being sent. Notifications on the status of the request are sent back
to the telephone in the form of tones. Indication from a Central
Office that a call is being offered to a telephone is arranged by
sending an alternating voltage down the access connection which in
turn causes the ringer in the telephone to sound. These access lines
have a unique address associated with them and can support a single
call.
However, with analogue or digital multi-line connections, or
Integrated Service Digital Network (ISDN) Basic or Primary Rate
Interfaces (BRI or PRI), several concurrent calls are possible and a
set of addresses are associated with them. The new ISDN access
connections are designed so that data exchanged with the network is
in multiplexed digital form, and there is an individual channel for
each of the potential connections, together with a separate channel
dedicated to sending and receiving call request and call alert data
as well as carrying packet switched user data. These call request and
call alert messages act as the equivalent of the pulses or tones that
are sent when dialing, and the ringing signal that is sent to a
telephone when a call is being made to it.
The operation of the call request is fairly simple in most cases and
is shown in Figure 17.
/--\
() ()
--____
/++\ \ ................................. /--\
/----\ \ ^ v ! () ()
A \ [-!-] [-!-] ! --
\->[CO ]=========[CO ]==\\ v ->-/ \
[---] [---] \\ [-!-] / /----\
\\=======[CO ]____/ B
[---]
Key: ___ Access Lines
=== Trunk Links (inter-CO user data links)
... Inter-CO signaling network links
CO Central Office (Telephone Exchange)
Figure 17
The user presses a sequence of numbers on a telephone handset
(labeled A), and the telephone passes a sequence of digits (either as
pulses or tone pairs) to the Central Office via the access line. The
Central Office contains a processor that will be notified that the
user has made a request and the digit string that is the sole
parameter of the request. This digit string is taken to be the unique
address of an access line connected either to itself or to another
Central Office. There is a hierarchical addressing scheme, so that
the digit string can be parsed easily. A call request to a terminal
(labeled B) connected to a remote Central Office can be routed by
examining the digit string passed; the Central Office will extract
the part of the passed address that corresponds to the remote Central
Office in question, and can route the request onward, forming an
inter-Switch call request and passing it via the signaling network.
At the same time it will allocate one of its available transmission
channels towards the remote Central Office.
11.1.2 Intelligent Network
This scheme has been used since the 1950s, and suffices for the
majority of calls. However, there are a range of other services that
can be (and have been) provided, enhancing this basic call
processing. Freephone or Premium Rate services (1-800 or 1-900
services) are good examples of the supplementary services that have
been introduced. Apart from the important feature that the cost of
these calls is varied so that the caller does not pay for a free-
phone call, or pays an extra charge for a premium rate call, they
have the similarity that the number dialed must be translated to
arrive at the "real" address of the destination terminal. They are
known as number translation services, and make up the bulk of all
supplementary services delivered today.
These were originally programmed into each Central Office, but the
complexity of maintaining the data tables on each processor grew
cumbersome, so a more general solution was sought. After a
considerable gestation period, the eventual solution was the
Intelligent Network. This takes the separation of Central Offices and
the network links interconnecting them a stage further.
The Central Offices are considered to provide the Call Control
Function (CCF). In addition, the Service Switching Function (SSF) is
provided to "enhance" the operation of these Switches by detecting
when a particular request has been made (such as by dialing 1-800).
If this pattern is detected, the equipment implementing the SSF will
send a specialized request message over the signaling network to a
separate computer that implements the Service Control Function (SCF).
This entity is responsible for querying service specific data (held
in a unit providing the Service Data Function, or SDF), performing
any digit translations necessary, and sending the details of how to
proceed back to the SSF, where they are obeyed and the call is put
through to the "real" destination. In many implementations, the SDF
is closely coupled to the SCF. This configuration is shown in Figure
18.
[---] [---] [---]
/--\ [SRF] [SCF] [SDF]
()/\()__ [|-!] [-!-] [-!-]
/__\ \ || \.............!......!........
\ || / ! ! /--\
__ \ [|-!] [-!-] ! ()/\()
\ \ \__[SSF] [CCF] ! ___/__\
[Fax]________[CCF]=========[---]==\\ [!--] / __
\\========[CCF]__/ \ \
[---]_______[Fax]
Key: ___ access relationship
=== trunk relationship
... signaling relationship
Figure 18
The advantage is that there can be a much smaller number of physical
units dedicated to the SCF, and as they are connected to the
signaling network they can be contacted by, and can send instructions
back to, all of the units providing the SSF and thus the CCF.
In another enhancement, a separate entity called the Special Resource
Function (SRF) was defined. Equipment implementing this function
includes announcement units to play recorded messages (for example,
prompts to enter digits) to callers. It will also include the tone
decoders needed to capture any digits pressed by the caller in
response to the prompts. It is connected to the rest of the PSTN
usually via trunk data links. It will also include a signaling
connection (directly or indirectly) back to the SCF, via the PSTN's
core signaling network.
