123 Coding Standards

The coding standards for software development have been

designed to ensure development of consistent structured

code which can be easily used and easily maintained over

the life of the code

13 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 IO drivers which interface to

the hardware It provides the following services to the tasks

and IO drivers

priority Preemptive scheduling and time slicing can be

optionally enabled

mail messages which can be sent and picked up and

semaphores which can be signaled and awaited

specified delay and which allow tasks and drivers to time

their activities and time out when an expected event

does not occur

size buffers as needed by tasks and drivers

Application tasks and IO 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

14 ISDN TELECOMMUNICATIONS STANDARDS

141 CCITT Standards

The Layer 2 Task implements CCITT 1988 (Blue Book)

specification Q921 (LAPD) and Layer 2 (LA B) of CCITT

specification x25

The Layer 3 Task implements the Protocol Control Proce-

dures of CCITT 1988 Specification Q931

The Layer 3 Task implements the circuit-switched proce-

dures described in Section 5 The Layer 3 Task implements

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

In terms of the specification and description language (SDL)

diagrams in the Q931 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 (Q921) but the Layer 3

aspects of this are not handled in the version of Q931 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

15 ISDN TELECOMMUNICATIONS SOFTWARE

The software packages described below are designed to be

easily ‘‘unbundled’’ and used independently by a software

developer

151 Layer 1 IO Driver

The Layer 1 IO Driver controls the HPC MICROWIRE

driver is responsible for the hardware initialization the con-

trol of the Serial Decoder the activation and the deactiva-

tion of the Layer 1 IO 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

152 Layer 2 IO Driver

The Layer 2 IO Driver controls the HDLCDMA controllers

onboard the HPC16400 and interfaces this hardware to the

Layer 2 Task This driver is responsible for the hardware

initialization the reception of frames toward the HPC the

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

153 Layer 2 Task

The Layer 2 Task implements the full LAPD protocol de-

fined in Q921 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 X25 LAPB processing for messages re-

ceived by a second HDLC controller connected to a bearer

B channel

154 Layer 3 Task

The Layer 3 Task implements the user side of the Protocol

Control Procedures of Q931 which are used to setup an-

swer suspend resume and disconnect a call Specifically

it implements all of

Figure 2 Q931 of Q931 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

155 Demonstration Call Control Task

The latest versions of Q931 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 Q931

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)

156 Management Entity Task

The Management Entity Task which is only generically de-

fined in Q921 and Q931 handles housekeeping functions

3