Digital Subscriber Line

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 various DSL protocols, referred to in the generic as xDSL, 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 Internet
service provider (ISP) connections. However, as DSL technologies are accepted into the home
and office, they will likely be used for primary and backup data transfer and for high-demand
services such as live video. DSL currently lags behind cable modem installations, but some
vendors, including Next Level Communications (
www.nlc.com
), have equipment in production
that demonstrates the long-term potential of this technology. The NLC-based systems
can provide voice (plain old telephone system, or POTS, based), video, and high-speed Internet
service over DSL technologies, and are priced competitively when compared to obtaining
these services independently.
The different DSL standards provide for varying amounts of upstream and downstream bandwidth
based on the equipment in use and the distances between this equipment. As a result of the
distance sensitivity of xDSL, connections typically must terminate within three miles of the central
office, but access technologies can 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 22.1 illustrates a typical installation of xDSL 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 feet and 18,000 feet.

xDSL installations
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 site, 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 22.1 shows the various xDSL technologies available.
Most vendors deploy one of the following two xDSL implementation models: ISP-based installation
(layer 3) and remote LAN (RLAN, or layer 2). The traditional ISP-based installation simply
substitutes ISDN or analog dial-up for xDSL. Because DSL is an always-on technology, there is no
call setup or teardown process, and the connection to the digital subscriber line access multiplexer
(DSLAM) is always active. There is a single link to the service provider, and all packets are routed
to their destination. RLAN, on the other hand, places the DSL connection on par with Frame
Relay or point-to-point links in the WAN. This provides more secure connectivity that can support
nonroutable protocols. This solution is being deployed for telecommuters as opposed to
Access
teminal
Central office
City 3-mile copper loop
No DSL service
Central office
City 3-mile fiber loop 1-mile
copper loop




The Various xDSL Technologies
Standard Characteristics
Asymmetric DSL (ADSL) There are a number of flavors to ADSL; the two most popular
are G.lite and G.dmt (discrete multitone). The G.lite specification
provides 1.5Mbps/384Kbps bandwidth and typically
invokes lower capital costs. The G.dmt specification can provide
8Mbps downstream and 1.5Mbps upstream.
High bit-rate DSL (HDSL) HDSL is similar to SDSL but uses double and triple pairs of
copper wire. Most other DSL technologies operate over a single
pair, which can simplify installation compared to HDSL.
HDSL typically provides distances reaching 15,000 feet.
ISDN-based DSL (IDSL) ISDN-based DSL typically allows the greatest distances but
is limited to 144Kbps.
Symmetric DSL (SDSL) Symmetric DSL provides 2Mbps bidirectional bandwidth
over a single pair of copper wires. Distances are typically
limited to 10,000 feet.
Very high bit-rate DSL (VDSL) VDSL can provide up to 52Mbps downstream bandwidth,
but its distance is limited to less than 4,500 feet. This is usually
the shortest range DSL service.

interoffice connections. Ultimately, designers might find that the consumer level of support currently
offered in DSL will be augmented, and the lower price for setup will encourage companies
to replace Frame Relay and leased-line installations for interoffice traffic with DSL as well.
Both of these implementation methods can make a modern network design perform better.
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—perhaps a distribution layer aggregation
point—and most vendors have multiple DSLAMs in the central office. An SVC-based solution
would also make a fault-tolerant design more successful.
Another concern with current DSL installations is that most products do not offer security
solutions. The RLAN model greatly reduces this risk because the links are isolated at layer 2,
but all connectivity must be provided by the head end, including Internet connectivity. For
Internet connections, the risk is significantly greater, especially when the bandwidth available
for an attack and the use of static IP addresses or address pools are considered. A number of
significant attacks have already occurred as a result of these issues, and although they should
not deter the use of the technology, the risks should be addressed with firewall technology.
A third consideration in DSL is the installation delay compared to other technologies.
Vendors are moving toward splitterless hardware so that the phone company does not have
to install a splitter in the home. The splitter divides the traditional phone signals from the
data stream and provides a jack for standard telephones—DSL transport data and voice
over the same twisted-pair wiring used for standard analog phone service. At present,
because the circuit to the home and the installation of the splitter need to be validated,
installations may require weeks to complete.