Method to transport a reference clock signal

Details Method to transport a reference clock signal Method to transport a reference clock signal

2001-06-15

2004-09-14

Matar, Ahmad F. (Department: 2642)

Telephonic communications

Centralized switching system

Class of service determination or transmission

C370S354000

Reexamination Certificate

active

06792101

ABSTRACT:

The present invention relates to a method to transport a reference clock as described in the preamble of claim
1
, to a method to synchronize a plurality of line terminals and to a method to synchronize a first line terminal in a first carrier unit with a second line terminal in a second carrier unit using such above method, as described in the preamble of, respectively, claim
3
and claim
4
, and to a central office and a carrier unit, realizing such a method as described in the preamble of claim
5
and claim
6
, and to a telecommunication network comprising such a central office and such a carrier unit as described in the preamble of claim
7
.
Such a method to transport a reference clock signal from a central office to a line terminal in a communication system wherein the central office communicates with the line terminal in a carrier unit via an optical link is already known in the art, e.g. from the
ITU
-
T Recommendation G
.983.1 (10/98) with title ‘
Broadband optical access systems based on Passive Optical Networks
(
PON
)’. According to paragraph 8.3.5 ‘Transport specific TC functions’ and more particular in paragraph 8.3.5.3 ‘Downstream PLOAM structure’ whereof the sub-paragraph 8.3.5.3.4 ‘Synchronization field (SYNC
1
-SYNC
2
)’ describes the purpose of the synchronization field SYNC
1
and SYNC
2
of the Physical layer Operation and Maintenance cell, shortly called hereafter PLOAM cell.
Such a physical layer operation and maintenance cell is a predefined part of one of the downstream information packets that is broadcasted on a predefined regular base from the central office to a plurality of line terminals in order to transport operation and maintenance information packets related to operation and maintenance functions.
Such functions are e.g.:
Performance monitoring;
Defect and failure detection;
System protection;
Failure or performance information;
Fault localization.
An optional Synchronization field of the PLOAM cell is described in the above-mentioned paragraph. The purpose of this field is to transport a 1 kHz reference signal provided at the optical line terminal OLT to the Optical Network Units ONU's.
It has to be remarked that the OLT and the ONU's are called hereafter and in the present claims, respectively, Central Office CO and Line Terminal LT.
As it is described in the Recommendation, a counter in the central office is incremented and reset on a regular base in order to generate a reference signal. At the central office, the value of that counter is taken right before transmission of the first PLOAM cell of a frame. The 15 least significant bits of the counter are placed in the 15 least significant bits of the SYNC
1
-SYNC
2
field of the PLOAM cell. The most significant bit of the counter is placed in the most significant bit of the SYNC
1
filed. Depending on the length of the counter other timing references can be obtained. At reception in the line terminal, this field is used to synchronize a local counter. The line terminal is then locked on the central office counter.
Thus a known method to transport a reference clock signal from a central office to a line terminal in e.g. a carrier unit via an optical link PON e.g. a PON system comprises the following steps:
a. retrieving the reference clock signal by the central office; and
b. synchronizing a first counter into the central office with the reference clock signal; and
c. regularly transferring counter values of the first counter as part of physical layer operation and maintenance cells over the optical link; and
d. adjusting a counter value of a second counter of the carrier unit when a counter value is received as part of a the physical layer operation and maintenance cell; and
e. deriving clock signals from the second counter for the line terminal.
A problem with this method is that the achieved accuracy is limited by two bytes i.e. the two predefined bytes SYNC
1
and SYNC
2
of each PLOAM cell in order to transport the reference clock. Indeed the number of bits that are representing the counter values of the first counter which are regularly transported are limited by the two predefined synchronization fields SYNC
1
and SYNC
2
.
An object of the present invention is to provide a method to transport a reference clock signal from a central office to a line terminal in a communication system according to the above known method but wherein an improved accuracy is reached.
According to the invention, this object is achieved with the method to transport a reference clock according to claim
1
, and with the method to synchronize a plurality of line terminals in a communication system and with the method to synchronize a first line terminal in a first carrier unit with a second line terminal in a second carrier unit that comprises such a method, according to, respectively, claim
3
and claim
4
, and with the central office and the carrier unit that realizes such a method according to, respectively, claim
5
and claim
6
, and with the telecommunication method according to claim
7
, that comprises such a central office and such a carrier unit.
Indeed, by realizing the above mentioned step c) by means of assigning a predefined value to a predefined message identifier filed of the physical layer operation and maintenance cell and by comprising the counter values of the first counter in a plurality of predefined message fields of the physical layer operation and maintenance cell and by realizing the above mentioned step d) by means of recognizing the predefined value of the predefined message identifier field and by, upon recognition, extracting the counter value from the plurality of predefined message fields, a counter value represented by a bigger number as two bytes can be transported from the central office to the line terminal.
It has to be explained that the above mentioned Recommendation G.983.1 defines in paragraph 8.3.5.3.7 messages in the PLOAM cells to transport e.g. all Operation and Maintenance related alarms or threshold-crossing. Also all ranging related messages are mapped in the message field of the PLOAM cell. The format of the PLOAM message is indicated in Table 11/G.983.1. Herein it is shown that a message identifier field indicates the type of message and a number of message fields are containing the message. Furthermore, it is shown in Table 17 of this Recommendation, which defines the messages, that message number
20
with message name ‘Vendor specific message’ describes a number of values for the message identifier field of the physical layer and operation cell, that are reserved for vendor specific messages from the central office to the line terminal. By assigning one of these vendor specific values to this predefined message identifier field it is allowed to comprise a vendor specific message in the message fields. According to the present invention the central office and the respective carrier units are instructed to associate the presence of a vendor specific value for the message identifier field with the transport of first counter value into a plurality of these message fields for synchronization purposes. Since the PLOAM cell contains up to 10 message fields an improved accuracy is realized with counter values of the first counter being represented by at least two bytes and by comprising these counter values in at least two of the predefined message fields.
Furthermore, when a counter value is received as part of physical layer and operation and maintenance cell by a carrier unit, the above mentioned step d) is realized i.e. adjusting a counter value of a second counter, by recognizing the predefined vendor specific value in the predefined message identifier field and by extracting the accurate counter value from the plurality of predefined message fields.
The method of the invention can be used to synchronize a plurality of line terminals in a communication system wherein a central office communicates with the plurality of line terminals in a carrier unit via an optical line. Indeed, the method comprises the step of transporting a reference clo

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