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Data Communication Basics. Chapter 6. Knowledge Concepts. Applications used in networking The 7-part data circuit Duplex data transmission Transmission channel Synchronous transmission OSI layer responsibilities Error control. Internet Application Software.

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knowledge concepts
Knowledge Concepts
  • Applications used in networking
  • The 7-part data circuit
  • Duplex data transmission
  • Transmission channel
  • Synchronous transmission
  • OSI layer responsibilities
  • Error control
internet application software
Internet Application Software

There are 4 important Internet application software tools:

  • the Web
  • electronic mail (e-mail)
  • FTP
  • Telnet
how the web works
How the Web Works
  • Each client computer needs an applications layer software package called a Web browser
  • Each server on the network needs an application layer software package called a Web Server
  • In order to get a page (file) from the Web, the user must first type the Internet Uniform Resource Locator (URL)
how the web works6
How the Web Works
  • In order for the request from the Web browser to be understood by the Web server, they must use the same standard protocol
  • The standard protocol for communication between a Web browser and a Web server is Hypertext Transfer Protocol (HTTP)
how the web works7
How the Web Works

An HTTP request from a Web browser to a Web server has three parts. Only the first part is required, the other two are optional

  • the request line
  • the request header
  • the request body
how the web works8
How the Web Works

Command URL HTTP version

GET http://tcbworks.cba.uga.edu/~adennis/res.htm HTTP/1.1

Date: Mon 03 Aug 1998 17:35:46 GMT

User-Agent: Mozilla/3.0

From: adennis@uga.cc.uga.edu

Referer: http://tcbworks.cba.uga.edu/~adennis/home.htm

]- Request Line

]- Date

]- Web browser (this is Netscape)

Request Header

]- User’s e-mail address

URL that contained the link to the requested URL

A Request from a Web browser to a Web server

using the HTTP standard

how the web works9
How the Web Works
  • Many people believe that the Web is anonymous
  • Every Web access must provide the Internet address of the requester’s computer; otherwise the server would not know where to send the requested page
how the web works10
How the Web Works
  • The format of an HTTP response from the server to the browser is very similar to the browser request
  • Only the last part is required, the other 2 are optional
    • the response status
    • the response header
    • the response body
slide11

A Response From a Web Server to a Web Browser

Using the HTTP Standard

HTTP version Status code Reason phrase

HTTP/1.1 200 OK

Date: Mon 03 Aug 1998 17:35:46 GMT

Server: NCSA/1.3

Location: http:// tcbworks.cba.uga.edu/~adennis/res.htm

Content-type: text/html

<html>

<head>

<title>Business Data Communications and Networking Web Resources </title>

</head>

<body>

<H2>Resources on the Web </H2>

<P>This section contains links to other resources on the WEB that pertain to

the field of data communications and networking </P>

</body>

</html>

]- Date

]- Web server

Response Header

]- URL

]- Type of file

Response

Body

e mail standards
E-mail Standards
  • Several standards have been developed to ensure compatibility between different e-mail software packages
  • 3 most commonly used standards are:
    • SMTP
    • X.400
    • CMC
  • All three e-mail standards work in the same basic fashion
e mail standards14
E-mail Standards
  • Each client computer in the LAN runs an application layer software package called a user agent, which formats the message into two parts:
    • The header (source & destination address)
    • The body (message)
e mail standards15
E-mail Standards
  • The user sends the message to a mail server that runs a special application layer software package called a message transfer agent, which reads the header and sends the message through the network to the receiver’s mail server, where it is stored in the receiver’s mailbox.
e mail standards16
E-mail Standards

The Simple Mail Transfer Protocol (SMTP) is one of the most commonly used e-mail standards simply because it is the e-mail standard used on the Internet

