INSTALLATION
DOC_DEV_XSRACK_User guide_A
Page 17
6
Preparing and checking the line
6.1
Type of cable
Twisted pair cable
The XSRACK is designed to be connected to one or several telephone grade twisted pairs.
The conductor diameter must be included between 0.4 mm and 1 mm.
A cable may be composed of several twisted pairs.
Each pair can usually be used for a different SHDSL transmission if necessary. However, care must be taken
to ensure that crosstalk between pairs is not excessive.
Cable made of quads
It often happens that the twisted pairs of the same cable are wound in groups of two pairs; a group of two
pairs rolled into each other is called a quad.
This type of cable is suitable. However, we will try to use only one pair per quad to avoid crosstalk (see below
6.2).
Shielded cable
It is better to use a shielded cable.
The shield must be connected to the earth at one of its ends.
The shield decreases the influence of the electromagnetic ambient noise on the SHDSL signal.
Moreover, the shield protects the XSRACK against lightning.
Electrical power cable
Two power conductors can be used instead of a twisted pair to set an SHDSL connection.
However, because the two wires are not twisted, the ambient electrical noise may disturb the transmission.
Compared to the transmission over a twisted pair, the maximum distance between two SHDSL switches is
decreased.
6.2
Crosstalk interference
If the cable is made of several pairs, a signal transmitted in one pair may disturb the signal transmitted in
another one, and, in some cases, may decrease the effective data rate of the SHDSL connection.
The closer the pairs, the greater the crosstalk. Thus, the risk of crosstalk is higher between two pairs of the
same quad.
This is why, if the cable is made up of quads, it is advisable to avoid using the two pairs of the same quad.
6.3
Shield earthing
A shielded cable provides better noise immunity and surges protection during thunderstorms.
The best protection is provided when the shield is earthed at each end of the line.
However, there may be a large potential difference between the connection points to the earth, especially when
the line is long.
Therefore, to avoid a large current flowing in the shield, it is recommended to connect the shield to the earth
at only one end of the cable.
