Voice over IP
Joanna Drapella
Marc 2002
Analytical Home Assignments in Internetworking
VT 2002, Period 3, 2G1305
Voice over IP – a sign of convergence of PSTN and PSN
Joanna Drapella
790224-A301
Marc 2002
Summary
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Introduction .................................................................................................................................. 2
Historical background .................................................................................................................. 2
Telephone network ....................................................................................................................... 2
Internet ......................................................................................................................................... 4
Expectations for a telephone connection ..................................................................................... 4
Technical problems in voice over IP............................................................................................ 5
Global interconnection ................................................................................................................. 5
Existing infrastructure (physical layer) ........................................................................................ 5
SS7 and H.323 (network and transport layer) .............................................................................. 6
New idea needed .......................................................................................................................... 7
Conclusion ................................................................................................................................... 7
Glossary ....................................................................................................................................... 8
Bibliography................................................................................................................................. 8
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Voice over IP
Joanna Drapella
Marc 2002
1. Introduction
Future research in telecommunications is oriented in the direction of integrated, universal services
and multiservice networks. But each of these services requires a different protocol, different
infrastructure, and each has different priorities. In this paper I would like to discuss how different
existing techniques, infrastructures and types of services try to find a common solution for this
problem. I will use Voice over IP as an example because in this case researchers need to implant the
ideas of a telephone switched network in a packet switched network and a real-time, guarantied
bandwidth of 64KHz, in a best effort system. The contradiction of these two ideas and the need to
put together all of the good points of both seems to be particularly interesting. On the other hand,
the idea of a very cheap and good quality phone calls excites me, as a client of a telecommunication
service provider and as a sociable human being. I do not want to concentrate only on technical
problems. I am also interested in the sociological and economical changes that the introduction to
the market of a cheap, high quality telephone conversation may cause. Another interesting problem
is the cooperation (or lack of cooperation) between different service providers and the reason for
this competition.
2. Historical background
Historically, the first existing type of network was the circuit switching network where for each
communication the operator allocates a given bandwidth from end to end. This circuit allocation
gives a guaranty of quality of the service, fixed bandwidth, and a real physical connection between
two ends. The opposite idea is the packet switching network. This network was designed for the
Internet within the idea of best effort. There is no physical connection established for the
communication and each packet (piece of information) "travels" separately. In this system resources
are better used in term of bandwidth and some dynamical changes in the connection are possible
because the connection is now "virtual" and not real.
These two types have been competing in term of quality and variety of service they could offer.
Today, voice over IP may end this competition and "destroy" PSTN. The success of internet and
packet switching networks has also changed the word of PSTN which more and more often uses
parts of the internet for long distance calls or even telecom routes and special routing tables like
SS7 described in the following chapter.
Telecom service providers, in most countries, are monopolists and very traditional. They are
obligated by the government to provide the same service, at the same price for all citizens, wherever
they live and whatever their expectations are. This stops most implementations of new telecom
solutions because of the high cost and non-homogeneity. As PSTN belongs to the state, its
geographical borders are strict. Some countries use high-technology solutions and others, less
developed old standards. The Internet broke all geographical and political borders which means that
any integration of telephone networks and the Internet automatically challenges the telecom’s
monopolist status.
3. Telephone network
At the very beginning of telephone history the architecture of a PSTN was very straight forward.
Twisted wires coming from each subscriber are regrouped and go to an exchange (end office). Tree
architecture is used. End offices are grouped in so called Class 3 Tolls, then class Primary,
Selectional, Regional etc. Few signals, coded by different voltage, could be sent. For example: start
connection, dial the number, wait for connection, and busy signal. This system developed a lot and
the architecture nowadays looks completely different.
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Voice over IP
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Marc 2002
Common Channel Signaling System No. 7 (SS7) is a global standard for telecommunications
defined by the International Telecommunication Union and established in mid-1970s. The standard
defines the procedures and protocol by which network elements in PSTN exchange information
over a digital signaling network to effect wireless (cellular) and wireline call setup, routing and
control. Each signaling point in the SS7 network is uniquely identified by a numeric point code.
Point codes are carried in signaling messages exchanged between signaling points to identify the
source and destination of each message. Each signaling point uses a routing table to select the
appropriate signaling path for each message.
