6.2 - Leased Lines

This section covers those services that can be rented from the network provider and that are augmented by the network and require converters to modify the signalling characteristics of the connected equipment.

The conversion equipment can usually be rented from the carrier as well as the line. However there are a small number of suppliers who sell these converters direct to the customers.

The DC services are for use over relatively short distances. The maximum lengths vary depending upon who is asked and vary between 2 miles and 11 miles. Generally these should be kept as short as possible as they utilise DC, consuming relatively large amounts of power.

The AC services are for longer distances. They can be easily boosted and are more efficient than the DC types. The costs of leasing are dependant upon the distance covered.

6.2.1 - DC5

Also known as Signalling System Direct Current No. 5 (SSDC5) this type of link is intended for the connection of physically close PBXs. Physically close usually means ‘on the same site’. The link is 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.

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

When specifying such a link the provider must know weither 2 or 4 wire DC5 is required.

4 Wire DC5
A	.................................................		Incoming Speech
B
A	.................................................		Outgoing Speech
B
E	.................................................		Signalling
M
2 Wire DC5
A	.................................................		2 Way Speech
B
E	.................................................		Signalling
M
Figure 4 - DC5 Circuit Wiring Schemes

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.