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Can someone explain what a CO trunk is for exactly?Helpful Member!(3) 

Irvineguy (MIS) (OP)
4 Sep 07 17:32
I've read the definition but still can't comprehend exactly what is a CO and what is is for.
Helpful Member!(2)  paterson (Programmer)
4 Sep 07 17:36
CO = Central Office.

Therefore, a CO trunk is a circuit (usually analog) with direct access to a Central office.  This CO trunk is how your phone equipment accesses the PSTN.  With access to the PSTN, you can make calls to anywhere in the world.  That is, anywhere in the world also connected to the PSTN.

PSTN= Public Switched Telephone Network.

If your phone equipment doesn't have access to the PSTN, then you have a glorified intercom system.

Insert Witty Signature Here.

LkEErie (Vendor)
4 Sep 07 21:30
Back in the day, a CO trunk was a special line that gave you dial tone in a special section of the Central Office.  The "trunk" had special characteristics in that it might be "ground start" or in a high capacity line finder group or a "rotary hunt group."

Most all of that special treatment went away with electronic CO's.  Now, when they talk about CO trunk and "lines" the term is pretty much interchangable.  The distinction is that key systems use(d) lines and PBX's use(d) trunks.  Certain Hybrids can use either, but again that distinction is pretty murky.

Lundah (TechnicalUser)
4 Sep 07 22:32
Depends on your perspective.  The way I look at it; in the PBX/Key System world, any circuit facing a Telco is a trunk, and anything going to a desktop device is a "line" (aka "station").

When you get into the carrier world, it depends on the switch interface, since a CO Switch (5ESS, DMS100, etc.) has a "line" side and a "trunk" side.  So from a Telco perspective, anything connecting to the "trunk" side (E&M, T-1, PRI, that sort of thing) is a trunk, while your POTS lines (1MB, 1FR, GR-303 groups, etc.) are "lines".

Ground Start 1MB's are an interesting hybrid of these terms, since in the PBX world they are almost universally referred to as trunks, but at the CO are a "line side" circuit.
Trotski (TechnicalUser)
5 Sep 07 3:31
I was taught that a circuit going to a hanset was a line and that connections between switching elements was termed a trunk(also called Junctions in the old days)
Lundah (TechnicalUser)
5 Sep 07 9:23
Also true.  About the only good rule to follow is line = terminates to a user/customer device, trunk = terminates to another switch.
ISDNman (Vendor)
7 Sep 07 1:54

exactly, If it is switch to switch it is a trunk. Tie lines are therefore trunks, for example.

Take a POTS circuit. Connect it to a phone and it is a "line". Connect that same Loop Start circuit to a PBX and it becomes a trunk.  Less loss is permitted on a trunk, but that is a very small difference (unless you are in the middle of nowhere in which case requesting a trunk will help with the loss since the tariffs are slightly different). Also you might not be able to gt a residence trunk in some states depending on the tariffs.

CO = Central Office = where your phone service enters the public switched telephone network.  At least with real phone lines. But with voip this is usually called a gateway.

OzzieGeorge (Programmer)
9 Sep 07 23:16
Co is an americanism which unfortunately is spreading. In America the local exchange was known as the Central Office (first started by AT&T I'm led to believe) Everywhere else in the world it was known as the exchange and they are referred to as exchange lines
392 (Instructor)
11 Sep 07 10:22
The way I always considered it was a trunk is a talk path between a customer's premise and the central office or another switch. It can take the form of a digital channel, analog dialtone or a hybrid such as a TIE line. But the talkpath is the key. In some corners, a trunk is sub-defined as a talkpath that does not return dialtone without signalling the far end that the near end requests connection, or never returns dialtone, only an open, connected talkpath.
Trotski (TechnicalUser)
12 Sep 07 5:35
now we are all lapsing into jargon, it does seem dependant on where and when you learnt your stuff as to what terms get used.
I,m getting my terms from the crossbar training I did 20 odd years ago (showing my age now!).
GMgerry (Programmer)
12 Sep 07 16:22
Speaking of Tie trunks, lets tell them what E&M stands for.
kwoerner (TechnicalUser)
13 Sep 07 11:12
ear and mouth?  
phdr (Vendor)
13 Sep 07 13:34
First answer  is the best.
....thanks Paterson....

