The Layer 3 Controller Task supports all the Network Layer

Peer-to-Peer messages defined in the CCITT Specification

Q931 ie

ALERT

Alerting

CALL PROC

Call Proceeding

CONN

Connect

CONN ACK

Connect Acknowledge

INFO

Information

PROG

Progress

SETUP

Setup

SETUP ACK

Setup Acknowledge

DISC

Disconnect

REL

Release

REL COM

Release Complete

RESUME

Resume

RESUME ACK

Resume Acknowledge

RESUME REJ

Resume Reject

SUSPEND

Suspend

SUSPEND ACK

Suspend Acknowledge

SUSPEND REJ

Suspend Reject

NOTIFY

Notify

STATUS

Status

STATUS EN

Status Enquiry

USER INFO

User Information

2421 Call Establishment And Clearing

The Layer 3 Controller Task’s primary responsibility is to

establish and clear user network connections on available

bearer channel facilties The Q931 CCITT Specifications

include Call Establishment and Clearing of both circuit-

switched and packet-switched calls Initially the HPC ISDN

Software Package only supports circuit-switched call proce-

dures on Basic Rate Interface (BRI) Bearer Channels The

Layer 3 Controller Task is responsible for Call Reference

assignment and maintenance The Layer 3 Controller Task

supports Call Establishment using both the Overlap and

Non-Overlap (enbloc) addressing modes

The procedure for establishing and clearing network con-

nections is defined in CCITT Specification Q931 It is impor-

tant to note that the Layer 3 Controller Task maintains an

associated state block for each network connection Primi-

tive mail messages arriving at the Layer 3 Controller Task

will be interrogated to determine which network connection

is to receive the mail The mail message is processed de-

pending on the state of the network connection This pro-

cessing typically includes the transmission of a Primitive to

another Layer Task and the appropriate update of the net-

work connection state block

2422 Call Suspension And Resumption

Call Suspension (SUSPEND) requires that the Bearer Chan-

nel facility and the Call Reference for a call be temporarily

relinquished The network connection is left intact but in the

suspend state The RESUME command reactivates the call

by obtaining a Bearer Channel facility and establishing a

new Call Reference The Suspend function is somewhat

analogous to the call HOLD feature

2423 Call Status And Notification

The Network can request the status of a network connec-

tion at any time via the USER INFO NOTIFY and STATUS

Commands The information includes Service Validation

and Channel Configuration

2424 Layer 3 Protocol Execption Management

The Layer 3 Controller is responsible for handling excep-

tions to the Network Control Protocol The primary Layer 3

Controller Task protocol exception is the expiry of the Layer

3 timer Such an exception requires the retransmission of

the particular command and may prompt a message to the

Management Entity Task

2425 Timer Support

The Layer 3 Controller supports the following system timers

per CCITT Specification Q931

T303

SETUP ACK Timer

T305

DISCONNECT ACK Timer

T308

RELEASE ACK Timer

T313

CONNECT ACK Timer

2426 SDL Updates

The Layer 3 Controller Task very closely follows the SDL

procedures illlustrated in CCITT Specification Q931 with a

few enhancements These enhancements are listed here

and fully defined in the Software User’s Manual

a Three new SDL States have been added to accommo-

date establishing the Data Link corresponding to a partic-

ular CES The new states are

b The Q931 NULLSTATE SDL now accepts a new com-

mand CCBROADCASTRESP This command is sent

from the Call Control Task to allow transistion from the

NULLSTATE(0) to the CALLPRESENT State(6) during a

Network Originated call via the Broadcast mechanism

25 APPLICATION TASKS

The Application Tasks are very dependent on both the ter-

minal equipment configuration and the far-end Network En-

tity software implementation

The HPC ISDN Software

Package includes two sample Application Tasks the Dem-

onstration Call Control Task and the Management Entity

Task Both of these tasks can be replaced or updated when

ported to a particular application These tasks are included

in the HPC ISDN Software Package primarily to verify the

operation of the OSI Layer Protocol Tasks and the HPC

Device Drivers

251 Demonstration Call Control Task

The Demonstration Call Control Task is closely coupled to

the specific facilities of an application The interaction be-

tween the Layer 3 Controller Task and Call Control is de-

fined in CCITT Specification Q931 In the HPC ISDN Appli-

cation the Call Control Task communicates with the Layer 3

Controller Task and the Tracer Task The availability of two

circuit switched voice bearer channels is simulated in the

Call Control Task The Call Control Task sends standard

Terminal Equipment prompts and messages to the Tracer

Task where they are displayed on a UART driven CRT The

Call Control Task has the following responsibilities

9