e mail standards17
E-mail Standards

TO: “Pat Someone” <someone@somewhere.com>

From: “Alan Dennis;”<adennis@uga.cc.uga.edu>

Date: Mon 03 Aug 1998 17:35:46 GMT

Subject: Sample Note

DATA: This is an example of an e-mail message

Message Header

]- Message Body

An example of an e-mail message using the SMTP standard

e mail standards18
E-mail Standards
  • The SMTP standards covers message transmission between message transfer agents
  • A different standard called Post Office Protocol (POP) defines how user agents operate and how messages to and from mail transfer agents are formatted
  • POP is gradually being replaced by a newer standard called Internet Mail Access Protocol (IMAP)
e mail standards19
E-mail Standards
  • Two other commonly used e-mail standards are X.400 and CMC, which are different from SMTP, POP and IMAP, so that they cannot be used interchangeably
  • The X.400 e-mail standards was developed in 1984
  • The Common Messaging Calls (CMC) standard is a simpler version of the X.400 standard, developed in 1994
e mail directories
E-mail Directories
  • Before you can send an e-mail message, you must know the receiver’s e-mail address
  • There are no universal e-mail directories
file transfer protocol ftp
File Transfer Protocol (FTP)
  • File Transfer Protocol (FTP) enables you to send and receive files over the Internet
  • There are 2 types of FTP sites:
    • Closed (requires users to have permission before they can connect and gain access to files)
    • Anonymous (permits any Internet user to login using the account name of Anonymous)
  • Many files and documents available via FTP have been compressed to reduce the amount of disk space they require
telnet
Telnet
  • Telnet enables users on one computer to login into other computers on the Internet
  • Telnet can be useful because it enables you to access your server or host computer without sitting at its keyboard
groupware24
Groupware

Software that helps groups of people to work together more productively

Group support system

groupware25
Groupware
  • Groupware allows people to exchange ideas, debate issues, make decisions, and write reports without actually having to meet face-to-face
  • There are 5 popular types of groupware:
    • Discussion groups
    • Document-based groupware
    • Group support systems
    • Videoconferencing
    • IM
discussion groups
Discussion Groups
  • Discussion groups are Internet users who have joined together to discuss some topic
  • Two discussion groups commonly used for business:
    • Usenet Newsgroups
    • Listservs
usenet newsgroups
Usenet Newsgroups
  • Usenet Newsgroups are the most formally organized of the discussion groups
  • The newsgroups are just a series of discussions about each topic
listservs
Listservs
  • Listserver (Listserv) group is similar in concept to the usenet newsgroups but is generally less formal
  • One part, the listserv processor, processes commands such as requests to subscribe, unsubscribe, or to provide more information about the listserv
  • The second part is the listserv mailer-- Anymessage sent to the listserv mailer is re-sent to everyone on the mailing list
document based groupware
Document-based Groupware
  • E-mail lacks a structured way to support an ongoing discussion
  • A document database (like Lotus Notes) designed to store and manage large collections of text and graphics was the first solution
document based groupware30
Document-based Groupware
  • One of Notes’ greatest strengths is its replication ability (the automatic sharing of information among servers when information changes)
  • More than 2 million people world-wide now use Lotus Notes (IBM product)
group support systems
Group Support Systems
  • Both e-mail and document-based groupware are designed to support individuals and groups working in different places and different times.
  • Group Support Systems (GSS) are software tools designed to improve group decision making.
group support systems32
Group Support Systems
  • In a GSS meeting, group members can discuss idea verbally as they could in any meeting room; however, they can also use the computer to type ideas and information, which are then shared with all other group members via the network.
  • With large groups, however, typing ideas is faster than talking because only one person can speak at a time
group support systems33
Group Support Systems
  • GSS enables users to make comments anonymously
  • These systems also provide tools to support voting and ranking of alternatives, so that more structured decision-making processes can be used
videoconferencing
Videoconferencing

Videoconferencing provides real-time transmission of video and audio signals to enable people in two or more locations to have a meeting

desktop videoconferencing
Desktop Videoconferencing
  • The fastest growing form of videoconferencing is desktop videoconferencing
  • Small cameras installed on top of each computer permit meetings to take place from individual offices
  • The key benefits of videoconferencing area the time and cost savings that can result
desktop videoconferencing36
Desktop Videoconferencing
  • The transmission of video requires a lot of network capacity
  • Like e-mail, most videoconferencing systems were originally developed by vendors using different formats so that many products were incompatible
  • Three commonly used standards for videoconferencing have the promise to reduce many incompatibilities once adopted: H.320, H.323, and MPEG-1 & -2
electronic commerce38
Electronic Commerce
  • Almost all large and medium-sized companies use the Internet for electronic commerce - doing business on the Internet
  • Most people automatically focus on the retail aspects of electronic commerce; that is selling products to individuals
electronic commerce39
Electronic Commerce

There are four major ways in which the Web can be used to support electronic commerce:

  • Electronic store
  • Electronic marketing
  • Information/entertainment provider
  • Customer service

Each requires a different configuration of hardware!

transmission channels
Transmission Channels
  • Data network is a 7 part data circuit:
    • DTE Physical interface
    • Originating DTE
    • Physical interface
    • Originating DCE
    • Transmission channel
    • Receiving DCE
    • Physical interface
    • Receiving DTE
data channel
Data Channel

