Ch_4-6_pp INSY

Chapter Four

Making Connections

Data Communications and Computer Networks: A Business Users Approach

Seventh Edition

After reading this chapter, you should be able to:

List the four components of all interface standards

Discuss the basic operations of the USB and EIA-232F interface standards

Cite the advantages of FireWire, SCSI, iSCSI,

Data Communications and Computer Networks: A Business User's Approach, Seventh Edition 2

Cite the advantages of FireWire, SCSI, iSCSI, InfiniBand, and Fibre Channel interface standards

Outline the characteristics of asynchronous, synchronous, and isochronous data link interfaces

After reading this chapter, you should be able to (continued):

Recognize the difference between half-duplex and full-duplex connections

Identify the operating characteristics of terminal-to-mainframe connections and why they are


to-mainframe connections and why they are unique compared to other types of computer connections

Introduction

Connecting peripheral devices to a computer has, in the past, been a fairly challenging task

Newer interfaces have made this task much easier


easier Lets examine the interface between a computer

and a device This interface occurs primarily at the physical

layer

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Interfacing a Computer to Peripheral Devices

The connection to a peripheral is often called the interface

The process of providing all the proper interconnections between a computer and a


interconnections between a computer and a peripheral is called interfacing

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Characteristics of Interface Standards There are essentially two types of standards

Official standards Created by standards-making organizations such as

ITU (International Telecommunications Union), IEEE (Institute for Electrical and Electronics Engineers), (now defunct) EIA (Electronic Industries Association), ISO


defunct) EIA (Electronic Industries Association), ISO (International Organization for Standardization), and ANSI (American National Standards Institute)

De facto standards Created by other groups that are not official standards

but because of their widespread use, become almost standards

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Characteristics of Interface Standards (continued)

There are four possible components to an interface standard: Electrical component: deals with voltages, line

capacitance, and other electrical characteristics Mechanical component: deals with items such as the


Mechanical component: deals with items such as the connector or plug description

Functional component: describes the function of each pin or circuit that is used in a particular interface

Procedural component: describes how the particular circuits are used to perform an operation

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Two Important Interface Standards

In order to better understand the four components of an interface, lets examine two interface standards EIA-232F an older standard originally designed


EIA-232F an older standard originally designed to connect a modem to a computer

USB (Universal Serial Bus) a newer standard that is much more powerful than EIA-232F

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An Early Standard: EIA-232F

Originally named RS-232 but has gone through many revisions

All four components are defined in the EIA-232F standard:


standard: Electrical Mechanical (DB-25 connector and DB-9

connector) Functional Procedural

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An Early Standard: EIA-232F

EIA-232F also used the definitions DTE and DCE An example of a DTE, or data terminating

equipment, is a computer


equipment, is a computer An example of a DCE, or data circuit-terminating

equipment, is some form of modem

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What is meant by duplexity?

EIA-232F defines a full-duplex connection. What does this mean?

A full-duplex connection transmits data in both directions and at the same time


directions and at the same time A half-duplex connection transmits data in both

directions but in only one direction at a time A simplex connection can transmit data in only

one direction Can you think of a modern example of each?

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Universal Serial Bus (USB)

The USB interface is a modern standard for interconnecting a wide range of peripheral devices to computers

Supports plug and play


Supports plug and play Can daisy-chain multiple devices USB 2.0 can support 480 Mbps (USB 1.0 is only

12 Mbps) USB 3.0 can support 4.8 Gbps

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Universal Serial Bus (USB) (continued)

The USB interface defines all four components The electrical component defines two wires

VBUS and Ground to carry a 5-volt signal, while the D+ and D- wires carry the data and signaling


the D+ and D- wires carry the data and signaling information

The mechanical component precisely defines the size of four different connectors and uses only four wires (the metal shell counts as one more connector)

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Universal Serial Bus (USB) (continued)


Universal Serial Bus (USB) (continued) The functional and procedural components are

fairly complex but are based on the polled bus The computer takes turns asking each

peripheral if it has anything to send


More on polling near the end of this chapter

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FireWire

Low-cost digital interface Capable of supporting transfer speeds of up to

800 Mbps Hot pluggable


Hot pluggable Supports two types of data connections:

Asynchronous connection Isochronous connection

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Thunderbolt

Digital interface currently found on Apple products

Capable of supporting transfer speeds of up to 10 Gbps


Uses same connector as existing Mini DisplayPort and similar protocol as PCI Express

Can daisy-chain devices and may get even faster with later versions

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SCSI and iSCSI SCSI (Small Computer System Interface)

A technique for interfacing a computer to high-speed devices such as hard disk drives, tape drives, CDs, and DVDs

