DOS/V Power Report 2004 March/VPR0403.ISO/talks/134/paper.dkb

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<article id="paper-134">
<articleinfo>
  <title>Voice telephony, the next peer-to-peer application?</title>
  <author>
    <firstname>Georg</firstname>
    <surname>Schwarz</surname>
  </author>
  <copyright>
    <year>2003</year>
    <holder>Georg Schwarz</holder>
  </copyright>
</articleinfo>


<section>
<title>A technical invention gives birth to a new industry</title>

<para>When in 1876 Alexander Graham Bell was granted a patent for the electrical
telephone he was not the first one, as we know today, to have come up with
such a device. Beyond doubt however, it was the historical achievement of that
enterprising American to clearly recognize the vast economic potential of that
invention, which soon formed the basis of a fast-growing, multi-billion dollar
industry.</para>

<para>Even though today's phones only quite remotely resemble the first models from
Bell's days, their basic technological principles of operation have remained
largely unchanged to date, and so has the core business model of telephone
companies. Today just like a hundred years ago telephone customers for a
monthly fee get connected to the operator's local switch and thus to the
telephone network. They are assigned a telephone number through which they can
be reached by other subscribers. When a call is placed it is the operator's
task to establish the callee's line corresponding to the dialed number, and to
set up a connection between the two parties. In the early days, this was done
over dedicated pairs of wires which were interconnected over the operator's
switchboards. Today, classical voice telephony typically uses TDM (time
division multiplexing) technology in the carriers' backbones. In any case
dedicated resources (wire pairs or fixed time slots) are temporarily assigned
exclusively to a connection for the duration of the call. Not surprising,
pricing models have come in use which charge callers by the length of a call,
with rates going up by distance to account for the longer "copper lines" being
occupied. The deployment of modern electronic switching technologies and large
bandwidth fiber networks as well as, probably most importantly, the fierce
competition in liberalized telecommunications markets has helped to bring down
the price per minute of a long distance or international phone call often to
only fractions of what they used to cost in former decades. Nevertheless, the
principle of charging by the minute remains unchallenged, and due to the
increased volume of calls still accounts for a considerable part of the
telecom industry's revenues.</para>
</section>

<section>
<title>The telco business model</title>

<para>The basis of the telco business model is the exclusive access to and control
over the customer a telephone company obtains when it connects a subscriber to
its network. All incoming and outgoing connections to and from that customer
will necessarily be established through the local switch and can thus be
recorded and potentially charged for by the telco. When a subscriber dials a
number it is the telephone company's switching equipment that performs the
task of setting up the connection to the called destination. Reachability of
subscribers on other operators' networks, for example in other countries, is
ensured by telephone companies forging interconnection agreements between each
other and taking care of handing over calls. Here as well the charging model
among operators follows the lines of "paying by the minute". For the end
customer, the subscriber, the local telco remains the only contractual partner
and service provider; it is the telco's role to offer voice services, set the
price tag and send the bill.</para>

<para>From the customer's perspective this rigid model has been softened in part by
liberalization of long distance markets. Subscribers in many countries for
their telephone calls can select other providers than their local telco and
make calls at these companies' tariffs. This new freedom of choice however is
not brought about by a change in technology but by incumbents being forced
through legislation to hand over calls to competitors at conditions and prices
defined by regulatory authorities.</para>

<para>Particularly in Germany liberalization has led to an unparalleled decline of
the price per minute of a long distance call. In spite of the increased
overall call volume these changes have resulted in a considerable reduction of
revenues from the voice minute business for ex-monopolist Deutsche Telekom.
Markets in many other countries have seen or will see a similar, albeit often
less dramatical development. The basic telco business model however remains
untouched. The new entrants, enabled by regulation, simply take away some of
the market share from the incumbents, but they operate at the same technology
and the same business principles. Most importantly, it is the operator of the
"last mile" that continues to ensure reachability of the subscriber under his
or her telephone number and that determines, within the framework set by
regulation, which services customers can use.</para>

<para>That "ownership of the customer" is a direct result of the technical voice
telephony network architecture; and it is that fact which enables operators to
control or at least participate in revenues from non-voice transport related
"value added" services such as directory assistance or other "premium
services". The operator of the last mile draws its dominating position within
the telephony value chain from its technical monopoly on the local call setup
and termination service. This holds true for all telephone networks, analog
(POTS) or digital (ISDN), fixed or mobile (GSM, etc.).</para>
</section>

