Overall Weight

Think about a radiation sensor you want to install on a manned mission to Mars. The launch function must not only boost every gram of weight into orbit but also send it on its way to another planet. Weight is absolutely critical in these situations. You will encounter other situations as well elsewhere, such as one that you have already considered. Think of the individual dosimeter. You want it worn by individual soldiers, so it should be comparable to the weight of a wristwatch, something you are already accustomed to doing now. So, remember this requirement:

• 8-1 (5-8) The BOSS Individual Radiation Dosimeter shall weigh no more than 4 ounces.

Size

You need to address the height, width, and thickness of the devices, or the radius if spherical or cylindrical, elliptical, or any other geometric shape that is required. For example, if you have a unit dosimeter, it may need to be installed in several different vehicles, like an MRAP (Mine-Resistant Ambush Protected—don’t you just love the convoluted way the military names things?) vehicle, M-1 tank, deuce-and-a-half truck, and AH-1 helicopters (AH = Attack Helicopter), for example. Naturally, you would need to list all the vehicles that need to house these unit radiation dose rate meters. Here we will talk about the dimension requirements:

• 8-2 The BOSS Unit Radiation Dose Rate meter shall be 10 inches high.

• 8-3 The BOSS Unit Radiation Dose Rate meter shall be 4 inches wide.

• 8-4 The BOSS Unit Radiation Dose Rate meter shall be 6 inches thick.

Other elements inside the device may drive the size of the device. Back in the old days, before light-emitting diode (LED) and liquid crystal display (LCD) monitors, tubes were necessary for TV and computer monitors and that constrained their minimum thickness. Radiation sensors have some similar restrictions in their minimum size.

Geometric Shape

What if the device will not be a standard box shape (i.e., nonrectangular)? Then you need to specify what shape it should be. It may even be nonplanar, spherical or cylindrical, elliptical, or any other geometric shape that is required. Think a jet’s wing. How would you specify that? (Yes, very carefully.) You might need some graphics to represent it, as words may be insufficient. You will get to that a bit more when you are exposed to modeling and graphical representations later in the book.

For now, consider the radiation sensor on the manned mission to Mars. You have learned that all you are allowed is a particular spot on a spherical surface area, with the sensor allowed to be no more than 2 inches thick. You should consider something like the following:

• 8-5 The BOSS Mars Radiation Dose Rate meter shall project a square with an inner radius of 10 inches and an outer radius of 12 inches onto the spherical surface with an angle of 30 degrees on the x- and y-axes.

Granted, that is a little involved, but a picture will help to represent it (see Figure 8-1).

Figure 8-1. Sensing area on the surface of Mars

Volume

You may have situations where the three dimensions may not be as important as the total volume. In that case, you might have something like the following:

• 8-6 The BOSS Unit Radiation Dose Rate meter shall be no larger than 30 cubic inches.

The various constraints placed on your particular piece of hardware drive the volume requirement.

Density

Do you notice how these several values, dimensions, weight, volume, density, and center of gravity are related? Of course, they are interdependent. Thus, you will need to consider all of them, potentially.

In certain cases, the density affects things like ability to float in a liquid. Think of getting a rock to float. Yes, they exist (e.g., pumice). In this case, the density needs to be less than 1 gram per cubic centimeter.

Assume you have a requirement for the individual dosimeter to float if it is separated from the soldier.

• 8-7 The BOSS Individual Radiation Dosimeter shall have a density of less than 0.95 g/cm³.

Center of Gravity

For those of you who do not remember, or know, what the center of gravity is, this is the point of the object where the weight can be concentrated for representing it in calculations. You might think it is in the center point of all the dimensions. That may be true if the density of the object is constant. What if it is not? Think of a rod. Assume the density of this rod is 2 Kg/M³ on the left half and 4 Kg/M³ on the right half. If you spun it around the middle point, it would wobble oddly because it is not spinning around the center of gravity. Take two eggs. Hard boil one and let it cool. Now spin each one. Do they spin the same? No. One has the yolk move to from one spot to another so it wobbles, while the hard-boiled one spins more evenly. Why? The hard-boiled one has a pretty uniform density, whereas the raw egg does not.

