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_ > < _| | / /__| | | | | __/ /\ / (_) | \__ \ __/ | |_ > < _
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\_\ " K e e p U p T h e N o i s e " /_/
Branches Of Signalling
======================
By: dialect
Explanations
============
1. Introduction
2. What is Signalling?
3. Steps in Placing a Call
Branches of signalling
======================
1. Common Channel Interoffice Signalling
2. Dtmf Signalling
3. MF signalling
4. SF signalling
5. Per-trunk Signalling
6. Ground Start Signalling
7. Brief of Loop Start Signnalling.
8. In-band - Out Band Signalling
9. Robbed Bit and Clear Channel signalling
Introduction
============
This text is to simply give you an overview of Signalling. To give you a sight into how mostly
all telephony equipment is or depends on signalling. Included are some explanations of basic
signalling factors and thier difference to each other. Of course more than i have listed but i
dont know that much and i'm not that smart but i will try to explain the branches and give
examples to further understand these subjects. Any questions email me :)
-------
What is Signalling?
===================
A telephone network with all these digital switches and gadgets would be pretty useless if there
wasnt a way to "communicate" between themselves to get the right data and also send out the right
data. You could say that signalling is maybe the Basis of the network that interacts or
communicates with the rest of the telephone network. Basically, sigalling is the exchange of
information between two or more points in a network that establishes, mantains (keeps control),
and disconnects the connections of everyday phonecalls using one of more form of signal
information for example the different tones used (which i will explain later.)
-------
Steps in placing a call
-----------------------
This is to my knowledge (i could always be wrong &#^$#$) the steps taken by a telco service to
letting you place your everday phonecalls. (yes even those evil outdials you overabuse. hehe)
Well anyway....
1). Of course, we start with taking the phone off the hook which completes a electrical
circuit through your pair of wires.
2). The current starts to flow and the CO (central office) notices or identifies the current.
3). The Co then connects a register to the line which has the simple task of kinda like
storing the digits you are about to push.
4). Then the CO provides a dialtone which signals to all of us meaning "start dialing"
------
The Branches or Factors of signalling
*************************************
Brief Views on the different Types
Common Channel Interoffice Signalling
=====================================
Today, most telephone carriers are using CCIS or Common Channel Interoffice signalling. Pretty
much, the point of CCIS is why use expensive trunking to send signalling? (bell is cheap) Instead
they want a totally different system for signalling. So since signalling information is basically
just data kind of like the on-hook off-hook conditions, signalling systems are packet data
networks. ss7 is a good example of CCIS. STP's are signalling Transfer points and are basically
used in the network to properly route packets. STp's are made in pairs because if one point
fails, then the other is there to get the data.
DTMF Signalling
===============
DTMF stands for Dual Tone Multi Frequency and it pretty much explains it all in the name. This
type of Tone uses 2 tones at the same time each at a different hertz to generate one smooth
sounding tone. You use dtmf signalling everyday when you push those little buttons of yours on
the phone. DTMF tones reduce dialing delay, especially it you have already preset a number to
dial. It basically uses equipment which reduces the amount of time and equipment needed at the
switches because its continuelsy doing the work for it when you're the one who's punching in the
digits. So all around. Dual tone multi frequencing allows an ordinary telephone to have the
functions of a computer terminal becuase of the way you have control of what your dialing at your
end of the line. Each Tone you produce is stored and then sent to the CO which properly routes
your call to the party you wish. Also, DTMF was made becuase of the way even a human voice can
produce a single tone which is called "false signalling". So, to prevent this "false signalling"
DTMF was born :) I've created a table which properly shows you the 2 tones that make up each
number on your phone.