Physical

Interface

Physical

Interface

DTE

DTE

DCE

Transmission Channel

DCE

  • DTE-Transmits, receives, performs error control
  • DCE-Provides interface between DTE and channel
  • Physical Interface is wire connection
data flow
Data Flow
  • One or Two Directional
    • Simplex - One-way Transmission (Radio, TV)
    • Half Duplex - Two-way Transmission
      • One-way at a Time
      • Control Signals Negotiate Sending and Receiving
      • 2 wire
    • Full Duplex - Bi-directional
      • Simultaneous Transmission
      • 4 wire
slide43

Error Correction

  • Message Acknowledgment
    • The mechanism used to effect retransmission is the positive or negative acknowledgment, often referred to as ACK and NAK, respectively
  • Retry Limit
    • To cut down on continual retransmission of messages, a retry limit—typically between 3 and 100—can be set. A retry limit of five means that a message received in error will be retransmitted five times; if it is not successfully received by the fifth try, the receiving station either disables the link or disables the sending station itself
slide44

Error Detection

  • Sequence Checks
    • Sequence check numbers can be assigned to each block of data so that the ultimate receiver can determine that all blocks have indeed arrived, and the blocks can be put back into proper sequence
  • Error Correction Codes
    • Some error-detection schemes allow the receiving station not only to detect errors but also to correct some of them. Such codes are called forward error-correcting codes, the most common of which are called Hamming codes
slide45

Error Detection

  • Parity Check
    • A parity check (also known as vertical redundancy check [VRC]) involves adding a bit—known as the parity bit—to each character during transmission
  • Longitudinal Redundancy Check (LRC)
    • With LRC, an additional, redundant character called the block check character (BCC) is appended to a block of transmitted characters, typically at the end of the block
  • Cyclic Redundancy Check (CRC)
    • A CRC can detect bit errors better than either VRC or LRC or both. The transmitting station generates the CRC and transmits it with the data
slide47

Ports

  • Need port fast enough for line
  • 232 Serial port to 115.2 kbps: only V.34, V.90 or ISDN
  • USB (universal serial bus)
    • 12 Mbps
    • Available on all new PCs
    • Fast enough for DSLs, cable modems
    • Faster version coming (USB-2, ~480 Mbps)
slide48

Ports

  • Firewire (IEEE 1394)
    • 400 Mbps and faster
    • Not available on most new PCs
    • Fast enough for DSLs, cable modems
  • Ethernet NIC (10 Mbps)
    • Network interface card used in PC networks
    • Printed circuit board
    • Must be installed inside PC systems unit
    • Fast enough for DSLs, cable modems
slide49

PC 232 Serial Ports

  • Ports
    • Connectors at back of PC
    • Plus related internal electronics to send/receive
  • PC 232 Serial Port
    • Follows EIA/TIA 232 standards
slide50

PC 232 Serial Ports: 9-Pin and 25-Pin Ports

  • 9 pins or 25 pins
  • Parallel ports have 25 holes

Pins

9-pin Serial Port

25-pin Serial Port

25-pin Parallel Port

Holes

slide51

232 Serial Ports: Send on One Pin Each Way

  • 9-Pin 232 Serial Ports
    • PC sends on Pin 3 (modem receives)
    • PC receives on Pin 2 (modem sends)
    • Pin 5 is a signal ground defining zero volts

PC

Modem

slide52

232 Serial Ports: Send on One Pin Each Way

  • 9-Pin 232 Serial Ports
    • Other pins are control signals to tell other side when it may transmit
    • Or tell PC what modem is hearing on the line (ringing, modem carrier signal)

PC

Modem

slide53

Serial and Parallel Transmission

  • Parallel
    • N bits per second on N wires
    • Parallel is faster than serial

1

1

1

1

0

0

Eight Bits

In Clock

Cycle One

Eight Bits

In Clock

Cycle Two

1

1

1

1

0

0

0

0

0

0

slide54

Serial and Parallel Transmission

  • Parallel Transmission is Only for Short Distances
    • Usually up to about 2 meters (6 feet)
    • Wire propagation speeds vary
    • Over long distances, bits from different clock cycles overlap

