JPH01264427A - System for switching transmission line - Google Patents

Abstract

PURPOSE:To prevent the hit of a transmission line when the line is switched by transmitting the same information to currently used transmission lines and a standby transmission line and shifting frame phases so that they can be synchronized to each other, and bits in the frames can be made the same in value, then, completely synchronizing the information string which can be made to the one to be switched on both transmission lines. CONSTITUTION:At a transmitting side device 1 of a transmission line switching system the information string of the currently used transmission lines 2a-2d are branched by means of a transmission line changeover switch 11 and transmitted in parallel to a standby transmission line 3. The information strings from the transmission lines 2a-2d and 3 are received by the interface circuit 14 of a receiving side device 4 and the frame phase of the information string received from the lines 2a-2d is synchronized to that of the information string received from the line 3 by a frame synchronizing means including an elastic store memory 20. Then the two information strings outputted by the frame synchronizing means are compared with each other through a changeover switch 24, variable delay circuit 25, variable delay memory 33, etc., and the difference in delay between the information strings is measured in the unit of frames. The measured difference in delay is used for correcting the delaying quantities of the transmission lines 2a-2d.

Description

[Detailed description of the invention] [Industrial application field] The present invention is utilized for time division multiplexed digital transmission.

In particular, the present invention relates to a transmission line switching method for switching between a working transmission line and a backup transmission line. The present invention is suitable for use in optical fiber communication equipment.

[Conventional technology]

FIG. 4 is a block diagram of a conventional digital transmission device.

In the transmitting side device 1, the digital information string is frame multiplexed by the multiplexing conversion device 10, and the transmission path changeover switch 11
Then, it is sent to the current transmission line 2 via the interface circuit 12.

In the receiving side device 4, the signal on the current transmission line 2 is received by the interface circuit 14, and is supplied to the multiplexing/demultiplexing device 38 via the transmission line changeover switch 36. The demultiplexer 38 is
Separate multiplexed information strings.

If the line or transmission equipment breaks down in the current transmission line 2,
When it is necessary to stop the operation for maintenance or when switching back after repairing a faulty part, the transmission line changeover switches 11 and 36 are used to switch the working transmission line 2 to the backup transmission line 3.

[Problem that the invention seeks to solve]

However, in the conventional transmission line switching system, switching from the working transmission line 2 to the backup transmission line was performed regardless of the main signal.

As a result, it is not possible to absorb the delay difference between the working transmission line 2 and the protection transmission line 3, causing instantaneous interruptions during switching, and loss of synchronization due to missing or duplicating main signals, resulting in normal transmission status. There was a drawback that it could not be maintained. In particular, in high-speed optical fiber communication devices, there is a propagation time difference greater than the frame length between the working transmission line and the protection transmission line, and there is a possibility that frames may be dropped or duplicated when switching between the working and protection transmission lines. This essentially results in a momentary interruption of the transmission path.

For example, in a backbone transmission line of several hundred Mb/s or more, even a very short momentary interruption when switching the transmission line has a major effect on all equipment and terminals in the Shimofuki group, resulting in deterioration of transmission quality. There is.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a transmission line switching method that solves the above problems, eliminates instantaneous interruptions when switching transmission lines, and can always maintain a normal transmission state.

[Means for solving problems]

In the transmission line switching system of the present invention, the transmitting side device is provided with means for branching the information string of the working transmission path and transmitting it in parallel to the protection transmission path, and the receiving side device is provided with a means for branching the information string of the working transmission path and transmitting it in parallel to the protection transmission path. A frame synchronization means including an elastic store memory that synchronizes the frame phase of the information string with the frame phase of the information string received from the current transmission path compares the contents of the two information strings output from this frame synchronization means, and a means for correcting the delay amount of the backup transmission line using this delay difference, and a means for correcting the delay amount of the backup transmission line based on the delay difference, and a means for measuring the delay difference of It is characterized by having a means for disconnecting the currently used transmission line.

The means for measuring the delay difference does not need to compare all the bits of the two information strings, but compares an arbitrary number of bits greater than or equal to -.

[For production]

The same information is transmitted on the working transmission line and the backup transmission line,
Synchronize the frame phases. Thereafter, the frame phase is shifted so that the bits within the frame have the same value, and the information strings to be switched are completely synchronized on both transmission paths. Furthermore, the protection transmission line is switched from the working transmission line in a completely synchronized state. This makes it possible to eliminate instantaneous interruptions when switching transmission paths.

