It is
usually referenced as Signaling System No. 7 or Signaling System #7, or simply
abbreviated to SS7. In North America it is often
referred to as CCSS7, an acronym for Common
Channel Signaling System 7. In some European countries, specifically the United
Kingdom, it is
sometimes called C7 (CCITT
number 7) and is also known as number 7 and CCIS7 (Common Channel Interoffice Signaling
7).
There
is only one international SS7 protocol defined by ITU-T in its
Q.700-series recommendations. There
are however, many national variants of the SS7 protocols. Most national
variants are based on two widely deployed national variants as standardized by ANSI and ETSI, which are in turn based on the
international protocol defined by ITU-T. Each national variant has its own
unique characteristics. Some national variants with rather striking
characteristics are the China (PRC) and Japan (TTC) national variants.
§ MTP2 (M2UA and M2PA)
§ MTP3 (M3UA)
There are two essential components to all telephone
calls. The first, and most obvious, is the actual
content—our voices, faxes, modem data, etc. The second is the information that
instructs telephone exchanges to establish connections and route the “content”
to an appropriate destination. Telephony signaling is concerned with the
creation of standards for the latter to achieve the former. These standards are
known as protocols. SS7 or Signaling System Number 7 is simply another set of
protocols that describe a means of communication between telephone switches in
public telephone networks. They have been created and controlled by various
bodies around the world, which leads to some specific local variations, but the
principal organization with responsibility for their administration is the
International Telecommunications Union or ITU-T.
Signalling System Number 7 (SS#7 or C7) is the protocol used by the telephone
companies for interoffice signalling. In the past, in-band signalling
techniques were used on interoffice trunks. This method of signalling used the
same physical path for both the call-control signalling and the actual
connected call. This method of signalling is inefficient and is rapidly being
replaced by out-of-band or common-channel signalling techniques.
To understand SS7 we must first understand something of the basic inefficiency
of previous signaling methods utilized in the Public Switched Telephone Network
(PSTN). Until relatively recently, all telephone connections were managed by a
variety of techniques centered on “in band” signaling.
A network utilizing common-channel signalling is actually two networks in one:
1. First there is the circuit-switched "user" network
which actually carries the user voice and data traffic. It provides a physical
path between the source and destination.
2. The second is the signalling network which carries the call
control traffic. It is a packet-switched network using a common channel
switching protocol.
SS7
PROTOCOL SUIT
The SS7 network is an
interconnected set of network elements that is used to exchange messages in
support of telecommunications functions. The SS7 protocol is designed to both
facilitate these functions and to maintain the network over which they are
provided. Like most modern protocols, the SS7 protocol is layered.
Physical
Layer (MTP-1)
This defines the physical and electrical characteristics of the signaling links
of the SS7 network. Signaling links utilize DS–0 channels and carry raw
signaling data at a rate of 56 kbps or 64 kbps (56 kbps is the more common
implementation).
Message
Transfer Part—Level 2 (MTP-2)
The level 2 portion of the message transfer part (MTP Level 2) provides
link-layer functionality. It ensures that the two end points of a signaling
link can reliably exchange signaling messages. It incorporates such
capabilities as error checking, flow control, and sequence checking.
Message
Transfer Part—Level 3 (MTP-3)
The level 3 portion of the message transfer part (MTP Level 3) extends the
functionality provided by MTP level 2 to provide network layer functionality.
It ensures that messages can be delivered between signaling points across the
SS7 network regardless of whether they are directly connected. It includes
such capabilities as node addressing, routing, alternate routing, and congestion
control.
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Signaling Connection Control Part (SCCP)
The signaling connection control
part (SCCP) provides two major functions that are lacking in the MTP. The first
of these is the capability to address applications within a signaling point.
The MTP can only receive and deliver messages from a node as a whole; it does
not deal with software applications within a node.
While MTP network-management
messages and basic call-setup messages are addressed to a node as a whole,
other messages are used by separate applications (referred to as subsystems)
within a node. Examples of subsystems are 800 call processing, calling-card
processing, advanced intelligent network (AIN), and custom local-area signaling
services (CLASS) services (e.g., repeat dialing and call return). The SCCP
allows these subsystems to be addressed explicitly.