As an example of the way that these different functional entities
interact, the SCF can ask an SSF handling a call to route the caller
temporarily through to an SRF. In response to instructions sent to it
from the SCF over the signaling network, the SRF can play
announcements and can collect digits that the user presses on their
terminal in response to prompts they are played. Once these digits
have been collected they can be passed on to the SCF via a signaling
message for further processing. In normal operation, the SCF would
then ask the SSF to dissolve the temporary connection between the
user's terminal and the SRF. This allows the collection of account
numbers or passwords (or PINs) and forms the heart of many "Calling
Card" services.
This pattern of user interaction is also used in a wide variety of
other services where extra account information and PINs are needed.
They are collected as just described and can be checked against the
correct values stored in the service database prior to allowing the
call to proceed.
The Intelligent Network functional entities can be realized as
physical units in a number of different combinations. A common
configuration is shown in Figure 19.
[---] [---] [---] [---]
/--\ [I.P] [SCP] [SDP] [SN ]
()/\()__ [|-!] [-!-] [-!-] [--|]
/__\ \ || \.............!.....!..... |
\ || / ! \ | /--\
__ \ [|-!] [-!-] \ | ()/\()
\ \ \__[SSP]=========[CO ]==\\ \ | ___/__\
[Fax]________[---] [---] \\ [!-|] / __
\\=======[CO ]__/ \ \
[---]_______[Fax]
Key: ___ Access Lines
=== Trunk Links (inter-CO user data links)
... Inter-CO signaling network links
SSP Service Switching Point - a unit that implements the
Service Switching Function
CCP Call Control Point - a unit that performs call control
functions.
This is normally a kind of Central Office (shown as CO
above)
SCP Service Control Point - a unit implementing the Service
Control Function. NOTE that this is connected to the SS7
Network and uses this connection for all of its
communications.
I.P Intelligent Peripheral - a unit that contains specialized
resources (like announcement units, tone decoders).
In effect, it implements Special Resource Functions.
SN Service Node
Figure 19
This diagram also shows a unit called a Service Node, or SN. This
contains components that realize all of the operational Intelligent
Network functions (SSF, SCF, SDF, and SRF). It is sometimes more
convenient to have all of these elements in one node (for example,
for operations and maintenance reasons), particularly within smaller
PSTNs or where there is a relatively low level of requests for
particular services. Another difference is that, as they are all co-
located, proprietary protocols can be used for internal
communication, rather than the full Intelligent Network Application
Part (INAP) protocol used over the core signaling network between
discrete units. It also differs from the "unbundled" approach in that
it is connected to the COs within a PSTN as a peripheral, having only
an access connection to a Central Office; there is no connection to
the core signaling network. Other than this, it operates in a similar
way, and can provide the same kinds of services. Information on the
specification of the Intelligent Network can be found in the ITU
recommendations [1], while two books ([2] and [3]) describe the
system, its history, operation, and the philosophy behind it.
11.2 Call Center Features
A Call Center is a system that allows a company to be organized with
a group of similar individuals (agents), all of whom can either make
calls to, or take calls from, customers. The system distributes
incoming calls to the agents based on their availability and
automates the placement of outgoing calls, selecting an agent to
handle the call and routing the call to them only once the call
request has been made of the PSTN.
The incoming call distribution feature ("automatic call
distribution", or ACD) is usually coupled with a call queuing scheme.
In this scheme, the callers are connected temporarily with an
announcement unit that normally plays music. The calls are treated in
sequence so that (once the caller is at the front of the queue) the
ACD system selects the next available agent and routes the call
through to them.
Another feature connects a customer making an incoming call to a unit
that asks them for some information on the purpose of their call,
selecting the agent to handle the call based on the particular area
of expertise needed; to do this, the agents are further categorized
by their knowledge (or "Skill Set"). If this skill set categorization
is used then by implication there will be separate queues for each of
the skill sets. This user selection scheme can be used independently
of the others. For example these so-called "voice navigation systems"
can be used to select a particular department extension number, based
on the function required by the customer; as such, they can automate
the job of company telephone receptionist in routing incoming calls.
Where possible, the information gleaned from the customer can be
provided to the selected agent, usually via a separate networked
computer connection. Similarly, if an outgoing call is being made to
one of a list of customers, information on the customer and the
purpose of the call can be provided to the agent selected to handle
the call. Such configurations are generally called "Computer
Telephony Integration" or CTI systems. Strictly, a CTI system can be
arranged to handle routing of incoming calls and automation of
outgoing calls only (also known as computer integrated telephony
features), without the agents having access to a network of
computers. However, the business case for combining the telephony
functions of the call center with provision to the agents of
computers with customer information can be compelling.
