ericsson review - ericssonhistory.com

21
HENNING ANDERSEN
VILLYK. PEDERSEN
Jydsk Teleton A/S, Arhus. Denmark
Fig. 3
The four types of bit pairs and the corresponding
phase shift angle of the carrier
transit point before being transferred
from one link to another.
In older signalling systems there is always
an unambiguous physical connection
between each speech connection
and the signals associated with it. Consequently
this signalling method is called
channel-associated signalling, fig. 1.
The signals are usually transmitted in
the speech channel or in a band that belongs
to the channel but lies outside the
transmitted speech band. In signalling
system No. 6, on the other hand, a separate
signalling channel is used, which is
common for a large number of speech
channels, and which transmits the signals
required for all these channels. This
method is therefore called common
channel signalling, fig. 2.
Thus a signal that is transmitted in the
common signalling channel is not
physically tied to a particular speech
channel, and hence each signal must be
provided with a speech channel label
that indicates the speech channel to
which the signal belongs.
All channel-associated signalling systems
require that the in-band signals are
transmitted without errors in both directions
if it is to be possible to establish a
connection. This provides an automatic
check that there are no faults in the
speech path before the call is set up.
Since the signals in system No. 6 are
transmitted over a separate channel it is
possible that everything functions normally
except the speech transmission.
Consequently with this system the
speech paths are checked with a tone of
2000 ±20 Hz before the call is set up.
The system is designed primarily for
analogue transmission systems, since it
was assumed during the development of
the system that this type of transmission
would be predominant on international
lines during the estimated life of the
system. However, a modified version of
the system can be used for digital
transmission systems (PCM).
Analogue version
In the analogue version a data link is
used that consists of a normal 4-wire
circuit for telephony. The transmission
speed is 2400 bits/second. Both ends of
the data link are connected to so-called
four-phase modems, with the binary
data signals grouped in bit pairs (dibits)
(00, 01, 11 and 10), where each pair corresponds
to one of the four phase positions
of the signal carrier. The pair that is
the next to be transmitted initiates a shift
to the phase angle required for sending
the information, fig. 3.
There is a constant bit stream in both
directions over the data link. It is filled
with non-informative signals, so-called
Fig. 2
Common channel signalling in accordance with
CCITT system No. 6
© The common signalling channel is a data link
consisting of a normal 4-wire circuit
@ Four-phase modem for the transmission of bit
pairs (dibits)