                             Lines versus Trunks
                         by pbxphreak <chris@lod.com>

The term line refers to more than one type of circuit in most cases, it
includes a connection configured to support a normal voice calling load
generated by one individual. But in the case of a PBX, the term line is
usally corresponds to one connection from the PBX to a desktop. In the
case of a centrex, a line is normally one physical connection from the
customer site to the CO. With a key system, a line corresponds to one
telephone number, but it might also be reffered to as a trunk.

The term trunk normally refers to a circuit configured to support the
calling loads generated by a group of users. Possibly numbering many
thousdands. Usually a general-use circuit from a PBX to a CO would be
described and billed as a trunk. Connections between COs or offices
higher in the network would also be referred as trunks. These trunks
are physically identical to lines.

The ability of any given switching system, such as a CO or a PBX, to
establish connections is limited. For example, although a PBX might be
able to support 200 connections or ports, it might only actually provide
80 paths at one time. In such a case, if 80 people were to connect to 80
other people (some of them possibly off site), that would account for 160
of the ports, and if any of the remaining telephones or ports attempted to
access service, the would fail. That is, a user could pick up the telephone
and not recieve a dialtone. Some systems are configured so that no such
failures can happen. In the previous example, if only 160 physical
connections were made to the PBX, then it could provide service at the same
time to all of them. Such a configuration is described as blocking.

Normally, a PBX's connections to the CO are configured so that a much higher
utilization than 10 minuties per hour is achieved on those ports, and a
primary benefit of a PBX is the ability to buy fewer telco connections than
one has telephones. The CO must be configured so that it can provide
connection services to such trunks at this higher utilization rate so it uses
more of the CO's overall switching and connection capacity (CO's are not
normally configured as nonblocking switches). So, the telco will naturally
bill a PBX trunk at a higher rate than a single business line, even though
the PBX trunk might be physically identical to that single line.

The final comparison of lines versus trunks would be as follows. A line is
an end point from a central switching service, such as a Co or a private
automated branch exchange (PABX). The line is an end point on the pair of
wires regardless of where it resides. A line carries one single conversation
at a time on the physical channel capacity. It is a billable location for
the telephony companies.

A trunk connects two switching systems. The trunk might be a single circuit
carrying a single call at a time, or might be a bundled service that is
multiplexed and carries multiple conversations going at the same time. The
difference is that a trunk will be used for switching and routing decisions
from the switching offices (CO, or PABX). The trunk is continually rather
than occasionally used. It is a billable address that can have additional
subaddressing capabilities behind it. In the telephone company world, it is
the connection between and among other offices. In the private user world, it
might be a sinle connection to the PABX from the CO. These disctinctions
offer some variations in billing and utitilizing them. Below are some common
configurations.

DID:
---

DID refers to direct inward dialing, from a callers point of view, this
service is in place if the caller can dial a 10 digit number rom the outside
and reach a specific person without an operator (live or automated). Centrex
normally supports this capability without andy additional configuration, and
everyone has their own telephone number. A true key system (where telephone
numbers are normally shared) can only allow DID if any given telephone number
has only a single apprearance.

But DID is ually referred to in the context of a PBX. It is a specific PBX
feature that musyt be enabled and configured, with elements set up both
within the PBX and also with the telco. Consider as an example a new site
intended to support 1100 employees, each with his/her own telephone connected
to a PBX.

The first step in arranging DID is to reserve the telephone numbers for all
those employees. Lets say the main company telephone number is 555-1234. The
telecommunications manager will request a block of DID numbers from the telco
probably about 2000. The telco might say, "Your DID numbers are 555-2200
through 555-4199". Notice that while there is a good chance the block will
have the same exchange as the main number, it probably will not inslude the
main number. The company will pay for those numbers on a montly basis, but
they will not cost anywhere near as much as actual telephone lines. So far
the only thing arranged is the reservation of the block of numbers themselves
These numbers will not be given out by the telco to anyone else. The telecom-
munications manager will assign each employee one of the numbers in the DID
block.

Next, the telecommuncations manager must determine how many trunks (or DID
lines) in the trunk group will be required to support the calls from outside
to the companys employees. These are inbound only, and are in addition to the
normal in/out or inbound trunks that serve the main operator, so they must
be engineered to a very low level of blocking. With DID, the telco passes on
to the customer PBX the responsibility of handling answer supervision.
Eg. Busy signals.

So if an external customer calls "Jane" at extension 2313, the customer will
dial 555-2313. The telco CO will seize the next available trunk in the DID
group and signal along that there is a call for extension 2313. At that point
if extension 2313 is busy, the PBX must deal with it, and the CO is merely
passing along the signals. Possible PBX actions include forwarding to a
message center, generating a busy signal, or forwarding the call to a
specified alternate extension.

DID is most often used to reduce or eliminate the manpower required for a
central answering position. The more calls customers can place directly, the
fewer must be answered by the company operator. On the other hand, some
companies prefer to have all incoming calls answered by someone trained in
the way the company wants its telephonees to be answered.

