1. Chapter 5: Multiplexing: Sharing a Medium
Data Communications and
Computer Networks: A
Business User’s Approach
Third Edition
Chapter 5:
Multiplexing: Sharing a Medium

2. After reading this chapter, you should be able to:
Objectives
After reading this chapter, you should be able to:
Describe frequency division multiplexing and list its applications, advantages, and disadvantages
Describe synchronous time division multiplexing and list its applications, advantages, and disadvantages
Outline the basic multiplexing characteristics of both T-1 and ISDN telephone systems
Data Communications & Computer Networks: A Business User's Approach, Third Edition

3. Objectives (continued)
Describe statistical time division multiplexing and list its applications, advantages, and disadvantages
Cite the main characteristics of wavelength division multiplexing and its advantages and disadvantages
Describe the basic characteristics of discrete multitone
Cite the main characteristics of code division multiplexing and its advantages and disadvantages
Apply a multiplexing technique to a typical business situation
Data Communications & Computer Networks: A Business User's Approach, Third Edition

4. Introduction
Under the simplest conditions, a medium can carry only one signal at any moment in time
For multiple signals to share one medium, the medium must somehow be divided, giving each signal a portion of the total bandwidth
The current techniques that can accomplish this include frequency division multiplexing, time division multiplexing, and wavelength division multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

5. Frequency Division Multiplexing
Assignment of non-overlapping frequency ranges to each “user” or signal on a medium
Thus, all signals are transmitted at the same time, each using different frequencies
A multiplexor
Accepts inputs and assigns frequencies to each device
Is attached to a high-speed communications line
Data Communications & Computer Networks: A Business User's Approach, Third Edition

6. (continued) Frequency Division Multiplexing
Corresponding multiplexor, or demultiplexor
Is on the end of the high-speed line
Separates the multiplexed signals
Data Communications & Computer Networks: A Business User's Approach, Third Edition

7. (continued) Frequency Division Multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

8. (continued) Frequency Division Multiplexing
Analog signaling is used to transmit signals
Broadcast radio and television, cable television, and AMPS cellular phone systems use frequency division multiplexing
Oldest multiplexing technique
Involves analog signaling  more susceptible to noise
Data Communications & Computer Networks: A Business User's Approach, Third Edition

9. Time Division Multiplexing
Sharing signal is accomplished by dividing available transmission time on a medium among users
Digital signaling is used exclusively
Time division multiplexing comes in two basic forms:
Synchronous time division multiplexing
Statistical, or asynchronous time division multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

10. Synchronous Time Division Multiplexing
The original time division multiplexing
Multiplexor
Accepts input from attached devices in a round-robin fashion
Transmits data in a never ending pattern
T-1 and ISDN telephone lines are common examples of synchronous time division multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

11. (continued) Synchronous Time Division Multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

12. (continued) Synchronous Time Division Multiplexing
If one device generates data at a faster rate than other devices, then the multiplexor must either
Sample incoming data stream from that device more often than it samples other devices OR
Buffer faster incoming stream
If a device has nothing to transmit,
Multiplexor must still insert a piece of data from that device into the multiplexed stream
Data Communications & Computer Networks: A Business User's Approach, Third Edition

13. (continued) Synchronous Time Division Multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

14. (continued) Synchronous Time Division Multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

15. (continued) Synchronous Time Division Multiplexing
So that the receiver may stay synchronized with the incoming data stream, the transmitting multiplexor can insert alternating 1s and 0s into the data stream
Data Communications & Computer Networks: A Business User's Approach, Third Edition

16. T-1 multiplexor stream is a continuous series of frames
T-1 Multiplexing
T-1 multiplexor stream is a continuous series of frames
Data Communications & Computer Networks: A Business User's Approach, Third Edition

17. ISDN multiplexor stream is also a continuous stream of frames
ISDN Multiplexing
ISDN multiplexor stream is also a continuous stream of frames
Each frame contains various control and sync info
Data Communications & Computer Networks: A Business User's Approach, Third Edition

18. SONET/SDH Multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

19. Statistical Time Division Multiplexing
Statistical multiplexor - transmits only the data from active workstations
If a workstation is not active, no space is wasted on the multiplexed stream
A statistical multiplexor
Accepts incoming data streams
Creates a frame containing only the data to be transmitted
Data Communications & Computer Networks: A Business User's Approach, Third Edition

20. (continued) Statistical Time Division Multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

21. (continued) Statistical Time Division Multiplexing
To identify each piece of data, an address is included
Data Communications & Computer Networks: A Business User's Approach, Third Edition

22. (continued) Statistical Time Division Multiplexing
If data is of variable size, length is also included
Data Communications & Computer Networks: A Business User's Approach, Third Edition

23. (continued) Statistical Time Division Multiplexing
More precisely, the transmitted frame contains a collection of data groups
Data Communications & Computer Networks: A Business User's Approach, Third Edition

