Switching and signaling ovt, Winter training .bsnl .awesome knowledge ,telecommunication, networking ,switching mechanism

1. Welcome to the Presentation of
Overview of
Telecom Switching Network
RTTC, Kalyani

2. Agenda
1. PSTN Architecture
2. Switching Network in 2nd Generation Land
Line Switch
3. Signaling
a) Subscriber Line Signaling
b) Inter-Exchange Signaling
i) CAS
ii) CCS#7

3. 2nd Generation Exchange
LME

4. 2nd Generation LL Exchange
Switch
MDF
Pill
ar DP
Box
Pill
ar
DP
Box

5. LL Exchange Connectivity
Exch.
A
Pillar
DP
Box
Pillar
DP
Box
Exch.
B

6. LL Exchange Connectivity with TAX
Local
Ex. A
Local
Ex. B
L2 TAX
L1 TAX
L2 TAX
Dumdum Jadavpur
Cossipur Kalighat
TB, BBD Bag

7. LL Exchange Connectivity with different TAX
TAX
Kolkata
LME,
CHN
LME,
KOL
TAX
Chennai

8. 2nd Generation PSTN
T
A
X
LME
SDH
NW
DWDM N/W
T
A
X
LME LME LME
SDH
NW

9. Local Main Exchange
DLU
DLU
SN
CP & CCNC
LTG
PCM Link Other
Exchange
LTG
LTG
PCM Link
PCM Link
PCM Link

10. DIGITAL SWITCHING
• A Digital switching system, in general, is one in which signals
are switched in digital form. These signals may represent
speech or data. The digital signals of several speech samples
are time multiplexed on a common media before being
switched through the system.
• To connect any two subscribers, it is necessary to interconnect
the time-slots of the two speech samples which may be on
same or different PCM highways. The digitalised speech
samples are switched in two modes, viz., Time Switching and
Space Switching. This Time Division Multiplex Digital Switching
System is popularly known as Digital Switching System.

11. Digital Switching
For example, PCM samples appearing in TS6 of I/C PCM
HWY1 are transferred to TS18 of O/G PCM HWY2, via the
digital switch.

12. Time & Space Switching
The interconnection of time-slots, i.e., switching of digital
signals can be achieved using two different modes of
operation. These modes are: -
i. Space Switching
ii. Time switching
Usually, a combination of both the modes is used.

13. Cont.
In the space-switching mode, corresponding time-slots of I/C and O/G
PCM highway are interconnected. A sample, in a given time-slot, TSi of
an I/C HWY, say HWY1, is switched to same time-slot, TSi of an O/G HWY,
say HW Y2. Obviously there is no delay in switching of the sample from
one highway to another highway since the sample transfer takes place in
the same time-slot of the PCM frame.
Time Switching, on the other hand, involves the interconnection of
different time-slots on the incoming and outgoing highways by re-
assigning the channel sequence. For example, a time-slot TSx of an I/C
Highway can be connected to a different time-slot., TSy, of the outgoing
highway. In other words, a time switch is, basically, a time-slot changer.

14. Digital Space Switching
Principle
The Digital Space Switch consists of several input highways, X1,
X2,...Xn and several output highways, Y1, Y2,.............Ym, inter
connected by a crosspoint matrix of n rows and m columns.
The individual crosspoint consists of electronic AND gates.
The operation of an appropriate crosspoint connects any
channel ‘a’ of I/C PCM highway to the same channel ‘a’ of O/G
PCM highway during each appropriate time-slot which occurs
once per frame as shown in the following figure. During other
time-slots, the same crosspoint may be used to connect other
channels. This crosspoint matrix works as a normal space
divided matrix with full availability between incoming and
outgoing highways during each time-slot.

15. Cont.
Each crosspoint column, associated with one
O/G highway, is assigned a column of control memory.
The control memory has as many words as there
are time-slot per frame in the PCM signal. In practice, this
number could range from 32 to 1024.
Each crosspoint in the column is assigned a
binary address, so that only one crosspoint per column is
closed during each time-slot.
The binary addresses are stored in the control
memory, in the order of time-slots.

16. Space Switch

17. Cont.
A new word is read from the control memory during each time-slot, in a
cyclic order. Each word is read during its corresponding time-slot,
i.e.,Word 0 (corresponding to TS0), followed by word 1 (corresponding to
TS1) and so on. Thus, the cross point corresponding to the address, is
operated during a particular time-slot. This cross point operates every
time the particular time-slot appears at the inlet in successive frames.
normally, a call may last for around a million frames.
As the next time-slot follows, the control memory is also advanced by one
step, so that during each new time-slot new corresponding words are
read from the various control memory columns. This results in operation
of a completely different set of cross points being activated in different
columns.

18. Illustration
• Consider the transfer of a sample arriving in
TS7 of I/C HWY X1 to O/G HWY Y3. Since this
is a space switch, there will be no reordering
of time i.e., the sample will be transferred
without any time delay, via the appropriate
cross point. In other words, the objective is to
connect TS7 of HWY X1 and TS7 of HWY Y3.
• The central control (CC) selects the
control memory column corresponding
output highway Y3. In this column, the
memory location corresponding to the TS7 is
chosen. The address of the cross point is
written in this location, i.e., 1, in binary, is
written in location 7.This cross point remains
operated for the duration of the time-slot
TS7, in each successive frame till the call
lasts.
• For disconnection of call, the CC erases the
contents of the control memory locations,
corresponding to the concerned time-slots.
The AND gates, therefore, are disabled and
transfer of samples is halted.

19. Practical Space Switch
• In a practical switch, the digital bits are transmitted in parallel rather than serially,
through the switching matrix.
• In a serial 32 time-slots PCM multiplex, 2048 Kb/s are carried on a single wire
sequentially, i.e., all the bits of the various time-slots follow one another. This single
wire stream of bits, when fed to Serial to Parallel Converter is converted into 8-wire
parallel output.
• This parallel output on the eight wires is fed to the switching matrix. It can be seen that
during one full time-slot period, only one bit is carried on the each output line, whereas
8 bits are carried on the input line during this period. Therefore, bit rate on individual
output wires, is reduced to 1/8th of input bit rate=2048/8=256Kb/s .
• Due to reduced bit rate in parallel mode, the cross point is required to be operated only
for 1/8th of the time required for serial working. It can, thus, be shared by eight times
more channels, i.e., 32 x 8 = 256 channels, in the same frame.
• However, since the eight bits of one TS are carried on eight wires, each cross point have
eight switches to interconnect eight input wires to eight output wires. Each cross point
(all the eight switches) will remain operated now for the duration of one bit only, i.e.,
only for 488 ns (1/8th of the TS period of 3.9 µs) .

20. Serial parallel converter

21. • For example, to connect 40 PCM I/C highways, a
matrix of 40x 40 = 1600 cross points each having a
single switch, is required in serial mode working.
• Whereas in parallel mode working, a matrix of (40/8
x 40/8) = 25 cross point is sufficient. As eight
switches are required at each cross point 25 x 8 =
200 switches only are required.
• Thus, there is a reduction of the matrix by 1/8th in
parallel mode working, hence reduction in size and
cost of the switching matrix.

22. Digital Time Switch : Principle
A Digital Time Switch consists of two memories, viz., a speech or buffer
memory to store the samples till destination time-slots arrive, and a
control or connection or address memory to control the writing and
reading of the samples in the buffer memory and directing them on
to the appropriate time-slots.
• Speech memory has as many storage locations as the number of
time-slots in input PCM, e.g., 32 locations for 32 channel PCM
system.
• The writing/reading operations in the speech memory are
controlled by the Control Memory. It has same number of memory
locations as for speech memory, i.e., 32 locations for 32 channel
PCM system.
• A Time-Slot Counter which usually is a synchronous binary
counter, is used to count the time-slots from 0 to 31, as they occur.
At the end of each frame, It gets reset and the counting starts again.
It is used to control the timing for writing/reading of the samples in
the speech memory.

23. Time Switch
Time switch can operate in two modes, viz.,
I. Output associated control
ii. Input associated control

24. Output associated control
In this mode of working , the samples of I/C PCM are written
cyclically in the speech memory locations in the order of
time-slots of I/C PCM, i.e., TS1 is written in location 1, TS2 is
written in location 2, and so on.
The contents of speech memory are read on output PCM in
the order specified by control memory. Each location of
control memory is rigidly associated with the corresponding
time-slot of the O/G PCM and contains the address of the TS
of incoming PCM to be connected to.

25. Illustration
• Consider the objective that TS4 of incoming PCM is to be
connected to TS6 of outgoing PCM. In other words, the
sample arriving in TS4 on the I/C PCM has to be delayed
by 6 - 4 = 2 time-slots, till the destination time-slot, viz.,
TS6 appears in the O/G PCM. The required delay is given
to the samples by storing it in the speech memory. The
I/C PCM samples are written cyclically i.e. sequentially
time-slot wise in the speech memory locations. Thus, the
sample in TS4 will be written in location 4.
• The reading of the sample is controlled by the
Control Memory. The Control Memory location
corresponding to output time-slot TS6, is 6. In this
location, the CC writes the input time-slot number, viz.,4,
in binary. These contents give the read address for the
speech memory, i.e., it indicates the speech memory
locations from which the sample is to be read out, during
read cycle.
• When the time-slot TS6 arrives, the control memory
location 6 is read. Its content addresses the location 4 of
the speech memory in the read mode and sample is read
on to the O/G PCM.
• In every frame, whenever time-slot 4 comes a new
sample will be written in location 4. This will be read
when TS6 occurs. This process is repeated till the call
lasts.

26. It may be noticed that the writing in the speech memory is
sequential and independent of the control memory, while
reading is controlled by the control memory, i.e., there is a
sequential writing but controlled reading.

27. Input associated control
Here, the samples of I/C PCM are written in a controlled way,
i.e., in the order specified by control memory, and read
sequentially.
Each location of control memory is rigidly associated with
the corresponding TS of I/C PCM and contains the address of
TS of O/G PCM to be connected to.

28. Input associated controlled Time Switch
• The previous example with the same
connection objective of connecting TS4 of I/C
PCM to TS6 of O/G PCM may be considered
for its restoration. The location 4 of the
control memory is associated with incoming
PCM TS4. Hence, it should contain the address
of the location where the contents of TS4 of
I/C PCM are to be written in speech memory.
• A CC writes the number of the destination TS,
viz., 6 in this case, in location 4 of the control
memory. The contents of TS4 are therefore,
written in location of speech memory.
• The contents of speech memory are read in
the O/G PCM in a sequential way, i.e., location
1 is read during TS1, location 2 is read during
TS2, and so on. In this case, the contents of
location 6 will appear in the output PCM at
TS6. Thus the input PCM TS4 is switched to
output PCM TS6. In this switch, there is
sequential reading but controlled writing.

29. Time Delay Switching
The writing and reading, of all time-slots in a frame, has to
be completed within one frame time period (before the
start of the next frame). A TS of incoming PCM may,
therefore, get delayed by a time period ranging from 1 TS to
31 TS periods, before being transmitted on outgoing PCM.
For example, consider a case when TS6 of incoming PCM is
to be switched to TS5 in outgoing PCM. In this case switching
can be completed in two consecutive frames only, i.e., 121
microseconds for a 32 channel PCM system. However, this
delay is imperceptable to human beings.

30. Non-Blocking feature of a Time
Switch
In a Time Switch, there are as many memory locations in the control
and speech memories as there are time-slots in the incoming and
outgoing PCM highways, i.e., corresponding to each time-slot in
incoming highway, there is a definite memory location available in
the speech and control memories.
Similarly, corresponding to each time-slot in the outgoing highway
there is a definite memory location available in the control and
speech memories.
This way, corresponding to free incoming and outgoing time-slots,
there is always a free path available to interconnect them.
In other words, there is no blocking in a time switch.

31. Two Dimensional Switching
It becomes necessary to employ a number of stages, using small switches as
building blocks to build a large network. This would result in necessity of
changing both the time-slot and highway in such a network.
Hence, the network, usually, employs both types of switches viz., space switch
and time switch, and. therefore, is known as two dimensional network.
A two-stage two-dimensional network, TS or ST, is basically suitable for very
low capacity networks only. The most commonly used architecture has
three stages, viz., STS or TST. However, in certain cases, their derivatives,
viz., TSST, TSSST, etc., may also be used.
In a TST network, there are two time stages separated by a space stage.
The former carry out the function of time-slot changing, whereas the latter
performs highway jumping.

32. Switching Network Configuration of some
Modern Switches
• E10B - T-S-T
• EWSD - T-S-S-S-T
• AXE10 - T-S-T
• CDOT(MBM) - T-S-T
• 5ESS - T-S-T
• OCB 283 - T

33. Switching
Any query, please ….

34. SIGNALLING IN TELECOMMUNICATION:
CAS and CCS#7

35. Introduction
A telecommunication network establishes and realizes
temporary connections, in accordance with the instructions
and information received from subscriber lines and inter-
exchange trunks, in form of various signals. Therefore, it is
necessary to interchange information between an exchange
and its external environment i.e. between subscriber lines
and exchange, and between different exchanges. Though
these signals may differ widely in their implementation they
are collectively known as telephone signals.

36. A signalling system uses a language which enables two
switching equipments to converse for the purpose of setting
up calls. Like any other language. it possesses a vocabulary of
varying size and varying precision, i.e. a list of signals which
may also vary in size and a syntax in the form of a complex
set of rules governing the assembly of these signals.
This handout discusses the growth of signalling and various
type of signalling codes used in Indian Telecommunication.

37. Basic Phases of a Call
i. A request for originating a call is initiated when the calling
subscriber lifts the handset.
ii. The exchange sends dial-tone to the calling subscriber to
indicate to him to start dialing.
iii. The called number is transmitted to the exchange, when
the calling subscriber dials the number.
iv. If the number is free, the exchange sends ringing current
to him.
v. Feed-back is provided to the calling subscriber by the
exchange by sending …

38. Basic Phases of a Call
a) Ring-back tone, if the called subscriber is free
b) Busy tone if the called subscriber is busy
or
c)Recorded message, if provision exists, for non completion
of call due to some other constraint
vi. The called subscriber indicates acceptance of the
incoming call by lifting the handset
vii. The exchange recognizing the acceptance terminates the
ringing current and the ring-back tone, and establishes a
connection between the calling and called subscribers.

39. Basic Phases of a Call
viii. The connection is released when either subscriber
replaces the handset. When the called subscriber is in a
different exchange, the following inter-exchange trunk
signal functions are also involved, before the call can be
set up.
ix The originating exchange seizes an idle inter exchange
trunk, connected to a digit register at the terminating
exchange.
x. The originating exchange sends the digit. The steps iv
to viii are then performed to set up the call.

40. Subscriber Line signalling
Types:
Calling Subscriber Line Signaling :
In automatic exchanges the power is fed over the subscriber’s loop by
the centralized battery at the exchange. Normally, it is 48 V. The power
is fed irrespective of the state of the subscriber, viz., idle, busy or
talking.
i) Call request
When the subscriber is idle, the line impedance is high. The line
impedance falls, as soon as, the subscriber lifts the hand-set, resulting
in increase of line current. This is detected as a new call signal and the
exchange after connecting an appropriate equipment to receive the
address information sends back dial-tone signal to the subscriber.

41. ii) Address signal
After the receipt of the dial tone signal, the subscriber
proceeds to send the address digits. The digits may be
transmitted either by decade dialing or by multifrequency
pushbutton dialling.
iii) End of selection signal
The address receiver is disconnected after the receipt of
complete address. After that connection is established or if
the attempt has failed the exchange sends any one of the
following signals.

42. 1. Ring-back tone to the calling subscriber and
ringing current to the called subscriber, if the
called line is free.
2. Busy-tone to the calling subscriber, if the
called line is busy or otherwise inaccessible.
3. Recorded announcement to the calling
subscriber, if the provision exists, to indicate
reasons for call failure, other than called line
busy.