CO = Central Office.

It IS a Phone Number. One CO per Phone Number.
One Pair of Wires per CO.
JUST like at your house. Two wires per Phone number
one number per jack in your house. (Sure that can vary)

OH.... we can get into T1, PSTN,
Ear&Mouth (how I was taught to Understand)
.....Understand being the Key !!
IrvineGuy needs the qwik answer.
Qwik Answer:
One CO per Phone number (Check Phone Bill)
CO is also known as TRUNK (TRK).
Your phone system will have:
 CO Card's
or Trunk card's
Usually "CO" or "TRK" is actually IN the Cards Name!
...these cards let you wire up CO's or TRK's
(2 wires each) FROM the Phone Company TO your
equipment ( your Cards Ports.)
These cards usually hold 4 or 8 CO's (Ports),
so you buy them incrementally as you add CO's.
(I almost said add lines, and yes card are 2 or 3
Ports also)

More info, more confusing?
letman2 (TechnicalUser)
19 Sep 07 10:59
from Wikpedia
In yet another story, the proposed labels were R and T for reception and transmission of signaling. However R and T were already used as labels for the wire pair carrying voice signals. Thus a letter within each of the word recEive and transMit was chosen.
E&M is actually the abbreviated form for "earth" and "magneto" from early and unspecified telephony times.

We used to say e to me and m to you
nytalkin (Vendor)
26 Jan 08 12:05
actually TIP and RING are parts of the orignal plugs used in switchboards.  Visualize the plug.   The TIP of the plug and Ring between the TIP and the SLEEVE and finally the SLEEVE of the plug.


SYQUEST (TechnicalUser)
26 Jan 08 23:23
Back to the subject of TRUNK... One item nobody has mentioned so far is transmission level in regards to PBX TRUNKS. The transmission level is(was) one of the special characteristics of TRUNK LINEs. It was considered a designed circuit, when a VFR (voice frequency repeater) was added to the loop to over come loop loss levels. In the day PBX trunks usually were designed to a -3.0dB or -3.5dB loss @ a 600 ohm termination. This was done to improve voice transmission to that far end PBX station at the other end of the building. This was done on both cord switchboards, SXS, Crossbar, and electronic PBXs. OPS (off-premise station) circuit designs used VFRs also for the same reason.

You can still order this type of local dialtone in most areas, but the monthly rate is higher than a bare POTS line, and some telcos will limit the loss to -5.0dB before adding a VFR. In analogue telco, level and loss are a big deal...

Helpful Member!  jerryreeve (Vendor)
28 Jan 08 16:46
Sysquest has some good info but note: should be 3.0dB or 3.5 dB loss (negative loss is a gain and commonly gets people confused, most measurements on lines/trunks are measureing a loss and when a gain is found it is recorded as a negative loss) same for limiting loss to 5.0dB before adding VFR.

the correct termination is important since different locations and stations may use different terminations, 600 ohm is the most common with 900 following a close second, be careful you do not double terminate when taking a reading, if you interrupt the signal flow you need to terminate, if you clip on or monitor without breaking the circuit then use the bridge termination (10,000 ohms)

FYI 3db of loss is 1/2 the power 3 db of gain is twice the power.  10 db loss is 1/10 the power and 20 db of gain is 100 times the power.

Communications Systems Int'l

ISDNman (Vendor)
28 Jan 08 20:23


I did mention the difference in loss in my post, though it was only casually mentioned ;)

Trotski (TechnicalUser)
29 Jan 08 5:32
Ah yes less loss and the ever popular is it TX in RX out or TX out RX in ?

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