by Donn Le Vie, Jr.

Here's the scenario: You're finishing up your latest HTML Help project...no more late nights or weekends...back to a "normal" 50-hour work week. That's when the development team lead strolls into your office and says she just got your manager's okay for you to help the development team "put together the functional requirements specification template for the next major project."

"A what?" you ask with a look of semi-shock. Panic sets in. "What did I do to deserve this? I don't even know where to start! Maybe someone on the TECHWR-L list can help...."

For technical writers who haven't had the experience of designing software requirements specifications (SRSs, also known as software functional specifications or system specifications) templates or even writing SRSs, they might assume that being given the opportunity to do so is either a reward or punishment for something they did (or failed to do) on a previous project. Actually, SRSs are ideal projects for technical writers to be involved with because they lay out the foundation for the development of a new product and for the types of user documentation and media that will be required later in the project development life cycle. It also doesn't hurt that you'd be playing a visible role in contributing to the success of the project.

This article will describe what an SRS is and why it's important, discuss how and why technical writers should be involved with them, and discuss the critical elements for writing an SRS. Although this article does not attempt to address all aspects of developing SRSs, it aims to help you determine the scope for such a project, to provide some guidelines for writing SRSs, and to provide additional resources. Hopefully with this information, you'll not be asking, "Why me?" but proclaiming "Why not me?"

What is a Software Requirements Specification?

An SRS is basically an organization's understanding (in
writing) of a customer or potential client's system
requirements and dependencies at a particular point in
time (usually) prior to any actual design or development
work. It's a two-way insurance policy that assures that both
the client and the organization understand the other's
requirements from that perspective at a given point in
time.

The SRS document itself states in precise and explicit
language those functions and capabilities a software system
(i.e., a software application, an eCommerce Web site, and so
on) must provide, as well as states any required constraints by
which the system must abide. The SRS also functions as a
blueprint for completing a project with as little cost growth
as possible. The SRS is often referred to as the "parent"
document because all subsequent project management documents,
such as design specifications, statements of work, software
architecture specifications, testing and validation plans, and
documentation plans, are related to it.

It's important to note that an SRS contains functional and
nonfunctional requirements only; it doesn't offer design
suggestions, possible solutions to technology or business
issues, or any other information other than what the
development team understands the customer's system requirements
to be.

A well-designed, well-written SRS accomplishes four major
goals:

• It provides feedback to the customer. An SRS is the
  customer's assurance that the development organization
  understands the issues or problems to be solved and the
  software behavior necessary to address those problems.
  Therefore, the SRS should be written in natural language
  (versus a formal language, explained later in this
  article), in an unambiguous manner that may also include
  charts, tables, data flow diagrams, decision tables, and so
  on.
• It decomposes the problem into component parts. The
  simple act of writing down software requirements in a
  well-designed format organizes information, places borders
  around the problem, solidifies ideas, and helps break down
  the problem into its component parts in an orderly
  fashion.
• It serves as an input to the design specification. As
  mentioned previously, the SRS serves as the parent document
  to subsequent documents, such as the software design
  specification and statement of work. Therefore, the SRS
  must contain sufficient detail in the functional system
  requirements so that a design solution can be devised.
• It serves as a product validation check. The SRS also
  serves as the parent document for testing and validation
  strategies that will be applied to the requirements for
  verification.

SRSs are typically developed during the first stages of
"Requirements Development," which is the initial product
development phase in which information is gathered about what
requirements are needed--and not. This information-gathering
stage can include onsite visits, questionnaires, surveys,
interviews, and perhaps a return-on-investment (ROI) analysis
or needs analysis of the customer or client's current business
environment. The actual specification, then, is written after
the requirements have been gathered and analyzed.

Why Should Technical Writers be Involved with Software
Requirements Specifications?

