FIELD OF THE INVENTION The invention refers to a "Digital Subscriber Line" (DSL) or "Asymmetric Digital Subscriber Line" (ADSL) system, fit to transport digital data together with analogue telephony signals by a conventional wire pair between a subscriber and a local exchange.

BACKGROUND OF THE INVENTION For e.g. "always on" access to the Internet these days, (A)DSL connections are getting more and more popular. For combining and/or separation of the ADSL (data) and (voice band) telephony signals, frequency splitters may be used at both ends of the local loop. The current ADSL system is suitable for a maximum downstream bit-rate of 8 Mbit/s, more advanced systems currently use 24 Mbit/s. To achieve that bit rate the system uses a high bandwidth on the local loop and high transmit levels. The high transmit levels and the used modulation method (Discrete Multi Tone, DMT), having very large amplitude peaks in the signal, require very good splitters (filters) to block these signals sufficiently in order to prevent that they disturb the telephony service and vice versa. Besides their costs, the splitters have to be installed, both at the user's and at the local exchange's side, and require additional cabling and/or wiring at the local exchange's "Main Distribution Frame" (MDF) and at the user's premises.

The term Main Distribution Frame (MDF) is used by the skilled person to designate a frame with mounted thereon connection blocks on which incoming cables with subscriber lines and other cables with connections to the local voice telephony exchange (LTE) are terminated in a telephone exchange building and between which exchangeable connection wiring can be added to connect the subscriber lines to the LTE. The Main Distribution Frame (MDF) takes up a considerable part of the space and cost of a telephone exchange location (local exchange or central office). In the early days (pre-ADSL) a telephone exchange location contained only the MDF and a local voice telephony exchange (LTE). The LTE provided switching facilities to the main telephone network and the MDF served to allow technicians to add selected connecting wiring between the LTE and the cables with subscriber lines.

Although the function of the MDF is rather trivial from a transmission perspective, the sheer volume of wiring and connections makes the MDF a costly and space consuming part of a telephone exchange location. At large exchange buildings the MDF is more than a hundred meters long and several meters high and comprises one or more frames filled with a large number of "MDF blocks" to which on one physical side cables with subscriber lines and cables to the LTE are permanently connected and on the other physical side terminals for exchangeable wiring between these two are provided.

With the emergence of (A)DSL further equipment has appeared at telephone exchange locations (local exchange or central office). So called "DSL Access Multiplexers" (DSLAMs) have been added, which connect on one side to a data network and on the other side a plurality of so-called DSLAM ports that interface to respective subscriber lines. The DSLAMs are arranged to (de- multiplex), intermix and/or separate voice data traffic. Each DSLAM port services one DSLAM user and comprises one ADSL modem designed e.g. for 8 Mbits/sec downstream bit-rate (i.e. to the subscriber) even if the relevant user only needs a relatively smallband (<8 Mbits/second) "always on" connection. Like the original LTE the DSLAMs are connected to the subscriber lines via the MDF. Cables from the DSLAMs are connected to the MDF blocks on the permanent wiring side and on the other side of the MDF blocks exchangeable wiring is provided to interconnect to MDF blocks that are connected to subscriber lines, so as to provide selected subscribers with (A)DSL service. Moreover, because the signals comprised of mixed voice signals and (A)DSL digital signals cannot be handled by conventional local voice telephony exchanges, the DSLAMs contain splitters to generate output signals from which all but the voice signals have been filtered out. Cables carrying these filtered signals (and in the opposite direction voice signals from the voice network for the subscriber line) are likewise connected to the MDF blocks on the permanent wiring side and on the exchangeable wiring side these MDF blocks are connected to other MDF blocks that are connected to the original LTE.

Due to the use of splitters, two extra connections at the local exchange's MDF will be used, viz. one connection for the DSL part and one for the voice part, which will require substantial additional space at the MDF. Moreover, the 8 Mbits capability results in a rather high energy consumption and a large physical size of the used ADSL modems.

SUMMARY OF THE INVENTION

It is an object of the invention to solve the problems discussed in the preceding. It is an alternative object of the invention to provide for a DSL system which is more efficient, for example in terms of MDF of use, in servicing "smallband DSL" users. One aspect is to connect a conductor that a carries a modulated carrier signal to a connection at the MDF for voice data band signals between the LTE and the user termination equipment, or to a connection on the user termination equipment side of the LTE. The digital data signals are converted to the modulated carrier signal in a way so that the modulated carrier has a frequency spectrum above the voice band and a bandwidth and/or format selected thus that no substantial mutual interference between the voice band signals and the converted data signals will occur in the local telephony exchange (LTE). In this way additional connections at the MDF back and forth to a splitter are avoided.

