2. 2
Terminal-Host Communication
īŽ Components
īŽ Terminal
īŽ Host (locus of processing)
īŽ Transmission line (here, phone line and modem)
īŽ Telephone line acts as a point-to-point link
Phone
Line
Modem Modem
Terminal
Host
3. 3
Terminal Emulation
īŽ People Already Have PCs
īŽ Host operating systems only work with terminals
īŽ Do not want to buy a terminal as well
īŽ PCs can emulate (act like) terminals
īŽ Only requires software (a communication
program)
īŽ Turns an expensive PC into a cheap terminal
4. 4
VT100 Terminals
īŽ VT100 Emulation Only Needs Software
īŽ Communications program
īŽ Terminal emulation software
īŽ Most Hosts Support VT100 Terminals
īŽ Lowest Common Denominator
īŽ Slow: maximum speed of 19 kbps, usually slower
īŽ Uses inefficient asynchronous ASCII transmission,
discussed later
īŽ No graphics or even multiple fonts: plain text only
īŽ No color
5. 5
Terminal Emulation Software
īŽ File Transfer
īŽ Transfer whole files with error correction
īŽ Upload: from PC to host
īŽ Download: from host to PC
īŽ Terminal emulation program and host file
transfer program must support the same file
transfer protocol standard
īŽ Kermit
īŽ XMODEM, YMODEM
īŽ IBM 3270 Terminals
7. 7
Digital and Binary
īŽ Digital Transmission
īŽ Can have multiple voltage levels, say 4
īŽ Change to one at start of each bit cycle
īŽ If 4, changes can represent 2 bits each:
īŽ 00, 01, 10, 11
00
11
01
10
Voltage
Level
Time
8. 8
Baud Rate and Bit Rate
īŽ Baud Rate
īŽ Number of times line changes per second
īŽ Let baud rate be 4 (4 changes per second)
īŽ Let bits per line change be 2
īŽ Bit rate = 8 bits per second
īŽ Bit rate = x2 Baud rate in this example
00
11
01
10
One Second
9. 9
Wave Characteristics
īŽ Amplitude (power)
īŽ Frequency (cycles per second, Hertz)
īŽ Wavelength (meters)
Amplitude
(power)
Wavelength
(meters)
Frequency (Hz)
One Second
10. 10
Wave Characteristics
īŽ Phase
īŽ Fully cycle is 360 degrees
īŽ Phase is degrees different from reference wave
īŽ Human ears cannot hear. Equipment can
Reference
Wave
180
degrees
out of
phase
0o
180o
0o
90o
180o
270o
0 and rising
Highest
0 and falling
Lowest
11. 11
Wave Characteristics
īŽ Amplitude
īŽ Frequency and Wavelength
īŽ Not independent
īŽ As frequency rises, wavelength falls
(Shorter guitar strings produce higher notes)
īŽ Their product is constant--the speed of light,
sound, etc.
īŽ Phase
16. 16
Modem Standards
īŽ Modems at Two Ends Must Communicate
īŽ Must follow same standards
īŽ Most modem standards set by ITU-T
īŽ Multiple category of standards:
īŽ Modem speed (modulation)
īŽ Error correction and compression
īŽ Facsimile
īŽ Etc.
īŽ When buying a modem, must check for
standard(s) followed in each category
17. 17
Modem Speed Standards
īŽ Set by the ITU-T
īŽ Govern how modulation is done
īŽ Standards for speed governs modulation for
data transmission
īŽ V.92 56.6 kbps plus quick connect,
modem on hold, PCM upstream
īŽ V.90 56.6 kbps
īŽ V.34 28.8 kbps/33.6 kbps
īŽ V.32 bis14.4 kbps
19. 19
Error Correction and Compression
īŽ ITU-T Standards
īŽ V.42 Error detection and correction
īŽ V.42 bisData compression (up to 4:1)
īŽ V.44 Data compression (20 to 120%
more than V.42 bis)
īŽ Independent of speed standards (but V.44 only
with V.92)
īŽ Microcom Standards
īŽ Microcom Network Protocol (MNP)
īŽ Both error correction and compression
īŽ Several levels
īŽ Independent of speed standards
20. 20
Modem Intelligence
īŽ Computer Can Send Commands to Modem
īŽ Dial a number, including how long to wait, etc.
