Cable modems, DSL answer the call

Cable modems, DSL answer the call

Two services that deliver up to 10 times as much bandwidth as ISDN step up in digital comm arena

By Kevin Jonah

Special to GCN

In digital communications parlance, the convergence buzzword simply means that cable, telephone and other communications providers are zeroing in on a single goal: low-cost, high-bandwidth, persistent Internet connections.




That particular comm scene, once owned by Integrated Services Digital Network connectivity, is up for grabs. The two main contenders, cable modem and digital subscriber line services, both provide up to 10 times as much bandwidth as ISDN. They are always online and usually bill at a flat rate rather than by usage. They represent a low-cost long-haul networking option for branch offices, small organizations, telecommuters and applications that require a full-time WAN link.

The problems that plagued ISDN in the early 1990s, and in some respects still do, are also problems to some degree for the new fat pipes: limited availability, varying standards and fluctuating quality of service. Often the choice of a fat pipe is limited by geography and by the local telecommunications and cable infrastructures.

Cable modems, which got their start as consumer Internet access devices, deliver potentially the biggest bandwidth bang for the buck for telecommuters and small offices'with some catches. The biggest catch: Cable modem bandwidth is shared.

A cable modem connection generally is good for up to 1.5 Mbps, better than T1 speed. The rate can be pushed as high as 10 Mbps depending on how an Internet provider connects to the cable network. If the provider has a dedicated cable system connection, the rate can rise to 52 Mbps, but it typically is somewhat slower. Interactive bandwidth'the speed at which data travels from a cable modem to a server or computer elsewhere on the Internet'usually is restricted to about 2.5 Mbps.

Band demand

That bandwidth, subject to demand on the local cable, goes to everyone connected to the same cable TV junction point. As more users hook up cable modems, available bandwidth drops in proportion.

The reason PC access caps out at 10 Mbps is that the connection to a cable modem goes through an Ethernet card. That adds another wrinkle for telecommuters, who must configure their PCs with LAN cards.

Some cable modem users report good speed when the connection is up but complain about downtime of up to 50 percent. Each time the service experiences a brownout , they must shut down for about 10 minutes and reboot to re-establish service.

Then there's the availability issue. Cable modems are available in many metropolitan areas and suburbs, but obviously only where cable companies have invested in digital and fiber-optic infrastructures.

To offer cable modem service, a cable company must have digital service in place as well as so-called fiber to the curb, connecting the local cable access boxes to the central switch. Areas with only a coaxial cable infrastructure are in for a long wait for cable modem service.

The telephone industry's answer to cable modems is DSL, which essentially spells the end of ISDN within a few years. DSL is cheaper and faster than ISDN. It also represents a foot in the door for competitive local exchange carriers and small telecommunications and networking companies, which can compete for DSL users on a nearly equal footing with the regional Bell operating companies.

DSL comes in three flavors: asymmetric DSL (ADSL), synchronous DSL (SDSL), and ISDN DSL (IDSL).

The first two work over standard twisted-pair copper wiring. In some cases, existing voice circuits can carry voice and DSL service simultaneously.

Depending on the provider, however, it might be necessary to install a separate line or a line splitter. An emerging standard, known variously as Universal DSL, DSL Lite or G.Lite, will eliminate the need for a splitter and will be turned on from the phone company's central office without an installer visit. That will make it cheaper, but also slower, than other classes of DSL service.

Universal DSL will be the first truly standard form of DSL; other DSL services vary with the provider's central office equipment'the digital subscriber line access multiplexer, or DSLAM. Agencies that want to deploy DSL to a number of geographic locations likely cannot standardize on one type of DSL modem or router unless they buy the service from a single company with coverage in all the areas.

It's probably wise for agency managers to regard the DSL connection point at each site as the provider's own premises equipment and plan to let the provider manage it.

A DSLAM at the phone company's central office connects all of its attached DSL circuits to an asynchronous transfer mode network.

At the other end of the ATM pipe, another DSLAM splits up the transmissions and routes them appropriately.

Depending on the provider, all this equipment might fall under the purview of one, two, three or four entities. The local phone company owns the local loop. The DSLAM and ATM network might belong to a specific DSL provider. The back-end DSLAM connections can belong to the local phone company or a competitive local carrier, and the WAN connection belongs to the Internet provider or private network provider.

The separation of services usually is transparent to the user. But it can lead to bizarre technical-support situations, as when the local phone company and the DSL provider send out trucks for a curbside conference about which one should handle a particular connectivity problem.

The level of DSL service available at a given site depends on how far it is, as the wire lies, from the phone company central office. The maximum distance tops out around 18,000 feet unless the phone company extends the loop with fiber-optic cable. The highest rates for SDSL and ADSL are often attainable only at half that distance; beyond 9,000 feet, bandwidth gradually degrades.

The good news is that, unlike the cable modem case, all the bandwidth is on a dedicated pipe to the DSLAM and the ATM network. Apart from Internet bottlenecks, the actual throughput of a DSL connection should run close to the advertised data rate.

DSL works not just for Internet access but for any dedicated WAN connection. Telecommuters can be hard-wired into their agency WAN rather than having to come in across the Internet through a virtual private network. It is obviously cheaper, however, to have them use the Internet as their WAN.

Most current consumer DSL service is one form or another of ADSL, the service most directly comparable in cost to cable modem. The asymmetric part of ADSL indicates that the transmission and reception speeds are different. Expect download speeds of up to 6.1 Mbps and transmission rates of up to 640 Kbps, depending on the class of ADSL and distance to the central office.

Most ADSL services assign a dynamic IP address to each user at boot-up'a temporary address from a pool of available ones. That's because ADSL is a client connection, not a peer-to-peer connection.

Offices that want static network addresses, or want better transmission speeds for a branch office or a telecommuter center, should opt for SDSL.

SDSL is essentially the same as the original DSL service, known as high-rate DSL, or HDSL, and it often is used by phone companies for T1-class data service. HDSL and SDSL both give the same transmission rate in both directions.'Their maximum speed is somewhat lower than that of ADSL, around 1.5 Mbps in the United States.

SDSL also is generally priced higher than ADSL, but it has static IP addresses, a plus for sites that run their own Web servers.

The stepchild of DSL is IDSL, which uses the same wiring as ISDN and provides the same data rate, 128 Kbps. It is superior to ISDN for dedicated network connections and has no per-call billing.

Standing in the wings is very-high-rate DSL, or VDSL, an emerging standard for faster transmissions over shorter distances. It will support up to 55 Mbps over distances up to 1,000 feet from the DSLAM, making it ideal for campus and metropolitan area networks.'But VDSL is far from ready and probably will not be available until after ADSL is widely deployed.


Kevin Jonah, a Maryland network manager and free-lance writer, writes about computer technology.

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