WO2009030875A1

WO2009030875A1 - Xdsl bypass test facility

Info

Publication number: WO2009030875A1
Application number: PCT/GB2008/002820
Authority: WO
Inventors: David Mather; Ian Miles Standish; David Thomas Lambert
Original assignee: Tyco Electronics Uk Ltd
Priority date: 2007-09-05
Filing date: 2008-08-20
Publication date: 2009-03-12
Also published as: GB0717243D0; GB2452506B; GB2452506A

Abstract

A device and method are described for providing test access for checking a transmission line carrying at least a plurality of digital channels, the transmission line (8, 18) having a plurality of twisted pairs, the device being for use with a test head (20) for determining a quality of the transmission line. A connection matrix (30) is provided for switching one of a plurality of the twisted pairs to a test bus (32), and a high pass filter (24) is coupled for maintenance of the digital channels, wherein the high pass filter is located in the connection matrix.

Description

XDSL BYPASS TEST FACILITY

TECHNICAL FIELD

The present invention relates to testing cables carrying telecommunications services in a first higher frequency band such as xDSL (e.g. ADSL). The cables may also carry other services in other frequency bands, e.g. in a lower frequency band such as telephone services. More particularly, the present invention relates to a method and device for providing access for testing a transmission line carrying a plurality of digital transmissions. The transmission line may also cany an analog signal. More particularly, the present invention relates to a method and system for gaining metal access to conductors of cables to allow measuring disturbances in transmissions via these conductors connected to subscriber equipment and a high speed xDSL modem. A telephone network may also be connected to the cables.

TECHNICAL BACKGROUND xDSL (such as ADSL) allows high speed data transmission on legacy twisted pair telephone lines by using a different frequency band than for telephone services and by using signal processing techniques such as modulation of a carrier or carriers. Various xDSL technologies are known, e.g. HDSL, SDSL, ADSL, VDSL. It is also known to provide both telephone services and data services by an xDSL connection using digital transmissions, e.g. to provide telephone services by Voice over IP (VoIP) over xDSL.

Plain Old Telephone Systems (POTS) equipment in the telecommunications industry requires a test facility that can locate faults in the local loop through the Central Office (CO) equipment. Test facilities to locate faults in the local loop that affect high data rate systems such as xDSL typically need to be located between the CO equipment and the local loop. As shown in Figure 1 an xDSL link such as an ADSL link can comprise an ADSL modem 2 for supply digital signals to a digital telephone 3 by VoIP for example and/or to a network with a computer 5. The system may include an ADSL filter 6 called a "splitter" for separating analog transmissions from the high speed transmissions in a higher frequency band. The digital ADSL transmissions re transferred to the ADSL modem 2 and the analog transmission go to a subscriber telephone 4 and/or to a telephone modem 7 at the subscriber premises. The telephone modem 7 and the ADSL modem 2 both require channel equalisation. This is usually done at switch on of the modem.

The splitter 6 is connected via a length of twisted pair cable 8 to a device with test access, i.e. for allowing checking of the transmissions on the cable 8 which is carrying a plurality of digital and/or analogue channels. In some systems this device can include a TAM switch (Test Access Matrix Switch) generally indicated by the reference number 12. Conventionally, a test access matrix (TAM) is a device to test problems in a DSL and POTS system. The TAM typically switches one of a plurality of two wire telecommunications circuits, e.g. twisted pairs to a two or four wire test bus. It can usually do a look in and a look out test to determine problems in the system. The tests can usually be done remotely and optionally non-intrusively.

The TAM 12 is connected on one side to a DSLAM or MSAN 16. This is connected to a high speed network via a link 19 and to the telephone network via link 17. The TAM 12 includes a master control unit 10. The DSLAM 16 is a Digital Subscriber Line Access Multiplexer for multiplexing transmissions exchanged through a high speed network such as a fiberoptic network (not shown). An MSAN is a MultiService Access Node. The master control unit 10 is connected to suitable test device such as an LDU (Loop diagnostic unit) 14 if the POTS service is to be tested or to a test head designed for testing the ADSL service, e.g. a QT2000. The master control unit 10 connects different lines to different test heads.

It is known that the twisted pair cable must be tested without interrupting the broadband ADSL link and the narrow band telephone link. In this way it is possible to maintain a telephone communication while a check is being carried out on the broadband link. Similarly it is possible to maintain a broadband ADSL link and to check telephone voice signals. A major problem in the prior art is due to the fact that when the communication line is being checked, known testing devices do not always function correctly or that the telephone link and/or broadband link is destabilised by the connection of the test equipment.