-----------------------
So for example: | | Of course the 11,12,13,14 buttons
1336hz + 697hz = 2 | 1 2 3 11 | 697 hz you do not see on your phone.
etc... etc... | | They arent used much and i guess
| | you could call them your silver
| 4 5 6 12 | 770 hz box tones.
| |
| |
| 7 8 9 13 | 852 hz
| |
| |
| * 0 # 14 | 941 hz
| |
-----------------------
1209 1336 1447 1633
MF Signalling
=============
See, DTMF is used for subsriber lines while MF (multifrequency) signalling is used only between
CO switches or for interoffice Trunking. (see my trunking txt for interoffice trunks.) MF came
out before DTMF and is now being phazed out by ss7. In MF signalling, the human voice could
possibly simulate one the control jobs that mf is used for. So thats why its used for between
switch signalling. The frequencies are different for Multi Frequency Signalling.
900 + 700 = 1 1300 + 1100 = 6 All in Hertz
1100+ 700 = 2 1500 + 700 = 7 ------------
1100+ 900 = 3 1500 + 900 = 8
1300+ 700 = 4 1500 + 1100 = 9
1300+ 900 = 5 1500 + 1300 = 0
SF Signalling
=============
Single Frequency signalling is used to exchange on and off conditions. With Single Freqeuncy,
when a customers line is idle, a continuos tone is sent and when the tone goes off, the CO knows
the the customer is seizing the line. So if the CO provides a tone , that is interpreted as a
ringing condition. And if the tone is absent or not there, it is interpreted as an idle
condition. All DTMF, MF and SF signalling are called "facility independant signalling formats"
which simply means that these tones can be sent over pairs, coax, and microwave fibers.
Brief of Loop start Signalling
------------------------------
Loop start signalling is commonly used on residential or subscriber lines, and only requires one
pair of wires. And you might know them as tip and ring. One lead of the pair is tip and the other
is ring. I'll give an example: Lets say a phone is on hook. (not in use) This means the contact
is open and allows no current to flow in any direction. Cuasing a loop to constantly keep
checking for that voltage it needs.
Ground Start Signalling
=======================
Ground start signalling uses DC loop currents and one twisted pair as does loops start
signalling. But ground start signalling is used primarily with PBX's. using loop start signalling
with pbx's would bring up problems that ground start signalling could easily take care of. For an
example i'll use the following : lines between a CO and a PBX. The ground start line between the
a CO and PBX is reffered to as a trunk on the PBX side and aline on the Central OFfice end.
Another example. Lets say a PBX which does not have the ability to detect dial tones sends an
off-hook condition over a loops start line. Then that arouses a delay which can result in losing
the stored digits, which means no call. Ground start wouldnt let that happen.
Per-Trunk Signalling
====================
This is pretty simple to think of. Lets say there is a path for voice travel over a line. And if
a call tries to go to the first open trunk for a call, what happens if the trunk is already
seized and supporting a call. Well then it has to go through the path and pick up another trunk.
And of course, that means more of a delay. That is why common channel signalling is used over
per-trunk signalling which is pretty much an old way of handling calls now.
In Band and Out Band Signalling
===============================
In-band signalling is also said to be robber bit signalling becuase the bits used for signaling
occupy the same (in band) bits that originally can be used for another purpose like Voice travel.
So if the signalling bits dont use the voice bits for transmission , the signalling bits are
said to be "out-of-band" from the voice bits.
Robbed Bit and Clear Channel signalling
=======================================
You could almost relate these signalling methods with the In-band- out-band. In-band being the
robbed bit and out-band being the Clear Channel signalling. Pretty much the only difference is
that the signalling bits if needed robbs the voice bits to gain connections. And Clear Channel
signalling is the fact that no signalling appears in a voice channel. ISDN calls for channel
signalling.
*******
That pretty much explains briefly some of the signalling
factors that occur and are used to send the right data
from CO to CO and subscriber line to subscriber line. Pretty
much new kinds of signalling exist now and are being used over
some of these. and i hope to get info on them soon. You should
read some old Telephone Digest mags or your local telephony mag
at Barnes & Nobles. They have some pretty nifty stuff there to
check out. Over and out.
============================
Email: dialect@linenoise.org
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