Multiplex communications – Diagnostic testing – Fault detection
Patent
1996-06-28
1998-03-03
Nguyen, Chau
Multiplex communications
Diagnostic testing
Fault detection
370506, 370522, H04J 312
Patent
active
057243425
DESCRIPTION:
BRIEF SUMMARY
This application claims benefit of international application PCT/F194/00506 filed Nov. 10, 1994.
BACKGROUND OF THE INVENTION
The invention relates to a method for receiving a signal used in a synchronous digital telecommunications system.
The current digital transmission network is plesiochronous, that is, 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 the higher-level signal has to be demultiplexed through each intermediate level down to the 2 Mbit/s level to extract the 2-Mbit/s signal. For this reason, especially the construction of branch connections requiring several multiplexers and demultiplexers has been expensive. Another disadvantage of the plesiochronous transmission network is that equipments from two different manufacturers are not usually compatible.
The above mentioned drawbacks, among other things, have led to the introduction of the new synchronous digital hierarchy SDH specified e.g. in the 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 with a single clock. However, if some of the nodes would lose connection to the common clock, this would result in problems in connections between the nodes. It is also necessary that the phase of the frame can be detected easily at the reception. On account of the above-mentioned factors, the SDH telecommunication utilizes a pointer, i.e. a number that indicates the phase of payload within a frame. The pointer thus points that byte in the STM frame from which the payload begins.
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 the column. Rows 1-3 and rows 5-9 of the N.times.9 first columns comprise a section overhead SOH, and the row 4 comprises an AU pointer. The rest of the frame structure is formed a section having the length of N=261 columns and containing the payload section of the STM-N frame. The row of the STM-1 frame is thus 270 bytes in length, as described above. The payload section comprises one or more administration units AU.
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 up. This requires negative justification: one byte is transferred from the received virtual container into the overhead section of the frame to be transmitted while the pointer value is decreased by one. If the rate of the received virtual container is lower than the clock rate of the node, the data buffer tends to be emptied, which calls for positive justification: a stuff byte is added to the received virtual container and the pointer value is incremented by one.
The frame structures described briefly above and the SDH system itself are described more closely e.g. in Finnish Patent Application 922 567 and in the publications mentioned in the patent application, which are referred to for a more detailed description.
The above-mentioned pointer mechanism allows flexible phase shift of different units within the STM fram
REFERENCES:
patent: 5210762 (1993-05-01), Weeber et al.
patent: 5455832 (1995-10-01), Bowmaster
patent: 5471477 (1995-11-01), Dries et al.
patent: 5539750 (1996-07-01), Kivi-Mannila et al.
Johansson Sixten
Oksanen Toni
Sihvola Tuomo
Nguyen Chau
Nokia Telecommunications Oy
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