Another aspect is to convert, e.g. by means of a "DSLAM port multiplicator", the (broadband) bitstream from and/or to each (single) DSLAM port into a number (N) of smallband bitstreams and to exchange those bitstreams to and/or from N different smallband users by means of smallband DSL modems at the side of the central office and at the side of the relevant (smallband) users. DSLAM ports of the type that are used to service broadband (A)DSL subscribers may be used. Each small band bit streams is obtained by converting to a modulated carrier signal in a way so that the modulated carrier has a frequency spectrum above the voice band and a bandwidth and/or format selected thus that no substantial mutual interference between the voice band signals and the converted data signals will occur in the local telephony exchange (LTE). Conductors that carry the modulated carrier signal a connected to a connection at the MDF for voice data band signals between the LTE and the user termination equipment, or to a connection on the user termination equipment side of the LTE.

The smallband DSL modems to be used are known as such from e.g. WO99/29097. The digital subscriber line modem converts digital data signals into a format of constant envelope modulated data signals. The constant envelope modulation lessens unwanted intermodulation distortion due to non- linearities of the POTS telephone equipment. An improved splitterless DSL system is disclosed in EP04076738 in which the data signals are sent in the format of a carrier signal modulated by shift keying codes (e.g. PSK). At least when the user termination equipment or the local exchange are in a state to exchange said analog voice band signals, the data signals are sent in the format of shift keying codes having a restricted shift. The shift keying codes are limited to codes which result in a restricted offset of the modulated carrier signal, e.g. limited to shift keying codes having a restricted "Running Digital Sum" (RDS). This is realized by imposing limitations on the shifts and the combinations of successive shifts that may be used.

The smallband DSL modems to be used are compact and require low power consumption and have low heat excitation due to their low bitrate. Moreover the smallband DSL modems do not require splitters at the side of the central station and/or at the user's side. The absence of splitters saves space and wiring at the MDF. The smallband DSL modems etc. may be fit for installation at the MDF itself, saving wiring and/or wiring modifications and saving DSLAM space elsewhere in the local exchange building.

The DSLAM port multiplicator may multiply one single user ADSL DSLAM-port to a large number (e.g. 20-100) of smallband DSL ports, each equipped with a small and lean modem designed for splitterless operation. The "DSLAM port multiplicator" is small enough to be installed -e.g. by means of the standard MDF connection contact (e.g. Krone's LSA Plus ® contact system, see e.g. http://www.krone-asiapac.com/tech/lsa-plus/lsa-plus.asp) at the MDF, saving cabling, space, MDF wiring, cost of labour etc.

As the "DSLAM port multiplicator" is physically small and has low energy consumption, optionally, it could be installed in an existing cable cabinet outside the local exchange and be "fed" by an ordinary ADSL link from the local exchange. No installation of e.g. fiber to the cabinet is needed. Users outside the ADSL range could be served with an always on connection. Because of the length of the cable sections, higher bitrates could be possible (e.g. down: 80 - 160 kbit/s and up: 40 - 80 kbit/s), dependant of the location of the cabinet. In principle, it will be able to expand the ADSL area with some kilometres, using conventional (e.g. existing) twisted wire pairs.

In an embodiment the smallband DSL modems are installed on MDF blocks. The MDF blocks have a face that contains terminals for connecting exchangeable wiring. Preferably the smallband DSL modems are installed on an opposite face of the MDF block. When the smallband DSL modems are installed on the MDF blocks the number of conductors that needs to run from a DSLAM port to the MDF can be significantly reduced. Preferably, connections for these conductors are provided on the same face of the MDF block as the terminals for connecting exchangeable wiring. Thus the opposite face can be kept free of connections. This simplifies the MDF. In a further embodiment a hinged connection is provided between the MDF rack and the plane of the MDF block that has a face with terminals and a face with the smallband DSL modems. This is made possible because the terminal(s) for connecting to the DSLAM port are on the face with the other terminals. The hinged connection support easy access to the smallband DSL modems for service purposes.

FIGURES and EXEMPLARY EMBODIMENTS Figure 1 shows a prior-art ADSL system. Figure 2 shows an embodiment comprising a DSLAM port multiplicator within the central office. Figure 3 shows an embodiment comprising a DSLAM port multiplicator within a cabinet outside the central office. Figures 4a,b show an MDF block.