<section>
<title>The success of the Internet</title>

<para>With the advent of the Internet in the 1980s and, driven by the success of the
World Wide Web, increasingly in the 1990s for the first time in history a
world-wide communications infrastructure emerged whose philosophy and whose
operational and technological concepts do not follow the approach of a
"carrier-centric network". Traditional communications networks such as the
world-wide telephone network (including mobile networks) or the telex network
(or to a certain extend even radio and television broadcast networks) have
been set up by one or several operators with the aim of connecting subscribers
and offering them certain communications services. Consequently, these
networks are designed along the lines of the carrier-centric network
architecture model. They are typically architectured to offer a specific
service only, e.g. voice telephony. End user devices (e.g. telephones) have a
network interface and protocol with rather limited capabilities. They must be
capable of selecting the service offered by the network (i.e. dial a
destination number or pick up an incoming call) as well as some means of using
that service. The service creation is always done by the provider, i.e. the
network.</para>

<para>In contrast, the Internet is based on a completely different approach.
Conceived as a robust means of interconnecting computer systems over large
distances and enabling the remote use of these computing resources, it
followed the approach of the "simple" network. With TCP/IP, the network
protocol the Internet is built on, the "intelligence", e.g. error correction,
the resolution of network addresses, all "higher level" services, in short
almost the entire service creation occurs on the hosts connected to the
network. One basic design goal of TCP/IP and a fundamental assumption of the
Internet is that every host connected to the net can communicate with any
other host. Likewise, there is no intrinsic limitation or preference from the
network architecture which services a host connected to the Internet can use,
or who will provide these services. Once a host has Internet connectivity, it
is (ignoring for the moment artificial, deliberate restrictions through
firewalls etc.) up to the operator of the host, not of the network
infrastructure, which other hosts (servers) to connect to for whatever
services and in turn which services to offer to whatever other hosts on the
net. In this respect, the Internet's design as a simple, universal network is
fundamentally opposed to the "operator-centric", special-purpose network
approach, where service creation occurs within the network and is provided and
controlled by the network operator. It is undoubtedly this property, which
enables new Internet services to be deployed by practically anyone without the
network having to be upgraded first, that has greatly contributed to the
tremendous success the Net has seen over the past decades.</para>

<para>From a traditional telecommunications network provider's point of view, the
Internet is a commercial accident, if not a nightmare, since it does away with
the operator's monopoly or even preferred position on service creation.
Successful online businesses such as Ebay, for example, need not share their
revenues with local ISPs or telcos as it would have been the case in a
carrier-centric network architecture (for example Deutsche Telekom's BTX
service from the 1980s and early 1990s or similar "video text" systems). With
its roots as a research network there has never been such a thing as a
"business case for the Internet"; otherwise the Net would most likely not look
the way it does, if it existed at all.</para>
</section>

<section>
<title>The impact of IP on voice telephony</title>

<para>The success of the Internet in the 1990s led to the establishment on IP as the
universal, ubiquitous communications protocol. With the availability of
suitable devices, e.g. workstations or personal computers with audio support,
and sufficient bandwidth between Internet sites such as universities, people
pursued the idea of using these communication lines for voice (and later
video) conversations. Soon a number of applications and protocols were
developed, most of them more or less incompatible with each other. When the
popularity of the Internet rose as more and more people got "online" and the
typical dialup bandwidth increased due to better and cheaper modem technology
"Internet telephony" found its way into the commercial software and service
provider market. The motivation was and still is to use the Internet as a
means of toll bypassing, especially in countries with high long distance and
international telephone rates due to limited competition in that market.</para>

<para>In the mid 1990s Internet telephony was sometimes portrayed as the imminent
death to telephone operators. That scenario did not materialize for a number
of reasons: Thanks to liberalization and the abundance of available fiber
capacities prices for long distance and even international telephone calls
have dropped sharply in many markets reducing the financial incentive of toll
bypassing. Secondly, until only very recently, almost all end users have been
using dialup connections for Internet access, preventing most of them from
having a permanent online connection that would have given then reliable
reachability for incoming Internet telephony calls. Also, while narrow-band
Internet access in principle is sufficient for good quality voice
communications over IP it requires some prioritization mechanism which are not
available in today's Internet. Thus Internet telephony was quickly dismissed
as unreliable and qualitatively inferior. Last, the lack of standardized
Internet telephony protocols that would have ensured interoperability between
various applications as well as the technical problems of determining one's
intended conversation partner's availability and location on the Internet
greatly limited their usefulness. Consequently, so far, for end customers such
applications are a niche product at best.</para>