US 2005231882 discloses a conventional device for controlling a transmission line carrying a plurality of digital and/or analog channels, comprising a measuring block for evaluating performance, searching for faults and establishing the quality of the line and services transmitted via said line. The device also comprises a switching module for selectively linking the measuring block exclusively to the transmission channels that are to be controlled and for keeping the other channels of the transmission line active.

SUMMARY OF THE INVENTION

It is an object of the present invention provide a method and a device providing access for selectively checking in a lower frequency band, e.g. DC or analog voice telephone transmissions signals or for checking in a different higher frequency band such as broadband xDSL transmissions operating in a first higher frequency, without disturbing operation of at least one telecommunications service in the communication system. Generally, there is an intrusive connection to the twisted pairs as far as one of the services is concerned, but non-intrusive for the other service which is bypassed through to the subscriber. The test head can be a high impedance test head for non- intrusive testing.

An advantage of the present invention is that network devices that need channel equalisation such as xDSL modems do not have to resynchronize when the test access for testing in a low frequency band, e.g. DC tests or tests for a voice frequency path, is switched in. This especially important when real-time streaming services (e.g. video) are transmitted over the DSL channel.

The invention relates to a test access device for providing test access to thereby allow checking of a transmission line transporting a plurality of digital channels alone and/or analogue transmission channels. The test access device is for use with a test head designed to test the quality of the line and/or transmitted services. The test head can be integrated in the test access device or can be separate. The test access device can comprise a switching module capable of selectively connecting a test bus for the test head only to the transmission channels to be checked, the switching module being adapted to keep other channels in the transmission line active, e.g. by using appropriate high pass, band pass or low pass filters.

A particular advantage of the present invention is that testing can be carried out without risking an interruption of the xDSL service (e.g. without a need for resynchronisation of the DSL link). The test access device according to the invention comprises the switching module implemented as a connection matrix for selectively gaining metal access to twisted pairs of a multipair telecommunications cable. The selective access is preferably controlled remotely, e.g. from a central office.

In one aspect of the invention a high pass filter is located in the connection matrix. Optionally, at least a part of a low pass filter may also be located in the connection matrix. The connection matrix may comprise first selection devices for selectively gaining metal access to a twisted pair of a multipair telecommunications cable and for providing the signals on these twisted pairs to the high pass filter. The connection matrix may also comprise first selection devices for selectively gaining metal access to a this twisted pair of a multi-pair telecommunications cable and for providing the signals on this twisted pair to one of a plurality of high pass filters via first lines and second selection devices for selecting which of the outputs of the first lines is to be supplied to a test head. The connection matrix may also comprise first selection devices for selectively gaining metal access to a twisted pair of a multi-pair telecommunications cable and for providing the signals on this twisted pair to one of a plurality of high pass filters via first lines, the first lines being connected in series to at least part of a low pass filter, and second selection devices for selecting which of the outputs of the at least parts of the low pass filters is to be supplied to a test head. An advantage of this arrangement is that the location of the high pass filter in the connection matrix defines limits for the distance between the location of the AC path in the higher frequency band and the location where the line under test is interrupted. Also this arrangement can have the advantage that it defines the limits of the distance between the AC path and the low pass filter (e.g. inductor) path for test head isolation. In accordance with another aspect of the present invention an impedance calibration is provided for the high pass filter. This can be of importance for the proper functioning of the test head.

Another aspect of the present invention describes limits on the parasitic shunt capacitance of the low pass filter (e.g. inductor) that is used in the low pass filter (e.g. inductor) path. Yet another aspect of the invention describes limits on the value of the capacitor in the AC modem path. Still another aspect of the invention is the implementation of two inductor paths in order to offer simultaneous look-in and lookout test access for the voice frequency test head.

The device, according to the invention, check at low frequencies without disturbing broadband channels operating in a higher frequency band. The low frequency testing may be at DC or in a narrow band channel that may be analog reserved for voice, fax or data transmission, or an ISDN type digital transmission or any other service transported within the frequency band less than 20 kHz. This same device or a separate dedicated device can be configured to check broadband channels without interrupting the telephone channel. It can also be configured check the narrow band channel without disturbing broadband channels.