Designed to support devices of a more permanent nature


nature SCSI is a systems interface

Need SCSI adapter iSCSI (Internet SCSI)

A technique for interfacing disk storage to a computer via the Internet

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InfiniBand and Fibre Channel InfiniBand a serial connection or bus that can carry

multiple channels of data at the same time Can support data transfer speeds of 2.5 billion bits (2.5

gigabits) per second and address thousands of devices, using both copper wire and fiber-optic cables

A network of high-speed links and switches


A network of high-speed links and switches Fibre Channel also a serial, high-speed network that

connects a computer to multiple input/output devices Supports data transfer rates up to billions of bits per

second, but can support the interconnection of up to 126 devices only

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Asynchronous Connections

A type of connection defined at the data link layer

To transmit data from sender to receiver, an asynchronous connection creates a one-


asynchronous connection creates a one-character package called a frame

Added to the front of the frame is a start bit, while a stop bit is added to the end of the frame

An optional parity bit can be added which can be used to detect errors

Asynchronous Connections (continued)



The term asynchronous is misleading here because you must always maintain synchronization between the incoming data stream and the receiver


Asynchronous connections maintain synchronization by using small frames with a leading start bit

Synchronous Connections

A second type of connection defined at the data link layer

A synchronous connection creates a large frame that consists of header and trailer flags, control


that consists of header and trailer flags, control information, optional address information, error detection code, and data

A synchronous connection is more elaborate but transfers data in a more efficient manner

Synchronous Connections (continued)


Isochronous Connections

A third type of connection defined at the data link layer used to support real-time applications

Data must be delivered at just the right speed (real-time) not too fast and not too slow


(real-time) not too fast and not too slow Typically an isochronous connection must

allocate resources on both ends to maintain real-time

USB and Firewire can both support isochronous

Terminal-to-Mainframe Computer Connections

Point-to-point connection a direct, unshared connection between a terminal and a mainframe computer

Multipoint connection a shared connection


Multipoint connection a shared connection between multiple terminals and a mainframe computer

The mainframe is the primary and the terminals are the secondaries

Terminal-to-Mainframe Computer Connections (continued)



To allow a terminal to transmit data to a mainframe, the mainframe must poll the terminal

Two basic forms of polling: roll-call polling and hub polling


hub polling In roll-call polling, the mainframe polls each

terminal in a round-robin fashion In hub polling, the mainframe polls the first

terminal, and this terminal passes the poll onto the next terminal

Making Computer Connections In Action

A laptop computer has many different types of connectors, or connections

While every laptop can be different, if anyone has a laptop in class, maybe someone will


has a laptop in class, maybe someone will volunteer to use theirs for show-and-tell

Making Computer Connections In Action (continued)

Power cord connection (why does the power cord have a big brick on it?)

USB connectors (one or more) RJ-11 (telephone jack) RJ-45 (LAN jack)


RJ-45 (LAN jack) PC Card / SmartCard DisplayPort (to connect your laptop to a video

device) Media card slot (SD, SDHC, xD, etc) DB-15 (to connect to an external monitor or

video projector)


A company wants to transfer files that are typically 700K chars in size

If an asynchronous connection is used, each character will have a start bit, a stop bit, and


character will have a start bit, a stop bit, and maybe a parity bit

700,000 chars * 11 bits/char (8 bits data + start + stop + parity) = 7,700,000 bits


If a synchronous connection is used, assume maximum payload size 1500 bytes

To transfer a 700K char file requires 467 1500-character (byte) frames


character (byte) frames Each frame will also contain 1-byte header, 1-

byte address, 1-byte control, and 2-byte checksum, thus 5 bytes overhead


1500 bytes payload + 5 byte overhead = 1505 byte frames

467 frames * 1505 bytes/frame = 716,380 bytes, or 5,731,040 bits


or 5,731,040 bits Significantly less data using synchronous

connection

Summary Connection between a computer and a peripheral is often

called the interface Process of providing all the proper interconnections between

a computer and a peripheral is called interfacing The interface between computer and peripheral is composed

of one to four components: electrical, mechanical, functional,


of one to four components: electrical, mechanical, functional, and procedural

A DTE is a data terminating device Computer

A DCE is a data circuit-terminating device Modem

Summary (continued) Two interface standards worthy of additional study: Universal

Serial Bus, and EIA-232F EIA-232F was one of the first highly popular standards Universal Serial Bus is currently the most popular interface

standard Half-duplex systems can transmit data in both directions, but

in only one direction at a time


in only one direction at a time Full-duplex systems can transmit data in both directions at the

same time Other peripheral interfacing standards that provide power,

flexibility, and ease-of-installation include FireWire, SCSI, iSCSI, InfiniBand, and Fibre Channel

