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How did DSL evolve? Its development is written in the annals of security monitoring systems. Monitoring companies needed instant communication with the alarms in buildings being guarded. Using regular telephone lines was an option, but the RBOCs (regional Bell operating companies) didn't like the idea of keeping a circuit nailed up 24x7 while being able to bill only $20 per month for it. A solution was to use dry copper pairs that did not pass through the switched telephone network. Circuits didn't need to be nailed up, and most areas had enough spare copper pairs to accommodate the needs of the security companies.

Enter those copper pairs used for security systems. The dry copper lines used for monitoring weren't connected to the PSTN (public switched telephone network), so they could be pushed to very high speeds. Thanks to low tariffs, these lines were also very inexpensive.

Manufacturers began creating devices that would use these inexpensive copper pairs to push data at nearly DS-1 speeds for a mere fraction of the cost of a DS-1 line, and DSL was born.

DSLAM Dance

As with analog connections, devices are necessary at both ends to make DSL work. DSLAMs (DSL access multiplexers) are placed at the CO (central office) to terminate the service provider's end of the connection. Because the DSLAM is located at the CO, all the DSL lines coming into that location can be multiplexed onto an ATM OC-3 (or higher) link that goes to the service provider's network. And since most DSLAMs are modular, they can accommodate interface modules for every flavor of DSL supported by the manufacturer (for more information on the types of DSL available, see "DSL: A Flavor for Every Taste").

DSLAMs incorporate ATM or IP as their transmission method of choice, but ATM has until recently been the popular format because of its QoS (Quality of Service) abilities. Within the last year, however, vendors have been using IP all the way through the network, providing QoS via bandwidth shaping and protocols, such as MPLS (Multiprotocol Label Switching). ATM-based devices support CBR (continuous bit rate), VBR (variable bit rate) and UBR (unspecified bit rate) traffic classes.

The actual bandwidth available from end to end is determined by two factors: the type of DSL being used and the distance the signal has to travel from the DSLAM to the modem or IAD (integrated access device). Bandwidths range from 128 Kbps to 52.8 Mbps. These bandwidths can be the same in both upstream and downstream directions (symmetric) or greater downstream than upstream (asymmetric).

The customer end of the DSL line requires a modem for simple connections or a full-blown IAD for more complex situations, such as when integrating more than data on the line. Residential customers who have DSL installed are acquainted with the modem aspect of these devices; they are plug-and-play, and act only as a bridge for the DSL connection, with the actual routing of packets happening at the DSLAM.

The Cart Pulls the Horse

DSL deployment is growing exponentially: At the end of 1999, more than half a million lines had been installed from more than 6,000 central offices. At the end of 2000, almost 2 million lines had been installed from more than 15,000 central offices. Cahners In-Stat Group reports that new DSL subscribers worldwide will top 23 million by 2004.

Much of this growth can be attributed to enterprises. In fact, many vendors believe business use will surpass residential use within the next year. In response, suppliers of DSL equipment are creating many new models of IADs to be deployed within the enterprise (for an overview of DSL IADs, see "Dishing the Goods on DSL Services").

Interoperability Hurdles

However, while DSL offers greater bandwidth at a lower cost, the lack of interoperability between DSL CPE (customer premises equipment) and DSLAMs means enterprise customers may not have a choice of what equipment is deployed to their locations.

Until last year, most vendors created their own DSLAMs and the modems that worked with the DSLAMs. More recently, however, these vendors have started building interoperability into their products.

Don't get your hopes up, however: The day when enterprise customers can purchase the CPE gear they want and feel confident it will work with their service providers' installed DSLAMs is still a long way off. This is counter to most enterprise customers' frame relay or ATM experiences, in which those customers have generally been able to use whatever gear they feel fits their needs.

In the past, deployed DSL equipment acted as bridges. However, as features are added to those bridges to turn them into IADs, enterprises now have choices. So, when comparing service providers, take a look at what CPE devices they sell and support and be sure the feature set meets your needs.

More Than Data

Advances in DSL technology have allowed other kinds of transmission besides broadband data over copper pairs. The most obvious in both residential and enterprise environments is voice channel. In residential modems, an added POTS line was included with the DSL data.

Because they work at different frequencies, both services can be combined on the same line without interfering with one another. Thus, residential requirements can be accommodated with just one phone line running over an asymmetric DSL link.

Getting Better

Enterprise voice needs go beyond a single phone line, however, and with vendors promising voice delivery over just about every piece of equipment in the LAN and WAN, DSL modem makers are starting to beef up their offerings in this area.

Using symmetric DSL technologies, you can combine as many as 32 channels of compressed voice with data. ATM-based DSLAMs use AAL2 (ATM Adaptation Layer 2) to carry voice calls.

In addition, because DSL IADs are not bridges, features such as VPNs (virtual private networks), firewalls, DHCP and NAT (network address translation) servers are being built into them. And for those enterprise customers that need more bandwidth than is available with a single DSL connection, bonding doubles the bandwidth by tying two DSL lines together.