Need info on E&M signaling

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Hello All,

 I have some basic knowledge about E&M, and have practically used it.
But concept wise, I am not very clear... So i am posting a few queries
related to E&M to get a better picture.

1. Normally I have used E&M in a back-toback configuration. i.e. I have
an E&M interface set to say type 4, and another interface also set to
type 4. Both of these are on the same VoIP gateway. And hence no PBX
coming into picture here. Thus, I guess I am using two signaling units,
rather than a signaling unit and a trunk unit ( Hope I am right here
!!!!). I connect the two using a cross E&M cable ( i.e. E<-->M, M<-->E,
.........). My query is that will the cross cable be still used if,
instead of the other interface on the VoIP gateway, I connect it to a
PBX. Or will it HAVE to be a straight cable.

2. With using the circuit diagramof Type II, i tried to visualise the
back to back configuration. What diagram I obtained was as follows:

Signaling Unit 1<------------> Signaling Unit 2

E ( open)------------------------M ( GND)
SG--------------------------------SB ( -48V)

M ( GND)------------------------E ( open)
SB ( -48V)-----------------------SG

My Question is if either side has to indicate Offhook, then it will
always be that E Lead has to be asserted. Have I understood it right.
Whereas the basic working principle of E&M ( be it E&M Wink or
immediate) states that once the side that is initiating the call
asserts the M Lead, and then dials the digits, the Recieving end will
assert the E Lead to indicate the call is connected. So i am not able
to correlate this principle with the figure shown above.

Thanks in advance,

Re: Need info on E&M signaling

sandeep wrote:
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Just remember that E and M stand for recEive and transMit.
The M lead does something and the E lead listens for something to happen.

Emoveray ethay Igpay Atinlay otay eplyray

Re: Need info on E&M signaling

hmm, I learned E for Ear and M for Mouth as a way to remember


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Re: Need info on E&M signaling

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Ear = Receive, Mouth = Transmit
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Re: Need info on E&M signaling

Dana wrote:
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I am aware that E receives and M transmits. But how will u guys explain
the circuit with respect to type II. I have shown the type II connected
in a back-to-back configuration, and hence uses cross cable to connect
two signaling units.

And do you guys mean that always the M lead on one end will be
connected to the E lead on the other and Vice versa.

Could you guys refer some sites where they have good documentation on
the different type of E&M and the different E&M signaling protocols. I
searched thru google, but found info where M lead on one end will be
connected to M lead on the other end. With this I came up with a
circuit diagram as above.

Thanks in advance,

Re: Need info on E&M signaling

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The E lead from the Signaling Unit is connected to the E of the
Trunk Unit, same with M leads.  The thing is, the Signaling Unit
is configured as a "Line" interface and the Trunk Unit is a
"drop" interface.

Type II and Type IV E&M, if I remember right, can be connected
back to back with a crossover cable, but the others cannot.  An
applique unit, which does signaling crossover, can be used to
accomplish back-to-back operation of Signaling Units.

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I suppose it would be loads of fun for me to use google's
advanced web search to find good documentation on E&M signaling,
but I worked with that crap for 40 years and have retired.
Which is to say that 1) I don't really want waste my time
remembering trivia about E&M signaling, and 2) if I did that
*you* would be robbed of the opportunity to learn all that
trivia that I am so happy to be forgetting as fast as I can!

Learn how to use it well.  Change the "results" to 100 instead
of 10.  Then try searching on something like "e&m type signaling
tutorial".  That looks simple, but it is a bit more complicated
than it appears.  The "&" between E&M means that the search will
return for any whitespace or punctuation, or none, between the E
and the M.  You could use E.M too!  The reason for putting the
word "type" in there is because that is likely to trigger on
sites that say something about the various types of E&M
Signaling, rather than just mentioning one of them.  The
"tutorial" is the same thing, though if you don't find anything
with that search you'll want to try it again without that word.

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Another good possibility is to see if any local library has an
old copy of "Note On The Network", which was a Bell publication
that came out periodically for years (the name may change a
little though, and for example later versions were "Notes on the
BOC Network" or something like that.  Whatever, it has some
great little diagrams and a very good discussion of E&M
signaling.  The 1979/1980 edition was the best of them all...

Floyd L. Davidson            <
Ukpeagvik (Barrow, Alaska)               

Re: Need info on E&M signaling

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Your graphic that illustrates a back to back analog E&M signaling layout
for a Type II E&M interface is correct.

An incoming Off Hook from the West direction will provide a contact
closure between the E & SG leads of the Signaling unit.

The -48VDC SB battery supply from the intervonnected Signalling unit in
the East direction will then flow over the E/SG contacat closure towards
the West Signaing unit on it's M lead.

A signaling transition from East direction to the West replicates the

Advantages of Type II signaling, both signaling device power supplies can
remain electrically isolated from each other. As opposed to Type I, where  
in even today's Special Service intallation practice calls for a common
bus to link the Trunk & Line signaling unit power supplies.

Type I E&M units require the use of a Pulse Link Repeater (PLR) to inter-
connect two Type I E&M signaling (Line) devices. The Type II design you
show is easy, and simple to maintain. You would be hard pressed to locate
a PLR these days, most being bay mounted equipment. Tellabs & Wescom once
sold 400 type mounted PLR's in the analog's heyday during the 1980's.

Today there are current circuit designs that use DX (DX-2 E&M Lead
extension) signaling units in a back to back configuration. eliminating
external signaling leads where a back to back channel bank design exists.
I had on not too long ago on a Automatic Ringdown circuit (PLAR).

All the above & SONET too...


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