īŽ Called intelligent modems
īŽ Hayes Developed the first Command Set
īŽ Most modems follow the same command set
īŽ We call them âHayes compatibleâ
īŽ Commands start with âATâ
īŽ Other Standards for Fax Modems
īŽ Class 1 and Class 2: extensions to Hayes
21. 21
Telephone Bandwidth is Limited
īŽ Telephone Transmission
īŽ Cuts off sounds below 300 Hertz
īŽ Cuts off sounds above about 3,400 Hz
īŽ Bandwidth is the difference between the highest
and lowest frequencies (3400-300): about 3,100
Hz
Sound
Loudness
Frequency (Hz)
0 300 3400 20,000
Bandwidth
3,100 Hz
22. 22
Telephone Bandwidth is Limited
īŽ Speed is Limited
īŽ Maximum speed is related to bandwidth
(Shannonâs Law)
īŽ Maximum speed for phone lines for transmission
is a little over 30 kbps
īŽ So modems canât get much faster
23. 23
Another Look at Compression
īŽ With 4:1 Compression, a V.34 Modem Can
Receive Data at 115.2 kbps from the PC
īŽ However the ~30 kbps limit of the phone
system is not exceeded. Still transmit at 33.6
kbps.
115.2 kbps 33.6 kbps
~35 kbps
Maximum
Compression
in Modem
24. 24
56 kbps Analog Modems
īŽ From home, you transmit
īŽ Analog-to-Digital Converter (ADC)
īŽ Filters your signal to a bandwidth of ~3.1 kHz
īŽ This limits you to 33.6 kbps
Telephone
Network
ADC
PC
V.34
modem
33.6 kbps
25. 25
56 kbps Analog Modems
īŽ But ISP Can Connect Digitally
īŽ Signal travels through phone system at 56 kbps
īŽ At user end, digital-to-analog converter (DAC)
īŽ Sends signal to analog modem at wide bandwidth
īŽ Modem can receive at 56 kbps
Telephone
Network
DAC
PC
56 kbps
modem
ISP
Digital
Link
56 kbps
26. 26
56 kbps Modems
īŽ What they can do
īŽ Send at 33.6 kbps (V.92 with PCM upstream can
go up to 48 kbps)
īŽ Receive at 56 kbps (V92 with V.44 compression
can go up to 120 kbps)
īŽ Problems
īŽ past: competing standards from Rockwell, U.S.
Robotics (V.90 ended them)
īŽ present: ISPs must support V.92 (all support V.90)
īŽ Users and ISPs
īŽ Users V.90 analog modem or V.92
īŽ ISPs V.90 digital modem or V.92
27. 27
56 Kbps Modems
īŽ Telephone company
īŽ No changes needed, although ...
īŽ Many not have an internal ADC conversion
between ISP and customer (some do)
īŽ May not have long transmission line from last
switch to the customer premises (local loop)
īŽ Not all phone lines to customer premises will
support 56 kbps modems
īŽ Even when they do, speeds may only be 40-50
kbps
28. 28
Half-Duplex Transmission
īŽ Sender and receiver must take turns sending
īŽ Like an old one-lane road
īŽ No interruption for error handling or flow control
A B A B
Time 1
Only one side
May communicate
A does
Time 2
Only one side
May communicate
B does
29. 29
Full-Duplex Transmission
īŽ Both Sides May Transmit Simultaneously
īŽ Needed for error correction, flow control
īŽ 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
30. 30
Asynchronous Transmission
īŽ ASCII Character Set
īŽ 7-bit is the standard
īŽ 8-bit extended ASCII is popular
īŽ Bits transmitted backward
īŽ Parity for Error Detection
īŽ Only for 7-bit ASCII
īŽ Start/Stop Bits for Framing
īŽ Each frame is exactly 10 bits long
31. 31
Asynchronous Transmission
īŽ ASCII Character Set
īŽ Created for sending printed American text
īŽ Each character is a 7-bit code (e.g., 1010101)
īŽ This allows 2^7 or 128 possible characters
īŽ Printing characters: A, a, !, <, %, etc.
īŽ Control codes: XOFF tells other side to pause
32. 32
Asynchronous Transmission
īŽ 8-bit ASCII
īŽ Used in PCs: 8 bits per character (10101010)
īŽ Used for word processing format codes
īŽ Used in graphics that stores data in bytes
33. 33
PC Serial Port
īŽ Bit Transmission of ASCII Characters
īŽ Transmits last bit first
īŽ If you wish to send 1111000,
īŽ The serial port transmits 0001111
34. 34
Parity
īŽ For 7-bit ASCII Only (No Parity = 8-bit ASCII)
īŽ Transmit an 8th bit per character
īŽ Even parity: sum of data and parity bits is
even
īŽ To send 1110000 (odd), send 00001111
īŽ To send 1111000 (even), send 00011110
īŽ Odd parity: sum is odd
īŽ If error is detected, the character is simply
discarded. No way to ask for retransmission
35. 35
Start and Stop Bits
īŽ When the Data Line is at Rest
īŽ It is kept in the â1â state
īŽ So â11110000â would look like 111111100001111
īŽ â00001111â would also look like 11111100001111
īŽ How can you tell where a character begins?
