asymmetrical Digital Subscriber Line or ADSL. It is asymmetrical because it offers a higher downstream data rate from the server compared to its upstream rates, from you back to the server.

When telephone companies first offered DSL services, they needed to send out a technician on each job to install the splitters required to separate the analog and digital signals. The need for a technician added hundreds of dollars to the cost of setting up an ADSL connection. To eliminate the need for the splitter and create a common consumer standard for ADSL, several manufacturers in the telecommunications industry banded together to create the Universal ADSL Working Group, the organization that defined the G.Lite standard under the formal designation G.992.2.

ADSL can move downstream data at speeds up to 8Mbps. Upstream, however, the maximum rate is about 640Kbps to 1Mbps. ADSL doesn't operate at a single rate as does T1. Its speed is limited by distance, with longer distances imposing greater constraints. It can push data downstream at the T1 rate for up to about 18,000 feet from the central office. At half that distance, its downstream speed potential approaches 8.5Mbps. To distinguish the more general form of ADSL from G.Lite, this full-fledged ADSL is sometimes called ADSL Full Rate or G.dmt. Its official designation is G.992.1.

G.Lite is one variety of ADSL. G.Lite allows for a bandwidth downstream of up to 1.544Mbps. Upstream the asymmetrical system allows for a bandwidth of up to 512Kbps. The maximum length of a G.Lite connection stretching between the central office and your home or business is 18,000 feet.

Table 13.4 summarizes the downstream speeds and maximum distances possible with ADSL technology.

Table 13.4. ADSL Downstream Data Rates
Equivalent Service Downstream Data Rate Distance
G.Lite 1.544Mbps 18,000 feet
T1 1.544Mbps 18,000 feet
E1 2.048Mbps 16,000 feet
DS1 6.312Mbps 12,000 feet
ADSL 8.448Mbps 9000 feet

Another kind of DSL offers the same speed in both the upstream and downstream directions. Termed Symmetrical Digital Subscriber Line (SDSL), this kind of service is best suited for companies offering Web services from their own servers. The data speed of the SDSL system ranges from 160Kbps up to 1.544Mbps. The maximum usable rate depends on the distance of the subscriber from the central office. The lowest speed occurs at a maximum range of the SDSL system, which is 24,000 feet (about 4.5 miles or 7.2 km).

The acronym SDSL has been used in the past to describe another service called Single-line Digital Subscriber Line, which was an alternative to multiline data services for high-speed operation.

The modulation system used by all DSL services operates at frequencies above the baseband used by the ordinary telephone service, which is why a single line can carry high-speed digital signals and ordinary telephone signals simultaneously. In typical DSL implementations, the DSL signals start at about 50KHz, leaving the lower frequencies for carrying conventional voice signals.

The splitter that divides the signal between voice and data on the subscriber's premises is the equivalent of a stereo speaker's crossover; the splitter combines a high-pass filter to extract a data-only signal and a low-pass filter to extract the voice-only signal. Using the G.Lite system, you install the required splitters (sometimes called filters) by simply plugging them in using modular plugs and jacks like ordinary telephone equipment.

VDSL

The next step above ADSL is the Very-high-data-rate Digital Subscriber Line (VDSL). A proposal only, the service is designed to initially operate asymmetrically at speeds higher than ADSL but for shorter distances, potentially as high as 51.84Mbps downstream for distances shorter than about 1000 feet, falling to one-quarter that at about four times the distance (12.86Mbps at 4500 feet). Proposed upstream rates range from 1.6Mbps to 2.3Mbps. In the long term, developers hope to make the service symmetrical. VDSL is designed to work exclusively in an ATM network architecture. As with ADSL, VDSL can share a pair of wires with an ordinary telephone connection or even ISDN service.

SDS 56

Switched Data Services 56 (sometimes shortened to Switched-56) is an archaic connection system that yielded a single digital channel capable of a 56Kbps data rate葉he same as with a modem but with true digital signals. The Switched-56 signals traveled through conventional copper twisted-pair wiring (the same old stuff that carries your telephone conversations). For most telephone companies, it was an interim service to bridge the gap between POTS and ISDN service areas.

With Switched-56 you needed special head-end equipment葉he equivalent of a modem葉o link the wire to your computer. To take advantage of the connection, you also needed to communicate with someone who also had SDS 56 service.

In some locales, SDS 56 was no more expensive than an ordinary business telephone line. Installation costs, however, could be substantially higher (PacBell, for example, at one time charged $500 for installation), and some telephone companies added extra monthly maintenance charges in addition to the normal dial-up costs. With modern modems promising the same speed with no extra charges, little wonder Switched-56 gets discussed in the past tense.

ISDN

The initials stand for Integrated Services Digital Network, a first attempt at bringing true digital communications to the home through existing telephone lines. Although the service is still available, it's essentially irrelevant because DSL offers more speed at about the same cost.