The invention also relates to a method for checking a transmission line carrying a plurality of digital transmission channels and/or analogue transmission channels including selectively connecting the test head only for low frequency testing, e.g. DC or testing of a low frequency narrow band transmission channel while maintaining the broad band connection. When testing DC or the narrow band connection, a link to the subscriber, e.g. through the narrow band transmission channel, can be maintained and/or a link to the central office may be maintained thus allowing both sides to be tested independently. Similarly, the method includes selectively connecting a test head only to the broadband transmission channels. A narrow band connection to the subscriber can be maintained during this test. When testing the broadband, e.g. xDSL connection, a link to the subscriber via the broadband transmission channel can be maintained or a link to the high speed network can be maintained thus allowing both sides to be tested independently.

Advantages and embodiments of the invention are described in the following description with reference to the attached figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically functional elements of an ADSL and telephone communications network with which the present invention can be applied.

FIGs. 2a, b and c show schematically embodiments of a device according to the present invention. FIG. 3 shows schematically a further embodiment of a device according to the present invention.

FIG. 4 shows schematically yet another embodiment of a low pass filter arrangement for use with a device according to the present invention.

FIG. 5 shows an impedance calibration circuit for the high pass filter in accordance with another embodiment of the present invention

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described with respect to particular embodiments and with reference to certain drawings but the invention is not limited thereto but only by the claims. The drawings described are only schematic and are non-limiting.

Furthermore, the terms first, second, third and the like in the description and in the claims, are used for distinguishing between similar elements and not necessarily for describing a sequential or chronological order. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

Moreover, the terms top, bottom, over, under and the like in the description and the claims are used for descriptive purposes and not necessarily for describing relative positions. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other orientations than described or illustrated herein.

It is to be noticed that the term "comprising", used in the claims, should not be interpreted as being restricted to the means listed thereafter; it does not exclude other elements or steps. It is thus to be interpreted as specifying the presence of the stated features, integers, steps or components as referred to, but does not preclude the presence or addition of one or more other features, integers, steps or components, or groups thereof. Thus, the scope of the expression "a device comprising means A and B" should not be limited to devices consisting only of components A and B. It means that with respect to the present invention, the only relevant components of the device are A and B.

Similarly, it is to be noticed that the term "coupled", also used in the claims, should not be interpreted as being restricted to direct connections only. Thus, the scope of the expression "a device A coupled to a device B" should not be limited to devices or systems wherein an output of device A is directly connected to an input of device B. It means that there exists a path between an output of A and an input of B which may be a path including other devices or means.

The present invention relates to providing metal access for checking of an in- service transmission line connecting a subscriber to any type of high speed network through a link operating in one frequency band, e.g. an xDSL link such as for provision of data and/or voice communications, e.g. VoIP. The subscriber may also be connected through the same transmission line to another service in another frequency band, e.g. to a telephone network. The following description relates to an illustrative application of the invention in which a transmission line connects a subscriber to a high speed network, e.g. TCP/IP based, e.g. through an ADSL link. As an example the transmission line can also carry services of a public switched telephone network. In the remainder of the description, identical numeric references will be used to denote similar elements. The present invention will be described with reference to a system like the one of

Figure 1 which illustrates an ADSL line but the present invention is not limited to ADSL nor is it limited to the specific architecture of Figure 1. In particular although the invention will be described with reference to a TAM switch 12, the present invention can be any device providing suitable test access. Hence when referring to the TAM switch 12, this should be interpreted widely as a test access matrix device that is able to provide the necessary access to each twisted pair of a multi-pair cable for test purposes whether this is by means of a matrix switch or otherwise.

The TAM 12 for allowing checking of the transmissions on the cable 8 can includes devices for selectively accessing one of the analog and xDSL transmissions while allowing the other service to be continued normally. By judicious use of an arrangement of high pass and low pass filters it is possible to route the xDSL transmissions through while allowing access for testing in a different frequency band, e.g. at lower frequencies, such as at DC or to allow access for testing the telephone transmissions. Also by judicious use of an arrangement of high pass and low pass filters it is possible to route lower frequency transmissions, e.g. the telephone transmissions through while allowing access for testing at the higher frequencies, e.g. of xDSL transmissions. The test access devices according to the present invention may be dedicated to one such arrangement and hence for providing one type of test. These test access devices may enable the high or low frequency transmission channel on the line to remain active. To achieve this the TAM 12 may include a test access device 15 according to the present invention for connection to a multi-pair cable 8 from the subscriber in a local loop (see Fig. 1), a back plane 13 and a connection board 11 for connection to a multipair cable 18 and to the MSAN or DSLAM 16 and for use with a test head 14. A schematic representation of such a test arrangement is shown in Figs. 2a to c.

