Chapter 4

Checksum

(1 byte) 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.

Stop-and-Wait ARQ

, the sender stops and waits for a response from the receiver after each data packet. ACK (acknowledgement) or NAK (negative acknowledgement) HALF DUPLEZ 4.4

Repeater

Amplifiers To avoid attenuation, telephone circui.On analog circuits, it is important to recognize that the noise and distortion are also amplified, along with the signal. A repeater receives the incoming signal, translates it into a digital message, and retransmits the message. Because the message is recreated at each repeater, noise and distortion from the previous circuit are not amplified.

SDLC

Each SDLC frame begins and ends with a special bit pattern (01111110), known as the flag. Look at FIGURE 4..9

Sources and Prevention of Error

Errors occur in all networks. Errors tend to occur in groups (or bursts) rather than 1 bit at a time. The primary sources of errors are impulse noises (e.g., lightning), cross-talk, echo, and attenuation. can be prevented by shielding cables

Transmission Efficiency and Throughput

Every protocol adds additional bits to the user's message before sending it (e.g., for error detection). Synchronous transmission provides greater efficiency than does asynchronous transmission. Small frame sizes are therefore better suited to error-prone circuits, and large frames, to error-free circuits.

What is an example of the ethernet 803.3ac frame?

Figure 4.10a shows an Ethernet 803.3ac frame. The frame starts with a 7-byte preamble which is a repeating pattern of ones and zeros (10101010).

Data Link Protocol

Figure 4.7

What is the key to select the best control technique?

Find the crossover point between controlled and contention: crossover point at around 20 computers page 122

What is the accuracy of CRC?

If burst longer than CRC, the CRC is not perfect but close to it. CRC 16 will detect 99.998 percent of all burst erros longer than 16 bits, CRC-32 detects 99.99999 oercebt if burst errors longer than 32 bits

Information vs 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.

Cyclical Redundancy Check

It 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

What are the major functions of a data link layer?

Media Access Control-controllling when computers trasmit Error Control- detecting and correctining transmission erros Message Delineation-idetifying the begging and end of a message

Media Access Control

Media access control refers to controlling when computers transmit: rollcalling,hubpolling, contention

Error Prevention

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

What is continuos ARQ also referred to as?

Sliding Window-visual imagery the early network : visual sender having messages to send stacked from first to last passes ACK then window covers it indicating its been sent or recieved FLOW CONTROL- the computer sending the message is not transmitting too quickly for the reciever

synchronization

The recognition of the start and stop of each message Look at Figure 4.8

What happens to the remainder of the cyclical redundancy check?

The remainder, R, is appended to the message as the error-checking characters before transmission. The receiving hardware divides the received message by the sameG, 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.Cyclical redundancy check performs quite well. The most commonly used CRC codes are CRC-16 (a 16-bit version), CRC-CCITT (another 16-bit version), and CRC-32 (a 32-bit version). The probability of detecting an error is 100% for all errors of thesame length as the CRC or less. For example, CRC-16 is guaranteed to detect errors

ARQ (Automatic Repeat Request)

The simplest, most effective, least expensive, and most commonly used method for error correctioWith retransmission, a receiver that detects an error simply asks the sender to retransmit themessage until it is received without error

Point-to-Point Protocol (PPP)

WANs. It is designed to transfer data over a point-to-point circuit but provides an address so that it can be used on multipoint circuits. Look at Figure 4.11

How does asynchronous transmission work?

With asynchronous transmission, each character is transmitted independently of all other characters. To separate the characters and synchronize transmission, a start bit and a stop bit are put on the front and back of each individual character. For example, if we are using 7-bit ASCII with even parity, the total transmission is 10 bits for each character (1 start bit, 7 bits for the letter, 1 parity bit, 1 stop bit). The start bit and stop bit are the opposite of each other.

Parity Checking

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 is either even or odd # (4.3 figure): ONLY DETECTS ODDS SO ODD PARITY OR EVEN PARITY

Error Detection and Correction

all error-detection schemes attach additional error-detection data, based on a mathematical calculation, to the user's message. Parity, checksum, and CRC are the most common error-detection schemes.

synchronous transmission

all the letters or data in one group of data are transmitted at one time as a block of data. This block of data is called a frame.

Asynchronous vs Synchronized?

asynchronous data transmission means each character is transmitted as a totally independent entity with its own start and stop bits to inform the receiving computer that the character is beginning and ending. Synchronous transmission means whole blocks of data are transmitted as frames after the sender and the receiver have been synchronized.

