Figure 2. The components of an SS7 signalling network [4]
2 SS7 architecture
2.1 The components of SS7 [1][2][4]
A telecommunication network consists of a number of switches and application
processors intrconnected by transmission circuits. The SS7 network exists
within the telecommunication network and control it. SS7 schieves this
control by creating and transfering call processing, network management,
and maintenance message to the network's various components.
An SS7 network has three distinct components:
1) SSP - Service Switching
Point,
2) STP - Signal Transfer
Point;
3) SCP - Service Control
Point.
Figure 2. The components of an SS7 signalling network [4]
Service Switching Points
SSPs are telephone switches interconnected by SS7 links. The SSPs perform call processing on calls that originate, tandem, or terminate at that site. As part of this call processing, the SSP may generate SS7 messages to transfer call-related information to other SSPs, or to send a query to a SCP for instructions on how to route a call. An SCP sends a response to the originating SSP containing the routing number(s) associated with the dialed number. An alternate routing number may be used by the SSP if the primary number is busy or the call is unanswered within a specified time. Actual call features vary from network to network and from service to service[3][4].
Signaling Transfer Points
STP are switches that relay messages between network switches and databases. Their main function is to route SS7 messages to the correct outgoing signaling link, based on information contained in the SS7 messages address fields. Because it acts as a network hub, an STP provides improved utilization of the SS7 network by eliminating the need for direct links between signaling points. An STP may perform global title translation, a procedure by which the destination signaling point is determined from digits present in the signaling message. An STP can also act as a "firewall" to screen SS7 messages exchanged with other networks[3][4].
Service Control Points
SCPs contains centralized network databases for providing enhanced services. The SCP accepts queries from an SSP and returns the requested information to the originator of the query.
2.2 SS7 Reliability[4]
To meet the stringent reliability requirements of public telecommunications networks, a number of safeguards are built into the SS7 protocol:
SS7 is designed to provide an internationally standardized, general-purpose signaling system; however, SS7 was not intended to be used as the signaling standard for access to the telephone network from PBXs or from telephone sets. To satisfy this latter need, the Integrated Services Digital Network - Access Protocol (ISDN-AP) has been developed. Together, SS7 and the ISDN-AP provide the end-to-end signaling required to deliver enchanced features to users. As an interim step, many telephone exchange carriers use proprietary access signaling to provide enchanced services.
2.4 SS7 Protocol Stack mapped onto the OSI model
The SS7 standard was designed to map onto the OSI 7-Layer Reference
Model. The comparison of the OSI Reference Model and the SS7 protocol stack
is illustrated in figure 2.4.
The bottom half of the SS7 protocol consists of the Massege Transfer
Part (MTP). three are three levels to the MTP: Level 1 corresponds to the
OSI Layer 1 (Physical Layer); Level 2 corresponds to OSI Layer 2 (Data
Link Layer); and, Level 3 corresponds to the bottom of OSI Layer 3 (Network
Layer).
The upper half of the SS7 protocol consists of several parts.The SS7
Signaling Connection Control Part (SCCP) corresponds to the top of OSI
Layer 3. The ISDN-User Part (ISDN-UP) maps onto OSI Layer 3 as well, and,
in addition it maps onto Layer 4 (Transport Layer), Layer 5 (Session Layer),
Layer 6 (Presentation Layer), and Layer 7 (Application Layer). The transaction
Capabilities Application Part (TCAP), the Application Service Elements
(ASE), and the Operations, Maintenance and Administration Part (OMAP)
of SS7 protocol all map onto OSI Layer 7 as well[3][4].
Figure 2.4 Similarities of the SS7 Protocol and the OSI model
[4]
3 Message Transfer Part[2][3][4]
3.1 MTP Level 1
The MTP Level is known as the signaling data link. MTP Level 1 is equivalent
to the OSI model Physical Layer (Layer 1). This layer defines the physical,
electrical, and functional characteristics of the signaling inks connecting
SS7 components.