Summary (continued) While much of an interface standard resides at the physical

layer, a data link connection is also required when data is transmitted between two points on a network Three common data link connections include asynchronous

connections, synchronous connections, and isochronous connections

Asynchronous connections use single-character frames and


Asynchronous connections use single-character frames and start and stop bits to establish the beginning and ending points of the frame

Synchronous connections use multiple-character frames, sometimes consisting of thousands of characters

Isochronous connections provide real-time connections between computers and peripherals and require a fairly involved dialog to support the connection

Summary (continued) A point-to-point connection is one between a

computer terminal and a mainframe computer that is dedicated to one terminal

A multipoint connection is a shared connection between more than one computer terminal and a


between more than one computer terminal and a mainframe computer

Chapter Five

Making Connections Efficient: Multiplexing and Compression


Seventh Edition


Describe frequency division multiplexing and list its applications, advantages, and disadvantages

Describe synchronous time division multiplexing and list its applications, advantages, and


and list its applications, advantages, and disadvantages

Outline the basic multiplexing characteristics of T-1 and SONET/SDH telephone systems

Describe statistical time division multiplexing and list its applications, advantages, and disadvantages


Cite the main characteristics of wavelength division multiplexing and its advantages and disadvantages

Describe the basic characteristics of discrete multitone


multitone Cite the main characteristics of code division

multiplexing and its advantages and disadvantages

Apply a multiplexing technique to a typical business situation


Describe the difference between lossy and lossless compression

Describe the basic operation of run-length, JPEG, and MP3 compression


JPEG, and MP3 compression

Introduction

Under simplest conditions, medium can carry only one signal at any moment in time

For multiple signals to share a medium, medium must somehow be divided, giving each signal a portion of the total bandwidth


portion of the total bandwidth Current techniques include:

Frequency division multiplexing Time division multiplexing Code division multiplexing

Frequency Division Multiplexing

Assignment of nonoverlapping frequency ranges to each user or signal on a medium Thus, all signals are transmitted at the same time,

each using different frequencies


A multiplexor accepts inputs and assigns frequencies to each device

Frequency Division Multiplexing (continued)

Each channel is assigned a set of frequencies and is transmitted over the medium

A corresponding multiplexor, or demultiplexor, is on the receiving end of the medium and


on the receiving end of the medium and separates the multiplexed signals

A common example is broadcast radio

Frequency Division Multiplexing (continued)


Frequency Division Multiplexing (continued) Analog signaling is used in older systems;

discrete analog signals in more recent systems Broadcast radio and television, cable television,

and cellular telephone systems use frequency division multiplexing


division multiplexing This technique is the oldest multiplexing

technique Since it involves a certain level of analog

signaling, it may be susceptible to noise

Time Division Multiplexing

Sharing of the signal is accomplished by dividing available transmission time on a medium among users

Digital signaling is used exclusively Time division multiplexing comes in two basic


Time division multiplexing comes in two basic forms: Synchronous time division multiplexing Statistical time division multiplexing

Synchronous Time Division Multiplexing

The original time division multiplexing The multiplexor accepts input from attached

devices in a round-robin fashion and transmits the data in a never -ending pattern


the data in a never -ending pattern T-1 and SONET telephone systems are common

examples of synchronous time division multiplexing

Synchronous Time Division Multiplexing (continued)

Figure 5-2 Several cash registers and their multiplexed stream of transactions


transactions


If one device generates data at faster rate than other devices, then the multiplexor must either sample the incoming data stream from that device more often than it samples the other devices, or buffer the faster incoming stream


devices, or buffer the faster incoming stream If a device has nothing to transmit, the

multiplexor must still insert something into the multiplexed stream


Figure 5-3Multiplexor transmission stream with


stream with only one input device transmitting data


So that the receiver may stay synchronized with the incoming data stream, the transmitting multiplexor can insert alternating 1s and 0s into the data stream



Figure 5-4Transmitted frame with added synchroni-


synchroni-zation bits

T-1 Multiplexing

The T-1 multiplexor stream is a continuous series of frames

Note how each frame contains the data (one byte) for potentially 24 voice-grade telephone


byte) for potentially 24 voice-grade telephone lines, plus one sync bit

It is possible to combine all 24 channels into one channel for a total of 1.544 Mbps

T-1 Multiplexing (continued)

Figure 5-4T-1 multiplexed data stream


SONET/SDH Multiplexing

Similar to T-1, SONET incorporates a continuous series of frames

SONET is used for high-speed data transmission


transmission Telephone companies have traditionally used a

lot of SONET but this may be giving way to other high-speed transmission services