ISDN predates DSL. Its start came in November 1992 when AT&T, MCI, and Sprint embraced a standard they called ISDN-1. Today, two versions of ISDN are generally available. The simplest is the Basic Rate Interface (BRI), which takes advantage of the copper twisted-pair wiring that's already in place, linking homes and offices to telephone exchanges. Instead of a single analog signal, an ISDN line uses what is called "2B1Q line coding" to carry three digital channels: two B (for Bearer) channels that can carry any kind of data (digitally encoded voice, fax, text, and numbers) at 64,000bps, and a D (or Delta) channel, operating at 16,000bps, that can carry control signals and serve as a third data channel. The three channels can be independently routed to different destinations through the ISDN system.

The maximum distance an ISDN line can stretch from the central office is 18,000 feet (about 3.4 miles or 5.5 km). To accommodate longer runs, this distance can be doubled by adding a repeater in the middle of the line. A repeater is an amplifier that regenerates the digital signals, erasing the signal distortion that arises on long lines.

A single BRI wire enables you to transfer uncompressed data bidirectionally at the 64,000bps rate, exactly like a duplex modem today but with higher speed and error-free transmission, thanks to its all-digital nature. Even during such high-speed dual-direction connections, the D channel would still be available for other functions.

The more elaborate form of ISDN service is called the Primary Rate Interface (PRI). This service delivers 23 B channels (each operating at 64,000 bits per second) and one D channel (at 16,000 bits per second). As with normal telephone service, ISDN service is billed by time in use, not the amount of data transmitted or received.

The strength of BRI service is that it makes do with today's ordinary twisted-pair telephone wiring. Neither you nor the various telephone companies need to invest the billions of dollars required to rewire the nation for digital service. Instead, only the central office switches that route calls between telephones (which today are mostly plug-in printed circuit boards) need to be upgraded.

Cable Services

The chief performance limit on telephone service is the twisted-pair wire that runs from the central office to your home or business. Breaking through its performance limits would require stringing an entirely new set of wires throughout the telephone system. Considering the billions of dollars invested in existing twisted-pair telephone wiring, the likelihood of the telephone company moving into a new connection system tomorrow is remote.

Over the past two decades, however, other organizations have been hanging wires from poles and pulling them underground to connect between a third to a half of the homes in the United States幼able companies. The coaxial cables used by most such services have bandwidths a hundred or more times wider than twisted pair. They regularly deliver microwave signals to homes many miles from their distribution center.

Tapping that bandwidth has intrigued cable operators for years, and the explosive growth of the Internet has set them salivating. The advent of digital cable has made Web connections an option for most cable subscribers.

Architecture

The cable television system differs from the international telephone system in several ways. Most cable television systems are local. They are meant to cover only a limited geographic range. Cable systems do not interconnect. Each cable operator plucks the signals it needs from the air, either from distant broadcast stations or from satellite. There is no great cable web that allows you to directly link with another cable user anywhere in the world.

Moreover, cable systems are designed differently. Telephone systems are point to point, caller to caller, one on one. Cable systems are designed for broadcast in a one-to-many fashion. Cable systems send the same signals to each of their subscribers. The wiring for telephone systems resembles a star with a center hub (the central office) and individual nodes, each connected directly to the hub with its own (albeit low bandwidth) wire. The cable system is a spine熔ne big, wide bandwidth wire carrying signals for all subscribers, each of which taps into the same spine for the same signals.

The essence of the cable design is that all users share the same bandwidth. The cable is piled with as many as 500 television channels葉hat's a full 3GB of bandwidth-ignoring guard bands. When operators put Internet signals on the cable system, users have to share that bandwidth, too. How wide a slice of bandwidth each user gets depends primarily on how many users are sharing. If you're the only one to log on to the Internet on your cable system, you can get speeds that T1 users would envy. Log on when Microsoft offers free downloads of a new version of Internet Explorer or a beer company runs an online bikini competition, and you're apt to get more free beer than bandwidth through the cable (that is, about zero). With a high-speed DSL telephone line, you are guaranteed bandwidth. With cable, you share and take your chances. You could do better熔r much, much worse. In technical terms, the telephone system guarantees Quality of Service (QoS). The cable system may not (although some operators are making QoS guarantees to lure you over to coaxial cable).

Cable systems allow for individual addressing of subscribers' equipment. Each cable box has an electronic serial number that the cable operator's equipment can address individually (for example, to alter the services you are authorized to receive). But this individual addressing does not provide an individual channel to your home. The signals to control your cable box are broadcast to all subscribers on the cable system. Only your box with the correct serial number responds to commands sent to it.

Standards

During the first few years of cable-based data services, the one element lacking was standardization. Each cable operator used its own equipment designs to adapt data signals to the cable medium. The adapters used by different cable operators were incompatible, as were the data signals from the head-end, the cable company's equivalent of the telephone company's central office. Although such proprietary designs gave cable companies a measure of security葉hey helped thwart widespread hacking葉hey also made equipment more expensive and employee training more difficult.

Several efforts at developing cable standards started in the mid1990s. The first major effort started at the IEEE, which formed a new working group called the Cable TV Media Access Control and Physical Protocol Working Group in May 1994, to define a standard for cable modems. The IEEE group members had difficulty agreeing to any single standard, however, and the group missed its original target of December 1995, for publishing a specification.