There are three kinds of signaling points in the SS7 network:
SSP
(Service Switching Point) are switches that originate, terminate, or tandem calls. An SSP
sends signaling messages to other SSPs to setup, manage, and release voice circuits required to
complete a call. An SSP may also send a query message to a SCP to determine how to route a
call. A SCP sends a response to the originating SSP containing the routing number(s) associated
with the dialed number.
STP
(Signal Transfer Point) is a packet switch network. STP routes each incoming message to an
outgoing signaling link based on routing information contained in the SS7 message. Because it
acts as a network hub, a STP provides improved utilization of the SS7 network by eliminating
the need for direct links between signaling points.
SCP
(Service Control Point) in a centralized database storing information about all possible routes
and connections.
The hardware and software functions of the SS7 protocol are divided into functional abstractions
called "levels" that can be associated with OSI layers.
The Message Transfer Part is divided into three levels. The lowest level, MTP Level 1, is equivalent
to the OSI Physical Layer. MTP Level 1 defines the physical, electrical, and functional
characteristics of the digital signaling link.
MTP Level 2 ensures accurate end-to-end transmission of a message across a signaling link. Level
2 implements flow control, message sequence validation, and error checking. When an error occurs
on a signaling link, the message (or set of messages) is retransmitted. MTP Level 2 is equivalent to
the OSI Data Link Layer with the difference that OSI link layer do not provide end-to-end
connection.
MTP Level 3 provides message routing between signaling points in the SS7 network. MTP Level 3
re-routes traffic away from failed links and signaling points and controls traffic when congestion
occurs. MTP Level 3 is equivalent to the OSI Network Layer.
The higher layers do not have any equivalent in OSI representation and have very specific functions
and handle connections with other telephone systems or define the protocol and procedures used to
set-up, manage, and release trunk circuits that carry voice and data calls over PSTN.
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Voice over IP
Joanna Drapella
Marc 2002
In the original PSTN, signaling (e.g., call setup) and talking utilized the same common trunk from
the originating switching system to the terminating switching system. This process seized the trunks
in all of the switching systems involved. If the terminating end was busy, all of the trunks were set
up unnecessarily. With SS7 system, a talking path was not assigned until all signaling had been
satisfactorily completed.
4. Internet
I will not describe details of Internet architecture and protocols because this is not the subject of this
paper. The most important part for VoIP is to understand IP (network layer), UDP, and TPC
(transport layer) protocols and their incompatibility with real-time service.
The Internet, different from PSTN, is a homogenous structure where between two communicating
hosts the only device an IP packet finds on its way is a router or a switch. There is no tree hierarchy
like in the telephone network. Or, to be more precise, we can distinguish LAN, AS, and WAN as
three levels in the Internet, but even these Internets have fractal rather than tree structure. IP is only
responsible to deliver the package to the destination address. There is no real (used and working)
option in IP that makes sure that a datagram arrives with a given maximal delay or arrives in a
given order because IP was not designed to fulfill those functions. Another important characteristic
of IP is that if an error occurs, or there is no place in the buffer queue, a package is discarded and
never reaches it's destination.
UDP is a simple, light protocol that does not do anything more that IP in term of service quality.
TCP tries to correct some errors that occur and resends a lost packet. This protocol establishes a
virtual connection between two machines that may look similar to a PSTN, but it is not the same
because the long delay caused by acknowledgments and retransmission goes completely against
real-time service.
5. Expectations for a telephone connection
Everyone who uses a telephone knows that in terms of service quality we want to have the best
possible audio quality of the signal and possibly no delay between the person speaking and us
hearing the voice. Nobody likes either white noise or an echo effect. On the other hand, phone
conversations, especially long distances, are pretty expensive, so we also want to reduce the cost of
the communication.
In theory those expectations correspond perfectly with the characteristics of the internet as a low
cost, digital transmission from end-to-end that allows digital control of the noise and echo,
"infinite" bandwidth compared to PTSN with a fix value of 64KHz. But this would be too simple to
be true.
Telephone networks are one of the oldest and the most popular telecommunication infrastructures.
A telephone connection exists in almost every house, in every country. This means that if we want
to implement VoIP and make it accessible for everyone it should look like a telephone and be used
like a telephone.