SDH is the European equivalent to SONET

SONET/SDH Multiplexing (continued)

Figure 5-6SONET STS-1 frame layout


Statistical Time Division Multiplexing

A statistical multiplexor transmits the data from active workstations only

If a workstation is not active, no space is wasted in the multiplexed stream


in the multiplexed stream

Statistical Time Division Multiplexing (continued)

Figure 5-7Two stations out of four transmitting via a statistical


via a statistical multiplexor


A statistical multiplexor accepts the incoming data streams and creates a frame containing the data to be transmitted

To identify each piece of data, an address is


To identify each piece of data, an address is included


Figure 5-8Sample address and data in a statistical


statistical multiplexor output stream


If the data is of variable size, a length is also included

Figure 5-9Packets of


Packets of address, length, and data fields in a statistical multiplexor output stream


More precisely, the transmitted frame contains a collection of data groups

Figure 5-10Frame layout


Frame layout for the information packet transferred between statistical multiplexors

Wavelength Division Multiplexing

Wavelength division multiplexing multiplexes multiple data streams onto a single fiber-optic line

Different wavelength lasers (called lambdas)


Different wavelength lasers (called lambdas) transmit the multiple signals

Wavelength Division Multiplexing (continued)

Each signal carried on the fiber can be transmitted at a different rate from the other signals

Dense wavelength division multiplexing


Dense wavelength division multiplexing combines many (30, 40, 50 or more) onto one fiber

Coarse wavelength division multiplexing combines only a few lambdas

Wavelength Division Multiplexing (continued)

Figure 5-11Fiber optic line using wavelength division multiplexing and supporting multiple-speed transmissions


transmissions

Discrete Multitone

Discrete Multitone (DMT) a multiplexing technique commonly found in digital subscriber line (DSL) systems

DMT combines hundreds of different signals, or


DMT combines hundreds of different signals, or subchannels, into one stream

Interestingly, all of these subchannels belong to a single user, unlike the previous multiplexing techniques

Discrete Multitone (continued)

Each subchannel is quadrature amplitude modulated (recall eight phase angles, four with double amplitudes)

Theoretically, 256 subchannels, each


Theoretically, 256 subchannels, each transmitting 60 kbps, yields 15.36 Mbps

Unfortunately, there is noise, so the subchannels back down to slower speeds

Discrete Multitone (continued)

Figure 5-12256 quadrature amplitude modulated streams combined into one DMT signal for DSL


DSL

Code Division Multiplexing

Also known as code division multiple access An advanced technique that allows multiple

devices to transmit on the same frequencies at the same time


the same time Each mobile device is assigned a unique 64-bit

code

Code Division Multiplexing (continued)

To send a binary 1, a mobile device transmits the unique code

To send a binary 0, a mobile device transmits the inverse of the code


the inverse of the code To send nothing, a mobile device transmits

zeros


Receiver gets summed signal, multiplies it by receiver code, adds up the resulting values Interprets as a binary 1 if sum is near +64 Interprets as a binary 0 if sum is near -64


Interprets as a binary 0 if sum is near -64

Code Division Multiplexing (continued) For simplicity, assume 8-bit code Example

Three different mobile devices use the following codes:

Mobile A: 11110000


Mobile A: 11110000 Mobile B: 10101010 Mobile C: 00110011

Assume Mobile A sends a 1, B sends a 0, and C sends a 1

Signal code: 1-chip = +N volt; 0-chip = -N volt


Example (continued) Three signals transmitted:

Mobile A sends a 1, or 11110000, or ++++---- Mobile B sends a 0, or 01010101, or -+-+-+-+


Mobile C sends a 1, or 00110011, or --++--++ Summed signal received by base station: -1, +1,

+1, +3, -3, -1, -1, +1


Example (continued) Base station decode for Mobile A:

Signal received: -1, +1, +1, +3, -3, -1, -1, +1 Mobile As code: +1, +1, +1, +1, -1, -1, -1, -1


Product result: -1, +1, +1, +3, +3, +1, +1, -1 Sum of Products: +8 Decode rule: For result near +8, data is binary 1


Example (continued) Base station decode for Mobile B:

Signal received: -1, +1, +1, +3, -3, -1, -1, +1 Mobile Bs code: +1, -1, +1, -1, +1, -1, +1, -1


Product result: -1, -1, +1, -3, -3, +1, -1, -1 Sum of Products: -8 Decode rule: For result near -8, data is binary 0

Comparison of Multiplexing Techniques


CompressionLossless versus Lossy

Compression is another technique used to squeeze more data over a communications line If you can compress a data file down to one half

of its original size, file will obviously transfer in less time


less time Two basic groups of compression:

Lossless when data is uncompressed, original data returns

Lossy when data is uncompressed, you do not have the original data

CompressionLossless versus Lossy (continued)

Compress a financial file? You want lossless

Compress a video image, movie, or audio file? Lossy is OK


Examples of lossless compression include: Huffman codes, run-length compression, and

Lempel-Ziv compression Examples of lossy compression include:

MPEG, JPEG, MP3

Lossless Compression

Run-length encoding Replaces runs of 0s with a count of how many 0s.

