multiplexing concepts and introduction to bisdn · • explain how to use the nyquist sampling...

Multiplexing Concepts and Introduction to BISDN

Professor Richard Harris

Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 2

Objectives

• Define what is meant by multiplexing and de-multiplexing

• Identify the main types of multiplexing– Space Division– Time Division– Frequency Division

• Explain the concept of Pulse Code Modulation• Explain how to use the Nyquist Sampling Theorem in

multiplexing.


Presentation Outline

• Multiplexing concepts– Space Division– Time Division– Frequency Division– Wavelength Division

• Pulse Code Modulation• Nyquist Sampling Theorem• BISDN Networking concepts


Additional References

[1] Hioki - Chapter 12


Multiplexing

• Multiplexing is the process of combining two or more signals and transmitting them over a single transmission link.

• Demultiplexing is the reverse process, that is the process of separating the multiplexed signals at the receiving end of the transmission link.

• Multiplexing results in the efficient use of the communications link.• Trunk circuits used within the PSTN use multiplexing techniques to

combine several signals. • Relationship with space, time and bandwidth.

– Space is required for the medium to exist in.– Time is required for multiple signals to be transmitted and received.– Bandwidth is the most important. Bandwidth is required to accommodate

the related frequency components of the multiplexed signals.


Multiplexing Classifications

• There are four fundamental multiplexing classifications:– Space-division multiplexing (SDM)– Frequency-division multiplexing (FDM)– Time-division multiplexing (TDM)– Wavelength (WDM)


Space-division Multiplexing

• Space-division multiplexing is the combination of physically separate signals being transmitted on separate cables into a bundled cable.

• Essentially, we are avoiding having to dig separate trenches and have different telephone poles for each customer in the communication network!!!

• Subscriber loops and trunk circuits are combined and share common trenches or conduits.


Bell System FDM Groups

Voice channel 1 voice channel

Channel group(12 voice channels)

12 voice channels

Super group(5 channel groups)

60 voice channels

Jumbo group(6 master groups) 3,600 voice channels

Jumbo groupmultiplex

(3 jumbo groups)10,800 voice channels

Master group(10 supergroups)

600 voice channels


Time Division Multiplexing

• Distribution of signals is now in the time domain rather than the frequency domain.

• Note that FDM is an analogue process while TDM is actually a digital process.

• In TDM, several analogue signals are sampled and converted to digital bit streams via A/D converters using Pulse Code Modulation. (See following slides.)

• The signals are interleaved from several sources to produce a PCM signal.


Time Division Multiplexing - 2

• Prior to 1960, telecommunications was predominantly analogue transmission with FDM serving as the major form of multiplexing. Since then, time-division multiplexed PCM (pulse-code modulation) has dominated and is still the preferred method of transmission onto Public Switched Telephone Network(PSTN) trunk circuits.

– TDM involves the distribution of multiple signals in the time domain, whereas in FDM these signals are distributed in the frequency domain.

– Another major distinction is that FDM is an analogue process, whereas TDM is a digital process.

• In TDM, several analogue signals are sampled and converted to digital bit streams through the use of analogue-to-digital (A/D) converters. The process of converting the analogue signal into an encoded digital value is referred to as pulse-code modulation (PCM).

• In TDM, signals from several sources are digitised and interleaved to form a PCM signal. The time-division multiplexed PCM signal is then transmitted onto a single channel.



• When the digital bit stream is received, the reverse process is performed.

• The bit stream is de-multiplexed and converted back to the original analogue signals.

• The analogue signals may then be heard through a telephone handset speaker.



Output

Input signal A

Input signal B

Input signal C

Input signal D

B

C

D

AA

B

CD

Time

Out

put v

alue

Figure 2.20 - sampling offour input devices



Slot3

Slot2

Slot 1

Slot1

Figure 2.22: The capacity of the transmission facility is dividedinto slots.

Slot3

Slot2

Slot1

Slot3

Slot2


Analogue to Digital Conversion

• Historically source signals (voice and video) were in analogue format, however, more recently, digital video sources have become available.

• To digitally transmit analogue source signals, the signal has to be transformed via an analogue to digital conversion.

• Three important methods of analogue to digital conversion are:– pulse-code modulation– differential pulse-code modulation– delta modulation


Digital Pulse Modulation - 1

• The process of analogue-to-digital conversion is sometimes referred to as digital pulse modulation.

• The first operation performed in the conversion of an analogue signal into digital form involves the representation of the signal by a sequence of uniformly spaced pulses, the amplitude of which is modulated by the signal. (See next slide.)

• The pulse-repetition frequency must be chosen in accordance with the Nyquist sampling theorem. (What’s this?)

• In both pulse-code modulation and differential pulse-code modulation, the pulse repetition frequency, or the sampling rateis chosen to be slightly greater than the Nyquist rate (i.e. greater than twice the highest frequency component) of the analogue signal.



+

=



Error

-0.060

-0.040

-0.020

0.000

0.020

0.040

0.060

Error

Quantisation Error

-5

-4

-3

-2

-1

0

1

2

3

4

5

00.0

40.0

80.1

20.1

6 0.2 0.24

0.28

0.32

0.36 0.4 0.44

0.48

0.52

0.56 0.6 0.64

0.68

0.72

0.76 0.8 0.84

0.88

0.92

0.96 1

Series2

Original Signal

Time

-5

-4

-3

-2

-1

0

1

2

3

4

5

00.040.080.1

20.16 0.20.24 0.280.3

20.36 0.4 0.4

40.480.520.56 0.60.640.68 0.720.7

6 0.80.840.8

80.920.96 1

Valu

e

Time

Sampled Signal



• In delta modulation, the sampling rate is chosen to be much greater than the Nyquist rate. The reason for this is to increase correlation between adjacent samples derived from the information-bearing analogue signal and thereby to simplify the physical implementation of the delta modulation process.