The prior system shown in figure 1 comprises a central office, including local telephony exchange circuitry LTE and a main distribution frame MDF. The LTE has a connection to a voice telephone network (e.g. a core network connection or multiplexed voice network connection). Furthermore the LTE has a plurality of analog voice connections (only one shown) coupled to the MDF.

The MDF is shown symbolically as a pair of solid lines, each with a series of smaller lines perpendicular to the solid line on one side of the solid line. The smaller lines symbolize the terminals for making exchangeable connections to the MDFs. The side of the solid line with perpendicular lines illustrates the side of the MDF blocks where exchangeable interconnections are made. The side of the solid line without perpendicular lines symbolizes the side of the MDF blocks where more permanent cable connections are made (to subscriber lines and to the LTE).

Different ones of the solid lines are drawn to symbolize MDF blocks that are connected to subscriber lines and to the LTE respectively. Figure 1 shows how exchangeable wiring is connected to the MDF to connect MDF terminals for the incoming subscriber lines to MDF terminals for a DSLAM and to connect other MDF terminals for the DSLAM to MDF terminals for the LTE.

A DSLAM (DSL Access Multiplexer) is provided in the ADSL system for data transmission. The DSLAM is coupled on one side to a data network and its other side contains a plurality of DSLAM ports. As far as relevant here, the DSLAM contains a multiplexer MP from the data network to the DSLAM ports via respective broadband digital select modems (BBDSLM) (only one shown). Typically, the multiplexer MP and the DSL modems function in two directions, both to pass data from the data network to the DSLAM ports and to pass data from the DSLAM ports to the data network. Although only transmission in one direction will be discussed at some points, it will be understood that this does not exclude transmission in the other direction.

Voice connections can be established from the LTE, via the MDF, to and/or from user telephone terminals, if desired via the user's private telephone exchange PTE (a PTE is shown but it will be understood that wire connections may be used instead). Data can be exchanged between the central office and users by means of one or more DSLAMs, comprising a multiplexer MP, connecting e.g. a 155 Mb/s data connection with a number of broadband DSL modems BBDSLM, each fit for a bitstream of e.g. 8 Mb/s.

Each DSLAM port, formed by such a BBDSLM, and servicing one user, is connected to the relevant user, via a signal splitter SSpI, fit to combine a voice signal and a data signal into a "voice + data" signal, as well as to split a "voice + data" signal into a voice signal and a data signal. The splitter's data port is connected with the analog data port of the BBDSLM, the splitter's voice port is connected with the relevant terminal at the LTE side of the MDF, and the splitter's "voice + data" port is connected with the terminal of the MDF connected to the relevant user's copper pair.

At the side of the user, the "voice + data" port is connected to user's copper pair, while the voice port is connected to the user's telephone sets or fax machines, answering machines etc, if desired via the user's home exchange PTE. The data port of the splitter is connected to the user's broadband DSL modem BBDSLM, which is connected to the user's computer or digital network. Voice signals may be exchanged from the LTE to the user's telephone equipment PTE via the LTE side of the MDF, the voice port of the SSpI, the "voice + data" port of the SSpI, the users' side of the MDF, the relevant twisted wire pair to the user's premises, the "voice + data" port of the user's SSpI, the voice port of the same SSpI and the user's telephone system PTE. The same path will be used —in inverse order- for voice signals from the user's equipment PTE to the LTE.

Data signals may be exchanged from the analog side of the BBDSLM assigned to the relevant user, via the data port of the SSpI, the "voice + data" port of the SSpI, the users' side of the MDF, the relevant twisted wire pair to the user's premises, the "voice + data" port of the user's SSpI, the data port of the same SSpI and the user's computer system. The same path will be used —in inverse order- for data signals from the user's computer equipment to the BBDSLM assigned to the same user.

Figure 2 shows an embodiment comprising DSLAM port multiplicators DPM (one shown) within the central office. The DPM forms a small band DSLAM ("small" and "broad" are used as relative terms to distinguish the bandwidth at the DSLAM ports to the DPM (or to broadband subscribers) from the bandwidth at the DPM ports to the user termination). Different from the prior-art system is that the signal splitters SSpI have disappeared (both at the side of the central office and at the user's side). Further each DSLAM port, formed by a BBDSLM (only one shown) is connected with a "DSLAM port multiplicator" DPM, expanding the relevant DSLAM port (read BBDSLM) to several (e.g. 20 to 100) smallband ports of e.g. 80 kB/s), each smallband servicing one smallband user. Each DPM may comprise a one (additional) broadband DSL modem for converting the analog (data) signals from the (e.g.) existing (installed base) BBDSLM to digital signals and vice versa, and a (digital) data multiplexer MP2, converting an e.g. 8 Mb/s datastream into a plurality of smalϊband (e.g. 80 kb/s) datastreams, to be modulated/demodulated in the relevant sinallband DSL modems SBDSLM.