<para>Nevertheless, Voice over IP (VoIP), i.e. the technology of transmitting voice
conversations over an IP network (not necessarily the Internet), quietly
matured over the years. While originally the notion of saving call charges
prevailed, later the focus of VoIP deployment shifted towards reducing
operational cost by replacing proprietary and thus expensive to maintain
classical telephone systems and, since data application and networks had
become an essential part of enterprises, by doing away with the need for
separate on-site telephony network infrastructure and support staff.
Standardization efforts for VoIP led to the development of the H.323 family of
protocols by the ITU and somewhat later SIP (Session Initiation Protocol) by
the IETF (Internet Engineering Task Force). While both protocols are similar
in their basic approach towards VoIP by separating call setup between
terminals (e.g. IP telephones) from the actual exchange of media (e.g. an
audio stream of a conversation) most people will probably agree that although
H.323 due to its earlier start today still enjoys the bigger market
penetration, SIP, thanks to its simple yet elegant, "Internet-like" design
will ultimately prevail.</para>

<para>The emergence of these open VoIP standards has created the opportunity for new
entrants to the previously highly proprietary telephony equipment market (and
likewise poses a severe long-term threat for vendors of traditional telephony
systems). As with other IP-based technologies, companies have realized the
potential of Open Source products (and in particular Linux) as a basis for
their development activities. Some of them, for example Cisco Systems with
their VOCAL project, have recognized the Open Source model as a means of
efficient and quality product development.</para>

<para>IP telephony equipment not only holds the promise of reducing investment and
maintenance costs but moreover, thanks to its open nature, greatly facilitates
the development of new services particularly in the field of Computer
Telephony Integration (CTI), for example the seamless integration of a call
center into an enterprise database and directory environment. In the domain of
voice carriers, the development of robust VoIP equipment together with the
tremendous growth of data traffic has led to the concept of the Next
Generation Network, or NGN for short. The idea of the NGN consists of replacing
the traditional TDM-based voice backbone of a carrier with a VoIP-based
backbone, ideally using a single IP network for both voice and data services.
In the latter scenario of a "converged" network voice telephony becomes just
another IP service, reversing the situation of the early days of data
communications when data connections were realized on top of voice networks.</para>

<para>Today, even incumbent carriers are seriously considering or are already
starting to deploy Next Generation Networks, often encouraged by vendors of IP
equipment who have realized their opportunity of extending their scope of
business. Proponents of NGN technology point out that it not only helps the
operators to reduce cost but enables the deployment of new services. For
consumers however, the transition to NGN first of all does not bring about any
significant change as long as customer network access and the end user
equipment remain traditional telephony technology. In other words, the mere
transition to NGN does not touch upon the traditional telco business model
since it does not affect ownership of the customer but rather provides
operators with a new, potentially more cost effective network technology. In
this respect, the introduction of NGN is somewhat comparable with the
replacement of analog switching technology by digital carrier voice networks.
It did enable a number of new services, but did not challenge the telephony
value chain.</para>

<para>A real change however is brought about the increasing availability of
broadband Internet access, typically via DSL or cable, for both enterprise and
residential customers. Access is typically charged by traffic volume (or
customers just pay a flat monthly fee), no longer by connection time. This
allows users to have permanent or quasi-permanent connectivity to the Internet
instead of "dialing up to the Net". Always-on access already did and will
continue to change the way people make use of the Internet.</para>

<para>Broadband Internet access eliminates many of the obstacles Internet telephony
has been struggling with in the past. Since it provides "always-on"
functionality it has the potential of delivering constant availability for
incoming communications requests just like a traditional telephone line.
Broadband access, together with fast, cheap personal computers boosts the use
of large-bandwidth applications, so ISPs are impelled to provide these
customers with adequate IP backbone capacity. As a result, the available
bandwidth is typically significantly larger than that required for an audio
stream. New popular real-time applications such as online gaming create
additional demand for high-quality IP connectivity. So even with the absence
of any strict quality of service mechanisms for IP broadband Internet access
typically allows for IP-based voice telephony of good or, with increasing
available bandwidth, even excellent quality.</para>

