Synchronous multiplexers | Asynchronous multiplexers | Quasi-synchronous multiplexer

Quasi-synchronous multiplexer , Synchronous multiplexers | Asynchronous multiplexers ?
E LINES
As a matter of fact, Europeans use a version of T lines called E lines. The T lines and E lines are conceptually identical but their capacities and number of voice channels which they can carry will be different.
Table 5.4 shows the E lines and their capacities.
Table 5.4.

 
SUMMARY
 
■          Multiplexing may be defined as a technique which allows many users to share a common communication channel simultaneously. There are two major types of multiplexing techniques. They are as under:
(i)         Frequency division multiplexing (FDM),
(ii)        Time division multiplexing (TDM)
■          The TDM system can be used to multiplex analog or digital signals, however it is more suitable for the digital signal multiplexing.
■          The signaling rate of a TDM system is defined as the number of pulses transmitted per second. It is represented by “r“.
■          TDM system is supposed to have its signaling rate as high as possible. It is evident from the expression above that the signaling rate can be increased by increasing the sampling rate fs and/ or the number of input signals N.
■          The minimum transmission bandwidth of a PAM-TDM channel is given by
BW =  (signaling rate)
■          Therefore, minimum transmission bandwidth BW >  x 2N f_m
Hence, minimum transmission bandwidth BW = Nf_m.
■          The multiplexed PAM signals can be received properly if and only if the transmitter and receiver commutators are synchronized to each other in terms of the speed and the position. In order to ensure synchronization, a marker pulse is introduced at the end of each frame in the transmitted signals.
■          Crosstalk basically means interference between the adjacent TDM channels it is the unwanted coupling of information from one channel to the other. The guard time TA, is the time spacing introduced between the adjacent TDM channels.
■          The communication channel over which the TDM signal is travelling should ideally have an infinite bandwidth in order to avoid the signal distortion. However, in practice, all the communication channels have a finite bandwidth. Such channels are known as the bandlimited channels.
■          One more cause for the crosstalk between the adjacent TDM signals is the use of bandlimiting filters. Because of these filters, the shapes of the TDM pulses are distorted and they get overlapped and crosstalk will take place.
■          The crosstalk resulting from the pulse overlap can be reduced by introducing guard time of sufficient duration between the adjacent TDM pulses. The guard time is denoted by T_g.
■          Advantages of TDM
(i)         Full available channel bandwidth can be utilized for each channel.
(ii)        Intermodulation distortion is absent.
(iii)       TDM circuitry is not very complex.
(iv)       The problem of crosstalk is not severe.
■          Disadvantages of TDM
(i)         Synchronization is essential for proper operation.
(ii)        Due to slow narrowband fading, all the TDM channels may get wiped out.
■          The digital multiplexing will enable us to combine many digital signals such as computer outputs, digital voice, digitized facsimile and TV signals.
■          The digital data can be multiplexed by using a bit-by-bit interleaving procedure. This can be achieved by using a selector switch which sequentially selects a bit from each input and places it over the high speed transmission line. At the receiving end the bits received on the common line are separated out and delivered to their respective destinations.
■          Digital multiplexing is based on the principle of “interleaving symbols” from two or more digital signals. This is similar to TDM but it does not require the periodic sampling and waveform preservation. The signals which are to be multiplexed may come from digital data sources or from analog sources that have been digitally coded.
■          A digital multiplexer is used to merge the input bits from different sources to form one signal from transmission via a digital communication system. The multiplexed signal consists of source digits interleaved bit by bit or word by word.
■          The various digital sources that are to be multiplexed will have different bit rates. In practice, the bit rate variation poses the most serious design problem and leads to the three categories of multiplexers, as under:
(i)         Synchronous multiplexers
(ii)        Asynchronous multiplexers
(iii)       Quasi-synchronous multiplexers.
■          The bandwidth efficiency of a multiplexing system is defined as,
Bandwidth Efficiency =
or    Bandwidth Efficiency  =
Thus bandwidth efficiency of a digital multiplexing system is only 11.77% which is extremely poor as compared to a much higher bandwidth efficiency of an analog multiplexing system (typically above 85%).
■          Advantages of Digital Multiplexing
(i)         Hardware cost reduction due to the use of digital ICs.
(ii)        Powder cost reduction due to use of regenerative repeaters.
(iii)       More flexibility as compared to the analog multiplexers.
■          DS0, DS1, DS2, … etc. are the names of the services. The telephone companies use the T lines (T — 0, T — 2) … etc.) to implement these services.
The T lines have capacities which precisely match with the bit rates of the corresponding services.
■          When a large number of PCM signals are to be transmitted over a common channel, multiplexing of these PCM signals is required.
■          When the PCM-TDM system is being used for the telephony, it is expected to transmit certain signaling and supervisory signals alongwith the speech information. The signaling information consists of the signals such as a call is being initiated or a call is being terminated, or the address of calling party etc.
SHORT QUESTIONS WITH ANSWERS
Q.1.     What do you mean by Multiplexing?
Ans.    Multiplexing may be defined as a technique which allows many users to share a common communication channel simultaneously. There are two major types of multiplexing techniques. They are as under:
(i)         Frequency division multiplexing (FDM),
(ii)        Time division multiplexing (TDM).
Q.2.     Explain Frequency Division Multiplexing (FDM).
Ans.    This technique permits a fixed frequency band to every user in the complete channel bandwidth. Such frequency slot is allotted continuously to that user. As an example consider that the channel bandwidth is 1 MHz. Let there be ten users, each requiring upto 100 kHz bandwidth. Then the complete channel bandwidth of 1 MHz can be divided into ten frequency bands, i.e. each of 100 kHz and every user can be allotted one independent frequency band. This technique is known as Frequency Division Multiplexing (FDM).
Q.3.     What is Transmission Bandwidth of a PAM/TDM Channel?
Ans.    The minimum transmission bandwidth of a PAM-TDM channel is given by
BW =  (signaling rate)
Therefore, minimum transmission bandwidth BW >  x 2N f_m
Hence, minimum transmission bandwidth BW = Nf_m.
Q.4.     What is Crosstalk in PAM/TDM System?
Ans.    Crosstalk basically means interference between the adjacent TDM channels. Infact, it is the unwanted coupling of information from one channel to the other. The guard time `T_g‘ is the time spacing introduced between the adjacent TDM channels.
Q.5.     Write the Advantages of TDM.
Ans.    (i)         Full available channel bandwidth can be utilized for each channel.
(ii)        Intermodulation distortion is absent.
(iii)       TDM circuitry is not very complex.
(iv)       The problem of crosstalk is not severe.
Q.6.     Write the Disadvantages of TDM.
Ans.    (i)         Synchronization is essential for proper operation.
(ii)        Due to slow narrowband fading, all the TDM channels may get wiped out.
Q.7.     Explain the Principle of Digital Multiplexing.
Ans.    Digital multiplexing is based on the principle of “interleaving symbols” from two or more digital signals. This is similar to TDM but it does not require the periodic sampling and waveform preservation. The signals which are to be multiplexed may come from digital data sources or from analog sources that have been digitally coded.
A digital multiplexer is used to merge the input bits from different sources to form one signal from transmission via a digital communication system. The multiplexed signal consists of source digits interleaved bit by bit or word by word. The important functions that must be performed by a multiplexer are as under:
(i)         To establish a frame. A frame consists of at least one bit from every input.
(ii)        A number of unique bits slots within the frame should be assigned to each input.
(iii)       To insert control bits for frame identification and synchronization.
(iv)       To make allowance for any variations of the input bit rates.
Q.8.     How can you classify Digital Multiplexers?
Ans.    The various digital sources that are to be multiplexed will base different bit rates. In practice, the hit rate variation poses the most serious design problem and leads to the three categories of multiplexers, as under:
(i)         Synchronous multiplexers
(ii)        Asynchronous multiplexers
(iii)       Quasi-synchronous multiplexers.
Q.9.     Write the Advantages of Digital Multiplexing.
Ans.    (i)         Hardware cost reduction due to the use of digital ICs.
(ii)        Powder cost reduction due to use of regenerative repeaters.
(iii)       More flexibility as compared to the analog multiplexers.
Q.10.   What is Channel Associated Signaling? Explain.
Ans.    When the PCM-TDM system is being used for the telephony, it is expected to transmit certain signaling and supervisory signals alongwith the speech information. The signaling information consists of the signals such as a call is being initiated or a call is being terminated, or the address of calling Party etc. In analog system such a signaling information is transmitted over a separate channel other than the voice channel. But in the 1’1 carrier system which is a digital system, a separate channel is not used. In this system, the signaling information is sent using the same data bit slots which are used to send the voice information. The technique used is “bit slot sharing”. In the “bit slot sharing” method, for the first five frames, all the 24 channels are encoded into an 8 bit digital code. However in the sixth frame, all the channels are coded into a 7 hit code and the LSB (least significant bit) of each channel is used to transmit the signaling information. This is called as “channel associated signaling”. This pattern is repeated after every six frames.
QUESTIONS

