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DESCRIPTION
This invention relates to the transmission of data over the public switched
network, and more particularly to improved credit card authorization
transactions.
The traditional approach to credit card authorization transactions is to
provide a service establishment, such as a store, with a special
authorization terminal equipped with an internal asynchronous modem which
is connected to a local telephone line. When a credit card is moved
through a slot on the terminal past a magnetic stripe reader, information
is collected from the magnetic stripe of the card. The terminal then goes
off-hook on the telephone line and dials a number previously programmed
into the terminal. Equipment, including a modem, at the called site
answers the call. The answering modem may be part of a Value Added Network
(VAN), such as those whose services are provided under the service marks
Compuserve and Tymnet, or part of the credit card company's own private
network. Typically, a number of such modems, all terminating respective
lines which are called when a credit card authorization transaction is
required, are connected to a Packet Assembler/Disassembler (PAD) or other
multiplexer or concentrator which may be connected via a leased line or
network to a host computer at a central location. After the call is
answered, data communication is established. The data transmission for
dial-up credit card authorizations is most often governed by the Visa
protocol. Information from the magnetic stripe, information previously
programmed into the terminal, and information keyed into the terminal by
the merchant about the specific sale are transmitted up-line to the host
where the request is processed and an authorization code or other
information (e.g., a request to call for voice authorization) is
transmitted back to the terminal. Upon receiving the response, the
terminal goes on-hook and the call is terminated. Many terminals can be
programmed to dial different numbers based upon the information contained
in the magnetic stripe on the card being processed.
The most common access provided by a VAN is a local telephone number which
is then pre-programmed into the terminal by the institution providing the
authorization service. The VAN sets up distinct telephone line groups,
each with its own telephone number, for each major customer. This is done
in each of the major cities. It is not necessary for a VAN to provide a
group of lines in every central office. It is possible in a particular
city for the call from a service establishment to go over trunk lines
between central offices to a particular central office at which the VAN
has lines. However, the local calling area for local lines typically does
not cover an entire city, and most major cities may require multiple
installations, with different groups of terminals in the same city thus
having to dial different numbers. Suburbs, rural communities, smaller
cities, and other areas in which the traffic cannot justify special
installations of the type described must be serviced by having the
terminal dial an 800 number, the VAN having appropriate equipment at a
more central site for handling incoming calls. In cities with measured
rate local service, a typical credit card authorization transaction costs
the service establishment five to seven cents for the local call to a line
at the VAN's equipment location. (A credit card authorization may require
only several seconds to execute, but there is a minimum charge for a local
call.) The VAN typically charges the credit card company about six cents
for processing the transaction (plus an additional fifteen cents or so
that it must pay for 800 service if the incoming call is to an 800 number
which the VAN is using.)
There are at least four problems with this arrangement. First, the charges,
especially the five to seven cents paid by the service establishment for a
call which lasts only several seconds, are excessive. Second, some credit
card companies, such as American Express, would prefer to absorb the
local-call charges incurred by the service establishment, if they could be
made reasonable. (There are some cases even now for which the credit card
company absorbs all transaction authorization costs--but the charges are
even higher than they would otherwise have to be. To avoid having the
service establishment charged for a local call, the service establishment
dials an 800 number, and the VAN is thus able to pass along to the credit
card company all of the costs incurred--including the extra charges for
800 service. All of this is required, including the technologically
unnecessary 800-call charges, just so that the service establishment will
not be billed for a local call.) Third, because the service establishments
in each major city have to have terminals which dial a local telephone
number which is unique to that city, or unique to a local calling area
within that city, corresponding to the VAN's lines in that city or local
calling area, the credit card company must program terminals differently
for each major city or other territory associated with a specific number.
It would obviously make things much easier if the same telephone number
could be used throughout the country. Fourth, the geographical area served
by local lines is often too small to justify installation of equipment,
thus forcing the credit card company to use the more expensive 800 access.
This in turn means that a smaller percentage of transactions are
authorized in order to keep costs to a minimum.
There are other problems with this kind of standard network approach, but
there also has been some progress made in connection with these other
problems. Perhaps the area of most concern is the speed of a transaction.
In order to speed up the handling of calls placed by service
establishments, American Express, as just one example, provided its own
VAN type network some time ago. Modems and PADs were placed on local lines
in different cities, and the PADs communicated over leased lines to an
authorization host using the X.25 protocol. The PADs were equipped to
support the Visa polling protocol; a PAD polled the terminals, and then
communicated rapidly with the host using the X.25 protocol.