0000000000000010000000001100000000000000000000111000000000001^

(30 0s)


(30 0s)

14 9 0 20 30 0 11

Lossless Compression (continued)

Run-length encoding (continued) Now replace each decimal value with a 4-bit

binary value (nibble) Note: If you need to code a value larger than 15,

you need to use two consecutive 4-bit nibbles


you need to use two consecutive 4-bit nibbles The first is decimal 15, or binary 1111, and the

second nibble is the remainder For example, if the decimal value is 20, you would

code 1111 0101 which is equivalent to 15 + 5

Lossless Compression (continued)

Run-length encoding (continued) If you want to code the value 15, you still need

two nibbles: 1111 0000 The rule is that if you ever have a nibble of 1111,

you must follow it with another nibble


you must follow it with another nibble

Lossy Compression

Relative or differential encoding Video does not compress well using run-length

encoding In one color video frame, not much is alike


But what about from frame to frame? Send a frame, store it in a buffer Next frame is just difference from previous frame Then store that frame in buffer, etc.

5 7 6 2 8 6 6 3 5 66 5 7 5 5 6 3 2 4 78 4 6 8 5 6 4 8 8 55 1 2 9 8 6 5 5 6 6First Frame

5 7 6 2 8 6 6 3 5 66 5 7 6 5 6 3 2 3 78 4 6 8 5 6 4 8 8 55 1 3 9 8 6 5 5 7 6Second Frame

Lossy Compression (continued)


First Frame Second Frame

0 0 0 0 0 0 0 0 0 00 0 0 1 0 0 0 0 -1 00 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 1 0Difference


Image Compression One image (JPEG) or continuous images

(MPEG) A color picture can be defined by red/green/blue,


or luminance/chrominance/chrominance which are based on RGB values

Either way, you have 3 values, each 8 bits, or 24 bits total (224 colors!)


Image Compression (continued) A VGA screen is 640 x 480 pixels

24 bits x 640 x 480 = 7,372,800 bits Ouch! And video comes at you 30 images per second


Double Ouch! We need compression!


JPEG (Joint Photographic Experts Group) Compresses still images Lossy JPEG compression consists of 3 phases:


JPEG compression consists of 3 phases: Discrete cosine transformations (DCT) Quantization Run-length encoding


JPEG Step 1 DCT Divide image into a series of 8x8 pixel blocks If the original image was 640x480 pixels, the new

picture would be 80 blocks x 60 blocks (next


slide) If B&W, each pixel in 8x8 block is an 8-bit value

(0-255) If color, each pixel is a 24-bit value (8 bits for red,

8 bits for blue, and 8 bits for green)

80 blocks



60 blocks

640 x 480 VGA Screen ImageDivided into 8 x 8 Pixel Blocks


JPEG Step 1 DCT (continued) So what does DCT do?

Takes an 8x8 array (P) and produces a new 8x8 array (T) using cosines


T matrix contains a collection of values called spatial frequencies

These spatial frequencies relate directly to how much the pixel values change as a function of their positions in the block


JPEG Step 1 DCT (continued) An image with uniform color changes (little fine

detail) has a P array with closely similar values and a corresponding T array with many zero values


values An image with large color changes over a small

area (lots of fine detail) has a P array with widely changing values, and thus a T array with many non-zero values

120 80 110 65 90 142 56 100


652 32 -40 54 -18 129 -33 84111 -33 53 9 123 -43 65 100-22 101 94 -32 23 104 76 10188 33 211 2 -32 143 43 14132 -32 43 0 122 -48 54 11054 11 133 27 56 154 13 -94-54 -69 10 109 65 0 17 -33199 -18 99 98 22 -43 8 32

Lossy Compression (continued) JPEG Step 2 -Quantization

The human eye cant see small differences in color

So take T matrix and divide all values by 10 Will give us more zero entries


Will give us more zero entries More 0s means more compression!