In Fig. 2a the cable 8 is sectioned and the telephone transmissions are carried though a low pass filter 22 without interruption to a cable 18 (see Fig. 1 for cables 8 and 18). The xDSL transmissions which operate in a higher frequency band do not pass through the low pass filter 22 significantly. Two high pass filters 24 are used for connection to test head 20 for testing the xDSL channels and signals. In Fig. 2b high pass filter 24 maintains the xDSL transmissions between cables 8 and 18 and the test head 20 is connected to cable 8, 18 via low pass filters 22 to allow testing at a lower frequency, e.g. at DC or to test the voice channels and signals. This latter scheme will be called "DSL bypass". The devices shown in Figs 2a and b may be implemented as insertable boards or cards 15 (see Fig. 1), e.g. a PCB onto which are mounted the relevant components. Each PCB can typically accommodate tens or hundreds of connections to twisted pair cables, e.g. to 200. Hence system is scalable by adding more devices 12. Fig. 2 c shows a particular arrangement of the low and high pass filters of Fig. 2b in accordance with an embodiment of the present invention. Fig. 2c shows multi-pair cables 8, 18 connected to a connection matrix 30. The connection matrix 30 provides metal access selectively to each of the conductors of any of the twisted pairs in the multipair cable 8, 18. The connection matrix 30 switches one of a plurality of twisted pair to allow testing by the test head 20 via a two or four wire test bus 32. Hence the connection matrix 30 switches signals from one of a plurality of twisted pairs to a test bus 32. To achieve this aim the connection matrix 30 can be provided with a first selection device 27 which can selectively switch metal access to conductors of the twisted pair cables of cables 8, 18 and to selectively provide signals from these conductors to high pass filters 24 via lines 34. These high pass filters 24 maintain the xDSL connection between the cables 8 and 18 and therefore to the subscriber. The high pass filters 24 may be made of one or more capacitors, e.g. in a capacitor bank. The location of these high pass filters 24 in the connection matrix reduces the distance between the conductors of the cables 8, 18 and the high pass filters 24 as is explained below. At least a first part 25 of a low pass filter 22 is connected in series with the lines 34. The first part 25 of the low pass filter 22 is also located in the connection matrix 30. This first part 25 of the low pass filter 22 may be an inductor. The location of at least part 25 of the low pass filter 22 in the connection matrix 30 reduces the distance between the high pass filter 24 and the at least part 25 of the low pass filter 22 as is explained below.

The connection matrix 30 has a plurality of high pass filters 24 and a plurality of low pass filters 25. The connection matrix 30 can also include a second selection device 28 for selectively connecting the outputs of certain of the lines 34 to a test bus 32 or to a further part 26 of the low pass filters 22. An advantage of splitting the low pass filters 22 into a first part 25 and a second part 26 is that by using the second selection device 28 a reduced number of low pass filter components are necessary while still maintaining a part 25 of the low pass filter 22 close to the high pass filters 24. The selection devices 27 and 28 may be operated remotely e.g. via a controller

36. Hence the selection devices 27 and 28 may comprise remotely operated switching devices such as relays. The controller 36 may be connected to a suitable control centre such as a Central Office by any suitable means, e.g. a wireless and/or wired and/or fibreoptic network. The low and and/or high pass filters 22, 24, 25, 26 may be implemented as discrete components such as a capacitor bank and an inductor chain. These may be associated with suitable selection device (not shown) that allow remote switching in of capacitors and or inductors to change the values of the capacitors or inductors (see below for explanation of effect of changing component values in the filters). Remote change of the values of the capacitor bank or the inductor chain can be done via the controller 36. Hence the present invention includes programmable high or low pass filters.

The low and/or high pass filters 22, 24, 25, 26 may be implemented as a digital filter or digital filters that can be configured as a function of the checking operation being made on the line. The configuration can be done remotely via the controller 36. Such a digital filter may be programmed so as to maintain a narrow band link on the cable 8 and a broadband link between the DSLAM 16 and the subscriber. Alternately, such a filter can be programmed to selectively connect cable 8 for checking the narrow band link while maintaining the broadband link between the DSLAM or MSAN and the cable 8.