To further speed up a transaction, the American Express modems were
provided with ground start interfaces. A call incoming from a service
establishment could actually be answered by the modem before it rang. A
call arriving on a ground start line is "announced" by the central office
placing a ground on the tip side of the line, the tip normally being open
when the line is not in use. The grounding of the tip is followed shortly
by ringing, but it is often possible to answer an incoming call before
ringing is even detected by the called modem. (The ground start technique
was originally used to resolve the long-standing problem of "collision"
where a party on a PBX dials a 9 to get an outgoing line and instead ends
up with an incoming call because the PBX equipment was unable to recognize
from the ground start signal on the line that it had ben seized by the
central office.) Another way in which the transaction time was reduced was
to start the modem "training" period, by which the two modems at the ends
of the line get into sync, even before the two-second billing protection
interval had expired. (The FCC requires that data transmission begin only
after an initial two-second interval, but there is nothing in the FCC
regulations which prohibit early start of the sync process.)
These improvements, in use for more than a year and thus part of the prior
art, illustrate the kinds of things which were done to improve the overall
credit authorization process, but they have nothing to do with the subject
invention and the solutions to the problems enumerated above (other than
that the subject invention also utilizes modems, and toward that end it is
advantageous to use the fastest modems available).
The solutions to the problems discussed above are predicated on some of the
peculiarities in telephone service which resulted from break-up of the
Bell System. To understand the way in which advantage is taken of the
telephone scheme which now exists in the United States, it is necessary to
first describe that scheme, something which is difficult to do without
reference to drawings. Suffice it to say in this general description, and
in the context of a credit card authorization network, that the invention
contemplates a credit card company, such as American Express, becoming an
Interexchange Customer (IC) subscribing to Switched Access Services, with
each of its modems, situated at the location of one of its access
facilities, having an associated dedicated circuit for interfacing with a
respective Feature Group B or D trunk accessed by a terminal calling
through an Exchange Carrier, with each modem thus interfacing with a trunk
facility (as opposed to the usual line) and communicating through a PAD or
the like over a communication channel to a cost.
Further objects, features and advantages of my invention will become
apparent upon consideration of the following detailed description in
conjunction with the drawing, in which:
FIG. 1 depicts the public telephone switched network as it existed before
the Bell System break-up;
FIG. 2 depicts the public telephone switched network as it existed before
the Bell System break-up and after the entry of other common carriers;
FIG. 3 depicts a typical present-day Local Access and Transport Area
(LATA);
FIG. 4 shows a prior art credit card authorization scheme;
FIG. 5 shows how a service establishment can be relieved from paying for a
local call during a credit card authorization transaction;
FIG. 6 depicts an arrangement which, while not an embodiment of the
invention, will serve as a bridge to an understanding of the invention;
FIGS. 7A and 7B depict two approaches to setting up a credit card
authorization system, those of the prior art and the invention;
FIG. 8 is a functional representation of the manner in which standard
system blocks can be modified to implement the invention, and thus
represents the preferred embodiment of the invention;
FIG. 9 is a chart which will facilitate an understanding of FIG. 8; and
FIG. 10 is a flow chart which depicts operation of protocol logic block 92
of FIG. 8.
The public telephone switched network, as it existed before the Bell System
break-up, is shown in grossly simplified form in FIG. 1. Each central
office 10 provided service over telephone lines 14 to telephone equipments
12. Central offices were connected to each other by trunks 16. Throughout
the drawings, trunks are distinguished from telephone lines by the use of
heavy lines. Each central office included a switch 8 for effecting
connections between lines and lines, and lines and trunks. (Other
switching facilities were provided for effecting trunk-trunk
interconnections.) In general, a trunk is a communication path in a
network which connects two switching systems. A trunk circuit, associated
with the connection of a trunk to a switching system, serves to convert
between the signal formats used internally in the switching system and
those used in the transmission circuit, and it performs logic and
sometimes memory functions associated with supervision. A line, on the
other hand, is a pair of wires carrying direct current between a central
office and a customer's terminal; a line-side connection is a connection
of central office equipment to a line.
As the public switched telephone network grew over the years, numerous
interfaces and protocols developed and became standard. An interface is a
shared boundary defined by common physical interconnection
characteristics, signal characteristics, and meanings of interchanged
signals. (In telephony, the term "signaling" means the transmission of
information to establish, monitor, or release connections and provide
network control.) Lines interfaced to central offices in ways which were
distinct from those in which trunks interfaced to central offices. The
physical connections were different, and even the number of wires could be
different. Certainly, the signalings were different. A protocol consists
of procedures for communication between a sender and a receiver, of
supervisory and address information, in order to establish and maintain a
communications path. It is an agreed-to set of procedures so that
communications between two ends will be intelligible in both directions
(transmit and receive).
