Networking Chapter 4

Two Commonly used Controlled Access Techniques

1. Access Requests 2. Polling

Three Functions of a Data Link Protocol

1. Controls when computers transmit. (media access control) 2. Detects and corrects transmission errors. (error control) 3. Identifies the start and end of a message by using a PDU. (message delineation)

Two Categories of Network Errors

1. Corrupted data (data that have been changed) 2. Lost data

Each Communication Protocol has 2 types of bits

1. Information bits 2. Overhead bits

Two Sublayers of the Data Link Layer

1. Logical Link Control 2. Media Access Control

Three Error-Detection Methods

1. Parity Checking 2. Checksum 3. Cyclic Redundancy Checking

Major types of error prevention - 2

1. Shielding 2. Moving cables

Two types of Automatic Repeat reQuests

1. Stop and Wait ARQ 2. Continuous ARQ

SDLC - Frame Check Sequence Field

A 32-bit CRC code (some older versions use a 16-bit CRC).

Checksum

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.

ACK

Acknowledgement

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.

Synchronous Transmission

All the letters or data in one group of data are transmit- ted at one time as a block of data.

White Noise

Also called Gaussian Noise - Caused by the thermal agitation of electrons and therefore is inescapable.

Impulse Noise

Also called spikes. The primary source of errors in data communications. It is heard as a click or a crackling noise and can last as long as 1/100 of 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.

Sliding Window

Another name for continuous ARQ - Because of the visual imagery the early network designers used to think about continuous ARQ. Visualize the sender having a set of messages to send in memory stacked in order from first to last. Now imagine a window that moves through the stack from first to last. As a message is sent, the window expands to cover it, meaning that the sender is waiting for an ACK for the message. As an ACK is received for a message, the window moves forward, dropping the message out of the bottom of the window, indicating that it has been sent and received successfully.

When are start and stop bits used

Asynchronous Transmission - Added to the beginning and end of each individual character being transmitted. Opposite of each other, typically start bit = 1 and the stop bit = 0

ARQ

Automatic Repeat reQuest

Controlled Access - Access Request Technique

Client computers that want to transmit send a request to transmit to the device that is controlling the circuit (e.g., the wireless access point). The controlling device grants permission for one computer at a time to transmit. When one computer has permission to transmit, all other computers wait until that computer has finished, and then, if they have something to transmit, they use a contention technique to send an access request.

Repeater

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 recreated at each repeater, noise and distortion from the previous circuit are not amplified. This provides a much cleaner signal and results in a lower error rate for digital circuits.

Contention

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 LANs.

Frame

Each block of data sent during Synchronous Transmission.

Flow Control

Ensuring that the computer sending the message is not transmitting too quickly for the receiver.

HDLC

High-Level Data Link Control - Formal 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.

SDLC - Control Field

Identifies the kind of frame that is being transmitted, either information or supervisory.

SDLC - 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.

LAP-B

Link Access Protocol - Balanced = Uses the same structure as HDLC but is a scaled-down version of HDLC (i.e., provides fewer of those benefits mentioned that are "beyond the scope of this book").

LAP-M

Link Access Protocol for Modems - The packets that are retransmitted are only those containing an error.

LLC

Logical Link Control - The data link layer's connection to the network layer above it.

MAC

Media Access Control - Controls the physical hardware.

NAK

Negative Acknowledgement

Cross-talk

Occurs when one circuit picks up signals in another. A person experiences cross-talk during telephone calls when she or he hears other conversations in the background. 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.

Hub Polling

Often Called token passing. One device 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.

Asynchronous Transmission

Often 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. It is typically used on point-to-point full-duplex circuits (i.e., circuits that have only two computers on them), so media access control is not a concern.

Parity Checking

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 1's in the byte (including the parity bit) either an even number or an odd number. Only detects errors when there are an odd number of them.

