Evolution of the North American Digital Hierarchy

As explained in Chapter 5, "T1 Technology," DS1 technology was introduced to groom several telephony pairs in a single circuit (24 DS0s), which resulted in savings for telephone companies (telcos) in copper, office space, and other equipment.

In the same way, DS1c (48 DS0s), DS2 (96 DS0s), DS3 (672 DS0s), and DS4 (4032 DS0s) were defined to groom more and more telephony circuits. The DS0, DS1, DS1c, DS2, DS3, and DS4 hierarchy is the NADH, also referred to as the plesiochronous digital hierarchy (PDH). Plesiochronous means almost synchronous, and you will see that the DS1s inside the same DS3 do not need to share the same clock. Europe (International Telecommunication Union [ITU]) and Japan have their own PDH hierarchies, which differ by the speed and the number of DS0s that are supported.

DS3 is a North American standard (American National Standards Institute [ANSI] T1.404, ANSI T1.107).

In parallel, it became apparent that there was a need to offer not only telephony pairs to customers but also digital circuits to enterprises, as shown in the following example.

As a small startup company, you began with a single digital dataphone service (DDS) connection for WAN connectivity. At first, this circuit was more than adequate to provide the bandwidth that your company required. About six months later, you added two new sites and you graduated to Switched 56 (SW56) to cut down on costs.

About a year later, with your company growing rapidly, you decided that it was time to get a combination of T1 and Fractional T1 (FT1) circuits to accommodate the increased demand. Your applications now include Internet Web and FTP services, video conferencing, and telecommuting for your employees. You are adding more and more T1 circuits to keep up with your explosive growth. The costs are adding up quickly, and you need to find a way to eliminate this growing problem. You decide to investigate the costs of a T3 circuit from your service provider.

Simply put, after five to ten T1s, your costs will compare to that of leasing a T3. This of course varies from provider to provider. The point is that if you are to be spending the money, why not spend the same amount on more bandwidth?

Service providers can also offer FT3. An FT3 is a T3 in which only a subset of the 28 T1s are active, and a client is charged only for the T1s that are being used. It also offers the ability to quickly increase the bandwidth offered to the customer by activating more T1s.

For speeds above T3, Synchronous Optical Network (SONET) is now the preferred choice (Chapter 14, "SONET and SDH").

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