Unfortunately, much of the time, systems architects and
programmers write SRSs with little (if any) help from the
technical communications organization. And when that assistance
is provided, it's often limited to an edit of the final draft
just prior to going out the door. Having technical writers
involved throughout the entire SRS development process can
offer several benefits:

• Technical writers are skilled information gatherers,
  ideal for eliciting and articulating customer requirements.
  The presence of a technical writer on the
  requirements-gathering team helps balance the type and
  amount of information extracted from customers, which can
  help improve the SRS.
• Technical writers can better assess and plan
  documentation projects and better meet customer document
  needs. Working on SRSs provides technical writers with an
  opportunity for learning about customer needs
  firsthand--early in the product development process.
• Technical writers know how to determine the questions
  that are of concern to the user or customer regarding ease
  of use and usability. Technical writers can then take that
  knowledge and apply it not only to the specification and
  documentation development, but also to user interface
  development, to help ensure the UI (User Interface) models
  the customer requirements.
• Technical writers, involved early and often in the
  process, can become an information resource throughout the
  process, rather than an information gatherer at the end of
  the process.

In short, a requirements-gathering team consisting solely of
programmers, product marketers, systems analysts/architects,
and a project manager runs the risk of creating a specification
that may be too heavily loaded with technology-focused or
marketing-focused issues. The presence of a technical writer on
the team helps place at the core of the project those user or
customer requirements that provide more of an overall balance
to the design of the SRS, product, and documentation.

What Kind of Information Should an SRS Include?

You probably will be a member of the SRS team (if not, ask
to be), which means SRS development will be a collaborative
effort for a particular project. In these cases, your company
will have developed SRSs before, so you should have examples
(and, likely, the company's SRS template) to use. But, let's
assume you'll be starting from scratch. Several standards
organizations (including the IEEE) have identified nine topics
that must be addressed when designing and writing an SRS:

1. Interfaces
2. Functional Capabilities
3. Performance Levels
4. Data Structures/Elements
5. Safety
6. Reliability
7. Security/Privacy
8. Quality
9. Constraints and Limitations

But, how do these general topics translate into an SRS
document? What, specifically, does an SRS document include? How
is it structured? And how do you get started? An SRS document
typically includes four ingredients, as discussed in the
following sections:

1. A template
2. A method for identifying requirements and linking
   sources
3. Business operation rules
4. A traceability matrix

Begin with an SRS Template

The first and biggest step to writing an SRS is to select an
existing template that you can fine tune for your
organizational needs (if you don't have one already). There's
not a "standard specification template" for all projects in all
industries because the individual requirements that populate an
SRS are unique not only from company to company, but also from
project to project within any one company. The key is to select
an existing template or specification to begin with, and then
adapt it to meet your needs.

In recommending using existing templates, I'm not advocating
simply copying a template from available resources and using
them as your own; instead, I'm suggesting that you use
available templates as guides for developing your own. It would
be almost impossible to find a specification or specification
template that meets your particular project requirements
exactly. But using other templates as guides is how it's
recommended in the literature on specification development.
Look at what someone else has done, and modify it to fit your
project requirements. (See the sidebar called "Resources for
Model Templates" at the end of this article for resources that
provide sample templates and related information.)

Table 1 shows what a basic SRS outline might look like. This
example is an adaptation and extension of the IEEE Standard
830-1998:

Table 1 A sample of a basic SRS outline

1. Introduction 1.1 Purpose 1.2 Document conventions 1.3 Intended audience 1.4 Additional information 1.5 Contact information/SRS team members 1.6 References 2. Overall Description 2.1 Product perspective 2.2 Product functions 2.3 User classes and characteristics 2.4 Operating environment 2.5 User environment 2.6 Design/implementation constraints 2.7 Assumptions and dependencies 3. External Interface Requirements 3.1 User interfaces 3.2 Hardware interfaces 3.3 Software interfaces 3.4 Communication protocols and interfaces 4. System Features 4.1 System feature A 4.1.1 Description and priority 4.1.2 Action/result 4.1.3 Functional requirements 4.2 System feature B 5. Other Nonfunctional Requirements 5.1 Performance requirements 5.2 Safety requirements 5.3 Security requirements 5.4 Software quality attributes 5.5 Project documentation 5.6 User documentation 6. Other Requirements Appendix A: Terminology/Glossary/Definitions list Appendix B: To be determined

Table 2 shows a more detailed SRS outline, showing the
structure of an SRS template as found on Ken Rigby's
informative Web site at

http://neon.airtime.co.uk/users/wysywig/srs_mt.htm.
Reprinted with permission.