DOD:
---

DOD refers to direct outward dialing. If an employee can dial and reach an
outside number without and internal operator, then the company has
implemented DOD. In the past, when old telephone systems were available with
less technology, it was not uncommon for a company to route all of its
outbound calls through an internal operator. The operator's responsibility
was both to screen calls and to route the calls over the right facilities,
for examples a WATS line. (see below for WATS line). With the advent of
modern PBX's and Centrex, such limitations can be programmed on a telephone
by a telephone or even user-by-user basis, eliminating the need to involve
an operator in inbound calls. DOD is a term not often used these days because
few companies consider not providing it.

FX:
--

FX refers to a foreign exchange circuit. Foreign refers to a CO other than
the local Co, not to a location outside the country.

Consider the case of an airline that wishes to locate all of its reservation
clerks in Atlanta. It cannot expect all of its customers to pay long distance
charges to make reservations. What are the alternatives? One possibility is
a group of 800 circuits. It will probably have a large number of those, but
800 trunks cover large areas (and are prices accordingly). What about service
for customers calling from large citys like Chicago? Perhaps a more focused
service might be more cost effective.

Think of an FX line (or trunk) as two-thirds of a dedicated point to point
connection. It starts at the customers location, connects to the local CO and
extends from there to another foreign CO anywhere in the country. There is a
fixed montly charge for all that mileage, but there are no usage-sensitive
charges for these miles. At the foreign CO, it is open. It has a telephone
number associated with that foreign CO. Calls made to that number ring at
the customers lcoation. Calls made from the customers location over the FX
line to proceed to the foreign Co, incurring only local charges for the call
from the foreign Co to the called location.

FX lines are often used by companies to provide local numbers that customers
can call in cities where the companies do not in fact have offices. In the
airlines case, it could arrange a group of FX lines from its Atlanta offices
to a Chicago CO. All of thelines could share one Chicago local telephone
number. People from anywhere could call the number, but normally only people
in Chicago would, because it would appear only in their telephone book, and it
would be a local call only for them. If the airline wished to allow it,
service representatives could also place calls from Atlanta to Chicago over
the FX lines. The calls would be billed though they were placed within
Chicago. Perhaps calls notifying customers of changing flight information
might be places in this way.

OPX:
---

OPX refers to off-premises extension. An OPX line permits a telephone not a
company's location to function to all intents and purposes as though it is
located at a company's location. This capability becomes particularly
interesting with the recent increase in telecommuting. Suppose an employee
plans to work at home. One of the problems to overcome in such a case is the
isolcation such a worker might experience. Providing the employee with a
telephone that look slike an internal line at the company might help to reduce
the problem. Others calling the line within the company will dial an internal
extension, which will ring at the employees home, and if the employee whishes
to make a long distance call, he/she ually just dials 9 and then the rest of
the number just as at a desk at the company's location.

As with an FX line, an OPX connects from the company's location to the local
CO then continues via whatever connecting Co's are neccassary until it
terminates directly on at telephone at another location. A key difference
from an FX, is that on the PBX and OPX is connected and configured as a
telephone rather than a trunk. This results in a limitated type of service
provided and normally only an analog telephone can be used at the end of an
OPX because the digital signalling between a PBX and its old style telephones
will probably not successfully make it throught the various analog and
digital circuits that make up the OPX. This limitation is not normal. It just
imposes on the telecommunications manager the need to configure the PBX to
support a certain number of analog telephones as well as the digital
telephones that might be used in house.

Tie Lines:
---------

A tie line also sometimes called a tie trunk refers to a private point to
point circuit used to connect to voice facilities. For example, a decicated
link between customers PBXs at two different locations would referred to as
a tie line. Other examples of tie lines might be a link between a PBX and a
centrex sysem, or one between two centrex systems. In all of these cases, it
would be equally correct to refer to the circuits as private or leased lines.
If ones of the connected systems is not a voice system, the term tie line
would not be used. Tie stands for terminal interface equipment.

WATS:
----

WATS is an abbreviation for wide area telephone service. WATS lines come in
two flavours: in-WATS and out-WATS. Another name for in-WATS is 800 service.
When most people refer to a WATS line, they mean and out-WATS facility. Both
services are merely billing arrangements for reduced billing of long-distance
calls based on a fixed montly free and discounts for larger calling volumes.
800 service also has the characteristic of reversing the charges to the
called party.

In the past, WATS lines have been separate facilities (physically identical
to local PBX trunks or private lines). Their coverage was also banded, and
a user may have had a WATS line that only reached adjacent states, or all
of the lower 48 states, or some intermediate variation. For out-WATS, either
the PBX had to be smart enough to recognize the dialed area and choose the
correct outgoing facility or users had to dial special codes to select the
right WATS line.

WATS service has never been free, although some of the older tariffs did
specify certain volumes aove which all calls were free, which was very large.
Those tariffs are long gone and all calls are now charged on a per minute
basis. The only factor is the per minute charge, which does decrease as the
calling volume increases.

One significant improvment is that WATS-type volume discount billing can now
be setup on existing trunks, no longer are seperate facilities into the local
CO required for such an arrangement.