〔Example〕

FIG. 1 is a block diagram of a digital transmission device according to an embodiment of the present invention. In this embodiment, the present invention is applied to a frame multiplex transmission apparatus using an optical fiber transmission line.

In this device, in order to switch between the working transmission lines 2a to 2d and the protection transmission line 3, which transmit information sequences in units of frames of a fixed length, the transmitting side device 1 is configured to
A to 2d, for example, a transmission line changeover switch 11 that branches the information string of the working transmission line 2d and transmits it in parallel to the protection transmission line 3, and the receiving side device 4 receives information from the protection transmission line 3. Frame synchronization means for synchronizing the frame phase of the information sequence with the frame phase of the information sequence received from the current transmission line, that is, the interface circuit 14, the frame synchronization circuit 17, the elastic store memory 20, the station clock source 21, and the elastic store memory. means for comparing the contents of two information streams outputted by 20 and measuring the delay difference between these information streams on a frame-by-frame basis, that is, a variable delay circuit 25, an exclusive OR circuit 27, a control circuit 30, and this delay difference. A variable delay memory 33 for correcting the delay amount of the backup transmission line 3 is provided, and the transmission side device 1 and the reception side device 4
and a means for disconnecting the current transmission line after correction by the variable delay memory 33, that is, a transmission line changeover switch 11 and a control circuit 13 disposed in the transmitting side device 1, and a control circuit disposed in the receiving side device 4. 30, 32, a transmission line changeover switch 36, and a data link 39 that connects the control circuit 13 and the control circuit 32.

The transmitting side device 1 further includes a multiplexing conversion device 10 and an interface circuit 12 provided corresponding to each of the working transmission lines 2a to 2d and the protection transmission line 3.

The receiving device 4 further includes a demultiplexing circuit 38 .

In this embodiment, any working transmission line 2a to 2d is connected to the protection transmission line 3, and if there is an unused working transmission line, that transmission line is used as a protection transmission line and is disconnected from other transmission lines. You can switch at a moment's notice. This selection is made by the control signal 31 from the control circuit 30.
This is done by operating 4.

Here, the operation of this device will be described using a case where the working transmission line 2d is switched to the protection transmission line 3 as an example.

The multiplex conversion device 10 outputs the frame multiplexed signal into which the frame pattern has been inserted to the transmission path changeover switch 11. The transmission line selector switch 11 connects the working transmission line 2d and the protection transmission line 3 in parallel under the control of the control circuit 13, and branches the information string of the working transmission line 2d to the protection transmission line.

The interface circuit 12 converts electrical signals into optical signals and sends them to each transmission path.

The interface circuit 14 converts the optical signal into an electrical signal for each transmission path, synchronizes the bits, and outputs a clock 15d,
Play 16. The frame synchronization circuit 17 receives the information string received by the interface circuit 14 and the clock 15d.
, 16 and each frame pulse 18d
, play 19. elastic store memory 20
stores received information sequences using clocks 15d and 16, respectively, with the phase of frame pulses 18 (!, 19 as a reference).The information stored in the elastic store memory 20 is based on the station frame pulse 230 supplied from the station clock source 21. Similarly, based on the phase, the station clock source 2
It is read out by the station clock 22 supplied from 1.

As a result, the frame phases of the received information sequences between the working transmission line 2d and the protection transmission line 3 match.

The changeover switch 24 receives a control signal 31 from the control circuit 30.
As a result, only the received information sequences of the working transmission line 2d and the protection transmission line 3 are connected to the variable delay circuit 25.

The output of this variable delay circuit 25 is supplied to an exclusive OR circuit 27. The control circuit 30 changes the delay amount of the variable delay circuit 25 using control signals 28 and 29 while referring to the output of the exclusive OR circuit 27.
control so that the output is always "0".

That is, the delay amount of one variable delay circuit 25 is set to the minimum, and the delay amount of the other variable delay circuit 25 is increased in units of frames so that the input of the exclusive OR circuit 27 always has the same bit code. That is, the output of the exclusive OR circuit 27 is controlled to always be "0". Even if the delay amount of the other variable delay circuit 25 is maximized, if the output of the exclusive OR circuit 27 does not always become "0", the delay amount of the other variable delay circuit 25 is minimized, The delay amount of the one variable delay circuit 25 is similarly increased.

In this way, the output of the exclusive OR circuit 27 is always "
0'', the control circuit 30 calculates the difference (in frame units) between the delay amounts of both variable delay circuits 25 to the current transmission line 2d.
It is detected as the transmission delay difference between the transmission line 3 and the backup transmission line 3.