Table 2 A sample of a more detailed SRS outline

1. Scope	1.1 Identification. Identify the system and the software to which this document applies, including, as applicable, identification number(s), title(s), abbreviation(s), version number(s), and release number(s). 1.2 System overview. State the purpose of the system or subsystem to which this document applies. 1.3 Document overview. Summarize the purpose and contents of this document. This document comprises six sections: Scope Referenced documents Requirements Qualification provisions Requirements traceability Notes Describe any security or privacy considerations associated with its use.
2. Referenced Documents	2.1 Project documents. Identify the project management system documents here. 2.2 Other documents. 2.3 Precedence. 2.4 Source of documents.
3. Requirements	This section shall be divided into paragraphs to specify the Computer Software Configuration Item (CSCI) requirements, that is, those characteristics of the CSCI that are conditions for its acceptance. CSCI requirements are software requirements generated to satisfy the system requirements allocated to this CSCI. Each requirement shall be assigned a project-unique identifier to support testing and traceability and shall be stated in such a way that an objective test can be defined for it. 3.1 Required states and modes. 3.2 CSCI capability requirements. 3.3 CSCI external interface requirements. 3.4 CSCI internal interface requirements. 3.5 CSCI internal data requirements. 3.6 Adaptation requirements. 3.7 Safety requirements. 3.8 Security and privacy requirements. 3.9 CSCI environment requirements. 3.10 Computer resource requirements. 3.11 Software quality factors. 3.12 Design and implementation constraints. 3.13 Personnel requirements. 3.14 Training-related requirements. 3.15 Logistics-related requirements. 3.16 Other requirements. 3.17 Packaging requirements. 3.18 Precedence and criticality requirements.
4. Qualification Provisions	To be determined.
5. Requirements Traceability	To be determined.
6. Notes	This section contains information of a general or explanatory nature that may be helpful, but is not mandatory. 6.1 Intended use. This Software Requirements specification shall ... 6.2 Definitions used in this document. Insert here an alphabetic list of definitions and their source if different from the declared sources specified in the "Documentation standard." 6.3 Abbreviations used in this document. Insert here an alphabetic list of the abbreviations and acronyms if not identified in the declared sources specified in the "Documentation Standard." 6.4 Changes from previous issue. Will be "not applicable" for the initial issue. Revisions shall identify the method used to identify changes from the previous issue.

Identify and Link Requirements with Sources

As noted earlier, the SRS serves to define the functional
and nonfunctional requirements of the product. Functional
requirements each have an origin from which they came, be it a
use case (which is used in system analysis to identify,
clarify, and organize system requirements, and consists of a
set of possible sequences of interactions between systems and
users in a particular environment and related to a particular
goal), government regulation, industry standard, or a business
requirement. In developing an SRS, you need to identify these
origins and link them to their corresponding requirements. Such
a practice not only justifies the requirement, but it also
helps assure project stakeholders that frivolous or spurious
requirements are kept out of the specification.

To link requirements with their sources, each requirement
included in the SRS should be labeled with a unique identifier
that can remain valid over time as requirements are added,
deleted, or changed. Such a labeling system helps maintain
change-record integrity while also serving as an identification
system for gathering metrics. You can begin a separate
requirements identification list that ties a requirement
identification (ID) number with a description of the
requirement. Eventually, that requirement ID and description
become part of the SRS itself and then part of the Requirements
Traceability Matrix, discussed in subsequent paragraphs. Table
3 illustrates how these SRS ingredients work together.

