Distribution Cabling Telephones are connected back to their

Distribution Cabling Telephones are connected back to their exchange via a pair of wires in a cable. Such pairs are always twisted so that each wire remains close to its partner. This is important as each wire is then as likely to receive the same amount of interference from other pairs as its partner. As such interference will be in the same direction in each wire of the pair, they will be in opposition at the telephone or exchange and not cause a current that will flow around the circuit. Crosstalk is due mainly to differences in the wire to wire capacitances in which cancellation cannot be perfect. To reduce crosstalk pairs are generally laid into a cable using a twisting process which means that any two pairs do not lay alongside each other for too long. Most small cables have a core of a small number of pairs twisted together, followed by layers of pairs which then twist in opposite directions and have different lengths of twist or "lay". Internal cables are generally finished with a white or grey PVC sheath. External cables usually have black polythene or PVC sheaths to resist sun and ultra violet damage. 1

Telephone circuits can be made up of many different cable lengths. However all the cables in the circuit need to have similar characteristic impedances. If a change of impedance were to occur, then some reflection of the signal would occur at that point and this would cause a significant loss in the line. Cables used for telephony generally have a characteristic impedance of about 1200 ohms. Cables designed for data work often have lower characteristic impedances. Such cables are not really suitable over longer lengths. 2

Cables from an exchange usually start with quite high numbers of pairs. These pairs are then connected to a number of smaller cables radiating out from the "main" cable. In turn further smaller cables may be employed, the whole arrangement mirroring a tree trunk, branches and twigs. In a public exchange system the "main" cables break down into smaller cables in roadside cabinets or pillars. 3

Cables start at the exchange "Main Distribution Frame" or "MDF". They are terminated on fuse mountings on the line side of the MDF. It is useful to remember that external cable pairs generally "count down", both on the MDF and at DPs. (Note that internal exchange cables generally "count up" when they terminate on frames). Each cable pair therefore has an MDF "bar and pair" number eg B 45 when it leaves the exchange. When the pair arrives at the first DP it is terminated on the "exchange" or "E side" of the DP. 4

Pairs leaving the DP are terminated on the "distribution" or "D" side of the DP. Connections from the "E" side to the "D" side pairs are made by : 1) using "jumpers" or straps 2) terminating the D side cables from the apparatus directly onto the incoming "E" side cables. 3) crimping the E and D side pairs directly together. This may leave an untidy looking cabinet but is the preferred method for new work as it is by far the most reliable and fault free method. Note that any point in the cable routing the "E" side looks back towards the exchange and the "D" side looks on towards the telephone. 5

DP Numbering on the DFR On the DFR, cables leaving the MDF terminate in the first DP on the route and this DP is given a number eg DP 3. The cable leaving DP 3 and continuing on then terminates in a sub DP 3 A. This will continue via DP 3 B, 3 C etc. It is possible that new DPs can be cut into cable lengths eg If an additional DP were to be provided between DPs 3 C and 3 D, then it would be numbered DP 3 CA. If a new DP is cabled away from an existing DP, then it will take the next free letter eg DP 3 Q is a new DP connected back to DP 3 A. Some cables from have been provided predominantly for S&T. These have been historically labelled alphabetically. Eg DPT Labelling has started at Z and is working backwards. This practice should continue. 6

This is an example of a DP record. It is kept as a paper copy at the exchange but the master is maintained as an Excel spreadsheet by the Recorder. The spreadsheet is also available on the telecoms website for the benefit of the S&T group. 7

The permanent cable pairs coming from MDF A 17 to A 36 are shown in colour as terminating on E sides 1 to 20. This is permanent data and will not change unless extra pairs are provided later. Any circuits using these pairs are shown in black. In this case, all the circuits terminate in the DPs vicinity and the cables to the phones are connected directly to the E side pairs. 8

This card is an example of a simple DP where a 10 pair cable enters on the E sides and a 10 pair cable leaves on the D side. The coloured entries describe the permanent cable pair terminations. The black entries show the circuits using the pairs. The four arrow symbols indicate that the pairs are connected straight through. 9

This DP example is a little more complex. There is a 10 pair E side cable entering the DP and a ten pair D side cable leaving. However the pairs are not all connected straight through. It can be seen that, say, 551 enters and leaves on pair 4 but that 552 enters on pair 3 but leaves on pair 5. This change of pair is shown in the two Cross Connect columns. The pairs shown in red go directly from the DP to S&T equipment. They are shown as D sides as they certainly do not go back to towards the exchange. This requires that the circuits be connected from D side to D side. Eg The SPT (signal post tele) Sig post 9 arrives from the signal box on D side 1 and leaves the DP on D side 12. 10

This example shows a DP with a 30 pair cable entering on the E side and a 10 and a 5 pair cable leaving on the D side. Some circuits are connected straight through while others are cross connected. 11

This is an example of a DP provided predominantly for S&T. It is labelled alphabetically rather than numerically. The S&T circuits are connected directly from the DP E sides to the signal box apparatus. It is perfectly correct for these DPs to be used for telecom purposes eg pair 1 is used for the PA press button on the signal box phone. 12