Multiplex communications – Fault recovery – Bypass an inoperative channel
Patent
1995-09-13
1998-01-27
Safourek, Benedict V.
Multiplex communications
Fault recovery
Bypass an inoperative channel
370400, H04L 122
Patent
active
057128456
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to a method for controlling conditional connections in a synchronous digital telecommunications system, such as the SDH or SONET system.
The current digital transmission network is plesiochronous, i.e. each 2 Mbit/s basic multiplex system has a dedicated clock independent of any other system. It is therefore impossible to locate a single 2 Mbit/s signal in the bit stream of a higher-order system, but to extract the 2 Mbit/s signal the higher-level signal has to be demultiplexed through each intermediate level down to the 2 Mbit/s level. For this reason, it has been expensive to construct especially branch connections requiring several multiplexers and demultiplexers. Another disadvantage of the plesio-chronous transmission network is that equipments from two different manufacturers are usually not compatible.
The above drawbacks, among other things, have led to the introduction of the new synchronous digital hierarchy SDH specified in CCITT Recommendations G.707 to G.709 and G.781 to G.784. The synchronous digital hierarchy is based on STM-N transfer frames (Synchronous Transport Modules) located on several levels of hierarchy N (N=1,4,16 . . . ). Existing PCM systems, such as 2, 8 and 32 Mbit/s systems, are multiplexed into a synchronous 155.520 Mbit/s frame of the lowest level of the SDH (N=1). Consistently with the above, this frame is called the STM-1 frame. On the higher levels of hierarchy, the bit rates are multiples of the bit rate of the lowest level. In principle, all nodes of the synchronous transmission network are synchronized into one clock. If some of the nodes should, however, lose connection with the common clock, it would lead to problems in the connections between the nodes. The phase of the frame must also be easy to recognize in the reception. For the reasons stated above, the SDH telecommunications have introduced a pointer, which is a number indicating the phase of the payload within the frame, i.e. the pointer indicates that byte in the STM frame from which the payload begins.
FIG. 1 illustrates the structure of an STM-N frame, and FIG. 2 illustrates a single STM-1 frame. The STM-N frame comprises a matrix with 9 rows and N.times.270 columns so that there is one byte at the junction point between each row and column. Rows 1-3 and 5-9 of the N.times.9 first columns comprise a section overhead SOH, and row 4 comprises an AU pointer. The rest of the frame structure is formed of a section having the length of N.times.261 columns and containing the payload section of the STM-N frame.
FIG. 2 illustrates a single STM-1 frame which is 270 bytes in length, as described above. The payload section comprises one or more administration units AU. In the example shown in the figure, the payload section consists of an administration unit AU-4, into which a virtual container VC-4 is inserted. (Alternatively, the STM-1 transfer frame may contain three AU-3 units, each containing a corresponding virtual container VC-3). The VC-4 in turn consists of a path overhead POH located at the beginning of each row and having the length of one byte (9 bytes altogether), and of a payload section in which there are lower-level frames also comprising bytes allowing interface justification to be performed in connection with mapping when the rate of the information signal to be mapped deviates to some extent from its nominal value. (Mapping of the information signal into the transmission frame STM-1 is described e.g., in patent applications AU-B-34689/89 and FI-914746.
Each byte in the AU-4 unit has its own location number. The above-mentioned AU pointer contains the location of the first byte of the VC-4 container in the AU-4 unit. The pointers allow positive or negative pointer justifications to be performed at different points in the SDH network. If a virtual container having a certain clock frequency is applied to a network node operating at a clock frequency lower than the above-mentioned clock frequency of the virtual container, the data buffer will be filled u
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patent: 5151902 (1992-09-01), Grallert
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patent: 5311551 (1994-05-01), Eng
patent: 5428612 (1995-06-01), Scheffel et al.
CCITT Blue Book, Recommendation G.709: "Synchronous Multiplexing Structure", May 1990.
SDH--Ny Digital Hierarki, pp. 43-49, TELE Feb. 1990.
Nokia Telecommunications Oy
Safourek Benedict V.
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