īŽ Solution
īŽ Add a start bit (always 0) to change the line state
īŽ 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)
36. 36
The Final Asynchronous Frame
īŽ Always 10 bits
īŽ Start, 7 data bits, parity, stop, or
īŽ Start, 8 data bits, stop
0 1 1 1 0 0 0 1 1 1
Start Parity
Stop
7-bit ASCII Character
0 1 1 1 0 0 0 1 1 1
Start
Stop
8-bit ASCII Character
37. 37
Flow Control
īŽ Ask the Other Device to Pause (or Slow
Down)
īŽ ASCII
īŽ In asynch, usually done by sending ASCII control
codes
īŽ XOFF tells other side to pause
īŽ XON tells the other device to resume
īŽ Serial Port
īŽ Signals on the pins control when PC, modem can
transmit
38. 38
3-38
Signal and Propagation
A signal is a disturbance in the media that propagates
(travels) down the transmission medium to the receiver
If propagation effects are too large, the receiver will not be
able to read the received signal
39. 39
3-39
Binary-Encoded Data
īŽ Computers store and process data in binary
representations
īŽ Binary means âtwoâ
īŽ There are only ones and zeros
īŽ Called bits
1101010110001110101100111
40. 40
3-40
Binary-Encoded Data
īŽ Non-Binary Data Must Be Encoded into
Binary
īŽ Text
īŽ Integers (whole numbers)
īŽ Decimal numbers
īŽ Alternatives (North, South, East, or West, etc.)
īŽ Graphics
īŽ Human voice
īŽ etc.
Hello 11011001âĻ
41. 41
Layering Perspective
īŽ Where is binary data encoding done?
īŽ It is done at the application layer, not at the
physical layer.
īŽ Where is signaling done
īŽ It is done at the physical layer
3-41
43. 43
3-43
On/Off Signaling
On/off signaling is used in optical fiber
The light is turned on during a clock cycle for a 1
The light is turned off during a clock cycle for a 0
There are two signaling statesâon and off
This is called binary signaling
This is a simple type of signaling
44. 44
3-44
Binary Voltage Signaling in 232 Serial Ports
The high state (0) is anything from +3 to +15 volts
The low state (1) is anything from -3 to -15 volts
1
45. 45
3-45
Relative Immunity to Errors in Binary Signaling
Binary signaling gives some immunity to errors.
This is one of its major attractions.
47. 47
3-47
Unshielded Twisted Pair (UTP) Wiring
īŽ UTP Characteristics
īŽ Inexpensive and to purchase and install
īŽ Dominates media for access links between computers
and the nearest switch
48. 48
3-48
Unshielded Twisted Pair (UTP) Wiring
īŽ Standards
īŽ The TIA/EIA-568 standard governs UTP wiring in the
United States
īŽ In Europe, the comparable standard is ISO/IEC 11801
49. 49
3-49
4-Pair UTP Cord with RJ45 Connector
3.
8-pin
RJ-45
Connector
2.
8 Wires
organized
as 4
twisted
pairs
Industry standard pen
1.
UTP cord
UTP cord
50. 50
3-50
Unshielded Twisted Pair (UTP)
Wiring
īŽ Cord Organization
īŽ A length of UTP wiring is a cord
īŽ Each cord has eight copper wires
īŽ Each wire is covered with dielectric
(nonconducting) insulation
īŽ The wires are organized as four pairs
īŽ Each pairâs two wires are twisted around each
other several times per inch
īŽ There is an outer plastic jacket that encloses the
four pairs
51. 51
3-51
Unshielded Twisted Pair (UTP)
Wiring
īŽ Connector
īŽ RJ-45 connector is the standard
connector
īŽ Plugs into an RJ-45 jack in a NIC, switch, or wall
jack
RJ-45
Jack
RJ-45
Jack
8-pin RJ-45 connectors
52. 52
3-52
Attenuation and Noise
Power
Distance
3.
Noise Floor
(Average Noise level)
2.
Noise
4.
Noise Spike
1.
Signal
2.
Signal-
to-Noise
Ratio (SNR)
5.
Error
1. The signal attenuates (falls in power) as it propagates
2. There is noise (random energy) in the wire that adds to the signal
3. The average noise level is called the noise floor
4. Noise is random. Occasionally, there will be large noise spikes
5. Noise spikes as large as the signal cause errors
6. You want to keep the signal-to-noise ratio high
53. 53
3-53
Limiting UTP Cord Length
īŽ Limit UTP cord length to 100 meters
īŽ This keeps the signal-to-noise ration (SNR) high
īŽ This makes attenuation and noise problems
negligible
īŽ Note that limiting cord lengths limits BOTH noise
and attenuation problems
100 Meters Maximum
Cord Length
54. 54
3-54
UTP Wiring
īŽ Electromagnetic Interference (EMI)
īŽ Electromagnetic interference is electromagnetic
energy from outside sources that adds to the signal
īŽ From fluorescent lights, electrical motors,
microwave ovens, etc.
