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Introduction

One aspect of real-time transmission stands out

as especially important, the use of IP as the

foundation for telephony services. The idea is

endorsed by many operators around the world.

The functionality is often referred to as Voice

over IP(VoIP) and on more general, non-techni-

cal occasions IP Telephonyor Internet Tele-

phony. Although it was designed and optimised

to transport data, IP has successfully carried

audio and video since its inception. In fact,

researchers began to experiment with audio

transmission across ARPANET before the Inter-

net of today was in place. By the 1990s, com-

mercial radio stations were sending audio across

the Internet, and software was available that

allowed an individual to send audio across the

Internet or to the standard telephony network.

Commercial operators also began using IP tech-

nology internationally to carry ordinary tele-

phone calls, i.e. voice.

Network Characteristics

There are mainly two different network categories,

namely circuit switched and packet switched net-

works. The problem with the deployment or inte-

gration of voice and data transfer is that they

belong to the circuit switched and packet switched

category, respectively. To get a better understand-

ing, the two are described below.

Circuit Switching

The traditional networks built for transmitting

voice are connection-oriented. This means that

to initiate a call the caller has to invoke a certain

procedure where a specific route through the

entire network is established before any trans-

mission of voice information is carried out. The

network signalling system, which is put into

action when the caller dials the number of the

destination, establishes this route. The route is

then used by all information (the pulse code

modulated voice) during the call.

After finalizing the call the signalling system

revokes all the resources that have been allo-

cated on the links and in the switches along the

route. The resources (capacity) of the transmis-

sion network that are allocated during a call are

dedicated entirely for that specific call, i.e. even

if nothing is sent at the moment the resources

cannot be used for other connections. The capac-

ity allocated is for an ordinary telephone call

64 kb/s. This is due to the fact that the voice is

sampled with a frequency of 8 kHz and that

each sample is coded by 8 bits, i.e. 64 kb/s. The

advantage of the approach to allocate capacity

along the route is that no variations in delay

between sender and receiver are introduced; the

delay that will exist will be fixed and determinis-

tic in its nature.

Packet Switching

The data networks have mainly been built to be

connectionless. This means that no connection

or resources are allocated for a specific transmis-

sion. The information that is to be sent is divided

into small segments, called packets, which are

sent out on the network independently. All trans-

missions on such a network have to compete for

the available resources in some fair manner.

Two different approaches are taken within this

concept, datagram and virtual circuit. In the case

of datagram the packets from one transmission

are sent as if each of them belonged to a new

independent transmission. This could very well

mean that the packets will be received at the des-

tination in another order than they were trans-

mitted at the sender. For example, after some

time the link that seems to be the closest and/or

most efficient will be congested or a switch

overloaded along the route. The next packet in

order to traverse the network will then be routed

along a different route in order to avoid this con-

gestion. Due to the new route, this packet could

reach the receiver before some of the packets on

the congested route.

Voice Transmission over Internet

JOHAN M KARLSSON

The idea of combining both data and voice information in the same network can be traced back to the
days of early discussions of ISDN. The envisioned deployment of ISDN was however more technically
challenging than planned. The next step into convergence was the introduction of the Internet Protocol
(IP). IP was able to route packets through diverse networks without prior knowledge of the service
involved. Mostly transfer of files and other text-based information took place. Later, a discussion of
differentiated services was introduced, which opened for different kinds of services to be transmitted
over the packet switched networks. This paper focuses on the transfer of real-time data and in specific
voice over IP (VoIP). Examples of VoIP applications and implementation issues are discussed. The
requirements for voice coders and acceptable time delays are mentioned. Further, the specific problems
of sending voice samples over a packet switched network as well as some of the protocols supporting
Johan M Karlsson (40) obtained this are evaluated.
his MSc and PhD degrees from
Lund Institute of Technology,
Sweden. He is working on
mobile telecommunications
and wireless communications
issues. Quality of service
aspects and performance of
traffic and networks, as well as
protocol issues are his main
research topics. Karlsson is
also program director for PCC
(Personal Computing and
Communications), which is
the largest Swedish research
initiative in its field.

[email protected]

Telektronikk 2/3.2001