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Voice over IP
Joanna Drapella
Marc 2002
6. Technical problems in voice over IP
There are three different types of VoIP communications: Computer to computer, computer to
telephone, and telephone to telephone.
In computer to computer communications we do not need to deal with the connection to the PSTN
that in may simplify the protocols but on the other hand the Internet was not designed for real-time
data transmission. This means that questions of appropriate audio code format, digital compression,
and the correct transport and network protocols need to be solved.
But computer to computer "telephone" conversations are not an interesting service to offer to a
client because people would rather use a telephone to talk. That is why we need to provide VoIP
service on the phone devices to make its usage more natural. But this requires a connection to the
PSTN and translation of Internet protocols to telecommunication protocols. Both have very
different backgrounds, use different technologies and infrastructures. This is what makes this
project a real challenge.
7. Global interconnection
In a competitive marketplace, there are a number of reasons why service providers need to
interconnect with each other. First of all, new entrants need access to the networks of incumbents so
that they can resell services. Another important problem is the so called "last mile". Competitive
voice, data, and wireless carriers need to deliver services to a user’s home. And finally, all carriers
need access to each other's back-office systems to fulfill number portability mandates and to
exchange the forms and messages involved in fulfilling customer orders.
Thus, in the telecom industry, interconnection or e-commerce is not just a way to make business
relations easier. It is, literally, the key to competition. Without those linkages, competition would
simply not be possible.
On a national level, interconnection is complicated enough, involving a tangled web of relationships
among different types of providers. Diverse languages, cultures, markets, regulatory environments,
and technical idiosyncrasies further complicate interconnection on a global scale. That's why the
"technique" used to achieve interconnection is all-important. That technique becomes either the
facilitator of or obstacle to competition and the spread of communications services around the
world.
8. Existing infrastructure (physical layer)
Today, we experience several co-existing "information" (telecommunication) networks. Almost in
every home you can find a television cable, phone twisted wire, and, more and more often, an RJ45
Internet socket. To this you can add systems such as television/radio broadcasting and wireless
telephony systems such as GSM. Those 5 different technologies are probably very appropriate for
each service they provide. And it is easy to understand, from the historical point of view, that there
was no cooperation between the designers of those networks. Each of them was developed for
different reasons, in different times and there was no motivation to integrate different parts of this
infrastructure.
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Voice over IP
Joanna Drapella
Marc 2002
Now the situation looks different. Not surprisingly the motivation is money. The cost of civil
engineering work increased incredibly and all companies try to avoid digging in the center of a city
to put in a new link. For new coming wire services, as Internet, the cost of putting Ethernet in every
existing house is too high. Internet providers started searching for a new solution and the idea was
to use existing equipment – as twisted wire and television cable to provide Internet connections.
This new situation encouraged signal processing and electronic research to develop modems and
made other telecommunication operators interested in providing Internet services. On the other
hand, the Internet required larger bandwidth, which was not an existing problem either for
telephony pr for TV and radio broadcasting. But, the discovery of broadband support on the
physical layer turned out to be extremely interesting also for other services (like optical fiber as a
telephone long distance link).
This step in convergence between CSN and PSN allowed CSN to use new technologies such as
broadband links and for the Internet to be present in every house using existing cables. The next
step will be to integrate mobile telephony in this false multi-services network for a mobile Internet.
This is future of UMTS and 4G cellular phones.
9. SS7 and H.323 (network and transport layer)
As I already mentioned, the integration of services on the physical layer works relatively well.
Unfortunately, this is not the case for the dialog between telephone and Internet protocols. The
difference of basic concepts between CSN and PSN, and the differences in aspects of these services
cause problems that I mentioned in the chapter "technical problems in voice over IP".
The main problem is to address the transport of packet-based PSTN signaling over IP networks,
taking into account the functional and performance requirements of the PSTN signaling. In order to
work with the PSTN, IP networks need to transport signaling such as integrated service digital line
or SS7 messages between IP nodes such as a signaling gateway, a media gateway controller, a
media gateway, or an IP–based database.