How does density affect the device? That depends on if it has to move in certain ways. Think of the jet mentioned earlier. Would you want the center of gravity on one of the wings? No, this would cause disastrous effects on the aircraft. Therefore, you should have something like this:

• 8-8 The XF-36 jet fighter shall have its center of gravity along the centerline of the fuselage.

This will not be the only requirement related to this, but this is just to illustrate one example here.

Human Portable

Is this device something that a person will need to carry? If so, that limits the size and weight of a device.

Guidance for carrying equipment for extended periods of time (like backpacking) is to have no more than a third of your weight. Therefore, if you weigh 150 pounds, you should carry only 50 pounds total. The military does not always follow that guidance, sometimes because of mission necessity.

Assume the following:

• 8-9 DRAFT The BOSS Unit Radiation Dosimeter shall weigh no more than forty pounds.

This is much higher than it should be, but it means that an individual of 120 pounds could carry it. It also assumes that they would carry nothing else. If however, their mission profile required them to carry 20 pounds of personal gear, food, and water, then the requirement will be as follows:

• 8-10 The BOSS Unit Radiation Dosimeter shall weigh no more than 20 pounds.

This device is still too large, coupled with the need for the military to carry more than 20 pounds of gear routinely. The weight they must carry now includes the armor they wear to protect against both firepower and explosives.

Safety Features

What particular items should you consider? Maybe you do not want any sharp edges that could hurt someone or could catch on items in its environment.

Here is the safety features example:

• 8-11 The BOSS Unit Radiation Dosimeter shall not cause any electrical discharge to the outside of the device to prevent someone holding the device from being shocked.

Storage

Does your device need any special storage when waiting for shipment? An article was written about the new main battle tanks developed during the 1970s that were left out in the elements of Detroit with no protection from the northern winters. Some of the pieces may not have survived as well as originally anticipated, based on local news media reports.

• 8-12 The BOSS Unit Radiation Dosimeter shall be stored inside a warehouse so that it is not exposed to inclement weather.

Inclement weather may be imprecise. For this exercise, you should assume that a definition was provided earlier in the set of requirements or a glossary that defined what this is. Ideally, it is not raining or snowing inside. As an aside, the building that housed the space shuttle before launch actually had clouds form inside of it.

Packaging, Cooling, Heating, and Integration Constraints

Think of what you might have to do for digital, analog, and power circuits. Think of a satellite, how it is heated when exposed to the sun, and how it is cooled to near absolute zero when in the dark. What kind of insulation is required? How is it heated in the dark and cooled in the sunlight?

These are very specialized requirements, so here you see a modified a statement found in the NASA-GSFC Nano-Satellite Technology Development, SSC98-VI-5, document:

• 8-13 Since the top and bottom of the BOSS spacecraft are insulated, the inside of the cylindrical solar array shall not be insulated allowing internal heat transfer between the internal equipment and the solar array.

Again, this is an example of a specialized situation. While you may not be fully experienced in this technology, as you will be in some cases, you get the gist of the process. (GSFC means Goddard Space Flight Center.) However, if you are developing equipment that operates wherever the U.S. Army might operate from the heat of the desert to the colds of Alaska, hot and cold are environments that must be considered. Thus, while the satellite requirement is more severe, it is really only a wider range, so the requirement may not be so specialized after all.

Power Consumption

What is the power consumption for your hardware? Think of a trip to the Kuiper belt, about 2.8 to 5.1 billion miles away. Even assuming near escape velocity from the solar system, a spacecraft would travel for four to seven years at almost 94,000 miles an hour. Think of the power consumption you must have for that long of a trip (assuming the system achieves the necessary velocity). You need to have the ability to send messages on a periodic basis. What power do you need to transmit from there? That will drive how much power consumption you must have.