24. Wavelength Division Multiplexing
Wavelength division multiplexing multiplexes multiple data streams onto a single fiber optic line
Different wavelength lasers (called lambdas) transmit the multiple signals
Each signal carried on the fiber can be transmitted at a different rate from the other signals
Data Communications & Computer Networks: A Business User's Approach, Third Edition

25. (continued) Wavelength Division Multiplexing
Dense wavelength division multiplexing combines many (30, 40, 50, 60, more?) onto one fiber
Coarse wavelength division multiplexing combines only a few lambdas
Data Communications & Computer Networks: A Business User's Approach, Third Edition

26. (continued) Wavelength Division Multiplexing
Data Communications & Computer Networks: A Business User's Approach, Third Edition

27. Discrete Multitone (DMT)
A multiplexing technique commonly found in digital subscriber line (DSL) systems
DMT combines hundreds of different signals, or subchannels, into one stream
Each subchannel is quadrature amplitude modulated
recall - eight phase angles, four with double amplitudes
Theoretically, 256 subchannels, each transmitting 60 kbps, yields Mbps
Unfortunately, there is noise
Data Communications & Computer Networks: A Business User's Approach, Third Edition

28. Code Division Multiplexing
Also known as code division multiple access
Advanced technique that allows multiple devices to transmit on the same frequencies at the same time
Each mobile device is assigned unique 64-bit code
Chip spreading code
To send a binary 1, mobile device transmits the unique code
To send a binary 0, mobile device transmits the inverse of code
Data Communications & Computer Networks: A Business User's Approach, Third Edition

29. Code Division Multiplexing (continued)
Receiver
Gets summed signal
Multiplies it by receiver code
Adds up resulting values
Interprets as a binary 1 if sum is near +64
Interprets as a binary 0 if sum is near –64
Data Communications & Computer Networks: A Business User's Approach, Third Edition

30. Code Division Multiplexing Example
For simplicity, assume 8-chip spreading codes
3 different mobiles use the following codes:
Mobile A:
Mobile B:
Mobile C:
Assume Mobile A sends a 1, B sends a 0, and C sends a 1
Data Communications & Computer Networks: A Business User's Approach, Third Edition

31. (continued) Code Division Multiplexing Example
Signal code: 1-chip = +N volt; 0-chip = -N volt
Three signals transmitted:
Mobile A sends a 1, or , or
Mobile B sends a 0, or , or
Mobile C sends a 1, or , or
Summed signal received by base station: +3, -1, -1, +1, +1, -1, -3, +3
Data Communications & Computer Networks: A Business User's Approach, Third Edition

32. (continued) Code Division Multiplexing Example
Base station decode for Mobile A:
Signal received: +3, -1, -1, +1, +1, -1, -3, +3
Mobile A’s code: +1, -1, +1, +1, +1, -1, -1, +1
Product result: +3, +1, -1, +1, +1, +1, +3, +3
Sum of Product results: +12
Decode rule: For result near +8, data is binary 1
Data Communications & Computer Networks: A Business User's Approach, Third Edition

33. (continued) Code Division Multiplexing Example
Base station decode for Mobile B:
Signal received: +3, -1, -1, +1, +1, -1, -3, +3
Mobile B’s code: -1, +1, +1, -1, +1, +1, +1, -1
Product result: -3, -1, -1, -1, +1, -1, -3, -3
Sum of Product results: -12
Decode rule: For result near -8, data is binary 0
Data Communications & Computer Networks: A Business User's Approach, Third Edition

34. Comparison of Multiplexing Techniques
Data Communications & Computer Networks: A Business User's Approach, Third Edition

35. Business Multiplexing in Action
XYZ Corporation has two buildings separated by a distance of 300 meters
A 3-inch diameter tunnel extends underground between the two buildings
Building A has a mainframe computer and Building B has 66 terminals
List some efficient techniques to link the two buildings
Data Communications & Computer Networks: A Business User's Approach, Third Edition

36. (continued) Business Multiplexing in Action
Data Communications & Computer Networks: A Business User's Approach, Third Edition

37. (continued) Business Multiplexing in Action Possible Solutions:
Connect each terminal to mainframe computer using separate point-to-point lines
Connect all terminals to mainframe computer using one multipoint line
Connect all terminal outputs and use microwave transmissions to send data to the mainframe
Collect all terminal outputs using multiplexing and send data to mainframe computer using conducted line
Data Communications & Computer Networks: A Business User's Approach, Third Edition

38. Frequency division multiplexing Synchronous time division multiplexing
Summary
Frequency division multiplexing
Synchronous time division multiplexing
Basic multiplexing characteristics of T-1 and ISDN telephone systems
Statistical time division multiplexing
Wavelength division multiplexing
Discrete multitone
Data Communications & Computer Networks: A Business User's Approach, Third Edition

39. Code division multiplexing
Summary (continued)
Code division multiplexing
Applying multiplexing techniques to typical business situations
Data Communications & Computer Networks: A Business User's Approach, Third Edition