Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TELECOMMUNICATIONS CABLE
This invention relates to telecommunications cable.
In the telecommunications cable industry, specific
designs of cable have conventionally been used for inside
buildings. A conventional cable design, which has been
employed for voice frequency ranges and low speed data, e.g.,
up to about 4 or 4.5 megabits, is an unshielded cable having
up to six pairs of individually insulated conductors
surrounded by a jacket, and wherein the material of the
jacket and also of the conductor insulation is a polyvinyl
chloride base compound. By unshielded cable throughout this
specification is meant a cable which has no metallic sheath
between the core and the jacket. In such a cable, the
conductors of each conductor pair are twisted together with a
twist length, referred to as "twist lay", of between 3.70 and
5.70 inches. While the above design of cable operates
satisfactorily within the voice frequency range, it is being
found to be unsatisfactory for various reasons above this
range, and has limitations for use with digital systems and
local area networks. In particular, attenuation of signals
at around 16 megabits is undesirably high as is the amount of
crosstalk experienced. There is also a high signal
distortion in the high frequency ranges used for digital
systems. Further to this, at 4 megabits, for digital use,
the practical use of the above cable is limited to a certain
"reach", i.e., a length of about 750 feet of cable between
two computers; this length decreases to about 300 feet at 16
megabits for one link. The reach is decreased further as the
number of computers connected within a network is increased.
The practical limit with 100 computers is 150 feet at 16
megabits.
The above problems inherent in use of the
conventional unshielded cable have been known since the
advent of digital systems and much consideration has been
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given to enabling this cable to be used without its
limitations for digital as well as voice frequency use. As a
recent example of this, in October 1989, McGraw Hill Inc., a
respected authority in the telecommunications industry,
S issued in its "Datapro Reports on PC Communications", Vol. 5,
No. 10, on page 3, an article under "Industry Trends",
entitled "U-B and Proteon Break the 16 Mbps/UTP Barrier".
This article disclosed that Ungermann-Bass (U-B) and Proteon
had stated that they could use unshielded twisted pair wiring
for transmitting 16 megabits on the token ring LAN system.
Although sceptics have believed that standard telephone
wiring could not be used at 16 megabits token ring systems,
U-B and Proteon had showed (according to this article) that
using suitable electronics in a system hub or by using a
suitable filter, the standard wiring could be used in the
required manner. Thus, in October 1989, no suitable
unshielded conducted pair cable had been devised to operate
in a commercially satisfactory manner up to at least 16
megabits and, to overcome the longstanding problem, special
electronics or filters had had to be designed. In fact,
above 4 megabits usage, the only satisfactory cable to date
has been a shielded cable which, because of the shielding,
avoids high frequency problems found in use of the
conventional unshielded cable.
The present invention seeks to provide an
unshielded telecommunications cable which minimizes the
degree of attenuation and crosstalk while providing a
~ ;zed "reach" up to at least 16 megabits.
Accordingly, the present invention provides an
unshielded telecommunications cable having a nominal
characteristic impedance of 100 ohms and a core comprising a
maximum of 6 pairs of individually insulated conductor wires,
the wire insulation formed from a flame retardant polyolefin
base compound and with the insulated conductors of each pair
twisted together with a maximum twist lay of 2.3 inches and
the core surrounded by a flame retardant jacket.
2019447
In the cable structure according to the invention,
the polyolefin insulation provides a low dielectric constant,
and a low dissipation factor which is found to be suitable
for providing acceptable low attenuation up to about 16
- megabits. In addition, the small twist lay minimizes
crosstalk at the above voice frequencies for digital
transmission but also provides a surprising and unexpected
result at those higher frequencies. This surprising result
is that below 2.30 inches twist lay, the electrical
characteristics are such that electromagnetic interference is
reduced to a commercially acceptable level, even though the
cable is unshielded. Indeed, the inventive cable has an
electromagnetic interference level which meets the EMI
requirements per FCC, Part 15, Subpart J. This surprising
result enables the inventive cable to be used successfully
both for the voice frequency range and for data frequency
ranges up to at least 16 megabits.
In addition, it has been found that the cables
constructed according to the invention have an extensive
reach which is completely acceptable for commercial use, this
reach varying for a four-pair conductor cable of 24 AWG
conductors, from about 990 feet at 4 megabit rate to
approximately 525 feet at the 16 megabit rate. In addition,
the near-end crosstalk is minimized to a commercially
acceptable level and the cable is capable of producing a high
digital performance. (Worst case signal to noise is 12 dB at
the highest frequency.) This is as measured upon an
oscilloscope for a set number of passes across the screen for
a certain length of cable.
