In developing the first version of a generic implementation of X.25, Levels 2 and 3, three development techniques are examined: table driven finite state machine implementation, an integrated testing environment, and top down design. A finite state machine (FSM) is a powerful tool for both specifying and implementing protocols. This technique was used in the X.25 specification. A table driven implementation of the FSM was chosen to facilitate changes and simplify coding. The effect that this technique have on program size, speed of execution, coding time, and debugging time are investigated

Hamilton, R. L.

English

NASA Technical Reports Server

Development Techniques for Generic Software
Richard L. Hamilton
Bell Laboratories
Holmdel, New Jersey 07733
1. INTRODUCTION
In developing the first version of a generic implementation of X.25,
Levels 2 and 3, we examined three development techniques: table-driven
finite state machine implementation, an integrated testing environment,
and top-down design. While not designed as an experiment, we monitored
the project closely and compared the product with other implementations
of X.25 at Bell Laboratories to evaluate potential benefits and
penalties.
2. TECHNIQUES
2.1 Finite State Machine
A finite state machine (FSM) is a powerful tool for both specifying and
implementing protocols. This technique was used in the X.25
specification and has been discussed in the literature[1,2,3,4]. A
table-driven implementation of the FSM was chosen to facilitate changes
and simplify coding. We were interested in what effect this technique
would have on program size, speed of execution, coding time, and
debugging time.
2.2 Testing Environment
Contrary to common practice, we made a testing environment before
coding. The complexities of a communications protocol, especially
X.25, require careful attention to the problems of verifying that an
R. Hamilton
Ben Labs
I o f 2 0
https://ntrs.nasa.gov/search.jsp?R=19820016145 2020-03-21T08:59:09+00:00Z
implementation of that protocol does in fact perform correctly. In
addition, we felt that the process of verification should start as
early as possible in the development process. The testing environment,
which runs under the UNIX* operating system, let us test the FSM and
its tables very early in the coding process. We were able to integrate
new modules easily and test them thoroughly using this tool.
2.3 Top Down Design
In designing and implementing a solution, we followed a top-down
approach. This made it possible to have a "running" version at all
times, with unwritten modules replaced by dummy routines. This was not
rigorously followed in coding because it was often more sensible to
code all of the routines that performed one function even if that meant
coding some low-level functions early. Doing this still let us always
have a running version, but simplified testing.
3. MEASUREMENTS
Our main method for evaluating these techniques was comparison with
existing implementations of X.25 at Bell Laboratories. We measured the
size and execution speed of both our implementation and the existing
ones and ran some simple complexity metrics.
* UNIX is a Trademark of Bell Laboratories
R. Hamilton
Bell Labs
2 of 20
We used the testing environment to help modify and transport existing
implementations of both Level 2 and Level 3 to a new environment, which
gave us the opportunity to compare our versions with the existing ones
in terms of the ease of making modifications. We kept a log of program
bugs found and the effort it took to fix them, for all of the
implementations, and tried to characterize the types of problems found.
4. CONCLUSION
A combination of a table-driven finite state machine realization, a
comprehensive testing environment, and a top-down approach was used to
produce an implementation of X.25, Levels 2 and 3. In comparison with
other, ad hoc, X.25 implementations, we found that our solution ran as
much as 20% faster, but was about 35 to 40 percent bigger. We were
able to explain all but 11% of that difference in terms of added
function or added flexibility. A McCabe complexity metric showed
little difference between the implementations.
Comparison of time spent debugging showed that our approach was
superior to the ad hoc methods, both in terms of number of errors
detected and time taken to correct those errors. Even so, the testing
environment was shown to be a significant aid in debugging the other
implementations when compared to other testing techniques. Although
not intended as a controlled experiment, the data collected during
development support using these techniques in similar circumstances.
R. Hamilton
Bell Labs
3 of 20
REFERENCES
[1] Bochmann, Gregor V., "A General Transition Model for Protocols
and Communication Services," IEEE Transactions on Communications,
vol. COM-28, no. A, April 1980.
[2] Bochmann, Gregor V. and Tankoano Joachim, "Development and
Structure of an X.25 Implementation," IEEE Transactions on
Software Engineering, vol. SE-5, no. 5, September 1979.
[3] Bochmann, Gregor V. and Carl A. Sunshine, "Formal Methods in ,
Communication Protocol Design," IEEE Transactions on
Communications, vol. COM-28, no. A, April 1980.
[A] Danthine, Andre A. S., "Protocol Representation with Finite-State
Models," IEEE Transactions on Communications, vol. COM-28, no. A,
April 1980.
R. Hamilton
Bell Labs
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Development techniques for generic software

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