Telephonic communications – Diagnostic testing – malfunction indication – or electrical... – Of trunk or long line
Reexamination Certificate
1998-11-03
2002-04-16
Nguyen, Duc (Department: 2643)
Telephonic communications
Diagnostic testing, malfunction indication, or electrical...
Of trunk or long line
C379S001040, C379S015030, C379S022040, C379S022020
Reexamination Certificate
active
06373923
ABSTRACT:
BACKGROUND TO THE INVENTION
This invention relates, in general, to a method of and apparatus for testing a telecommunication link and is particularly, but not exclusively, applicable to a method of and apparatus for testing a copper pair (connecting a telephone exchange to a subscriber unit) to determine its ability to support high frequency data transmissions that are ancillary to its originally designated function of supporting voice telephony.
SUMMARY OF THE PRIOR ART
The connection between individual telephone subscribers, whether these are domestic or business subscribers, and a local telephone exchange has traditionally been provided using copper cables consisting of a number of unshielded twisted-pair wires, usually known as “copper pairs”. More explicitly, network topology has a multi-pair cable emanating from an exchange, which multi-pair cable is gradually split out to provide one (and sometimes a plurality) of single copper pairs at a customer's premises. When these copper pairs were first deployed in local areas, it was assumed that they would be used for transmission of voice signals only; these wireline connections were therefore expected to operate in a frequency range of less than about 4 kilohertz (kHz). Consequently, the planning rules adopted for such wireline (i.e. copper pair) systems were based on easily controlled and measured parameters, such as loop resistance and low frequency attenuation. In the UK, the normal planning limits are 1000 Ohms (&OHgr;) loop resistance and seven decibel (7 dB) attenuation at 1 kHz.
These planning limits are achieved by a suitable choice of conductor gauge or diameter depending on the route distance between the exchange and the customer. Longer routes clearly require larger conductors in order to meet the resistance and attenuation limits. Conductors tend to be between 0.3 mm and 0.9 mm in diameter, with increasingly larger diameter conductors being used the further the cable extends from the exchange. This allows for bundles of narrow gauge pairs to be grouped together at an exchange thus minimising cable handling problems.
As the number of new subscribers obtaining telephone services from operators utilising optical feeders increases, telephony providers, whose systems are largely constructed of copper pairs, are increasingly looking to the provision of wideband and broadband services to their customers over their copper pair links. With the advent of wideband and broadband services (such as internet access, video-on-demand and digital data transmissions) as well as increases in the volume of telephony services and traffic, telephony providers are necessarily considering the testing of individual links between exchanges and subscribers in order to ascertain whether or not each link will support the provision of such services. In particular, lines must presently be tested to see if they will support present ISDN services while, as time passes, further tests will more frequently need to ascertain whether or not these twisted pairs will support broadband services requiring technologies such as asynchronous digital subscriber line signalling (ADSL), high speed digital subscriber line signalling (HDSL) and very high speed digital subscriber line signalling (VDSL); these transmission techniques are generically termed xDSL transmissions. Indeed, the more exotic forms of xDSL use wideband techniques for enhanced data capacity (presently up to about ten megabits per second, 10 Mbps), with such wideband techniques distributing information across a number of sub-carriers, e.g. as supported by discrete multitone (DMT) and orthogonal frequency division multiplexing (OFDM).
In contrast with audio signals that have a frequency of less than about 4 kHz, broadband signals may be in the range 25 kHz to 10 MHz, and more usually exceed 1 MHz in order to support broadband applications.
One of the key basic parameters for establishing the suitability of a particular copper pair for carrying such broadband services is its transmission length (arising as a consequence of signal attenuation increasing with transmission length). Unfortunately, this is not readily deducible from the records of a particular operator, even if they are accurate. This is because, although the records show duct routes and section lengths, they do not necessarily indicate how a cable is routed through the duct. For example, it is often found that a copper pair in a specific cable will transverse the full length of the duct to a splice point and then return along the same duct as a pair in another possibly smaller cable.
Furthermore, it will be understood that cabling diameters also play a significant role at high frequencies since signal transmission in cable (at these higher frequencies) is principally through the so-called “skin effect”, while loss (i.e. signal attenuation) is also skin effect dependent. Specifically, loss is proportionally greater in smaller gauge cables. While these effects are not important with voice signals, these effects are considerable in relation to higher frequency transmissions because the associated error in the determination can yield misleading results. More particularly, the attenuation at low frequencies (used for narrowband voice communication below about 4 kHz) is primarily dominated by direct current (DC) resistance that is inversely proportional to the cross-sectional area of the wires. At relatively high frequencies (such as employed in xDSL), the dominant skin effect exhibits attenuation that is inversely proportional to the circumference of the wires and also proportional to the square root of the frequency. Thus, the amount of attenuation increases with frequency.
It is equally misleading to use measurements based on grid references in order to predict lengths, because the necessary scaling factor of actual cable length to direct distance is unknown in any specific instance. For example, in the UK, the average scaling factor is probably somewhere between 1.4 and 2.0; the resulting distances are often enough to render a link unsuitable for the provision of wideband or broadband services, while the uncertainty in ascertaining actual cable route lengths makes this method highly inaccurate. In addition, end-to-end connections may include sections of aluminium (or differing numbers of junctions) which will have different transmission characteristics to the copper sections. Aluminium was used in this way when copper prices made copper less economic than aluminium.
Of course, there are other factors that contribute to attenuation or loss, but these are of relatively minor importance and deserve just a passing note, namely loss caused within the dielectric between copper pairs and impedance mismatch at discontinuities, such as at joints between cabling sections.
One alternative approach is for operators to dispatch staff to a customer's premises to undertake one or a series of measurements of the copper cable and its performance, so called “truck roll”. This is a time consuming and consequently expensive solution, especially if the customer decides not to take the service, or takes it only for a short period.
A variation on full truck roll is for the operator to take field-based sample measurements of cable lengths and performances from the exchange to the local telephone cabinet, from where individual copper pairs are directed to individual subscribers. As the cables from the exchange to the cabinet are shared this would reduce the cost per customer line, but would only give an indication in relation to a few of the copper pairs from any particular exchange. Equally this latter method gives no indication of the length and performance of the copper drops from the cabinet into a customer's premises. This method is, therefore, again rather inaccurate.
Time domain reflectometry (TDR) is a technique primarily used for determining a discontinuity or breakage in a cable. However, this may be usable to test a link from an exchange to a subscriber unit. Unfortunately, this method is not conducive to copper pairs since there can
Grant Michael Francis
Williamson Roger James
Lee,Mann,Smith,McWilliams,Sweeney & Ohlson
Nguyen Duc
Nortel Networks Limited
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