Selection and Testing of Electronic Components for LM

Cross Stranding of
Telephone Cable
Sigurd Nordblad
LM Ericsson have developed a new manufacturing process and constructed new
machines for the manufacture of pair cables. The process, which is called cross
stranding, combines two methods: twinning and stranding of groups in one operation
and repeated changing of the relative positions of the pairs during the stranding.
The changing can either be carried out systematically in accordance with a set
plan or at random, so-called randomized cross stranding. The main purpose of the
cross stranding is to reduce the extreme values of the crosstalk and thus improve
the quality of the cable.
In the article the cross stranding technique is described with the emphasis on the
manufacture of pair cables with randomized changing, but cross stranding can also
be used with advantage for stranding single conductors, triples, quads, quintuples
etc.
The cross stranding technique has now been introduced at most of the telecommunication
cable factories owned by the Ericsson Group. At the Piteå plant, which
wasstartedin 1972, the entire production is based on this technique. Manufacturers
outside the Group also use the technique.
UDC 621.315.2
621.391.31
Fig. 1
Cross stranded cables
Top, 150-pair cable with 25-pair groups
Bottom, 50-pair cable with 10-pair groups, jelly-
In addition to line attenuation, characteristic
impedance and line resistance,
the crosstalk characteristics of a cable
have a very great influence on its field of
use. This applies particularly in the case
of trunk cables but also for subscriber
cables.
One reason why the subscriber cables
of today should have a low level of
crosstalk is that modern telephone sets
can then be utilized more efficiently.
The usefulness of a telephone set is limited
by such factors as the crosstalk
level in the cable network. A reduction
of the crosstalk means that greater distances
can be spanned or that the conductor
size can be reduced.
High frequency systems, which are used
nowadays to an ever increasing extent,
SIGURD NORDBLAD
Sieverts Kabelverk AB
also require cables with improved
electrical characteristics.
Previously the pairs, single conductors,
quads etc. of a cable have usually been
assembled in concentric layers. The
pairs were then parallel in each layer
and were adjacent to the same pairs
along the whole length of the cable.
Subsequently the unit cable was introduced,
but the units were still built up of
concentric layers. Efforts to improve the
cable characteristics have been concentrated
on improving the precision of
the wire drawing, insulation etc. and on
suitable selection of lay lengths, i.e. improvements
within the pairs, and very
little attention has been paid to the effect
of the cabling method on the
electrical characteristics.
Crosstalk occurs mainly between adjacent
pairs and it is obvious that the
crosstalk increases when the pairs are
adjacent over a long distance. Using the
conventional layer stranding technique
the pairs are placed adjacently and as
close as possible along the whole
length of the cable. The crosstalk level
between pairs varies in a cable; high
level between adjacent pairs and very
low level between separated pairs.
However, in a telephone system the
worst values often constitute a technical
limit and a number of very good values
does not alter this fact.