The variable delay memory 33 has a memory capacity that is more than twice the transmission delay difference and an even number multiple of the frame length. The control circuit 30 sets the variable delay memory 33 on the current transmission line 2d side in advance so that the delay amount is half the memory capacity using the control signal 34d. Furthermore, if the delay of the backup transmission line 3 is smaller than the delay of the working transmission line 2d based on the detected transmission delay difference, the control circuit 30 adds a new delay amount to the delay amount preset in the working side variable delay memory 33. The detected transmission delay difference is added to the delay amount, and this delay amount is set in the spare variable delay memory 33 by the control signal 35. In addition, the delay of the backup transmission line 3 is greater than the delay of the working transmission line 2.
If the delay is larger than the delay of d, the roughly detected transmission delay difference is subtracted from the delay amount preset in the variable delay memory 33 on the active side, and this delay amount is used as the control signal 35.
is set in the variable delay memory 33 on the spare side. Thereby, the transmission line delay difference between the working transmission line 2d and the protection transmission line 3 is absorbed by the variable delay memory 33.

After this, the control circuit 30 controls the transmission line changeover switch 36 using the control signal 37, and switches the working transmission line 2d to the protection transmission line 3 at high speed. Following this, the control circuit 32
, sends a signal to the control circuit 13 via the data link 39, controls the transmission line changeover switch 11, and switches the current transmission line 2.
d is disconnected, and transmission line switching is completed.

FIG. 2 shows the format of the information string.

This information string is made up of frames, and each frame is made up of a frame synchronization bit F and a main information bit. The variable delay circuit 25 compares one or more arbitrary bits D included in the main information (2). However, bit D needs to be at the same bit position in each frame. All bits in the main information I can also be compared.

Figure 3 shows the phase relationship of information strings in each circuit, and (a
) is the output of the elastic store memory 20 on the active side,
(b) is the output of the variable delay memory 33 on the active side, (C) is the output of the elastic store memory 20 on the spare side, (
d) shows the output of the variable delay memory 33 on the reserve side. In this figure, Δ~N are frames, D.

is the allowable delay difference between transmission paths, and D is the delay 1! due to the variable delay memory 33 on the active side. , D2 is the delay difference between transmission paths, D3 is the delay amount due to the variable delay memory 33 on the standby side, and D3
=DI +D2 , α indicates the allowable arrival range of the first bit of frame G on the spare side.

This figure shows a case where the capacity of the variable delay memory 33 is for six frames and the delay of the backup transmission line 3 is smaller than that of the working transmission line 2d. This transmission delay difference is absorbed by the variable delay memory 33.

〔Effect of the invention〕

As explained above, the transmission line switching method of the present invention can switch the transmission line from the working transmission line to the protection transmission line without momentary interruption. Therefore, it is possible to prevent deterioration of transmission quality due to instantaneous interruptions.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a digital transmission device according to an embodiment of the present invention. FIG. 2 is a diagram showing the format of an information string. FIG. 3 is a diagram showing the phase relationship of information strings. FIG. 4 is a block diagram of a conventional digital transmission device. DESCRIPTION OF SYMBOLS 1... Sending side device, 2.2a-2d... Working transmission line, 3... Backup transmission line, 4... Receiving side device, 10.
...Multiplex conversion device, 11.36...Transmission path changeover switch, 12.14...Interface circuit, 13.3
0.32... Control circuit, 17... Frame synchronization circuit, 20... Elastic store memory, 21...
Station clock source, 24... Changeover switch, 25... Variable delay circuit, 27... Exclusive OR circuit, 33...
Variable delay memory, 38... Demultiplexing circuit, 39...
data link. Patent applicant: Nippon Telegraph and Telephone Corporation, agent: Naotaka Ide, patent attorney

Claims (1)

[Scope of Claims] 1. In a transmission line switching system comprising switching means for a working transmission line and a backup transmission line, each of which transmits an information string in units of frames of a fixed length, the transmitting side device is provided with a switching means for transmitting information on the working transmission line. means for branching the information sequence and transmitting it in parallel to the protection transmission line, and having the receiving side device transfer the frame phase of the information sequence received from the protection transmission line to the frame phase of the information sequence received from the working transmission line. a frame synchronization means including an elastic store memory for phase synchronization; a means for comparing the contents of two information streams outputted by the frame synchronization means and measuring a delay difference between these information streams in units of frames; means for correcting the delay amount of the backup transmission line based on the difference, and means for disconnecting the working transmission line after the correction by the correction means is provided on both the sending side device and the receiving side device. A transmission path switching method characterized by: