Multiplexing and multiplexing systems

 Explain the need for multiplexing

 Distinguish between multiplexing techniques
 Explain the key features of FDM and TDM
 Distinguish between synchronous and asynchronous TDM
 Understanding the PDH system
 When the bandwidth of a medium is greater than individual
signals to be transmitted through the channel, a medium can
be shared by more than one channel of signals. The process
of making the most effective use of the available channel
capacity is called Multiplexing
 utilize the full capacity of a data link
 when many nodes compete to access the network, some
efficient techniques for utilizing the data link are very
essential
 a technique that allows simultaneous transmission of multiple
signals across a single data link.
 Frequency-division multiplexing (FDM):
• It is most popular and is used extensively in radio and TV
transmission. Here the frequency spectrum is divided into several
logical channels, giving each user exclusive possession of a
particular frequency band.
 Time-division Multiplexing (TDM):
• It is also called synchronous TDM, which is commonly used for
multiplexing digitized voice stream. The users take turns using
the entire channel for short burst of time.
 Statistical TDM:
• This is also called asynchronous TDM, which simply improves on
the efficiency of synchronous TDM.
• available bandwidth of a single physical medium is subdivided
into several independent frequency channels.
• Independent message signals are translated into different
frequency bands using modulation techniques, which are
combined by a linear summing circuit in the multiplexer, to a
composite signal.
• The resulting signal is then transmitted along the single channel
by electromagnetic means
• The carriers used to modulate the individual message signals are
called sub-carriers
 At the receiving end the signal is applied to a bank of band-
pass filters, which separates individual frequency channels.
The band pass filter outputs are then demodulated and
distributed to different output channels
 If the channels are very close to one other, it leads to inter-
channel cross talk. Channels must be separated by strips of
unused bandwidth to prevent inter-channel cross talk. These
unused channels between each successive channel are
known as guard bands
 all signals operate with same frequency at different times

 This is a base band transmission system, where an electronic

commutator sequentially samples all data source and combines
them to form a composite base band signal, which travels
through the media and is being demultiplexed into appropriate
independent message signals by the corresponding commutator
at the receiving end.
 The incoming data from each source are briefly buffered. Each
buffer is typically one bit or one character in length. The buffers
are scanned sequentially to form a composite data stream. The
scan operation is sufficiently rapid so that each buffer is emptied
before more data can arrive. Composite data rate must be at
least equal to the sum of the individual data rates. The composite
signal can be transmitted directly or through a modem.
• the composite signal has some dead space between the successive
sampled pulses, which is essential to prevent interchannel cross talks.
• Along with the sampled pulses, one synchronizing pulse is sent in
each cycle. These data pulses along with the control information form
a frame.
• Each of these frames contain a cycle of time slots and in each frame,
one or more slots are dedicated to each data source. The maximum
bandwidth (data rate) of a TDM system should be at least equal to the
same data rate of the sources.
• Synchronous TDM is called synchronous mainly because each time
slot is preassigned to a fixed source. The time slots are transmitted
irrespective of whether the sources have any data to send or not.
Hence, for the sake of simplicity of implementation, channel capacity
is wasted. Although fixed assignment is used TDM, devices can
handle sources of different data rates. This is done by assigning fewer
slots per cycle to the slower input devices than the faster devices
 also known as asynchronous TDM or Intelligent TDM. It
dynamically allocates the time slots on demand to separate
input channels, thus saving the channel capacity.
 In case of statistical TDM, the data in each slot must have an
address part, which identifies the source of data. Since data
arrive from and are distributed to I/O lines unpredictably,
address information is required to assure proper delivery
 digital signals can be classified from the point of view of
their generation and the relation between their clock signals:
• Isochronous signal
• Anisochronous signals
• Homochronous signals
• Heterochronous signals
Isochronous signal

 the time interval between two significant moments is

theoretically equal with a unitary time interval or with a
multiple of this
Anisochronous signals

 the time interval separating two significant moments it is not

necessarily related to a unitary interval or to a multiple of
this;

 the symbols of a non-isochronous binary signal do not have

the same duration.
Homochronous signals

 isochronous signals with the same rate and constant phase

relation, can be divided in:

• Mesochronous signals –isochronous signals with the same rate

and non-constant phase relation –constant average phase
relation;
• Synchronous signals –isochronous signals with the same rate and
constant phase relation.
HETEROCHRONOUS SIGNALS
 isochronous signals with different rates and variable phase
relation;

 Plesiochronous signals –signals with the same nominal rate,

all the variations of this rate being maintained between
specified limits;
• for ex. signals with identical nominal rates from different sources.
MULTIPLEXING OF
PLESIOCHRONOUS DIGITAL SIGNALS
 Can be realized in two possible ways:

• generation of signals with high stability of the clock frequency

and use of some buffers;

• very high price and periodical loss of information;

• use of the justification (stuffing) method;

• without information loss;