As the years went by and AT&T faced competition from other common carriers
(OCCs), the public telephone switched network developed as shown, once
again grossly simplified, in FIG. 2. Bell System central offices were
still connected by trunks 16 to other central offices. An OCC such as MCI
or Sprint would provide switches 18 which could be accessed from central
office lines. The OCC switches would be interconnected via their own
network trunks 24. In order to gain access to a common carrier other than
AT&T, a telephone subscriber would be connected over a telephone line to
his central office, through his central office and perhaps others, and
finally over another telephone line assigned to the OCC.
The quality of communication was generally not as good when going through
the facilities of an OCC. This was the case even though the trunks
actually used by the OCCs were leased from AT&T. The reason had to do with
the fact that poor performance is attributable most often to the inferior
transmission performance on line-side connections. A typical AT&T Bell
System call involved two lines 14, one at each end of the overall
communication path; all other interfaces along the way involved trunk
connections. An OCC, on the other hand, had an additional two line
interfaces, as shown by the numerals 20 and 22 in FIG. 2. Because each
central office was connected over lines, not trunks, to an OCC switch,
there were four line segments in each call, not just two.
In order that the common carriers other than AT&T be able to provide the
same superior service following break-up of the Bell System, and in order
that equal access to the local exchange users be given to all long
distance Interstate Carriers, several access arrangements were made
available. These access arrangements are known as Switched Access
Services. The term "Interexchange Customer" (IC) is used to denote any
subscriber of Switched Access Services, including an Interexchange
Carrier.
The geographic areas served by the Bell Operating Companies have been
divided into Local Access and Transport Areas (LATAs). A typical LATA is
shown in FIG. 3. A LATA is an area within which a Bell Operating
Company--an Exchange Carrier (EC)--may offer telecommunication services.
Interexchange Carriers and other ICs provide services between LATAs. The
Interexchange Carriers are, of course, AT&T, MCI, Sprint, and others. The
specific switched access arrangements offered by the Bell Operating
Companies are known as the Feature Groups. An End Office (EO) is a Bell
Operating Company switching system within a LATA where customer station
loops (lines) are terminated for purposes of interconnection to each other
and to trunks; a call may go directly from an End Office or be tandemed
through a second office known as an Access Tandem (AT) to reach the IC. (A
Tandem is a switching system in the message network that establishes
trunk-to-trunk connections). The important thing to note is that, as shown
in FIG. 3, telecommunications within a LATA are handled by an EC, whereas
telecommunications from one LATA to another are handled by an IC.
An IC under the present scheme designates a location within a LATA for the
connection of its facilities with those of the Bell Operating Company
which serves that LATA. (There are about two hundred LATAs in the United
States, and each of the Bell Operating Companies serves all or portions of
multiple LATAs.) The location of interconnection designated by the IC is
called a Point Of Presence (POP), and typically it is at a building that
houses an IC's switching system or facility node. An IC may have more than
one POP within a LATA. In FIG. 3 a POP is shown by the numeral 30. An
aggregate of one or more IC trunks is shown by the numeral 32. Instead of
the POP being connected to a single central office, what usually happens
is that it is connected by trunks to an Access Tandem (AT). The AT, shown
by the numeral 26 in FIG. 3, is in turn connected to multiple central
offices by means of trunks. An Access Tandem is a Bell Operating Company
switching system that provides a traffic concentration and distribution
function for inter-LATA traffic originating/terminating within a LATA. The
AT thus provides the IC with access to more than one End Office within the
LATA. (A central office is an End Office.)
For present purposes, what is important is that the EC offers the IC a
choice of four switched access arrangements, called Feature Groups. Each
IC, based on its own technical needs and business considerations, selects
the access arrangement that it wants. The access arrangement involves a
multiplicity of interfaces represented in FIG. 3 by the single trunk 28,
it being understood that the drawing is only symbolic and there are in
fact as many connections as there are maximum number of simultaneous calls
that the IC expects to handle.
Feature Group A is a two-wire line-side connection between the IC and the
EC. Feature Group A is not of particular interest because it is a
line-side connection; it will be recalled from the discussion of FIG. 2
that it is line-side connections, depicted by the numerals 20 and 22, that
put a long-distance carrier at a disadvantage in the first place prior to
the Bell System break-up. (Feature Group A is also the only one of the
four access arrangements for which the calling party is billed any local
tariffed charges, i.e., message units.)
