PROPUESTA DEL ESTUDIO

Earlier we discussed the interaction of bandwidth, latency, and throughput rate. All of these items are also related to information fidelity, which brings up the issue of data compression. When we speak or write, we format information in ways that allow the receiver to receive and process the information in the way the human brain is wired to operate. Language, rules of grammar, sentence structure, punctuation, adjectives, and adverbs all serve to make our meaning clear. They also use up a lot of time and bandwidth. When young people text each other, they poke their cell phones with their thumbs at blinding speed and use abbreviations and grammar that is impenetrable to their elders. What they are doing, from a technical point of view, is encoding for information compression. Because the available bandwidth limits the rate of symbol transmission, the flow of this critically important information is slowed to an unacceptable level by the normal overhead associated with academically acceptable grammar, spelling, and so forth. The encoding is a form of data compression to remove redundancy from the data, thereby allowing the information rate to data rate ratio to increase. The same function is served by digital data compression techniques used for speech and video compression. Figure 2.7

shows the information flow from originator to user including data compression (by any means). Note that the signals received by the receiver will also include interfering signals and noise and that the receiver itself generates noise.

Figure 2.7 Any data compression approach will be subject to errors because of the impact of interference and noise on the decompression.

The problem, of course, is that any coding used will have some impact on information fidelity. Ideally, the communicator uses a lossless code in which all of the information is preserved through the encoding and decoding process. However, now add the impact of the transmission of the encoded information from the sender to the receiver. First consider digital communication media. As the range increases or interference (intentional or unintentional) occurs, bit errors are created at the point at which the receiver must determine whether a one or zero has been received. Figure 2.8 shows the relationship between the bit error rate and E_b/N₀. E_b/N₀ is the received predetection signal-to-noise ratio (RFSNR) adjusted for the ratio of bit rate to RF bandwidth. To be transmitted, digital data must be carried by a modulation, which requires demodulation to recreate the original digital ones and zeros. Each modulation has a different curve in this figure, but all have about the same shape. In radio transmission, the system is typically designed so that a 10–3 to 10−7 bit error rate is required. In this range, most modulations provide an error to RFSNR slope of about one order of magnitude of bit error change to 1 dB of change in RFSNR. For transmission within a cable (such as in a telephone network), much higher SNR may be practical, and the slope of this curve steepens.

We will be talking about forward error correction in Chapter 5. Now just consider that error correction and detection codes (EDC) add extra information to transmitted signals to allow some level of errors to be removed at the receiver location.

Figure 2.8 _{The bit error rate in a demodulated digital signal is a function of Eb/N0.}

The point of this discussion is that there will probably be bit errors. These bit errors will degrade the transmitted information by reducing the accuracy of the conversion from the code back to the basic form of the information. For example, when video compression is used, every bit error degrades the reconstructed picture quality. Note that analogous phenomena occur any time encoding is used, all the way down to the young people texting. One misplaced thumb hit degrades the information fidelity by an amount that is proportional to the power (i.e., the data compression ratio) of the code. This shows the interdependence of the first four rows of Table 2.1. If the connectivity is over a network that is under attack by an enemy or through a high interference environment, the network and the way it is employed must be robust enough to deliver the necessary information fidelity using the available bandwidth, acceptable latency, and required throughput rate.

Message security is important any time there is a reason to prevent someone else from

knowing the information you are sending. This is most obvious for military communication in which an enemy can do your forces great harm by knowing the plans and orders transmitted by command and control communication. By breaking the naval ENIGMA code during the World War II, the allied forces were able to locate (and thus sink) Axis submarines, which changed the whole course of the war. Before the code was broken, ships from Canada to England were sunk twice as fast as they could be built. After the code was broken, submarines were sunk twice as fast as they could be built. Another obvious requirement for message security is the transmission of confidential financial information. Most of us are so afraid of identity theft that we do not transmit credit card numbers or Social Security numbers unless we are confident in the security of the media.

that the information be in digital form and that a series of random bits be digitally added to the message (1 + 1 = 0 and so forth). At the receiving end, the same random bit series is added to the received message to recover the original message. This does not typically require an increase in the required bandwidth or slow the throughput rate. However, some encryption systems are subject to increases in bit error rates when bit errors are present. One system (many years ago) was carefully measured and it was found that it increased the bit error rate by two orders of magnitude when the encryption was used (i.e., apparently, the decrypter converted one error into 100 errors). According to Figure 2.8, this required two more decibels of received signal power to provide adequate information fidelity.

In Figure 2.9, note that the information flow path starts with compression and then goes to encryption, error correction coding, and transmission. At the receiver, the received information is first subjected to the correction of errors. This is necessary because both the decryption and decompression change the data bits and cause problems related to the number of bit errors present. Note that the EDC also returns the data to its original format. Decryption is after EDC and before decompression because the same code that is encrypted must be decrypted.

A related issue is authentication to prevent an enemy from entering your network to insert false information. High-level encryption provides excellent authentication, but proper use of prescribed authentication procedures are also important.

Transmission security requires that an enemy not be able to detect or locate your

transmitters. This is quite different from message security in that an enemy may well be able to read the content of your messages under certain circumstances even if you use transmission security measures adequate to provide acceptable protection in expected tactical situations. Transmission security measures include limitation of radiated energy, geometrically narrowing transmission paths, and spectrum spreading. Later in this chapter, we will discuss all of these issues in the context of their impact on the effectiveness of information flow.

Interference rejection and jamming resistance are two sides of the same issue.

Communication jamming is the process of deliberately creating undesired (interfering) signals in an enemy’s receiver to degrade or eliminate the flow of information. The main difference is that deliberate jamming may be more sophisticated.

Figure 2.9 The information flow has compression as the first function. EDC is performed between the encryption and decryption, allowing as many errors as possible to be removed before the decryption and finally the decompression functions.

Techniques to reduce the impact of interference (either accidental or deliberate) include some related to received signal strength and some related to special modulations. Whichever approaches are used, it is necessary that the network connecting EW assets provide adequate interference protection to allow adequate information fidelity.