X.25 Physical Layer

Physical Layer

The Physical Layer used for X.25 can also be employed by a number of other protocols, such as PPP, Cisco HDLC (cHDLC) or Frame Relay.

There are a number of interfaces that can be used for X.25, such as:

  • V.24 (RS-232)
  • X.21 (X.24/V.11)
  • V.35
  • RS-422
  • RS-530
  • E1

Getting the physical layer right is essential for communication, and is thus the most common cause of problems when getting a new X.25 installation working.

One end of the link must be a DTE and the other a DCE. The normal interface for X.25 terminals and hosts is DTE; FarSync X.25 cards can also provide a DCE interface when an appropriate cross-over cable is used in addition – in this case the FarSync card must generate a clock.


Normally one end of the link (the DCE), and only that end, generates the clock. However, at some combinations of speed and cable length, this can give rise to phase shifting between the clock and the data, as a result of propagation delay. For this reason, it’s sometimes necessary to use Dual-Clocking; when both ends of the link generate the clock to be used for their transmit data circuit, and thus receive the clock for the receive data circuit. This ensures that the clock signal and transmit data circuits remain in phase with each other.

In practice, Dual-Clocking is almost never required for X.25 simply because X.25 links faster than 256kbps are rare.

V.24 or RS-232

V.24, otherwise known as RS-232, is the most common interface for X.25, but is only suitable for low speed links. The V.24 specification covers speeds up to 19.2kbps, but in practice it’s usually possible to use it up to 64kbps on short cables.


X.21 is suitable for much higher speeds than V.24; the FarSync X.25 software can be used with links up to 8Mbps when using X.21.

Note that the interface most commonly referred to as X.21 is only really part of the protocol defined in the ITU (CCITT) X.21 Recommendation – X.21 itself is really a circuit switching protocol. However, circuit switching X.21 is so rare (and indeed now obsolete) that the the term X.21 is most commonly used for the just physical interface. It would perhaps be more accurate (and, in the opinion of the author, less confusing) to refer to the X.21 interface as X.24, but alas that does not happen in practice.

X.21 interfaces typically have a 15-pin D-type connector – male (plug) for the DTE and female (socket) for the DCE. All the circuits are on balanced pairs, with the electrical characteristics of V.11; this gives better noise immunity as high speeds.

X.21 should not be confused with X.21bis – which is effectively the same as V.24.


V.35 is another high-speed interface, using a MRAC-35 connector that looks a bit like a small brick.


RS-422 is electrically the same as V.11, so it is much like X.21.

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X.25 Networking Guide