1

1

1

1

0

0

1

1

1

1

0

0

0

0

0

0

slide55

PC 232 Serial Ports

  • PC 232 serial ports are binary because there are only two states (voltage levels)
  • PC 232 serial ports are serial because data is sent on only one wire at a time
data link layer introduction
Data Link Layer Introduction
  • The data link layer sits between the physical layer and the network layer
  • The data link layer accepts messages from the network layer and controls the hardware that actually transmits them
  • Both the sender and receiver have to agree on the rules or protocols that govern how their data link layers will communicate with each other
the data link layer
The Data Link Layer
  • Provides protocols that deliver reliability to upper layers
  • Layer for which network architecture standards are defined
  • Organizes the physical layer bit stream into structured frames
  • Frames add addressing and error checking information (detection, notification, recovery)
  • Frames are built inside the NIC for the type of network architecture
  • NIC cards are assigned an address by the NIC manufacturer
the ethernet frame
The Ethernet Frame
  • Preamble
    • Synchronizes signal frequency
  • Destination Address
    • Receiving station(s) by MAC
  • Source Address
    • Sending station(s) by MAC
  • Type/length
    • Upper layer protocol/data length indicator
  • Data
    • At least 64 bytes
  • Frame Check Sequence
    • Uses checksum for error control
peer to peer networks
Peer-to-Peer Networks

Peer-to-peer networks do not require a dedicated server. All computers run special network software that enables them to function as both a client and as a server

When would a peer-to-peer network be a possible solution?

p2p in the news
P2P In the News
  • Allows a user on a network to draw on spare processing power and/or storage capacity of other computers
  • UC Berkeley research project SETI@home
  • Analyzes interstellar radio signals that may contain evidence of alien civilization
seti@home
SETI@Home
  • Found spikes of energy that may prove extraterrestrial life
p2p companies
P2P Companies
  • Applied MetaComputing (creates distributed apps)
  • Flycode (free digital videos)
  • Groove networks (Collaboration)
  • MangoSoft (share browser caches)
  • Napster (music)
  • United Devices (share your PC & earn webmiles)
costs and benefits of client server architectures
Costs and Benefits of Client-Server Architectures
  • Client-server architectures have some important benefits compared to host-based architectures
    • Client-server architectures are scaleable
    • Client-server architectures can support many different types of clients and servers
    • Because no single host computer supports all the applications, the network is generally more reliable
client server architectures71
Client-Server Architectures
  • Client-server architectures have critical limitations
  • Updating the network with a new version of the software is more complicated
  • Cost
  • Trouble-shooting problems

Why would troubleshooting be more difficult in client-server

Networks?

client server architectures72
Client-Server Architectures
  • Middleware does two things:

1. It provides a standard way of communicating that can translate between software from different vendors

2. It manages the message transfer from clients to servers so that the clients need not know the specific server that contains the application’s data

two tier three tier and n tier architectures
Two-tier, Three-tier, and N-tier Architectures
  • There are many ways in which the application logic can be partitioned between the client and the server
    • Two-tiered
    • Three-tiered
    • N-tiered
two tier three tier and n tier architectures74
Two-tier, Three-tier, and N-tier Architectures

Two-tiered client-server architecture

two tier three tier and n tier architectures75
Two-tier, Three-tier, and N-tier Architectures

Three-tiered client-server architecture

n tier architectures
N-tier Architectures

N-tiered client-server architecture

two tier three tier and n tier architectures77
Two-tier, Three-tier, and N-tier Architectures
  • The primary advantage of n-tiered client-server architecture is that it separates out the processing that occurs to better balance the load on the different servers; it is more scaleable
  • There are two primary disadvantages to an n-tiered architecture

1. Greater load on the network

2. More difficult to program and test software for a n-tiered architectures

thin clients versus fat clients
Thin Clients versus Fat Clients
  • Another way of classifying client-server architectures is by examining how much of the application logic is placed on the client
  • A “thin client” places little or no logic on the client, and are easier to manage
  • A “fat client” places all or almost all of the application logic on the client
  • There is no direct relationship between thin/fat clients and 2-/3-/n-tiered architectures
data link layer osi model
Data Link Layer (OSI Model)
  • Manages the basic transmission circuit established in layer 1 and transforms it into a circuit that is free of transmission errors
  • Frames the information
  • Performs error detection, correction and retransmission of data
  • Divided into two sublayers
    • MAC - Media access control
    • LLC - Logical link control
data link protocol
Data Link Protocol

A data link protocol provides three functions:

  • Controls when computers transmit (media access control)
  • Detects and corrects transmission errors (error control)
  • Identifies the start and end of a message (message delineation)
data link functions
Data Link Functions
  • Media access control
    • .. who can transmit at a given time
  • Error control
    • .. how the receiver determines if a transmission error has occurred and corrects it
  • Message delineation
    • .. how the receiver knows where a character or message begins or ends
media access control
Media Access Control
  • The need to control when devices transmit
  • Needed when several devices share the same communication circuit:
    • Point-to-point configuration with half-duplex transmission, requiring devices to take turns
    • Multi-point configurations with several devices sharing the same line
  • Critical function in local area networks
  • Two types: Controlled access and Contention
media access control controlled access
Media Access Control - Controlled Access
  • Mainframe Networks Use Controlled Access
    • Front-end Processor
      • Controls the Circuit
      • Determines Which Devices Can Access the Media
      • Controls When the Devices Can Access the Media
    • X-on/x-off
      • One of the Oldest Protocols for Media Access Control
      • Dates Back to the Teletype
      • Used for the Transmission of Text Messages Only
      • Mostly Used in Communications Between a Computer and It’s Printer
media access control controlled access86
Media Access Control - Controlled Access