43. Ring back tone and ringing current are always transmitted
from the called subscriber local exchange and busy tone and
recorded announcements, if any, by the equipment as close
to the calling subscriber as possible to avoid unnecessary
busying of equipment and trunks.
iv) Answer Back Signal
As soon as the called subscriber lifts the handset, after
ringing, a battery reversal signal is transmitted on the line of
the calling subscriber. This may be used to operate special
equipment attached to the calling subscriber, e.g., short-
circuiting the transmitter of a CCB, till a proper coin is
inserted in the coin-slot.

44. v) Release signal
When the calling subscriber releases i.e., goes on hook, the
line impedance goes high. The exchange recognizing this
signal, releases all equipment involved in the call. This signal
is normally of more than 500 milliseconds duration.
vi) Permanent Line (PG) Signal
Permanent line or permanent glow (PG) signal is sent to the
calling subscriber if he fails to release the call even after the
called subscriber has gone on-hook and the call is released
after a time delay. The PG signal may also be sent, in case
the subscriber takes too long to dial. It is normally busy tone.

45. Called subscriber line signals
i) Ring Signal
On receipt of a call to the subscriber whose line is free, the
terminating exchange sends the ringing current to the called
telephone. This is typically 25 or 50Hz with suitable
interruptions. Ring-back tone is also fed back to the calling
subscriber by the terminating exchange.
ii) Answer Signal
When the called subscriber, lifts the hand-set on receipt of
ring, the line impedance goes low. This is detected by the
exchange which cuts off the ringing current and ring-back
tone.

46. iii) Release Signal
If after the speech phase, the called subscriber goes on hook
before the calling subscriber, the state of line impedance
going high from a low value, is detected. The exchange sends
a permanent line signal to the calling subscriber and releases
the call after a time delay, if the calling subscriber fails to
clear in the meantime.

47. iv) Register Recall Signal
With the use of DTMF telephones, it is possible to enhance
the services, e.g., by dialing another number while holding
on to the call in progress, to set up a call to a third
subscriber. The signal to recall the dialling phase during the
talking phase, is called Register Recall Signal. It consists of
interruption of the calling subscriber’s loop for duration less
than the release signal. it may be of 200 to 320 milliseconds
duration.

48. Inter-exchange Signaling
Inter exchange signalling can be transmitted over a channel
directly associated with the speech channel, channel-
associated signalling (CAS) , or over a dedicated link
common to a number of channels, common channel
signalling (CCS). The information transmitted for setting up
and release of calls is same in both the cases.
Channel associated signalling requires the exchanges, to
have access to each trunk via the equipment which may be
decentralised, whereas, in common channel signalling, the
exchange is connected to only a limited number of signalling
links through a special terminal.

49. Channel- Associated Signalling
In the PCM systems the signalling information is conveyed on
a separate channel which is rigidly associated with the
speech channel. Hence, this method is known as channel
associated signalling (CAS). Though the speech sampling rate
is 8 Khz, the signals do not change as rapidly as speech and
hence, a lower sampling rate of 500 Hz, for digitisation of
signals can suffice. Based on this concept, TS 16 of each
frame of 125 microseconds is used to carry signals of 2
speech channels, each using 4 bits.
Hence, for a 30 channel PCM system, 15 frames are required
to carry all the signals. To constitute a 2 millisecond

50. multiframe of 16 frames. F 0 to F 15. TS 16 of the frame F 0 is used for
multiframe synchronisation. TS 16 of F1 contains signal for speech
channels 1 and 17 being carried in TS 1 and TS 17, respectively, TS16
of F2 contains signals of speech channels 2 and 18 being carried in TS2
and TS 18, respectively and so on, Both line signals and address
information can be conveyed by this method.
Although four bits per channel are available for signalling only two bits
are used.
However, the utilisation of such a dedicated channel for signalling for each
speech channel is highly inefficient as it remains idle during the speech
phase. Hence, another form of signalling known as common-channel
signalling evolved.

51. COMMON CHANNEL SIGNALING SYSTEM No. 7 (CCS#7)
The CCITT has, therefore, specified the common channel
signalling system no.7 (CCS-7). CCS-7 is optimised for
application in digital networks. It is characterised by the
following main features :
i) Internationally standardized (national variations possible).
suitable for the national, international and intercontinental
network level.
ii) Suitable for various communication services such as
telephony, text services, data services digital network (ISDN).
iii) High performance and flexibility along with a future-
oriented concept which well meet new requirements.