In cable structures according to the invention, the
maximum twist lay of 2.3 inches may be in a single direction
in the core or may oscillate from one direction to another
around the core, i.e., in the manner commonly referred to as
the 'S-Z' twist.
2019447
One embodiment of the invention may be described by
way of example with reference to the accompanying drawings,
in which:
Figure 1 is an isometric view of part of a cable
according to the embodiment;
Figure 2 is a graph which compares attenuation
characteristics of prior art cables and cables according to
the embodiment;
Figure 3 is a graph comparing near-end crosstalk
characteristics of prior art cables and cables according to
the embodiment;
Figure 4 is a graph comparing the reach of a prior
art cable with a cable according to the embodiment;
Figure 5 is a representation of an eye pattern
developed through a set number of passes across an
oscilloscope screen for a certain length of prior art cable
and compared with the pattern for a cable according to the
embodiment.
In the embodiment as shown in Figure 1, an
unshielded inside building telecommunications cable 10 having
a nominal characteristic impedance of 100 ohms comprises a
core 12 formed from four pairs of individually insulated
conductors 14, the conductors in each pair being twisted
together with a twist lay not exceeding 2.30 inches. In this
particular embodiment, the twist lay is in the range 1.00 to
2.00 inches. The twist lay is in one direction only, but
could, alternatively, change direction at specific intervals
to provide what is commonly referred to as 'S-Z' twist.
The insulation 16 surrounding each of the
conductors 14 is formed from a flame retardant polyolefin
base compound, which, for flame retardancy requirements, is
suitable for a non-plenum rated cable. This particular
compound has a maximum dielectric constant of 2.5 at 1 MHz
with the following formulation:-
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Material % Total Wt
Base resin polyolefin 40 - 65
Halogenated flame retardant 25 - 40
Antimony trioxide 10 - 20
5 Stabilizer and lubricants 0.5 - 0.2
Any formulation according to the above will meet
electrical requirements and also Underwriters' Laboratory
1666 Flammability Tests on two pair and higher construction.
In the above typical formulation, the base resin
polyolefin may be any suitable polyolefin material such as
high or low density polyethylene or an EVA or EEA copolymer
or compounds thereof. The halogenated flame retardant
material may be decabromodiphenyl-oxide, or
ethylenebistetrabromo-phthalimide, or ethylenebisdibromo-
norbornane dicarboximide. In addition, the stabilizer may,
for instance, be a phenolic or phosphite base antioxidant and
the lubricant may be a polyethylene wax.
The core 12 is surrounded by a jacket 20 of a flame
retardant material which in this case is a polyvinyl chloride
compound. The jacket could, however, be formed from another
suitable flame retardant material such as a flame retardant
polyolefin compound, a vinyl base compound, or a
fluoropolymer compound, e.g., a polytetraflorethyline base
compound or a polyvinyledene-fluoride base compound.
Two cables were constructed according to the
embodiment. Cable 1 made according to the embodiment had 24
AWG insulated conductors within the core, and Cable 2
differed from Cable 1 solely in that the conductors were of
22 AWG.
A series of tests were conducted to compare certain
electrical and other properties of Cables 1 and 2 with a
conventional unshielded inside building cable having a
nominal characteristic impedance of 100 ohms and having four
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pairs of individually insulated conductors of 24 AWG. In
this standard cable, referred to as Cable 3 in the tests, the
twist lay of each pair was above 3.S inches with the
insulation on each pair being formed from a polyvinyl
chloride compound. The core comprising the four pairs of
conductors in Cable 3 was surrounded by a jacket comprising a
polyvinyl chloride base compound. In addition, for various
of the tests, a Cable 4 was included. This cable was a
standard shielded cable having a core formed from four
twisted pairs of conductors of 22 AWG and, of course, having
a metal shield between the insulated conductors of the core
and the jacket material. Cable 4 had a nominal
characteristic impedance of 150 ohms.
