Will digital copper catch on? ISDN

Communications News, May, 1998 by Robyn Aber

Adoption has been slow for both ISND and xDSL. Yet faster access on the last mile is a priority for remote workers and branch offices. Two experts take a look at how we got here, and ask whether it's going to get better.

It's too bad that, unlike Douglas MacArthur's old soldiers, old local loop technologies don't just fade away. They hang on, making the decision-making and technology integration processes of network managers complex and confusing. Established ISDN services and emerging xDSL technologies are perfect examples.

Developed close to 20 years ago, the integrated services digital network (ISDN) is a system of telephone networks that allows data, images, voice, and video to be transmitted simultaneously on a single digital channel. It can operate through the standard telephone jacks in offices and homes using existing copper telephone wiring (usually without special conditioning). Since ISDN is entirely digital, there is no analog to digital conversion, and users benefit from the full bandwidth of each channel.

Typical telephone systems require a separate line for each device--phone, fax, computer or whatever--that is used at the same time. ISDN eliminates this requirement by combining separate data signals into a single channel. Its use of digital lines means that noise and interference are virtually eliminated.

In ISDN systems, 64 Kbps circuit-switched bearer channels (B channels) carry both voice and data. A 16 Kbps or 64 Kbps delta channel (D channel) handles out-of-band signaling and control. This out-of-band signaling capability allows call setup within 1 to 3 seconds--compared to as much as 40 seconds for analog modem calls. A new feature called always on/dynamic ISDN (AO/DI), not yet widely available, is intended to eliminate ISDN's call set-up time. AO/DI uses ISDN's D channel packet service to maintain an "always on," low speed (9.6 Kbps), TCP/IP connection between a user and an information service provider or corporate LAN.

There are three types of ISDN service: basic rate interface (BRI), primary rate interface (PRI) and broadband ISDN (BISDN). BRI offers 144 Kbps symmetrical service. PRI is for users with greater capacity requirements and allows aggregation of multiple B channels with bandwidth equivalent to T1 facilities. Unlike dedicated, point-to-point T1 lines, ISDN is a switched service and relies on an intelligent switching network. Still evolving, B-ISDN will support as much as 622 Mbps using ATM technology as a switching infrastructure. Its implementation depends on the availability of optical fiber networks.

In an ISDN network, the technology is present at both ends of the "call." All ISDN connections (voice and data) flow through a Class 5 ISDN switch. BRI connections via ISDN terminal adapters or access routers are used at smaller sites, such as residences and small businesses or enterprise branches, and PRI connections via routers or access concentrators are used for communications at larger corporate sites.

xDSL services offer point-to-point, broadband, dedicated (`always on') access. They allow multiple forms of data, voice, and video to be carried over existing copper wire on the local transmission loop between a customer's site and a network service provider's central office or local exchange. Technically, xDSL refers to physical transmission interfaces. But generally, when it's discussed, the assumption is that there is a full-service network that includes routing and switching behind the access network.

The "x" in xDSL stands for the various (and growing) kinds of digital subscriber line technologies including: ADSL/RADSL (asymmetric & rate adaptive digital subscriber line), ADSL Lite, HDSL (high bit rate DSL), SDSL (single pair DSL) and VDSL (very high bit rate DSL). Each technology offers different speeds (ranging from 1.54 Mbps to 52 Mbps downstream from the network to the user and from 640 Kbps to 2.5 Mbps upstream), varying levels of symmetry, and assorted distances (up to 18,000 feet) over which optimal speed can be delivered. As a result, each type of xDSL addresses a different market segment. But the factor that unifies them is their ability to deliver broadband local access services.

One of the best features of xDSL technologies is their ability to transport large amounts of information across existing copper telephone lines. This is possible because xDSL modems leverage signal-processing and modulation techniques to insert and extract more digital data onto analog lines. Because of these techniques, a larger data payload can be carried over greater distances than is usually the case on ordinary copper pair wires.

xDSL differs from ISDN in several significant ways:

* xDSL is an overlay technology, not an intelligent switching system like ISDN. As such, xDSL can be deployed on an as-needed, where-needed basis;

* Unlike ISDN, xDSL does not require its technology to be present at both ends of the call to realize the benefits of high bandwidth (identical deployments are not needed at both the sending and destination sites);