Browse Prior Art Database

Rate Based End-to-End Clock Synchronization

IP.com Disclosure Number: IPCOM000112252D
Original Publication Date: 1994-Apr-01
Included in the Prior Art Database: 2005-Mar-27
Document File: 6 page(s) / 201K

Publishing Venue

IBM

Related People

Lin, BC: AUTHOR [+3]

Abstract

All the electronic components of a digital system, such as a computer, are orchestrated by a central crystal oscillator, so-called clock. The coordination is via a sequence of precisely timed electrical pulses generated from the oscillator. Each component performs operations at the correct timing based on the sequence of electrical pulse in order to cooperate with other modules to produce a particular function. Usually, a system has its own clock which is independent from the clock used in the other system. However, networking has changed entirely the complexity and the definition of a digital system. The boundary of a digital system is no longer clear.

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Rate Based End-to-End Clock Synchronization

      All the electronic components of a digital system, such as a
computer, are orchestrated by a central crystal oscillator, so-called
clock.  The coordination is via a sequence of precisely timed
electrical pulses generated from the oscillator.  Each component
performs operations at the correct timing based on the sequence of
electrical pulse in order to cooperate with other modules to produce
a particular function.  Usually, a system has its own clock which is
independent from the clock used in the other system.  However,
networking has changed entirely the complexity and the definition of
a digital system.  The boundary of a digital system is no longer
clear.

      Typical computer applications such as file transfer and E-mail
do not require end-to-end clock synchronization and are not sensitive
to the time delay between sender and receiver.  The clock information
of one system does not have to be known by its partner at the other
end.  The sender and the receiver complete the operations relying on
an asynchronous handshaking protocol.

      The newly blooming multimedia applications put a new set of
requirements to the existing network which are:

o   Real time data delivery,

o   Constant data rate,

o   Clock synchronization may be required during a session's
    lifetime.

Especially, the most popular Person-to-Person conferencing standard,
CCITT H.261/H.221, is defined given that a TDM network is used.  TDM
networks require the receiving system to understand the bit pattern
that the sending system puts on the wire.  The receiving station
clocks in the bit stream and interpret their meanings according to
their position in the sequence.  That requires the synchronization of
the two clocks running at both ends.  The above scenario becomes more
complicated if the end-to-end systems are connected to different type
of networks; one is on an asynchronous packet-switching networks and
the other one is on the edge of a synchronous ISDN network.  That
will extend the Multimedia applications to link the stations
connected to a packet-switching network to the stations within an
ISDN communication cloud.  In order not to underflow or overflow the
intermediate synchronous nodes, the end station has to produce the
same data rate.  Clock synchronization becomes critical since the
rate is measured based on the clock running in the system.
Unfortunately, the crystal is not perfect.  Frequency generated by
the crystal oscillator cannot be kept constant over time.  It wanders
in a range of tens ppm (parts per million) over a relatively long
period of time, hours or days.  In other words, during a period of
few hours, the frequency generated by a crystal oscillator is
considered to be stable, and then, it changes gradually over a long
period of time.

      What happens if the clocks do not synchronize is clear.  The
data buffer in the receiving system will overflow if...