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Notes: SFS/TN/005ilter Study
Names: »3783A «
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SFS/TN/005 1983-05-30
INTERFACE FLEXIBILITY Page #
T̲A̲B̲L̲E̲ ̲O̲F̲ ̲C̲O̲N̲T̲E̲N̲T̲S̲
Page
1 GENERAL .......................................
2
1.1 INTRODUCTION ...............................
2
1.2 REFERENCES .................................
2
2 ANALYSIS ......................................
3
3 HARDWARE ......................................
3
3.1 ELECTRICAL CHARACTERISTICS .................
3
3.2 FRAME FORMATS ..............................
5
4 SOFTWARE .......................................
5
1̲ ̲ ̲G̲E̲N̲E̲R̲A̲L̲
1.1 I̲N̲T̲R̲O̲D̲U̲C̲T̲I̲O̲N̲
This technical note represents the output of work package
330, Interface flexibility, within the framework of
the Security Filter Study, performed under contract
no. FK8219 between the Air Materiel Command of the
RDAF and Christian Rovsing A/S.
The aim of this technical note is to clarify the functional
participation between common and interface specific
hardware and software for the purpose of optimising
the filter specification in terms of interface flexibility.
The term "interface" covers here the electrical and
functional characteristics within the limits of level
1 to 3 as described below:
Level 1: The physical, electrical, functional and
procedural characteristics to establish,
maintain and disconnect the physical link.
Level 2: The link access procedure for data interchange
across the link.
Level 3: The packet format and control procedures
for the exchange of packets containing
control information and user data.
1.2 R̲E̲F̲E̲R̲E̲N̲C̲E̲S̲
This technical note is based on the preliminary Line
Terminator Specification given in Technical note no.
4:
Configuration Study, section 2.
2̲ ̲ ̲A̲N̲A̲L̲Y̲S̲I̲S̲
The interface to the communication lines is provided
by the Line Terminator. This module performs all interface
specific functions and is therefore the only module
which is affected by the particular electrical specifi-
cations and the particular protocol and procedures
used for the communication.
Considering the design of the Line Terminator (see
figure overleaf) the characteristics of level 1 is
implemented by the serial interface controller.
The higher levels of the interface is handled by the
microprocessor i.e. by firmware.
3̲ ̲ ̲H̲A̲R̲D̲W̲A̲R̲E̲
3.1 E̲L̲E̲C̲T̲R̲I̲C̲A̲L̲ ̲C̲H̲A̲R̲A̲C̲T̲E̲R̲I̲S̲T̲I̲C̲S̲
The electrical characteristics are determined by the
electrical line interface of Line Terminator being
part of the box called Electrical Protection. The X.25
recommendation specifies electrical characteristics
according to recommendation V.11 which describes a
balanced double-current interchange circuit.
It can be foreseen, however, that other electrical
characteristics should be optional in order to ease
integration with existing equipment. Obvious options
are:
o V.10 characteristics (unbalanced)
o MIL-STD-188C characteristics (low level)
These options (strap-selectable) can be incorporated
into the design at virtually no additional cost.
Figure
3.2 F̲R̲A̲M̲E̲ ̲F̲O̲R̲M̲A̲T̲S̲
The baseline frame format is the format specified in
the X.25 recommendation. There are available micro-
circuits which are directly suited for this application,
and they are even programmable to accommodate a large
class of formats. They do, however, not directly accommodate
e.g. the Link 1 format.
This particular problem can be overcome e.g by a separate,
very simple adaptor in front of the Line Terminator
which performs the frame synchronisation and provides
the Line Terminator with the synchronisation pulse.
With this adaptor-concept, virtually all types of byte-oriented
frame formats can be accommodated.
4̲ ̲ ̲S̲O̲F̲T̲W̲A̲R̲E̲
The firmware of the Line Terminator is significantly
affected by the particular protocol for the communication
line being serviced.
This means that specific firmware must be used for
e.g. X.25 lines and Link 1 lines.
