Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04J—MULTIPLEX COMMUNICATION
  • H04J3/00—Time-division multiplex systems
  • H04J3/16—Time-division multiplex systems in which the time allocation to individual channels within a transmission cycle is variable, e.g. to accommodate varying complexity of signals, to vary number of channels transmitted
  • H04J3/1605—Fixed allocated frame structures
  • H04J3/1611—Synchronous digital hierarchy [SDH] or SONET

Definitions

• the present invention relates to systems and equipment for telecommunications transmission and can be applied to transmission via any suitable medium, such as radio, optical fibre, coaxial cable.
• the invention is based on the creation of a signal structure which provides many of the benefits of SDH (synchronous digital hierarchy) , while permitting the use of the extensive infrastructure of the existing PDH (plesiochronous digital hierarchy) for signal distribution.
• SDH synchronous digital hierarchy
• PDH plesiochronous digital hierarchy
• SDH was created by CCITT standards operating a 140 Mbit/s rate with the aim of improving the flexibility and facilities which transmission networks can provide.
• traffic in SDH carries "overheads" which are additional to those of the PDH and can be used for a variety of management purposes.
• Higher traffic rates have been defined for SDH than for PDH, with the result that while SDH networks can carry PDH traffic, the reverse is not true.
• a telecommunications system arranged to operate at a rate of below 140 Mbit/s in a synchronous digital hierarchy overheads carry information in virtual containers, at least some of the virtual containers have a bit rate which is compatible with a plesiochronous digital hierarchy in the access part of the virtual container.
• the virtual container may have a PDH rate of the order of 8,448, 34,368, 6,000 or 45,000 kbit/s.
• the signal structure may be modified to include framing information similar to that of the PDH over and above framing information required to support the virtual container.
• the framing information may replace the conventional H4 frame in an SDH frame and the information may also include a multiframe indicator to mark one of up to 32 frames.
• the framing information may be verified as it passes through the network.
• Some of the non-essential information from the virtual container may be omitted to form a "compressed" virtual container.
• the non-essential information omitted from the virtual container is preferably from the POH.
• FIG. 1 shows an SDH frame which has a repetitive structure with a period of 125 microseconds - the same as for PCM (pulse code modulation) - and consists of nine equal length segments. At the gross transport rate of 155.52 Mbit/s for the base synchronous at the start of each segment.
• Figure 1 (b) shows how the SDH frame at STM-1 is conventionally represented, with the segments displayed to form 9 rows and 270 columns. Each byte is equivalent to 64 kbit/s and so each column of nine bytes is equivalent to 576 kbit/s.
• SDH signals at STM-N are created by byte interleaving N of the STM-1 signals together, together with some detailed processing for transmission purposes.
• the first nine columns of the frame contain a section overhead (SOH) for transport-support features such as framing, management operations channels and error monitoring, with the first segment containing the frame word for demultiplexer alignment.
• SOH section overhead
• the remaining columns can be assigned in many ways to carry lower bit-rate signals, such as 2,048 kbit/s ("2
• SUBSTITUTE SHEET Mbit/s each signal with its own overheads.
• payload capacity is allocated in an integral number of columns, inside which are management overheads associated with the particular signal and is represented as in Fig. 2 to which reference is now made.
• the first level of division is the administrative unit (AU) , of which there are two sizes and which is considered as the unit of provision of bandwidth in the main network. Its capacity may be used to carry a high bit-rate signal such as 45 Mbit/s or 140 Mbit/s (for the two sizes of AU respectively, AU-3 and AU-4) ; Figure 2 shows an AU-4 which occupies all of the payload capacity of an STM-1.
• An AU may be further divided to carry lower-rate signals, each within a tributary unit (TU) of which there are several sizes; for example, a TU-12 carries a single 2 Mbit/s signal and a TU-2 carries a N. American or Japanese 6 Mbit/s signal.
• a specific quantity of one or more TUs can be notionally combined into a tributary unit group or TUG, for planning and routing purposes. No overheads are attached to create this item, so its existence relies on network management tracking its path.
• TUG-2 should carry 3 x 2 Mbit/s in the form of 3 x TU-12.
• subdivisions of capacity may individually float between the payload areas of adjacent frames to allow for clock differences and wander as payloads traverse the network and are interchanged and multiplexed with others.
• each subdivision can readily be located by means of its own pointer embedded in the overheads.
