The Internet boom has exposed the soft, white underbelly of the Net: agonizingly slow access through traditional analog modems. No matter where they live or what they do, Internet users have the same complaints. You can turn gray waiting for graphics-laden Web pages to display. Huge files download like mud. And forget about viewing animated or audio-enhanced Web sites or plowing through mountains of e-mail messages daily.
Sometimes slow downloads and displays simply aren't under your control, but result from poor connectivity at the other end. It is everybody's wish that the Information Highway will someday consist exclusively of fiber-optic cables. But that day is not in the near future. The government is not planning to dig a trench from New York to San Francisco, fill it with fiber-optic cables, and call it a data highway. Nevertheless, much of the data highway already exists in the vast web of fiber-optic strands, coaxial cables, radio waves, satellites, and lowly copper wires now spanning the globe.
More often, however, the bottleneck is at the user end of the connection. This last-mile user connection is the connection from the Telephone Company's (Telco) End-Office (EO) to the user's home. Unfortunately, the latest analog modems offer bandwidth of only upto 33.6 kbps.
Studies have shown that users are "consuming" much more information than they are "producing". The Telecommunications industry is taking advantage of this fact, and it's developing new asymmetric technologies that give huge downstream bandwidth to the user, but allow only a fraction of it for upstream bandwidth from the user. [Halfhill]. In fact, to the users' advantage, there is a race among Cable Networks companies, Telephone companies, and Satellite companies, each giving their answer to the users's request for more bandwidth.
One day you might be tuning into the Internet Channel on your PC. Of all the high-speed Internet access solutions, cable TV systems are probably the most talked about. That's partly because they leverage existing broadband cable TV networks and partly because they promise to deliver high-speed access at an affordable price.
But there are considerable technical hurdles: While satellites are only one-way devices, cable modems can work in both directions if cable operators make their one-way networks interactive. Once that's accomplished, the technology could offer the best price/performance combination of any Internet access method to date, delivering close to 30-Mbps speeds at less than $50 per month--about three times the cost/performance factor of ISDN access [Gillett].
Today, making the cable-to-PC connection requires a cable modem to modulate and demodulate the cable signal into a stream of data. The similarity with analog modems ends there, however. Cable modems also incorporate a tuner (to separate the data signal from the rest of the broadcast stream); parts from network adapters, bridges, and routers (to connect to multiple computers); network-management software agents (so the cable company can control and monitor its operations); and encryption devices (so your data isn't intercepted or sent someplace else by mistake).
Each cable modem has an Ethernet port that connects to the computer (or network) on one side and to the cable connection on the other. You install an Ethernet adapter in the PC, then connect it to the cable's Ethernet port via a standard RJ-45 connector; you configure the PC with standard TCP/IP software. As far as your PC is concerned, it's hooked directly to the Internet via an Ethernet cable. There are no phone numbers to dial and no limitations on serial-port throughput (as is the case with ISDN modems). What you do get is lots of speed: Downlinks vary from 500 Kbps to 30 Mbps, while uplinks can, potentially, range from 96 Kbps to 10 Mbps [Halfhill].
In each community, cable operators install a head end that receives both satellite and broadcast TV signals. Coaxial cable carries these signals to each subscriber's home, as shown in figure 23.3.1. Depending on the number of homes and the distance between them, the operator may need to install amplifiers and filters to maintain signal strength.
Figure 23.3.1: Existing Uni-directional Cable Networks,
© VideoTron
Typical cable systems serve between 500 to 2,500 homes on one line [Debevc]. Because the cable is broadband, it carries multiple signals, or channels. Most of these channels are devoted to TV programs, although many cable operators also carry radio stations. A TV channel occupies 6 MHz of the spectrum, and sometimes cable operators multiplex several channels into one. If the cable system were used strictly for data, it could deliver gigabytes of that data per second over hundreds of individual networks, with Ethernet-equivalent throughput.
But TV signals consume most of the potential bandwidth. And most cable systems send these signals in one direction only: from the head end to your home. Internet access, obviously, is two-way: Every mouse click, every command and keystroke must travel back "upstream."