In the following the low and high pass filters 22, 24, 25, 26 will be described mainly with reference to lumped circuit filters made from discrete components which are not programmable but the present invention is not limited thereto. During practical implementation of such a test facility as can be used with the system of Fig. 1 some problems have been encountered. These problems are solved by the arrangement of Fig. 2c. One problem with conventional telephone and xDSL e.g. ADSL systems is that the modems often have only rudimentary capabilities to adapt to a varying channel. Although designs have been proposed that allow continuous adaptation to channel characteristics, i.e. channel equalisation as is known for wireless connections, in practice these are rarely used for fixed lines. Typically the modems will only equalise on switch on. During testing, the high frequency (xDSL) channel can remain available and active, but the modems lose synchronisation and need to be retrained to adapt to the characteristics of the new circuit arrangement. Although the modems (CO (Central

Office) and subscriber side) remain active and communicate with each other, subscribers will temporarily lose service. This has been found to be especially problematic when real time streaming services, such as digital TV, are being transmitted.

The present invention is based on experimental work on communication networks including a first xDSL service such as an ADSL service operating in a higher frequency band (e.g. ADSL 1 or ADSL 2) and a second telephone service operating in a lower frequency band. In particular the present invention relates to a method and apparatus to provide access for testing or monitoring at a low frequency band, e.g. DC or in the voice frequency band of a twisted pair cable whilst a service in another frequency band such as an xDSL signal (e.g. an ADSL signal) on the twisted pair remains uninterrupted and the system is not disturbed by this test.

A particular embodiment of the present invention hat can be in the form of a board or card 15, e.g. an insertable board or card, is shown schematically in Figure 3. The board 15 may be used in combination with other boards such as connection board 11 and the backplane 13 (Fig. 1) to provide a connection matrix 30 (Fig. 2c). This board 15 implements part of the schematics of Fig. 2c. The board 15 has a plurality of connections 39 for connecting to a conductor of an individual twisted pair of cable 8, 18 and first selection devices 27, e.g. relays, for selecting one of these and connecting this conductor of a selected twisted pair cable to a line 34. The connections 39 to the cable 18 may be brought to the board 15 via the board 11, whereas the connections 39 to the cable 8 may be directly onto board 15. Other arrangements are included within the scope of the present invention. For example, board 15 may slot into a connector on board 11 for example or both boards 11 and 15 may slot into the backplane 13.

For example starting from the bottom of Fig. 3, the first relay 270 on the left hand side of Fig. 3 connects to a twisted pair from cable 8 and the relay 271 immediately above that connects to a twisted pair from cable 18 (see Fig. 1 and 2c). Lines 34 are connected to relays 270, 271. A high pass filter 24 is connected between the lines 34 to maintain a high frequency service such as an xDSL service. Hence, one line 34 from each of relays 270 and 271 are selectively connected to a single high pass filter 24 to thereby bypass the xDSL service across the high pass filter 24 from cable 8 to cable 18. Relays 270 to 273 have connections 39 to two twisted pairs in each of the cables 8 and 18. The conductors from any one of these twisted pairs can be brought to either of the high pass filters 240 or 241 by selective use of relays 274 and 275 in conjunction with relays 270-273. To maintain the normal functioning of all other twisted pairs not being tested the rest position of the connection matrix 30 switches all services through directly.

The high pass filter 24 can be one or more capacitors, e.g. in a capacitor bank. The capacitors may be programmable. The high pass filter 24 allows the ADSL transmissions to pass through to the subscriber but blocks the telephone services or any low frequency, e.g. DC test signals or test signals below 20 KHz. These are accessed via low pass filters 25 for delivery to a test head. The low pass filter 25 may be the first part of a low pass filter 22. The low pass filter 25 is preferably located close to the high pass filter 24 in accordance with an embodiment of the present invention, e.g. located on board 15 in the connection matrix 30. The low pass filter 25 may be part of a programmable low pass filter 22. Further selection devices 28 allow selective connection of the outputs of two of lines 34 or the outputs of two of the low pass filters 25 to a test bus 32 and hence make these outputs available to a test head 20 (see Fig. 4). The test bus outputs of board 15 of Fig. 3 are (from top to bottom) DSLLINA and

DSLLINB from relay 280, and DSLLINE and DSLLINF from relay 281. The four test lines allow both a look-in and a look-out test at the same time.