• the pointer is used to find the floating part of the AU or TU, which is called a virtual container (VC) .
• the AU locates a higher order VC and the TU pointer locates a lower-order VC.
• an AU-3 contains a VC-3 plus a pointer and a TU-2 contains a VC-2 plus a pointer.
• a VC is the payload entity which travels across the network, being created and dismantled at or near the service termination point.
• PDH traffic signals are mapped into containers (E) of appropriate size for the bandwidth required, using single-bit justification to align the clock rates where necessary.
• Ql ⁇ TUTE SHEET overheads are then added for management purposes, creating a VC and these overheads are removed where the VC is dismantled and the original signal is restored.
• the STM-1 rate of 155.52 Mbit/s contains 63 of the CV-12, each of which contains a single 2 Mbit/s signal.
• the STM-1 signal Before transmission, the STM-1 signal has scrambling applied overall, apart from a few bytes of overhead which must be left unscrambled for subsequent demultiplexing.
• the SDH structure described above is based on the SONET structure which originated in the USA but is based on n x 155.52
• SDH traffic be split into its constituent VC components and an appropriate number of these should be assembled into a signal structure which shares key attributes with SDH but employs a bit rate which is compatible with PDH.
• Each of these bit rates has a tolerance defined by CCITT around the nominal bit rate quoted and each is assigned a notional "order" in the hierarchy, such that order two is 6 or 8 Mbit/s, order three is 34 or 45 Mbit/s.
• the present invention is suitable for structures at 8 and 34 Mbit/s to carry as many VCs as are practicable within those overall rates.
• 8 Mbit/s is to carry 4 x CV-11 or 3 x VC-12 or 1 x VC-2 or 1 x TUG-2.
• 34 Mbit/s is to carry 14 x VC-12 or appropriate numbers of VC of other size, or of TUG-2.
• a "compressed" VC may be carried. These are not as yet defined in standards but may be created by omitting some non- essential information from the VC (normally from the POH) , thereby reducing its bit rate in order to allow a larger number of containers (C) to be carried. In practice, the achievable increase is of value mostly for submarine cable applications at 140 Mbit/s and compressed VC which are created for this purpose
• SUBSTITUTE SHEET may with advantage be carried over existing PDH transmission systems below 140 Mbit/s, in order to reach conveniently the "frontier stations" where 140 Mbit/s submarine cables are terminated.
• the equipment may be configured to support either an SDH- like signal structure as outlined above, or a true PDH signal structure as defined by CCITT. This provides maximum flexibility in application and is well within the capability of modern ASIC technology.
• the signal structure may be modified to include framing information similar to that of the PDH, over and above framing information required to support VC.
• the latter requires essentially a frame with a 125 microsecond period, plus a multiframe indicator to mark one of up to 32 frames.
• the former requires a specific frame alignment word of around 12 bits with a period matching that of the level of the PDH hierarchy which is being simulated.
• 8 Mbit/s SDH-like signal structure can be created to carry 4 x VC-12, equivalent to 4 x 2 Mbit/s plus overheads and would have an aggregate bit rate in the region of 10 Mbit/s.
• a variant of the 34 Mbit/s system which carries 14 x VC-12 would alternatively carry 16 x VC-12 at about 40 Mbit/s.
• a further feature of the invention is the use of a signal label technique similar to that described in British Patent Specification No. 2196517 for management configuration of remote equipments, between different options for signal structure. Because the possible signal structures outlined above for one nominal PDH rate are so different and may have different exact bit rates (eg. 8 or 10 Mbit/s for the nominally 8 Mbit/s version) , it is not simple to command changes between them by utilising any overhead capacity in them. Instead, where CMI for example is used as the line code (typically for optical fibre links) , a change between the two possible forms of line code symbol for each of binary logic one or zero, can be used as the basis for a control signal to command a particular structure to be used. Other variants will become apparent to those skilled in the art.

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• Engineering & Computer Science (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Time-Division Multiplex Systems (AREA)

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• 1992
  • 1992-10-09 GB GB929221232A patent/GB9221232D0/en active Pending
• 1993
  • 1993-10-07 WO PCT/GB1993/002081 patent/WO1994009577A1/en active Application Filing
  • 1993-10-07 AU AU51174/93A patent/AU5117493A/en not_active Abandoned
  • 1993-10-09 CN CN 93114669 patent/CN1094877A/zh active Pending

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