As may be seen from Figure 2, the attenuation
characteristics of the various cables were compared. This
comparison was made over a range from 0 to 20 MHz for one
hundred meters of each cable. As may be seen from Figure 2,
the standard cable with the 24 AWG conductors, i.e., Cable 3,
had an attenuation characteristic which increased up to
slightly below 15 dB/100 metres at 20 MHz whereas the
standard Cable 4, the shielded cable operating at a nominal
characteristic impedance of 150 ohms, had an attenuation at
20 MHz of about 5 dB/100 metres.
In comparison, Cable 1 constructed according to the
embodiment and with 24 gauge conductors, had an attenuation
of slightly below 10 dB/100 metres at 20 MHz while the 22
gauge cable of the embodiment (Cable 2) had an attenuation of
approximately 7 dB/100 metres.
It is clear from these attenuation results that
Cable 1 of the embodiment has a distinct attenuation
advantage over standard Cable 3 at 20 megabits which is above
the range normally expected for use with data processing at
this time. It is also noticeable that the 22 gauge
unshielded cable of the embodiment (Cable 2) is comparable
for its losses with the standard shielded cable (Cable 4),
20~4~
even though this has the added advantage of the 150 nominal
characteristic impedance.
The attenuation results shown by Figure 2 indicate
that the embodiment with regard to Cables l and 2 provides
acceptable losses while approaching the low losses available
with the use of the 150 nominal characteristic impedance
Cable 4. Hence the cables of the invention which are
directly comparable with Cables 3 and 4 show a distinct
advantage at least for attenuation over the standard Cable 3
and enable the embodiment to be used with acceptable
attenuation up to 20 MHz or even higher frequencies.
In a further test, Cables 1 and 2 were compared
with standard cables 3 and 4 for near-end crosstalk.
The results of this may be seen from Figure 3 in
which Cables l and 2 have directly comparable characteristics
while having a distinct crosstalk isolation advantage over
Cable 3 between 0 and 20 MHz. At the 20 MHz range, there is
a 33% crosstalk isolation improvement in Cables l and 2 over
Cable 3. Cable 4 has a further 15 dB advantage over both of
Cables 1 and 2 by virtue of individual pair shielding. The
reason for the improvement of Cables l and 2 over Cable 3 in
this respect is the small twist lay below 2.30 inches in
Cables 1 and 2 which, in this embodiment, is approximately
2.00 inches.
It was also found that with the unshielded cables,
Cables 1 and 2 had a far greater reach than Cable 3. Figure
4 illustrates this particular point in which the reach of
Cable 3 is compared directly with that of Cable 1 at
different frequencies. For instance, as shown in Figure 4,
at 4 megabits, whereas Cable 3 had a reach of approximately
770 feet, Cable l had a reach of approximately 990 feet for
an improvement over Cable 3 of approximately 30%. The reach
of both of the cables dropped as the frequency increased
until, at 16 megabits, Cable 3 had a reach of approximately
2019~47
300 feet while Cable 1 had a reach of approximately 525 feet,
which is an improvement of approximately 70% over Cable 3.
In the results of Figure 4, those for the 4 and 16 megabits
were obtained using the IBM Token Ring System, whereas the
results at the 10 megabits frequency were obtained with the
Ethernet Lattisnet System. At 10 megabits, Cable 3 had a
reach of approximately 600 feet, whereas the reach of Cable 1
was approximately 825 feet.
As shown by Figure 5, signal degradation along
Cable 1 was compared with that for Cable 3 for 500 feet of
cable using the Ethernet Lattisnet System at 10 megabits.
The curves for each cable were produced upon an oscilloscope
for 700 passes across the screen for a certain length of
cable, each oscillate trace being a function of the encoding
technique which, in this case, is the known Manchester
encoding technique. The curve structure produced for each
cable is referred to as an "eye pattern" which is the result
of superimposing all possible pulse sequences during a
defined period of time. For the transmission to be error
free then each eye formed by a curve should be completely
open. As may be seen from Figure 5, the eye pattern of the
curve for Cable 1 is extremely open compared to that for
Cable 3, thereby indicating that the signal trace varied
extremely little in the case of Cable 1 whereas greater
variation was apparent for Cable 3. A conclusion which can
be drawn from this is that the degradation of the signal over
the length of Cable 1 was far less than was found with Cable
3.
Further to the above comparisons between cables
which show clearly that the cables according to the
embodiment are superior to Cable 3, it has also been found
rather surprisingly, that the cables according to the
embodiment have an electromagnetic interference level which
meets the EMI requirements per FCC, Part 15, Subpart J. As a
result, cables of the embodiment may be used successfully up
to at least 16 megabit range.