Two-Way StreetTo become interactive, cable operators must allocate spectrum on the cable for upstream signals so you can send data from the PC back to the Internet. This is shown in figure 23.3.2. Typically, the upstream signal is transmitted via a low-frequency band that hasn't previously carried a TV channel. Why? Mainly because these low frequencies are noisy: Ham and CB radios, household appliances, lights, and other devices generate interference, which must be filtered somewhere between the head end and the cable recipient [Robertson, Atkinso, Cocoros].
Figure 23.3.2: Internet over a two-way Cable Network,
© VideoTron
Another problem: All homes (or offices) connected to the cable drop share this one transmission channel. Downstream transmissions have to be broadcast over a separate channel. And as is the case with an Ethernet network, too many nodes competing for bandwidth slow network performance. If your neighbors do lots of downloads, your throughput will suffer unless the cable operator provides additional capacity or extra routers and channels.
Cable operators will also have to modify their cable amplifiers to separate the upstream and downstream signals. In some regions, they'll end up replacing most amplifiers, putting fiber closer to each home.
Finally, cable operators will have to set up a community-wide Internet point of presence (POP) to serve all the networks associated with a particular head end. This will require the cable companies to plan very carefully and to gain an enormous understanding of TCP/IP networking. They'll have to set up routers and servers at the head end and at strategic places around the cable system to manage Internet traffic.
The Institute of Electrical and Electronic Engineers (IEEE) has assigned a working group committee, 802.14, that's trying to define a common media-access control scheme for sending data over cable. The committee is presently selecting the best elements from among 17 different proposals and hopes to have its recommendations finished by year's end.
Instead of a modem moving bits between analog telephone lines and digital applications, ISDN is a digital solution from end to end. Rather than connecting to the Internet at 28.8 Kbps, you can reach speeds of 56 to 128 Kbps, depending on where you live, where your Internet service provider (ISP) is located, and what equipment lies in between.
This extra bandwidth is useful for everything from downloading files to browsing graphics- and multimedia-rich Web pages. If you need to download and upload data, then ISDN offers the best connection speed; alternatives such as satellite or cable links are currently one-way technologies that either require a separate dial-in line for uploads (satellite) or don't traditionally handle them at all (cable).
Figure 23.3.3: ISDN connection over existing telephone lines
If you frequently connect to the Internet but don't like waiting for data modems to negotiate the call, then you'll like ISDN's almost instantaneous connection: Being completely digital, it takes only seconds. In fact, of all the high-speed Internet access methods, ISDN is furthest along in its deployment, availability, and products.
ISDN BasicsThe notion behind ISDN is simple: It's a standard, two-wire phone line that carries three separate digital signals. This is called 2B+D in phone company parlance; it's also known as Basic Rate Interface, or BRI [Gillett]. Your phone company uses one of these channels (the "D" or delta channel) to handle control and signaling information, such as identifying the calling party's phone number and the amount of bandwidth the line occupies.
The two remaining channels (the "B" or bearer channels) can carry voice, data, or both. B-channels can be either 56 Kbps or 64 Kbps apiece, depending on the equipment at your local phone company switch or central office. The two B-channels can also be bonded to act as a single 128-Kbps channel. (Bonding is an acronym for Bandwidth On Demand Interoperability Group.) Some ISDN equipment even supports dynamic bonding, which you use when you require the extra bandwidth--for downloading a large file, for example. With dynamic bonding, you pay for the connectivity only when you need it. Once you've downloaded the file, your ISDN router disconnects automatically, saving you the telephone connect charges.
Another ISDN flavor is Primary Rate Interface, or PRI. This high-volume ISDN can handle either 23 or 30 bearer channels. It's often called 23B+D (used in the U.S.) or 30B+D (used in Europe and Japan) and can run at 1.544 megabits per second and 2.048 Mbps, respectively (over the B-channels). High-capacity PRI is most useful for connecting large networks or for ISPs linking their backbone networks, though. For small-office and home users, BRI is clearly the way to go.
Asymmetric Digital Subscriber Line (ADSL) is the current darling of the phone companies for delivering advanced digital services. It offers the promise of high-speed transmissions yet allows the phone companies to use the copper wiring that already connects to each home. Someday, ADSL could be the darling of consumers as well: Because ADSL works over a pair of ordinary phone wires (figure 23.3.4), you can use your existing analog phones and maintain a high-speed data connection at the same time.