An impatient cable industry got tired of waiting for engineers to come up with a standard, so a group of cable television operators, including such major players as Comcast, Cox, MSOs, TCI, and Time Warner, started their own standardization effort. The companies formed an independent limited partnership called Multimedia Cable Network System Partners, Ltd. (usually shortened to MCNS), which Continental Cablevision and Rogers Cablesystems then joined. In little more than a year, the partnership sifted through 12 proposals to produce a draft standard that it published as the Data Over Cable Service Interface Specification (DOCSIS) in March, 1997. The industry research organization CableLabs took over the administration of the specification and developed a compliance program. By March 1998, the group had developed an interoperability certification program and, in March 1998, the ITU endorsed the DOCSIS standard as an official standard ITU J.112.

A revised version of the DOCSIS specification, version 1.1, was released in April 1999. The new standard better defined signal parameters to guarantee the bandwidth of the signal and minimize delays. In addition, the revision added standards for new services, including voice-over-Internet telephones and constant bit-rate services. A European version of DOCSIS, called EuroDOCSIS, is used with the different television standards prevailing there. Another update, to DOCSIS 2.0, is in the works. The complete DOCSIS specification is available online at www.cablemodem.com/specification.

DOCSIS fits data onto cable by taking over a single television channel for downstream data and a second channel for upstream data. Each television channel has a bandwidth of 6MB. In general, the system can use any channel in the VHF or UHF ranges (from 50 to 864MHz) for downstream data, but upstream data is restricted to lower frequencies (5 to 42MHz), which makes the adapters less expensive to manufacturer.

The DOCSIS standard allows cable operators flexibility in choosing either 64- or 256-state quadrate amplitude modulation for the downstream data signals they provide. Typically cable operators use 64-state QAM, with which a single 6MHz downstream television channel can carry data at a rate of about 27Mbps. Upstream, most cable operators have a choice of 16-state QAM or quadrature phase-shift keying, a more robust but slower modulation scheme most choose to ride over the higher noise levels prevalent at lower frequencies on cable. In this asymmetrical system, cable operators often limit upstream bandwidth from individual users, often to as little as 320Kbps, although the standard allows upstream rates as high as 10Mbps. At low-demand times, you might score downstream bandwidth approaching the full 27Mbps rate. Typically, however, cable services deliver downstream data at 1 to 3Mbps.

To allow multiple users to share the same bandwidth, DOCSIS uses time division multiple access (TDMA) technology. That is, each user gets a fraction of the total bandwidth in which to transmit and receive data. To gain access to the cable, DOCSIS uses the Ethernet MAC (Media Access Control) layer.

DOCSIS includes provisions for each user to have a 14-bit subscriber ID, which allows cable operators to individually tailor service much as they do premium cable television channels. For example, individual users may be assigned different bandwidths.

Before the wide adoption of DOCSIS, some cable companies deployed telco-return modems, which used the cable company's high-speed coaxial cable for downstream data but made the upstream link through a conventional phone line (with all its bandwidth constraints). With the move to DOCSIS, most cable modems now use only a cable connection.

Because all signals ride across the same coaxial cable, all your neighbors have access to the packets of data you send and receive. To maintain privacy, your cable modem automatically encrypts everything you send and receive using the Data Encryption Standard (DES) algorithm.

Satellite Services

The same technology used by direct-broadcast satellite television, through which you can grab viewable video directly from orbiting satellites, also works for data. Using a small parabolic antenna pointed at a geosynchronous satellite orbiting about 24,000 miles away, you can tap into the Internet at speeds well beyond the capabilities of dial-up telephone connections. Instead of television signals, the satellite simply beams a stream of data down to earth.

The leading satellite service, DirecPC from Hughes Electronics, initiated service in 1997. It bills itself as the fastest Internet service available nationwide. Although slower than either DSL or cable modems, it holds the advantage of availability預nywhere you can see the southern sky, you can make a connection to DirecPC.

Satellite systems are inherently asymmetrical. You don't transmit your needs to the satellite妖oing so would require an uplink and a much larger antenna. Instead, you use the satellite connection only for a downlink, to receive data. To send data, your computer connects to the Web through a conventional dial-up modem. The satellite-based downlink operates at 400Kbps while your phone-based uplink struggles along at modem speed, 14.4 to 56Kbps.

Satellites best fit a broadcast model. That is, they dump out their signals across wide areas for consumption by the multitudes rather than directly targeting individuals. By limiting the downlink bandwidth to 400Kbps, they maximize the number of subscribers that can share the system. In addition, DirecPC attempts to maximize the speed and usefulness of its product using push technology. The system pushes out selected Web and newsgroup information, and your computer captures it as it is sent out, spooling the data to disk. When you want to access one of the pushed Web sites or newsgroups, you can read it almost instantly from the cache. So your computer won't clog up by trying to cache the entire Internet, the system allows you to choose which sites and groups to cache locally.

DirecPC uses a 21-inch elliptical antennae designed for roof mounting. The DirecPC antenna is a single-purpose device and can be used only for data, not satellite television reception. The system also requires a receiver (sometimes called a modem) that may be installed as an expansion board inside your computer or as a standalone external peripheral.

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