<para>This development provides companies with the opportunity of offering telephony
services over IP to broadband Internet subscribers, bypassing the local
telco's monopoly on the last mile. While it might seem natural for the local
ISP to come up with such a "value added" service offering, and in fact many
already do, there is no such strict technical requirement. Just as with any
other IP services, in principle anyone with adequate Internet connectivity can
offer them.</para>

<para>Companies like Vonage in the US have built their business around that idea.
They are providing their customers with a SIP-based VoIP terminal adapter to
be connected the residential IP (cable or DSL) router. Up to two conventional
telephones can be plugged into that adapter, making it a small IP-enabled home
PBX. Using any broadband Internet access (which Vonage does not provide) users
can make outgoing calls via Vonage's servers. Vonage provides a gateway to the
PSTN (i.e. the plain old telephone network) enabling their customers to use
their telephones just as if they were connected to a conventional phone line.
Moreover, Vonage provides conventional telephone numbers for their
subscribers, enabling them to receive incoming phone calls which are
terminated at their local SIP adapter. Ideally, users do not even realize that
they are using Voice over IP for their telephone calls. Vonage thus has
assumed exactly the role of the traditional telco, just making use of a
different access technology. Since the company is in control of their
subscribers' incoming and outgoing calls the traditional telecom business
model still applies, albeit with a new provider.</para>

<para>Other players will soon pursue a far more radical approach. With the ever-
increasing popularity of Internet-based messaging and online gaming it is only
a matter of time until high quality voice functionalities are integrated into
these applications or even new IP-enabled voice communications consumer
devices will appear on the market that have the potential of substituting
conventional voice telephony. Microsoft, for example, has started to offer a
voice communications kit for their Xbox gaming machine. It enables subscribers
of Microsoft's XBox Live online gaming service to make voice conversations
with one another over that platform. Besides, that company is busily
integrating telephony functionality into their Windows CE operating system.</para>
</section>

<section>
<title>Do consumers need a telephony service provider?</title>

<para>With growing proliferation of always-on broadband Internet access so-called
Integrated Access Devices (IADs) will increasingly appear on the market which
allow people to use their Internet access for voice telephony much like they
do today with the conventional telephone network. The interesting question
will be whether these devices will come bundled with specific service
offerings and whether customers will be forced to use specific platforms or
providers for their Internet-based telephone calls. Under the latter scenario
the dominating position within the telecommunications value chain would just
pass from the network operators to user device (or software) vendors.</para>

<para>The acceptance of any public communications network highly depends on the
number of communications partners users can reach over it. In the case of the
Internet the open, standardized nature of its communications protocols and its
liberal policies ensured success over closed, proprietary data communications
platforms. With the development of SIP (Session Initiation Protocol) in the
past years a powerful IETF standard for Internet-based messaging has been made
available for IP-enabled communications devices and applications. In some
respect, SIP extends the philosophy of electronic mail (i.e. Internet-based
non-real-time messaging) to all sort of real-time messaging, including but by
far not limited to voice telephony. Strictly speaking, SIP as a messaging
protocol deals with the setup and control of communication sessions only, not
with the actual media exchange (for the latter job, a stack of other suitable
protocols has been standardized). This approach helps to maintain a simple,
yet powerful protocol and ensures its far-ranging applicability. SIP's
addressing scheme is based on the proven, scalable, existing world-wide DNS
(domain name service) system. SIP addresses, which assume the role of the
telephone numbers from traditional telephony, thus bear a high degree of
similarity to email addresses, and since they are independent from the actual
communications media (voice, video, text, etc.) they can naturally provide the
basis for true unified messaging (UM).</para>

<para>SIP solves the problem of mobility and locating users that do not necessarily
have a fixed IP address by allowing them to temporarily register with a SIP
server (which can be located anywhere on the Net), much like connecting to a
POP3 or IMAP server to retrieve mail. The SIP server will then forward any
incoming connection requests for that user to the present location on the
Internet. Since only the session setup and control is handled by the SIP
server while the actual media stream is typically passed directly between the
two parties' computers or IP phones SIP servers require relatively little
resources. SIP services, just like web or email services can thus be offered
by anyone with Internet access and control over a DNS domain. Due to the
flexibility of the SIP protocol users can easily combine SIP services (e.g.
voice mail) from any provider. Whether providers will succeed in bundling
services and assuring customer loyalty becomes primarily an issue of
marketing, not of technology. Bare voice transport as simply yet another
comparatively low-bandwidth service in an open IP environment will be a
commodity for which consumers will no longer be willing to pay by the minute.</para>

