Appendix C. A brief ISDN glossary 227
defines seven layers of communication functions that take place at each end of
a communication link. Although the OSI model is not always clearly
organized in the same well-defined layers, many if not most products
involved in telecommunication can be described in relation to the OSI model.
It is also valuable as a uniform reference that furnishes a common ground for
education and discussion in the communications field. Its seven layers are:
— Layer 7: The application layer. This is the layer at which communication
partners are identified, quality of service is identified, user authentication
and privacy are considered, and any constraints on data syntax are
identified. (This layer is not the application itself, although some
applications can perform 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 presentation
format to another (for example, from a text stream into a popup window
with the newly arrived text). Sometimes called the syntax layer.
— Layer 5: The session layer. This layer sets up, coordinates, and terminates
conversations, exchanges, 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 if all packets have arrived) and error checking.
It ensures complete data transfer.
— Layer 3: The network layer. This layer handles the routing of the data
(sending outgoing transmissions in the right direction to the right
destination and receiving 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, performing bit-stuffing for strings of 1s 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.
• PPP - The Point-to-Point Protocol. A TCP/IP protocol used in the dial-up
networking environment. PPP works on the OSI layers 1 and 2. It is a full-
duples protocol that can be used on various physical media, including twisted
pair or fiber optic lines, or satellite links. It uses a variant of High Speed Data
Link Control (HDLC) for packet encapsulation.
• PPTP - POint-to-Point Tunneling Protocol. A variation of the PPP protocol. It
is a protocol (set of communication rules) that allows corporations to extend
their own corporate network through private "tunnels" over the public
Internet. In effect, a corporation uses a wide-area network as a single large
local-area network. Companies no longer need to lease their own lines for
wide-area communication, but can securely use the public Internet. This kind
of interconnection is known as a virtual private network (VPN).
• PRI - See Primary rate interface.
• Primary rate interface - Access to the ISDN network comes in two different
flavors, known as basic-rate and primary-rate access. The primary-rate
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