Pulse or digital communications – Testing
Reexamination Certificate
1999-07-22
2003-07-08
Ghayour, Mohammad H. (Department: 2734)
Pulse or digital communications
Testing
C375S257000, C375S229000
Reexamination Certificate
active
06590930
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to measuring performance of networks, and specifically to measuring the performance of links within a local area network.
BACKGROUND OF THE INVENTION
As customer requirements for rates of data delivery over Local Area Networks (LANs) increase, network installers and maintainers are increasing the rates at which data is transferred. Links in such networks commonly comprise cables and associated connectors within the network. Older cables and connectors, which may have been installed to handle 1 MHz or 10 MHz frequencies, are required to cope with 100 MHz or even 1 GHz. As frequencies are increased, signal degradation increases significantly due to, amongst other causes, frequency-dependent attenuation. Signal processing techniques are available to correct for such degradation, at least in part.
At higher frequencies, cable maintenance and diagnosis of problems, whether of older cables or of cables specifically installed for these higher frequencies, becomes significantly more demanding. Telecommunications Systems Bulletin 67 (TSB-67), issued by the Electronics Industry Association (EIA) of Washington, D.C., and the Telecommunications Industry Association (TIA) of Arlington, Va., specifies requirements that are to be met for category-5 cables used within a LAN operating under one of the Ethernet 100BASE standards. Some of the category-5 cable types covered in TSB-67 are: unshielded twisted pair (UTP), shielded twisted pair (STP), screened twisted pair (SCTP), and foiled twisted pair (FTP). TSB-67 specifies that amongst the parameters that are to be measured in determining compliance with the requirements are the physical length and the attenuation of each cable within the LAN.
Each particular type of cable has a nominal attenuation, also termed a nominal insertion loss, in dB/100 m. The nominal attenuation of UTP category-5 cable in dB/100 m is given by the equation:
Nominal
Attenuation
=
2.1
·
f
0.529
+
0.4
f
[
dB
/
100
m
]
wherein f is the frequency in MHz. For example, the nominal attenuation of UTP category-5 cable at 100 MHz is 24 dB/100 m.
In addition to the nominal attenuation of the cable, in practice there are also “flat” (frequency-independent) attenuations caused by, for example, connectors or an electrical interface to the cable. Other fixed attenuations are frequency-dependent, such as are caused by magnetics (transformers) feeding the cable.
An approximate effective length (L
E
) of a specific cable may be defined as:
L
E
=
Actual
attenuation
[
dB
]
-
Fixed
attenuation
[
dB
]
Nominal
cable
attenuation
[
dB
/
100
m
]
·
100
m
(
1
)
In practice, the effective length of a cable is a more useful measure than the physical length, since the effective length incorporates the actual cable attenuation in the measurement, and so gives a better measurement of the quality of the connection formed by the cable. When a link is formed by more than one cable, the effective length of the link, defined by summing the effective lengths of the cables based on equation (1), is similarly a more useful measure than the physical length.
Fluke Corporation, of Everett, Washington, produces a set of meters called a DSP-2000 for measuring parameters of LAN cables. Wavetek Corporation, of San Diego, Calif., produces a similar set of meters, called an LT-8000. Both sets of meters are operated in substantially the same manner. A cable to be tested is removed from the network, e.g., by disconnecting one end of the cable at a cable closet and by disconnecting the other end of the cable at a user's work station. One of the meters in the set is connected to one end of the cable, and the other meter in the set is connected to the other end of the cable. An alternative method of measurement connects one meter to an end of the cable, and disconnects the other end of the cable, leaving it as an open circuit. The length of the cable is measured by finding the time taken for a pulse to travel along the cable using a direct path when two meters are used, or a reflected path when one meter is used. The cable length is found assuming a velocity of propagation for the pulse based on nominal physical properties of the type of cable used.
Use of such meters involves disruption to the network, as each cable is disconnected then reconnected. Furthermore, apart from the time taken by the meters to test each cable, the time taken for the physical disconnection and reconnection can be considerable, especially for a medium- to large-size LAN comprising many hundreds or even thousands of cables. Measurements made on the cables while they are disconnected from the network do not necessarily provide a good measure of the performance of a link formed by one or more of the measured cables. For example, reconnecting the cable(s) back into the network to re-form the link may create one or more poor connections, so that signal degradation over the link is higher than would be expected from the cable measurements.
SUMMRY OF THE INVENTION
It is an object of the present invention to provide methods and apparatus for measuring an aspect of the performance of a link within a network without the necessity of removing the link from the network. Preferably, the aspect that is measured comprises an effective length of the link.
It is a further object of some aspects of the present invention to provide methods and apparatus for measuring the performance of a link within a local area network while the link is transmitting data.
It is a yet further object of some aspects of the present invention to provide methods and apparatus for locally or centrally measuring the performance of a link within a local area network.
In preferred embodiments of the present invention, a network, preferably a local-area network (LAN), comprises a link terminated at a first end by a data signal transmitter and at a second end by a data signal receiver. The receiver processes signals received from the transmitter in order to improve recovery of the signals from degradation due to transmission over the link. Coefficients generated by the receiver in order to perform the processing are used by a link length estimator (LLE) within the network to calculate an effective length of the link providing the signals. The effective length (as explained above in the Background of the Invention) is a measure of the attenuation generated within the link. For a link attenuating at the nominal attenuation of the link, the effective length is equal to the physical length of the link.
In some preferred embodiments of the present invention, the receiver processes the signals using an adaptive equalization technique, whereby equalization coefficients in a filter of a receiver, most preferably comprising a forward equalizer and a decision feedback equalizer, are adaptively set in order to optimize recovery of the received signals. The equalization coefficients thus determined are used by the LLE to determine the effective link length. U.S. patent application Ser. No. 09/070,466, which is assigned to the assignee of the present invention and which is incorporated herein by reference, describes a filter which uses such a technique.
Thus, the effective length of the link (and the physical length if so desired) may be measured without disconnecting the link from the network and without disruption of network operation, unlike methods known in the art which disconnect the link and so disrupt network operation. Furthermore, since the link is not disassembled while measurements are made, there is no possibility of introducing problems into the network on reassembly of the link. Measuring the performance of a network, by measuring effective lengths of links within the network without removing the links, significantly speeds up maintenance and diagnosis of problems of networks, compared to maintenance
Ghayour Mohammad H.
Ladas & Parry
Mysticom Ltd.
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