<para>In short, SIP has the potential of turning the Internet into a universal, open
platform for real-time messaging. All it takes for broadband Internet users to
participate is a suitable SIP client (i.e. a software application running on
the user's PC or a messaging device such as an IP phone) and a SIP address and
corresponding SIP server to register with. The latter service can be expected
to be (and in fact already is) available for free or a very small monthly fee
from many providers; or users with their own DNS domain might even run their
own SIP server, ensuring their reachability. SIP makes voice telephony or
other types of real-type communications a peer-to-peer application,
essentially returning to the original aim of the Internet: enabling direct
communications between any of the systems connected to the network.</para>

<para>This implies the question whether there is still a role for a telephony
service provider. Even if the use of SIP addresses will gain significant
popularity for voice communications over the Internet it will probably be a
long way until they become an accepted mainstream form of communications. It
is hard to imagine that Internet telephony will replace the traditional phone
network altogether in any foreseeable future, nor that people and businesses
will completely cease using traditional telephone numbers for their
reachability. Therefore, even those users who completely switch to Internet
telephony will continue to subscribe to some form of gateway to and from the
PSTN. Such gateways however can in principle be located anywhere in the Net
and run by anyone, not necessarily just the local ISP or telco. The speed at
which non-traditional addresses, i.e. SIP addresses or some equivalent
addressing scheme, become accepted as a replacement for telephone number for
voice communications is one important factor that determines how fast telcos
will loose their influence.</para>

<para>Aside from the social acceptance of new telephone "numbers" there is another
issue that makes it doubtful whether a true global peer-to-peer scenario will
find widespread use outside the group of Internet hobbyists. Since the
Internet so far does not provide any global user authentication mechanisms
peer-to-peer telephony is destined to become an easy victim to all sorts of
malicious or junk calls, especially since IP-based phone calls could easily be
automated and placed at negligible cost. Although people have been putting up
with that very problem when it comes to email the effect would probably be far
more damaging on real-time communications. Moreover, unlike electronic mail,
which was a completely new service when the general public became aware of the
Internet in the 1990s, Internet voice telephony must stand the comparison with
traditional telephony in terms of quality, security and reliability if it is
to replace it. Some sort of caller authentication mechanism therefore
represents an important prerequisite. Such a requirement however does not only
exist for telephony but for many other applications as well. This makes it
easily conceivable that such authentication services for Internet applications
and users will successfully be introduced in the future. Whoever provides and
has control over these services will assume part of the role that telephone
companies still hold in the traditional telephony world.</para>
</section>

<section>
<title>Can Open Source make a difference?</title>

<para>Since with IP as the network protocol the service creation occurs on the
terminal device (e.g. the IAD, the PC or the IP-enabled telephone) it is these
devices, no longer the network, that determine what services and providers the
users see and can subscribe to. Domination over these devices thus implies
control over the telecommunications services market.</para>

<para>Open Source can give enterprises and consumers the option to make use of the
opportunities an IP-based communications infrastructure offers. In a closed
source environment software vendors will be tempted to promote or even enforce
the use of a limited set of services from specific providers only according to
their economic and strategic interests, limiting consumers' choice and
competition. On the service provider side, the Open Source model and the
existing code base will help enterprises to rapidly develop new services and
to tailor and combine services according to their clients' needs. Open Source
can thus be expected to extend its success from traditional non-voice Internet
services, many of which, such as DNS or electronic mail, rely almost entirely
on Open Source implementations, to IP telephony services. Security and
reliability requirements, which are particularly high for telephony services,
also favor the Open Source development model for such applications.</para>
</section>