But this is too lossy And dividing all values by 10 doesnt take into

account that upper left of matrix has more action (the less subtle features of the image, or low spatial frequencies)

1 3 5 7 9 11 13 153 5 7 9 11 13 15 175 7 9 11 13 15 17 197 9 11 13 15 17 19 219 11 13 15 17 19 21 2311 13 15 17 19 21 23 2513 15 17 19 21 23 25 27



13 15 17 19 21 23 25 2715 17 19 21 23 25 27 29

U matrix

Q[i][j] = Round(T[i][j] / U[i][j]), for i = 0, 1, 2, 7 andj = 0, 1, 2, 7


JPEG Step 3 Run-length encoding Now take the quantized matrix Q and perform

run-length encoding on it But dont just go across the rows


Longer runs of zeros if you perform the run-length encoding in a diagonal fashion


Figure 5-13Run-length encoding of a JPEG image



How do you get the image back? Undo run-length encoding Multiply matrix Q by matrix U yielding matrix T Apply similar cosine calculations to get original P


Apply similar cosine calculations to get original P matrix back

Business Multiplexing In Action

Bills Market has 10 cash registers at the front of their store

Bill wants to connect all cash registers together to collect data transactions


to collect data transactions List some efficient techniques to link the cash

registers

Business Multiplexing In Action (continued)

Possible solutions Connect each cash register to a server using point-to-

point lines Transmit the signal of each cash register to a server

using wireless transmissions


using wireless transmissions Combine all the cash register outputs using

multiplexing, and send the multiplexed signal over a conducted-medium line

Summary

For multiple signals to share a single medium, the medium must be divided into multiple channels

Frequency division multiplexing involves assigning nonoverlapping frequency ranges to different signals Uses analog signals


Uses analog signals Time division multiplexing of a medium involves

dividing the available transmission time on a medium among the users Uses digital signals

Summary (continued) Synchronous time division multiplexing accepts input

from a fixed number of devices and transmits their data in an unending repetitious pattern

Statistical time division multiplexing accepts input from a set of devices that have data to transmit, creates a frame


set of devices that have data to transmit, creates a frame with data and control information, and transmits that frame

Wavelength division multiplexing involves fiber-optic systems and the transfer of multiple streams of data over a single fiber using multiple, colored laser transmitters

Discrete multitone is a technology used in DSL systems

Summary (continued) Code division multiplexing allows multiple users to share

the same set of frequencies by assigning a unique digital code to each user

Compression is a process that compacts data into a smaller package

Two basic forms of compression exist: lossless and


Two basic forms of compression exist: lossless and lossy

Two popular forms of lossless compression include run-length encoding and the Lempel-Ziv compression technique

Lossy compression is the basis of a number of compression techniques

Chapter Six

Errors, Error Detection, and Error Control


Seventh Edition


Identify the different types of noise commonly found in computer networks

Specify the different error-prevention techniques, and be able to apply an error-


techniques, and be able to apply an error-prevention technique to a type of noise

Compare the different error-detection techniques in terms of efficiency and efficacy

Perform simple parity and longitudinal parity calculations, and enumerate their strengths and weaknesses


Cite the advantages of arithmetic checksum Cite the advantages of cyclic redundancy

checksum, and specify what types of errors cyclic redundancy checksum will detect


cyclic redundancy checksum will detect Differentiate between the basic forms of error

control, and describe the circumstances under which each may be used

Follow an example of a Hamming self-correcting code

Introduction

Noise is always present If a communications line experiences too much

noise, the signal will be lost or corrupted Communication systems should check for


Communication systems should check for transmission errors

Once an error is detected, a system may perform some action

Some systems perform no error control, but simply let the data in error be discarded

White Noise

Also known as thermal or Gaussian noise Relatively constant and can be reduced If white noise gets too strong, it can completely

disrupt the signal


disrupt the signal

White Noise (continued)


Impulse Noise

One of the most disruptive forms of noise Random spikes of power that can destroy one or

more bits of information Difficult to remove from an analog signal


Difficult to remove from an analog signal because it may be hard to distinguish from the original signal

Impulse noise can damage more bits if the bits are closer together (transmitted at a faster rate)

Impulse Noise (continued)


Crosstalk

Unwanted coupling between two different signal paths For example, hearing another conversation while

talking on the telephone


Relatively constant and can be reduced with proper measures

Crosstalk (continued)


Echo

The reflective feedback of a transmitted signal as the signal moves through a medium

Most often occurs on coaxial cable If echo bad enough, it could interfere with


If echo bad enough, it could interfere with original signal

Relatively constant, and can be significantly reduced

Echo (continued)


Jitter

The result of small timing irregularities during the transmission of digital signals

Occurs when a digital signal is repeated over and over


and over If serious enough, jitter forces systems to slow

down their transmission Steps can be taken to reduce jitter

Jitter (continued)