X-ON/X-OFF Communication with a printer

Time

Send X-ON

Send X-OFF

0%

Buffer Utilization

100%

Low Limit

High Limit

media access control polling
Media Access Control - Polling
  • Polling - the Process Of:
    • Sending a Signal to a Terminal That Either
    • Gives It Permission to Transmit
    • Or Asks It to Receive a Transmission
  • Fep or Host Polls the Client Terminal/pc
  • Store and Forward
    • Clients Store the Messages Until Polled
    • Responds With the Message or With a Pass
media access control polling88
Media Access Control - Polling
  • Roll Call Polling
    • Front-end Processor Polls Consecutively Through the List of Clients
    • Can Be Modified for Some Terminals to Have Priority
    • Loses Time Because of Wait - Some Terminals Off/no Messages to Send
    • Must Have a “Time-out Period” Set

1, 2, 3, 1, 4, 5, 1, 6, 7, 1, 8, 9, ...

Roll call list

media access control polling89
Media Access Control - Polling
  • Hub “Go-ahead” Polling
    • Used in Multi-point Configurations
    • Either a Front-end Processor Passes the Poll to the Most Remote Device on the Circuit Which Sends It’s Message and Passes the Poll to the Next Device or One Computer Starts the Process and Passes It to the Next Computer
    • Token Passing
    • Requires Intelligent Workstations
media access control contention
Media Access Control - Contention
  • Opposite of Controlled Access
  • Devices Wait Until the Circuit Is Free
  • Requires “Collision” Protection or Recovery
  • Works Better on Small Networks With Low Usage
  • Commonly Used in Ethernet LANs
media access control relative performance
Media Access Control - Relative Performance
  • Number of messages transmitted affects performance depending on the MAC Protocol used
  • Controlled access
    • large networks
    • high usage
    • response time increases slowly
  • Contention
    • small networks
    • low usage
    • collisions are costly in terms of throughput
    • wasted circuit capacity
  • Find the crossover point

Contention

long

Response Time

Controlled

access

short

low

high

Traffic

Figure 4-1

network errors
Network Errors
  • Human Errors
    • controlled by application programs
  • Transmission Errors
    • controlled by network hardware and software
  • Networks should be designed to:
    • prevent
    • detect
    • correct
network errors93
Network Errors
  • Corrupted data
    • data that has been altered or changed
  • Lost data
    • data that is misdirected or dropped from the transmission circuit
  • Burst errors
    • many bits changed or lost during one occurrence
what causes errors
What Causes Errors?

Line noise and Distortion cause errors

Source of ErrorWhat Causes ItHow to Prevent It

Line Outages

White Noise

Impulse Noise

Cross-Talk

Echo

Attenuation

Intermodulation

Noise

Jitter

Harmonic Distortion

Storms, Accidents

Movement of electrons

Sudden increases in electricity

(e.g. lightening, voltage changes)

Multiplexer guardbands too small,

or wires too close together

Poor connections

Gradual decrease in signal

over distance

Signals from several circuits combine

Analog signals change phase

Amplifier changes phase

Unable to prevent it

Increase signal strength

Shield or move the wires

Increase the guardbands, or

move or shield the wires

Fix the connections, or

tune equipment

Use repeaters or amplifiers

or reduce frequency or

Increase signal strength

Move or shield the wires or

adjust the equipment

Tune equipment

Tune equipment

error prevention
Error Prevention
  • Shielding
  • Moving Cables
  • Changing Multiplexing Techniques
  • Improving Connection Quality
  • Adding Amplifiers and Repeaters
  • Conditioning (Equalization)
error prevention digital repeaters
Error Prevention - Digital Repeaters

Regenerative Digital Repeater

0 1 0 1 1 0

0 1 0 1 1 0

0 1 0 1 1 0

error detection
Error Detection
  • To detect/correct errors, it is necessary to send extra bits along with the data
  • The more extra bits sent, the greater the error protection

Efficiency of

data throughput

Error detection/correction

error detection98
Error Detection

There are three common error detection methods.

