sages that only have headers In the transmit direction this

feature allows large contiguous buffers to be broken up into

smaller send buffers without having to copy them following

a header Issues specific to the HDLCDMA Channels are

defined below

HDLCDMA Channel

2

the LAPB Channel

requires

frame sizes to be nominally 130 bytes 2 bytes of header

and 128 bytes of information Provision can be made for

messages with up to 1026 bytes 2 bytes header and 1024

bytes of information

The presentation of data between the Layer 2 Driver and

Layer 2 Controller is identical regardless of which channel

the frames are associated with

233 Terminal Device Driver and Tracer

The Terminal Device Driver interfaces to the HPC onboard

UART The associated SRQ Driver Task referred to as Tra-

cer serves primarily as a high-level demonstration vehicle

Tracer can field mail messages from any other task in the

system as well as keystroke mail messages from its accom-

panying ISR Tracer’s responsibilities include the following

functions

face

sages

The Telephone Simulation function of Tracer allows the

user to enter ‘‘telephony-like’’ keystroke characters that are

passed to Tracer then on to the ISDN layer tasks for pro-

cessing Menu responses are fielded by Tracer to select

various levels of the Trace function as well as to enter and

exit the Telephone Simulation mode

Depending on the level of trace that is selected Tracer re-

ceives mail messages from the system tasks and properly

formats them on the CRT display Tracer offers various lev-

els of trace capability Trace can be turned off all together

in which case only the application layer Telephone Simula-

tion inputs will be displayed Trace can display all messages

from every layer or it can be set to ‘‘zoom’’ to display only

the messages at a particular layer Messages will generally

have address fields and data fields

The Terminal Driver’s Interrupt Service Routine (ISR) han-

dles keyboard characters from the UART and mails them to

the Tracer SRQ Task for further processing The ISR also

handles transmission completion of a character that has

been sent to the CRT

The data structures and hardware interface requirements

for the Terminal Device Driver and capabilities of Tracer

are defined in the Software User’s Manual

24 ISDN LAYER PROTOCOL TASKS

The ISDN Layer Protocol Tasks provide implementation of

the DATA LINK Layer 2 and the NETWORK Layer 3 in ac-

cordance with the protocol definitions of the CCITT Specifi-

cations The two Layer Protocol Tasks (the Layer 2 Control-

ler Task and the Layer 3 Controller Task) are designed to

satisy the ISDN Basic Rate Interface (BRI) Terminal Equip-

ment requirements They are independent of user applica-

tions and hardware environment The PHYSICAL Layer 1

implementation is defined in the IO Device Driver section

of this document Implementation of layers above the NET-

WORK Layer 3 are specific to user applications Two such

layer tasks are provided the Demonstration Call Control

Task and the Management Entity Task These tasks are

defined in the Application Task section of this document

The purpose of the Layer 2 Controller Task is to provide the

NETWORK Layer 3 with an error free sequenced data

frame service The Layer 2 Controller Task uses CCITT

Specifications Q921 and X25 and the primary functional

specifications The Layer 2 Controller Task satisifies the

Link Access Procedures for both the D Channel and the B

Channel (LAPD and LAPB) Design considerations have

also been included for the future implementation of V120

the new CCITT rate adaption scheme

The Layer 2 Controller Task’s data frame delivery service

allows the Layer 3 Controller Task to confidently setup and

teardown user voice and data calls on the available facili-

ties The Layer 3 Controller Task uses CCITT Specification

Q931 as the primary functional specification Note that the

X25 Layer 3 packet processor task is not included in the

initial software package

The Layer Protocol Tasks require a somewhat non-conven-

tional task architecture in order to simultaneously manage a

significant number of multiple logical connections This

event-driven state-machine architecture requires that a

state memory block be created and maintained for each

logical connection When a Layer Protocol Task ‘‘wakes up’’

due to the arrival of mail the message’s address is interro-

gated to determine which logical connection is to receive

the mail The particular logical connection’s state block is

retrieved and the mail message is processed per the CCITT

Specification requirements depending on the state of the

particular logical connection Typically processing the mail

message results in sending a Primitive message to another

task and updating the logical connection’s state block The

Layer Protocol Task then returns to its mail box to pick up

any subsequent mail

The interface between all of the ISDN Layer Tasks is delib-

erately achieved via the System Mail Utilities This ensures

a distinct uniform layering mechanism in the event that ap-

plication programmers wish to replace layers with their own

implementations

241 Layer 2 Controller Task

The primary job of the ISDN Data Link Layer 2 is to deliver

error-free sequenced data frames to the Network Layer 3

The Layer 2 Controller Task implements the following Layer

2 Link Access Procedures (LAP) for the HPC ISDN Software

Package

Since the Q921 LAPD requirements were derived from the

X25 LAPB requirements most of the same Layer 2 Control-

ler Task routines can be used to implement both LAPB and

LAPD Design considerations have been made to allow fu-

ture implementation of V120

The Layer 2 Controller Task communicates with the Layer 2

DMAHDLC Controller Device Driver Task and the Manage-

ment Entity Task via the System Mail Utilities These tasks

interrogate the mail message headers to determine whether

to process the frames using LAPB or LAPD procedures The

7