PPP

Point-to-Point Protocol - developed in the early 1990s and is often used in 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. The frame starts with a flag, and has a one-byte address (which is not used on point-to-point circuits). The control field is typically not used. The protocol field indicates what type of data packet the frame contains (e.g., an IP packet).

Error-Detection

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. If the two match, the message is assumed to be correct. If they don't match, an error has occurred.

Controlled Access - Polling Technique

Sends a signal to a client computer that gives it permission to transmit. The clients store all messages that need to be transmitted. Periodically, the controlling device (e.g., a wireless access point) polls the client to see if it has data to send. If the client has data to send, it does so. If the client has no data to send, it responds negatively, and the controller asks another client if it has data to send.

Intermodulation Noise

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. This type of noise is similar to harmonics in music. 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.

SYN

Synchronization Character

SDLC

Synchronous Data Link Control - mainframe protocol developed by IBM in 1972 that is still in use today. It uses a controlled-access media access protocol.

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. The distance between the amplifiers depends on the amount of attenuation, although 1- to 10-mile intervals are common.

Roll-Call Polling

The controller works consecutively through a list of clients, first polling client 1, then client 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

Error Rate

The error rate might be stated as 1 in 500,000, meaning there is 1 bit in error for every 500,000 bits transmitted.

Attenuation

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. As the medium absorbs power, the signal becomes weaker, and the receiving equipment has less and less chance of correctly interpreting the data. This power loss is a function of the transmission method and circuit medium. High frequencies lose power more rapidly than do 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.

Media Access Control

The need to control when computers transmit. Becomes important when several computers share the same communication circuit, such as a point-to-point configuration with a half-duplex configuration that requires computers to take turns, or a multipoint configuration in which several computers share the same circuit.

Synchronization

The recognition of the start and stop of each message takes place for each individual character because the start bit is a signal that tells the receiver to start sampling the incoming bits of a character so the data bits can be interpreted into their proper character structure. A stop bit informs the receiver that the character has been received and resets it for recognition of the next start bit.

Echoes

The result of 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. Echoes can also occur in fiber-optic cables when connections between cables are not properly aligned.

Continuous ARQ

The sender does not wait for an acknowledgment after sending a message; it immediately sends the next one. Although the messages are being transmitted, the sender examines the stream of returning acknowledgments. If it receives an NAK, the sender retransmits the needed messages.

Stop and Wait ARQ

The sender stops and waits for a response from the receiver after each data packet. After receiving a packet, the receiver sends either an acknowledgment (ACK), if the packet 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

SDLC - Flag

The special bit pattern that each SDLC frame begins and ends with (01111110)

Transmission Efficiency

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).

Throughput

The total number of information bits received per second, after taking into account the overhead bits and the need to retransmit frames containing errors.

Go-Back-N ARQ

The first packet with an error and all those that followed are retransmitted.

TRIB

Transmission Rate of Information Bits - The effective number of information bits that is transmitted over a communication circuit per unit of time.

Line Noise

Undesirable electrical signals (for fiber-optic cable, it is undesirable light). Noise is introduced by equipment or natural disturbances, and it degrades 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).

Controlled Access

Used by most wireless LANs. The wireless access point controls the circuit and determines which clients can transmit at what time.

Overhead bits

Used for purposes such as error checking and marking the start and end of characters and packets.

SDLC - Information Frame

Used for the transfer and reception of messages, and the frame numbering of contiguous frames.

Information bits

Used to convey the user's meaning.

SDLC - Supervisory Frame

Used to transmit acknowledgments (ACKs and NAKs).

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.

SDLC - Message Field

Variable length and is the user's message.

Efficiency of Data Throughput

Varies inversely as the desired amount of error detection is increased.

Ethernet

Very popular LAN protocol, conceived by Bob Metcalfe in 1973 and developed jointly by Digital, Intel, and Xerox in the 1970s. Since then, Ethernet has been further refined and developed into a formal standard called IEEE 802.3ac. There are several versions of Ethernet in use today. Ethernet uses a contention media access protocol.