CONTENTS OF
TELEPHONES UK

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SSDC5 is the abbreviation for Signalling System Direct Current No. 5.  This type of link is intended for the connection by a private circuit of PBXs which are in close proximity to one another. This generally means that they are located on the same site and within a distance of 100 metres. For long distances SSAC15 private circuits are used instead. The link comprises of either 2 or 4 wires for transmission and a further pair for signalling. These are sometimes referred to as 2 wire DC5 and 4 wire DC5.

2 wire DC5 uses the same pair for both transmission and reception and is therefore more susceptible to crosstalk between the signals. 4 wire DC5 uses one pair for transmission of voice and another pair for reception. This reduces the crosstalk or interference, between the incoming and outgoing speech. 

A	--------------- Transmit ---------------		Incoming Speech	--------------- Receive ---------------	A
B					B
A	--------------- Receive ---------------		Outgoing Speech	--------------- Transmit ---------------	A
B					B
E	---------------		Signalling	---------------	M
M	---------------			---------------	E

A	---------------		2 Way Speech	---------------	A
B	---------------			---------------	B
E	---------------		Signalling	---------------	M
M	---------------			---------------	E

The pairs that carry the speech are denoted as A and B. The signalling pair are denoted E for Ear and M for Mouth. This gives rise to the other common term for this type of link: E&M. Signals are received on the E wire and sent on the M wire. At the destination these wires are crossed over so that the E wire is receiving the signals from the other M wire and vice versa.

The E and M wires are electrically isolated from each other, with signals being sent as the presence or absence of a earth on the wire.

The following table lists the earth on/off states on the E and M wires at different states in a call.

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