• The distinguishing feature between pulse-code modulation and differential pulse-code modulation is that in the latter case, additional circuitry (designed to perform linear prediction) is used to exploit the correlation between adjacent samples of the analogue signal so as to reduce the transmitted bit rate.

• Pulse-code modulation is viewed as a benchmark against which other methods of digital pulse modulation are measured in performance and circuit complexity.



• Comparison of the three basic forms of digital pulse modulation


Pulse-Code Modulation (PCM)

• PCM was developed in 1937 at the Paris Laboratories of AT&T. Alex H. Reeves was the inventor.

• Reeves conducted several successful transmission experiments across the English Channel using various modulation techniques, including pulse-width modulation (PWM), pulse-amplitude modulation (PAM) and pulse-pulse modulation (PPM).

• Circuitry was quite complex and expensive in the early stages ofdevelopment.

• In the 1960s, the evolution of the semiconductor industry permitted low cost circuits to be fabricated. PCM became the preferred method of transmitting over the PSTN.


Pulse-Code Modulation - 2

• PCM is a method of serially transmitting an approximate representation of an analogue signal.

• The PCM is itself a succession of discrete numerically encoded binary values derived from digitising the analogue signal.

• The maximum expected amplitude of the analogue signal is quantised. That is, divided into discrete numerical levels. The number of discrete levels depends on the resolution (number of bits) of the analogue-to-digital (A/D) converter used to digitise the signal.

• If an 8-bit A/D converter is used, the analogue signal is quantised into 256 (28) discrete levels.

• quantising range = 2(no. of A/D converter bits)



• The essential operations in the transmitter of a PCM system are sampling, quantising, and encoding.

• The quantising and encoding operations are usually performed in the same circuit, which is called an analogue-to-digital converter.

• The essential operations in the receiver are regeneration of impaired signals, decoding and demodulation of the train of quantised samples.

• These operations are usually performed in the same circuit, which is called a digital-to-analogue converter.

• At intermediate points along the transmission route from the transmitter to the receiver, regenerative repeaters are used to reconstruct (regenerate) the transmitted sequence of coded pulses to reduce the effects of signal distortion and noise.



Sampler Encoder

Decoder

Input signal

Quantiser

Schematic Diagram of PCM Communication

Sampler

PAM WaveReceived signal

Transmission Link



• The basic elements of a PCM system - expanded


Sampling Theorem - Nyquist

• The sampling theorem for band-limited signals of finite energy may be stated in two equivalent parts:– A band-limited signal of finite energy, which has no frequency

components higher than W Hertz, is completely described by specifying the values of the signal at instants of time separated by 1/2W seconds.

– A band-limited signal of finite energy, which has no frequency components higher than W Hertz, may be completely recovered from a knowledge of its samples taken at the rate of 2W per second.

• Part 1 of the theorem is exploited in the transmitter; part 2 of the theorem is exploited in the receiver.

• The sampling rate 2W is called the Nyquist rate, and its reciprocal 1/2W is called the Nyquist interval.


Nyquist Sampling Theorem

• If a signal is sampled at a rate that is at least twice the highest frequency that it contains, the original signal can be completely reconstructed.

• A signal containing frequency components up to 4 kHz can therefore be recovered with minimal distortion. Since the bandwidth of the telephone lines is 300 to 3400 Hz, 8 kHz is easily twice the highest frequency component within this range.

• Sampling at a rate of 8kHz means one sample every 125 microseconds (1/8000 = 125 x 10-6 sec)

Go back to Pulse Modulation


BISDN Networking Concepts

• Multiplexing defines the way in which information streams are carried through one media.

• There are many different multiplexing schemes in use today and these include:– Frequency– Wavelength– Code Division– Time Division

• The most important ones are highlighted above.• In time division, two basic principles can be identified

– Synchronous Time Division (STD)– Asynchronous Time Division (ATD)


Synchronous Time Division

• In STD time is divided into frames.• Each frame is divided into slots.• Identically numbered slots in consecutive frames

identify an STD channel, eg. with a 64kb/s transmission rate.

• A physical channel is identified by the position of the slot in the frame.

Frame Frame Frame Frame22 22 22 22 22


Asynchronous Time Division

• Once again, time is divided into slots.• Each slot carries an information unit of variable length (also

called a frame or packet) or of constant length - called a cell.• Packets or cells may appear completely asynchronously.• They carry a Virtual Channel Identifier (VCI) within their header.• Units with identical VCI’s form a virtual channel.

Slot

InformationField Cell Header

SynchronisationCell

SynchronisationCell


Transfer Modes

• STM - Synchronous Transfer Mode– Multiplexing of physical connections based on STD.– Circuit switching principle– Constant bitrate traffic streams– Integration of traffics with different but constant bitrates.

• PTM - Packet Transfer Mode– Multiplexing of virtual connections based on ATD– Packet switching principle with variable block length (packets)– Bursty traffic streams– Flow control, error recovery– Connectionless (datagram) transfer

• ATM - Asynchronous Transfer Mode– Multiplexing of virtual connections based on ATD– Packetised switching of fixed sized cells– Bursty traffic streams– Lightweight protocols: No flow control– Integration of traffics with arbitrary cell rates

multiplexing concepts and introduction to bisdn · • explain how to use the nyquist sampling...

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