There are two hierarchial structures that exist for digital networks:
Plesiochronous hierarchies
Synchronous hierarchies
To further complicate things, North American standards (usually derived from US standards bodies) are different from the International CCITT (now ITU-T) recommendations.
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. As such, the user's clock rate is propagated (plus a little "Jitter") 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 North American 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
The CCITT Digital hierarchy's basic level is the DS0 rate of 64 KBPS. These signals are usually delivered from the provider on Twisted-Pair or Coaxial cables.
Name Rate ---- ----------- DS0 64 KBPS E1 2.048 MBPS E2 8.448 MBPS E3 34.368 MBPS E4 139.264 MBPS
In the later 1980s, synchronous network hierarchies were defined. In Synchronous networks, all multiplex functions operate using clocks derived from a common source.
This 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, etc). Some typical rates are listed below:
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
This system is based upon a fundamental rate of 155.520 MBPS, three times that of the SONET system. This fundamental signal is called STM-1 (Synchronous Transport Module, Level 1). The typical transmission media is defined to be fiber, but the Broadband ISDN specification does define a User-Network Interface (UNI) STM-1 (155.520 MBPS) operating over coaxial cables. Some typical rates within this hierarchy:
Name Rate ------ --------------- STM-1 155.520 MBPS STM-3 466.560 MBPS STM-4 622.080 MBPS STM-16 2488.320 MBPS STM-64 9953.280 MBPS STM-256 39813.120 MBPS
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