Signaling and Network Control

NETW 704
Signaling &
Network Control
Intelligent Networks
Dr. Eng. Amr T. Abdel-Hamid
Winter 2006
Amr Talaat, 2006
SS7 in the Converged World
The "Converged World" of Next Generation Networks
(NGNs) brings with it the promise of voice, video, and
data over a single broadband network.
This transition from the traditional circuit-switched
networks to packet-switched networks has been
underway for many years, and Voice over IP (VoIP) is
now leading the transition.
The immediate benefits of NGNs are decreased cost of
infrastructure and improved ease of management.
Longer-term benefits include the ability to rapidly deploy
new services.
Switched Circuit Network (SCN)
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NGN Architecture
Media Gateway (MG) handles the media, or bearer, interface. It
converts media from the format used in one network to the format
required in another network. For example, it can terminate the TDM
trunks from the PSTN, packetize and optionally compress the audio
signals, and then deliver the packets to the IP network using the Real
Time Protocol (RTP).
Media Gateway controller (MGC) (also known as a Call Agent)
contains the call processing. In addition, it manages the resources of
the MGs that it controls.
Signaling Gateway (SG) sits at the edge of an IP network and
terminates circuit-switched network signaling, such as SS7 or ISDN,
from the circuit-switched network. It transports, or backhauls, this
signaling to the MGC or other IP-based application endpoint.
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NGN Architecture
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Signaling Transport (SigTran) protocol
protocol defined by RFC 2719 in the
The protocol framework identified
three necessary components for the
SigTran protocol stack:
 A set of adaptation layers that
telephony signaling protocols
 A common signaling transport
requirements of transporting
telephony signaling
 IP network protocol
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The Working Group began evaluating the two commonly used
transport protocols, User Datagram Protocol (UDP) and Transport
Control Protocol (TCP) against these requirements.
UDP was quickly ruled out because it did not meet the basic
requirements for reliable, in-order transport.
TCP met the basic requirements, it was found to have several
limitations, such as:
 Head-of-line blocking: Because TCP delivery is strictly
sequential, a single packet loss can cause subsequent packets
to also be delayed. The analysis showed that a 1% packet loss
would cause 9% of the packets being delayed greater than the
one-way delay time.
 Timer granularity: The retransmission timer is often large
(typically one second) and is not tunable.
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TCP further limitations:
Also, because of a timer granularity issue and the lack of
a built-in heartbeat mechanism, it takes a long time to
detect failure (such as a network failure) in a TCP
A new transport protocol, Stream Control Transmission
Protocol (SCTP) was developed for transporting SCN
signaling. Note that SCTP is a generic transport that can
be used for other applications equally well.
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RFC 3309 is the most updated version that describes this
SCTP provides the following features:
Acknowledged error-free, nonduplicated transfer of user data
Data segmentation to conform to path MTU size (dynamically assigned)
Ordered (sequential) delivery of user messages on a per "stream" basis
Option for unordered delivery of user messages
Network-level fault tolerance through the support of multihoming
Explicit indications of application protocol in the user message
Congestion avoidance behavior, similar to TCP
Bundling and fragmenting of user data
Protection against blind denial of service and blind masquerade attacks
Graceful termination of association
Heartbeat mechanism, which provides continuous monitoring of reachability
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SCTP is a connection-oriented protocol.
Each end of the connection is a SCTP endpoint. An
endpoint is defined by the SCTP transport address,
which consists of one or more IP addresses and an
SCTP port. The two endpoints pass state information in
an initialization procedure to create an SCTP
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SCTP uses streams as a means of decreasing the impact of headof-line blocking.
Streams provide the ability to send separate sequences of ordered
messages that are independent of one another.
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SCTP provides the ability to have multiple streams within
an association. Each stream provides reliable delivery of
ordered messages that are independent of other streams.
Packet 2 is dropped again. However, because packets 3,
4, and 5 belong to a different stream, they can be
delivered to the application without delay.
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User Adaptation (UA) Layers
The User Adaptation (UA) layers encapsulate different SCN
signaling protocols for transport over an IP network using
UA layer is unique in terms of the encapsulation because of
the differences of the signaling protocols themselves,
following are some common features among all UA layers:
Support for seamless operation of the UA layer peers over an IP
Support for the primitive interface boundary of the SCN lower
layer, which the UA layer replaces. For example, M2UA supports
the primitive interface boundary that MTP Level 2 supports.
Support for the management of SCTP associations.
Support for asynchronous reporting of status changes to layer
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User Adaptation (UA) Layers
MTP Level 3 User Adaptation (M3UA) layer is defined for the
transport of SS7 User Part messages (such as ISUP, SCCP, and
SCCP User Adaptation (SUA) layer is defined for the transport of
SCCP User Part messages (such as TCAP)
MTP Level 2 User Adaptation (M2UA) layer is defined for the
transport of MTP Level 3 messages.
MTP Level 2 Peer Adaptation (M2PA) layer is defined for the
transport of MTP Level 3 data messages over SCTP. M2PA
effectively replaces MTP Level 2. It provides the ability to create
an IP-based SS7 link.
The ISDN User Adaptation (IUA) layer is defined for the transport
of Q.931 between an ISDN SG and a MGC.
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UA Common Terminology
The UAs introduce some new terminology that did not exist in
the SS7 world.:
 Application Server (AS): A logical entity that serves a
specific Routing Key. An example of an Application Server is
a virtual switch element that handles all call processing for a
unique range of PSTN trunks. Another example is a virtual
database element, handling all HLR transactions for a
particular SS7 combination. The AS contains a set of one or
more unique ASPs, of which one or more is normally actively
processing traffic.
 Application Server Process (ASP): A process instance of an
Application Server. An ASP serves as an active or backup
process of an Application Server (for example, part of a
distributed virtual switch or database).
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UA Example
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MTP Level 3 User Adaptation
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SCCP User Adaptation
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MTP Level 2 User Adaptation
The M2UA protocol defines the layer split between MTP Level 2
and MTP Level 3. M2UA is defined by RFC 3331.
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MTP Level 2 Peer Adaptation
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