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 B45 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 DP3.
The cable leaving DP3 and continuing on then terminates in a sub DP3A.
This will continue via DP3B, 3C etc.
It is possible that new DPs can be cut into cable lengths eg If an additional DP
were to be provided between DPs 3C and 3D, then it would be numbered
DP3CA.
If a new DP is cabled away from an existing DP, then it will take the next free
letter eg DP3Q is a new DP connected back to DP3A.
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
DP 2
Parkend
sheet 1
E Sides : These records look back towards the exchange
E side
cable from
MDF or E side cable E side cable
D sides
type
Colours
A17
A18
A19
A20
A21
A22
A23
A24
A25
A26
A27
A28
A29
A30
A31
A32
A33
A34
A35
A36
20 pair
cable
from
MDF
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s/r
b/bn
s/r
b/bn
s/r
g/bk
o/w
s/r
b/bn
s/r
b/bn
s/r
b/bn
s/r
b/bn
o/w
b/bn
s/r
o/w
g/bk
location Signal box
Jumpers
E side
Circuit Details
E side
Pair No.
348 PKD S/Box Locking Room
347 Parkend Signal Box
336 Parkend Signal Box
E1
E2
E3
E4
E5
E6
E7
E8
E9
E10
E11
E12
E13
E14
E15
E16
E17
E18
E19
E20
E Cross D Cross
Conn.
Conn. D side
to
to
Pair No.
>>>>
>>>>
>>>>
issue date
16/03/2010
D Sides : These records look on towards the distribution network
D side
Circuit Details
D side cable
Colours
Apparatus Details or
D side cable to sub DP
348 PKD S/Box Locking Room
347 Parkend Signal Box
336 Parkend Signal Box
1
11
10
20
DP Layout
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
DP 2
Parkend
sheet 1
E Sides : These records look back towards the exchange
E side
cable from
MDF or E side cable E side cable
D sides
type
Colours
A17
A18
A19
A20
A21
A22
A23
A24
A25
A26
A27
A28
A29
A30
A31
A32
A33
A34
A35
A36
20 pair
cable
from
MDF
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s/r
b/bn
s/r
b/bn
s/r
g/bk
o/w
s/r
b/bn
s/r
b/bn
s/r
b/bn
s/r
b/bn
o/w
b/bn
s/r
o/w
g/bk
location Signal box
Jumpers
E side
Circuit Details
E side
Pair No.
348 PKD S/Box Locking Room
347 Parkend Signal Box
336 Parkend Signal Box
E1
E2
E3
E4
E5
E6
E7
E8
E9
E10
E11
E12
E13
E14
E15
E16
E17
E18
E19
E20
E Cross D Cross
Conn.
Conn. D side
to
to
Pair No.
>>>>
>>>>
>>>>
issue date
16/03/2010
D Sides : These records look on towards the distribution network
D side
Circuit Details
D side cable
Colours
Apparatus Details or
D side cable to sub DP
348 PKD S/Box Locking Room
347 Parkend Signal Box
336 Parkend Signal Box
1
11
10
20
DP Layout
The permanent cable pairs coming from MDF A17 to A36 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
DP 5F
Norchard
sheet 1
E Sides : These records look back towards the exchange
E side
cable from
MDF or E side cable E side cable
D sides
type
Colours
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
10 pair cable
from DP5EA
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b/bk
o/bk
g/bk
bn/bk
s/bk
b/r
o/r
g/r
y/r
c/bk
E side
Circuit Details
591 Lineside Test Tele North
NCD-PKD o/g 1 jnc 1
NCD-PKD o/g 2 jnc 2
PKD-NCD o/g 1 jnc 3
PKD-NCD o/g 2 jnc 4
597 Whitecroft
PKD alarm extension
Resvd DP5C Norchard to PKD 1
Resvd DP5C Norchard to PKD 2
Resvd Sig Box NCD to PKD Token
location North of Tuft's Bridge
Jumpers
D Sides : These records look on towards the distribution network
E Cross D Cross
E side Conn. Conn. D side
Pair No.
to
to
Pair No.
E1
E2
E3
E4
E5
E6
E7
E8
E9
E10
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
<<<<
<<<<
<<<<
<<<<
<<<<
<<<<
<<<<
<<<<
<<<<
<<<<
issue date 01/11/2010
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D side
Circuit Details
591 Lineside Test Tele North
NCD-PKD o/g 1 jnc 1
NCD-PKD o/g 2 jnc 2
PKD-NCD o/g 1 jnc 3
PKD-NCD o/g 2 jnc 4
597 Whitecroft
PKD alarm extension
Resvd DP5C Norchard to PKD 1
Resvd DP5C Norchard to PKD 2
Resvd Sig Box NCD to PKD Token
D side cable
Colours
Apparatus Details or
D side cable to sub DP
r/s
o/w
g/bk
bn/b
s/r
b/bn
o/w
g/bk
bn/b
s/r
10 pair cable
to DP5G
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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
DP 3N
Norchard
sheet 1
E Sides : These records look back towards the exchange
E side
cable from
MDF or E side cable E side cable
D sides
type
Colours
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
10 pairs
in cable
from
DP3M
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o/w
s/r
g/r
o/r
b/r
bn/r
b/w
g/w
bn/w
s/w
E side
Circuit Details
LJN Concentrator Alarm & Clock
552 Diesel Depot Site Hut
551 Lydney Junction Platform
553 Lydney Junc Exchange
Tel Nor S/B & L/Jcn S/B
587 Lineside Test Tele South
546 Lydney Junc Sig Box
Token Nor S/B & L/Jcn S/B
location
Jumpers
D8
D5
>>>>
D7
>>>>
>>>>
>>>>
D12
D13
D14
<<<<
E3
<<<<
E5
E1
<<<<
<<<<
D1
D2
D3
issue date
27/11/2011
D Sides : These records look on towards the distribution network
E Cross D Cross
E side Conn. Conn. D side
Pair No.
to
to
Pair No.
