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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.
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.
|8P8C or 8P4C|
|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)|
of the NT
|Max # of|
|Max # of TEs|
|Short Passive Bus||120m (120nf/km) /|
on the bus
|anywhere on the bus||12||8|
|Extended Passive Bus||450m (120nf/km) /|
|at one end||within 25m (120nf/km) /|
of the opposite end
|Long Passive Bus|
|600m (120nf/km) /|
|at one end||the opposite end||2||1|
BRI patch cablesBRI 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 crossoverIn 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
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 crossoverPRI 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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