Chapter 4
*Impulse noise*
(sometimes called spikes) is the primary source of errors in data communications. Some of the sources of impulse noise are voltage changes in adjacent lines, lightning flashes during thunderstorms, fluorescent lights, and poor connections in circuits.
*White noise* or *gaussian noise*
(the familiar background hiss or static on radios and telephones) is caused by the thermal agitation of electrons and therefore is inescapable. Even if the equipment was perfect and the wires were perfectly insulated from any and all external interference, there still would be some white noise. White noise usually is not a problem unless it becomes so strong that it obliterates the transmission. In this case, the strength of the electrical signal is increased so it overpowers the white noise; in technical terms, we increase the signal to noise ratio.
Compare and contrast roll-call polling, hub polling (or token passing), and contention.
*Roll call polling*, the front end processor works consecutively through a list of clients, first polling terminal 1, then terminal 2, and so on, until all are polled. *Roll call polling* can be modified to select clients in priority so that some get polled more often than others. For example, one could increase the priority of terminal 1 by using a polling sequence such as 1, 2, 3, 1, 4, 5, 1, 6, 7, 1, 8, 9. *Hub polling* is often used in LAN multipoint configurations (i.e., token ring) that do not have a central host computer. One computer starts the poll and passes it to the next computer on the multipoint circuit, which sends its message and passes the poll to the next. That computer then passes the poll to the next, and so on, until it reaches the first computer, which restarts the process again. *Contention* is the opposite of controlled access. Computers wait until the circuit is free (i.e., no other computers are transmitting), and then transmit whenever they have data to send. Contention is commonly used in Ethernet local area networks.
What are three ways of reducing errors and the types of noise they affect?
*Shielding* (protecting wires by covering them with an insulating coating) is one of the best ways to prevent impulse noise, cross-talk and intermodulation noise. *Moving cables* away from sources of noise (especially power sources) can also reduce impulse noise cross-talk and intermodulation noise. For impulse noise, this means avoiding lights and heavy machinery. Locating communication cables away from power cables is always a good idea. For cross-talk, this means physically separating the cables from other communication cables. Cross-talk and intermodulation noise is often caused by improper multiplexing. *Changing multiplexing techniques* (e.g., from FDM to TDM), or changing the frequencies or size of the guardbands in frequency division multiplexing can help. Many types of noise (e.g., echoes, white noise, jitter, harmonic distortion) can be caused by *poorly maintained equipment* or poor connections and splices among cables. The solution here is obvious: tune the transmission equipment and redo the connections. To avoid attenuation, telephone circuits have *repeaters or amplifiers* spaced throughout their length.
Which is the simplest (least sophisticated) protocol described in this chapter?
An argument could be made for SDLC, HDLC, or PPP. Each of these are similar in many ways.
How does cyclical redundancy checking (CRC) work?
Cyclical redundancy check (CRC) adds 8, 16, 24 or 32 bits to the message. With CRC, a message is treated as one long binary number, P. Before transmission, the data link layer (or hardware device) divides P by a fixed binary number, G, resulting in a whole number, Q, and a remainder, R/G. So, P/G = Q + R/G. For example, if P = 58 and G = 8, then Q = 7 and R = 2. G is chosen so that the remainder R will be either 8 bits, 16 bits, 24 bits, or 32 bits. The remainder, R, is appended to the message as the error checking characters before transmission. The receiving hardware divides the received message by the same G, which generates an R. The receiving hardware checks to ascertain whether the received R agrees with the locally generated R. If it does not, the message is assumed to be in error.
How do information bits differ from overhead bits?
Information bits are those used to convey the user's meaning. Overhead bits are used for purposes such as error checking, and marking the start and end of characters and packets.
What does the data link layer do?
The data link layer controls the way messages are sent on the physical media. The data link layer handles three functions: media access control, message delineation, and error control. The data link layer accepts messages from the network layer and controls the hardware that actually transmits them. The data link layer is responsible for getting a message from one computer to another without errors. The data link layer also accepts streams of bits from the physical layer and organizes them into coherent messages that it passes to the network layer.
Which is better, controlled access or contention? Explain.
The key consideration for which is better is throughput -- which approach will permit the largest amount of user data to be transmitted through the network. In most of the 1990s, contention approaches worked better than controlled approaches for small networks that have low usage. In this case, each computer can transmit when necessary, without waiting for permission. In high volume networks, where many computers want to transmit at the same time, the well-controlled circuit originally prevented collisions and delivered better throughput in such networks. Today contention-based systems have been improved to the point where they deliver substantially better throughput and are competitive because of hardware cost considerations.
Under what circumstances is forward error-correction desirable?
Forward error correction is commonly used in satellite transmission. A round trip from the Earth station to the satellite and back includes a significant delay. Error rates can fluctuate depending on the condition of equipment, sun spots, or the weather. Indeed, some weather conditions make it impossible to transmit without some errors, making forward error correction essential. Compared to satellite equipment costs, the additional cost of forward error correction is insignificant.
How does forward error correction work? How is it different from other error-correction methods?
