1. Lecture 4 Continuation of transmission basics Chapter 3, pages 75-96
Dave Novak
School of Business
University of Vermont
Sources: 1) Network+ Guide to Networks, Dean 2013
2) Comer, Computer Networks and Internets, 2004
3) Other sources cited within the lecture slides

2. Objectives Line coding Modulation Multiplexing Broadband –vs- Baseband
AM, FM, Phase Shift
Multiplexing
FDM, TDM, WDM
Broadband –vs- Baseband

3. How do computers communicate?
At a very basic level computers use binary digits (bits) to represent data / information
Bits are transmitted over some medium
Electrical current over copper cable
Pulses of light over fiber optic cable

4. How do computers communicate?
How can data be represented by electrical signals?
Can be generally explained via local asynchronous communication (RS-232)
Example: electrical voltage over copper wires

5. How do computers communicate?
Simple electronic communication systems
Electric current is used to encode data
For example: Negative voltage represents a 1 and positive voltage represents a 0
Transmit a “1” by transmitting negative voltage over a copper wire

6. Local asynchronous communication
RS-232 (EIA) emphasizes need for standards and illustrates how they are used in networking
Most widely accepted standard for transferring data across copper wires
Defines serial, asynchronous communication
Serial –
Asynchronous -

7. Local asynchronous communication
There are limitations to hardware…
Electronic devices cannot produce an exact voltage or change from one voltage to another instantly
Wires are not perfect conduits
Signal loses energy as it travels
Takes time to change voltage

8. Local asynchronous communication
Transmission hardware is typically rated in baud
Baud = the signaling rate at which data are sent through a channel measured in transactions per second
In simple RS-232 baud rate = the bit rate, as one bit is transferred per signal transition
9600 baud = 9600 bps
This is not true for more complex coding schemes

9. Local asynchronous communication
Bit rate –vs- baud rate – they are directly related to one another
Bit rate – number bits transmitted per sec
Baud rate – number of signaling elements per sec
Depending on the signaling level or modulation technique, more than one bit can be transferred per sec
Bit rate = baud/sec x # of bits/baud

10. Modems
Hardware circuit that accepts sequence of data bits and applies modulation to a carrier wave is called a modulator
Hardware circuit that accepts a modulated carrier wave and recreates the sequence of bits used to modulate the carrier is called a demodulator
To support full duplex transmission, each system needs both – these are combined into a single device called a modem

11. Modems
Different types of modems including RF (wireless) and optical fiber modems
Most familiar with 2-wire dialup modems
Half duplex – take turns sending info
Use a carrier that is an audible tone to mimic a telephone
Note that the term modem is not limited to the dialup device
Modern modems use a combination of modulation techniques to transmit multiple bits per baud

12. Some Basic Issues
Why don’t we just use simple RS232 to transmit 1’s and 0’s?
Desire to transmit large amounts of data over long distances at really high speeds
Multiple conversion processes as different types of data travel over different physical networks (for example sending analog data over a digital network)
Transmission errors
Increase bit transfer rate

13. Advanced transmission concepts
Line Encoding
Modulation
Multiplexing

14. Line encoding
Different line encoding schemes are used to transmit digital data using a digital signal
Improve bit-rate
Decrease bit-error rates
Digital data / digital signals
Encoding schemes can vary by
Layer 1 and 2 standards (which also impact media and distance)

15. Line Encoding

16. Modulation
Encode digital data onto a continuous analog carrier wave by modulating (altering one or more properties of the carrier wave) the signal
Digital data using analog signals
1) Frequency Modulation
2) Amplitude Modulation
3) Phase-shift Modulation
Analog data using digital signals
4) Pulse Code Modulation

17. Digital data / analog signal
a) Digital (binary) signal being represented
b) Amplitude modulation (AM)
c) Frequency modulation (FM)
d) Phase-Shift modulation (PSM)
Source:

18. Analog data / digital signal
Most common technique for encoding analog data using digital signals is Pulse Code Modulation (PCM)

