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Chapter Four
Making Connections
Data Communications and Computer Networks: A Business Users Approach
Seventh Edition
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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
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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
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to-mainframe connections and why they are unique compared to other types of computer connections
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Introduction
Connecting peripheral devices to a computer has, in the past, been a fairly challenging task
Newer interfaces have made this task much easier
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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
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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
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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
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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
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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:
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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
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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
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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
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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
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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)
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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
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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
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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
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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
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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
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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-
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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
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Asynchronous Connections (continued)
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Asynchronous Connections (continued)
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Asynchronous Connections (continued)
The term asynchronous is misleading here because you must always maintain synchronization between the incoming data stream and the receiver
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Asynchronous connections maintain synchronization by using small frames with a leading start bit
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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
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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
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Synchronous Connections (continued)
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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
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(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
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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
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Multipoint connection a shared connection between multiple terminals and a mainframe computer
The mainframe is the primary and the terminals are the secondaries
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Terminal-to-Mainframe Computer Connections (continued)
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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
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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
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Terminal-to-Mainframe Computer Connections (continued)
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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
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has a laptop in class, maybe someone will volunteer to use theirs for show-and-tell
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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)
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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)
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Making Computer Connections In Action (continued)
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
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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
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Making Computer Connections In Action (continued)
If a synchronous connection is used, assume maximum payload size 1500 bytes
To transfer a 700K char file requires 467 1500-character (byte) frames
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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
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Making Computer Connections In Action (continued)
1500 bytes payload + 5 byte overhead = 1505 byte frames
467 frames * 1505 bytes/frame = 716,380 bytes, or 5,731,040 bits
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or 5,731,040 bits Significantly less data using synchronous
connection
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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,
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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
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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
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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
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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
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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
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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
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between more than one computer terminal and a mainframe computer
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Chapter Five
Making Connections Efficient: Multiplexing and Compression
Data Communications and Computer Networks: A Business Users Approach
Seventh Edition
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After reading this chapter, you should be able to:
Describe frequency division multiplexing and list its applications, advantages, and disadvantages
Describe synchronous time division multiplexing and list its applications, advantages, and
Data Communications and Computer Networks: A Business User's Approach, Seventh Edition 2
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
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After reading this chapter, you should be able to (continued):
Cite the main characteristics of wavelength division multiplexing and its advantages and disadvantages
Describe the basic characteristics of discrete multitone
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multitone Cite the main characteristics of code division
multiplexing and its advantages and disadvantages
Apply a multiplexing technique to a typical business situation
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After reading this chapter, you should be able to (continued):
Describe the difference between lossy and lossless compression
Describe the basic operation of run-length, JPEG, and MP3 compression
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JPEG, and MP3 compression
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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
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portion of the total bandwidth Current techniques include:
Frequency division multiplexing Time division multiplexing Code division multiplexing
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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
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A multiplexor accepts inputs and assigns frequencies to each device
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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
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on the receiving end of the medium and separates the multiplexed signals
A common example is broadcast radio
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Frequency Division Multiplexing (continued)
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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
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division multiplexing This technique is the oldest multiplexing
technique Since it involves a certain level of analog
signaling, it may be susceptible to noise
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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
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Time division multiplexing comes in two basic forms: Synchronous time division multiplexing Statistical time division multiplexing
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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
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the data in a never -ending pattern T-1 and SONET telephone systems are common
examples of synchronous time division multiplexing
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Synchronous Time Division Multiplexing (continued)
Figure 5-2 Several cash registers and their multiplexed stream of transactions
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transactions
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Synchronous Time Division Multiplexing (continued)
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
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devices, or buffer the faster incoming stream If a device has nothing to transmit, the
