Point-to-Point Communication

1
Point-to-Point Communication
2
Terminal-Host Communication

• Components
• Terminal
• Host (locus of processing)
• Transmission line (here, phone line and modem)
• Telephone line acts as a point-to-point link

Host
Terminal
Phone Line
Modem
Modem
3
Terminal Emulation

• People Already Have PCs
• Host operating systems only work with terminals
• Do not want to buy a terminal as well
• PCs can emulate (act like) terminals
• Only requires software (a communication program)
• Turns an expensive PC into a cheap terminal

4
VT100 Terminals

• VT100 Emulation Only Needs Software
• Communications program
• Terminal emulation software
• Most Hosts Support VT100 Terminals
• Lowest Common Denominator
• Slow maximum speed of 19 kbps, usually slower
• Uses inefficient asynchronous ASCII transmission,
  discussed later
• No graphics or even multiple fonts plain text
  only
• No color

5
Terminal Emulation Software

• File Transfer
• Transfer whole files with error correction
• Upload from PC to host
• Download from host to PC
• Terminal emulation program and host file transfer
  program must support the same file transfer
  protocol standard
• Kermit
• XMODEM, YMODEM
• IBM 3270 Terminals

6
Analog and Digital Transmission
Analog Signal
Digital Signal (1101)
Modem
Modulation
Analog Signal
Digital Signal (1101)
Modem
Demodulation
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Digital and Binary

• Digital Transmission
• Can have multiple voltage levels, say 4
• Change to one at start of each bit cycle
• If 4, changes can represent 2 bits each
• 00, 01, 10, 11

11
Voltage Level
10
01
00
Time
8
Baud Rate and Bit Rate

• Baud Rate
• Number of times line changes per second
• Let baud rate be 4 (4 changes per second)
• Let bits per line change be 2
• Bit rate 8 bits per second
• Bit rate x2 Baud rate in this example

11
10
01
00
One Second
9
Wave Characteristics

• Amplitude (power)
• Frequency (cycles per second, Hertz)
• Wavelength (meters)

Frequency (Hz)
Wavelength (meters)
One Second
Amplitude (power)
10
Wave Characteristics
0o 90o 180o 270o
0 and rising Highest 0 and falling Lowest

• Phase
• Fully cycle is 360 degrees
• Phase is degrees different from reference wave
• Human ears cannot hear. Equipment can

Reference Wave
0o
180 degrees out of phase
180o
11
Wave Characteristics

• Amplitude
• Frequency and Wavelength
• Not independent
• As frequency rises, wavelength falls(Shorter
  guitar strings produce higher notes)
• Their product is constant--the speed of light,
  sound, etc.
• Phase

12
Frequency Modulation
Vary the frequency (wavelength) to represent 1
and 0
Wavelength
Low Frequency (0)
Wavelength
High Frequency (1)
0
1
Frequency Modulation (1011)
1
1
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Amplitude Modulation
Amplitude (low)
Low Amplitude (0)
Amplitude (high)
High Amplitude (1)
Amplitude Modulation (1011)
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Phase Modulation
In Phase (0)
180 degrees out of phase (1)
Frequency Modulation (1011)
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Complex Modulation
Vary both amplitude and phase Several values (not
just two) on each dimension
In Phase
90 Degrees Out of Phase, High Amplitude
High Amplitude
Low Amplitude
180 Degrees Out of Phase
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Modem Standards

• Modems at Two Ends Must Communicate
• Must follow same standards
• Most modem standards set by ITU-T
• Multiple category of standards
• Modem speed (modulation)
• Error correction and compression
• Facsimile
• Etc.
• When buying a modem, must check for standard(s)
  followed in each category

17
Modem Speed Standards

• Set by the ITU-T
• Govern how modulation is done
• Standards for speed governs modulation for data
  transmission
• V.92 56.6 kbps plus quick connect,
  modem on hold, PCM upstream
• V.90 56.6 kbps
• V.34 28.8 kbps/33.6 kbps
• V.32 bis 14.4 kbps

18
Modem Speed Standards

• Most data modems are also fax modems
• V.14 14.4 kbps
• V.29 9,600 bps

19
Error Correction and Compression

• ITU-T Standards
• V.42 Error detection and correction
• V.42 bis Data compression (up to 41)
• V.44 Data compression (20 to 120 more than
  V.42 bis)
• Independent of speed standards (but V.44 only
  with V.92)
• Microcom Standards
• Microcom Network Protocol (MNP)
• Both error correction and compression
• Several levels
• Independent of speed standards

