Side_1_360

ogy [8]. The H.248 is a joint venture between
IETF and ITU’s H.323. The main features are a
greater scalability and to address the technical
requirements of multimedia conferencing.

There are also some discussion that the tech-
nologies will coexist for a long time, however
not competing, instead taking their special parts
of the system. In such a scenario the H.323 will
become the enterprise legacy standard, while
MGCP and H.248 will be used between carriers’
call agents and other media gateways. The SIP
will dominate the connections between the call
agents and between call agents and residential IP
phones.

Tariff Arbitrage

In today’s markets for packet telephony one of
the main factors is the tariff arbitrage across the
data networks. In most international markets,
particularly highly regulated ones, communica-
tion carriers have tariff structures that are artifi-
cially high as compared to deregulated markets.
Additionally, these markets generally offer
lower tariff structures for data connections. Sev-
eral smaller operators have begun to exploit
these market disparities and provide users with
significant savings on their long distance calls.
Internet phones and VoIP gateways (as earlier
described) are two products to fully exploit these
disparities. Tariff arbitrage products are however
purely tactical infrastructure plays, which will be
short lived as the international communication
carriers embark upon a process of deregulation
over the near future. As artificial tariff dispari-
ties evaporate, the value proposition will
implode. Manufacturers who want to survive
this transition must be able to adapt to the
changing market condition as they change.

VoIP Supporting Protocols

Most protocols used for delivering the IP pack-
ets now also containing voice-coded information
were developed with data applications in mind,
such as email and file transfer. On higher layers
real time protocols are needed to support multi-
media applications, such as VoIP. Some exam-
ples to be mentioned are;

• IP Multicasting;


• Real-time Transport Protocol (RTP);


• RTP Control Protocol (RTCP);


• Resource Reservation Protocol (RSVP);


• Real-time Streaming Protocol (RTSP) [9].

The objective of IP Multicastingis to send one

packet and have it received by many destinations.

This feature could be used for services like news

broadcasts, stock quotes and distance learning.

The concept was first introduced in 1989 [10],

and involved end terminals have to support the

Internet Group Management Protocol (IGMP)

[10]. The IGMP enables multicast routers to

identify which stations are members of multicast

addresses. Specific IP addresses (224.0.0.0

through 239.255.255.255) are reserved to support

multicasting [11]. The Real-time Transport Pro-

tocolprovides end-to-end service for data requir-

ing real time support. The IP protocol is deployed

Figure 6 The protocols that

comprise the ITU IP telephony

standard (H.323)

Figure 7 The VoIP packet

and its sections. The IP, UDP

and RTP headers, which all

encapsulate the actual voice

sample, are also shown

audio/video application control & signalling data appl

video

codec

audio

codec

RTCP

H.225

(registr.)

H.225

(signalling)

H.245

(control)

T.120

(data)

RTP

UDP TCP

IP

IP Header

(20 octets,

plus options

and padding)

UDP Header

(8 octets)

RTP Header

(12 octets, plus

extensions)

Voice sample

(20 octets)

Ver IH Service Type Total length

Identifier

Time to live Protocol Header Checksum

Source Address

Destination Address

Options + padding

Timestamp

Synchronization Source Identifier (SSRC)

Contributing Source Identifier (CSRC)

Header Extension

20 ms Voice Sample

Source Port

Length

Destination Port

Checksum

VPXC Payload Sequence Number

Flag Fragment