Next, consider simpler systems. You might have several subsystems within your entire system, you will need to consider the power consumption of each, and you will need to consider whether they work in different modes, where they may consume different amounts for the various modes. You will need a good mission profile to estimate the frequency of each mode.

For this example, consider one system only.

• 8-14 The BOSS Individual Radiation Dosimeter shall consume 0.01 watts per exposure.

The reason for this type of requirement might be driven by the small battery that might have to last 60 days without recharging or replacement, or some other constraint that the mission places on this. This also indicates that requirements can have some interdependency—in this case, power consumption versus the life of the energy source.

Material

Your environment may put certain constraints on the materials. You might need to specify whether it is plastic, metal, ceramic, or some special material depending on your situation. What if it needs to operate in the containment dome of a nuclear power plant? What kinds of material would you need for nuclear hardening? Think of a satellite in orbit that is exposed to significant changes in temperatures, micro-meteors, and solar flares, to name a few. There are only certain materials built to survive that.

For this example, you should consider something a little less severe.

• 8-15 The BOSS Individual Radiation Dosimeter material shall cause no reaction when exposed to human skin.

If, however, the specific compound causes most soldiers to break out in a rash, the soldiers would be inclined not to wear it and possibly forget it or lose it, defeating the purpose of having it .

Surface Coefficient of Friction

If you are going to have something in motion, you will want to define what the friction is to minimize the impact to the speed, unless you are in outer space, where there is no air to worry about. (Of course, you have other problems, like micro-meteors.)

Of course, there are some instances where you want surface friction. What if your tires have the tread worn off and you are driving in a heavy rainstorm? You will have a hard time gripping the road. Therefore, the amount of surface coefficient of friction is important.

• 8-16 The XF-36 jet fighter tires on its landing gear shall have surface coefficient of friction of 0.7 on dry pavement.

Of course, you would have to define the other coefficients for other conditions, such as wet, rainy, snow, and ice.

Physical Robustness

Physical robustness includes the steps taken to protect your system. Think of the plastic protector you place over the screen of your phone so it doesn’t get scratched. If you wear eyeglasses, do you put a coating on it to protect it from scratching, or even to protect against intense light? Is the watch you wear needed for diving? Then you need it to be waterproof. Or depending on the depth you will dive, you may need it strengthened against certain pressure. What kinds of things must you protect it against? Then identify those elements and write requirements to address them.

If you do need them, find the resident expert to help you craft good requirements as the requirements are very environment dependent.

Reliability

This was talked about in Chapter 5, so we will not discuss it here. You learned about what makes a system reliable, hardware and software, and the learning measurements of reliability. This will not be repeated here; just remember that it does relate to physical characteristics.

Throughput

You will see this in the “Throughput Characteristics ” section in this chapter, not here. The reason for this is because of the importance of throughput.

Do these physical characteristics apply to computer systems? If you are talking about the “box” that contains the computer components, then yes. However, software and operating systems have additional characteristics that you need to consider. This will be presented next.

Physical Computer Characteristics

Remember when every time you picked up a box that contained a software program, you saw somewhere on the box a list of minimum requirements describing what your computer needed to have in order to run the application? Here is a list of physical characteristics that you may need to specify. You should consider some or all of the following characteristics:

• On what microprocessor or microprocessors can the software run?

• How much physical memory (RAM—in this case, random access memory) must you have at a minimum for it to work?

• How much disk storage capacity must you have at a minimum for it to work?

• What devices can it run on? Laptops, desktops, phones, tablets?

• Is it designed to run on a stand-alone machine, or must it work attached to a server or even connected to the Internet (think of World of Warcraft)?

• Must this run on a network ? If yes, then:

  • What kinds are supported?

  • LAN?

  • WAN?

  • Storage area network (SAN)?

  • Metropolitan area network (MAN)?

  • Wireless or wired?

• If this is client-server, what is on the client versus what is on the server?