Forward error correction uses codes containing sufficient redundancy to prevent errors by detecting and correcting them at the receiving end without retransmission of the original message. The redundancy, or extra bits required, varies with different schemes. It ranges from a small percentage of extra bits to 100 percent redundancy, with the number of error detecting bits roughly equaling the number of data bits.
Under what conditions is media access control unimportant?
With point-to-point full duplex configurations, media access control is unnecessary because there are only two computers on the circuit and full duplex permits either computer to transmit at anytime. There is no media access control.
How do amplifiers differ from repeaters?
An amplifier takes the incoming signal, increases its strength, and retransmits it on the next section of the circuit. They are typically used on analog circuits such as the telephone company's voice circuits. On analog circuits, it is important to recognize that the noise and distortion are also amplified, along with the signal. Repeaters are commonly used on digital circuits. A repeater receives the incoming signal, translates it into a digital message, and retransmits the message. Because the message is re-created at each repeater, noise and distortion from the previous circuit are not amplified.
Briefly describe how checksum works.
Checksum error checking adds a checksum (typically 1 byte) is added to the end of the message. The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and then using the remainder as the checksum. The same approach is used at the receiving end. If the receiver gets the same result, the block has been received correctly.
Describe the frame layouts for SDLC, Ethernet, and PPP.
Each SDLC frame begins and ends with a special bit pattern, known as the flag. The address field identifies the destination. The length of the address field is usually 8 bits but can be set at 16 bits; all computers on the same network must use the same length. The control field identifies the kind of frame that is being transmitted, either information or supervisory. An information frame is used for the transfer and reception of messages, frame numbering of contiguous frames, and the like. A supervisory frame is used to transmit acknowledgments (ACKs and NAKs). The message field is of variable length and is the user's message. The frame check sequence field is a 16-bit or 32-bit cyclical redundancy checking (CRC) code. For a typical Ethernet packet, the destination address specifies the receiver, while the source address specifies the sender. The length indicates the length in 8-bit bytes of the message portion of the packet. The LLC control and SNAP control are used to pass control information between the sender and receiver. These are often used to indicate the type of network layer protocol the packet contains (e.g., TCP/IP or IPX/SPX as described in Chapter 6). The maximum length of the message is 1492 bytes. The packet ends with a CRC-32 frame check sequence used for error detection. The PPP frame is similar to the SDLC frame. The frame starts with a flag and has a one-byte address. It also contains a control field which is rarely used. The protocol field indicates what type of data is contained. The message portion is variable in length and may be up to 1,500 bytes long. The frame check sequence is either CRC-16 or -32. The frame ends with a flag.
Errors normally appear in ______________________________, which is when more than one data bit is changed by the error-causing condition.
Errors normally appear in bursts, which is when more than one data bit is changed by the error-causing condition.
Briefly describe how even parity and odd parity work.
Even parity is when the seven bits of an ASCII character have an even (2, 4, or 6) number of 1s, and therefore a 0 is placed in the eighth parity position. Odd parity is when the seven bits of an ASCII character have an odd (1, 3, 5, or 7) number of 1s, and therefore a 1 is placed in the eighth parity position.
What is media access control, and why is it important?
Media access control handles when the message gets sent. Media access control becomes important when several computers share the same communication circuit, such as a point-to-point configuration with a half duplex line that requires computers to take turns, or a multipoint configuration in which several computers share the same circuit. Here, it is critical to ensure that no two computers attempt to transmit data at the same time -- or if they do, there must be a way to recover from the problem. Media access control is critical in local area networks.
Briefly define noise.
Noise consists of undesirable electrical signals, or, in the instance of fiber optic cable, undesirable light. Noise is typically introduced by equipment or natural disturbances, and it can seriously degrade the performance of a communication circuit. Noise manifests itself as extra bits, missing bits, or bits that have been "flipped," (i.e., changed from 1 to 0 or vice versa).
Define two fundamental types of errors.
There are two fundamental types of errors: human errors and network errors. Human errors, such as a mistake in typing a number, usually are controlled through the application program. Network errors, such as those that occur during transmission, are controlled by the network hardware and software. There are two categories of network errors: corrupted data (data that have been changed) and lost data.
Describe three approaches to detecting errors, including how they work, the probability of detecting an error, and any other benefits or limitations.
Three common error detection methods are parity checking, longitudinal redundancy checking, and polynomial checking (particularly checksum and cyclic redundancy checking). One of the oldest and simplest error detection methods is parity. With this technique, one additional bit is added to each byte in the message. The value of this additional parity bit is based on the number of 1's in each byte transmitted. This parity bit is set to make the total number of ones in the byte (including the parity bit) either an even number or an odd number. Any single error (a switch of a 1 to a 0 or vice versa) will be detected by parity, but it cannot determine which bit was in error. But, if two bits are switched, the parity check will not detect any error. Parity can detect errors only when an odd number of bits have been switched; any even number of errors cancel each other out. Therefore, the probability of detecting an error, given that one has occurred, is only about 50 percent. Many networks today do not use parity because of its low error detection rate. Polynomial checking adds a character or series of characters to the end of the message based on a mathematical algorithm. With the checksum technique, a checksum (typically one byte) is added to the end of the message. The checksum is calculated by adding the decimal value of each character in the message, dividing the sum by 255, and using the remainder as the checksum. The receiver calculates its own checksum in the same way and compares it with the transmitted checksum. If the two values are equal, the message is presumed to contain no errors. Use of checksum detects close to 95 percent of the errors for multiple bit burst errors. The most popular polynomial error checking scheme is cyclical redundancy check (see the answer to # 16 below for more discussion). The probability of detecting an error is nearly 100% or, in some cases, 100%.