multiplexor must still insert something into the multiplexed stream
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Synchronous Time Division Multiplexing (continued)
Figure 5-3Multiplexor transmission stream with
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stream with only one input device transmitting data
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Synchronous Time Division Multiplexing (continued)
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
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Synchronous Time Division Multiplexing (continued)
Figure 5-4Transmitted frame with added synchroni-
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synchroni-zation bits
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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
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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
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T-1 Multiplexing (continued)
Figure 5-4T-1 multiplexed data stream
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SONET/SDH Multiplexing
Similar to T-1, SONET incorporates a continuous series of frames
SONET is used for high-speed data transmission
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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
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SONET/SDH Multiplexing (continued)
Figure 5-6SONET STS-1 frame layout
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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
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in the multiplexed stream
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Statistical Time Division Multiplexing (continued)
Figure 5-7Two stations out of four transmitting via a statistical
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via a statistical multiplexor
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Statistical Time Division Multiplexing (continued)
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
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To identify each piece of data, an address is included
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Statistical Time Division Multiplexing (continued)
Figure 5-8Sample address and data in a statistical
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statistical multiplexor output stream
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Statistical Time Division Multiplexing (continued)
If the data is of variable size, a length is also included
Figure 5-9Packets of
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Packets of address, length, and data fields in a statistical multiplexor output stream
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Statistical Time Division Multiplexing (continued)
More precisely, the transmitted frame contains a collection of data groups
Figure 5-10Frame layout
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Frame layout for the information packet transferred between statistical multiplexors
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Wavelength Division Multiplexing
Wavelength division multiplexing multiplexes multiple data streams onto a single fiber-optic line
Different wavelength lasers (called lambdas)
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Different wavelength lasers (called lambdas) transmit the multiple signals
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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
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Dense wavelength division multiplexing combines many (30, 40, 50 or more) onto one fiber
Coarse wavelength division multiplexing combines only a few lambdas
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Wavelength Division Multiplexing (continued)
Figure 5-11Fiber optic line using wavelength division multiplexing and supporting multiple-speed transmissions
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transmissions
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Discrete Multitone
Discrete Multitone (DMT) a multiplexing technique commonly found in digital subscriber line (DSL) systems
DMT combines hundreds of different signals, or
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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
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Discrete Multitone (continued)
Each subchannel is quadrature amplitude modulated (recall eight phase angles, four with double amplitudes)
Theoretically, 256 subchannels, each
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Theoretically, 256 subchannels, each transmitting 60 kbps, yields 15.36 Mbps
Unfortunately, there is noise, so the subchannels back down to slower speeds
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Discrete Multitone (continued)
Figure 5-12256 quadrature amplitude modulated streams combined into one DMT signal for DSL
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DSL
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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
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the same time Each mobile device is assigned a unique 64-bit
code
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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
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the inverse of the code To send nothing, a mobile device transmits
zeros
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Code Division Multiplexing (continued)
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
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Interprets as a binary 0 if sum is near -64
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Code Division Multiplexing (continued) For simplicity, assume 8-bit code Example
Three different mobile devices use the following codes:
Mobile A: 11110000
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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
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Code Division Multiplexing (continued)
Example (continued) Three signals transmitted:
Mobile A sends a 1, or 11110000, or ++++---- Mobile B sends a 0, or 01010101, or -+-+-+-+
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Mobile C sends a 1, or 00110011, or --++--++ Summed signal received by base station: -1, +1,
+1, +3, -3, -1, -1, +1
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Code Division Multiplexing (continued)
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
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Product result: -1, +1, +1, +3, +3, +1, +1, -1 Sum of Products: +8 Decode rule: For result near +8, data is binary 1
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Code Division Multiplexing (continued)
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
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Product result: -1, -1, +1, -3, -3, +1, -1, -1 Sum of Products: -8 Decode rule: For result near -8, data is binary 0
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Comparison of Multiplexing Techniques
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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
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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
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CompressionLossless versus Lossy (continued)
Compress a financial file? You want lossless
Compress a video image, movie, or audio file? Lossy is OK
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Examples of lossless compression include: Huffman codes, run-length compression, and
Lempel-Ziv compression Examples of lossy compression include:
MPEG, JPEG, MP3
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Lossless Compression
Run-length encoding Replaces runs of 0s with a count of how many 0s.
0000000000000010000000001100000000000000000000111000000000001^
(30 0s)
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(30 0s)
14 9 0 20 30 0 11
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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
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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
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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
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you must follow it with another nibble
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Lossy Compression
Relative or differential encoding Video does not compress well using run-length
encoding In one color video frame, not much is alike
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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.
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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)
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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
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Lossy Compression (continued)
Image Compression One image (JPEG) or continuous images
(MPEG) A color picture can be defined by red/green/blue,
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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!)
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Lossy Compression (continued)
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
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Double Ouch! We need compression!
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Lossy Compression (continued)
JPEG (Joint Photographic Experts Group) Compresses still images Lossy JPEG compression consists of 3 phases:
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JPEG compression consists of 3 phases: Discrete cosine transformations (DCT) Quantization Run-length encoding
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Lossy Compression (continued)
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
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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)
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80 blocks
Lossy Compression (continued)
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60 blocks
640 x 480 VGA Screen ImageDivided into 8 x 8 Pixel Blocks
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Lossy Compression (continued)
JPEG Step 1 DCT (continued) So what does DCT do?
