Telco - Tutorial Ss7 Ip ...

[ Pobierz całość w formacie PDF ]
© 2001 Performance Technologies, Inc. All Rights Reserved.
Performance Technologies, Inc
.
www.pt.com
Table of Contents
Interworking Switched Circuit and Voice-over-IP Networks ........................................................ 3
Introduction............................................................................................................................... 3
Signaling in Switched Circuit and VoIP Networks........................................................................ 3
Signaling in Switched Circuit Networks.................................................................................... 3
Signaling in VoIP Networks ......................................................................................................4
Sigtran Protocols ...................................................................................................................... 6
Performance Considerations for SS7 over IP .......................................................................... 6
Security Requirements for SS7 over IP ................................................................................... 6
SCTP: Stream Control Transmission Protocol ............................................................................ 7
Transporting MTP over IP ............................................................................................................ 8
M2UA: MTP2 User Adaptation Layer ....................................................................................... 9
M2PA: MTP2 User Peer-to-Peer Adaptation Layer ................................................................. 9
M3UA: MTP Level 3 User Adaptation Layer .......................................................................... 10
Transporting SCCP over IP .......................................................................................................11
SIP, PINT, SPIRITS, ENUM, TRIP ............................................................................................ 11
SIP Protocol Components ...................................................................................................... 12
SIP-T ...................................................................................................................................... 12
PINT and SPIRITS ................................................................................................................. 13
ENUM ..................................................................................................................................... 13
TRIP ....................................................................................................................................... 13
Bibliography ............................................................................................................................... 13
Abbreviations.......................................................................................................................... 14
For more Information.............................................................................................................. 14
Page
2
© 2001 Performance Technologies, Inc. All Rights Reserved.
08/22/01
Performance Technologies, Inc
.
www.pt.com
Interworking Switched Circuit and Voice-over-IP
Networks
Introduction
Telephone companies offload voice calls from
public switched telephone networks
(PSTNs) to
voice-over-Internet Protocol
(VoIP) networks because it is cheaper to carry voice traffic over
Internet Protocol (IP) networks than over
switched circuit networks
. In the future, IP telephony
networks are expected to enable innovative new multimedia services while working seamlessly
with legacy telephone networks.
A VoIP network carries voice traffic cheaper than a switched circuit telephone network because
IP telephony
networks make better use of available bandwidth. In a public switched telephone
network, for example, a dedicated 64 kilobits per second (kbps) end-to-end circuit is allocated for
each call. In a VoIP network, digitized voice data is highly compressed and carried in
packets
over IP networks. Using the same bandwidth, a VoIP network can carry many times the number
of voice calls as a switched circuit network with better voice quality. The savings realized in using
VoIP networks are often passed onto users in the form of lower costs.
In 2001, US telephone companies are expected to offload between 15 and 20 percent of
percentage is rising each year as VoIP network infrastructure is rolled-out. Other countries, such
as China, carry an even higher percentage of voice traffic over domestic and international VoIP
networks.
In addition to voice data,
signaling
data is exchanged between switched circuit telephone
networks and VoIP networks. Signaling information is used to setup, manage and release voice
calls, and support telephony services such as caller ID, toll-free calling, and mobile authentication
and roaming services.
The remainder of this tutorial introduces the topic of interworking the public switched telephone
network and next-generation VoIP networks to support voice calls and telephony services.
Signaling in Switched Circuit and VoIP Networks
Signaling in Switched Circuit Networks
Switched circuit telephone networks use a signaling protocol called
Common Channel Signaling
System #7
(more commonly called
SS7
or
C7
). For more information, refer to the
SS7 tutorial
on
the Performance Technologies Web site (www.pt.com). In the public switched telephone network,
signaling end points
send and receive SS7 signaling messages. There are three kinds of
signaling end points: a
Service Switching Point
(
SSP
or
central office switch
), a
Signal
Transfer Point
(
STP
) and a
Service Control Point
(
SCP
).
Page
3
© 2001 Performance Technologies, Inc. All Rights Reserved.
08/22/01
Performance Technologies, Inc
.
www.pt.com
Figure 1: SS7 Signaling End Points (SEPs) in a Switched Circuit Network
In SS7 networks,
ISUP
(
Integrated Services Digital Network
(
ISDN
)
User Part
) signaling
messages are used to setup, manage and release trunk circuits that carry voice calls between
central office switches. ISUP messages also carry caller ID information, such as the calling
party's telephone number and name. ISUP is used for both ISDN and non-ISDN calls between
central office switches.
TCAP
(
Transaction Capabilities Application Part
) signaling messages support telephony
services, such as toll-free (freephone), calling card, local number portability and mobile (wireless)
roaming and authentication services. Mobile services are enabled by information carried in the
Mobile Application Part
(MAP) of a TCAP message. TCAP supports non-circuit related
information exchange between signaling points using the
Signaling Connection Control Part
(
SCCP
) connectionless service.
Signaling in VoIP Networks
VoIP networks carry SS7-over-IP using protocols defined by Signaling Transport (
sigtran
)
working group of the Internet Engineering Task Force (IETF), the international organization
responsible for recommending Internet standards. The sigtran protocols support the stringent
requirements for SS7/C7 signaling as defined by International Telecommunication Union (ITU)
Telecommunication Standardization Sector.
Page
4
© 2001 Performance Technologies, Inc. All Rights Reserved.
08/22/01
Performance Technologies, Inc
.
www.pt.com
In IP telephony networks, signaling information is exchanged between the following functional
elements:
Media Gateway:
A media gateway terminates voice calls on inter-switch trunks from the
public switched telephone network, compresses and packetizes the voice data, and
delivers compressed voice packets to the IP network. For voice calls originating in an IP
network, the media gateway performs these functions in reverse order. For ISDN calls
from the PSTN, Q.931 signaling information is transported from the media gateway to the
media gateway controller (described below) for call processing.
Media Gateway Controller:
A media gateway controller handles the registration and
management of resources at the media gateway(s). A media gateway controller
exchanges ISUP messages with central office switches via a signaling gateway
(described below). Because vendors of media gateway controllers often use off-the-shelf
computer platforms, a media gateway controller is sometimes called a
softswitch
.
Signaling Gateway:
A signaling gateway provides transparent interworking of signaling
between switched circuit and IP networks. The signaling gateway may terminate SS7
signaling or translate and relay messages over an IP network to a media gateway
controller or another signaling gateway. Because of its critical role in integrated voice
networks, signaling gateways are often deployed in groups of two or more to ensure high
availability.
A media gateway, signaling gateway or media gateway controller (softswitch) may be separate
physical devices or integrated in any combination.
Figure 2: Example of a VoIP Network Configuration
Page
5
© 2001 Performance Technologies, Inc. All Rights Reserved.
08/22/01
[ Pobierz całość w formacie PDF ]