For a commercial application you could find on your desk, you might find something like this requirement for the sample application:

• 8-17 The BOSS Application shall require the following parameters to run on a system:

  • 500 MHz or faster processer

  • 256 MB of RAM, with 512 MB recommended

  • 3.0 GB available disk space

  • 1024 by 576 resolution monitor or higher

  • Window 7 or Windows Server 2008 or higher

Of course, your requirements will vary, but you get the idea. Plus this gets very dated quickly, as it is just an example for a snapshot in time. Do not criticize how antiquated it may be. The original requirement could be written some years before. However, it does say higher, so it allows for updated requirements.

Throughput

Margaret Rouse, in her “throughput” definition on Tech Target’s Search Networking, defines throughput as the amount of work that a computer can do in a given time period, in computer technology. In data transmission, throughput is the amount of data moved successfully from one place to another in a given time period. When discussing throughput, delays in the passing of such information are important and need to be addressed. This is called latencyand is present as a subsection.

The Open Process Framework (OPF ) web site’s “Throughput Requirements” article gives some of the following examples of throughput requirements:

• “The application shall be able to successfully process a minimum of 150,000 customer orders per day including credit card authorizations under average loads.”

• “The missile avionics system shall update the position of the ailerons 20 times a second.”

• “At least 98% of the time, the application shall be able to successfully display the results of a keyword search in no more than 4 seconds.”

Remember the discussion of performance in Chapter 4; you saw some examples, like this:

• 8-18 (4-32) The FBI BOSS Records Management Search Function shall return the results within 4 seconds, 80% of the time.

• 8-19 (4-41) The FBI BOSS Records Management Search Function shall return the results within 10 seconds, 80% of the time when there are 100 searches initiated within 10 minutes.

These examples show how a given computer can perform; the latter talks about specific transactions. Also, notice that you were provided these examples in different functional areas or topics elsewhere in your requirements areas. This demonstrates that the distinction between boundaries is blurry. It also helps to reiterate that there is no one way to organize the data. Do whatever way works best for you, unless your organization has a specific structure or has it mandated to you by a governing organization like the DoD.

Back to throughput: you will have more requirements besides the areas talked about previously. You will need to address data transmission. This has been done as well with the following example from earlier:

• 8-20 (5-50) The BOSS Unit Radiation Dosimeter shall have the ability to download up to 1000 transactions to the BOSS Dosimetry Archive Laptop in 5 minutes.

IBM, on its Transaction Processing Facility (TPF ) Product Information Center web site, has an article, “System throughput (messages per second),” that defines the number of messages processed over a given interval of time as system throughput. It goes on to say that a business enterprise must identify its projected peak message rate in order to assess whether the system is an appropriate solution. This definition matches the definition in this text and is an important approach to take when defining throughput.

You need to emphasize system throughput for networks. This is an important aspect of WAN and LAN needs. Think of a regional company that does many transactions on a daily basis. Now look at a tool reseller. The corporate headquarters (HQ) buys the tools wholesale from the manufacturers, and each remote location sells the tools to their local customers.

You need to know the size of each record that is affected with each transaction. Assume that 1,000 bytes are captured with each buy and sell. It is possible that large wholesale record sizes are different from selling one tool or a small purchase. You will have to determine that for your situation. However, for simplicity of the example, you will use 1,000 for each.

In this tool example, assume you have 2,400 different tools in stock. On a daily basis, your company restocks 20 tools. Each of 50 sites averages 250 sales a day.

That works out that each LAN at the regional office has 250 sales by 1,000 bytes per sale, or 250,000 bytes sent in.

To get frequency, you need the time aspect also. If each transaction is when the sale is made, then, through the course of nine hours, that is 9 hours by 60 minutes by 60 seconds, or 32,400 seconds. Divide that into 250,000 bytes. That works out to 7.72 bytes per second. Not a very strenuous need.

Now you need to see how much is coming to the BOSS headquarters, the central point of the WAN; 50 times that 7.72 bytes per second is 385.80 bytes per second.

However, what if all the work is sent at 4 p.m. over the course of one hour? The same calculation gives 69.44 bytes per second per office and 3,472.22 bytes per second at HQ. Still that is not a significant value.

