A DATA TRANSMISSION SYSTEM FOR THE TRANSMISSION OF A LARGE NUMBER OF TELEPHONE CHANNELS AND A METHOD IN CONNECTION THEREWITH

Abstract: A data transmission system is used for the transmission of a large number of telephone channels between nodes in a transmission network built as a Synchronous Digital Hierarchy (SDH), where a pulse frame (e.g. STM-1) contains a large number of bytes, each of which can be used for the transmission of a telephone channel or of overhead signals for the administration of the system, and where signals from a Plesiochronous Digital Hierarchy (PDH) are introduced into the pulse frame of the Synchronous Digital Hierarchy, so that the signals from the Plesiochronous Digital Hierarchy when introduced do not occupy all the bytes in said pulse frame. At least some of the bytes of the pulse frame which are not occupied by said introduction, are used for the transmission of user-specified data signals. A method of transmitting user-specified data signals in such a transmission network comprises transmitting the user-specified data signals in one or more if the bytes in the pulse frame which are not occupied by the introduction of the PDH signals into the SDH.

Description: A data transmission system for the transmission of a large number of telephone channels and a method in con ¬ nection therewith The invention concerns a data transmission system for the transmission of user-specified data signals m a trans¬ mission network designed as a Synchronous Digital Hierar¬ chy (SDH) . A pulse frame (e.g. STM-1) contains a large number of bytes, each of which is used for transmitting a telephone channel or overhead signals for administering the system, and signals from a Plesiochronous Digital Hi¬ erarchy (PDH) are inserted into the pulse frame for the Synchronous Digital Hierarchy, so that the signals from the Plesiochronous Digital Hierarchy when inserted do not occupy all the bytes in said pulse frame. The invention moreover concerns a method of transmitting user-specified data signals in such a transmission net- work. The Synchronous Digital Hierarchy (SDH) is a very widely used transmission system for the transmission of a large number of telephone channels between nodes in a transmis- sion network. The system permits transfer of asynchronous signals, such as e.g. signals from a Plesiochronous Digi¬ tal Hierarchy (PDH) of 140, 34 and 2 Mbits/s, said sig¬ nals being inserted or mapped into the SDH system. Stan¬ dards prescribe how to place the individual signals in the individual bytes in the pulse frame of the SDH sys¬ tem. Frequently, it is desirable to be able to transmit a user channel together the data proper (i.e. the telephone channels) . It may e.g. be a monitoring channel for the equipment which is connected to a 2 Mbits/s section, it being desirable to transmit the channel together with the associated 2 Mbits/s signals. So far, this has just been possible by using overhead bytes, as e.g. the overhead bytes called D1-D12 in the SDH system may be used in cer- tain situations for transferring user channels. However, these can just be used for transferring data between two adjacent elements in the network, as each element decides for itself what it wants to use them for, and in certain situations they are reserved in advance for other pur- poses. It is thus not possible to use these bytes when other network elements are passed en route. Furthermore, the capacity of these bytes is rather limited, so that only small amounts of data can be transferred in this way. An example of this use of overhead bytes is known from WO 94/22249. However, when the PDH signals are inserted into the pulse frame of the SDH system, they will not use all the bytes of the pulse frame because of the absent synchronization of the two systems. Thus, there is a plurality of empty bytes which are merely used as a filler between the bytes which contain PDH signals. It is known from US 4 964 122 to use such bytes to trans- fer parity information, but this is information that only relates to the actual transmission of the data proper, and the bytes concerned are not accessible to a user. The object of the invention is to provide a data trans- mission system where one or more user channels may be transmitted together with the telephone channels in the SDH system without otherwise affecting the system or its transmission capacity, and without the information being lost in network elements which are passed en route. This is achieved according to the invention m that at least some of the bytes in the pulse frame which are not occupied by said insertion, are used for the transmission of user-specified data signals. In case of inserting e.g. PDH signals at 2 Mbits/s into a virtual container VC-12 in the Synchronous Digital Hier¬ archy, the user channel together with the telephone chan¬ nels m the VC-12 may also be connected through equipment which is not adapted to transfer the user channel, if only the VC-12 is not "opened" en route. The function may be used e.g. for ensuring that a monitoring channel for equipment connected to a 2Mbιts/s section accompanies the associated telephone channels m the network. Further, this user channel/monitoring channel is easily accessed at the location (s) where the telephone channels are taken out of the system. As stated in claim 2, it may be expedient in this situa- tion to place the user channels in one or more of the bytes which are called fill bytes or stuffing bytes (R) m this insertion. A particularly expedient embodiment of the invention is obtained when, as stated in claim 3, the user channels are placed m byte No. 34 and/or byte No. 104 in the VC- 