SSDC5 is the abbreviation for Signaling 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 signaling. 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.

The pairs that carry the speech are denoted as A and B. The signaling 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.