Browse Prior Art Database

Redundant Self Monitoring Network Clocking System

IP.com Disclosure Number: IPCOM000107611D
Original Publication Date: 1992-Mar-01
Included in the Prior Art Database: 2005-Mar-22
Document File: 5 page(s) / 219K

Publishing Venue

IBM

Related People

Rogers, LS: AUTHOR

Abstract

Disclosed is a system to provide a redundant self-monitoring clocking system for synchronous communications networks.

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 35% of the total text.

Redundant Self Monitoring Network Clocking System

       Disclosed is a system to provide a redundant
self-monitoring clocking system for synchronous communications
networks.

      Digital communications systems that carry synchronous
information must be able to provide accurate clocking signals at its
input and output ports.  To provide long-term error-free
transmission, the clock signals must be maintained within less than
one part per million over the entire network.  The clocking system
disclosed here will provide such function plus a capability to self
monitor and report system anomalies.

      Error-free synchronous communications services, such as voice,
video, and synchronous data, require that information enter the
system at exactly the same rate it leaves at the destination.  If the
play-out rate is not exactly equal to the input sample rate (clock
offset), then buffer overrun or underrun will occur with a resulting
error.  Although an occasional error would likely not be noticeable
to a human listener, the effect on voice band data and facsimile
transmissions would be significant.  Synchronous data transmissions
would be directly effected.  The extent of these errors would depend
on the coding being used, the length of the error, and the
application.  If clock offset is not to limit overall system error
performance, its magnitude must be maintained to less than a few
parts per million.

      For many years, digital communications systems have maintained
synchronous operation to allow time division multiplexing and small
or zero input-output frequency offset.  This was accomplished by
establishing a highly accurate atomic clocking facility.  The
frequency of this master clock was distributed throughout the digital
network, via isochronous communications links.  Various lower level
(less accurate) clocking facilities existed in geographic regions,
all linked to the master.  The network would supply this clock to
terminal and PBX equipment to drive the sample and play-out rates.

      In theory, the above system would provide zero frequency
offset.  However, in practice, the maintenance and administration of
the system is a continuing problem. Equipment maintenance and repair
requires skilled craft people with extensive test equipment.

      In an effort to reduce the operational and interconnect
problems, most systems operators have chosen to change to more
recently available technologies and move to a plesiochronous system.
Each carrier would establish regional clocks, each accurate and
traceable to the world standard.  When operating properly, all clocks
would be accurate within less than one part per billion.  The
frequency of these clocks is distributed throughout the region to the
attached terminal equipment to synchronize sampling and data rates.

      Plesiochronous clocking systems reduce but do not eliminate the
complexity required to operate, maintain, and distribute clockin...