LPAD frames are mailed to the Q931 Layer 3 Controller

Task while the LAPB frames are mailed to the X25 Layer 3

Task

The HPC16400 HDLC hardware handles the Layer 2 HDLC

Procedures which includes bit stuffing address recognition

and Frame Check Sequence generation and detection The

Layer 2 Controller Task is responsible for the Layer 2 ‘‘Data

Link Procedure’’ which includes the following functions

To accomplish these functions the Layer 2 Controller sup-

ports the full set of Layer 2 Peer-to-Peer messages defined

in the CCITT Specification Q921 These messages are list-

ed below and defined further in the Software User’s Manual

UI

Unnumbered Information Frames

UA

Unnumbered Acknowledge

SABM(E) Set Asynchronous Balanced Mode (Extended)

DISC

Disconnect Command

DM

Disconnect Mode

I

Acknowledged Information Frames

RR

Receiver Ready

RNR

Receiver Not Ready

REJ

Request Recrimination of Frames

FRMR

Unrecoverable Error Frame Reject

The Layer 2 Controller Task also supports the primitives

required to communicate with the other ISDN tasks

2411 Layer 2 Data Transmission

Layer 2 peer-to-peer Data Transmission is supported with

two modes Unacknowledged Data Mode and Multi-Frame

Acknowledged Data Mode

The Unacknowledged Data

Mode is used primarily for setting up logical connections

and for peer-to-peer Management Entity communication

This mode uses the Unnumbered Information (UI) and the

Unnumbered Acknowledge (UA) messages

The Multi-

Framed Acknowledged Mode is established by the Set

Asynchronous Balanced Mode (SABM) command

This

mode provides the mechanism for acknowledgement of

data frame transport in each direction The Multi-Frame Ac-

knowledged Mode is terminated with the Disconnect (DISC)

command The response to the DISC message can be ei-

ther an Unnumbered Acknowledge (UA) message or a Dis-

connect Mode (DM) message The actual Layer 2 data

frames are transmitted in the Information (I) messages

while in the Multi-Framed Acknowledged Mode

The Layer 2 Controller is responsible for avoiding message

congestion and buffer overflow A Layer 2 entity can issue

the Receive Ready (RR) command to its peer to indicate

that it is ready to continue data transmission Likewise the

Layer 2 Controller can issue the Receiver Not Ready (RNR)

command to its peer to indicate that it is not ready for data

transmission

2412 Layer 2 Protocol Exception Management

The Layer 2 Controller Task is responsible for handling ex-

ceptions to the Data Link Protocol These exceptions are of

two types recoverable and unrecoverable Recoverable ex-

ceptions in the receive direction are typically failed frames

which are handled by requesting the retransmission of the

failed frame with the Reject (REJ) command Recoverable

exceptions in the transmit direction include the expiry of a

Layer 2 Timer Timer expiry requires the retransmission of

the frame that was not acknowledged in time and all subse-

quent frames Timer expiry also prompts a message to the

Management Entity Unrecoverable exceptions result in the

Frame Reject (FRMR) response A message to the Man-

agement Entity Task is also sent in this case

2413 Layer 2 LAPD-Specific Functions

The following Layer 2 Controller Task functions are LAPD

specific These functions involve establishing and maintain-

ing multiple logical data link connections Note that a LAPB

connection will be maintained as a special independent logi-

cal connection

A two byte address is required for each logical data link

This address is referred to as the Data Link Connection

Identifier (DLCI) The DLCI consists of a Service Access

Point Identifier (SAPI) and a Terminal Endpoint Identifier

(TEI) The Layer 2 Controller Task is responsible for sup-

porting the TEI Assignment Procedure and the TEI Verifica-

tion Procedure These procedures are both initiated by the

Management Entity The Layer 2 Controller Task supports

both the Automatic and Non-Automatic TEI Assignment

Procedures

Establishment of the LAPD multi-frame acknowledged data

transmission

mode

requires

an

extended

command

(SABME) to prompt the peer entity that the frames are in-

tended for a particular logical data connection identified by

the accompanying DLCI The Layer 2 Controller Task main-

tains each logical link’s state and data frames independent-

ly as explained earlier in this section

The Layer 3 Controller Task addresses and maintains inde-

pendent logical connections via an identifier called a Con-

nection Endpoint Suffix (CES) Since the CES is different

from the Layer 2 Terminal Endpoint Identifier (TEI) a map-

ping function is required The Layer 2 Controller Task main-

tains a CES – TEI translation procedure to properly address

Layer 3 logical entities

242 Layer 3 Controller Task

The Layer 3 Controller Task implements the application in-

dependent portion of the ISDN NETWORK Layer 3 protocol

per the Q931 CCITT Specification The primary responsibili-

ty of the Layer 3 Controller Task is to establish a network

access connection link between a terminal and its peer in

the Central Office

The Layer 3 Controller Task communicates with both the

Layer 2 Controller Task and the Call Control Task by send-

ing primitives via the System Mail Utilities The Layer 3 Con-

troller Task also communicates with the Management Entity

Task The HPC ISDN Layer 3 Controller Task is responsible

for the following NETWORK functions

8