Apparatus depending on timing source for synchronization

Details Apparatus depending on timing source for synchronization Apparatus depending on timing source for synchronization

1999-08-09

2003-09-30

Olms, Douglas (Department: 2661)

Multiplex communications

Communication techniques for information carried in plural...

Combining or distributing information via time channels

C370S507000, C375S358000, C375S357000

Reexamination Certificate

active

06628674

ABSTRACT:

BACKGROUND 0F THE INVENTION
1. Field of the Invention
The present invention generally relates to a dependent synchronous apparatus and an SDH (synchronous digital hierarchy) apparatus having the dependent synchronous apparatus, and particularly relates to a dependent synchronous apparatus capable of identifying a direction of a timing dependency and an SDH apparatus having such a dependent synchronous apparatus.
2. Description of the Related Art
SDH networks employ a dependent-synchronization scheme. SDH networks are a synchronous network, and, thus, it is vital to maintain synchronization at all times. To this end, various measures are taken in order to avoid severing of a synchronous network caused by master-clock failures or failures of communication lines for transferring timing signals.
FIG. 1
is an illustrative drawing showing a schematic configuration of an SDH network.
The network of
FIG. 1
includes a primary master clock
19
and a secondary master clock
20
. More than one master clock is provided in order to insure redundancy.
The network also includes network elements NE
1
through NE
4
. A network element that receives an output of a master clock directly therefrom is referred to as a gateway network element (GNE) of a synchronous system. In
FIG. 1
, the network element NE
1
and the network element NE
2
are gateway network elements.
Each network element is provided with a function to select a timing source so that the network element operates in synchronism with the selected timing source. The timing-source selection function of each network element NE usually allows a plurality of timing sources to be selected and given priority. Prioritized timing sources are monitored all the time with regard to quality thereof, and a timing source having the highest quality is selected for use. If more than one timing source exhibits the same quality, a timing source having the highest priority is selected for use. If the timing source currently in use is degraded in terms of quality thereof, a timing source having the next highest quality automatically replaces the current timing source.
Quality of timing sources is transferred via an overhead in the case of an STM-n (synchronous transfer mode-n).
FIG. 2
is an illustrative drawing showing a format of an STM-
1
overhead.
An STM-
1
overhead is comprised of
3
rows of RSOH (regenerator section over head), a pointer row, and
6
rows of MSOH (multiplex section over head). RSOH is terminated at each transit apparatus, and RSOH is terminated at a transit apparatus connected to a terminal. A pointer in the pointer row indicates a start position of a signal in the payload.
FIG. 3
is an illustrative drawing showing an S
1
byte of an SOH (section over head). Here, the S
1
byte is a Z
1
#1 byte of an old system.
As shown in
FIG. 3
, quality of a timing source is transferred by using SSMB, which is inserted into four lower bits of the S
1
byte.
As will be described later, SSMB is defined by binary codes shown in ITU-T, G.708. These binary codes define quality of a SDH timing with respect to each combination of the four bits of SSMB. For example, “0010(02h)” indicates that a timing source has a quality equivalent to that of “G.811” (a primary timing source using a cesium atomic oscillator), and “1111” indicates that the timing source should not be used for synchronization.
The network element NE
1
of
FIG. 1
is a gateway network element of the synchronous system, and, thus, selects an external clock input A from the primary master clock
19
as a timing source of a first priority. When quality of the external clock input A from the primary master clock
19
degrades, a transfer line G is selected as a timing source of a second priority since a clock in synchronism with the secondary master clock
20
is now necessary.
The network element NE
2
needs a clock from the network element NE
1
such that the clock is in synchronism with the primary master clock
19
, and, thus, selects a transfer line B as a timing source of a first priority. When quality of the transfer line B degrades, a transfer line F is selected as a timing source of a second priority since a clock in synchronism with the secondary master clock
20
is now necessary.
The network element NE
3
needs a clock from the network element NE
2
such that the clock is in synchronism with the primary master clock
19
, and, thus, selects a transfer line C as a timing source of a first priority. When quality of the transfer line C degrades, a transfer line E is selected as a timing source of a second priority since a clock in synchronism with the secondary master clock
20
is now necessary.
The network element NE
4
needs a clock from the network element NE
3
such that the clock is in synchronism with the primary master clock
19
, and, thus, selects a transfer line D as a timing source of a first priority. When quality of the transfer line D degrades, an external clock input H from the secondary master clock
20
is selected as a timing source of a second priority since a clock in synchronism with the secondary master clock
20
is now necessary.
A synchronous network is established in such a manner as described above.
In
FIG. 1
, the primary master clock
19
may be comprised of a cesium atomic oscillator, and the secondary master clock
20
may be comprised of a rubidium atomic oscillator. A description of such a configuration will be given in the following.
The network element NE
1
transmits an SSMB code “02h” to the next network element NE
2
as an S
1
byte signal of MSOH via the transfer line B. Here, the SSMB code “02h” indicates a timing quality of the primary master clock
19
that is connected to the network element NE
1
and comprised of a cesium atomic oscillator. By the same token, the network element NE
2
transmits the SSMB code “02h” to the network element NE
3
as a S
1
-byte signal of MSOH via the transfer line C. The network element NE
3
transmits the SSMB code “02h” to the network element NE
4
as a S
1
-byte signal of MSOH via the transfer line D. Further, an SSMB code transferred on the transfer lines E, F, and G is set to “0Fh” in order to prevent a synchronous loop.
FIG. 4
is an illustrative drawing showing a situation where a failure occurs in the primary master clock
19
or on the output transfer line thereof, and the external clock input A from the primary master clock
19
has a degraded quality. In such a situation, switching of a timing source is effected in each network element, so that the SDH network as a whole is synchronized with the secondary master clock
20
.
When a failure occurs with respect to the external clock input A form the primary master clock
19
, the network element NE
1
detects the failure, and lapses in a hold-over status. The network element NE
1
changes the SSMB code from “02h” to “0BhSETS(synchronization equipment timing source)” in response to the change of quality of the timing source, and sends this SSMB code to the network element NE
2
via the transfer line B. The network element NE
2
transmits the SSMB code “0Bh” to the network element NE
3
via the transfer line C. By the same token, the network element NE
3
transfers the SSMB code “0Bh” to the next network element NE
4
via the transfer line D. Consequently, the entire network is put in a hold-over status originating from the network element NE
1
.
In such a status, the network element NE
4
compares the timing quality (SSMB code “0Bh”) of the transfer line D having the first priority with a timing quality (SSMB code “04h”) of the external clock input H having the second priority and originating from the secondary master clock
20
, which is comprised of a rubidium atomic oscillator. The network element NE
4
selects the timing source of the second priority because of a higher quality thereof, and sends the SSMB code “04h” to the network element NE
3
via the transfer line E. The network element NE
3
attends to a similar comparison to select the timing source of the second priority, and sends the SSMB code “04h” to the network element NE

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