12 container, it being possible to transmit e.g. up to 7 user channels with 24 kbits/s, as stated in claim 4. As mentioned, the invention also concerns a method of transmitting user-specified data signals in such a trans¬ mission network. When the user-specified data signals are transmitted in one or more of the bytes in the pulse frame which are not occupied by said insertion, the above-mentioned advantages are obtained. Claims 6-8 define expedient embodiments of the method. The invention will be described more fully below with reference to the drawing, in which fig. 1 shows how an STM-1 frame in an SDH system is de¬ signed, fig. 2 shows how 3 TUG's are multiplexed into a VC-4, fig. 3 shows how the TU-12's and the TUG-2's are multi¬ plexed into a TUG-3, fig. 4 shows the linkage of VC-12's from 4 successive VC- 4's, and fig. 5 shows the structure of fig. 4 in greater detail, A Synchronous Digital Hierarchy (SDH) is a digital trans¬ mission system which is used e.g. in connection with the transfer of a large number of telephone channels between nodes in a telecommunications network. Like many other signals transmitted in a telecommunica¬ tions network, SDH signals are a serial flow of logic 1's and O's that may be divided into a sequence of bytes hav¬ ing 8 bits each. The signals are structured so that the transmitted bit flow may be subdivided into a plurality of channels for different applications. The basic struc¬ ture of an SDH signal is a so-called Synchronous Trans- port Module at level 1 (STM-1), which is shown in fig. 1, from which it appears that the STM-1 signal may be illus¬ trated as a frame 1 having 9 rows and 270 bytes in each row. The signals are transmitted one row at a time with the uppermost row first, and each row is transmitted from the left to the right. Each byte is transmitted with the most significant bit first. As appears from the figure, the first 9 bytes 2 in each row are used by the SDH system itself even for overhead 4, 6 and pointers 5, respectively. The remaining 261 bytes 3 in each row constitute the transport capacity of the SDH system, but with part thereof being used also for overhead. The STM-1 frame is transmitted 8000 times per second, corresponding to a duration of 125 μs of each frame, and since each frame contains 9 rows each having 270 bytes of 8 bits each, the data rate is thus 155.520 Mbits/s. The 125 μs correspond to the sampling time in a digital telephone channel. A telephone channel is digit ¬ ized with 8 bits, and this means that each byte in an STM-1 signal may be a telephone channel. The transport capacity of the 9 rows of 261 bytes each constitutes a so-called virtual container designated VC- 4. Frequently, the SDH system is used for transporting e.g. PDH signals, and in that case a VC-4 may e.g. con- tain a PDH channel of 140 Mbits/s, or it may be subdi ¬ vided into a plurality of smaller virtual containers. It may e.g. contain 3 VC-3 each having a PDH channel of 34 Mbits/s or 63 VC-12 each having a PDH channel of 2 Mbits/s. An insertion structure, a so-called mapping, is defined for each of these signals, indicating how the signal is to fill the allocated location in the frame. Fig. 2 thus shows how 3 VC-3 containers may be mapped into a VC-4. This is done by subdividing VC-4 into 3 units called TUG-3, each of which may contain a VC-3. As will be seen, the first three columns are used for over¬ head and stuffing bytes, while the 3 TUG-3 units are mul¬ tiplexed into the remaining columns. If 2 Mbits/s channels are to be transferred, each TUG-3, instead of a VC-3, contains 7 TUG-2 units, each of which is in turn divided into 3 TU-12 units. It appears from fig. 3 how the TU-12's and the TUG-2's are multiplexed into TUG-3. It also appears that each TU-12 consists of 4 columns of 9 bytes each in each SDH frame, i.e. a total of 36 bytes for each 125 μsec. Thus, a VC-12 might be included in principle m each TU- 12; but to utilize the space better for overhead informa¬ tion, the TU-12's (i.e. 36 bytes) are linked in 4 succes- sive VC-4 containers in practice. Hereby, each byte in¬ tended for overhead may be used for various items of overhead information, but, then, each of these is trans¬ ferred only m every fourth VC-4. This linkage is shown This structure is shown more fully in fig. 5. I repre¬ sents the bits which are used for the information proper, i.e. the telephone channels. R represents bits which are introduced to make the bit number come right when mapping the 2 Mbits/s channel into the VC-12. These bits are called stuffing bits. If a whole byte exclusively con¬ sists of stuffing bits, the byte might be called a stuff¬ ing byte. The other designations are various items of overhead information. According to the invention, some of the bytes exclusively consisting of stuffing bits are used for transmitting user channels. These may e.g. be the bytes which are des¬ ignated by 10 and 11 in fig. 5. In practice, 6 bits are used in each of the two bytes, the remaining two bits in each byte being used for parity comparison. It is hereby possible to transfer 7 user channels with 24 kbits/s. One of the advantages of this system is that, when trans- mitting in the SDH system, the channels are packed to¬ gether with the other data, which means that the channels will be fed with certainty to the point where the VC-12 is unpacked, and this will usually also be the point where the information is to be used. Although a preferred embodiment of the present invention has been described and illustrated, the invention is not retricted to this, but may be embodied in other ways within the scope of the subject-matter defined in the following claims.

