ISDN Datasheet, PDF (8/14 Pages) Texas Instruments – ISDN Basic Rate Interface Software for the HPC16400E High Performance Data Communications Microcontroller

2 0 Functional Description (Continued)
sages that only have headers In the transmit direction this
layer tasks are provided the Demonstration Call Control
feature allows large contiguous buffers to be broken up into
Task and the Management Entity Task These tasks are
smaller send buffers without having to copy them following
defined in the Application Task section of this document
a header Issues specific to the HDLC DMA Channels are
defined below
The purpose of the Layer 2 Controller Task is to provide the
NETWORK Layer 3 with an error free sequenced data
HDLC DMA Channel 2 the LAPB Channel requires
frame service The Layer 2 Controller Task uses CCITT
frame sizes to be nominally 130 bytes 2 bytes of header
Specifications Q 921 and X 25 and the primary functional
and 128 bytes of information Provision can be made for
specifications The Layer 2 Controller Task satisifies the
messages with up to 1026 bytes 2 bytes header and 1024
Link Access Procedures for both the D Channel and the B
bytes of information
Channel (LAPD and LAPB) Design considerations have
The presentation of data between the Layer 2 Driver and
Layer 2 Controller is identical regardless of which channel
also been included for the future implementation of V 120
the new CCITT rate adaption scheme
the frames are associated with
The Layer 2 Controller Task’s data frame delivery service
2 3 3 Terminal Device Driver and Tracer
allows the Layer 3 Controller Task to confidently setup and
teardown user voice and data calls on the available facili-
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
 Management of the Telephone Simulation User Inter-
face
te  Display Management of the System Trace Mail Mes-
sages
 Proper Display of Task-Related Information
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
le ^{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-
o _{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
s ^{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
b The data structures and hardware interface requirements
for the Terminal Device Driver and capabilities of Tracer
are defined in the Software User’s Manual
2 4 ISDN LAYER PROTOCOL TASKS
The ISDN Layer Protocol Tasks provide implementation of
O ^{the DATA LINK Layer 2 and the NETWORK Layer 3 in ac-}
ties The Layer 3 Controller Task uses CCITT Specification
Q 931 as the primary functional specification Note that the
X 25 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
2 4 1 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
 LAPB per the X 25 CCITT Specification
 LAPD per the Q 921 CCITT Specification
 V 120 Terminal Adaption capability
Since the Q 921 LAPD requirements were derived from the
X 25 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-
cordance with the protocol definitions of the CCITT Specifi-
ture implementation of V 120
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 I O Device Driver section
The Layer 2 Controller Task communicates with the Layer 2
DMA HDLC 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
of this document Implementation of layers above the NET-
WORK Layer 3 are specific to user applications Two such
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