Table 3 This sample table identifies requirements and
links them to their sources

ID No.	Paragraph No.	Requirement	Business Rule Source
17	5.1.4.1	Understand/communicate using SMTP protocol	IEEE STD xx-xxxx
18	5.1.4.1	Understand/communicate using POP protocol	IEEE STD xx-xxxx
19	5.1.4.1	Understand/communicate using IMAP protocol	IEEE STD xx-xxxx
20	5.1.4.2	Open at same rate as OE	Use Case Doc 4.5.4

Establish Business Rules for Contingencies and
Responsibilities

"The best-laid plans of mice and men..." begins the famous
saying. It has direct application to writing SRSs because even
the most thought-out requirements are not immune to changes in
industry, market, or government regulations. A top-quality SRS
should include plans for planned and unplanned contingencies,
as well as an explicit definition of the responsibilities of
each party, should a contingency be implemented. Some business
rules are easier to work around than others, when Plan B has to
be invoked. For example, if a customer wants to change a
requirement that is tied to a government regulation, it may not
be ethical and/or legal to be following "the spirit of the
law." Many government regulations, as business rules, simply
don't allow any compromise or "wiggle room." A project manager
may be responsible for ensuring that a government regulation is
followed as it relates to a project requirement; however, if a
contingency is required, then the responsibility for that
requirement may shift from the project manager to a regulatory
attorney. The SRS should anticipate such actions to the
furthest extent possible.

Establish a Requirements Traceability Matrix

The business rules for contingencies and responsibilities
can be defined explicitly within a Requirements Traceability
Matrix (RTM), or contained in a separate document and
referenced in the matrix, as the example in Table 3
illustrates. Such a practice leaves no doubt as to
responsibilities and actions under certain conditions as they
occur during the product-development phase.

The RTM functions as a sort of "chain of custody" document
for requirements and can include pointers to links from
requirements to sources, as well as pointers to business rules.
For example, any given requirement must be traced back to a
specified need, be it a use case, business essential,
industry-recognized standard, or government regulation. As
mentioned previously, linking requirements with sources
minimizes or even eliminates the presence of spurious or
frivolous requirements that lack any justification. The RTM is
another record of mutual understanding, but also helps during
the development phase.

As software design and development proceed, the design
elements and the actual code must be tied back to the
requirement(s) that define them. The RTM is completed as
development progresses; it can't be completed beforehand (see
Table 3).

What Should I Know about Writing an SRS?

Unlike formal language that allows developers and designers
some latitude, the natural language of SRSs must be exact,
without ambiguity, and precise because the design
specification, statement of work, and other project documents
are what drive the development of the final product. That final
product must be tested and validated against the design and
original requirements. Specification language that allows for
interpretation of key requirements will not yield a
satisfactory final product and will likely lead to cost
overruns, extended schedules, and missed deliverable
deadlines.

Table 4 shows the fundamental characteristics of a quality
SRS, which were originally presented at the April 1998 Software
Technology Conference presentation "Doing Requirements Right
the First Time." Reprinted with permission from the Software
Assurance Technology Center at NASA
(http://www.gsfc.nasa.gov/).
These quality characteristics are closely tied to what are
referred to as "indicators of strength and weakness," which
will be defined next.

Table 4 The 10 language quality characteristics of an
SRS

SRS Quality Characteristic	What It Means
Complete	SRS defines precisely all the go-live situations that will be encountered and the system's capability to successfully address them.
Consistent	SRS capability functions and performance levels are compatible, and the required quality features (security, reliability, etc.) do not negate those capability functions. For example, the only electric hedge trimmer that is safe is one that is stored in a box and not connected to any electrical cords or outlets.
Accurate	SRS precisely defines the system's capability in a real-world environment, as well as how it interfaces and interacts with it. This aspect of requirements is a significant problem area for many SRSs.
Modifiable	The logical, hierarchical structure of the SRS should facilitate any necessary modifications (grouping related issues together and separating them from unrelated issues makes the SRS easier to modify).
Ranked	Individual requirements of an SRS are hierarchically arranged according to stability, security, perceived ease/difficulty of implementation, or other parameter that helps in the design of that and subsequent documents.
Testable	An SRS must be stated in such a manner that unambiguous assessment criteria (pass/fail or some quantitative measure) can be derived from the SRS itself.
Traceable	Each requirement in an SRS must be uniquely identified to a source (use case, government requirement, industry standard, etc.)
Unambiguous	SRS must contain requirements statements that can be interpreted in one way only. This is another area that creates significant problems for SRS development because of the use of natural language.
Valid	A valid SRS is one in which all parties and project participants can understand, analyze, accept, or approve it. This is one of the main reasons SRSs are written using natural language.
Verifiable	A verifiable SRS is consistent from one level of abstraction to another. Most attributes of a specification are subjective and a conclusive assessment of quality requires a technical review by domain experts. Using indicators of strength and weakness provide some evidence that preferred attributes are or are not present.

