Data Transmission

1
Data Transmission

• The basics of media, signals, bits, carries, and
  modems
• (Part I)

2
Physical Layer

• The lowest layer in any network architecture
  model
• It is concerned with the transparent transmission
  of raw bits across a communications medium
• It deals with the physical characteristics
  (mechanical, electrical, procedural) of data
  transmission and communication.
• It is responsible for
• providing basic signaling (control, data)
• signal modulation
• encoding/decoding
• activate/deactivate physical medium (PM)
• bit-timing (clocking)
• mapping between different formats

3
Physical Layer
4
Data Communication

• Source initiates the communication.
• Destination address (identifier) is required for
  the network to establish a communication path
  between the source and the destination
• Destination must be prepared to receive data
• Source and destination hosts may belong to
  different types of networks.
• Line speed and/or packet formats mismatches
  should be taken care of (i.e., via fragmentation,
  conversion, etc.)
• Example consider a file transfer (e.g., FTP)

5
Analog and Binary Data
Binary Data
Analog Data
1101011000011100101
Smoothly changing among an infinite number of
states (loudness levels, etc.)
Two statesOne state represents 1The other
state represents 0
6
Binary Data and Binary Signal
7
Binary Data and Binary Signal
Time is divided into clock cycles The State is
held constant within each clock cycle. It can
jump abruptly at the end of each cycle. One bit
is sent per clock cycle.
15 Volts (0)
Clock Cycle
0
0
0 Volts
1
Transmitted Signal
-15 Volts (1)
8
Binary Data and Digital Signal
11
11
10
10
01
01
01
00
Client PC
00
Server
In binary transmission, there are two states. In
digital transmission, there are few states (in
this case, four). With four states, two
information bits can be sent per clock cycle. 00,
01, 10, and 11 Binary transmission is a special
case of digital transmission.
9
Baud Rates for Digital Signals
Baud Rate of Clock Cycles/Second
11
11
10
10
01
01
01
00
Client PC
00
Server
Suppose that the clock cycle is 1/10,000
second. Then the baud rate is 10,000 baud (10
kbaud). The bit rate will be 20 kbps (two
bits/clock cycle times 10,000 clock cycles per
second). (The bit rate gives the number of
information bits per second.)
10
In Summary- Bit Rate and Baud Rate

• Two terms frequently used in data communication
• Bit rate the number of bits sent in one second,
  usually expressed in bits per second (bps)
• More important to know how long it takes to
  process each piece of information
• Duration of a bit (bit interval) the time
  required to send one single bit. bit interval
  1/bit rate
• Example bit rate 55.6 kbps,
• duration of a bit 1/55600
  second
• 18 microseconds

11
Baud Rate

• Refers to the number of signal units per second
  that are required to represent those bits.
• Related to the bandwidth -- the fewer signal
  units required, the more efficient the system and
  the less bandwidth required to transmit more bits
• More important to know how efficiently we can
  move those data from place to place
• Bit rate Baud rate the number of bits
  represented by each signal unit
• Example An analog signal carries 4 bits in each
  signal element. If 1000 signal elements are sent
  per second, then baud rate 1000 bauds per
  second,
• bit rate 1000 4 4000 bps

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Perspective

• Analog Data
• Smooth changes among an infinite number of
  stateslike hands going around an analog clock
• Digital Data
• Few states
• In a digital clock, each position can be in one
  of ten states (the digits 0 through 9)
• Binary Data
• Two states (a special case of digital)

13
Basic Idea For Transmission Media

• Encode data as energy and transmit energy
• Decode energy at destination back into data
• Energy can be electrical, light, radio, sound,
  ...
• Each form of energy has different properties and
  requirements for transmission

14
Transmission Media

• Transmitted energy is carried through some sort
  of medium Transmitter encodes data as energy and
  transmits energy through medium
• Requires special hardware for data encoding
• Requires hardware connection to transmission
  medium
• Media can be copper, glass, air, ...

15
Copper Wires

• Twisted pair a pair of insulated copper wires.
  Can run for a few kms (oldest and most common).
  Several standards STP and UTP (Unshielded
  Twisted Pair).

16
4-Pair Unshielded Twisted Pair Cable with RJ-45
Connector
17
4-Pair Unshielded Twisted Pair Cable with RJ-45
Connector
18
Copper Wires (continued)

• Coaxial cable copper wire surrounded by an
  insulator encased by another copper conductor
  (mesh) covered by a protective plastic sheath

19
Glass Fibers

• Thin glass fiber carries light with encoded data
• Plastic jacket allows fiber to bend (some!)
  without breaking
• Fiber is very clear and designed to reflect light
  internally for efficient transmission
• Light emitting diode (LED) or laser injects light
  into fiber
• Light sensitive receiver at other end translates
  light back into data

20
Glass Fibers (continued)

• Very reliable and high capacity
• Can run up to tens of kms
• Not affected by electromagnetic inference
• Easy to install but a costly technology

