Nortel Meridian Meridian 1 Feature Description And Administration page 17

These protocols are based on a model containing seven layers of protocol
developed by the International Standards Organization (ISO). This
seven-layer model, called the Open Systems Interconnection model, has been
adopted by the International Telegraph and Telephony Consultative
Committee (CCITT). It is the basis for building protocol structures for ISDN
service. ISDN currently uses four of the seven layers:
• layer 1—physical layer
• layer 2—link layer
• layer 3—network layer
• layer 7—application layer
Each layer uses the series of services provided by the layer beneath it, and
builds on these services to perform a communications function for the layer
above. For example, layer two builds on the services from layer one and
provides the combined services to layer three. This layered approach splits
the complex protocols into a series of easily managed blocks, each of which
can be modified without affecting protocols in another layer.
Layers one through three control the set-up of connections. These layers also
supervise the transmission of information between terminals and the packet
-switched and circuit-switched networks.
The PRA protocol layers are implemented on Meridian 1 and Succession
CSE 1000 as follows:
• Layer 1 (the Physical Layer) is handled by the Primary Rate Interface
(PRI) card.
• Layer 2 (the Link Layer) is handled by the D-channel Interface.
• Layer 3 (the Network Layer) is handled by Meridian 1 or Succession
CSE 1000 software.
• Layer 7 (the Application Layer) is also handled by Meridian 1 or
Succession CSE 1000 software.
The Application Layer uses the Transaction Capability Application Part
(TCAP) and the Remote Operation Service Element (ROSE) to process
applications. The transport of applications has three main parts:
• a simple interface allowing applications to send and receive data
International ISDN PRI Feature description and administration
ISDN product overview Page 17 of 1814