Technical data

isdnio(7I) Ioctl Requests SunOS 5.5
The CCITT recommendations on ISDN Basic Rate interfaces, I.430, identify several “refer-
ence points” for standardization. From (Stallings89);
“Reference point T (terminal) corresponds to a minimal ISDN network termina-
tion at the customer’s premises. It separates the network provider’s equipment
from the user’s equipment. Reference point S (system) corresponds to the inter-
face of individual ISDN terminals. It separates user terminal equipment from
network-related communications functions. Reference point R (rate) provides a
non-ISDN interface between user equipment that is not ISDN-compatible and
adaptor equipment. ... The final reference point...is reference point U (user).
This interface describes the full-duplex data signal on the subscriber line.”
Some older technology components of some ISDN networks occasionally steal the low
order bit of an ISDN B-channel octet in order to transmit in-band signalinginformation
between switches or other components of the network. Even when out-of-band signaling
has been implementedin these networks, and the in-band signalingis no longer needed,
the bit-robbing mechanism may still be present. This bit robbing behavior does not appre-
ciably affect a voice call, but it will limit the usable bandwidth of a data call to 56000 bits
per second instead of 64000 bits per second. These older network components only seem
to exist in the United States of America, Canada and Japan. ISDN B-channel data calls
that have one end point in the United States, Canada or Japan may be limited to 56000
bps usable bandwidth instead of the normal 64000 bps. Sometimes the ISDN service pro-
vider may be able to supply 56kbps for some calls and 64kbps for other calls. On an inter-
national call, the local ISDN service provider may advertise the call as 64kbps even
though only 56kbps are reliably delivered because of bit-robbing in the foreign ISDN that
is not reported to the local switch.
A Basic Rate Interface implements either a Terminal Equipment (TE) interface or a Net-
work Termination (NT) interface. TE’s can be ISDN telephones, a Group 4 fax, or other
ISDN terminal equipment. A TE connects to an NT in order to gain access to a public or
private ISDN network. A private ISDN network, such as provided by a Private Branch
Exchange (PBX), usually provides access to the public network.
If multi-point configurations are allowed by an NT, it may be possible to connect up to
eight TE’s to a singleNT interface. All of the TE’s in a multipoint configuration share the
same D and B-channels. Contention for B-Channels by multiple TEs is resolved by the
ISDN switch (NT) through signalingprotocols on the D-channel.
Contention for access to the D-channel is managed by a collision detection and priority
mechanism. D-channel call control messages have higher priority than other packets.
This media access function is managed at the physical layer.
A BRI-TE interface may implement a “Q-channel”, the Q-channel is a slow speed, 800
bps, data path from a TE to an NT. Although the structure of the Q-channel is defined in
the I.430 specification, the use of the Q-channel is for further study.
A BRI-NT interface may implement an “S-channel”,the S-channel is a slow speed, 4000
bps, data path from a NT to an TE. The use of the S-channelis for further study.
7I-152 modified 7 Apr 1994