line coding techniques in DCS

Topics: Modulation, Data transmission, Multiplexing Pages: 19 (641 words) Published: July 14, 2014
EE4512 Analog and Digital Communications

Chapter 7
Multiplexing Techniques

Chapter 7

EE4512 Analog and Digital Communications

Chapter 7
Multiplexing Techniques
• Time Division Multiplexing
• Pages 364-368

Chapter 7

EE4512 Analog and Digital Communications

Chapter 7

• Time division multiplexing (TDM) combines several low,
fixed and predefined bit rate sources into a single high
speed bit stream for transmission over a single digital
communication channel:

Time slots

Multiplexer

Demultiplexer

EE4512 Analog and Digital Communications

Chapter 7

• The TDM time slots have to be chosen properly. If the time slots are too small (for example, 1 bit) then the multiplexer and demultiplexer must switch rapidly. If the time slots are too large (for example, 64 Kb) then the data must be

buffered and delay would be produced. TDM is used for
baseband (not bandpass) data transmission.
Time slots

Multiplexer

Demultiplexer

EE4512 Analog and Digital Communications

Chapter 7

• The T1 TDM system for telephone networks uses a 193 bit
frame where each frame has 24 8-bit slots and 1-bit in each
frame for signaling. The T1 bit rate rb = 1.544 Mb/sec.

S&M Figure 7-3
The duration of each frame Tf = 193 b / 1.544 Mb/sec =
125 µsec or 8 k samples/sec. Here all the data sources
have the same data rate.

EE4512 Analog and Digital Communications

Chapter 7

• If the data sources have different rates a multiplexer
scheme must reconcile the disparate rates.

S&M Figure 7-4
The data rates are in the lowest possible ratio of 8:16:24 or 1:2:3 for a total of 1 + 2 + 3 = 6 slots. The slots are
apportioned to the channels as: a b b c c c and the channel
data rate is 48 kb/sec.
Another example is data rates of 10, 15, 20, and 30 Kb/sec
which reduces to 2:3:4:6 (the LCD is 5) for 2 + 3 + 4 + 6 =
15 slots and the channel data rate is 45 kb/sec.

EE4512 Analog and Digital Communications

Chapter 7

• If data arrives asynchronously from variable rate sources, a statistical multiplexer with input buffers is used. The design is performed by observation and tested in simulation.

S&M Figure 7-5

EE4512 Analog and Digital Communications

Chapter 7

• The statistical TDM packet consists of a start flag, address field, control field, information bits, error control, and an end flag.
S&M Figure 7-6

TCP/IP data packet

EE4512 Analog and Digital Communications

Chapter 7

• TDM with four equal data rate sources rb = 250 b/sec and a transmission rate rTDM = 1 kb/sec

0110

EE4512 Analog and Digital Communications

Chapter 7

• TDM with unequal data rate sources rb = 250, 250 and 500 b/sec and a transmission rate rTDM = 1 kb/sec
rb = 500 b/sec (input)

rTDM = 1 kb/sec

1 msec delay

rb = 500 b/sec (output)

EE4512 Analog and Digital Communications

Chapter 7
Multiplexing Techniques
• Frequency Division Multiplexing
• Pages 368-370

Chapter 7

EE4512 Analog and Digital Communications

Chapter 7

• Frequency division multiplexing (FDM) divides the total
bandwidth available to the system into non-overlapping
frequency sub-bands for transmission over a single digital
communication channel.

EE4512 Analog and Digital Communications

Chapter 7

• FDM usually utilizes guard bands to separate the digital data transmissions. FDM is used for bandpass (not
baseband) data transmission

EE4512 Analog and Digital Communications

• FDM for FM broadcasting over a cable system:
WPTS
Pittsburgh
WQAN
Scranton

WXTU
S&M
Figure 7-7a

Chapter 7

EE4512 Analog and Digital Communications

Chapter 7

• FDM employs a common shared bandwidth but here with
no apparent guard bands.
S&M Figure 7-7b

EE4512 Analog and Digital Communications

Chapter 7

• Optical FDM is usually called wavelength division
multiplexing (WDM) and utilizes separate wavelengths
(λ) of light.

EE4512 Analog and Digital Communications...
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