Asterisk vs ISDN

Some information on how to conect Asterisk to the ISDN
Chapter 7:

General: Cabling

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general: Cabling

This chapter is not directly related to Asterisk, but there seems to be some demand regarding cabing, so here you go:


The U-interface

This is the cable coming in to your house and going in to the NT.
This hopefully shouldn't worry you as you probably got the NT from your telco which usually doubles as the demarcation point.
A BRI NT usually has a UK0 port on a 6P2C modular socket, like a POTS line. Often the release tab of the plug is off-center, however.

Old PRI NTs use a 4-wire UK2 port in various forms.
Newer PRI NTs are actually DSL modems (Usually G.SHDSL), just with a G.703 E1/T1/J1 port on the other side, instead of the usual Ethernet.
There are also UG2 lines using fibre optics, possibly carrying more than one PRI.


The S/T-interface

That's what comes out of the NT and connects to your equipment.
There are some general points for all normal use cases (both BRI and PRI): So a normal Ethernet patch cable will be fine.

In the special case that an interface can be operated in the opposite mode of what it is wired for, a crossover cable or adapter would be needed. See below for BRI and PRI crossover.


BRI (S0)


8P8C or 8P4C
modular/western plug
1+power source 3 /
power sink 3
The data pairs usually also carry phantom
power to supply telephones from the NT
7-power source 2(optional/rare)

Bus cabling

The S0 Interface is generally a bus. That means you (can) have an fixed installation part and devices patched to that installation. This is especially true for ptmp lines.
ConfigurationMaximum installed
cable length
of the NT
of TE(s)
Max # of
Max # of TEs
Short Passive Bus120m (120nf/km) /
220m (30nf/km)
on the bus
anywhere on the bus128
Extended Passive Bus450m (120nf/km) /
825m (30nf/km)
at one endwithin 25m (120nf/km) /
50m (30nf/km)
of the opposite end
Long Passive Bus
600m (120nf/km) /
1100m (30nf/km)
0.6mm wire
at one endthe opposite end21
The long passive bus can be used on ptp lines or on ptmp lines in the special case that only one TE is connected.
Patch cables from socktes to TEs should not be longer than 10m.
If the NT is connected via a patch cord to a socket on the bus, the patch cable should not be longer than 1m.


The bus always has to be terminated at both ends with a 100Ω resistor between Rx+ and Rx- as well as between Tx+ and Tx-.
A short or extended passive bus is usually terminated by the NT on one end and on the last socket at the other end. The termination on the NT can usually be switched off in case you want to place it in the middle of a short passive bus, in which case the first and the last socket must be terminated.
Many PC cards also contain jumperable terminators that come in handy if you just want to connect them directly to the NT (as the only device; see long passive bus above). If you have a terminated fixed installation, make sure the termination on the card is disabled.
Like most other buses, the S0-bus must be terminated exactly once on each end. No more. No less.

BRI patch cables

BRI cables come in both round and flat models and often only have 4-wires ending on 8P4C modular plugs.
The outer 4 pins are not usually needed. By specification they can carry additional power supplies but usually the two data pairs carry phantom power for phones.
The length of patch cables connecting devices to the bus must not exceed 10m.

Straight BRI cable
There have been some BRI cables around with the wires reversed between the two plugs leading to myths about this being the correct wiring. It is not! And it's not any sensible type of cross-over cable, either.
If you have only one device connected and use such a wrong cable, it will work without issues. However, if you connect a second device using a correct cable, things will fail.
So if you've got such cables you might consider to get rid of them to avoid any headache in the future.

Wrong BRI cable

BRI crossover

In some special cases a crossover cable or adapter has to be used.
This will be the case when in interface is not operated in the mode it was wired for and either cannot be jumpered to care for that situation or changing jumpers would be too much hassle.
The popular case is when you use your good old ISDN card to connect a telephone or your old PBX to it, simulating a line.
Some cards (all the cheap ones with HFC-S chip) can be run in NT mode. However, the connector is hardwired for TE operation so you either need (to make) a crossover adapter to connect a TE to it using a straight cable, or you use a crossover cable to connect them.
To do so the inner and outer pins of the 8P4C connector need to be crossed.

BRI Crossover cable

PRI (S2m)


8P8C modular/
western plug
D-Sub 9
PRIs are always ptp, so nothing special here, really.
As noted above, just use straight Ethernet patch cables.
The maximum cable length for the S2m interface is 250m over Cat2 (or better) cable.

In some (hopefully very) rare occasions you might find some non-standard connectors like a Sub-D, e.g. as listed above, or even two coaxial connectors like BNC or TRS sockets. You might even find PRIs using the BRI pin-out. So if you use rare or ancient equipment, better check the manual.


PRI crossover

PRI hardware can often be operated in both TE and NT mode and often can be jumpered accordingly.
But if there are no jumpers available to care for the situation or maybe you're just too lazy to open the case, you can use a crossover cable instead.
In this case the pair 1-2 has to be crossed with the pair 4-5.

PRI crossover cable


Note: Ethernet crossover cables will never work.


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