The other three Feature Groups involve trunk-side connections.
Feature Group B has an associated universal 7-digit (950-0/1XXX) access
code and is used for the purpose of originating or terminating calls to or
from subscribers. The XXX code is unique to each IC and, most importantly,
it is the same throughout the country in all LATAs in which the IC has a
presence. Feature Group B access arrangements include trunk signaling,
trunk protocols, trunk transmission and trunk testing, and they provide
answer and disconnect supervision. There can be two-wire and four-wire
trunk terminating equipment, and, in general, there are supplemental
features (as there are in the other Feature Groups) which are offered that
allow an IC to specify substitutions for, or additions to, the standard
arrangements as defined by the appropriate tariff.
Feature Group C exists now but is transitional. AT&T, whose trunk-side
connections are presently Feature Group C, will convert to Feature Group D
as it becomes available. Feature Group B involves 2-stage dialing, the
kind of arrangement which existed before subscribers could select an IC
other than AT&T. With an arrangement such as that shown in FIG. 2, a first
number is dialed in order to gain access to a line which is connected to
the OCC facility. after a connection is established, a second number is
dialed to tell the OCC the destination of the call. Feature Group D, on
the hand, provides true "Equal Access" in that a customer can
pre-subscribe to the long-distance carrier of his choice. By dialing the
digit 1, his call will be connected to a Feature Group D trunk at the
selected IC's Point Of Presence. There is only one number dialed, that of
the destination. While the subject invention is certainly applicable to
Feature Group D service, the illustrative embodiment of the invention is
described in terms of Feature Group B service.
Once a caller gains access to an IC's facilities, any subsequent dialing
procedures are as specified by the IC. In-band tone dialing is usually
employed, but in any event the EC is transparent to address signaling and
data communications between the subscriber and the IC. As far as the
interface between the EC and the IC is concerned, the IC can specify the
type of supervisory signaling and interface to be used between the Bell
Operating Company access facilities and the IC facilities at the IC's
point of presence. The signaling options and interfaces that are available
vary with the particular Feature Group and tariff.
A prior art credit card authorization scheme is shown in FIG. 4. A credit
card authorization terminal 30 is connected via an ordinary telephone line
34 to a central office 10. The authorization terminal includes a standard
modem 32. At the beginning of the authorization process, the terminal
dials a number which is associated with a line connected to a particular
Value Added Network. The connection is established through central office
10. In the VAN 40, line 36 is connected to modem 38. Using the Visa
protocol, the two modems communicate with each other. Modem 38 is
connected via a digital interface to a Packet Assembler/Disassembler 42 or
some other multiplexer or concentrator within the VAN. The PAD establishes
communication with a host over leased line 44 or some other communication
channel.
As described above, the service establishment pays for a local call to the
VAN. The VAN charges the credit card company for handling the call. There
is no way to reduce the charges paid by the service establishment because
a local call through the central office is being placed. It is also
apparent that depending upon the location of the VAN in a particular city,
the authorization terminals must have different numbers pre-programmed in
their automatic dialers.
What is shown in FIG. 5 is the only way that a service establishment can be
relieved of paying for a local call. Also, the scheme of FIG. 5 must be
employed when there is no VAN presence near the credit card authorization
terminal. In this case the terminal establishes a call through the dial
network 46, and over line 34 and trunk 48 to an AT&T facility 50. The AT&T
lines are extended as an 800 call to modems in VAN 40. In this case the
service establishment does not pay for the call; AT&T pays the Bell
Operating Company. The credit card company picks up all charges of AT&T
and the VAN. The arrangement of FIG. 5 is hardly preferred because of the
cost of an 800 call.
The arrangement of FIG. 6 is not an embodiment of the invention. However,
it will serve as a bridge to an understanding of the invention; it is the
kind of system which might be devised by a "telephone man". Lines 14 are
connected to authorization terminals and they are within the LATA of the
Exchange Carrier. Some of the EC trunks, Feature Group B or D, are
extended to the Point Of Presence of an IC. A switch 54 is provided for
extending trunks 28 to their destinations, in this case lines connected to
modems and a PAD. Communications originate in the terminal, and a
terminating modem in the POP is required to communicate with the modem in
the terminal. Modems interface with lines, not trunks. A switch is the
standard mechanism for interconnecting lines and trunks. Since only trunks
come into the POP, a switch is necessary to connect an incoming trunk to a
modem line (just as the switch of a central office connects a trunk and a
subscriber line).