19. Multiplexing
Technique that allows multiple signals to be transmitted simultaneously over a single medium
Media is separated into multiple channels or subchannels
This can be done virtually and/or physically
Individual signals from different sources can be combined into a single complex signal and then the separate signals are recovered at the receiving end
Multiplexing depends on signal type (analog / digital) and the media used

20. Multiplexing
Why is multiplexing so important in data communications?

21. Basic concept of multiplexing
Image Source:

22. Multiplexing Frequency Division Multiplexing (FDM)
Wave Division Multiplexing (WDM)
Same concept as FDM but applied to fiber where optical signals are used
Time Division Multiplexing (TDM)

23. Frequency Division Multiplexing (FDM)
Inherently an analog technology
Uses different frequency ranges over single medium
Total bandwidth is divided into subchannels consisting of smaller segments of available bandwidth
Carrier wave used by each sender/receiver pair operates within a unique frequency band to avoid interfering with other transmissions

24. Frequency Division Multiplexing (FDM)
Source:

25. Wave Division Multiplexing
Frequency division applied to light waves as opposed to radio frequencies
Combining separate wavelengths of data into a light stream on a single fiber carrier

26. Time Division Multiplexing (TDM)
Primarily a digital technology that separates different data streams by time (as opposed to frequency)
1) Synchronous time division (STD) (slotted)
2) Statistical

27. Time division multiplexing
Synchronous time division (slotted)
Sources takes turns in a round robin fashion
Great for telephone transmission

28. Time division multiplexing
Statistical multiplexing
Take turns like STD, but does not waste slots if source has no data to send

29. Some Basic Issues
To transmit an analog signal, the bandwidth of the signal must match the bandwidth of the transmission channel
To transmit a digital signal, the bit rate of the signal must be within the bit rate range of the transmission channel

30. Form of transmission
Baseband and broadband are different forms of transmission
Baseband transmission uses digital signaling
Broadband transmission uses analog signaling

31. Baseband
Baseband systems can transmit one signal / one channel at a time
Each transmission requires full use of the medium so when one node is transmitting the other nodes must wait their turn
Ethernet uses baseband signaling
Why would the most popular LAN technology use this signaling technique?

32. Baseband Characteristics
Converts digital signal to voltage without using different frequency channels
Single frequency
Bi-directional signal flow (but not simultaneously)
Entire bandwidth of cable used to transmit single data stream
Multiple signals can be sent using Time Division Multiplexing (TDM)

33. Baseband
If baseband requires the use of the entire cable for single transmission, how can multiple computers on a LAN communicate simultaneously?

34. Broadband
Broadband systems can transmit multiple signals over many different channels simultaneously
Each channel uses a different frequency band to transmit
Cable TV uses broadband signaling

35. Broadband Characteristics
Communications medium split into multiple channels
Multiple signals are transmitted over a single medium simultaneously
Can send and receive simultaneously
Analog transmission
Signal flow is unidirectional on each channel
Uses Frequency Division Multiplexing (FDM)

36. Broadband
If broadband only supports uni-directional transmission, how do 2 (or more) devices communicate?

37. Broadband versus baseband
Broadband networks carry more data than baseband networks
Using analog signaling over multiple frequency bands
Broadband networks are more expensive and complex than baseband networks
Use multiplexing
Broadband networks are better suited for long-distance communication than baseband networks

38. Broadband, Baseband, Analog, Digital
What is the signaling technique used on communication channel?
Phone line (DS0) – broadband channel
T-1 carrier – baseband channel
Ethernet – baseband
WiFi - broadband
What are the form of the data transmitted over the channel?
Can be either analog or digital

39. Broadband, Baseband, Analog, Digital
Transmit analog data over broadband channel
Transmit analog data over baseband channel
Transmit digital data over broadband channel
Transmit digital data over baseband channel

40. Summary Transmission basics Analog –vs- Digital
Simple transmission RS232
Line coding
Modulation
AM, FM, Phase Shift
Multiplexing
FDM, TDM, WDM
Broadband –vs- Baseband