  • parity checking
  • longitudinal redundancy checking
  • polynomial checking
    • checksum
    • cyclic redundancy checking
error detection99
Error Detection
  • Parity checking
    • one extra bit is added to each byte in the message
    • value is based on the number of 1’s in the byte
    • set to make the total number of 1’s even or odd
    • only specifies an error has occurred
    • 50% error detection rate
parity checking
Parity Checking

Assume we are using even parity with 7-bit ASCII.

The letter V in 7-bit ASCII is encoded as 0110101.

Because there are four 1s (an even number), parity is set to zero.

This would be transmitted as: 01101010.

Assume we are using odd parity with 7-bit ASCII.

The letter V in 7-bit ASCII is encoded as 0110101.

Because there are four 1s (an even number), parity is set to one.

This would be transmitted as: 01101011.

Assume we are using even parity with 7-bit ASCII.

The letter W in 7-bit ASCII is encoded as 0001101.

Because there are three 1s (an odd number), parity is set to one.

This would be transmitted as: 00011011.

parity checking101
Parity Checking

Transmitter

Receiver

odd parity

(W) 0 0 0 1 1 0 1 0

(W) 0 0 0 0 1 0 1 0

A 0 was appended as the parity because the total number of 1’s were odd.

A transmission error changed this bit to a 0, making the total number of 1’s even. Retransmission is requested.

error detection102
Error Detection
  • Longitudinal Redundancy Checking (LRC)
    • used with parity - creates 98% error detection
    • adds one additional byte to the end of the message - Block Check Character (BCC)

Network with Odd Parity

7-bit ASCII & Longitudinal

Redundancy Checking

Sending the message “DATA”

(D) 1 0 0 0 1 0 0 1

(A) 1 0 0 0 0 0 1 1

(T) 1 0 1 0 1 0 0 0

(A) 1 0 0 0 0 0 1 1

(BCC) 1 1 0 1 1 1 1 1

Determined Horizontally

Parity Bit Row

Determined Vertically

error detection103
Error Detection
  • Polynomial Checking
    • adds a character or series of characters to the end of the message based on one of several mathematical algorithms
    • done on blocks of data
      • CHECKSUM
      • CYCLICAL REDUNDANCY CHECK (CRC)
polynomial checking
Polynomial Checking
  • CHECKSUM
    • one byte is added to the end of the message
    • calculated by adding the decimal value of each character in the message and dividing the sum by 255, the remainder is the checksum
    • remainder is transmitted along with the message
    • this remainder is checked on both sides
    • 95% error detection rate for multiple-bit burst errors
polynomial checking105
Polynomial Checking
  • CYCLICAL REDUNDANCY CHECK (CRC)
    • adds 8, 16, 24 or 32 bits to the message
    • message us treated as one long binary polynomial number (P) divided by a fixed binary number (G)
    • the remainder is attached to the message
    • 99+% error detection rate

where:

P R

= Q +

G G

P = the message to be send

G = polynomial agreed upon by both

parties as the divisor

chosen so R is multiple of 8 bits

Q = quotient (whole number P/G)

R = remainder of P/G

parity checking106
Parity Checking

Base 10 Example of Polynomial Checking

Data to be sent: P = 247

Mutually Agreed Upon Divisor: G = 5

3. Receiver obtains data

calculates the

remainder

R = 2

then no action

R 2

then requests

retransmission of data

2. Transmitter

sends data

and remainder

2 247

1. Transmitter

calculates

the remainder

49 R2

5) 247

20

47

45

2

error correction
Error Correction
  • Retransmission
    • simplest, most effective
    • least expensive, most common
    • Automatic Repeat reQuest (ARQ)
      • Stop and Wait ARQ
      • Continuous ARQ
error correction108
Error Correction
  • Stop and Wait ARQ - sender stops and waits for a response from the receiver
    • ACK - acknowledgment - no errors detected
    • NAK - negative acknowledgment - message contained an error
error correction109
Error Correction
  • Continuous ARQ - sender continues with transmission and examines return acknowledgments at the same time
    • called sliding window
    • LAP-M retransmit only those packets with errors
    • Go-Back-N ARQ - retransmit from point of error
continuous arq sliding window
Continuous ARQ (sliding window)

Transmitter

Receiver

Go-Back-N ARQ

MSG3

MSG2

MSG1

MSG0

NAK2

MSG5

MSG4

MSG3

MSG2

LAP-M ARQ (Link Access Protocol for Modems)

