1. Important Hardware Modems (amplitude and frequency Modulation)
FAX machines
Multiplexers (time and frequency division multiplexing)
Inverse Multiplexers
Front-end communications processors
Remote intelligent controllers/line splitter (concept of polling)
Line interface module/Terminal Controller
Port Sharing Device.
Cellular Phones (frequency reuse)
[Note: Main source of pictures in this document is Fitzgerald and Dennis, Wiley, 1998]

2. Modems
Modem is an acronym for Modulator/ Demodulator, and takes digital electrical pulses from a computer, terminal, or microcomputer and converts them into a continuous analog signal, for transmission over an analog voice grade circuit.
It then re-converts the analog signal to its original digital format.
Most modems accept commands from a computer keyboard.

3. Modem Operation

4. Modulation
Modulation is the technique that modifies the form of a digital electrical signal so the signal can carry information on a communications media.
There are three fundamental methods of analog modulation of an analog signal:
Amplitude Modulation (AM)
Frequency Modulation (FM)
Phase Modulation(PM)

5. Amplitude Modulation

6. Frequency Modulation and FSK
(Frequency Shift Keying)

7. Phase Modulation and PSK

8. Sending Multiple Bits Simultaneously
In practice, the maximum number of bits that can be sent with any one of these techniques is about five bits. The solution is to combine modulation techniques.
One popular technique is quadrature amplitude modulation (QAM) involves splitting the signal into eight different phases, and two different amplitudes for a total of 16 different possible values.

9. Sending Multiple Bits Simultaneously
Trellis coded modulation (TCM) is an enhancement of QAM that combines phase modulation and amplitude modulation.
The problem with high speed modulation techniques such as TCM is that they are more sensitive to imperfections in the communications circuit.

10. Bit Rate Versus Baud Rate Versus Symbol Rate
The bit rate and the symbol rate (or baud rate) are the same only when one bit is sent on each symbol. If we use QAM or TCM, the bit rate would be four to eight times the baud rate.

11. Modem Standards
There are many different types of modems available today.
Most modems support several standards so that they can communicate with a variety of different modems.
Better modems can change data rates during transmission to reduce the rate in case of noisy transmission (fast retrain).
28800 bps
V.34 Modem
                       
Time Elapsed: 30 seconds
(using a Half-fast modem)
V.32 Modem
14400 bps
Time Elapsed: 30 seconds
(good time to go for coffee)
V.90 Modem
50000 bps
Time Elapsed: 30 seconds
OK! I get the picture!
                       
                        
Source:

12. Modem Standards Modem Standard Maximum Symbol Rate Bits per Data Rate(
bps)
V.22
1200 1
2400
V.32 4
9600
V.32bis 6
14,400
V.34
3429
8.4
28,800
V.34+
9.8
33,600
V.42, V.42bis
N/a
56,000
V Digital N/a ,000
downstream
More information:

14. Digital Transmission of Analog Data
Analog voice data can be sent over digital networks using a pair of special devices called CODECs (Coder/Decoder).

15. Pulse Amplitude Modulation (PAM)
Analog voice data must be translated into a series of binary digits before they can be transmitted.
With Pulse Amplitude Modulation, the amplitude of the sound wave is sampled at regular intervals and translated into a binary number.
The difference between the original analog signal and the translated digital signal is called quantizing error.

16. Pulse Amplitude Modulation/Code Modulation
Source: Goldman, Wiley.

17. Pulse Amplitude Modulation
For standard voice grade circuits, the sampling of 3000 Hz at an average of 2 samples/Hz would result in a sample rate of 6000 times per second.

18. Reducing Quantizing Error
There are two ways to reduce quantizing error and improve the quality of the PAM signal.
Increase the number of amplitude levels
Sample more frequently (oversampling).

19. Pulse Code Modulation
Pulse Code Modulation is the most commonly used technique in the PAM family and uses a sampling rate of 8000 samples per second.
Each sample is an 8 bit sample resulting in a digital rate of 64,000 bps (8 x 8000).
A T-1 channel (capacity = Mbps) can carry 24 simultaneous voice conversations.
Kai Larsen: For more information on PCM, see Stallings (2003) “Data & Computer Communications” Sixth Edition (available in my office).

20. Multiplexing There are three major types of multiplexers
Time division multiplexers (TDM)
Frequency division multiplexers (FDM)
Statistical time division multiplexers (STDM)

21. Multiplexed Circuit

22. Opticom - 16E1 Multiple E1 and Ethernet Multiplexer
(Source: Opticom Communications)

23. Time Division Multiplexing

24. Time Division Multiplexing
Time division multiplexing shares a circuit among two or more terminals by having them take turns, dividing the circuit “vertically.”
Time on the circuit is allocated even when data are not transmitted, so that some capacity is wasted when a terminal is idle.
Time division multiplexing is generally more efficient and less expensive to maintain than frequency division multiplexing, because it does not need guardbands.

25. TIME DIVISION MULTIPLEXING1
Signal 1
Signal 2
Signal 3
Signal 4
Multiplexed Signal

26. Frequency Division Multiplexing (FDM)

27. Frequency Division Multiplexing (FDM)
Frequency division multiplexers are somewhat inflexible because once you determine how many channels are required, it may be difficult to add more channels without purchasing an entirely new multiplexer.