What is transmission efficiency?
Transmission efficiency is defined as the total number of information bits (i.e., bits in the message sent by the user) divided by the total bits in transmission (i.e., information bits plus overhead bits).
Compare and contrast stop-and-wait ARQ and continuous ARQ.
With stop-and-wait ARQ, the sender stops and waits for a response from the receiver after each message or data packet. After receiving a packet, the receiver sends either an acknowledgment (ACK) if the message was received without error, or a negative acknowledgment (NAK) if the message contained an error. If it is an NAK, the sender resends the previous message. If it is an ACK, the sender continues with the next message. Stop-and-wait ARQ is by definition, a half duplex transmission technique. With continuous ARQ, the sender does not wait for an acknowledgment after sending a message; it immediately sends the next one. While the messages are being transmitted, the sender examines the stream of returning acknowledgments. If it receives an NAK, the sender retransmits the needed messages. Continuous ARQ is by definition a full duplex transmission technique, because both the sender and the receiver are transmitting simultaneously (the sender is sending messages, and the receiver is sending ACKs and NAKs).
Is there any difference in the error rates of lower-speed lines and of higher-speed lines?
Yes, normally lower speed lines have higher error rates because (1) leased lines can be conditioned to prevent noise, but dial-up lines can not and (2) dial-up lines have less stable transmission parameters.
*Line outages*
are a catastrophic cause of errors and incomplete transmission. Occasionally, a communication circuit fails for a brief period. This type of failure may be caused by faulty telephone end office equipment, storms, loss of the carrier signal, and any other failure that causes a short circuit. When constructing and designing redundant networks that are fault survivable, this is usually called designing for the "farmer with a back hoe" problem.
*Phase hits*
are short-term shifts "out of phase," with the possibility of a shift back into phase.
*Echoes*
can cause errors. Echoes are caused by poor connections that cause the signal to reflect back to the transmitting equipment. If the strength of the echo is strong enough to be detected, it causes errors. Echoes, like cross-talk and white noise, have such a low signal strength that they normally are not bothersome. In networks, echo suppressors are devices that reduce the potential for this type of error. Echoes can also occur in fiber optic cables when connections between cables are not properly aligned.
*Intermodulation noise*
is a special type of cross-talk. The signals from two circuits combine to form a new signal that falls into a frequency band reserved for another signal. On a multiplexed line, many different signals are amplified together, and slight variations in the adjustment of the equipment can cause intermodulation noise. A maladjusted modem may transmit a strong frequency tone when not transmitting data, thus producing this type of noise.
*Attenuation*
is the loss of power a signal suffers as it travels from the transmitting computer to the receiving computer. Some power is absorbed by the medium or is lost before it reaches the receiver. This power loss is a function of the transmission method and circuit medium. High frequencies lose power more rapidly than low frequencies during transmission, so the received signal can thus be distorted by unequal loss of its component frequencies. Attenuation increases as frequency increases or as the diameter of the wire decreases, or as the distance of the transmission increases. Repeaters can be used in a digital environment to correct for attenuation due to distance, where amplifiers can be used to boost diminishing or attenuating analog signals over longer distances. A repeater will perfectly replicate the incoming, distorted digital signal and send it on deeper into the network as if new. An amplifier will boost an attenuating analog signal, but also boost the error noise in the signal as it does so. Fewer repeaters are necessary as compared to amplifiers to correct for attenuation, thus helping to make digital more cost effective when compared to analog transmission in controlling for noise.
*Jitter*
may affect the accuracy of the data being transmitted because minute variations in amplitude, phase, and frequency always occur. The generation of a pure carrier signal in an analog circuit is impossible. The signal may be impaired by continuous and rapid gain and/or phase changes. This jitter may be random or periodic.
*Cross-talk*
occurs when one circuit picks up signals in another. It occurs between pairs of wires that are carrying separate signals, in multiplexed links carrying many discrete signals, or in microwave links in which one antenna picks up a minute reflection from another antenna. Cross-talk between lines increases with increased communication distance, increased proximity of the two wires, increased signal strength, and higher frequency signals. Wet or damp weather can also increase cross-talk. Like white noise, cross-talk has such a low signal strength that it normally is not bothersome.
*Harmonic distortion*
usually is caused by an amplifier on a circuit that does not correctly represent its output with what was delivered to it on the input side.