System Technical Performance

A system needs to be analyzed by a hierarchal evaluation technique, since no one system will provide a total solution in the Coast Guard’s dynamic environment.

System flexibility is intended to represent a system’s ability to support the full dynamic range of Coast Guard missions and environments. The “ability to support” can be characterized by the following twelve characteristics:

a. Coverage

Coverage is the geographic area in which a mobile user has access to the satellite system. Coverage can also be defined more stringently as the ability to focus required satellite capabilities when and where they are needed. The vendor typically provides a coverage diagram of

the service area. This could be a map or chart showing the geographic area in which the system operates. This area is composed of the “footprint(s)” of the satellite or satellites that make up the system. Exact coverage areas can be calculated using software and verified by field tests. [Ref. 8, p.7] The figure below shows the basic difference in footprint coverage for the three different satellite orbits.

Figure 2. Representative Satellite Coverage Patterns [From Ref. 2]

b. Accuracy

Accuracy is a measure of the absence of error.

Examples of accuracy in a voice or data system would be:

Can you understand what the person is saying? Do you recognize his/her voice? Is the data sent on one end of the system the same as the data received at the other end?

What is the Bit Error Rate (BER) for the system? These

parameters can all be measured in the lab or in the field.

[Ref. 9, p.7]

c. Availability

Availability is the amount of unit time on any give day that the system is available for use. Reasons for non-availability could include: the traffic exceeds the capacity of the system, the system is temporarily out of service, or a satellite is not in view. Failure of user equipment would not be a reason for system non-availability. Prediction of the number of satellites in view and system availability can be calculated using satellite software programs, and can also be tested in the field or in labs. [Ref. 8, p.7] The C2BST has agreed that the Coast Guard would use 99.7% availability as the baseline requirement for connectivity. [Ref. 5, p.14] This means that the Coast Guard will accept 65.7 hours, or approximately 2.7 days, per year of unscheduled down time.

The authors believe that a more stringent requirement of at least 99.9% be required. Doing this would require an unexpected downtime of less than 1 day per year (21.9 hours per year). Industry standards are already at 99.9% for network/server uptime and moving towards 99.999%.

d. Cost

This deals strictly with the costs associated with each system. These would include equipment costs and recurring service fees. Equipment costs would be life cycle costs such as: initial acquisition, installation, training and maintenance. These tend to vary for each mobile system. Recurring service fees would be the monthly access fees and usage fees based on airtime or the amount

of data sent. This data will be compiled based upon input from the system and service providers. [Ref. 8, p.7]

e. Interoperability

Interoperability is a measure of how well the system interfaces or integrates with existing systems. For example: Is it a circuit switched system that works with the Public Switched Telephone system? This would mean it might work like a telephone, fax or modem. Does it work like a packet switched system? How would we integrate it with existing Coast Guard systems? This can all be determined by lab testing. [Ref. 8, p.8] Interoperability is further described as the ability of systems, units, or forces to provide information services to, and accept information services from, other systems, units, or forces.

It is then desired to use the services to enable them to operate effectively together.

f. Latency

Latency is the end-to-end delay in the system.

In any transmission, this metric can be just as important as capacity or bandwidth of the channel. It is affected by a variety of things. The first and most obvious would be the length of the path. Other parts of the delay would be due to factors like the earth station location, buffering, system loading, and congestion. These factors can be measured in lab and field tests. [Ref. 9, p.8] This is the single largest hurdle to overcome with geostationary (GEO) satellite systems.

g. Reliability

Reliability is a measure of a system’s dependability. This can be evaluated in the lab and in the

field by monitoring and recording equipment failures. It can also be obtained through the provider by researching past performance of equipment. [Ref. 9, p.8]

h. Capacity

Capacity is the maximum rate of reliable information transmission. This term is also sometimes used in conjunction with bandwidth. Bandwidth is the width of the communications channel from its highest operating frequency to its lowest frequency, and is an indication of how much information can be transferred by that channel.

For analog voice, this would be measured in cycles per second, and for data or digital voice, this would be in bits per second. This is typically provided by the manufacturer, but can also be verified by lab testing.

[Ref 8, p. 8]

i. Throughput

Throughput is the actual rate of traffic through the system and is dependent on many factors. A system may claim 64 kbps, like a 64 kbps modem used at home, but the user may only see 28-52 kbps (throughput) because of telephone line quality or other factors. Throughput available to users can and will be affected by terminal power, number of users (demand), latency, the required BER, and the security required. Figure 4 below shows how throughput is an end-to-end measurement of system effectiveness. As shown in the graphs, as the number of users, security requirements, or the required BER go up, the throughput of the system will go down. To offset this, as shown in the second graph, transmission (TX) and/or receiver (RX) power can be boosted in order to get more

throughput. This is not easily done with shipboard applications due to limited space and weight considerations.

Figure 3. Throughput vs. variable parameters [From Ref. 4]

j. Ease of Use

How easy is the system to use and maintain? Is it like standard phones and PC’s? Or would users have to learn new systems? [Ref. 8, p.8] Ease of use may be one of the most important evaluation criteria in deciding between two systems. No system is effective in support of missions unless personnel use it. Therefore, the technical depth required to operate the system needs to be kept to a minimum.

k. Security

Security involves how the system will protect the privacy and integrity of user data, as well as the network itself. Specifically, security is the ability of a system to avoid, prevent, negate, or mitigate the degradation, disruption, denial, unauthorized access, or exploitation of communications services by adversaries or the environment.

This is typically done through some type of encryption scheme. There are three areas of security: authenticity, integrity, and data secrecy. Each system will have to be evaluated as to the level of security that is provided.

[Ref. 8, p.8]

• Data Secrecy is often addressed by clearance levels (TS, S, C, U). This ensures only certain personnel has authority to view specific material.

• Integrity helps to prevent unauthorized modification, and ensure data accuracy.

• Authenticity is proven by assigning passwords, pins, and tokens. It proves your identity by something you know, something you have, or something you are (biometrics), often used in combinations. [Ref. 9, p.5]

l. Maintainability

Maintainability is defined as the “ability” of an item to be retained in or restored to a specified condition when personnel with the right skills perform the maintenance. [Ref. 8, p.8]