 

What makes an SRS "good?" How do we know when we've written
a "quality" specification? The most obvious answer is that a
quality specification is one that fully addresses all the
customer requirements for a particular product or system.
That's part of the answer. While many quality attributes of an
SRS are subjective, we do need indicators or measures that
provide a sense of how strong or weak the language is in an
SRS. A "strong" SRS is one in which the requirements are
tightly, unambiguously, and precisely defined in such a way
that leaves no other interpretation or meaning to any
individual requirement.

The Goddard Space Flight Center (GSFC) studied dozens of
NASA requirements specifications that revealed nine categories
of SRS quality indicators. The individual components in each
category are words, phrases, and sentence structures that are
related to quality attributes. The nine categories fall into
two classes: those related to individual specification
statements, and those related to the total SRS document. Table
5 summarizes the classes, categories, and components of these
quality indicators. This table was also originally presented at
the April 1998 Software Technology Conference presentation
"Doing Requirements Right the First Time." Reprinted with
permission from the Software Assurance Technology Center at
NASA (http://www.gsfc.nasa.gov/).

Table 5 Quality measures related to individual SRS
statements

Imperatives: Words and phrases that command the presence of some feature, function, or deliverable. They are listed below in decreasing order of strength.
Shall	Used to dictate the provision of a functional capability.
Must or must not	Most often used to establish performance requirement or constraints.
Is required to	Used as an imperative in SRS statements when written in passive voice.
Are applicable	Used to include, by reference, standards, or other documentation as an addition to the requirement being specified.
Responsible for	Used as an imperative in SRSs that are written for systems with pre-defined architectures.
Will	Used to cite things that the operational or development environment is to provide to the capability being specified. For example, The vehicle's exhaust system will power the ABC widget.
Should	Not used often as an imperative in SRS statements; however, when used, the SRS statement always reads weak. Avoid using Should in your SRSs.
Continuances: Phrases that follow an imperative and introduce the specification of requirements at a lower level. There is a correlation with the frequency of use of continuances and SRS organization and structure, up to a point. Excessive use of continuances often indicates a very complex, detailed SRS. The continuances below are listed in decreasing order of use within NASA SRSs. Use continuances in your SRSs, but balance the frequency with the appropriate level of detail called for in the SRS. 1. Below: 2. As follows: 3. Following: 4. Listed: 5. In particular: 6. Support:
Directives: Categories of words and phrases that indicate illustrative information within the SRS. A high ratio of total number of directives to total text line count appears to correlate with how precisely requirements are specified within the SRS. The directives below are listed in decreasing order of occurrence within NASA SRSs. Incorporate the use of directives in your SRSs. 1. Figure 2. Table 3. For example 4. Note
Options: A category of words that provide latitude in satisfying the SRS statements that contain them. This category of words loosens the SRS, reduces the client's control over the final product, and allows for possible cost and schedule risks. You should avoid using them in your SRS. The options below are listed in the order they are found most often in NASA SRSs. 1. Can 2. May 3. Optionally
Weak phrases: A category of clauses that can create uncertainty and multiple/subjective interpretation. The total number of weak phrases found in an SRS indicates the relative ambiguity and incompleteness of an SRS. The weak phrases below are listed alphabetically.
adequate	be able to	easy	provide for
as a minimum	be capable of	effective	timely
as applicable	but not limited to	if possible	tbd
as appropriate	capability of	if practical
at a minimum	capability to	normal
Size: Used to indicate the size of the SRS document, and is the total number of the following: 1. Lines of text 2. Number of imperatives 3. Subjects of SRS statements 4. Paragraphs
Text Structure: Related to the number of statement identifiers found at each hierarchical level of the SRS and indicate the document's organization, consistency, and level of detail. The most detailed NASA SRSs were nine levels deep. High-level SRSs were rarely more than four levels deep. SRSs deemed well organized and a consistent level of detail had text structures resembling pyramids (few level 1 headings but each lower level having more numbered statements than the level above it). Hour-glass-shaped text structures (many level 1 headings, few a mid-levels, and many at lower levels) usually contain a greater amount of introductory and administrative information. Diamond-shaped text structures (pyramid shape followed by decreasing statement counts at levels below the pyramid) indicated that subjects introduced at higher levels were addressed at various levels of detail.
Specification Depth: The number of imperatives found at each of the SRS levels of text structure. These numbers include the count of lower level list items that are introduced at a higher level by an imperative and followed by a continuance. The numbers provide some insight into how much of the Requirements document was included in the SRS, and can indicate how concise the SRS is in specifying the requirements.
Readability Statistics: Measurements of how easily an adult can read and understand the requirements document. Four readability statistics are used (calculated by Microsoft Word). While readability statistics provide a relative quantitative measure, don't sacrifice sufficient technical depth in your SRS for a number. 1. Flesch Reading Ease index 2. Flesch-Kincaid Grade Level index 3. Coleman-Liau Grade Level index 4. Bormuth Grade Level index