ISUP user part defines the messages
and protocol used in the establishment and tear down of voice and data calls
over the public switched network (PSN), and to manage the trunk network on
which they rely. Despite its name, ISUP is used for both ISDN and non–ISDN
calls. In the North American version of SS7, ISUP messages rely exclusively on
MTP to transport messages between concerned nodes.
Transaction Capabilities Application Part (TCAP)
TCAP defines the messages and
protocol used to communicate between applications (deployed as subsystems) in
nodes. It is used for database services such as calling card, 800, and AIN as
well as switch-to-switch services including repeat dialing and call return.
Because TCAP messages must be delivered to individual applications within the
nodes they address, they use the SCCP for transport.
Operations, Maintenance, and Administration Part (OMAP)
OMAP defines messages and protocol
designed to assist administrators of the SS7 network. To date, the most fully
developed and deployed of these capabilities are procedures for validating
network routing tables and for diagnosing link troubles. OMAP includes messages
that use both the MTP and SCCP for routing.
Signaling
Elements/Endpoints
·
SSPs are end office or tandem switches that connect voice
circuits and perform the necessary signaling functions to originate and
terminate calls.
·
The STP routes all the signaling messages in the SS7 network.
·
The SCP provides access to databases for additional routing
information used in call processing. Also, the SCP is the key element for
delivering IN applications on the telephony network.
The
following sections explore the three signaling elements of the SS7 network in
more detail.
SSP
SSPs
are telephone switches that are provisioned with SS7 capabilities. End office
SSPs originate and terminate calls, and core network switches (STPs) provide
tandem or transit calls. The SSP provides circuit-based signaling messages to
other SSPs for the purposes of connecting, disconnecting, and managing voice
calls. Non-circuit based messages are used to query databases when the dialed
number is insufficient to complete the call.
End
office SSPs connect directly to users on their subscriber interfaces. The
protocols used can vary from analog to digital and can be based on ISDN Primary
Rate Interface (PRI) or channel-associated switching (CAS). The end office is
in charge of translating subscriber protocol requests into SS7 messages to
establish calls.
The
SSP uses the dialed number to complete the call, unless, for example, it is an
800, 8xx, 9xx, or Local Number Portability exchange (or is ported NXX). In the
latter case, a query is sent to an SCP requesting the routing information
(number) necessary to complete the call.
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1.
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The SSP uses the called number from the calling party or
routing number from the database query to begin circuit connection signaling
messages.
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2.
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Then
the SSP uses its routing table to determine the trunk group and circuit
needed to connect the call.
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3.
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At
this point, a signaling setup message is sent to the destination SSP
requesting a connection on the circuit specified by the originating SSP.
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4.
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The
destination SSP responds with an acknowledgment granting permission to
connect to the specified trunk and proceeds to connect the call to the final
destination.
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STP
STPs,
as illustrated, are an integral part of the SS7 architecture providing access
to the network. STPs route or switch all the signaling messages in the network
based on the routing information and destination point code address contained
in the message.
The
STP provides the logical connectivity between SSPs without requiring direct
SSP-to-SSP links. STPs are configured in pairs and are mated to provide
redundancy and higher availability. These mated STPs perform identical
functions and are considered the home STPs for the directly connected SSP or SCP.
The STP also is capable of performing global title translation, which is
discussed later in this section.
Circuit-based
messages are created on the SSP. Then, they are packetized in SS7 packets and
sent from the SSP. Usually they contain requests to connect or disconnect a
call. These packets are forwarded to the destination SSP where the call is
terminated. It is the STP network's job to properly route such packets to the
destination.
Non-circuit
based messages that originate from an SSP are database queries requesting
additional information needed to complete the call. It is the STP network's job
to properly route packets between the SSP and the database interface known as
the SCP. These packets are routed to the destination SCP and are addressed to
the appropriate subsystem database. The SCP is the interface to the database
that provides the routing number required to complete the call.
STPs
also measure traffic and usage. Traffic measurements provide statistics such as
network events and message types, and usage measurements provide statistics on
the access and number of messages per message type. This information is used by
the carrier's network planning teams to look at overall system capacity and
future planning.
