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L T -Line Termination
Typically, the L T consists of the "U" interface tranceiver
and power feeding functions on the ISDN line card. These
functions must interface to the switch at the "V" reference
point, which is not currently being standardized by CCITI. It
could be a proprietary backplane interface or a nationally
specified interface which would allow the L T to be physically
and electrically separated from the switch.
ISO Layered Protocol Model
The ISO (International Standards Organization) has defined
a 7 layer model structure which describes convenient break
pOints between various parts of the hardware and software
in any data communications system.
Layer 1: Physical layer, that is the hardware which transports
bits across interfaces. This includes ISDN transceivers,
modems etc., power supplies, methods of activating
and de-activating a transmission link, and also the transmission
medium itself, such as wire, fiber, plugs and sockets,
etc.
Layer 2: Data Link layer, which describes a basic framing
structure and bit assignments to enable higher layer messages
to be passed across a physical link. HOLC framing,
addressing and error control are the major elements of this
layer in ISDN.
Layer 3: Network layer, that is those parts of a message
associated with setting-up, controlling and tearing-down a
call through the network. These are all software control
functions, and generally this is the highest layer in the ISO
protocol model which is considered in chip development.
The top 4 layers relate to the structure of the actual application
programs;
Layer 4: Transport layer, concerned with defining sources
and destinations within an operating system for the transfer
of application programs.
Layer 5: Session layer.
Layer 6: Presentation layer.
Layer 7: Application layer.
These layers are generally running on a high level machine,
and discussion regarding this machine is outside the scope
of this document.
LAPD
Link Access Protocol in the "0" channel is the name given
to the packet-mode Signaling protocol defined in CCITI
specs 0920 and 0921 for the data link layer (layer 2) and
0930 and 0931 for the network layer (layer 3 in the ISO 7
layer reference model). At layer 2, LAPO uses the HOLC
framing format. This protocol defines the bits, bytes and sequence
of states necessary between the user and the network
to establish, control and terminate calls using any of
the 100 or more types of services which may be available
via an ISDN. If the users at both ends of the call are connected
to the ISDN and there is a through path for the 0
channel then end-to-end call control is available.
Because of this extensive range of services, implementation
of full LAPO requires considerable memory and processing
power. Standards work has recently focused on definition of
a minimal subset of LAPO to cover the basic requirements
of call control.
Activation/De-activation
Activation is the process of powering up the'S' and 'U'
interfaces from their standby (i.e. de-activated) states and
sending specific signals across the interfaces to get the
whole loop synchronized to the network. A small state machine
in each TE and the NT controls this sequence of
events, and uses timers to ensure that, if the activation attempt
should fail for any reason, the user or network is alerted.
At the end of a call an orderly exit from the network is
effected by sending de-activation sequences before any
equipment can power-down.
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