<section>
<title>What about mobile telephony?</title>

<para>So far, considerations have focused primarily on fixed line telephony
services. With mobile phone services, the situation is still somewhat
different, although ultimately a similar development can be envisioned.
With the proliferation of 2.5G and particularly 3G (i.e. GPRS and UMTS) mobile
services, network operators aim at selling data services to customers in order
to increase revenues. Although these network architectures are still well
within the traditional "carrier-centric" model, allowing operators to retain
ownership over the customer, they pave the way for acceptance of IP-enabled
mobile devices. Operators must make sure that new data communications
services, which one the one hand they hope will enable them to sell new
content-based services to subscribers, will on the other hand not lead to
customers making use of that data connectivity to substitute for traditional
services such as SMS or ultimately, with the help of VoIP-capable mobile
devices, voice telephony, i.e. services which measured by data volume today
are extremely highly-priced and thus profitable to operators. Such a
cannibalization scenario requires the availability of the respective
functionality in end user devices. Here again Open Source development might
empower consumers to make use of their devices in the best of their own
interests.</para>

<para>An even bigger threat for mobile network operators comes from the emergence of
WLAN (Wireless LAN) hotspots, not only to data services that the owners of
UMTS licenses have put their hopes on for consolidation of their business
cases, but beyond that to the traditional voice telephony business, i.e. their
present cash cow. Although it is still unclear today what the wireless data
market will look like and who will be the dominating players in a few years
(traditional network operators, local hotspot operators, or vendors of mobile
devices or respective software) there should be little doubt that the
availability of comparatively inexpensive always-on broadband network
connectivity and IP as an open network protocol will lead to a similar
development as in the fixed line world. Recent standardization efforts for
Fixed Wireless Broadband (IEEE 802.16a) and particularly Mobile Wireless IP
Access (IEEE 802.20) might pave the way for further deployment scenarios.</para>
</section>

<section>
<title>Outlook</title>

<para>The perspective of the increased proliferation of inexpensive broadband
"always-on" Internet access leading to a qualitatively new kind of access to
consumers and thus undermining traditional business models not only holds for
telephone companies. In principle, any established communications and
information infrastructure, whether it is for example radio or cable
television, is potentially affected by the competition of IP as an open,
universal network protocol. Internet radio has already developed into a
serious player in the media industry. Compared to conventional radio it offers
the advantage to the entertainment industry of not having to apply for scarce
frequency resources (and often licenses) and at the same time enables
potentially global reach. It is only a matter of typically available bandwidth
for digital television to become a mainstream Internet service. In the field
of Video on Demand distribution via Internet is already a reality, turning
broadband Internet access into a direct competition to plans of cable network
operators of selling to consumers personalized premium entertainment as a
lucrative value-added product for their networks. Just as telephone companies,
cable network operators are about to loose their technical monopoly for such
"higher-value" services and are endangered of seeing themselves confined to
the role of yet another high-speed Internet access provider, i.e. of having to
live from the revenues of a "bit transport provider".</para>

<para>In contrast to radio or television operators, the added value telephone
companies offer to their customers is not some content but merely the call
setup (i.e. location of the callee) and the voice transmission. The fact that
on the one hand this kind of service, measured by its data volume, is
extremely highly-priced and on the other hand could be provided by intelligent
end-user devices with the help of the existing DNS infrastructure, makes the
telco business model particularly vulnerable - or, depending on the
perspective, attractive - to substitution by IP-based technology and the
consequent recreation of the value chain.</para>

<para>Today's telcos will increasingly find themselves trapped by the success of
high-speed Internet access. Even if they recognize that threat there
ultimately seems to be little what they could do to prevent it. With a mix of
technical as well as legal and regulatory obstruction and clever marketing
they are arguably in a good initial position of delaying that process. Over
time however, pressure from other influential industries such as entertainment
or consumer device vendors as well from public and politics, who realize that
failing to adopt new communications schemes will put their countries behind
other societies, will prevail. That leaves for telcos the choice to either
transform themselves into an entertainment and service provider amongst a
competition of many others, or to confine themselves to the role of the
operator of the last mile, maybe providing basic IP connectivity in addition.</para>

<para>It will be interesting to see whether other players will be able to take over
the telecom companies' dominating role in telecommunications service markets
or whether consumers will be capable of making use of the freedom of choice
the Internet architecture model offers them. Just as with computing services
so far, Open Source has the potential to enable both consumers and businesses
to efficiently solve their communications needs, including voice telephony,
the way they want.</para>
</section>

<section>
<title>Acknowledgements</title>

<para>The author would like to thank his colleagues from Detecon's "Strategic
Telecommunications Services and Architectures" group for numerous valuable
discussions on that subject over the past months.</para>
</section>

</article>