This straight-forward approach allows a terminal to be connected to the
host. The service establishment need not be billed for the call because
with Feature Group B or D service, the IC pays the EC for each call which
is placed. The IC is the only source of billing to the calling party, and
with switching equipment having sufficient intelligence, it would be
possible for the IC to bill the credit card company for calls placed to
its host installation. The scheme is not feasible, however, because of the
cost of switch 54. A typical modem costs in the order of $500. A switch
for 100 lines costs in the order of $200,000. That makes the cost per
modem not $500, but $2,500. That is impractical. And there is no apparent
way to avoid the use of the switch. There is a modem in each authorization
terminal. There must be a modem at the other end of the connection. Modems
have line connections. The EC/IC interface is over a Feature Group B or D
trunk. There is no way that a trunk can be interfaced with a conventional
modem. Not only may the number of wires in the trunk be different from the
number of wires at the modem input, but the signaling requirements are
totally different. A costly switch is the device which allows a modem line
to be connected to a trunk.
In accordance with the principles of my invention, when it is employed in a
network for authorizing credit card transactions, the credit card company,
such as American Express, is given its own 3-digit Customer Identification
Code (CIC). It becomes an Interexchange Customer. At its POP, it has
dedicated modems and a PAD or other multiplexer or concentrator. But the
switch is eliminated. Instead, the modem arrangement provides a trunk-side
interface. In the illustrative embodiment of the invention, E&M signaling
and T1 interfaces were selected It will be recalled that with Feature
Group B or D, the IC can tell the Bell Operating Company the kind of
signaling and interface that it desires on its trunk facilities within the
bounds of the applicable tariff. By providing each modem with a trunkside
interface, the cost of the modem increases from perhaps $500 to $700,
considerably less than $2,500.
By a credit card company such as American Express becoming an IC, all of
its authorization terminals, in all LATAs in which it has a presence, need
dial the same number. That number, 950-0/1XXX, where XXX is American
Express's CIC number, always gains access to a Feature Group B trunk of
the local EC, and that trunk appears at an American Express POP in the
respective LATA. American Express, as an IC, simply need not bill the
"subscribers" which access its trunks, i.e., the service establishments.
The IC in this case simply absorbs all costs. More significantly, the
overall communications charges are greatly reduced. Whereas in the prior
art network of FIG. 4 the local telephone message unit charge to a
subscriber was in the order of five to seven cents for each call, with
Feature Group B or D service the IC pays the Bell Operating Company
charges which are based on time. The charge for a typical credit card
authorization is between one and two cents. Thus while the credit card
company absorbs all of the transaction costs, the cost associated with the
local telephone call part of the transaction is reduced very
substantially. (Whether the other costs, paid to the VAN, in the prior art
system of FIG. 4 are reduced in the scheme of the invention depends
primarily on efficiencies of the credit card company operations.)
In order for the scheme to work, each modem of the invention requires a new
interface which meets Feature Group B or D specifications, and the modem
must be capable of exercising trunk protocols. This is brought out in
FIGS. 7A and 7B. The former depicts the prior art approach and corresponds
to the system of FIG. 6; the latter corresponds to the system of the
invention to be described below.
The numeral 60 depicts symbolically the entrance to the EC network; with
reference to FIG. 3, the numeral 60 would be the Access Tandem 26. The EC
establishes a trunk connection. To do this there must be a trunk hardware
interface, shown symbolically by solid lines in FIGS. 7A and 7B. The
Access Tandem exercises a trunk protocol, and protocols, as opposed to
hardware interfaces, are depicted symbolically by dashed lines in FIGS. 7A
and 7B. There is not necessarily only one trunk protocol which can be used
with each trunk hardware interface, although signaling limitations of
particular hardware interfaces necessarily restrict the number of
applicable trunk protocols.
The typical modem 64 exercises a line protocol and it is provided with a
line hardware interface, as shown in FIG. 7A. In order to convert between
a trunk over which a trunk protocol is executed and a line over which a
line protocol is executed, traditional telephone practice would require
the use of a switch 62 by the IC, corresponding to switch 54 in FIG. 6.
The switch could have trunk hardware interfaces at the trunk side and
would execute a trunk protocol for communicating with the EC, and it would
have line hardware interface at the line side and would execute a line
protocol for communicating with the modem. As described above, it is the
cost of the switch which is the stumbling block. (FIG. 7A in fact depicts
present-day 800 service, and the similarity to FIG. 5 will be apparent.)
