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Figure 5.11. The ISDN protocol reference model.
Control plane protocols provide management services that control user data
services, such as call establishment and termination, transport channel provisioning,
and supplementary service management. User plane protocols transport user data.
The management plane provides traffic management services on the link. Both the
B and D channel information is transported over the same physical transport
medium. Although ISDN is a "transport" mechanism, which traditionally resides at
OSI-RM Layer 1, only the B channels operate exclusively at OSI-RM Layer 1. The
ISDN D channel operates at OSI-RM Layers 1, 2, and 3.
ISDN Access and Reference Points
With a TDM network, there are defined demarcations between the CPE and
LEC/CLEC equipment. Because ISDN is an international standard and the
CPE/telephone company demarcation changes depending on the PSTN system,
ISDN defines four demarcation reference points. These are sometimes referred to
as user premise interfaces (UPI). Along with these demarcation reference points,
ISDN also defines different types of CPE equipment. The type of CPE equipment
generally specifies the type of physical ISDN interface and functions the equipment
will support. First, let's look at the different types of ISDN CPE interfaces and their
functions:
• Terminal Equipment type 2 (TE2)—This is the Data Terminal Equipment
(DTE) that has no direct support for ISDN.
• Terminal Adapter (TA)—This provides ISDN access for TE2 equipment types.
This can either be an external adapter connected by a common interface,
such as an RS232 or V.35 interface port, or an internal BUS card.
• Terminal Equipment type 1 (TE1)—This is an ISDN-compatible device that
can directly support ISDN protocols, interfaces, and management functions.