20
Modem Intelligence

• Computer Can Send Commands to Modem
• Dial a number, including how long to wait, etc.
• Called intelligent modems
• Hayes Developed the first Command Set
• Most modems follow the same command set
• We call them Hayes compatible
• Commands start with AT
• Other Standards for Fax Modems
• Class 1 and Class 2 extensions to Hayes

21
Telephone Bandwidth is Limited

• Telephone Transmission
• Cuts off sounds below 300 Hertz
• Cuts off sounds above about 3,400 Hz
• Bandwidth is the difference between the highest
  and lowest frequencies (3400-300) about 3,100 Hz

Sound Loudness
Bandwidth 3,100 Hz
0
300
3400
20,000
Frequency (Hz)
22
Telephone Bandwidth is Limited

• Speed is Limited
• Maximum speed is related to bandwidth (Shannons
  Law)
• Maximum speed for phone lines for transmission is
  a little over 30 kbps
• So modems cant get much faster

23
Another Look at Compression

• With 41 Compression, a V.34 Modem Can Receive
  Data at 115.2 kbps from the PC
• However the 30 kbps limit of the phone system is
  not exceeded. Still transmit at 33.6 kbps.

35 kbps Maximum
Compression in Modem
33.6 kbps
115.2 kbps
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56 kbps Analog Modems

• From home, you transmit
• Analog-to-Digital Converter (ADC)
• Filters your signal to a bandwidth of 3.1 kHz
• This limits you to 33.6 kbps

Telephone Network
PC
V.34 modem
ADC
33.6 kbps
25
56 kbps Analog Modems

• But ISP Can Connect Digitally
• Signal travels through phone system at 56 kbps
• At user end, digital-to-analog converter (DAC)
• Sends signal to analog modem at wide bandwidth
• Modem can receive at 56 kbps

Digital Link
Telephone Network
PC
ISP
56 kbps modem
DAC
56 kbps
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56 kbps Modems

• What they can do
• Send at 33.6 kbps (V.92 with PCM upstream can go
  up to 48 kbps)
• Receive at 56 kbps (V92 with V.44 compression can
  go up to 120 kbps)
• Problems
• past competing standards from Rockwell, U.S.
  Robotics (V.90 ended them)
• present ISPs must support V.92 (all support
  V.90)
• Users and ISPs
• Users V.90 analog modem or V.92
• ISPs V.90 digital modem or V.92

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56 Kbps Modems

• Telephone company
• No changes needed, although ...
• Many not have an internal ADC conversion between
  ISP and customer (some do)
• May not have long transmission line from last
  switch to the customer premises (local loop)
• Not all phone lines to customer premises will
  support 56 kbps modems
• Even when they do, speeds may only be 40-50 kbps

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Half-Duplex Transmission

• Sender and receiver must take turns sending
• Like an old one-lane road
• No interruption for error handling or flow control

Time 1 Only one side May communicate A does
Time 2 Only one side May communicate B does
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Full-Duplex Transmission

• Both Sides May Transmit Simultaneously
• Needed for error correction, flow control
• Now almost universal in modem communication

A
B
A
B
Time 1 Both sides may communicate Both do
Time 2 Both sides may communicate A does
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Asynchronous Transmission

• ASCII Character Set
• 7-bit is the standard
• 8-bit extended ASCII is popular
• Bits transmitted backward
• Parity for Error Detection
• Only for 7-bit ASCII
• Start/Stop Bits for Framing
• Each frame is exactly 10 bits long

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Asynchronous Transmission

• ASCII Character Set
• Created for sending printed American text
• Each character is a 7-bit code (e.g., 1010101)
• This allows 27 or 128 possible characters
• Printing characters A, a, !, lt, , etc.
• Control codes XOFF tells other side to pause

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Asynchronous Transmission

• 8-bit ASCII
• Used in PCs 8 bits per character (10101010)
• Used for word processing format codes
• Used in graphics that stores data in bytes

33
PC Serial Port

• Bit Transmission of ASCII Characters
• Transmits last bit first
• If you wish to send 1111000,
• The serial port transmits 0001111

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Parity

• For 7-bit ASCII Only (No Parity 8-bit ASCII)
• Transmit an 8th bit per character
• Even parity sum of data and parity bits is even
• To send 1110000 (odd), send 00001111
• To send 1111000 (even), send 00011110
• Odd parity sum is odd
• If error is detected, the character is simply
  discarded. No way to ask for retransmission