52. iv) High reliability for message transfer.
v) Signalling on separate signalling links; the bit rate of the
circuits is, therefore, exclusively for communication.
vi) Signalling links always available, even during existing calls.
vii) Use of the signalling links for transferring user data also.
viii) Automatic supervision and control of the signalling
network.

53. CCS#7 Signalling terminology
In CCS7 the signalling messages are sent via separate
signalling links (See Figure bellow). One signalling link can
convey the signalling messages for many circuits.
The CCS7 signalling links connect signalling points (SPs) in a
communication network. The signalling points and the
signalling links form an independent signalling network
which is overlaid over the circuit network.

54. Signalling Link between Exchanges

55. Signalling Points (SP)
A distinction is made between signalling points (SP) and
signalling transfer points (STP).
The SPs are the sources (originating points) and the sinks
(destination points) of signalling traffic. In a communication
network these are primarily the exchanges. The STPs switch
signalling messages received to another STP or to a SP on the
basis of the destination address. No call processing of the
signalling messages occurs in a STP. A STP can be integrated
in a SP (e.g. in an exchange) or can form a node of its own in
the signalling network. One or more levels of STPs are
possible in a signalling network, according to the size of the
network.

56. All SPs in the signalling network are identified by means of a
code within the framework of a corresponding numbering
plan and, therefore, can be directly addressed in a signalling
message.
Signalling links
A signalling link consists of a signalling data link (two data
channels operating together in opposite directions at the
same date rate) and its transfer control functions. A channel
of an existing transmission link (e.g. a PCM30 link) is used as
the signalling data link. Generally, more than one signalling
link exists between two SPs in order to provide redundancy.

57. In the case of failure of a signalling link, functions of the
CCS7 ensure that the signalling traffic is rerouted to fault-
free alternative routes. The routing of the signalling links
between two SPs can differ. All the signalling links between
two SPs are combined in a signalling link set.
Signalling Routes
The route defined for the signalling between an originating
point and a destination point is called the signalling route.
The signalling traffic between two SPs can be distributed
over several different signalling routes. All signalling routes
between two SPs are combined in a signalling route set.

58. Signalling Modes
Two different signalling modes can be used in the signalling
networks for CCS7, viz. associated mode and quasi-
associated mode.
In the associated mode of signalling, the signalling link is
routed together with the circuit group belonging to the link.
In other words, the signalling link is directly connected to SPs
which are also the terminal points of the circuit group . This
mode of signalling is recommended when the capacity of the
traffic relation between the SPs A and B is heavily utilized.

59. Associated Mode of Signaling

60. In the quasi-associated mode of signalling, the
signalling link and the speech circuit group run along
different routes, the circuit group connecting the SP A
directly with the SP B.
For this mode, the signalling for the circuit group is
carried out via one or more defined STPs . This signalling
mode is favourable for traffic relations with low capacity
utilization, as the same signalling link can be used for
several destinations.

61. Quasi-associated mode

62. CCS#7 Signalling messages
Signal Units (SU)
The MTP transport messages in the form of SUs of varying
length. In addition to the message it also contains control
information for the message exchange.
There are three different types of SUs :
- Message Signal Units (MSU).
- Link Status Signal Units (LSSU).
- Fill-in Signal Units (FISU).

63. Using MSUs the MTP transfers user messages, that is,
messages from UPs and messages from the signalling
network management .
The LSSUs contain information for the operation of the
signalling link (e.g. of the alignment).
The FISUs are used to maintain the acknowledgement cycle
when no user messages are to be sent in one of the two
directions of the signalling link.

64. DPC & OPC
Destination Point Code (DPC) : (14 bits) identifies the SP to
which this message is to be transferred.
Originating Point Code (OPC) : (14 bits) specifies the SP from
which the message originates. The coding of OPC and DPC is
pure binary and using 14 bits linear encoding, it is possible to
identify 16,384 exchanges. The number of exchanges in DOT
network having CCS7 capability are expected to be within
this limit.

65. Any Query
Thank You