Multiplex communications – Communication techniques for information carried in plural... – Combining or distributing information via time channels
Utility Patent
1998-03-19
2001-01-02
Ngo, Ricky (Department: 2731)
Multiplex communications
Communication techniques for information carried in plural...
Combining or distributing information via time channels
C375S358000
Utility Patent
active
06169753
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a transmission device which is suitably used, for instance as a node device for a trunk network, and c signal transmission method in a synchronous network.
(2) Description of the Related Art
With the standardization of a synchronous digital hierarchy technology (abbreviated as SDH, hereinafter) in recent years, efforts have been made to introduce an SDH network in various countries.
Referring to
FIG. 15
which is a block diagram, there is shown an example of an SDH network paying particular attention to its clock supply system. An SDH network
100
shown in
FIG. 15
includes a plurality of network elements (abbreviated as NE, hereinafter)
102
and a plurality of slaves
103
, which are connected in series to a PRC
101
as a clock oscillator.
Specifically, a plurality of NEs
102
(e.g., N elements; N is 20 at the maximum) and one slave
103
are alternately connected in series to the PRC
101
. For the number of slaves
103
among the plurality of NEs
102
, K slaves (K is 10 at the maximum) are provided.
The PRC
101
is a high-quality clock oscillator, which is prepared on the SDH network
100
as prescribed by Recommendation G.811 of International Telecommunication Union (abbreviated as ITU).
A clock produced from the PRC
101
is used as a master clock and supplied via each NE
102
so as to be distributed to all the NEs
102
included in the SDH network
100
. Accordingly, synchronism can be obtained for the entire SDH network
100
.
The quality of a synchronizing clock is reduced each time it is relayed through the NE
102
. Thus, according to Recommendation of ITU or the like, the number of NEs relaying a clock signal (number of relaying) must be maintained at a fixed number. As prescribed by Recommendation G. 812, the slave
103
is provided so as to regain the clock quality when it is necessary to perform clock relaying by a number of times exceeding the above-noted fixed number.
For distributing a clock from the PRC
101
, for example as shown in
FIG. 16
, a STM-N signal for main signal transmission, a signal (2 Mb/s) for clock distribution or a signal (1.544 Mb/s) for hierarchy formation is used. In other words, by using a clock signal component contained in each of these signals, a master clock can be transmitted from the PRC
101
to NEs
102
a
and
102
b.
Upon receiving such a signal, according to setting, each of the NEs
102
a
and
102
b
selects one of the clock signals of several kinds of input signals based on a preset priority order. The selected clock signal is used as an internal operation clock
104
.
Specifically, in the NE
102
a,
among signals (STM-1 signal
121
, STM-1 signal
122
and clock distribution signal [EXT CLOCK; EXTERNAL CLOCK]
123
) containing clock signal components inputted from other NE (e.g., NE
102
b
), a priority order can be set beforehand for clock signals to be selected.
In the NE
102
a
shown in
FIG. 16
, the STM-1 signal
122
is first in the priority order, the STM-1 signal
121
is second and the clock distribution signal [EXT CLOCK; EXTERNAL CLOCK]
123
is third.
For clock selection, in each NE, a better quality clock is selected based on a synchronous status message half byte (abbreviated as SSMB) as clock quality information added to an input signal. In
FIG. 16
, “G. 811” indicating that a clock is from the PRC
101
or later-described “DNU” is set as an SSMB.
For example, an SSMB as a quality signal byte is taken out for the section overhead (abbreviated as SOH) of the STM-N signal. Then, for instance, the NE
102
a
automatically selects a clock to be selected from input signals (STM-1 signal
121
,
122
or clock distribution signal
123
) which can be selected as clocks according to the set clock selection priority order and the SSMB of each input signal.
Quality information regarding the selected clock is multiplexed to be an output signal and then outputted.
Accordingly, for instance when a clock signal contained in the STM-N signal is selected, in each of the NEs
102
a
and
102
b,
clock quality information is taken out from the quality signal byte contained in the SOH of the STM-N signal. Then, if the currently selected clock signal becomes defective for one reason or another, this clock signal is changed to another better quality clock signal in the choices. Thus, a redundant configuration may result.
In the clock distribution system, a clock system closed loop
124
like that shown in
FIG. 16
may be formed because of the redundant configuration or the complex structure of the network or setting mistakes, and so on, made by a maintenance engineer.
Because of the formation of this closed loop
124
, a distribution system cannot be correctly configured between the two opposing NEs
102
a
and
102
b,
and the distribution system is disconnected. Consequently, a master clock cannot be supplied. In other words, a clock distributed from the master clock cannot be selected in the NEs
102
a
and
102
b
having the closed loop
124
.
It is difficult for the maintenance engineer to identify such a clock closed loop
124
. In addition, since the NEs
102
a
and
102
b
use clocks received from each other as master clocks, a difference between these clocks will be increased if fluctuation occurs in a clock frequency contained in a signal following the deterioration of a transmission line caused by environmental changes such as temperature fluctuation. Consequently, a main signal system will be adversely affected. For instance, jitters or the like may occur.
In order to deal with the above-described situation, for example as shown in
FIG. 17
, clock quality indicated in the STM-1 signal outputted from the NE
102
b
to a clock receiving direction (direction from the NE
102
b
to the NE
102
a
) is forcibly displayed as Don't use (DNU; clock signal use not allowed). In this way, this clock cannot be used in the opposite NE
102
a.
The formation of a closed loop in the clock system between the opposing NEs can be prevented.
As described above, in the NEs each as a transmission device like that shown in
FIG. 17
, the formation of a clock closed loop can be prevented between the opposing NEs
102
a
and
102
b.
However, the formation of a clock system closed loop cannot be prevented if three or more NEs are used.
For example as shown in
FIG. 18
, in the case of an SDH network
100
′ configured by interconnecting three NEs
102
a
to
102
c,
if the supply system of a master clock supplied to the NE
102
a
is disconnected, a clock system closed loop
125
of NE
102
a
→NE
102
b
→NE
102
c
→NE
102
a
may be formed.
That is, as shown in
FIG. 17
, between the opposing NEs
102
a
and
102
b,
for example only a clock selected to be distributed from the NE
102
b
to the NE
102
a
cannot be selected when a signal of a direction from the NE
102
a
to the NE
102
b
is to be transmitted. Also, as shown in
FIG. 18
, in the NE
102
a,
a clock looped not only through the NE
102
b
as a station opposite the NE
102
a
but also through the NE
102
c
is one looped through a third station other than the opposing station. Therefore, these clocks cannot be identified and consequently a closed loop
125
is formed.
SUMMARY OF THE INVENTION
The present invention was made in order to solve the problems discussed above. It is an object of the present invention to provide a transmission device and a signal transmission method in a synchronous network, whereby the formation of a clock system closed loop can be prevented even among three or more transmission devises.
In order to achieve the objective, according to an aspect of the present invention, there is provided a transmission device which employs a network synchronization system for relay-transmitting a transmission signal from a transmitting side to a receiving side in synchronization with a clock signal. The transmission device comprises a clock extracting unit for extracting a plurality of clock signals from a plurality of received
Fujitsu Limited
Helfgott & Karas, PC.
Ngo Ricky
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