Networking —

Internet Access Technologies

Internet access has become an integral part of modern business. There are several ways to obtain Internet access. The type chosen will often depend on the cost as well as what technologies are available in the area you are located. This section explores some of the more common methods of obtaining Internet access.

xDSL Internet Access

DSL is an Internet access method that uses a standard phone line to provide high-speed Internet access. DSL is most commonly associated with high-speed Internet access; because it is less expensive than technologies such as ISDN, it is often used in homes and small businesses. With DSL, a different frequency can be used for digital and analog signals, which means that you can talk on the phone while you're uploading data.

DSL arrived on the scene in the late 1990s, and it brought with it a staggering number of flavors. Together, all these variations are known as xDSL:

  • Asymmetric DSL (ADSL) Probably the most common of the DSL varieties is ADSL. ADSL uses different channels on the line: One channel is used for POTS and is responsible for analog traffic, the second channel is used to provide upload access, and the third channel is used for downloads. With ADSL, downloads are faster than uploads.

  • Symmetric DSL (SDSL) SDSL offers the same speeds for uploads and for downloads, making it most suitable for business applications such as Web hosting, intranets, and e-commerce. It is not widely implemented in the home/small business environment and cannot share a phone line.

  • ISDN DSL (IDSL) ISDN DSL is a symmetric type of DSL that is commonly used in environments where SDSL and ADSL are unavailable. IDSL does not support analog phones.

  • Rate Adaptive DSL (RADSL) RADSL is a variation on ADSL that can modify its transmission speeds based on the signal quality. RADSL supports line sharing.

  • Very High Bit Rate DSL (VHDSL) VHDSL is an asymmetric version of DSL and, as such, can share a telephone line.

  • High Bit Rate DSL (HDSL) HDSL is a symmetric technology that offers identical transmission rates in both directions. HDSL does not allow line sharing with analog phones.

Why are there are so many DSL variations? The answer is quite simply that each flavor of DSL is aimed at a different user, business, or application.

Businesses with high bandwidth needs are more likely to choose a symmetric form of DSL, whereas budget-conscious environments such as home offices are likely to opt for an option that allows phone line sharing at the expense of bandwidth. In addition, some of the DSL variants are simply older technologies. While the name persists, they have been replaced with newer DSL implementations. When you're working in a home/small office environment, you should expect to work with an ADSL system.

Table 5 summarizes the maximum speeds of the various DSL options. Keep in mind that maximum speeds are rarely obtained.

Table 5 DSL Speeds

DSL Variation

Upload Speed

Download Speed

ADSL

1Mbps

8Mbps

SDSL

1.5Mbps

1.5Mbps

IDSL

144Kbps

144Kbps

RADSL

1Mbps

7Mbps

VHDSL

1.6Mbps

13Mbps

HDSL

768Kbps

768Kbps