MSG3

MSG2

MSG1

MSG0

NAK2

MSG6

MSG5

MSG4

MSG2

forward error correction
Forward Error Correction
  • Uses codes containing sufficient redundancy to prevent errors without retransmission
  • Essential for satellite transmissions, where propagation delay is significant
  • V.34 modem standard includes forward error checking
    • Hagelbarger code - corrects up to six consecutive bit errors provided at least 19 valid bits follow
    • Bose-Chaudhuri code - capable of correcting double errors and detecting up to four errors
    • Hamming code - associates even parity bits with unique combinations of data bits
    • Reed-Solomon code - nonbinary, multisymbol block code, capable of correcting 12 errors
forward error correction112
Forward Error Correction
  • Used with CD/ROM disk and audio compact disk technology
  • Data is encoded as a series of microscopic pits and flat spaces that are read by laser
  • This medium is very prone to errors - once data is stored, there is no way to correct it
  • Forward error correction is used to correct the stored errors when reading the CD
forward error correction113
Forward Error Correction
  • Used in main RAM memory
  • Alpha particles in the plastic encasement of memory chips cause soft errors
  • RAM memory actually contains more than the advertised amount
    • provide extra bits for forward error checking
  • Without this capability, PCs would not be reliable enough for general acceptance

1 MB = 220 bytes = 8,388,608 bits

16 MB = 134,217,728 bits

The MTBF, due to soft error, for

such RAM memory, would be:

1 million years

= 2.7 days

134,217,728

data link layer protocols
Data Link Layer Protocols
  • Focused on message delineation
    • indicates where the message starts and stops
    • and the parts of the message transmitted
    • needed to determine which part of the message is the error control portion
data link protocols
Data Link Protocols
  • Asynchronous Transmission
    • “start-stop” transmission
      • the sending device can transmit a character whenever convenient
    • a start and stop bit are appended to each character sent
    • typically used on point-to-point full duplex circuits

Figure 4-9

data link protocols116
Data Link Protocols
  • Synchronous Transmission
    • used for high-speed transmission of a block of data (frame or packet)
    • start/stop bits are NOT required
    • synchronization established by sending a group of SYN characters (1-8 SYN characters)
    • used in point-to-point and multipoint
      • for multipoint requires destination and source addresses
synchronous transmission
Synchronous Transmission

There are many protocols for synchronous transmission that fall into three broad categories:

  • Byte-oriented
  • Bit-oriented
  • Byte-count
data link protocols118
Data Link Protocols
  • Synchronous Transmission
    • Binary Synchronous Communication (BSC)
      • mainframe protocol developed by IBM in 1967
      • byte-oriented protocol
    • Synchronous Data Link Control (SDLC)
      • mainframe protocol developed by IBM in 1972
      • bit-oriented protocol
      • control fields and data do not have to be in 8-bit bytes

Start Flag

01111110

8 bits

Control

8 bits

Frame check SEQ

16 or 32 bits

End Flag

01111110

8 bits

Address

8 bits

Message (variable length)

synchronous transmission119
Synchronous Transmission
  • High-level Data Link Control (HDLC)
    • developed by the International Organization for Standardization (ISO)
    • similar to SDLC except the address and control fields are longer
  • LAP-B (Link Access Procedure-Balanced)
    • a scaled down version of HDLC
    • uses the same structure as HDLC
  • additional benefits of both are beyond the scope of this class
synchronous transmission120
Synchronous Transmission
  • Token Ring (IEEE 802.5)
    • developed by IBM in early 1980s
    • LAN protocol
    • supports transparency automatically
    • byte-orientated protocol
    • controlled media access
    • frame - starts and ends with a special electrical signal
synchronous transmission121
Synchronous Transmission
  • Ethernet (IEEE 802.3)
    • developed by Digital, Intel, and Xerox in 1970s
    • LAN protocol
    • byte-count protocol
    • supports transparency automatically
    • includes a field that specifies the length of the message portion of the packet
    • uses a contention media access protocol
synchronous transmission122
Synchronous Transmission

Token Ring format

Start Frame Destination Source Message End

delimiter control address address variable delimiter

1 byte 1 byte 6 bytes 6 bytes max of 4500 1 byte

Access Frame

control check sequence1 byte 4 bytes

Ethernet format

Destination Source Length SNAP Message CRC-32

address address 2 bytes control variable 4 bytes

6 bytes 6 bytes 5 bytes max of 1492

LLC

Control

3-bytes

synchronous transmission123
Synchronous Transmission
  • Serial Line Internet Protocol (SLIP)
    • byte-oriented protocol developed in 1980s
    • one of two widely used protocols to connect a client computer to an ISP
    • uses TCP/IP
    • designed for point-to-point telephone connections
    • Problems:
      • no error control
      • transparency is an issue