One of the primary goals in the development of the H.323 standard was interoperability with other
multimedia-services networks. This interoperability is achieved through the use of a gateway. A
gateway performs any network or signaling translation required for interoperability. H.323 specifies
4 kinds of components: terminals, gateways, gatekeepers and multipoint control units. An H.323
gateway provides connectivity between an H.323 network and a non–H.323 network, for example
PSTN. This connectivity of dissimilar networks is achieved by translating protocols for call setup
and release, converting media formats between different networks, and transferring information
between the networks connected by the gateway. Gatekeepers provide important services such as
addressing, authorization, and authentication of terminals and gateways; bandwidth management;
accounting; billing; and charging. Multipoint control units provide support for conferences (for
more than two terminals communicating).
Whereas H.323 is used to transport data over IP–based networks, RTP is typically used to transport
data via the user datagram protocol (UDP). RTP, together with UDP, provides transport-protocol
functionality.
H.323 has much larger usage than a simple VoIP transfer. It is used for all kinds of real time
multimedia transmissions. In the case of IP, telephony gateways connect to SS7 system and
translate the protocols.
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Voice over IP
Joanna Drapella
Marc 2002
10.New idea needed
It is always difficult to unite two very different systems and preserve only the best parts of both.
PSTN, using SS7, is not a real circuit switch networks any more because they adapted a lot of
solutions from the Internet. An important part of transmission is now based on packet switching
ideology. In the same time we see PSTN approaching the Internet and the Internet trying to include
interconnection with telephone network.
Very often we get stuck with the old solutions of trying to implement something new and I do not
think that works. Usually, to come up with a good service, an original, brilliant idea is needed. And
what happens now with VoIP is that researchers want to copy PSTN structure and protocol over IP
to allow easy phone to phone and computer to phone communications. This is a very nice shortterm solution because, in my opinion, circuit switching networks are history now. This entire
solution seems to me like building a hybrid by putting a car and a boat one on top of another. In one
case you get a car carrying a boat and in the other a boat carrying a car, but never a hybrid vehicle.
11.Conclusion
Even if I do not like the way telephone services over the Internet were developed, it works, and this
is probably the most important. More and more companies offer the software and even hardware
(like CISCO) for VoIP communications. But the problem is far from being solved. The audio
quality of Internet conversation is very poor. As a user I prefer to pay more and hear the "real"
voice of the person I am talking to. This is the human part of usage of a service, but this is also why
the service exists. So if the service is a good solution from the technological point of view but
nobody wants to use it, it makes no sense.
At the same time, integration of services in the width sense means that we should get all kinds of
services via one support, in this case via Internet. This means that PTSN will disappear completely.
I believe in the world where all devices will have an IP address and communicate via the Internet.
In this case all efforts made for create protocols that imitate PSTN on the Internet have no use.
My idea is of course completely unrealistic because all Internet and telephone providers will never
agree on drastic changes, and integration of services is one on those. And we should not forget that
we live in a real world and the physical infrastructure that I described in chapter "existing
infrastructure (physical layer)" already exists and it is much more difficult (impossible) to put in a
new infrastructure than to change a telecommunication protocol. Then we should add the historical
and political handicap of PSTN to understand the whole problem.
If the real solution for VoIP –cheap communication, good audio quality via a device and
infrastructure easily accessible for everyone- is found, PSTN will “die”. This will cause a hard
economical problem because the most important telephone companies are not private. I am afraid
that even when the solution comes, governments of all the semi-capitalistic European countries will
have a hard time allowing it to be put into place.
All of the problems described in this paper also apply to other real-time transmissions such as
videoconference, television, video and radio. But I think that the problem of VoIP is the most
interesting to study because it is a complex problem and it touches everyone who dreams about
getting lower telephone bills.
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12.Glossary
PSTN public switch telephone networks
VoIP voice over Internet Protocol
CSN circuit switch network
PSN
packet switch network
SS7
Common Channel Signaling System No. 7
AS
anonymous system
13.Bibliography
http://www.pt.com/tutorials/ss7/ - performance technologies: tutorial about SS7 protocol
http://www.iec.org/online/tutorials/ - international engineering consortium proposing tutorials about
latest technical and business issues.
http://www.protocols.com/voip.htm – various information about VoIP
Computer Networks and Internet by Douglas E. Comer
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