The first selecting devices 27 are arranged to be able to connect signals from one of the conductors of a twisted pair to one of a plurality of lines 34 connected to different high pass filters 24, e.g. 240 or 241. For example, the first selecting devices 27 can be arranged to be able to connect signals from one of the conductors of a twisted pair to one of two lines 34 connected to two different high pass filters 24, e.g. 240 or 241. The purpose of being able to select to one of a plurality of lines 34 connected to different high pass filters 24 is to shorten the routing on the board 15 and is explained in more detail below. It reduces the length of connection wiring on the board 15 between the twisted pair and the high pass filter 24. The board 15 also has two low pass filters 22 for each high pass filter 24 resulting in four low pass filters connectable to four relays 270, 271, 272, 273. The purpose of this arrangement is also explained below. First experiment

In the first experiment it was investigated what influence the positioning of the high pass and low pass filters 24, 22, respectively had on the system. As shown in Figure 2b, the high pass and low pass filters 24, 22 respectively are located at a position between the twisted pairs of cables 8, 18 and the test head 20. It has been discovered that the high pass filter 24 has to be close to the connection to the twisted pairs of cables 8, 18, e.g. by providing the high pass filter 24 in the connection matrix 30 as shown in Fig. 2c. In particular, it has been found that varying this distance varies the maximum bitrate of an ADSL2+ connection that can be operated in the "DSL Bypass" test mode without the need for retraining of the ADSL modems.

Test results of first experiment

From the test bus tree connections on a TAM daughter board, track length was added and an assessment made of the impact on performance. The assessment was carried out using the following standard arrangement (see Fig. 1) comprising:

A Spirent cable simulator with lkm of cable, -49.2dBm ADSL-A noise on the subscriber side and -41.2dBm ITU-Euro-K noise on the TAM side of cable 8. A Zyxel DSLAM was configured with FAST setup and data rates as reported. A Netgear DSL modem was used. A TAM master unit and sub-rack with DSL bypass board 11718-2 and a Teradyne LDU was used.

The modem was trained up at the maximum possible data rate which was assessed in 0.5Mbit/s increments for downstream data rate and lOOkbit/s increments for upstream data rate. For example with a conventional DSL bypass circuit, if the modem was trained up with 19.5Mbit/s downstream rate and 1000kbit/s upstream rate and a

DSL bypass test implemented the modem lost synchronisation and had to retrain. With lkm of cable the maximum achievable upstream rate was 1000kbit/s.

To simulate additional path length, a PCB with 250mm lengths of nominally lOOohm differential pair tracking was used and connected to the DSL bypass circuit with lOOohm twisted pair cable.

Tests were carried out with two different PCBs and the results are shown below in tables 1 and 2. Table 1

DSL Bypass board 1

*The 0mm added path length is in addition to the normal additional path length introduced by routing the line to the test path. This normal additional path length is between 190mm and 250mm.

Table 2

DSL Bypass Board 2

The step change in upstream performance was checked by using the additional PCB tracks in a different order between DSL bypass boards. So on DSL bypass board 1 the 250mm length was track 1, the 500mm length was tracks 1 and 2 and the 750mm length was tracks 1, 2 and 3. On DSL Bypass board 2 the 250mm length was track 3 and the 500mm length was tracks 1 and 3.

The conclusion from these tests is that a length of less than 350 mm, less than 300 mm or more preferably 250mm or less for routing the transmission line and the high pass filter 24 is preferred for xDSL modems with downstream data rates above 10Mbit/s. For ADSL-I modems where data rates are lower an instance of the invention with additional path lengths up to Im can be utilised. In a particular embodiment of the present invention two high pass filters 24 are implemented in the connection matrix 30, i.e. on one printed circuit board as shown in Fig. 3 and a selection device 27 for selective connection to lines 34 with different high pass filters 24 in order improve routing and reduce the distance between the transmission line 8 and the high pass filter 24. The high pass filter 24 may also be located between first and second selection devices 27, 28 of the connection matrix 30.

Experiment 2

In a second aspect of the present invention it has been discovered that at least a part 25 of the low pass filter 22 needs to be close to the high pass filter 24.

Relevant is the physical distance between the high pass filters 22, e.g. capacitors and at least a part 25 of the low pass filters 24, e. g. inductors on the board 15 of Figure 3.

Test results for experiment 2 The impact of moving the initial low pass filter 25 (an inductor) on the performance was determined. To do this an additional path length was inserted at the axial inductors.

This keeps the path length to the high pass filters (e.g. capacitors) the same but extends the path to all of the low pass filters (e.g. inductors). The same standard setup was used as for the first experiment. The results are given in Tables 3 and 4.

