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New Member

X21 and V24 termination - what are the differences?


can anyone explain (in as simple terms as possible!) about X21 and V24 standards? Essentially what I’m getting at is – when would you use X21 and why, and when would you use V24 and why?

We have a number of existing leased lines to our remote offices (speeds of 64K to 2M) which terminate on X21 interfaces (15 pin connectors)

As a temporary workaround for a new project, we need a 9.6K circuit (into a bank's network). The question is, can we use our existing infrastructure and have this terminate on an X21 interface or is there a reason why it should terminate on a v24 interface (25pin connector)?

The service provider seem to be suggesting that the termination for a 9.6k line will be V24 but we are having difficulty in getting them to explain why this should be and/or if there is the possibility to have the link terminate on our existing infrastructure (i.e. using X21) instead.

If anyone could clarify X21 and V24 termination for me, I'd be grateful!

New Member

Re: X21 and V24 termination - what are the differences?

The the V series of specifications relate to analogue technology and the X series relate to digital.

before digital exchanges you would use analogue leased lines , BT would sell eps42 analogue multidrop and eps 21 analogue point to point. These circuits would run at 9.6k bps using modems. the modems would have a V24 interface.

Along comes digital, more reliable circuits . BT would offer 9.6k 48k and 64k circuits these would be terminated on v24 v35 and x21.

So a customer could transfer from 9.6 analogue to a digital circuit by moving the v24 cable from a modem to NTU.A 9.6 k digital circuit would still occupy a 64k timeslot, this works by taken 6 bits of customer data and adding an alignment bit(alternating between zero and one) and a status bit( one for transmit).This would give 12.8 k, this would then be reiterated 5 times to give 64k.

the 48k circuit would also have an alignment and status bits added to fill up the 64 timelsot.

2m circuits used to provided on g703 termination, but changed to x21 in the late 90's.

V24 is unbalanced and uses 1 cable for xmit and 1 for recieve , they reference ground pin 7 .

x21 is balanced and uses 2 cables for xmit and 2 for recv and they reference each other.

A good book on layer 1 stuff is

ISDN Explained: Worldwide Network and Applications Technology, 3rd Edition

John M. Griffiths

ISBN: 0-471-97905-8


306 pages

March 1998

New Member

Re: X21 and V24 termination - what are the differences?

Thanks, that info is very helpful.

But what I still don't fully understand is - our existing circuits terminate on X21.

For the new 9.6k circuit, it will terminate on V24 but still connect to the same NTU as our existing circuits?

If that is the case, can they not just terminate it on an X21 connection as with our existing circuits? (Then we can use our existing cabling to our Cisco router)

Or am I missing the point somewhere?