One of the benefits to an ISDN-based SOHO solution is the use of a single line for voice and data. The installation may be configured to use both B channels (ISDN BRI) for data-only transmissions. A voice call can use either of the two channels, and this configuration will still provide data connectivity.

On the hosting side of ISDN connections, the designer has a number of options. Multiple ISDN BRI circuits may be terminated to Cisco’s 4000 series router. However, this solution would service only a few connections. Deployment of the 4000 or 7000 series routers with ISDN PRI connections could support a larger population of users. An alternative Cisco solution is the 3600 platform; however, this platform was unavailable when the current exam was developed.

	Some recommendations in this book suggest using end-of-life or discontinued equipment. This is due to the age of the examination objectives and is reflective of the current examination. Please consult Cisco’s Web site for the most recent information.

High-Density Solutions

Cisco also offers the AS5200, which may be used for termination of ISDN and analog phone connections and can provide service for fixed-location users. This platform yields the greatest flexibility of these solutions. Both the AS5100 (discontinued) and AS5200/AS5300/AS5800 products offer integrated modems, which may benefit administrators concerned with rack space. Integrated solutions typically benefit from lower total costs as well.

High-density solutions may also benefit large pools of mobile users. The smallest AS5200 configuration is typically 24 digital modems. Mobile user pools would not be served well with the 4000 or 7000 platform.

	Both the 4000/4000M and 7000/7010 models are classified end-of-life at this writing. Please check the Cisco Web site for current information.

Network Design with DSL Technologies

Digital Subscriber Line (DSL) technologies were developed to be the “magic bullet” of the telecommunications industry. Primarily designed to add bandwidth to the home without installing fiber optics, the xDSL protocols have the potential to provide 52Mbps over already installed copper wire—a marked increase in performance. This feat is accomplished with special encoding of the digital signal. At present, DSL technologies are being used as a replacement for ISDN and analog ISP connections. However, as DSL technologies are accepted into the home and office, it is likely that they will be used for primary and backup data transfer and for high-demand services such as live video.

	DSL technologies and cable modems are not included as an exam objective at present. This section is provided only as optional material for those readers interested in this technology.

The various DSL technologies, referred to in the generic as xDSL, provide for varying amounts of upstream and downstream bandwidth based on the equipment in use and the distances between that equipment. As a result of its distance sensitivity, xDSL typically must terminate within three miles of the central office, but access technologies may be employed to extend the range. Access products connect a remote termination device to the central office via fiber optics, which greatly extends the reach of xDSL. Figure 9.1 illustrates a typical installation of DSL with and without an access product. As shown, a home four miles away cannot obtain xDSL access without an access product. Please note that most xDSL technologies support distances between 1,800 and 18,000 feet.

As of this writing, vendors are deploying DSL at fairly low speeds and as an Internet connectivity solution. Most vendors provide 1.544Mbps downstream bandwidth, as viewed from the central office side, and 128Kbps to 384Kbps upstream. These bandwidths greatly surpass ISDN and analog offerings, but they cannot provide the multi-service goals of xDSL—primarily MPEG-2 video streaming. Table 9.2 shows the various xDSL technologies available.

FIGURE 9.1  xDSL installations

Most vendors are deploying xDSL from two perspectives. The first is the traditional ISP-based installation, which simply substitutes ISDN or analog dial-up for DSL. Because DSL is an always-on technology, there is no call setup or teardown process, and the connection to the DSLAM, or Digital Subscriber Line Access Multiplexer, is always active. The second connectivity model is RLAN, or Remote LAN. This model places the DSL connection on par with Frame Relay or point-to-point links in the WAN; however, the solution is being deployed for telecommuters as opposed to interoffice connections. Ultimately, designers may find that the consumer level of support currently offered in DSL will be augmented and the lower price will encourage replacement of frame and lease-line installations for interoffice traffic as well.

Both of these implementation methods can assist a modern network design. However, some caveats should be considered.

At present, most DSL vendors offer a single PVC with DSL installations. This limits connectivity options and makes redundancy difficult. A second PVC could provide a link to another head-end (distribution layer aggregation point), and most vendors have multiple DSLAMs in the central office. An SVC-based solution would also assist in designing fault-tolerance.