UNIX System Administration Handbook - Evi Nemeth [230]
Digital Subscriber Line (DSL) uses ordinary copper telephone wire to transmit data at speeds of up to 7 Mb/s (although typical DSL connections yield between 256 Kb/s and 768 Kb/s). Since most homes already have existing telephone wiring, DSL is a viable way to provide the “last mile” of connectivity from the telephone company to the home. DSL connections are usually terminated in a box that acts as a TCP/IP router and provides an Ethernet connection to other devices within the home. DSL is typically both cheaper and faster than ISDN, so it is now the preferred technology for home users.
Unlike regular POTS (Plain Old Telephone Service) and ISDN connections, which require you to “dial up” an endpoint, DSL is a dedicated network service that is always connected. This feature makes it even more attractive, because there is no setup or connection delay when a user wants to transfer data.
DSL comes in several forms, and as a result it’s often referred to as xDSL, with the x representing a specific subtechnology such as A for asymmetric, S for symmetric, H for high speed, or RA for rate adaptive. The exact technology variants and data transfer speeds available in your area depend on the central office equipment that your telephone company or carrier has chosen to deploy.
The race for “last mile” connectivity to hundreds of millions of homes is a hot one. It’s also highly politicized, well capitalized, and overpublicized. The DSL approach leverages the copper infrastructure that is common among the Incumbent Local Exchange Carriers (ILECs), who favored higher profit margins over investments in infrastructure as the networking revolution of the 1980s and 90s passed them by.
Cable television companies, which already have fiber infrastructure in most neighborhoods, are promoting their own “last mile” solutions, which yield similar (though asymmetric) high-bandwidth connections to the home. The cable modem industry has recently become enlightened about data standards and is currently promoting the Data Over Cable Service Interface Specification (DOCSIS) standard. This standard defines the technical specs for both the cable modems and the equipment used at the cable company, and it allows various brands of equipment to interoperate.
All in all, the fight between cable modem and DSL technologies largely boils down to “my marketing budget is bigger than yours.” DSL has something of an inherent advantage in that each connection is private to the particular customer; cable modems in a neighborhood share bandwidth and can eavesdrop on each other’s traffic.
15.8 WHERE IS THE NETWORK GOING?
When you look closely at the technologies described above, you’ll see one thing in common: the simple, inexpensive ones are succeeding, whereas the complex and expensive ones are dying quickly. Where does this put us down the road?
Ethernet has pummeled its rivals because it is incredibly inexpensive. It’s so simple to implement that today you can even buy microwave ovens with Ethernet interfaces. Ethernet has scaled well: in many organizations, 10 Mb/s Ethernet infrastructure from the early 1980s is still in production use, connected into 100 Mb/s and 1 Gb/s segments. 10 Gb/s Ethernet is already in development and should be widely available by 2004. We expect to see this trend continue, with faster and faster switching hardware to connect it all.
On the “connectivity to the home” front, DSL offers new life to the tired old Ma Bell copper plant. It’s likely that advances in technology will soon increase the speed of DSL by up to tenfold, which will offer a whole new world of options to homes and home offices.
Another exciting area of development is wireless networking.