SIGNALLING USEFUL TIPS for PAPER I
1. Signalling is either preceded by action or it is followed by an action or both.
2. Signalling can be classified generally as Subscriber line signalling and Interexchange
3. Signalling information can be categorised under 4 main heads.1.Call request and release
information 2.Selection (address) information 3.End of selection information 4.Supervisory
4. Call request and release information involves calling sub off hook, seizure, on hook etc.
5. Selection (address) information involves PTS signal in trunks & dial tone in subscriber
signalling, address information etc.
6. End of selection involves called line free or busy or the reason for non completion of call
7. Supervisory information involves off/on hook condition of the calling subscriber after the
connection has been setup.
8. PTS-Proceed to send – when the exge is ready to receive the address information, it sends
back a request known as PTS in trunk signalling.
9. In address signals, the digits may be transmitted either by decadic dialling or by multi
frequency pushbutton dialling.
10. In decadic dialling the address digits are transmitted as a sequence of interruption of the
11. Decadic dialling is relatively slow and signal cannot be transmitted during the speech
12. Constraints of decadic dialling has been overcome in multi frequency pushbutton dialling
13. Seven frequencies (3 frequencies in the HF group(high group) and 4 frequencies in the
LF(low frequency) group) are used.
14. DTMF dialling generates a signal comprising of 2 frequencies, one from each group.
15. HF group frequencies are 1209,1336, 1477 Hz.
16. LF group frequencies are 497, 770, 852, 941 Hz.
17. Since the frequencies in the DTMF are in the speech band, information may be transmitted
during the speech phase also.
18. Busy tone and recorded announcement is sent to the calling subscriber by the equipment as
close to the calling sub as possible to avoid unnecessary busying of equipment and trunks.
19. The inter exchange signalling may be 1.pulsed signal 2.continuous signal 3.compelled
20. Inter exchange signalling may be transmitted either by using in band signalling or by out of
21. The inband signals are transmitted using the same frequency as for speech signals in the
frequency range 2400hz or 2600hz.
22. Tone on condition limitation (indicating the end of signal) is over come in out of band of
23. In Out of band signalling the tone frequency range is 3825 hz which is beyond the speech
24. Inter exge signalling was subdivided into two categories as line signalling and register
25. In Line signalling, the signals operate throughout the duration of the call.
26. Register signalling operates during the setting up of a call essentially for transmitting the
27. Register signals can be transmitted in inband or out of band.
28. The register signals are PTS signal, address signals, signals indicating the result of the call
29. When more than two exchanges are involved in setting up of connection, the signalling
may be done in either end to end or link by link signalling.
30. In end to end signalling, in the case of 3 exges A,B,C initially the signal is between A& B
and then between A&C, after B,C connection is established.
31. In link by link signalling, initially the signalling is between A&B then between B&C, after
the B-C connection is established.
32. Generally supervisory and subscriber signalling is necessary on link by link basis.
33. Addressed component may be signalled either by end to end or link by link.
34. In R2 signalling, 2 out of 6 frequencies are used.
35. To make the system more reliable, compelled sequence is used.
36. R2 system is normally called compelled sequence multifrequency (CSMF) signalling.
37. In R2 signalling, 6 forward frequencies and 6 backward frequencies are available.
38. By using 2 out of 6, there will be 15 combination in the forward and 15 combination in the
39. In modified R2 signalling, 5 forward frequencies and 5 backward frequencies are used.
40. Using 2 out of 5, there will be 10 combinations in the forward and 10 combinations in the
41. Forward signalling frequencies are 1380,1500,1620,1740,1860.Hz.
42. Backward signalling frequencies are 1140,1020,900,780,660 Hz.
43. The forward signals are used for sending the digits dialled by the subscriber and calling
line category and address information of the calling subscriber.
44. Backward signals are transmitted as the acknowledgement to the forward signal.
45. Backward signals are used for demanding address information and callers category and for
sending condition and category of called line.
46. Backward signals are divided into 2 groups, viz GroupA and Group B.
47. GroupA signals are in the form of instructions from the terminating exchange to the
48. GroupB signals are used to give information about the condition of the called subscriber’s
49. MFC signalling minimize the post dialling delay and unpaid holding time of trunk and
junction line due to its high speed.
50. In satellite, semicompelled scheme is used, due to long propagation time.
51. In mod R2 signalling, interexchange line signalling is out of band signalling and
interexchange register signalling is inband signalling.
52. Digital signalling are of 2 types .1.CAS 2. CCS7 signalling.
53. In channel associated signalling, interexchange signallings are transmitted over a channel
directly associated with a speech channel.
54. In CCS7 signalling interexchange signalling can be transmitted over a dedicated link
common to a number of channels.
55. In CAS ,interexchange signalling is via timeslot 16& inter register signalling in the speech
56. Limitations of CAS signals are as follows:1.expensive 2.slow signalling 3.limited
57. Benefits brought to telephone subscriber consequent to introduction of cc7 are 1.Faster call
setup 2. Provision of new facilities 3. More call completion
58. Typical SU (signal unit) format of initial SU (C6) is heading -5 information- 4 label -
11 checkbits- 8
59. CCS6 uses fixed length of signal units and each unit has 28 bits.
60. CCS7 uses variable length of signal unit.
61. There are 4 levels in C7 system.1.Level1-Physical layer.2.Level2 –Data layer 3.Level 3-
Network layer.4.Level4 –User part.