1. What do you mean by multiplexing? Explain TDM and FDM.
2. Explain a PAM/TDM system in detail with a block diagram and write briefly about the following:

(i)         frame
(ii)        signaling rate
(iii)       transmission BW
(iv)       synchronization
(v)        crosstalk
(vi)       guard time

3. Explain the principle of digital multiplexing. What are the types of digital multiplexers?
4. Explain multiplexing hierarchy for digital communication.

OBJECTIVE TYPE QUESTIONS
Multiple Choice Questions

1. Which multiplexing technique transmits analog signal

(a)        FDM                                       (b)        TDM
(c)        WDM                          (d)       (a) and (h)

2. In TDM, the transmission rate of the multiplexed path is usually — the sum of the transmission rates of the signal sources.

(a)        greater than                             (b)        less than
(c)        equal to                                   (d)       1 less than

3. Guarud bands increase the bandwidth for

(a)        FDM                                       (b)        TDM
(c)        both (a) and (b)
(d)       none of the above

4. In T1 system the frame synchronization code repeats every

(a)        125 s                                     (b)        1.5 ms
(c)        1.2 ms                          (d)       150 s

5. A TDM link has 20 signal channels and each channel is sampled 8000 times/sec. Each sample is represented by 7 binary bits and contains an additional bit for synchronization. The total bit rate for the TDM link is

(a)        180 kbps                                  (b)        1280 kbps
(c)        1180 Mbps                              (d)       1880 Mbps
Answer

1. (a)                    2.         (a)                    3.         (c)                    4.         (a)
2. (b)