E1
E2
E3
E4
E5
E6
E7
E8
E9
E10
Bypass Crossing
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
D12
D13
D14
D side
Circuit Details
SPT Sig post 9
Tel ABCL
Bypass Power Alarm
551 Lydney Junction Platform
552 Diesel Depot Site Hut
Tel Nor S/B & L/Jcn S/B
553 Lydney Junc Exchange
LJN Concentrator Alarm & Clock
546 Lydney Junc Sig Box
Token Nor S/B & L/Jcn S/B
SPT Sig post 9
Tel ABCL
Bypass Power Alarm
D side cable
Colours
Apparatus Details or
D side cable to sub DP
10 pairs
in cable
to
DP3P
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conn directly by S&T Group
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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
DP 3HJ
Norchard
sheet 1
E Sides : These records look back towards the exchange
E side
cable from
MDF or E side cable E side cable
D sides
type
Colours
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
D12
D13
D14
D15
D16
D17
D18
D19
D20
D21
D22
D23
D24
D25
D26
D27
D28
D29
D30
30 pairs
in cable
from
DP3GC
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(b) b/w
(b) o/w
(b) g/w
(b) bn/w
(b) s/w
(b) b/r
(b) o/r
(b) g/r
(b) bn/r
(b) s/r
(o) b/w
(o) o/w
(o) g/w
(o) bn/w
(o) s/w
(o) b/r
(o) o/r
(o) g/r
(o) bn/r
(o) s/r
(g) b/w
(g) o/w
(g) g/w
(g) bn/w
(g) s/w
(g) b/r
(g) o/r
(g) g/r
(g) bn/r
(g) s/r
E side
Circuit Details
546 Lydney Junc Sig Box
525 St Mary's halt
522 Town Crossing & Station
551 Lydney Junction Platform
Token Nor S/B & L/Jcn S/B
587 Lineside Test Tele South
552 Diesel Depot Site Hut
Tel Nor S/B & L/Jcn S/B
Town Treadle North
553 Lydney Junc Exchange
LJN Concentrator Alarm & Clock
location Town Crossing North Side
Jumpers
E side
Pair No.
E1
E2
E3
E4
E5
E6
E7
E8
E9
E10
E11
E12
E13
E14
E15
E16
E17
E18
E19
E20
E21
E22
E23
E24
E25
E26
E27
E28
E29
E30
E Cross D Cross
Conn.
Conn. D side
to
to
Pair No.
D9
D1
D14
E2
E13
E8
>>>>
>>>>
D8
D3
D10
D7
>>>>
>>>>
D2
<<<<
<<<<
E10
E7
E1
E9
<<<<
<<<<
E3
issue date
01/11/2011
D Sides : These records look on towards the distribution network
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
D11
D12
D13
D14
D15
D side
Circuit Details
D side cable
Colours
Apparatus Details or
D side cable to sub DP
1
2
3
4
5
10 pairs
in cable
to
DP3K
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5 pairs
in cable
to DP3K
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525 St Mary's halt
552 Diesel Depot Site Hut
551 Lydney Junction Platform
Token Nor S/B & L/Jcn S/B
Town Treadle North
587 Lineside Test Tele South
546 Lydney Junc Sig Box
Tel Nor S/B & L/Jcn S/B
553 Lydney Junc Exchange
LJN Concentrator Alarm & Clock
522 Town Crossing & Station
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
Norchard
DP T
sheet 1
E Sides : These records look back towards the exchange
E side
cable from
MDF or E side cable E side cable
D sides
type
Colours
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
10 Pair
Cable
from
DP3R
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b/w
o/w
g/w
bn/w
s/w
b/r
o/r
g/r
bn/r
s/r
E side
Circuit Details
PA Press Button 529 Signal Box
Sig Post 13 SPT
TAC P
NSB Concentrator Alarm & Clock
(HL)2 GK
b wire N12, a wire B12
b wire (HL)2 GC, a wire TW Q
Resvd Sig Box NCD to PKD Token
Jumpers
E side
Pair No.
E1
E2
E3
E4
E5
E6
E7
E8
E9
E10
location
Norchard Signal Box
issue date
05/10/2011
D Sides : These records look on towards the distribution network
E Cross D Cross
Conn.
Conn. D side
to
to
Pair No.
D side
Circuit Details
D side cable
Colours
Apparatus Details or
D side cable to sub DP
PA Press Button 529 Signal Box
Sig Post 13 SPT
TAC P
NSB Concentrator Alarm & Clock
(HL)2 GK
b wire N12, a wire B12
b wire (HL)2 GC, a wire TW Q
Resvd Sig Box NCD to PKD Token
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