Global Title Translation
In
addition to performing basic SS7 packet routing, STPs are capable of performing
gateway services such as global title translation. This function is used
to centralize the SCP and database selection versus
distributing all possible destination selections to hundreds or thousands of
distributed switches. If the SSP is unaware of the destination SCP address, it
can send the database query to its local STP. The STP then performs global
title translation and re-addresses the destination of the database query to the
appropriate SCP.
Global
title translation centralizes the selection of the correct database by enabling
queries to be addressed directly to the STP. SSPs, therefore, do not have the
burden of maintaining every potential destination database address. The term global
title translation is
taken from the term global title digits, which is another term for dialed
digits.
The
STP looks at the global dialed digits and through its own translation table to
resolve the following:
·
The point code address of the appropriate SCP for the
database
·
The subsystem number of the database
The
STP also can perform an intermediate global title translation by using its
translation table to find another STP. The intermediate STP then routes the
message to the other STP to perform the final global title translation.
STP Hierarchy
STP
hierarchy defines network interconnection and separates capabilities into
specific areas of functionality. STP implementation can occur in multiple
levels, such as:
·
Local Signal Transfer Point
·
Regional Signal Transfer Point
·
National Signal Transfer Point
·
International Signal Transfer Point
·
Gateway Signal Transfer Point
The
local, regional, and national STPs transfer standards-based SS7 messages within
the same network. These STPs usually are not capable of converting or handling
messages in different formats or versions.
International
STPs provide international connectivity where the same International
Telecommunication Union (ITU) standards are deployed in both networks.
Gateway STPs can
provide the following:
·
Protocol conversion from national versions to the ITU
standard
·
Network-to-network interconnection points
·
Network security features such as screening, which is used to
examine all incoming and outgoing messages to ensure authorization
You can deploy and
install STP functions on separate dedicated devices or incorporate them with
other SSP functions onto a single end office or tandem
switch. Integrating SSP and STP functions is particularly common in
Europe and Australia. This is why fully associated SS7 or CCS7 (CCS7 is the
ITU-T version of SS7) networks are prevalent in those areas. Fully
associated SS7 occurs
when the same transmission channel carries the bearer's information and the
signaling information.
SCP
The SCP, as
shown provides the interface to the
database where additional routing information is stored for non-circuit based
messages. Service-provider SCPs do not house the required information; they do,
however, provide the interface to the system's database. The interface between
the SCP and the database system is accomplished by a standard protocol, which
is typically X.25. The SCP provides the conversion between the SS7 and the X.25
protocol. If X.25 is not the database access protocol, the SCP still provides
the capability for communication through the use of primitives.
The database stores
information related to its application and is addressed by a subsystem number,
which is unique for each database. The subsystem number is known at the SSP
level; the request originated within the PSTN contains that identifier. The
subsystem number identifies the database where the information is stored and is
used by the SCP to respond to the request.
The following
databases are the most common in the SS7 network:
·
800 DatabaseProvides
the routing information for special numbers, such as 800, 877, 888, 900, and
976 numbers. The 800 database responds to the special number queries with the
corresponding routing number. In the case of 800, 888, and 900 numbers, the
routing number is the actual telephone number at the terminating end.
·
Line Information
Database (LIDB)Provides subscriber or user information such as screening and
barring, calling-card services including card validation and personal
identification number (PIN) authentication, and billing. The billing features
of this database determine ways you can bill collect calls, calling-card calls,
and third-party services.
·
Local Number
Portability Database (LNPDB)Provides the 10-digit Location Routing Number (LRN)
of the switch that serves the dialed-party number. The LRN is used to route the
call through the network, and the dialed-party number is used to complete the
call at the terminating SSP.
·
Home Location
Register (HLR)Used in cellular networks to store information such as current
cellular phone location, billing, and cellular subscriber information.
·
Visitor Location
Register (VLR)Used in cellular networks to store information on subscribers
roaming outside the home network. The VLR uses this information to communicate
to the HLR database to identify the subscriber's location when roaming.