This is often further combined with a company's order and service
processing computer system. In this case, a call is treated as part
of a business transaction, with the information to be exchanged
captured as fields of a computer form. While such a computer system
is not, strictly, part of a call center, integrating the company
computer system with the call center is very common. This allows the
details of the call to be stored on a centralized database, allowing
further automated order processing, for example. It also allows the
call to be transferred from one agent to another where needed,
ensuring that the new agent has the information already captured.
This might be useful if someone with a different area of expertise
were to be needed to handle the customer's requirements.
Traditionally, Call Centers have been used to support teams of agents
working at a single site (or a small number of sites, with private
telephony trunks interconnecting them). The site Private Automatic
Branch eXchange (PABX) was integrated with a computer system to
provide these features to people at that site. There can be a
business case for provision of such features to distributed teams of
workers as well. In particular, the possibility of providing support
for people working from home has been seen as important. Some of the
Call Center features have been incorporated into public telephone
exchanges or Central Offices (COs) from many manufacturers as part of
their "Centrex" service offerings.
There are practical limitations in providing such features on COs.
Apart from the procedures needed to configure these features for any
telephone line that is to use them, the basic requirement that every
agent must have a connection to the supporting CO can limit its
usefulness. Another approach is to provide Call Center features via
the Intelligent Network. The features might thus be provided over a
Telephone Operator's entire network, and would mean that the Call
Center could be configured centrally while still allowing agents to
be located anywhere within the telephone network. It also means that
the supported company can pay for the Call Center features "as they
go" rather than having a high "up front" cost.
12. References
[1] ITU-T Q.12xx Recommendation Series, Geneva, 1995.
[2] I. Faynberg, L. R. Gabuzda, M. P. Kaplan, and N. J. Shah, "The
Intelligent Network Standards, their Application to Services",
McGraw-Hill, 1996.
[3] T. Magedanz and R. Popesku-Zeletin, "Intelligent Networks: Basic
Technology, Standards and Evolution", Intl. Thomson Computer
Press, 1996.
[4] Information processing systems - Open Systems Interconnection -
Specification of Abstract Syntax Notation One (ASN.1),
International Organization for Standardization, International
Standard 8824, December, 1987.
[5] McCloghrie, K., Editor, "Structure of Management Information for
Version 2 of the Simple Network Management Protocol (SNMPv2)",
RFC1902, January 1996.
[6] Kristol, D. and L. Montulli, "HTTP State Management Mechanism",
RFC2109, February 1997.
[7] Zimmerman, D., "The Finger User Information Protocol", RFC1288
December 1991.
Authors' Addresses
Steve Bellovin
AT&T Labs
Room E-215
180 Park Ave. Bldg. 103
Florham Park, NJ 07932-0000
USA
Phone: +1 973 360 8656
Fax: +1 973 360 8077
EMail: smb@research.att.com
Fred M. Burg
AT&T Labs
Room 1N-117
307 Middletown Lincroft Road
Lincroft, NJ 07738
USA
Phone: +1 732 576 4322
Fax: +1 732 576 4317
EMail: fburg@hogpb.att.com
Lawrence Conroy
Roke Manor Research Limited
IT&N-INIA Group
Roke Manor, Old Salisbury Lane,
Romsey, Hampshire SO51 0ZN
U.K.
Phone: +44 1794 833666
Fax: +44 1794 833434
EMail: lwc@roke.co.uk
Paul Davidson
Nortel
P.O.Box 3511 Station "C"
Mail Stop 242
Ottawa, Ontario, Canada K1Y 4H7
Phone: +1 613 763 4234
EMail: pauldav@nortel.ca
A. DeSimone
Lucent Technologies
Room 6H510
600-700 Mountain Avenue
Murray Hill, NJ 07974-0636
USA
Phone: +1 908 582 2382
Fax: +1 908 582 1086
E-Mail:tds@lucent.com
Murali Krishnaswamy
Bell Laboratories
Lucent Technologies
Room 2G-527a
101 Crawfords Corner Road
Holmdel, NJ 07733-3030
USA
Phone: +1 732 949 3611
Fax: +1 732 949 3210
EMail: murali@bell-labs.com
Hui-Lan Lu
Bell Laboratories
Lucent Technologies
Room 4K-309
101 Crawfords Corner Road
Holmdel, NJ 07733-3030
USA
Phone: +1 732 949 0321
Fax: +1 732 949 1196
EMail: hui-lan.lu@bell-labs.com
Henning Schulzrinne
Dept. of Computer Science
Columbia University
New York, NY 10027
USA
Phone: +1 212 939 7042 (@Bell Labs: 732 949 8344)
Fax: +1 212 666 0140
EMail: schulzrinne@cs.columbia.edu
Kamlesh T. Tewani
AT&T Labs
Room 1K-334
101, Crawfords Corner Rd.
Holmdel, NJ 07733
USA
Phone: +1 732 949 5369
Fax: +1 732 949 8569
EMail: tewani@att.com
Kumar Vishwanathan
Isochrone
EMail: kumar@isochrone.com
Full Copyright Statement
Copyright (C) The Internet Society (1998). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.