īŽ The problem is that UTP cords are like long radio
antennas
īŽ They pick up EMI energy nicely
īŽ When they carry signals, they also send EMI
energy out from themselves
55. 55
3-55
Electromagnetic Interference (EMI)
and Twisting
Interference on the Two Halves of a Twist Cancels Out
Twisted
Wire
Electromagnetic
Interference (EMI)
UTP is twisted
specifically to reduce EMI
56. 56
3-56
Crosstalk Interference and Terminal Crosstalk
Interference
Untwisted
at Ends Signal
Terminal Crosstalk
Interference
Crosstalk Interference
Terminal crosstalk interference
normally is the biggest EMI problem for UTP
57. 57
3-57
Interference Hierarchy
īŽ EMI is any interference
īŽ Signals in adjacent pairs interfere with one another
(crosstalk interference). This is a specific type of EMI
īŽ Crosstalk interference is worst at the ends, where the
wires are untwisted. This is terminal crosstalk
interferenceâa specific type of crosstalk EMI
EMI
Crosstalk Interference
Terminal Crosstalk
Interference
58. 58
3-58
Terminal Crosstalk Interference
īŽ Terminal crosstalk interference dominates
interference in UTP
īŽ Terminal crosstalk interference is limited to an
acceptable level by not untwisting wires more
than a half inch (1.25 cm) at each end of the
cord to fit into the RJ-45 connector
īŽ This reduces terminal crosstalk interference
to a negligible level.
1.25 cm or 0.5 inches
59. 59
Shielded Twisted Pair Wiring (STP)
īŽ We have been talking about unshielded twisted
pair wiring.
īŽ Is there a shielded twisted pair wiring?
īŽ Yes. It has a metal mesh shield around each pair to
reduce cross-talk interference
īŽ It also has a metal mesh shield around the
four pairs to reduce external EMI
īŽ It is no longer used extensively because UTP,
which is much less expensive, was found to be
good enough for normal environments
īŽ However, we will see that Cat 7 wiring uses STP
3-59
60. 60
3-60
UTP Limitations
īŽ Limit cords to 100 meters
īŽ Limits BOTH noise AND attenuation problems to an
acceptable level
īŽ Do not untwist wires more than 1.25 cm (a half
inch) when placing them in RJ-45 connectors
īŽ Limits terminal crosstalk interference to an acceptable
level
īŽ Neither completely eliminates the problems but
they usually reduce the problems to negligible
levels
2
63. 63
3-63
Optical Fiber Transceiver and Strand
An optical fiber strand has a thin glass core
This core is 8.3, 50, or 62.5 microns in diameter
This glass core is surrounded by a tubular glass cladding
The outer diameter of the cladding is 125 microns,
regardless of the coreâs diameter
The transceiver injects laser light into the core
64. 64
3-64
Optical Fiber Transceiver and Strand
When a light wave ray hits the core/cladding boundary,
there is perfect internal reflection. There is no signal loss
66. 66
3-66
Two-Strand Full-Duplex Optical Fiber Cord
with SC and ST Connectors
A fiber cord has
two-fiber strands
for full-duplex
(two-way)
transmission
SC Connectors
ST Connectors
Two
Strands
Cord
67. 67
3-67
Full-Duplex Optical Fiber Cord with SC and ST
Connectors
SC Connector
(push and click)
ST Connector
(bayonet connectors:
push and click)
In contrast to UTP, which always uses RJ-45 connectors,
there are several optical fiber connector types
SC and ST are the most popular
68. 68
3-68
Optical Fiber Strand
In optical fiber transmission, light is expressed in nanometers.
The transceiver transmits at 850 nm, 1,310 nm, or 1,550 nm
Shorter-wavelength (850 nm) transceivers are less expensive
Longer-wavelength (1,310 or 1,550 nm) light travels farther for a given speed
For LAN fiber, 850 nm provides sufficient distance and dominates
69. 69
3-69
Multimode Fiber and Single-Mode Fiber
Multimode fiber has a thick core (50 or 62.5 microns in diameter)
Light can only enter the core at certain angles, called modes
Modes traveling straight through arrive faster than
modes that bounce against the cladding several times
71. 71
3-71
Radio Propagation
Radio signals also propagate as waves.
As noted earlier, radio waves are measured in hertz (Hz),
which is a measure of frequency.
Radio usually operates in the MHz and GHz range.
73. 73
3-73
Wireless Propagation Problems
UTP and optical fiber propagation are fairly predictable.
However, radio suffers from many propagation effects.
This makes radio transmission difficult to manage.
74. 74
Topology
Network topology is the physical
arrangement of a networkâs computers,
switches, routers, and transmission lines
It is a physical layer concept:
īŦ Point-to-point
īŦ Star
īŦ Bus
īŦ Ring