Home > Articles > Networking > Wireless/High Speed/Optical

Data Communications: Use the Right Medium for your Message

Learn when to use twisted pair cable, satellites, or fiber-optics to get your message across. From economic considerations to bandwidth limitations, here's a guide to your options.
This chapter is from the book

In this chapter

  • Information as a Quantity
  • Bounded Medium
  • Unbounded Medium
  • Effects of Bandwidth on a Transmission Channel
  • Bandwidth Requirements for Signals
  • Carrier Systems
  • What You Have Learned

Messages represent information useful to people, but the sender and receiver might or might not be human. The medium must be suitable to convey the type of message. In this chapter, you will examine in more detail the types of messages and the media that carry them. Concerning the latter, you will also examine impairments that affect the data-transfer capability of different media and why certain types of media have a higher data-transfer capacity than other types of media.

Useful communication requires four elements, as shown in Figure 3.1:

  1. A message (information) to be communicated
  2. A sender of the message
  3. A medium or channel over which the message can be sent
  4. A receiver

Figure 3.1 The elements of communication.

Information as a Quantity

Information can be defined as "a numerical quantity that measures the uncertainty in the outcome of an experiment to be performed." This definition has an application in the sending of messages. For example, suppose you have a machine that can send only two symbols: A1 and A2. You can then say that the "experiment" is the accurate recognition of the two symbols (A1 and A2) being sent from one machine to another. As far as the receiving machine is concerned, it is just as likely to receive one symbol as the other. So, you can say that the "numerical quantity" in this experiment is a unit of information that allows a selection between two equally likely choices. This quantity, or unit of information, is usually called a bit (a contraction of the terms "binary digit"), and it has two possible values, 0 and 1. If these two values are used to represent A1 and A2, A1 could be represented by the bit value 0, and A2 could be represented by the bit value 1. The number of bits per symbol is 1, but you still need a way of selecting which symbol (bit) you want to use. A machine that needs only two symbols needs only a 1-bit select code (0 and 1).

A machine that uses only two symbols is not of much use for communication, but suppose the machine could use 128 symbols (like the standard ASCII character set). Then the number of equally likely choices to be handled would be 128, and the number of bits (information) required to represent each of those 128 symbols would be seven (refer to Table 1.1 in Chapter 1, "An Overview of Data Communications"). You can see, then, that if the knowledge (or intelligence) to be communicated can be represented by a set of equally likely symbols, the amount of information required per symbol to communicate that knowledge is necessarily dependent on the total number of bits of information.

The standard ASCII character set is particularly useful for selecting the information to be communicated because it can select 1 of 128 ASCII symbols with only one 8-bit byte, a common bit grouping in computers (the eighth bit is not used in this case). The extended ASCII character set that uses all 8 bits per byte supports 256 symbols, doubling the number of ASCII symbols.

Information Content of Symbols

In many information systems, not every symbol is equally likely to be used in a given communication. The English language is a good example. In a message written in English, the letter e is 12 times more likely to occur than the letter s. This uneven distribution is also characteristic of particular groups of letters and of words. This means that each of the 128 symbols in ASCII (or 256 symbols in extended ASCII) is not likely to occur an equal number of times in any given communication. For example, notice that the uses of the letters e and g and the letter combinations th and er are unequal in this paragraph.

In 1949, Claude Shannon published a book entitled The Mathematical Theory of Communication. In this book, he discussed the uncertainty or amount of disorder of a system, which he called entropy. Entropy can also be considered as a measure of randomness, and, as you will shortly note, it has a significant role in communications, both for verification of the accurate arrival of messages and as a mechanism for reducing the physical size of messages while retaining their meaning.

The entropy of a set of equally likely symbols (such as the digits 0–9 in a table of random numbers) is the logarithm to the base 2 of the number of symbols in the set. The entropy of the English alphabet, which contains 26 letters and a space, is then log2(27) = 4.76 bits per symbol. Because of the uneven use of letters in the English language, however, its entropy was estimated by Shannon as 1.3 bits per symbol.

Because the probability of occurrence of each character in the English alphabet differs, entropy of the alphabet is calculated as follows:


Here, Pi is the probability of occurrence of the ith character in the English alphabet. Note that the symbol H is used by mathematicians to represent entropy. The preceding calculation can be simplified as follows for any code or language:


Here, n represents n possible distinct characters or symbols in a language or code.

When entropy was computed for the English language, Shannon discovered that the language is about 70 percent redundant and that it should be possible to reconstruct English text accurately if every other letter is lost or changed due to noise or distortion. Obviously, redundancy is desirable to raise the chances of receiving a good message when the medium is noisy. (The words noise and noisy in this book refer to electrical noise—that is, an electrical signal that is not supposed to be present.)

Using Redundancy in Communications

So, you wonder, what does all this have to do with the real world of data communications? Quite a bit, because almost every scheme in current use for sending data uses redundancy in an attempt to verify that the data has been received exactly as sent (that is, no errors have been introduced by the sending mechanism, the transmission medium, or the receiver). The redundant information might consist simply of a retransmission of the entire original message.

Although it's simple to implement, retransmission is not efficient. Special techniques are therefore used to generate redundant information that is related to the message in a way that is known to both the sender and the receiver. The sender generates the redundant information during transmission and sends it with the message. The receiver regenerates and checks the redundant information when the message is received. This scheme is represented in Figure 3.2. Verification usually occurs at the end of each link in the chain making up the transmission path. The details of this process and various methods in current use are described in Chapter 10, "WAN Architectures and Packet Networks."

Using Redundancy for Data Compression

A second area where entropy plays a considerable role is as a foundation for data compression. Because the entropy of an alphabet indicates the average number of bits per symbol, this information provides software and hardware developers with a goal for implementing various data-compression schemes.

Figure 3.2 Error-checking points.

For example, if the uneven use (different probabilities) of the letters in the English alphabet results in an entropy of 1.3 bits per symbol, why use an 8-bit byte to transmit each character? This tells software and hardware designers that by compressing data and temporarily removing redundancies via the use of one or more algorithms prior to transmission, larger quantities of data can be transmitted per unit time. Recognizing the value of entropy, almost all modems today include a built-in data-compression mechanism.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020