CertiGuide to Network+  9  Chapter 0111: Wide Area Networking       9  VII  Wide Area Networking Technologies

VII  Wide Area Networking Technologies 1 X.25

These are the “dedicated” lines that originally formed the infrastructure of private networks. A company would call the telephone company and specify a line running from their building to whatever other location they chose (across town, in a different city, different state, etc.). The telephone company would determine the route for the line; configure the line (often physically laying some or all of the cable required to get from Point A to Point B) and its price. The entire line capacity would be available for use by the company that ordered the line.

These dedicated lines are useful because sometimes you just don’t want even the remote possibility that someone else’s traffic will interfere with yours. Alas, with this exclusive asset comes the cost disadvantage we mentioned in Chapter 0000. Point-to-Point leased lines tend to be the most expensive way at a given throughput rate, to get data from point A to point B. This is not just because of the telephone company’s management overhead of installing and supporting a line used by only one customer, but also because companies using leased lines pay for the available bandwidth whether they’re using it or not.

These lines are available in analog or digital versions, with digital lines typically being higher speed. A digital leased line is an informal name for what is officially known as Digital Data Service, hence the “DS” in the naming convention.

Today, you’re likely to see digital leased lines in common capacities such as:

There are several things to notice about this chart of “speeds and feeds”. A 64kb/sec DS0 line is the basic building block of capacity in the digital network. Immediately above the DS0 are the T-carrier and E-carrier lines. The T-carrier lines are based on North American standards, and the E-carrier lines on European standards.

The “OC” in some of the line types above stands for optical carrier, indicating the type of media used for this type of line. The OCx lines use SONET, a standard for synchronous data transmission over optical media. (The international equivalent to the ANSI SONET standard is SDH, or synchronous digital hierarchy). SONET provides standards for lines running at rates up to 9.953 Gb/sec.

It is also possible to get Fractional T1 service, which allocates to you only some of the 24 DS0 lines available in a T1 line.

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