Line noise and distortion

cause cause data communication erros TP and coaxial> fiber optic

error Control in practice (figure 4.6)

data link sofware Ethernet detect errors but not correct them: packet with error discarded. Wireless LAns and some Wans wehre erros are more likely, still perform both error detection and error correction: Correction-software at higher leverls: lost packetsm request the sender to retrasnmit them: done by transport layer using continuous ARQ

Efficiency

data throughput varies inversely as the desired amount of error detection is increased. amount of error detection increased: throughput of useful data reduced

Transmission efficiency

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) Look at Figure 4.12

Data Link Protocol is Also Referred to as?

fundamental "language" spoken by networks.

Ethernet

further refined and developed into a formal standard called IEEE 802.3ac. uses media access protocol

Attenuation

loss of power a signal suffers as it travels from the transmitting computer to the recieving computer: some power absorbed by medium: or as diameter of the wire dec

Message Delineation

means to indicate the start and end of a message. asyrncnous and syncrounous

burst error

more than 1 data bit is changed by the error-causing condition. In other words, errors are not uniformly distributed in time. Although an error rate might be stated as 1 in 500,000, errors are more likely to occur as 100 bits every 50,000,000 bits.

Cross-talk

occurs when one circuit picks up signals in another cross-talk during telephone calls when she or he hears other conversations in the background. microwave links in which one antenna picks up a minute reflection from another antenna. Cross-talk between lines increases with increased communication distance wet or damp weather: LOW SIGNAL

Asynchronous transmission

referred to as start-stop transmission because the transmitting computer can transmit a character whenever it is convenient, and the receiving computer will accept that character

What does the data link layer do?

responsible for moving messages from one device to another controls the way messages are sent on media Organizes physical layer bit streams into cohoerent messages for the network layer

Echoes

result of poor connections that cause the signal to reflect back to the transmitting equipment.: LOW SIGNAL FB CABLES

Error Detection Performance

send extra data with each message.These error-detection data are added to each message by the data link layer of the sender on the basis of some mathematical calculations performed on the message (in some cases, error-detection methods are built into the hardware itself).The receiver performs the same mathematical calculations on the message it receives and matches its results against the error-detection data that were transmitted with the message.

Intermodulation Noise

specidal type of cross-talk: harmony in music:signals from 2 circuits combine to form a new signal falls into frequency band reserved for another signal

Impulse noise

spikes) is the primary source of errors in datacommunications. It is heard as a click or a crackling noise and can last as long as 1/100of a second. Such a click does not really affect voice communications, but it can obliterate a group of data, causing a burst error. At 1.5 Mbps, 15,000 bits would be changed by a spike of 1/100 of a second. Some of the sources of impulse noise are voltage changes in adjacent lines, lightning flashes during thunderstorms, fluorescent lights, and poor connections in circuits: LOW SIGNAL

High-level data link control (HDLC)

standard developed by the ISO often used in WANs. HDLC is essentially the same as SDLC, except that the address and control fields can be longer. HDLC also has several additional benefits that are beyond the scope of this book, such as a larger sliding window for continuous ARQ. It uses a controlled-access media access protocol. One variant, Link Access Protocol-Balanced (LAP-B), uses the same structure as HDLC but is a scaled-down version of HDLC (

What is an example of synchronous transmission?

terminal or personal computer will save all the keystrokes typed by the user and transmit them only when the user presses a special "transmit" key. In this case, the start and end of the entire frame must be marked, not the start and end of each letter. Synchronous transmission is often used on both point-to-point and multipoint circuits. For multipoint circuits, each packet must include a destination address and a source address, and media access control is important.

White Noise or Gaussian Noise

the familiar background hiss or statis on radios and telephones inesepcable cuz of electrons

error rates

the pattern of errors that can be expected on the circuits they lease. For example, the error rate might be stated as 1 in 500,000, meaning there is 1 bit in error for every 500,000 bits transmitted.

continuous ARQ

the sender does not wait for an acknowledgment after sending a message; it immediately sends the next one: Message containing error(called Link Access Protocol for Modems [LAP-M]) or may be the first packet with an error and all those that followed it (called Go-Back-N ARQ). LAP-M is better because it is more efficient.THE SENDER AND RECIEVER AGREE ON THE SIZE OF THE SLIDING WINDOW: cannot send anymore packets in sliding window until thr reciever sends an ACK 4.5

Forward Error Correction

uses codes containing sufficient redundancy to prevent errors by detecting and correcting them at the recieving end without retransmission of the original message. ( must be mininmum # of error-free bits between bursts of errors)

Forward error correction

uses codes containing sufficient redundancy to prevent errors small percentage of redundancy to 100%. a minimum number of error-free bits between bursts of errors. by detecting and correcting them at the receiving end without retransmission of the original message USED IN SATELLITE TRANSMISSION