21
Multimode Single-Mode Fiber
22
Multimode Single-Mode Fiber
23
Multimode and Single-Mode Fiber

• Multimode
• Limited distance (a few hundred meters)
• Inexpensive to install
• Dominates fiber use in LANs
• Single-Mode Fiber
• Longer distances tens of kilometers
• Expensive to install
• Commonly used by WANs and telecoms carriers

24
Wireless

• Air is the medium
• Radio
• Data transmitted using radio waves
• Energy travels through the air rather than copper
  or glass
• Conceptually similar to radio, TV, cellular
  phones
• Can travel through walls and through an entire
  building
• omnidirectional (broadcast)

25
Radio Wave
Wavelength
Amplitude
Frequency Measured in Hertz (Cycles per Second) 2
Cycles in one Second, so 2 Hz
Wavelength Frequency Speed of Propagation
26
Wireless

• Satellite unidirectional, costly, propagation is
  a consideration

27
Wireless

• Microwave
• High frequency radio waves
• Unidirectional, for point-to-point communication
• Antennas mounted on towers relay transmitted data
  
• Infrared
• Infrared light transmits data through the air
• Similar to technology used in TV remote control
• Can propagate throughout a room (bouncing off
  surfaces), but will not penetrate walls

28
Wireless

• Laser
• Unidirectional, like microwave
• Higher speed than microwave
• Uses laser transmitter and photo-sensitive
  receiver at each end
• Point-to-point, typically between buildings
• Can be adversely affected by weather

29
Wireless Propagation Problems
30
Wireless Propagation Problems
31
Choosing A Medium

• Copper wire is mature technology, rugged and
  inexpensive maximum transmission speed is
  limited
• Glass fiber
• Higher speed
• More resistant to electromagnetic interference
• Spans longer distances
• Requires only single fiber
• More expensive less rugged

32
Choosing A Medium

• Radio and microwave don't require physical
  connection
• Radio and infrared can be used for mobile
  connections
• Laser also does not need physical connection and
  supports higher speeds

33
Data Transmission

• Data transmission requires
• Encoding bits as energy
• Transmitting energy through medium
• Decoding energy back into bits
• Energy can be electric current, radio, infrared,
  light
• Transmitter and receiver must agree on encoding
  scheme and transmission timing

34
Encoding--Using Electric Current To Send Bits

• Simple idea - use varying voltages to represent
  1s and 0s
• One common encoding use negative voltage for 1
  and positive voltage for 0
• In following figure, transmitter puts positive
  voltage on line for 0 and negative voltage on
  line for 1

35
Encoding Details

• All details specified by a standard
• Several organizations produce networking
  standards
• IEEE Institute for Electrical and Electronics
  Engineers
• ITU (International Telecommunications Union)
• EIA (Electronic Industries Association)
• Hardware adheres to standard interoperable

36
Transmission Modes

• Asynchronous and synchronous communications
• Asynchronous communication data are transmitted
  one character at a time
• transmitter and receiver do not explicitly
  coordinate each data transmission
• transmitter can wait arbitrarily long between
  transmissions
• receiver does not know when a character will
  arrive. May wait forever

37
Asynchronous Communication

• To ensure meaningful exchange
• start bit before character
• one or more stop bits after character
• 1s when idle
• Used, for example, when transmitter such as a
  keyboard, may not always have data ready to send

38
Synchronous Communication

• No start/stop bits. Sends bytes contiguously
• Periodically transmit clocking information
• Uses flags (special bit sequences) as delimiters
  for frames
• More efficient transmission

39
The RS-232C Standard

• Example use
• Connection to keyboard/mouse
• Serial port on PC (as opposed to parallel
  transmission)
• Specified by EIA
• Voltage is 15 or 15
• Cable limited to 50 feet
• Use asynchronous communication

40
Illustration Of RS-232

• Start bit
• Same as 0
• Not part of data
• Stop bit
• Same as 1
• Follows data

41
Duration Of A Bit In RS-232

• Determined by baud rate
• Typical baud rate 9.6 kbaud, 14.4 kbaud, 28.8
  kbaud
• bit_rate baud_rate
• Duration of a bit is 1/baud_rate
• Sender and receiver must agree a priori
• Receiver samples signal
• Disagreement results in framing error

42
Two-Way Communication

• Desirable in practice
• Requires each side to have transmitter and
  receiver
• Called full duplex

43
Illustration Of Full-Duplex Communication

• Transmitter on one side connected to receiver on
  other
• Separate wires needed to carry current in each
  direction
• Common ground wire

44
Electric Transmission

• In real world
• Electric energy dissipates as it travels along
• Wires have resistance, capacitance, and
  inductance which distort signals
• Magnetic or electrical interference distorts
  signals
• Distortion can result in loss or
  misinterpretation

45
Illustration Of Distorted Signal For A Single Bit

• In practice
• Distortion can be much worse than illustrated

46
Consequences

• RS-232 hardware must handle minor distortions
• Take multiple samples per bit
• Tolerate less than full voltage
• Can not use electrical current for long-distance
  transmission

47
Reading Materials

• Chapter 4
• Chapter 5 Sections 5.1-5.7