35
Start and Stop Bits

• When the Data Line is at Rest
• It is kept in the 1 state
• So 11110000 would look like 111111100001111
• 00001111 would also look like 11111100001111
• How can you tell where a character begins?
• Solution
• Add a start bit (always 0) to change the line
  state
• End with a stop bit (always 1) to guarantee at
  least a one-bit rest (1) against which to detect
  the next start bit (0)

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The Final Asynchronous Frame

• Always 10 bits
• Start, 7 data bits, parity, stop, or
• Start, 8 data bits, stop

Start
Parity
0 1 1 1 0 0 0 1 1 1
Stop
7-bit ASCII Character
Start
0 1 1 1 0 0 0 1 1 1
Stop
8-bit ASCII Character
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Flow Control

• Ask the Other Device to Pause (or Slow Down)
• ASCII
• In asynch, usually done by sending ASCII control
  codes
• XOFF tells other side to pause
• XON tells the other device to resume
• Serial Port
• Signals on the pins control when PC, modem can
  transmit

38
ISDN

• Integrated Serviced Digital Network
• Planned as eventual replacement for the worldwide
  analog telephone network
• All-digital, but expensive and difficult to
  install
• Integrated services
• Automatic number identification
• Call waiting
• Now available only locally (business and limited)
• will ISDN standardize worldwide? - NO!!!

39
Basic Rate Interface (BRI)

• Service Brought to Users Desk
• 3 Channels multiplexed onto one set of wires
• Really, bidirectional, so 6 channels!

D Channel control signals
B Channel
Terminal Adapter
2BD
1101
Multiplexed Onto One Set of Wires
B Channel
40
ISDN BRI

• B Channels
• 2 B-channels multiplexed to desktop
• 64 kbps each
• Simultaneous voice and data connections
• Sometimes, a single 128-kbps connection
• Moderately faster than modems and comparable
  increase in cost
• D Channel
• one 16 kbps channel, usually for control signaling

41
Connecting a Computer to the ISDN

• Requires a Terminal Adapter
• Contains a DSU (data service unit)
• Converts PC digital format to ISDN digital format

DSU
1101
1101
Terminal Adapter
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ISDN Costs for Internet Access

• Carrier Charges
• Installation of the ISDN Line by Carrier
• Monthly Service Charge to Carrier
• Traffic volume charges (per kilobit)
• Purchase or Lease of Terminal Adapter
• Internet Service Providers
• Charge extra for ISDN access
• Even more for 128 kbps service (2 B channels)
• ISDN fees do not go to ISP

43
Beyond ISDN

• Faster than analog phone lines but still slow
• Better dial-up connections . V.92-V.44
• Newer alternatives are growing
• Cable modems from cable television companies
• DSL lines from telephone companies
• Much faster
• 1-10 Mbps to customer premises
• Sometimes slower uplink (33.6 kbps - 1 Mbps)
• More expensive than dial-up
• equivalent to the cost of two phone lines
• installation costs and/or monthly lease of modem

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Beyond ISDN

• Cable Modems from Cable TV Companies
• High-speed digital access offered by cable
  television companies
• Really DSUs but called cable modems
• 500 kbps to 10 Mbps downstream (to you)
• Downstream speed shared by block of users
• ISDN-like speeds upstream (from you)
• Typical Cable ISP charges around 40.00
• Comcast and TCI major players
• 10 million users today and growing

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Beyond ISDN

• Digital Subscriber Link (DSL)
• High-speed digital access offered by telephone
  companies over the ordinary local loop
• Requires equipment at telephone company switch
• User needs DSL modem (really, a DSU)
• 128 kbps to perhaps 8 Mbps downstream
• ISDN-like rates upstream are common
• Several versions Asymmetric (ADSL), etc.
• Microsoft, Intel, Compaq, and others join Telcos
• Keep yourself informed ADSL Forum

46
Beyond ISDN

• Satellite/Wireless
• Satellite link instead of wire link to ISP
• Good for mobile users
• Good for intranets in a company with multiple
  dispersed sites
• ISDN speeds or higher
• High cost

47
Broadband Market Forecast

• Residential Use (2003)
• 80 of Households have computers online
• 77 dial-up connections
• 23 broadband (17 million)
• Forecast 2007 56 ( 41 million)
• Broadband
• 54 Cable modems
• 34 DSL
• 8 ISDN
• 4 Satellite/wireless
• Cnet Cable vs DSL