Figure 23.3.4: ADSL connection over existing telephone lines
To do this, ADSL carries three separate frequency channels over the same line. The first set of frequencies carries plain old telephone system (POTS) conversations. Another series of frequencies transmits a 16- to 640-Kbps data signal (different products use different speeds) that carries information upstream from your home to the Internet. Like ISDN, this is a digital signal; but unlike ISDN, each channel goes in only one direction. The third signal is a high-speed downstream connection, which can run anywhere from T1 speeds, 1.5 Mbps, on up to 9 Mbps [ADSL Forum].
The assumption here, of course, is that most of us will be downloading more from the Internet than we'll be transmitting to it. This is a huge gamble, one that cable and satellite vendors are also betting on. The difference is that satellite and cable technologies are much further along. One point in cable's favor is that there are many more cable modem vendors than ADSL modem vendors.
To use ADSL--once it's commercially available--you'll need an external ADSL modem; there will also be one in the phone company's central office. While ADSL modems are still being developed, one prototype, from Aware, has three connectors on the back of the unit: One goes to the wall jack and then out to the phone company; one is for a standard RJ-11 phone jack for analog phone service; and one is an Ethernet twisted-pair RJ-45 connector that hooks the ADSL modem to your computer equipment.
This means once you install an ADSL modem, you won't need special interface electronics to run your analog phones. That's a big plus, one that could speed ADSL's adoption as a single solution for home PC users and small businesses that don't want to install and pay for an extra data line. Also, most home PCs are located near phone wall jacks, which will make ADSL easier to install than, say, cable modems.
Another, similar technology, referred to as Very-high-data-rate Digital Subscriber Line (VDSL), promises even greater speeds. VDSL can operate at ultrafast rates, between 13 to 55 Mbps, but over about half the loop distances as ADSL[Sallee]. And VDSL and ADSL equipment aren't compatible, though they share many of the compression and modulation technologies to achieve such high throughput.
While vendors such as NII Norsat International have announced plans to offer Internet access via satellite, the only product shipping today is Hughes Network Systems' DirecPC. Configuring and maintaining satellite dishes takes patience and a tolerance for cutting-edge technology. And when all is said and installed, ISDN is probably still a better bet for individual users with access to ISDN service. That's because satellites don't deliver the throughput rates they promise, setup is a bear, and they don't do uploads: To transmit data to the Internet, you still need a separate dial-in account to an ISP.
Interestingly enough, eliminating our dial-in connection in favor of a satellite link accentuated other Internet bottlenecks. Many sites have poor connectivity: Either they link to the Internet via slow-speed lines or are choked with so many users that effective throughput was on a par with ISDN lines (figure 23.3.5). In fact, depending on traffic, the time of day, and the number of users accessing a site, your satellite link may not buy as much bandwidth as you'd expect.
Figure 23.3.5: DirectPC Satellite Network Structure,
© Hughes Network Systems, Inc.
Why do you need the analog line and modem? The satellite dish is a non-powered receiver: It can't transmit information back to the Internet. So every time you type at the keyboard, your modem needs to dial your ISP and send that information back to the Internet. The information you requested is then relayed via the satellite back to your computer. The result is a highly asymmetrical network: 400 Kbps downstream. Since a standard analog modem carries upstream messages, the maximum speed is 33.6 Kbps [DirectPC]. The speeds are well short of what cable modems and ADSL offer.
The connection between the dish and the PC is straightforward. The coaxial cable connects directly to an ISA adapter card, which is set up like a network adapter card (with a few caveats, which we'll discuss in a moment).
Among the three asymmetric public network technologies the most prominent seems to be the Cable TV networks. The huge downstream bandwidth they provide will allure most of the home users. The fact that some cable modems are already in the market, while most of them are in the final trial stages at different cable networks around the world, gives them a definite advantage. ADSL trials, on the other hand, are still in the early stages. Nevertheless, the dedicated bandwidth will attract many users concerned about privacy and need fixed bandwidth. As for the Satellite networks, they need to provide more and reliable bandwidth. These will be ideal for isolated areas where there are no telephone or cable networks installed.
Copyright © 1996 Theodoros P. David, All Rights Reserved
Theodoros P. David
<tdavid@vt.edu>
Last modified: Mon Dec 16 15:52:09 1996