Book Excerpt: Next-Generation Network Services

In this excerpt from a Cisco Press tome, we provide fundamental technical information about DSL technology.

by Robert Wood in Next-Generation Network Services, published by Cisco Press [May 9, 2006]

This is excerpt is from Chapter 8: Wireline Networks, pp. 475-490 of Next-Generation Network Services, published by Cisco Press

DSL equipment is a digital modem technology that uses existing twisted-pair telephone lines to transport high-bandwidth data, such as multimedia data and video, in addition to voice services to subscribers. DSL services are dedicated, point-to-point, public network access over twisted-pair copper wire on the local loop between a service provider's CO and the customer site, or on local loops created either intrabuilding or intracampus.

DSL Layer 1 technology is coupled with Layer 2 ATM and Layer 3 IP, and increasingly using Ethernet at Layer 2 instead of ATM. These are the common protocol building blocks with which to design DSL access networks for service providers. When DSL uses ATM, it depends on ATM Layer 2 switching beyond the DSL network itself and relies on TCP/IP for routing between networks and into the public Internet.

DSL data is not passed through expensive telephone switches at the service provider's CO. The data portion of a DSL line is aggregated into DSL termination equipment called a Digital Subscriber Line Access Multiplexer (DSLAM) in the CO, and then aggregated via a broadband remote aggregation system (BRAS) for routing through IP network infrastructure and on to ISPs. While the analog telephony portion of the DSL line continues to be separated (using splitters or microfilters) and delivered to the CO telephone switch, less expensive data communications equipment supports the larger volume of aggregate data traffic from this residential broadband data service.

DSL modems are rather complex and try to accommodate a patchwork of different copper lines by checking up to 256 different frequency pathways before deciding on the best frequency path to use within the twisted pair. Since DSL uses frequencies up to just above 1 MHz, this places limitations on DSL coverage due to increased signal attenuation with higher frequencies. This limits DSL distance and speed. DSL is only able to reach about 60 percent of the market without deploying DSLAM technology into the digital loop carrier domain.

Back at the CO, or in a DLC cabinet, SLIC, or other provider point of presence, the service provider uses a DSLAM to aggregate the xDSL connections from the neighborhoods and businesses. The DSLAM is usually a device that aggregates individual DSL lines at Layer 1. The DSLAM contains cards often referred to as DSL modems on which to terminate the subscriber side DSL modems. These DSLAM DSL modem cards come in various port densities, as each DSL remote subscriber is connected to one port of the DSLAM's modem cards. DSL uses the concept of modems because DSL signaling is essentially a conversion of electrical signals to sound tones, although these tones are in the inaudible range. The DSLAM is responsible for converting these frequency sound waves into electrical or optical signals that are delivered upstream toward the provider's core network and service platforms. More on the DSLAM is covered at the end of this section under "DSLAM Broadband Aggregation Layer."

Collectively and generally, the various DSL technologies are referred to as xDSL. Each type provides a particular requirement of bandwidth symmetry, speed, and distance. DSL is drawing significant attention from implementers and service providers, because it promises to deliver high-bandwidth data rates to dispersed locations with relatively small changes to the existing Telco local loop infrastructure.

DSL is broadly divided into asymmetric and symmetric categories. The following list introduces several of the current variations of DSL technology:

• Asymmetric DSL (ADSL)—It is called "asymmetric" because the download speed is greater than the upload speed. ADSL works this way because most Internet users look at, or download, much more information than they send, or upload. Another version of ADSL is Rate Adaptive DSL (RADSL). This is a popular variation of ADSL that allows the modem to adjust the speed of the connection depending on the length and quality of the line.
• Asymmetric DSL-Lite (G.Lite)—Often referred to as splitterless ADSL, this technology attempts to provide ADSL capabilities at reduced transfer rates, while still supporting analog telephony. G.Lite is often priced less to the customer using marketing terms like "DSL Lite."
• High Data Rate DSL (HDSL)—Providing transfer rates comparable to a T1 line (about 1.5 Mbps), HDSL receives and sends data at the same speed, but it requires two subscriber lines that are separate from a normal subscriber line. Four-wire HDSL is often used within service providers as an alternate form of T1 data delivery for businesses and high-end data subscribers. Sometimes referred to as repeaterless T1, HDSL saves providers both time and costs in provisioning T1 lines.
• High Data Rate DSL-2 (HDSL2)—HDSL2 uses a more aggressive modulation technique called pulse amplitude modulation 16 (PAM-16), which accomplishes up to 2 Mbps of symmetrical bandwidth on a single pair of wires. Two-wire HDSL now becomes applicable to residential users with requirements for symmetrical bandwidth.
• ISDN DSL (IDSL)—Geared primarily toward existing users of ISDN, IDSL is slower than most other forms of DSL, operating at a fixed rate of 144 Kbps in both directions. The advantage for ISDN customers is that they can use their existing equipment, but the actual speed gain is typically only 16 Kbps (ISDN runs at 128 Kbps).
• Symmetric DSL (SDSL)—Like HDSL, this version receives and sends data at the same speed. While early SDSL modems also require a separate line from your phone, the high data rate is accomplished through the use of a single line instead of the two lines needed by HDSL. SDSL is also the official designation of the European Telecommunication Standards Institute (ETSI) to develop a standard based on HDSL2, but is expected to be rate adaptable up to 2 Mbps while providing voice and ISDN services without the use of analog splitters.
• Multirate Symmetric DSL (MSDSL)—This is a symmetric DSL that is capable of more than one transfer rate. The service provider sets the transfer rate, typically with prorated pricing based on the service level.
• Very High Data Rate DSL (VDSL)—An extremely fast connection, VDSL includes an asymmetric and a symmetric version. VDSL achieves high rates by supporting a shorter distance specification for standard copper phone wiring. However, the maximum speed of VDSL technology will likely drive the technology to prominence within service provider networks.
• SHDSL—An ITU standard developed to replace or enhance many existing DSL technologies and transport options into one standard for better interoperability and manufacturing support. SHDSL is a symmetric service that supports multiple data rates from 192 Kbps to 2.3 Mbps. Other benefits are a 30 percent farther reach than SDSL, support for both IP and ATM, and spectral compatibility with ADSL. Mainly used for business-class users, SHDSL is expected to replace many of the proprietary SDSL implementations.

Table 8-2 provides a brief comparison of the various DSL technologies. Transmit and receive speeds are affected by distance and quality of the subscriber line, so the table attempts to list the theoretical maximums. Also, interoperability of subscriber DSL modems with the DSLAM can affect optimum results. For example, mixing vendor A's remote DSL modem with vendor B's DSLAM might impair full functionality due to minor inconsistencies between vendor's choices or versions of DSL chip sets within the DSL modems.

Table 8-2 Comparing DSL Technologies

As seen from the previous list and table, there are many varieties of DSL targeted at different requirements. The varieties exist because there is a definite segmentation of customer requirements, speeds, and willingness to pay. Asymmetric DSL varieties tend to target Internet use and residential pricing markets. Symmetric DSL tends to focus on the business market to exhibit performance similar to dedicated point-to-point circuits or for high-speed multimedia requirements.

The development and deployment of ADSL, SHDSL, and VDSL technologies and architectures is currently the primary focus in xDSL.

Reproduced from the book Next-Generation Network Services. Copyright 2006, Cisco Systems, Inc.. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240.

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