Private Line:
------------

Any circuit leased from a crrier from point on one customers premises to
another point on a customers premises (even the same premises) can be
described as a private line circuit. If an organization builds its own
facilities(eg. a mircowave link across a big city), these facilties would
also be described as private circuits. In either case, the alternative is
normally a dial-up link.

Many factors go into the decision as whether to setup a private facility.
Some reasons why a company might setup a private link include:

 - Private analog circuits can be tuned for higher performance than dial-up
   facilities can. Both in terms of speed and reliability.

 - Many types of digital facilities are only available on private basis.

 - Management and troubleshooting of private facilities can be more tightly
   controlled than in dial-up enviroment.

 - High volume of calls or data would generate higher charges on the public
   dial network than on a non usage sensitive private network.

Reasons to go with public dial network include:

 - Volumes of calls or data too low to justify a leased link.

 - Unwillingness or inability to coordinate and manage a private network.
   Like do you really want to be your own telephony company? I think not :)

 - A large number of small locations that would be uneconomical to connect
   with private links.

With few exceptions, there is not a "right" descision on this issue. What
might make the most sense today might be uneconomical tomorrow.

A classical example of this kind of change is the decision as to whether to
build a private voice communications network of tie lined connected PBXs
which is called a tandem network.

My Comments on Line and Trunk Networking:
----------------------------------------

Some of us might rememebr the early days of competition in the long distance
arena. Remember the way we had to conenct to the alternate long-distance
suppliers, like Sprint and MCI.

A customer might sign up with a long distance supplier other than AT&T prior
to 1984. This new customer of the competitors was offered several discounts
over the long distance tariffed rates from AT&T. So that the customer could
use the service, the long distance supplier would issue an 800 number to call
its network, or a special number. This special number was a seven digit
telephone number that could be a regular local number in the area, a 950-XXXX
number or foreign exchange telephone number from a major area. The choices
were based on the density of the carrier's service in the customers area.

The customer would issue this telephone number to all internal users. Along
with the 7 or 10 digit telephone number for entrance into the carriers
network (I will use MCI from this point on for simplicity), another 10 digit
number, called an autorization number, was issued. THis might be an unique
number for every indivdual in the organization or a global number used by the
entire organization.

The caller now wants to make a long distance call from his/her office. So the
sequence begins like this:

 - Pick up the phone and get a dial tone, then dial 9 for an outside line.

 - Dial the MCI toll free 800 number

 - Wait for connection. As the call proceeds, a ring tone is heard and then
   the MCI system answers and provides a computer tone which sounds much like
   steady high pitched tone.

 - On getting the computer tone, dial 1234567890 or wahtever 10 digit
   authorization number is assigned to the organization. 

 - After the computer acknowledges the 10 digit authorization code it will
   return a dial tone to you.

 - Now dial the 10 digit telephone number of the party you with to speak with
   Eg. (410)671-8192.

 - Wait for the call to proceed and ring. Hope and pray that the call is
   answered and the line is not busy and is clear enough to hold a
   conversation on. Otherwise, start all over from square one.

Users would obviously become very frustrated with this procedure. This is
especailly true if the called parties were busy or if the user (for instance
a telemarketing group) needed to make multiple calls. The need to dial 32
or 33 digits just to get a call through was frustrating. Especially since the
users did now follow the company guidelines and dialed AT&T directly, they
only had toi dial 12 digits. This was significant, particularly when there
were thousands of calls being made per month. The accumulated waste of time
might have cost the organizations more in productivity losses than MCIs
service saved them.

So why did MCI require all this digits in the past? The answr is simple. They
had no choice. When competition first began, AT&T was the owner of the Bell
system. To preclude the competitive threat, AT&T controlled how the network
was set up. MCI had to rent telephone li nes from the local Bell telephone
company. At the central office, these lines were connected from the CO to the
MCI computer. THe calls was a completed call the minute the computer answered
the incoming request. MCI did not get any of the information that is passed
along from CO to CO or from CO to long distance supplier, becuase they were
on the wrong side of the switching system. They were on the line side, not
the trunk side. AT&T was on the trunk side of the switch, so call the caller
ID information was paseed from switch to switch and no extra digits were
required.

When the Bell system was broken apart from the AT&T network, then equal
access was allowed. Prior to that, AT&T controlled the network and made sure
that equal access would not be a reality, or they priced the equal-access
connectivity so hight that no vendor could afford it.

Now that things are equal in the eyes of carriers such as MCI, Sprint, LDDS
Worldcom, etc. They can all be connected to the trunk side of the system. Now
caller ID information, called automatic idenfication of outward dialed or
automatic number identification (ANI) and many other names is passed on to
any carrier that is connected to the local or toll switches on the trunk
side of the network. Calls are passed from switch to switch, routed
through the network to and end point before termination takes place. The
world is a better place for this. MCI and its peers are now all able to
offer the same limited dial sequence that AT&T has always enjoyed. Now
they get even better access to the systems and are offered services called
feature groups, allowing for flate rate billing, call screening, and
multiplexed services on high speed trunks. This makes them as attractive
as any of the long distance services that they were competing with in the
past years.

Conclusion:
----------

This concludes my article. I hope you had fun reading. Cya next time :)
