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Digital Transmission Hierarchies

There are two hierarchical 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.

Plesiochronous 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. 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.

North American Digital hierarchy

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

International (CCITT) Digital hierarchy

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


Synchronous Hierarchies

In the later 1980s, synchronous network hierarchies were defined. In Synchronous networks, all multiplex functions operate using clocks derived from a common source.

North American SONET (Synchronous Optical NETwork)

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


International SDH (Synchronous Digital Hierarchy)

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