28. Wavelength Division Multiplexing (WDM)
…is a version of FDM used in fiber optic cables. WDM should reach 25 Terabits per second within three years.                                                      
Kai Larsen: SWDM= Selective Wave Division Multiplexing (DWDM) DSLAM=Digital Subscriber Line Access Multiplexer
Source:

29. Voice Circuit

30. FDM Example

31. Statistical Time Division
STDM provides more efficient use of the circuit and saves money. However, STDM introduces two complexities:
1. STDM can cause time delays, if all terminals decide to transmit simultaneously.
2. All data must be identified by an address that specifies the device to which terminal it belongs.
Most STDM multipexers do not send one bit at a time from each terminal, but send groups of characters at one time (say, a byte).

32. STDM
xxxxxxxxxxxxxxx

33. NASA’s Ground Network

34. Inverse Multiplexing
Inverse multiplexing (IMUX) combines several low speed circuits to appear as one high speed circuit.
One of the most common uses is to provide T-1 (1.544 Mbps) circuits for wide area networks, by combining 24 slower (64 Kbps) circuits.
T1 actually uses a multiplexer/inverse multiplexer pair.

35. Inverse Multiplexing

36. Front End Processors (FEP)

37. Front End Communications Processor (FEP)
Objective: Specialized computer to take away communications
related functions from the host computer.
Functions
Communications Line Control: (Polling/Selecting)
Message Formatting (Assembly/Disassembly)
Protocol Conversion
Message Editing: Compression/Decompression etc.
Message Buffering
Error Control
Message Recording/Trail Keeping
Statistics

38. Remote Intelligent Controller or Cluster Controller
Small computer which reduces load on host computer.
A group of terminals are connected to the host via the
cluster controller instead of being directly connected.
Functions
Control operation of cluster.
(e.g., data collection, screen formatting, etc.)
Some processing is done within cluster controller
Reduce need for interaction with host.
Reduce traffic
Step towards distributed processing.

39. Polling
Polling is the process of sending a signal to a client that gives it permission to transmit or to ask it to receive.
There are two common types of polling.
Roll call polling, the FEP works consecutively through a list of clients, first 1, then 2 etc. until all are polled.
Hub go-ahead polling (a.k.a. token passing) One computer starts the poll and passes it to the next system on the multipoint circuit, which sends its message or passes it to the next system, and so on.

40. Network Configuration
Network configuration is the basic physical layout of the network.
3 common types of circuits are used to configure networks:
Point-to-point circuits (or two-point) - sometimes called dedicated circuits.
Multipoint circuits (or multidrop).
Multiplexed circuits.
Most complex computer networks have many circuits, some of each type.

41. Multipoint Configuration

42. Example of a Bank Network

43. The Cellular Phone Network
                                                

44. How does the Cellular Phone Network Work?
Source: Carr and Snyder, Irwin.

45. Frequency Reuse
                                       4 6 1 2 7 5 3
This is a repeating pattern with 7 frequencies.
The same set of frequencies is repeated two cells away.
Source for monkey: gettyimages.com

46. Cellular Phone Standards
AMPS: Advanced Mobile Phone Services.
(Analog Standard in North America. Uses 30 KHz per channel.
Frequency band: MHz.)
TDMA: Time Division Multiple Access.
(From 3 to 15 times the capacity of analog; caller ID; text messaging; better battery life; seamless roaming between AMPS and PCS.)
CDMA: Code Division Multiple Access.
(All the advantages of TDMA with twice the capacity, better voice quality, and lower costs.)
GSM: Global System for Mobile Communications.
(Similar to TDMA. Pan-European Standard.)

47. Team exercise Form teams
Go to computer labs (10 minutes, no more), use WWW to do task
Either:
Map different cell-phone providers back to their underlying technologies
Find maps of coverage areas and/or cooperative deals between providers
Come back to classroom, 5 mins to write up findings
Enlighten the class with your findings…

48. TDMA
Utilizes time-sharing methods of centralized computers with large numbers of users.
Stems from time-division multiplexing used by phone companies to put multiple calls on each digital line.
Used in GSM
Conservative digital system that multiplies the number of users in an analog cellular channel by three.
Developers founded Qualcomm and developed CDMA.

49. CDMA
Differentiates calls by multiplying the digital signal with a code resembling white noise.
Spreads the signal across a 1.25-megahertz swath of cellular spectrum.
Allows many users to share the spectrum at the same time.
Each phone is programmed with a specific pseudonoise code to spread the signal over a wide frequency band.

50. CDMA continued…
Base station uses same code in inverted form (1's are 0's and 0's are 1's) to "despread" the original signal.
All other codes remain spread out and indistinguishable from background noise.
Keeps transmit power at lowest feasible level.

51. Cocktail Party Example
CDMA
Everyone can talk at once, but in different languages.
Everyone listens for messages in their own languages and ignores all other sounds as background noise.
If someone begins to yell they drown out surrounding messages reducing the number of sustainable conversations .
TDMA
Each person restricts talk to a specific time slot while everyone else is silent.
Reasonable as long as party was run by a dictator who controlled conversation with complex rules and a rigid clock.

52. Advantages of CDMA over TDMA
Longer battery life in cell phone.
Combines three strongest signals into one.
TDMA perceives the combination as interference.
CDMA does not divide time slots --> multipath signals come in time to strengthen message.
CDMA allows simple and soft hand-offs
Utilizes spread spectrum facilitating easy movement from one cell to another.

53. Summary
We studied some important concepts, and devices used in computer networks.
These are the building blocks of networks and are used to design networks.
General trends:
- Several devices are becoming smaller, faster and
more capable with developments in chip technology.
- Many devices are getting integrated into other
devices.
- Prices are falling rapidly.