Appendix C: A Brief ISDN Glossary
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ISDN PC Card – 17 Appendix C: A Brief ISDN Glossary
Layer 7: The application layer. This is the layer at
which communication partners are identified,
quality of service is identified, user authentica-
tion and privacy are considered, and any con-
straints on data syntax are identified. (This layer
is not the application itself, although some appli-
cations may perform application layer functions.)
Layer 6: The presentation layer. This is a layer,
usually part of an operating system, that converts
incoming and outgoing data from one presenta-
tion format to another (for example, from a text
stream into a popup window with the newly ar-
rived text). Sometimes called the syntax layer.
Layer 5: The session layer. This layer sets up, co-
ordinates, and terminates conversations, ex-
changes, and dialogs between the applications at
each end. It deals with session and connection
coordination.
Layer 4: The transport layer. This layer manages
the end-to-end control (for example, determining
whether all packets have arrived) and error-check-
ing. It ensures complete data transfer.
Layer 3: The network layer. This layer handles the
routing of the data (sending outgoing transmis-
sions in the right direction to the right destination
and receiving incoming transmissions at the
packet level). The network layer performs routing
and forwarding.
Layer 2: The data link layer. This layer provides
synchronization for the lower physical layer, per-
forming bit-stuffing for strings of 1’s in excess of 5,
for example. It furnishes transmission protocol
knowledge and management.
Layer 1: The physical layer. This layer conveys a bit
stream through the network at the electrical and
mechanical level. It provides the hardware a
means of sending and receiving data on a carrier.
Glossary
