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WAN Technologies: Important Points of Interest, Part 2 of 3

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In Part 2 of a three-part series, Sean Wilkins, co-author of CCNA Routing and Switching 200-120 Network Simulator, Academic Edition, reviews popular low-end and mid-grade offerings for Internet access: dial-up, ISDN, DSL, and cable.
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Overview

Part 1 of this series reviewed some of the common wide area networking (WAN) technologies that network engineers need to grasp in order to earn certification and/or find a job in the computing industry.

This article focuses on four additional technologies: dial-up, Integrated Services for Digital Network (ISDN), Digital Subscriber Line (DSL), and broadband cable. These technologies typically are or have been deployed in residences or in organizational branch office locations.

Dial-Up

The term dial-up originated with individuals and organizations that utilize the telephone voice network for data transmission. This connection method has been used for over 30 years. In the early growth of the Internet, the only common access method for the public was through an Internet service provider (ISP). Using a modem connected to any typical analog phone line, users "dialed up" the ISP via the telephone network and were connected through the ISP to the Internet.

In the really early years, the access rate provided by dial-up was limited to .1 to .3 kbps (300 baud), but it quickly evolved to reach speeds of 9,600–14,400 bps by the time Internet access started being available to the public (early 1990s). Modern modems support line rates up to approximately 56 kbps, with additional speeds available using different forms of compression.

Dial-up isn't just for Internet access; many organizations still use dial-up as a form of backup connection into remote sites. If a primary connection to a site fails, the dial-up line can be used to connect to the remote equipment for access and troubleshooting.

Integrated Services for Digital Network (ISDN)

ISDN was developed in the late 1980s as a method for integrating voice and data connections. Two different types of ISDN, Basic Rate Interface (BRI) and Primary Rate Interface (PRI), are commonly used as access methods:

  • BRIs are targeted at individual sites, offering two separate 64 kbps bearer (B) channels and a single 16 kbps data (D) channel. The two B channels can then be combined to offer an access rate of up to 128 kbps; this method was generally used to provide access for individuals or to remote offices. ISDN utilized a digital connection, rather than the analog connection utilized by dial-up.
  • PRIs are targeted at larger organizations, offering 23 separate 64 kbps B channels and a single 64 kbps D channel. These channels can be used separately or combined for connections between devices or offices.

Digital Subscriber Line (DSL)

DSL has been used for over 15 years. It was one of the main technologies that (in most cases) quickly replaced the aging dial-up Internet access method. Never limited to large organizations, DSL has been a very popular public access method for a couple of reasons:

  • The telephone company's DSL utilizes the same copper infrastructure that's used for phone lines, making the upgrade to support it rather easy, with changes mainly being required in the central office.
  • For individuals and organizations, the cost of a DSL connection was reasonable, and DSL offered a considerable increase in Internet access speed versus the very slow dial-up methods.

A number of different types of DSL can be implemented, with asynchronous DSL (ADSL) being the most popular. Other types include synchronous DSL (SDSL), high bit-rate DSL (HDSL), and very-high bit-rate DSL (VDSL), along with many others. ADSL and VDSL offer different access rates downstream and upstream, with a higher speed available downstream (from the Internet) than is available upstream (to the Internet). This characteristic works well for most connections because Internet access traffic generally involves more traffic coming downstream than travels upstream. Both HDSL and SDSL offer equal downstream and upstream, which means they're typically more expensive.

The main limitation to DSL technologies is the distance between the telephone central office (CO) and the remote site. The closer the remote site is to the CO, the higher the access rate that's possible; for example, ADSL is limited to about 18,000 feet.

Broadband Cable

Broadband cable access has been around for individuals and businesses almost as long as DSL, bringing a number of advantages and disadvantages. Both DSL and cable offer reasonably priced Internet access methods for both businesses and individuals. Unlike DSL, however, cable isn't as limited by distance from a central point because of the shielded nature of the cabling being used.

Like ADSL and VDSL, broadband cable connections have different downstream (higher) and upstream (lower) rates. The selection of one technology over the other primarily revolves around which technology is available and which offers the best deal for the greatest access rate based on the location. Broadband cable typically offers faster access rates than DSL alternatives for the price, and therefore cable connections are considered the preferred option when a high access rate is required.

Summary

The evolution of the technologies discussed here goes hand in hand with the evolution of many computing technologies. As the speed of chips and electronics has generally increased, so has the speed of networking connections. We've certainly come a long way from dial-up modems (with slow and very frustrating downloads) to our current state of widespread broadband. The future will bring additional speed and the promise that these technologies and their successors will spread to all those in search of them.

Part 3 concludes this series by examining satellite, wireless data, and leased line technologies.

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