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time between sending DMs is referred to as Tl, and is decided by the
network administration. This is simply a means of asking: is there anyone
there? When the DTE is eventually connected and operational then it
will respond to one of the DM frames by sending a SABM (Set
Asynchronous Balanced Mode). The SABM frame is an indication that
the DTE wants to bring the link into operation. The DCE will respond
with a UA (Unnumbered Acknowledgement) and the link is then
operational and Info frames can be exchanged. This is shown in Fig. 4.3.

Fig. 4.3 Starting the link

X.25 offers many types of frame and packet for use, but is not always
clear on the procedures for using them. The startup situation is one where
the recommendation does not say precisely what should happen, and
there are therefore some differences and indeed incompatibilities between
implementations.

Some implementations poll DISC frames instead of DM frames when
in the Disconnected Phase. Such implementations require a UA response
to the DISC before a SABM command can be transmitted.

In Disconnected Phase the DCE will respond to frames other than link
initialization commands by sending a DM. This is shown in Fig. 4.4.

The DCE may also attempt to start the link by sending a SABM, though
this is not usual. The response to a SABM may also be a DM rather than
a UA to indicate that the DTE or DCE is unable to operate the link. The
component would then remain in Disconnected Phase.

The link may be initiated by a SARM (Set Asynchronous Response
Mode) rather than a SABM, but use of this mode is declining. This is
discussed further in section 4.2.6.

4.2.2 Stopping the link

One end may want to stop the link, perhaps to perform an orderly
shutdown prior to the component stopping, or in response to a command
given by the site manager. The site manager commands were discussed in
Chapter 2.

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