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Signal type for analog phones connected to PSTN

Posted on 2010-08-21
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Ok, here my last question that I could not answer from my self-studying.
My Answer was CAS, but I am not so Sure.

. Analog phones connected to the PSTN typically use which of the following signal types?
A.      Loop start
B.      Ground Start
C.      CAS
D.      CCS
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Question by:netcomp
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Accepted Solution

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Jody Lemoine earned 668 total points
ID: 33493228
Standard phone connections aren't going to use CAS or CCS.  Both of these are channel signalling mechanisms that only apply to digital trunks.  Depending on the line, you're going to see loop-start or ground-start used with analogue circuits.  Just to follow the thought process here, why did you think CAS might be involved?
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Assisted Solution

by:Alex Bahar
Alex Bahar earned 668 total points
ID: 33494064
Answer A, B, and C.
Loop start and ground start are supervision signalling (off-hook, on-hook, answer etc) used by analog circuits.
CAS is address signalling. So, sending the called number using DTMF digits or DP (decadic pulse on an old rotary phone) is handled by CAS.

D is wrong. CCS  is used only by trunks. For example SS7-ISUP.
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Assisted Solution

by:feptias
feptias earned 664 total points
ID: 33503131
The answer is A.

Now, here's a new question: How many different answers can you get to a question from three different experts?
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by:netcomp
ID: 33503211
Feptias, you can get a max of three answers. :-), :-),

But, you can tell me why is it A. Man, you said so strongly too. " The answer is A"
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Expert Comment

by:feptias
ID: 33505278
Because jodylemoine is correct when he says CAS and CCS are descriptions used only for digital signalling protocols, so that eliminates C and D. I assumed you wanted one answer only, in which case it has to be A because loop start is much more common than ground start. Also, I have only ever seen ground start on old PABX's, never on an analogue phone. However, the signalling used for analogue lines does vary with location and I am only familiar with the UK.
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Expert Comment

by:Alex Bahar
ID: 33509345
CAS is originally an analog signalling. After the invention of TDM, it is also applied to digital trunks in the form of R1 and R2 register/address signalling and delay dial, wink etc supervision signalling. To differentiate, if CAS is used on E1/T1 it is called digital CAS. On POTS lines it is called analog CAS.
The first form of CAS signalling is called Decadic Pulse Signalling which is used by  rotary phones (POTS line, ordinary phone). It uses the same bit for supervision and address signalling. These terms are also called line and register signalling. Later DSP is invented and address signalling used DTMF and MF tones in CAS.
As the name implies it is channel associated signalling. So the bearer and signalling are on the same channel, i.e. same circuit. That is POTS, the ordinary telephone.
Reference http://www.pulsewan.com/data101/cas_basics.htm
See the paragraph
Loop disconnect Signalling (or decadic) is associated with a DC analogue CAS system and was used in the early CAS systems.  In this situation the local switch provides a DC voltage on all subscriber lines enough to power a telephone.  When the telephone is on-hook (idle), the loop inside the instrument is broken, and no lines current is drawn. When the subscriber goes off hook, initiating a call, current is drawn. The sending of the dial digits causes the loop to be opened and closed at a rate of 10 pulses per seconds. Thus each number in the dial (0 to 9) can be represented by a series of pules and the digit 0 to equal to 10 pulses. The decadic pulsing can be seen via the line signalling by the toggling of one of the ABCD bits (usually the A-bit).  
 
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Expert Comment

by:Alex Bahar
ID: 33509394
BTW I used to work at Nortel as a protocol development engineer in 1980s. I have developed software for analog and sigital CAS (Decadic pulse, DTMF, MF compelled and non compelled), SS7, and ISDN country specific variants for CIS (ex Russia) on DMS100 service provider class exchanges. I am confident enough.. LOL..
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Expert Comment

by:feptias
ID: 33509651
I stand corrected then. I've worked on analogue telephone systems for about 15 years and never heard anyone refer to CAS in the context of an analogue circuit, but you clearly have the references to back up what you say. Also, I can't argue that the description "channel associated signalling" makes sense in the context of all the signalling being on the same channel.

I still bet that the authors of the self-study guide were expecting the answer to be A. (Assuming that they only allow one answer). I was applying some exam technique to the problem as well as some telephony experience.
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Expert Comment

by:Alex Bahar
ID: 33509874
If we have to chose only one answer, then C) CAS would be my choice.
CAS signalling needs both Supervision signaling and address signaling to work.
Loop start and ground start are partially correct, because these provide only supervision signalling of CAS.
For address signalling of CAS, you still need DTMF or DP decadic pulse for sending called number.
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Expert Comment

by:feptias
ID: 33510083
http://en.wikipedia.org/wiki/Channel_Associated_Signaling

Ok, I know wikipedia is not infallible, but it does help to explain why I rejected C from the list.

netcomp, do you have access to the answers in your self study course? In fact, why are you asking EE for the answers - you should have done the studying and been able to answer them for yourself. No point getting a self study course if it doesn't teach you anything!
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