FIG. 7B depicts the approach taken in the invention. Nothing corresponding
to IC switch 62 is used. The EC provides Feature Group B service on T1
access facilities. A conventional T1 trunk hardware interface is used, and
a conventional E&M trunk protocol is employed. The particular interface
and protocol are not arbitrary and offer great advantages, as will be
described. At the other end of the T1 link is a conventional channel bank.
The T1 link has 24 channels. A channel bank, such as the standard Rockwell
D3/D4 channel bank, shown by the numeral 66 in FIG. 7B, includes an
individual card for each of the 24 channels in a T1 digital facility. The
channel bank thus includes a T1 trunk hardware interface. As will be
described, the channel bank is provided with an FXS interface card which,
in the illustrative embodiment of the invention, is optioned for ground
start. The modification to the channel interface card which is required in
order to implement the preferred embodiment of the invention will be
described below. The channel interface card and the modem have ground
start interfaces. The modem is designed to execute a trunk protocol, the
second modification to a standard system block which will be described
below. The net result is that at the two ends of the transmission path, at
the EC and the modem, a trunk protocol is executed, and thus the two ends
can communicate with each other. This is accomplished without the use of
an intervening IC switch.
FIG. 8 is a functional diagram showing how the preferred embodiment of the
invention is constructed from standard system blocks with only minor
modifications. The code at the bottom of FIG. 8, which represents the
symbolism used in the drawing for transmission, address and control
signaling, and physical interface signaling, allows symbolic
representation of a T1 digital facility. The facility provides for 24
individual time division multiplex channels with associated framing bits.
Each 64-kb channel has two low-speed bit streams for transmit and receive
signaling. The bits are referred to as transmit and receive A and B bits.
Thus there are up to four possible signaling states which can be
transmitted in each direction.
A TI access facility is selected because it facilitates a physical
interface conversion, as will become apparent from the description of FIG.
8. Feature Group B service, when provided on a T1 link, is always
provisioned with the E&M supervisory signaling format, thereby indicating
that an E&M physical interface would be appropriate; this is a requirement
of the applicable tariff. The physical analog interface required by the
selected modem, however, is a ground start loop interface; a ground start
interface provides for the fastest possible operation since there is no
need for the modem to wait for incoming ringing. Separate and apart from
protocol considerations, some way must be found to make compatible the E&M
trunk supervisory signaling at one end and the ground start loop signaling
at the other end.
A conventional channel interface card performs two functions. The first is
to encode/decode digital transmission on an individual T1 channel
from/into an analog signal. The second function of the channel interface
card is to translate physical interface signals from/into the appropriate
supervisory signaling. The supervisory signaling for each channel consists
of the A and B bits referred to above. The appropriate supervisory
signaling format is a function of the physical interface supported on the
channel interface card. In normal practice, the same kind of interface
cards are arranged at both ends of each channel, in channel banks. In the
system of FIG. 7B, where the T1 digital encoding is done directly by an EC
switch at one end, the appropriate channel interface card which would
normally be used would be selected based on the signaling specified by the
switch interface. This insures that the supervisory signaling is the same
at both ends. (The standard supervisory signaling formats are defined in
product descriptions of various channel bank and switch manufacturers as
well as AT&T, Bellcore and other telephone company technical
documentation.) Because E&M supervisory signaling is used at the EC end of
the transmission path in FIG. 7B, it would appear that an E&M interface
would be required for the modem. There is no E&M channel bank card,
however, which is available for use in a channel bank and which could be
adapted readily to provide a ground start interface for the modem.
An alternative to E&M supervisory signaling is FXS/FXO. FXS and FXO cards
are normally installed at opposite ends of the same channel, the FXS being
provisioned at the remote end and interfacing to the station equipment,
and the FXO interfacing to the central office equipment. An FXS channel
interface card can be purchased with a ground start or a loop start
option. Since a ground start interface is desired for the modem, and that
cannot be obtained with an E&M channel bank card, an FXS channel interface
card is used, as indicated in FIG. 7B. But with an FXS signaling format
being used in the channel bank, the logic circuitry on the FXS card must
be modified such that ground start loop interface signals to/from the
modem are translated to/from the E&M supervisory signaling format. In
other words, the EC utilizes an E&M signaling format, thinking that an E&M
channel bank card is at the other end of the T1 link. In fact, what is
there is an FXS channel bank card (because it can be connected readily to
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