9.6 Fractional T1

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With the introduction of FT1 services in the late 1980s, speculation by industry observers was that DDS would become obsolete. After all, the pricing structure of AT&T’s FT1 Service made 64 Kbps look like a bargain compared to DDS at 56 Kbps. Both types of services, however, are still available.

One reason for this coexistence is that DDS offers error correction, whereas T1 and FT1 do not. The CRC of AT&T’s ESF does not correct errors; it only monitors for error conditions. DDS also offers a higher availability rate than T1 and FT1. A 1,000-mile DDS interoffice channel has 99.95% availability, which translates into 4.38 hours of downtime per year, whereas a T1 interoffice channel at the same distance has 99.75% availability, which translates into 21.9 hours of downtime per year.

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