The smallband modems SBDSLM, both at the central office side and at the user's side are of a type as disclosed in WO99/29097 or, preferably, as disclosed in EP04076738. Such smallband modems do not need the use of splitters thanks to the measures, within those modems against unwanted intermodulation distortion between data and voice domains.

Due the fact that no signals splitters are needed, the analog data port of the SBDSLM can be simply connected with the connection wires of the line between the local telephone exchange LTE and the user telephone equipment PTE. As in this new configuration the data port simply can be connected with the voice line, without any need for splitters, there also is no need to interrupt the voice line, as in prior art system, where the voice connection is interrupted inside the MDF, to be able to split/combine the voice and data stream.

In figure 2 the analog data ports of the smallband modems SBDSLM are connected with the user connection wires within the central office station, viz. at the user side of the MDF.

In figure 3 the analog data ports of the smallband modems SBDSLM are not connected with the user wires within the central station but at a location outside the central office, viz. in an (e.g. existing) street cabinet SCab. In this configuration the broadband DSL modem within the "DSLAM port multiplicator" DPM is connected to the relevant (installed base) BBDSLM within the central office by means of a twisted wire pair (BB data), "feeding" the relevant BBDSLM within the street cabinet. The advantage of the configuration of figure 3 is that the maximum total (cable) distance between the central office and the user will be expanded with the (maximum) cable length of the BB data section, e.g. by several kilometres.

Preferably, power supply current (e.g. DC current) is fed through the BB data connections simultaneously with the data to the DPM, to supply the DPM.

Figures 4a,b show a front and back view of a new MDF block 40 for use in an MDF. The exchangeable wiring side 42 of the new MDF block 40 comprises terminals 48 for connecting exchangeable wiring to MDF blocks for subscriber lines. A DPM 44 has been included on (or inside) the new MDF block 40. The connection 49 to DSLAM from (and to) the new MDF block 40 is provided on the same side 42 as the terminals 48 for exchangeable wiring to the MDF block. Preferably both data and power supply for the SBDSLM are provided through the same connection, but alternatively separate terminals may be used. This can be done without occupying an unacceptable amount of space on this side 42 for exchangeable wiring to other MDF blocks because only a single connection is needed for connecting to the DSLAM for a plurality of subscriber lines.

In this way the new MDF block 40 is designed so that no wiring needs to be connected to the other side of the new MDF block 40, i.e. the side that is conventionally used for permanent wiring. Instead this side is used to provide space for DPM 44 for exchanging the DPM 44. Preferably the MDF block is hingedly connected to an element 46 with which the MDF block is attached to the MDF rack (not shown). In one embodiment element 46 is part of the MDF rack (not shown), in which case the hinged connection is made when the MDF block is mounted on the rack (or on a module of the rack). In one embodiment element 46 is part of an MDF block assembly, together with a hinge and the assembly is attached to the rack. In both cases element 46 may be part of a U -shaped profile (not shown) that bends back to the edge of MDF block 40 opposite to the edge to which mounting element 46 is hingedly connected.

The hinged connection makes it possible to rotate the MDF block, so that access is gained the rear side and to DPM 44 when DPM 44 has to be exchanged. Element 46 is located near one edge of MDF block 40. Preferably another, similar element (not shown) is provided near the opposite edge, coupled to MDF block 40 by means of a screw or snap coupling instead of a hinge, e.g. to the arm of the U shaped profile opposite the arm that is connected to the hinge. This serves to keep the MDF block 40 in place in the normal operating position, connected to the MDF frame (not shown) when no access to DPM 44 is needed.

Preferably, the hinged connection is provided near an edge of MDF block 40 that contains a slot 43 in MDF block 40, the slot being provided for passing exchangeable wiring/or wiring from the DSLAM from the back of the MDF block to its front side 42 for connection to terminals 48, 49 on the front side. As an alternative to slot 43 a number of holes through MDF block 40 may be provided. Thus, this type of wiring does not obstruct access to DPM 44. Preferably the holes or slot 43 are as close as structurally possible to the hinge, so that rotation about the hinge is impeded as little as possible by wiring.