Delay Distortion

Occurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal Can be reduced


Can be reduced

Attenuation

The continuous loss of a signals strength as it travels through a medium


Error Prevention

To prevent errors from happening, several techniques may be applied: Proper shielding of cables to reduce interference Telephone line conditioning or equalization


Telephone line conditioning or equalization Replacing older media and equipment with new,

possibly digital components Proper use of digital repeaters and analog

amplifiers Observe the stated capacities of the media

Error Prevention (continued)


Error Detection

Despite the best prevention techniques, errors may still happen

To detect an error, something extra has to be added to the data/signal


added to the data/signal This extra is an error detection code

Three basic techniques for detecting errors: parity checking, arithmetic checksum, and cyclic redundancy checksum

Parity Checks

Simple parity If performing even parity, add a parity bit such

that an even number of 1s are maintained If performing odd parity, add a parity bit such that


an odd number of 1s are maintained For example, send 1001010 using even parity For example, send 1001011 using even parity

Parity Checks (continued)

Simple parity (continued) What happens if the character 10010101 is sent

and the first two 0s accidentally become two 1s? Thus, the following character is received:

11110101


11110101 Will there be a parity error?

Problem: Simple parity only detects odd numbers of bits in error


Longitudinal parity Adds a parity bit to each character then adds a

row of parity bits after a block of characters The row of parity bits is actually a parity bit for


each column of characters The row of parity bits plus the column parity bits

add a great amount of redundancy to a block of characters


Both simple parity and longitudinal parity do not catch all errors

Simple parity only catches odd numbers of bit errors


errors

Longitudinal parity is better at catching errors but requires too many check bits added to a block of data

We need a better error detection method What about arithmetic checksum?

Arithmetic Checksum

Used in TCP and IP on the Internet Characters to be transmitted are converted to

numeric form and summed Sum is placed in some form at the end of the


Sum is placed in some form at the end of the transmission

Arithmetic Checksum

Simplified example:567234


3448

210Then bring 2 down and add to right-most position

102

12

Arithmetic Checksum

Receiver performs same conversion and summing and compares new sum with sent sum

TCP and IP processes a little more complex but idea is the same


idea is the same But even arithmetic checksum can let errors slip

through. Is there something more powerful yet?

Cyclic Redundancy Checksum

CRC error detection method treats the packet of data to be transmitted as a large polynomial

Transmitter takes the message polynomial and using polynomial arithmetic, divides it by a given


using polynomial arithmetic, divides it by a given generating polynomial

Quotient is discarded but the remainder is attached to the end of the message

Cyclic Redundancy Checksum (continued)

The message (with the remainder) is transmitted to the receiver

The receiver divides the message and remainder by the same generating polynomial


remainder by the same generating polynomial If a remainder not equal to zero results, there

was an error during transmission If a remainder of zero results, there was no error

during transmission


Some standard generating polynomials: CRC-12: x12 + x11 + x3 + x2 + x + 1 CRC-16: x16 + x15 + x2 + 1 CRC-CCITT: x16 + x15 + x5 + 1


CRC-CCITT: x16 + x15 + x5 + 1 CRC-32: x32 + x26 + x23 + x22 + x16 + x12 + x11 +

x10 + x8 + x7 + x5 + x4 + x2 + x + 1 ATM CRC: x8 + x2 + x + 1

Error Control

Once an error is detected, what is the receiver going to do? Do nothing (simply toss the frame or packet) Return an error message to the transmitter


Return an error message to the transmitter Fix the error with no further help from the

transmitter

Do Nothing (Toss the Frame/Packet) Seems like a strange way to control errors but

some lower-layer protocols such as frame relay perform this type of error control

For example, if frame relay detects an error, it simply tosses the frame


simply tosses the frame No message is returned

Frame relay assumes a higher protocol (such as TCP/IP) will detect the tossed frame and ask for retransmission

Return A Message

Once an error is detected, an error message is returned to the transmitter

Two basic forms: Stop-and-wait error control


Stop-and-wait error control Sliding window error control

Stop-and-Wait Error Control

Stop-and-wait is the simplest of the error control protocols

A transmitter sends a frame then stops and waits for an acknowledgment


waits for an acknowledgment If a positive acknowledgment (ACK) is received,

the next frame is sent If a negative acknowledgment (NAK) is received,

the same frame is transmitted again

Stop-and-Wait Error Control (continued)


Sliding Window Error Control

These techniques assume that multiple frames are in transmission at one time

A sliding window protocol allows the transmitter to send a number of data packets at one time


to send a number of data packets at one time before receiving any acknowledgments Depends on window size

When a receiver does acknowledge receipt, the returned ACK contains the number of the frame expected next

Sliding Window Error Control (continued)



Older sliding window protocols numbered each frame or packet that was transmitted

More modern sliding window protocols number each byte within a frame


each byte within a frame An example in which the packets are numbered,

followed by an example in which the bytes are numbered:


Notice that an ACK is not always sent after each frame is received It is more efficient to wait for a few received

frames before returning an ACK


How long should you wait until you return an ACK?