Conclusion

There's so much more we could say about requirements and
specifications. Hopefully, this information will help you get
started when you are called upon--or step up--to help the
development team. Writing top-quality requirements
specifications begins with a complete definition of customer
requirements. Coupled with a natural language that incorporates
strength and weakness quality indicators--not to mention the
adoption of a good SRS template--technical communications
professionals well-trained in requirements gathering, template
design, and natural language use are in the best position to
create and add value to such critical project
documentation.

Additional Resources

Brooks, Frederick P. Jr., No Silver Bullet: Essence and
accidents of software engineering, IEEE Computer, vol. 15, no.
1, April 1987, pp. 10-18.

Gause, Donald C., and Weinberg, Gerald M., Exploring
Requirements Quality Before Design, Dorset House Publishing,
NY, NY, 1989.

IEEE Std 830-1993, Recommended Practice for Software
Requirements Specifications, December 2, 1993.

Wiegers, Karl E. Software Requirements, Microsoft Press,
Redmond, WA, 1999.

Resources for Model Templates As previously noted, you should first look for SRS documents developed by your company. Not only are these documents readily available to you, but also they're likely for products that are similar to the product you're developing an SRS for. Additional resources include: IEEE Standard 830-1998 describes an SRS template (Table 1 is an adaptation and extension of this template). Ken Rigby's informative Web site at http://neon.airtime.co.uk/users/wysywig/srs_mt.htm includes sample specification templates and articles about writing various types of specifications. STC's Management Special Interest Group (SIG) offers a Reference Documents Page at http://www.stcsig.org/mgt/reference.htm that includes a sample template and other related resources. Other sites are also available on the Web that offer examples of actual SRSs and templates. A quick search for "software requirements specifications" at your favorite search engine should yield some useful results.

Donn Le Vie, Jr. has helped develop requirements specifications for the hardware and software fields, the earth and space sciences, and the eCommerce industry. Donn works for Intel Corporation's Communications Group, and Wireless Communication and Computing Group, where he provides process development and information engineering support for wireless/mobile products and storage area network projects that use the StrongARM? and XScale? microarchitectures. He has authored more than 60 technical/industry-focused articles, 700 general-interest articles, and two nonfiction books. His latest book, Designing Successful eBusiness Proposals, will be available later this year. He can be reached at
donald.s.levie@intel.com..

Writing and Editing

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