Digital Transmission Hierachies
There are two hierarchical structures that exist for digital networks: Plesiochronous hierarchies and Synchronous hierarchies.
In a Plesiochronous hierarchy, the higher level multiplex functions include "bit stuffing" techniques. This allows the input bit streams from I/O channels to use "free-running" clocks. The user's clock rate is propagated through the higher level multiplexer. Slip rates requirements between End-User multiplex equipment must still be met, for adequate performance of voice and particularly data.
The digital hierarchy starts off with a basic Digital Signal level of 64 KBPS (DS0). Thereafter, all facility types are usually referred to as "T x", where "x" is the Digital Signal level within the hierarchy (e.g. T1 refers to the DS1 rate of 1.544 MBPS). Up to the DS3 rate, these signals are usually delivered from the provider on Twisted-Pair or Coaxial cables.
North American T1 service providers often refer to the signal interface between the User and the Network as "DS-1" signals. In the case of User to User interfaces, the term "DSX-1" is used to describe those DS1 signals at the "cross-connect" point.
Name Rate
---- -----------
DS0 64 KBPS
DS1 1.544 MBPS
DS1C 3.152 MBPS
DS2 6.312 MBPS
DS3 44.736 MBPS
DS4 274.176 MBPS
Synchronous Hierarchies
In the late 1980s, synchronous network hierarchies were defined. In Synchronous networks, all multiplex functions operate using clocks derived from a common source.
This SONET (Synchronous Optical NETwork) system is based upon multiples of a fundamental rate of 51.840 MBPS, called STS-1 (Synchronous Transport Signal, Level 1). The facility designators are similar, but indicate the facility type, which is usually Fiber Optic Cable (e.g. OC-3 is an Optical Carrier supporting a STS-3 signal; while OC-12 supports a STS-12 signal).
Name Rate
------ ---------------
STS-1 51.840 MBPS
STS-3 155.520 MBPS
STS-9 466.560 MBPS
STS-12 622.080 MBPS
STS-48 2488.320 MBPS
STS-192 9953.280 MBPS
STS-768 39813.120 MBPS