62. Level 1,2,3 forms a MTP(message transfer part).
63. Under message transfer procedure in CCS system, synchronisation is needed at data link
level to recover bit timing.
64. Level 1 is the physical layer ie signalling data link layer for providing bidirectional data
link for sending signals between
2 points simultaneously.
65. Level 2 ensures reliable and error free transfer of signals between two points.
66. Level 3 is the network layer, deals with 1.Message handling and 2.Network management.
67. Message handling involves 1.Message discrimination 2.Message distribution 3.Message
68. The following functions are required under Message transfer procedure for sync of
signalling messages. 1Message transaction control 2.Message alignment 3.Message
69. N/w management involves signalling traffic management 2.signalling link management
3.signalling route management.
70. Level 4 is the user part which includes message, message codings and protocol to handle
71. Level 4 -user part may be ISUP (ISDN user part),TUP (Telephone user part),DUP (Data
72. In a CCS N/W ,signalling terminal consists of coder ,decoder ,input buffer o/p buffer, a
block storage & an error control section.
73. In c7 system, Signalling link code (SLC) is used for load sharing between various
signalling links inter connecting the signalling points involved.
74. In signalling route management a signalling route set test procedure is implemented to
check the availability of the signalling routes at each signalling points.
75. The function of signal transfer point is to transfer the signals from one signalling link to
another through conversion of the address labels in corresponding signal unit.
76. Signalling data link is constituted by digital interface & digital transmission media.
77. In c7, Signalling link code is used for load sharing ie. signalling traffic sharing between
various signalling links interconnecting the 2 signalling points.
78. Signalling link is shared by a large no of speech ckt in C7 which provides bi-directional
data link between CC s
(Central processor ) of the 2 xges.
79. Level3 is concerned with signalling message handling & n/w management fns.
80. In Per ckt load sharing method, all the messages pertaining to the same speech circuits
follow the same transmission path.
81. Level 1& 2 is specifically concerned with a single signalling link.
82. The message is sent in the form of signalling units.
83. In C7 , 3 types of signalling are used.1.MSU 2.LSSU 3.FISU.
84. MSU is the message signal unit and whenever there is a message MSU is transported
85. MSU has got variable length signal format.
86. C7 , Flag code of a SU is used to delimit (ie) open or close a SU.
87. Signalling field (SF) has got 8 or 16 bits and first 3 bits of first octet are used for
88. SI(service indicator octet) defines user part.
89. SIO field of a SU is used to represent the service indicator & sub service field.
90. Length indicator (LI) gives the no of octets between the checkbit field and length indicator
91. LI is zero for FISU &1or 2 for LSSU & greater than 2 for MSU.
92. The signal units are acknowledged as either faulty or positive on the basis of check bit.
93. FSN is the forward sequence number. It is of 7 bits assigned consecutively to each signal
unit to be transmitted.
94. FIB is used during general error correction in transmission and indicates whether a
signalling unit is transmitted in
first time or retransmission.
95. BIB is the backward indicator bit, is of one bit & during error correction and is for
96. BIB of a SU is used to request retransmission when a MSU has been received out of sync.
97. FIB of a signal unit is used to indicate the start of retransmission cycle
98. If FIB&BIB are inverted, then it is a case of retransmission.
99. BSN assumes the last correctly accepted FSN value of MSU.
100. Flag is of 8 bit of the format 01111110
101. 101.For telephone service in c7 the information fields of message signals units consists
1.calling line cat. 2.address signals 3.indicator bits 4 . label & heading .
102. BSN of the SU is used to identify the FSN of a correctly received message SU.
103. F -8 CRC -16 INF -8 LI 6 FIB 1 FSN -7 BIB -1 BSN-7 F8 represents FISU format.
104. LSSU is used to convey the status of the signalling link during alignment.
105. LSSU is transported during the alignment and when the bit error rate is increased beyond
106. LSSUis used to send congestion to the transmitting mode.
107. FISU is sent when there is no message and the signalling link is ok.
108. Any node of n/w or o&m centre that has access to the c7 n/w is signalling point.
109. There are 3 methods of association1.associated mode 2. Non associated mode 3.quasi
110. In Associated mode, signalling links are routed together with circuit group(speech ie traffic
111. In non associated mode, signalling messages are routed through several signalling links
according to the network conditions while the traffic ckts are routed directly.
112. In Quasi associated mode, signalling are transferred between 2 xges over 2 or more
common signalling links in
tandem between certain predetermined path where as traffic ckts are routed directly..
113. Under level 3 functions in c7, the message discriminator function depends on DPC
(destination point code).
114. The speech path continuity check is carried out automatically during the xge of signalling
115. In c7 N/W, Associated mode is employed for signalling between 2 exchanges with
comparatively large no speech circuits.
116. Ccs7 signalling system is a modular system providing for independent evaluation of its
constituent parts and independent definition of its subsystem for its application.
117. Two types of error rate monitors are used.1.signal unit error rate monitor(SUERM).
2.Alignment error rate monitor (AERM).
118. Two methods are used to detect and correct the errors.1.basic error detection /correction
2.preventive cyclic retransmission(PCR).
119. In ccs 7, PCR method of error correction is applicable for links with high propagation
delay & which is a forward error connecting method using only the +ve acknowledgment.
120. In level 4 functions, the 4 least significant bits of a ckt identification code CIC represent
signalling link code SLC.