Claims: P a t e n t C l a i m s 1. A data transmission system for the transmission of a large number of telephone channels between nodes in a transmission network designed as a Synchronous Digital Hierarchy (SDH), where a pulse frame (e.g. STM-1) con¬ tains a large number of bytes, each of which may be used for transmitting a telephone channel or overhead signals for administering the system, and where signals from a Plesiochronous Digital Hierarchy (PDH) are inserted into the pulse frame for the Synchronous Digital Hierarchy, so that the signals from the Plesiochronous Digital Hierar¬ chy when inserted do not occupy all the bytes in said pulse frame, c h a r a c t e r i z e d in that at least some of the bytes in the pulse frame which are not occu¬ pied by said insertion are used for transmitting user- specified data signals. 2. A data transmission system according to claim 1, where the signals from the Plesiochronous Digital Hierar¬ chy comprise signals at 2 Mbits/s which are inserted into a virtual container VC-12 in the Synchronous Digital Hi¬ erarchy, c h a r a c t e r i z e d in that the user- specified signals are placed in one or more of the bytes which are called fill bytes or stuffing bytes (R) in this insertion. 3. A data transmission system according to claim 2, c h a r a c t e r i z e d in that the bytes in which the user-specified signals may be placed are byte No. 34 and byte No. 104 in the virtual container VC-12. 4. A data transmission system according to claims 1-3, c h a r a c t e r i z e d in that up to 7 user channels with 24 kbits/s may be transmitted in said bytes. 5. A method of transmitting user-specified data signals in a transmission network designed as a Synchronous Digi ¬ tal Hierarchy (SDH) where a pulse frame (e.g. STM-1) con ¬ tains a large number of bytes, each of which may be used for transmitting a telephone channel or overhead signals for administering the system, and where signals from a Plesiochronous Digital Hierarchy (PDH) are inserted into the pulse frame for the Synchronous Digital Hierarchy so that the signals from the Plesiochronous Digital Hierar- chy when inserted do not occupy all the bytes in said pulse frame, c h a r a c t e r i z e d in that the user- specified data signals are transmitted in one or more of the bytes in the pulse frame which are not occupied by said insertion. 6. A method according to claim 5, where the signals from the Plesiochronous Digital Hierarchy comprise signals at 2 Mbits/s which are inserted into a virtual container VC- 12 in the Synchronous Digital Hierarchy, c h a r a c - t e r i z e d in that the user-specified signals are transmitted in one or more of the bytes which are called fill bytes or stuffing bytes (R) in this insertion. 7. A method according to claim 6, c h a r a c t e r - i z e d in that the user-specified signals are transmit¬ ted in byte No. 34 and/or byte No. 104 in the virtual container VC-12. 8. A method according to claims 5-7, c h a r a c t e r - i z e d in that the user-specified signals are transmit¬ ted as up to 7 user channels with 24 kbits/s.

Inventors: NIELSEN ANDERS BOEJE, NIELSEN, ANDERS, BOEJE
Applicants: DSC COMMUNICATIONS AS, NIELSEN ANDERS BOEJE, DSC COMMUNICATIONS A/S, NIELSEN, ANDERS, BOEJE
Application year: 1996
Main classification: H04J3
IPC codes: H04J3