ISDN Datasheet, PDF (4/14 Pages) Texas Instruments – ISDN Basic Rate Interface Software for the HPC16400E High Performance Data Communications Microcontroller

1 0 Architectural Description (Continued)
1 2 3 Coding Standards
1 5 1 Layer 1 I O Driver
The coding standards for software development have been
The Layer 1 I O Driver controls the HPC MICROWIRE
designed to ensure development of consistent structured
PLUS^TM interface and the onboard Serial Decoder This
code which can be easily used and easily maintained over
driver is responsible for the hardware initialization the con-
the life of the code
trol of the Serial Decoder the activation and the deactiva-
1 3 HPC EXECUTIVE
The HPC Executive provides an operating environment for
the Layer 2 and Layer 3 tasks the application tasks the
various support tasks and the I O drivers which interface to
the hardware It provides the following services to the tasks
tion of the Layer 1 I O device Use of the HPC Executive
mail and semaphore services makes this driver simple to
implement and easy to enhance by users that require addi-
tional Layer 1 hardware interfaces
1 5 2 Layer 2 I O Driver
and I O drivers
The Layer 2 I O Driver controls the HDLC DMA controllers
 Scheduling of tasks that are ready to run based on task
onboard the HPC16400 and interfaces this hardware to the
priority Preemptive scheduling and time slicing can be
Layer 2 Task This driver is responsible for the hardware
optionally enabled
initialization the reception of frames toward the HPC the
 Task-task and driver-task communication by means of
mail messages which can be sent and picked up and
semaphores which can be signaled and awaited
 Timers which are equivalent to mail messages with a
specified delay and which allow tasks and drivers to time
their activities and time out when an expected event
does not occur
 Memory management to allocate and deallocate fixed-
te ^{size buffers as needed by tasks and drivers}
Application tasks and I O drivers developed by users of the
HPC can easily be inserted in the HPC Executive environ-
ment and can take full advantage of its services
1 4 ISDN TELECOMMUNICATIONS STANDARDS
1 4 1 CCITT Standards
le ^{The Layer 2 Task implements CCITT 1988 (Blue Book)}
specification Q 921 (LAPD) and Layer 2 (LA B) of CCITT
specification x 25
The Layer 3 Task implements the Protocol Control Proce-
dures of CCITT 1988 Specification Q 931
The Layer 3 Task implements the circuit-switched proce-
dures described in Section 5 The Layer 3 Task implements
o the Protocol Control procedures and some of the Resource
Management The Call Control Task implements a demon-
stration version of the Call Control Procedures and the bal-
ance of the Resource Management
s ^{In terms of the specification and description language (SDL)}
diagrams in the Q 931 specification the Layer 3 Task imple-
ments Figure 38 (26 pages)
The establishment and release of logical links are fully cov-
ered in the Layer 2 specifications (Q 921) but the Layer 3
b ^{aspects of this are not handled in the version of Q 931 on}
which the Layer 3 Task is based Therefore additional Layer
3 states and SDL diagrams have been created and addition-
al software has been written to handle this requirement
1 5 ISDN TELECOMMUNICATIONS SOFTWARE
The software packages described below are designed to be
O ^{easily ‘‘unbundled’’ and used independently by a software}
transmission of frames away from the HPC and appropriate
error handling Use of the HPC Executive mail and sema-
phore services makes this driver simple to implement and
easy to replace with alternative drivers that a user may wish
to develop
1 5 3 Layer 2 Task
The Layer 2 Task implements the full LAPD protocol de-
fined in Q 921 providing error free in-sequence transmis-
sion reception and multiplexing of messages received by
an HDLC controller connected to the D signaling channel
The event-driven state machine architecture described
above enables a single software module to support simulta-
neous activity on multiple logical connections The Layer 2
Task also supports X 25 LAPB processing for messages re-
ceived by a second HDLC controller connected to a bearer
B channel
1 5 4 Layer 3 Task
The Layer 3 Task implements the user side of the Protocol
Control Procedures of Q 931 which are used to setup an-
swer suspend resume and disconnect a call Specifically
it implements all of Figure 2 Q 931 of Q 931 The event-
driven state machine architecture described above en-
ables a single software module to support simultaneous ac-
tivity relating to calls on both bearer B channels
1 5 5 Demonstration Call Control Task
The latest versions of Q 931 separate the Layer 3 proce-
dures into Protocol Control Procedures and Call Control
Procedures Call Control Procedures are application depen-
dent These procedures handle bearer channel selection
and actual establishment of the voice channel As Q 931
notes these procedures can also be considered to be part
of the Applications Layer The Call Control Task implements
a minimal subset of the Call Control Procedures for demon-
stration purposes In an actual application this task will be
replaced by an application-specific task tailored to the ca-
pabilities of the actual terminal equipment (number of termi-
nals handsets etc )
1 5 6 Management Entity Task
developer
The Management Entity Task which is only generically de-
fined in Q 921 and Q 931 handles housekeeping functions
3