BLOCK SCHEMATIC OF PDH
MULTIPLEXING –DEMULTIPLEXING
EQUIPMENTS
 Principle of the rate matching between the tributary and
multiplexer based on positive justification;
• The plesiochronous binary signal is written in the elastic memory
with a specific clock frequency, fi;
• The reading of the memory and the transmission of the signal in
the channel is realized with a higher clock frequency fo>fi;
• appears a clean out tendency of the elastic memory content;
• it is detected by using a phase comparator (compares fo and fi);
 When a phase difference threshold value is exceeded
(between signals fo and fi), the phase comparator generates
a blocking commands of the reading impulse;
• it is created a break in the line signal (one stuffing impulse is
inserted) which decreases the phase difference between the
clock signals;
• the stuffing impulse has no information.
 The justification (stuffing) is signaled to the reception side on
a link multiplexed with the data signal;
• the signaling of the justification (stuffing) is necessary to inform
the receiver about the exact moment and location of the
justification (stuffing);
• this information is necessary for suppression of the justification bits in
the receiver;
 Only the information bits are written in the memory at the
reception side with a frequency fo, the memory being read
with a frequency fi;
 The extraction of the justification (stuffing) impulses
generates a jitter in the output signal;
 Use of the elastic buffer for rate adaptation:
 Justification signaling
• Individual insertion;
• insertion of signaling justification bits is realized before multiplexing;
• complex method at transmission; flexible and low complexity method
at reception;
• Common insertion;
• the signaling information from all tributary signals are concentrated on
a common path which is then multiplexed with data;
• lower complexity at transmission, but higher complexity at reception.
 Individual insertion of the justification signaling;
• Signal diagram:
 MUX and DEMUX block schematics:
COMMON INSERTION OF THE
JUSTIFICATION SIGNALING.
 Signal diagram:
 MUX and DEMUX block schematics:
 The information related to the signaling commands is very
important for the functioning of the multiplexing equipments;
• if this information is erroneous other bits than the justification bits
will be extracted from the received signal;
• this will lead to loss of synchronization;
• redundant coding of the signaling information and error
correction of the signaling bits is used;
• repetition codes are used usually (these bits are transmitted several
times and the correct bits are decided based on a majority logic);
• for ex. justification signaling: c1c2c3=1 1 1; absence of justification:
c1c2c3= 0 0 0;
• c1c2c3 -justification signaling bits for one tributary/source.
COMPUTATION OF THE
JUSTIFICATION SIGNALING
 N0 is the total number of symbols of a transmission frame;
 Ns is the number of synchronization symbols;
 n0 is the number of information symbols;
 η is the frame efficiency.
 fsn is the nominal frequency of the locally generated clock;
 fpn is the nominal value of the tributary signal rate;
 the nominal frequency of the writing clock;
 fsn’ is the nominal reading frequency of the elastic buffer;
 fd is the mean justification frequency;
 fdmax is the maximum justification frequency;
 obtained when the reading frequency attains the maximum permitted
limit, and the writing frequency the minimum permitted limit.
 The structure of the secondary PDH frame

 The structure of the tertiary PDH frame

 The structure of the quaternary PDH frame;
 limited management and reconfiguration capabilities;
 low flexibility;
• designed only for circuit switching (voice transmission);
• it is relatively difficult to use this system for other services (for ex.
packet data);
 the insertion and extraction of a basic data stream requires
the demultiplexig and re-multiplexing of the entire multiplex
signal;
 Ex.:insertion/ extraction of a 2Mbps stream into / from a
multiplex signal having the bit rate 140 Mbps;
 In transmission systems with time division multiplexing it is
necessary;
 Identification at reception of the multiplexing order of the involved
tributaries;
 Identification of the first bit of the frame;

 In multiplexed digital signal it is inserted a special code group

named synchronization group;
 Relatively to this sequence is defined the order of the multiplexed
tributaries;
 The cyclic or group synchronization process;
 It achieves the alignment between the transmission and reception
side of a digital transmission system;
 It is maintained and restored the alignment, in case of losing this;
 in some situations could be necessary the use of two levels of
synchronization, namely: frame and word synchronization (characteristic for
the primary multiplex).
 Conditions imposed to the synchronization sequence:
 To reduce as much as possible the simulations (of this sequence) by the
transmitted data;
 The recognition (detection) probability of these sequences must be high in the
presence of bit errors;
 Methods for insertion of the synchronization sequence:
 Distributed allocation;
 is proper for channels with high level of bit errors (especially packet errors);
 the synchronization is reestablished faster in the presence of packet errors;
 the complexity of the method is higher;
 for low error probability the synchronization time is larger.

 Grouped allocation;
 is more sensitive to bit errors –especially to packet errors;
 the complexity of the method is lower;
 the synchronization time is lower for low error probability.
  The choice of a given method depends on:
 the technological complexity;
 error performances;
 the synchronization time.

 Synchronization group insertion methods:

 a) distributed insertion; b) grouped insertion.
THE SYNCHRONIZATION
EQUIPMENT
 The synchronization devices must fulfill the following
requirements:
 synchronization time at the connection establishment and after
the loss of synchronization as small as possible;
 minimum synchronization information in a frame in the condition
of an acceptable synchronization search time;
 the detection probability of the synchronization signal must be
high in the situation of bit errors;
 the time between two losses of the synchronization must be as large as
possible;
 the synchronization equipment must be as simple and reliable as
possible;
 The synchronization device has the following functions:
 establishment of the synchronization at the beginning of the
transmission;
 control of the synchronism state during the transmission;
 identification of the states when the synchronization is missing;
 reestablishment of the synchronization after the loss of this.
 Block schematic of a cyclic synchronization circuit;
 Positioning of the synchronization device inside the receiver;