mitzy miroy wrote:
>
> Can anyone tell me whether the D channel in ISDN uses in-band
> or out-of-band signaling? I'm having a heck of a time finding
> this.
>
> Thanks
It's a strangely worded question. You could say that ISDN uses out-of-band
signaling and that the signaling is carried on the D channel.
The entire D channel is used for signaling in most applications. ISDN Layer
2 signaling is provided by LAPD (Q.921). ISDN Layer 3 signaling is provided
by Q.931. Q.921 establishes a connection with the local switch. Q.931 is
end-to-end. Both of these run on the D channel. In most applications, the
only job of the D channel is to carry signalling.
The D channel provides signaling for the 2 B channels when ISDN BRI is used
and for the 23 B channels for PRI in the U.S. and 30 B channels for PRI in
Europe. The B channels carry user data.
So, from the point of view of the B channels, I guess the D channel
signaling could be considered out-of-band signaling. It could be compared to
the robbed-bit signaling used on T1. In the T1 Superframe format, signaling
is placed in the least significant bit position of every DS0 channel in the
6th and 12th frame of every Superframe. With Extended Superframe (ESF), the
least significant bits of every channel in the 18th and 24th frame are also
borrowed. So, robbed bit signaling could be considered in-band signaling,
and in comparison, signaling on a separate D channel could be conisdered
out-of-band signaling.
But, the D channel doesn't use a different path or require a separate
interface, as is the case in somes uses of the term "out-of-band."
Priscilla
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