What if they were all sent within one minute at 4 p.m.? That gives 4,166.67 bytes per second for each office and 208,333.33 bytes per second. Now you are getting some throughput. What if this was in Southern Africa and you had only 64 Kbps lines? Would that be practical? You say yes, because each line going from the office is less than 64 Kbps. That is true, but you have only one 64 Kbps line coming into the central server. Therefore, it does not work. Now you see where the issue might come in.

This also indicates that you need to see what peak throughput needs are. If HQ mandates the 4 p.m. load, then you must consider that approach. Alternatively, think of an eBay or Amazon amount of throughput. Then you have to figure out what times are peak times. Are they weekday normal working hours? Are they after work? How much does the average transactions go up?

So, what do you write for the requirements ? For your situation, you should consider the following:

• 8-21 The BOSS Tools Transaction Function at each regional office shall transmit 250 sales by 1,000 bytes per transaction daily throughout the day when the transaction occurs.

• 8-22 The BOSS Tools Transaction Function at the central office shall receive all regional office sales throughout the day when the transaction are transmitted.

You could have instead presented this as the actual transmission rates. However, if the sales force doubles or triples the number of tools or through changes in the sales workforce quadruples the number of sales per day by each regional office, it is much easier to update your values rather than recalculating the transmission rates each time. In addition, the values are not necessarily clear to everyone reviewing it to determine whether it is correct just by looking at the following requirement:

• 8-23 DRAFT The BOSS Tools Transaction Function at each regional office shall transmit 69.44 bytes per second per office per day.

While the information may be correct, how would a person know that by looking at it? This reiterates the artistry of requirements definition. There are multiple ways to craft a statement, but not all of them are the best way. Judgment comes into play.

To emphasize how important throughput is, we expanded on the topic here because of its importance. One lesson to remember is that one of the most challenging areas in development of systems is the connection between systems. The throughput necessary to support those connections is instrumental in the successfully communications between them.

Latency

As was introduced in this throughput section, you need to address the delays inherent in the movement of this data. It is important to define whether there are stakeholder restrictions to latency.

Andrew Heim defines latency as the amount of time it takes to complete an operation, according to his white paper “Make it Faster: More Throughput or Less Latency?” You must decide what units of measure time are most useful: milliseconds, microseconds, or nanoseconds.

You saw earlier when you examined search results, where latency was addressed, even though it was not represented in requirements there. Here you should have something like the following:

• 8-18 The FBI BOSS Records Management Search Function shall return the results within 4 seconds, 80% of the time.

• 8-19 The FBI BOSS Records Management Search Function shall return the results within 10 seconds, 80% of the time when there are 100 searches initiated within 10 minutes.

Latency may not be an issue for most cases. Experience on a particular system where, because of the data modeling of the system and an extra commercial off-the-shelf (COTS) package that added a layer between the user interface and the database, the system’s query results could take up to 15 minutes to be returned. If this was a rare occasion, once a month for one or two users, that might be acceptable. However, it happened almost daily for the majority of the users. That latency was unacceptable. Therefore, latency was important in the design of the next system.

Also, think about a nuclear power plant. If an error condition occurred, would you want the response to the operator delayed by seconds or even minutes? No, you want the latency to be almost nonexistent in the situation.

Therefore, determine the latency drivers in your system. The web site article referenced for the previous definition spends some time discussing the comparison and contrast between throughput and latency. If you are going to work on measurement and control systems, you might want to read it.

Exercise 1

Drawing on the “Physical Hardware Characteristics” section of this chapter, write a good mission profile to describe a medic assigned to a unit during a field exercise who needs to carry the BOSS Unit Radiation Dosimeter.

Exercise 2

Look at the performance requirements 4-119 through 4-139 in Chapter 4. How many of those 21 meet the definitions in A) and B) here?

1. Throughput is the amount of work that a computer can do in a given time period, in computer technology.

2. In data transmission, throughput is the amount of data moved successfully from one place to another in a given time period.