3. SIGNALING
A signaling is nothing but a language which enables
two switching equipment's to converse for the purpose
of setting up calls.
Types :
1. Subscriber line Signalling
2. Inter-Exchange Signalling
7. PCM in Telephony
Natural human voice spans a frequency range from 20Hz to 20KHz, however
conventional microphone system passes frequencies from 400Hz to 3.5KHz.
Highest Fm : ~ 4khz
The sampling theorem essentially says that a signal has to be sampled at least with
twice the frequency of the original signal to reconstructed at far end.
Therefore fs = 2fm = 8khz
In PCM every sample is converted into 8 bits ,so bitrate =n x fs = 8kx8 =64kbps
Bit duration in PCM is 488ns , 8 bits per sample = 8x488ns = 3.9ns (channel)
1 frame = 32 channels or time slots =32x3.9ns=125micro seconds .
Multi frame = 16 frames = 16x125 =2ms
9. Limitations of CAS:
Limited Signaling Information: CAS is limited by the amount of information that
can be signaled using the voice channel. Because only a small portion of the voice
band is used for signaling
Inefficient Use of Resources: CAS systems are inefficient because they require either
continuous signaling or, in the case of digital CAS, at regular intervals even without
new signals.
Signaling is limited: to call set-up and release phases only. This means that
signaling cannot take place during the call connection phase.
10. CCS#7
In CCS7 the signalling messages are sent via separate signalling links. One
signalling link can convey the signalling messages for many circuits
11.
12. Terminology in SS#7
Signalling points :
The SPs are the sources (originating points) and the sinks
(destination points) of signaling traffic. In a communication
network these are primarily the exchanges.
Each signaling SP may equipped with
Service Switching Point (SSP) : switches
Signal Transfer point (STP) : Router and or a gateway
Service Control Point (SCP) : Interface to applications
Signalling links : SP Interconnection
14. Functional Levels of CCS7
MTP1 = Message Transfer Part 1
MTP2 = Message Transfer Part 2
MTP3 = Message Transfer Part 3
SCCP = Signaling Connection Control Part
TCAP = Transaction Capabilities Application Part
MAP = Mobile Application Part
INAP = Intelligent Network Application Part
ISUP = ISDN User Part
15. Conclusion:
CCS 7 messages are exchanged over out-of-band (OOB)
signaling links, so the voice carrier channel is isolated from the
signaling channel. Hence out-of-band signaling channel is
inherently more secure and reliable with variety of features.