Sliding Window Error Control (continued) Using TCP/IP, there are some basic rules concerning

ACKs: Rule 1: If a receiver just received data and wants to send

its own data, piggyback an ACK along with that data Rule 2: If a receiver has no data to return and has just

ACKed the last packet, receiver waits 500 ms for another


ACKed the last packet, receiver waits 500 ms for another packet

If while waiting, another packet arrives, send the ACK immediately

Rule 3: If a receiver has no data to return and has just ACKed the last packet, receiver waits 500 ms

No packet, send ACK


What happens when a packet is lost? As shown in the next slide, if a frame is lost, the

following frame will be out of sequence The receiver will hold the out of sequence bytes in

a buffer and request the sender to retransmit the


a buffer and request the sender to retransmit the missing frame


What happens when an ACK is lost? As shown in the next slide, if an ACK is lost, the

sender will wait for the ACK to arrive and eventually time out


When the time-out occurs, the sender will resend the last frame

Correct the Error

For a receiver to correct the error with no further help from the transmitter requires a large amount of redundant information to accompany the original data


This redundant information allows the receiver to determine the error and make corrections

This type of error control is often called forward error correction and involves codes called Hamming codes

Correct the Error (continued) Hamming codes add additional check bits to a

character These check bits perform parity checks on various

bits Example: One could create a Hamming code in


Example: One could create a Hamming code in which 4 check bits are added to an 8-bit character We can number the check bits c8, c4, c2 and c1 We will number the data bits b12, b11, b10, b9, b7,

b6, b5, and b3 Place the bits in the following order: b12, b11, b10,

b9, c8, b7, b6, b5, c4, b3, c2, c1

Correct the Error (continued)

Example (continued): c8 will perform a parity check on bits b12, b11, b10,

and b9 c4 will perform a parity check on bits b12, b7, b6 and

b5


b5 c2 will perform a parity check on bits b11, b10, b7, b6

and b3 c1 will perform a parity check on bits b11, b9, b7, b5,

and b3 The next slide shows the check bits and their values


The sender will take the 8-bit character and generate the 4 check bits as described The 4 check bits are then added to the 8 data bits

in the sequence as shown and then transmitted


The receiver will perform the 4 parity checks using the 4 check bits If no bits flipped during transmission, then there

should be no parity errors What happens if one of the bits flipped during

transmission?


For example, what if bit b9 flips? The c8 check bit checks bits b12, b11, b10, b9 and c8

(01000) This would cause a parity error

The c4 check bit checks bits b12, b7, b6, b5 and c4


The c4 check bit checks bits b12, b7, b6, b5 and c4 (00101)

This would not cause a parity error (even number of 1s) The c2 check bit checks bits b11, b10, b7, b6, b3 and

c2 (100111) This would not cause a parity error


For example, what if bit b9 flips? (continued) The c1 check bit checks b11, b9, b7, b5, b3 and

c1 (100011) This would cause a parity error


Writing the parity errors in sequence gives us 1001, which is binary for the value 9

Thus, the bit error occurred in the 9th position

Error Detection In Action

FEC is used in transmission of radio signals, such as those used in transmission of digital television (Reed-Solomon and Trellis encoding) and 4D-PAM5 (Viterbi and Trellis encoding)


Some FEC is based on Hamming Codes

Summary

Noise is always present in computer networks, and if the noise level is too high, errors will be introduced during the transmission of data Types of noise include white noise, impulse noise,

crosstalk, echo, jitter, and attenuation


crosstalk, echo, jitter, and attenuation Among the techniques for reducing noise are proper

shielding of cables, telephone line conditioning or equalization, using modern digital equipment, using digital repeaters and analog amplifiers, and observing the stated capacities of media

Summary (continued)

Three basic forms of error detection are parity, arithmetic checksum, and cyclic redundancy checksum

Cyclic redundancy checksum is a superior error-


Cyclic redundancy checksum is a superior error-detection scheme with almost 100 percent capability of recognizing corrupted data packets

Once an error has been detected, there are three possible options: do nothing, return an error message, and correct the error

Summary (continued) Stop-and-wait protocol allows only one packet to

be sent at a time Sliding window protocol allows multiple packets

to be sent at one time Error correction is a possibility if the transmitted


Error correction is a possibility if the transmitted data contains enough redundant information so that the receiver can properly correct the error without asking the transmitter for additional information

Ch_4-6_pp INSY

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