ISDN Datasheet, PDF (9/14 Pages) Texas Instruments – ISDN Basic Rate Interface Software for the HPC16400E High Performance Data Communications Microcontroller

2 0 Functional Description (Continued)
LPAD frames are mailed to the Q 931 Layer 3 Controller
two types recoverable and unrecoverable Recoverable ex-
Task while the LAPB frames are mailed to the X 25 Layer 3
ceptions in the receive direction are typically failed frames
Task
which are handled by requesting the retransmission of the
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
 Data Transmission
 Protocol Exception Management
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
 LAPD-Specific Functions
2 4 1 3 Layer 2 LAPD-Specific Functions
To accomplish these functions the Layer 2 Controller sup-
ports the full set of Layer 2 Peer-to-Peer messages defined
The following Layer 2 Controller Task functions are LAPD
in the CCITT Specification Q 921 These messages are list-
specific These functions involve establishing and maintain-
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
te ^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
2 4 1 1 Layer 2 Data Transmission
le ^{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
o ^{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-
s 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
b ^{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)
O ^{command to its peer to indicate that it is not ready for data}
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
2 4 2 Layer 3 Controller Task
The Layer 3 Controller Task implements the application in-
dependent portion of the ISDN NETWORK Layer 3 protocol
per the Q 931 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
transmission
for the following NETWORK functions
2 4 1 2 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
 Call Establishment and Clearing
 Call Suspension and Resumption
 Call Status and Notification
 Protocol Exception Management
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