Table 3

PCBl, measurements

The same result was obtained if the low pass filters (e.g. inductors) were shorted out. Table 4 PCBl

The conclusion is that the physical distance between the high pass filters 22, e.g. capacitors and at least a part 25 of the low pass filters 24, e.g. inductors on the board 15 of Figure 3, in one instance of the present invention is 25mm maximum, preferably 10mm maximum or 6mm maximum.

In a particular implementation as shown in Figure 3 having two high pass filters 24, the low pass filters 22, e.g. inductors, have also been duplicated in order to minimize this distance, i.e. two low pass filters 22 for each high pass filter 24 making four low pass filters in total.

Experiment 3

In a third embodiment of the present invention it has been discovered that the low pass filter preferably presents a very high impedance in the high frequency band. If a standard wound inductor is used as a low pass filter, these can have a parasitic shunt capacitance which limits the high frequency impedance.

Experiment 3 test results:

The impact of a high pass parasitic filter in the low pass filter (e.g. inductor winding capacitance) on the performance was measured. High pass filters (e.g. capacitors) were inserted in parallel with the first low pass filter (a toroidal inductor) to simulate a more conventional winding with multiple overlapping layers. The shunt capacitance of this first toroid is around 15pF compared to around 50OpF for an over- wound inductor.

Measurements were made with the same standard setup as in experiment 1 and are reported below in table 5. Table 5 PCBl

This behaviour appears anomalous as 15OpF seems to degrade the downstream performance more than 30OpF or 45OpF. However, adding capacitance can produce a notch in the phase performance which will change position depending on the capacitance value. The 15OpF could therefore produce a notch at a more critical frequency then the 30OpF or 45OpF. Upstream performance was more straight forward with higher capacitance causing more degradation.

In a particular an embodiment of the present invention shown in Figure 4 this has been realized by using low pass filters i.e. a chain of inductors, of the second part 26 of the low pass filter 22. The first inductor e.g. L15, L16 of the second part 26 of the low pass filter 22 (= second inductor of the low pass filter 22, the first inductor being 25) is a single layer toroid. With this inductor the windings are separate to minimise shunt capacitance. The core has a high permeability to give a high inductance value with a low number of turns. The high permeability also gives very good high frequency (>25kHz) isolation. The coils are differentially wound to reduce the common mode impedance as seen by the test head but to maintain differential isolation and protect the modem. The second (L 12, L 14) and third (L 11 , L 13) inductors of the chain of the second part 26 of the low pass filter 22 are single layer toroids where the windings are separate to minimise shunt capacitance. However the core is much lower permeability to provide stable inductance when POTS line current is applied. The coils are differentially wound to reduce the common mode impedance as seen by the test head but to maintain differential isolation and protect the modem. In the device of Fig. 4 the output signals on test bus 32 may be provided to a test head 20 via suitable pre-amplifiers (not shown). The outputs on the test bus 32 of the second part 26 of the low pass filter 22 are (from the top) CALA, CALB, CALE and CALF. The twisted pair of cable 8 or 18 is tested between CALA and CALB, or CALE and CALF.

Experiment 4

In a fourth embodiment of the invention it has been found that there are specific limits on the characteristics of the high pass filter, e.g. on the value of the capacitor that forms the high pass filter. For the high frequency channel this capacitor would ideally be very large. However this would compromise the isolation that is required for the low frequency channel.

Accordingly, experiments were carried out of how a reduction in the capacitance affects the upstream performance. Four different capacitance values (high pass filter) were used with a low pass filter for ADSLl and ADSL2 tests. In an ADSLl or an ADSL2+ configuration, the model and type of modem, loop length, may affect the result.

Table 6

As you can be seen, 33nF capacitor stops upstream working altogether and with 66nF it only works at 128kbit/s upstream for ADSLl. At 330 nF even the downstream rate is less than optimal. A suitable minimum is therefore above the 330 nF value, e.g. 0.5 microF as this allows the full 19 Mbit/s downstream bitrate for ADSL2. The inductor minimum value is not really important for upstream as it is the capacitor value that will limit upstream performance.

From the above values of capacitance versus maximum bitrate and the isolation requirements for the voice frequency test head a suitable range of capacitance values in practice can be, e.g. between 0.5 and 5 microF, more preferred can be between 1 and 3 microF.

In a fifth aspect of the invention it has been discovered that by implementing two low pass filters 22 as shown in Fig. 3, either a look-in or a look-out test or look-in can be provided or a simultaneous look-in and look-out test access can be offered to the low frequency test head.

