WO1994009577A1

WO1994009577A1 - Improvements relating to telecommunication transmission systems

Info

Publication number: WO1994009577A1
Application number: PCT/GB1993/002081
Authority: WO
Inventors: Richard Munro Dorward; Stephen Patrick Ferguson
Original assignee: Gpt Limited
Priority date: 1992-10-09
Filing date: 1993-10-07
Publication date: 1994-04-28
Also published as: AU5117493A; CN1094877A; GB9221232D0

Abstract

In a telecommunications system arranged to operate at a rate of below 140 Mbit/s in a synchronous digital hierarchy in which 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.

Description

IMPROVEMENTS RELATING TO TELECOMMUNICATION TRANSMISSION SYSTEMS

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 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. In particular, 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.

According to the present invention in 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.

There is preferably an overhead capacity applied to the aggregate of all the virtual containers for network management purposes.

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.

SUBSTITUTE SHEET 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.

In order that the proposed invention can be understood, there follows an outline description of the key characteristics of SDH which are relevant to this invention and reference will be made to the two diagrammatic figures of the accompanying drawings in which Figure 1 is a time related framework for the transmission of data and Figure 2 is a conventional representation of an SDH frame with its associated segments identified.

Referring first to Figure 1 and particularly to Figure 1A, this 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. 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. For transporting PDH traffic signals, 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. For example, in Europe, the ETSI standard proposes that a TUG-2 should carry 3 x 2 Mbit/s in the form of 3 x TU-12.

At each level, 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. However, 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. For example, 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. Path

Ql ΓΠTUTE SHEET overheads (POH) 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. Other VC sizes exist, for example a VC-11 to carry the North American 1,544 kbit/s signal and a VC-2 which carries about 6.9 Mbit/s.

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

Mbit/s, in contrast with the SONET arrangement which is based on n x 51.84 Mbit/s (= n x 155.52 Mbit/s) .

3

In order to retain the advantages of SDH but allow transport via PDH, it is proposed that 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.

The need for such an arrangement has been addressed by CCITT and a possible SDH-like signal structure for use at 140 Mbit/s has been proposed for CCITT, although at this time it has not yet been defined in a fully functional form. The equipment which would perform the conversion between true SDH and SDH-like PDH at 140 Mbit/s is referred to as a "140 Mbit/s modem".

The requirement at 140 Mbit/s arises from the existence of a large amount of installed transmission equipment for this rate for trunk transmission purposes, such as on repeatered submarine cables. In many cases it is impractical to adapt existing systems to a higher bit rate because of the difficulty or cost, or both, of access and replacement of appropriate items.

The transport of a slightly reduced number of VC on existing 140 Mbit/s systems is an attractive alternative to the need for new transmission systems to cope with the higher 155 Mbit/s rate. In contrast, the present invention puts a very much reduced number of VC into the European lower order PDH bit rates of 8,448

SUBSTITUTE SHEET and 34,368 kbit/s ("8 Mbit/s" and "34 Mbit/s" respectively, primarily for applications in the customer access part of telecommunications networks. Similarly, it puts appropriate other quantities of VC into the North American PDH rates of 6,312 ("6 Mbit/s") and 44,736 kbit/s ("45 Mbit/s").

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.

There has been no previous recognition of the need for SDH- like signal structures at PDH rates below 140 Mbit/s. Attempts have been unsuccessful even to gain international standards acceptance for a true SDH structure at a lower rate than 155 Mbit/s, based on the USA SONET structure at 51.84 Mbit/s. In practice, there is a potential application for SDH-like systems at lower rates because of the growing interest in extending CV down to customer premises, particularly for business customers.

There is a considerable amount of low cost equipment available for PDH transmission at the low bit rates of 8 and 34 Mbit/s for example. Only a very limited amount of this equipment is as yet installed in the customer access part of telecommunications networks but for the relatively low traffic levels needed in the application, it is more economic than the use of SDH equipment.

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. In particular, 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. Similarly, 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.

Furthermore 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.

The use of such framing information is to allow the integrity of PDH signal to be verified as it traverses a network, by the use of existing commercial test equipment for PDH, attached to signal monitor points in PDH equipments along the path. The requirement for such a feature in SDH-like signals at rates below 140 Mbit/s is considerable because the path of these signals can be complex. True SDH signals do not need this facility because embedded overheads can be checked by each equipment along a path.

Where it is important that equivalent PDH traffic capacity be maintained via VC, but exact conformance with PDH rates is not essential, near-equivalents to the PDH bit rates can be supported. This situation can arise for example on optical fibre systems, where the approximate bit rates are set by the existing technology in a PDH system but the exact rate can be altered in a minor redesign or even simply by management command. This alteration by command is within the capability of modern ASIC technology, particularly by the use of digital clock extraction circuits which can be commanded to operate at different digital bit rates without the need for components to be changed. Such

SUBSTITUTE SHEET command flexibility is specially valuable for remote equipments.

By way of example, while 8,448 kbit/s carries 4 x 2 Mbit/s in true PDH form, it can carry only 3 x 2 Mbit/s in SDH-like form because each VC-12 which contains a single 2 Mbit/s occupies about 2.3 Mbit/s of capacity. A variant of the 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. Similarly, 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.

In all of these proposed signal structures, there is an overhead capacity applied to the aggregate of all the CV. This is used in a similar way to the related overheads carried in true SDH, for such purposes as network management communications, engineer order wires and signal identity.

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.

.ST.TUTE SHE-

Claims

1. A telecommunications system arranged to operate at a rate of below 140 Mbit/s in a synchronous digital hierarchy in which overheads carry information in virtual containers characterised in that 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.

2. A telecommunications system as claimed in Claim 1 characterised in that the virtual container has a PDH rate of the order of 8.448, 34,368, 6,000 or 45,000 kbit/s.

3. A telecommunications system as claimed in Claim 1 or Claim 2 characterised in that there is an overhead capacity applied to the aggregate of all the virtual containers for network management purposes.

4. A telecommunications system as claimed in any preceding claim in which the signal structure is modified to include framing information similar to that of the PDH over and above framing information required to support the virtual container.

5. A telecommunications system as claimed in Claim 3 or Claim 4 characterised in that the framing information replaces the conventional H4 byte in the SDH frame.

6. A telecommunications system as claimed in Claims 3, 4 or 5 characterised in that the framing information includes a multiframe indicator to mark one of up to 32 frames.

7. A telecommunications system as claimed in Claim 4 characterised in that the framing information is verified as it passes through the network.

SUBSTsT T^ SHE T

8. A telecommunications system as claimed in any preceding claim in which some of the non-essential information from the virtual container is omitted to form a "compressed" virtual container.

9. A telecommunications system as claimed in Claim 7 wherein the non-essential information omitted from the virtual container is from the POH.

^SUB^STITUTESHEET