Begin

11000000

1 byte

End

11000000

1 byte

Message

(variable)

synchronous transmission124
Synchronous Transmission
  • Point-to-Point Protocol (PPP)
    • byte-oriented protocol developed in the early 1990s to replace SLIP
    • better because it includes error control
    • supports network protocols beyond the Internet protocols
    • transparency is still a problem

Flag

01111110

1 byte

Control

1 byte

Message

(variable)

max 1500 bytes

Flag

01111110

1 byte

Address

1 byte

Protocol

2 bytes

CRC-16

2 bytes

Figure 4-13

data link protocol125
Data Link Protocol
  • Isochronous Transmission
    • combines the elements of both synchronous and asynchronous data transmission
    • required to have a start and stop bit
    • sender and receiver are synchronized
    • data can be transmitted at higher speeds
transmission efficiency
Transmission Efficiency
  • The number of bits of user information divided by the total bits sent:

BI

EC =

BT

where:

    • EC = efficiency of the code
    • BI = number of bits of user information
    • BT = number of total bits transmitted

(user information + overhead)

transmission efficiency127
Transmission Efficiency
  • Asynchronous Transmission
    • 7-bit ASCII characters
    • assuming 1 parity bit
    • 1 start bit and 1 stop bit

BI 7 7

EC = = = (70%)

BT 7 + 1 + 2 10

transmission efficiency128
Transmission Efficiency
  • Synchronous Transmission
    • 8-bit EBCDIC characters
    • assuming 100 character frame of information
    • 32-bit frame check sequence
    • 8-bits start and end flag
    • 8-bits address
    • 8-bits control

BI 800 800

EC = = = (92.6%)

BT 800 + 64 864

transmission efficiency129
Transmission Efficiency

What is the best packet size for transmission?

  • Why is the efficiency low for very small packets?
  • Why is the efficiency low for very large packets?
transmission efficiency130
Transmission Efficiency
  • Throughput - the total number of information bits received per second, after taking into account the overhead bits and the need to retransmit packets containing errors
  • Transmission Rate of Information Bits (TRIB) - the effective rate of data transfer or the actual throughput of the circuit
throughput trib
Throughput (TRIB)
  • Measures the effective rate at which information is transmitted over a link per unit of time
  • ANSI provides the basic definition for calculating TRIB
  • Divide the number of information bits transferred by the total time required for the transfer:

number of information bits transferred

TRIB =

total transfer time

throughput trib132
Throughput (TRIB)

TRIB = number of information bits accepted

total transfer time

TRIB = K(M - C)(1 - P)

M

R

where: K = information bits per character

M = packet length in characters

R = data transmission rate in characters per second

C = average # of non-information bits per block

P = probability that a block will require retrans because of error

T = time between blocks in seconds

+ T

throughput trib133
Throughput (TRIB)

The following TRIB example shows the calculation of throughput assuming a 4800 bits per second half-duplex circuit.

7(400-10)(1-0.01)

(400/600) + 0.025

where: K = 7 bits per character (information)

M = 400 characters per block

R = 600 characters per second (derived from 4800 bps

divided by 8 bits/character)

C = 10 control characters per block

P = 0.01 (10-2) or one retransmission out of 100 blocks

transmitted 1%

T = 25 milliseconds (0.025) turnaround time

TRIB= = 3908 bits per second

throughput trib134
Throughput (TRIB)

7(400-10)(1-0.01)

(400/600) + 0

If all factors in the calculation remain constant except for the circuit, which is changed into full duplex (no turnaround time delays, T=0) then the TRIB increases to 4054 bps.

Look at the equation where the turnaround value (T) is 0.025. If there is a further propagation delay time of 475 milliseconds (0.475), this figure changes to 0.500. For demonstrating how a satellite channel affects TRIB, the total delay time is now 500 milliseconds. Still using the figures above (except for the new 0.500 delay time), we reduce the TRIB for our half-duplex, satellite link to 2317 bps, which is almost one-half for the full-duplex (no turnaround time) 4054 bps.

TRIB= = 4054 bits per second

throughput trib135
Throughput (TRIB)

7(200-15)(1-0.02)

(200/1200) + 0.030

Consider a 9600 bps half-duplex circuit in which ASCII characters are sent with one parity bit for each character. Assume 200 characters per block of which 15 are for control, there is a 2% error probability, and a 30 millisecond turnaround time.

TRIB= = 6440 bits per second

important figures
Important Figures
  • Figure 6.1 p. 159
  • Figure 6.7 p. 170