In the above description discrete element filters have been described. Each or either of these can be replaced with a digital filter that could be made configurable depending upon the check type being made on the line. This filter may be made programmable so as to maintain the narrow band link and to connect the broadband link between the DSLAM and the test head. Alternately, this filter can be programmed to selectively connect narrow band link to the test head while maintaining the broadband link. It should be noted that attempting to provide a programmable low pass filter and a programmable high pass filter in parallel with a switching matrix can result in some of the distances mentioned above being difficult to realise. Accordingly it is preferred is dedicated checking devices are for either the telephone channels or the xDSL channels

In accordance with a sixth aspect of the present invention an impedance calibration circuit is provided for the high pass filter 24 in the connection matrix 30 as shown schematically in Fig 5. The outputs from the low pass filter 26 of Fig. 4 are CALA, CALB, CALE and CALF. The outputs CALA and CALF are supplied directly to the test head 20, e.g. via pre-amplifiers if necessary as outputs DSLOUTA and DSLOUTF. CALB and CALE are supplied to a relay 29 as shown in Fig. 5. In the normal position of relay 29, CALB is connected through as DSLOUTB and CALE is connected through as DSLOUTE and supplied to the test head 20. Operation of relay 29 swaps the inputs CALB and CALE. Relay 29 can be remotely operated, e.g. under the control of controller 36. Hence CALB is connected through as DSLOUTE and CALE is connected through as DSLOUTB and supplied to the test head 20. The test head can now test the impedance e.g. capacitance between the CALA and CALE and between CALF and CLAB which the impedance of the respective high pass filter 24. This allows the test head to adjust measurements based on the actual values of the high pass filter 24, i.e. actual values of the capacitance. This also allows the test head to adjust measurements based on the selected values of the high pass filter 24 when the high pass filter is a programmable filter.

Claims

1. Device for providing test access for checking a transmission line carrying at least a plurality of digital channels, the transmission line having a plurality of twisted pairs, the device being for use with a test head for determining a quality of the transmission line, the device comprising: a connection matrix for switching one of a plurality of the twisted pairs to a test bus, and a high pass filter coupled for maintenance of the digital channels, wherein the high pass filter is located in the connection matrix.

2. The test access device of claim 1, wherein connection of the test access device to the line does not require a ^synchronisation of a device requiring channel equalisation.

3. The test access device according to any previous claim, comprising a first selection device for selective connection of a twisted pair to a high pass filter.

4. The test access device according to claim 3 wherein the first selection device is for selective serial connection of a twisted pair to at least a first part of a low pass filter, the first part of the low pass filter also being located in the connection matrix.

5. The test access device according to claim 4, wherein there are plurality of first parts of the low pass filter further comprising a second selection device for selective connection an output of one of the at least parts of the low pass filter to a test bus.

6. The test access device according to claim 5, wherein the second selection device is for selective connection an output of one of the at least first parts of the low pass filter to a second part of the low pass filter.

7. The test access device according to any of the claims 3 to 6 wherein the first selection devices can be operated remotely.

8. The test access device according to any of claims 5 to 7 wherein the second selection devices can be operated remotely.

9. The test access device according to any of the previous claims wherein the high pass filter is programmable.

10. The test access device according to any of the claims 4 to 9, wherein the low pass filter is programmable.

11. The test access device according to any of the claims 4 to 10, wherein a value of a parasitic shunt capacitance of the at least first part of the low pass filter is selected to maintain full upstream and downstream bit rate.

12. The test access device according to 11, wherein the value of a parasitic shunt capacitance of the at least first part of the low pass filter is 15 pF or less.

13. The test access device according to any of the previous claims, wherein the high pass filter is a capacitor and the capacitor has a value in the range of 0.5 and 5 microF, or 1 and 3 microF.

14. The test access device according to any of the claims 4 to 13, wherein two low pass filter paths are provided allowing simultaneous look-in and look-out test access for a test head.

15. The test access device according to any previous claim further comprising a high pass filter impedance calibration circuit for measuring the impedance value of the high pass filter.

16. A method for providing test access for checking a transmission line carrying at least a plurality of digital channels, the transmission line having a plurality of twisted pairs, the method being for use with a test head for determining a quality of the transmission line, the method comprising: switching signals from one of a plurality of the twisted pairs to a test bus using a connection matrix, and maintenance of the digital channels by high pass filtering of the signals, wherein the high pass filtering is carried out in the connection matrix.