Takes an 8x8 array (P) and produces a new 8x8 array (T) using cosines
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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
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Lossy Compression (continued)
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
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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
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120 80 110 65 90 142 56 100
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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
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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
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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)
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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
Lossy Compression (continued)
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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
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Lossy Compression (continued)
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
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Longer runs of zeros if you perform the run-length encoding in a diagonal fashion
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Lossy Compression (continued)
Figure 5-13Run-length encoding of a JPEG image
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Lossy Compression (continued)
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
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Apply similar cosine calculations to get original P matrix back
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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
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to collect data transactions List some efficient techniques to link the cash
registers
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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
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using wireless transmissions Combine all the cash register outputs using
multiplexing, and send the multiplexed signal over a conducted-medium line
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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
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Uses analog signals Time division multiplexing of a medium involves
dividing the available transmission time on a medium among the users Uses digital signals
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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
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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
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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
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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
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Chapter Six
Errors, Error Detection, and Error Control
Data Communications and Computer Networks: A Business Users Approach
Seventh Edition
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After reading this chapter, you should be able to:
Identify the different types of noise commonly found in computer networks
Specify the different error-prevention techniques, and be able to apply an error-
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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
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After reading this chapter, you should be able to (continued):
Cite the advantages of arithmetic checksum Cite the advantages of cyclic redundancy
checksum, and specify what types of errors cyclic redundancy checksum will detect
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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
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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
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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
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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
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disrupt the signal
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White Noise (continued)
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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
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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)
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Impulse Noise (continued)
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Impulse Noise (continued)
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Crosstalk
Unwanted coupling between two different signal paths For example, hearing another conversation while
talking on the telephone
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Relatively constant and can be reduced with proper measures
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Crosstalk (continued)
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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
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If echo bad enough, it could interfere with original signal
Relatively constant, and can be significantly reduced
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Echo (continued)
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Jitter
The result of small timing irregularities during the transmission of digital signals
Occurs when a digital signal is repeated over and over
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and over If serious enough, jitter forces systems to slow
down their transmission Steps can be taken to reduce jitter
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Jitter (continued)
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Delay Distortion
Occurs because the velocity of propagation of a signal through a medium varies with the frequency of the signal Can be reduced
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Can be reduced
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Attenuation
The continuous loss of a signals strength as it travels through a medium
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Error Prevention
To prevent errors from happening, several techniques may be applied: Proper shielding of cables to reduce interference Telephone line conditioning or equalization
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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
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Error Prevention (continued)
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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
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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
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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
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an odd number of 1s are maintained For example, send 1001010 using even parity For example, send 1001011 using even parity
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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
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11110101 Will there be a parity error?
Problem: Simple parity only detects odd numbers of bits in error
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Parity Checks (continued)
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
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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
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Parity Checks (continued)
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Parity Checks (continued)
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Parity Checks (continued)
Both simple parity and longitudinal parity do not catch all errors
Simple parity only catches odd numbers of bit errors
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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?
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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
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Sum is placed in some form at the end of the transmission
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Arithmetic Checksum
Simplified example:567234
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3448
210Then bring 2 down and add to right-most position
102
12
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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
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idea is the same But even arithmetic checksum can let errors slip
through. Is there something more powerful yet?
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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
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using polynomial arithmetic, divides it by a given generating polynomial
Quotient is discarded but the remainder is attached to the end of the message
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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
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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
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Cyclic Redundancy Checksum (continued)
Some standard generating polynomials: CRC-12: x12 + x11 + x3 + x2 + x + 1 CRC-16: x16 + x15 + x2 + 1 CRC-CCITT: x16 + x15 + x5 + 1
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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
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Cyclic Redundancy Checksum (continued)
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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
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Return an error message to the transmitter Fix the error with no further help from the
transmitter
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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
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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
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Return A Message
Once an error is detected, an error message is returned to the transmitter
Two basic forms: Stop-and-wait error control
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Stop-and-wait error control Sliding window error control
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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
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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
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Stop-and-Wait Error Control (continued)
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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
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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
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Sliding Window Error Control (continued)
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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
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each byte within a frame An example in which the packets are numbered,
followed by an example in which the bytes are numbered:
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
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
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How long should you wait until you return an ACK?
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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
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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
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
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
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a buffer and request the sender to retransmit the missing frame
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Sliding Window Error Control (continued)
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Sliding Window Error Control (continued)
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
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When the time-out occurs, the sender will resend the last frame
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Sliding Window Error Control (continued)
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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
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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
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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
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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
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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
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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
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Correct the Error (continued)
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Correct the Error (continued)
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
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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?
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Correct the Error (continued)
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
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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
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Correct the Error (continued)
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
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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
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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)
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Some FEC is based on Hamming Codes
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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
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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
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Summary (continued)
Three basic forms of error detection are parity, arithmetic checksum, and cyclic redundancy checksum
Cyclic redundancy checksum is a superior error-
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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
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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
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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