A signaling link is a bidirectional transmission path for signaling,
comprising two data channels operating together in opposite directions
at the same data rate.
3.2 MTP Level 2
The MTP Level 2 together with MTP Level 1, provides signaling link for
reliable transfer of signaling messgaes between two directly connected
signaling points. MTP Level 2 maps onto Layer 2 of the OSI 7-Layer Model.
An SS7 message is called a signal unit (SU). There are three kinds
of signal units: Fill-In Signal Units (FISUs), Link Status Signal Units
(LSSUs), and Message Signal Units (MSUs). They are differentiated by the
value contained in the length indicator (LI) field:
3.3 MTP Level 3
The MTP Level 3 provides the functions and procedures related to message
routing and network management. MTP Level 3 handles these functions,
assuming that signaling points are connected with signaling links as described
in MTP Level 1 and Level 2. The MTP Level 3 maps onto Layer 3 of OSI 7-Layer
Model.
MTP Level 3 has signaling message handling functions, and signaling
network management functions. The signaling message handling functions
are made up of message routing, discrimination and distribution; these
functions are performed at each signaling point in signaling network. The
signaling network management functions provide the actions and procedures
required to active and maintain signaling service, and to restore normal
signaling conditions in the event disruption in the signaling network,
either in signaling links or at signaling points..
4 SCCP - Signaling Connection Control Part [1][2][3]
SCCP provides enhancements to the MTP Level 3 to provide connectionless
and connection-oriented network services, as well as to address translation
capabilities. The SCCP enhancements to the MTP services provide a network
service which is equivalent to the OSI Network Layer 3.
SCCP is used as the transport layer for TCAP-based services such as
freephone (800/888), calling card, wireless roaming, and personal communications
services (PCS).
4. 1 GTT - Global Title Translation
SCCP also provides the means by which an STP can perform global title translation, a procedure by which the destination signaling point and subsystem number (SSN) is determined from digits (i.e., the global title) present in the signaling message. The global title digits may be the dialed 800/888 number, calling card number, or mobile subscriber identification number depending on the service requested. Because an STP provides global title translation, originating signaling points do not need to know the destination point code or subsystem number of the associated service. Only the STPs need to maintain a database of destination point codes and subsystem numbers associated with specific services and possible destinations.
4.2 SCCP Message Format
SCCP messages are contained within the signaling information field (SIF) of an MSU. The SIF contains the routing label followed by the SCCP message contents. The SCCP message is comprised of a one-octet message type field followed by the mandatory fixed part (mandatory fixed-length parameters), mandatory variable part (mandatory variable-length parameters), and the optional part. Each optional part parameter is identified by a one-octet parameter code followed by a length indicator field.
5 Integrated Services Digital Network User Part (ISDN-UP)[2][4]
The basic functionof the ISDN-UP is the control of circuit switched
network connections between subscriber line exchange terminations. This
includes basic voice and data services, and supplementary services. The
ISDN-UP provides some of the functionality of OSI Layers 3 through 7.
.The ISDN User Part (ISUP) defines the protocol and procedures used
to set-up, manage, and release trunk circuits that carry voice and data
calls over the public switched telephone network (PSTN). ISUP is used for
both ISDN and non-ISDN calls. Calls that originate and terminate at the
same switch do not use ISUP signaling.
5.1 Basic ISUP Call Control
1.When a call is placed to an out-of-switch number, the originating SSP transmits an ISUP initial address message (IAM) to reserve an idle trunk circuit from the originating switch to the destination switch. The IAM includes the originating point code, destination point code, circuit identification code, dialed digits and, optionally, the calling party number and name.
2.The destination switch examines the dialed number, determines that it serves the called party, and that the line is available for ringing. The destination switch transmits an ISUP address complete message (ACM) to the originating switch (via its home STP) to indicate that the remote end of the trunk circuit has been reserved. The destination switch rings the called party line and sends a ringing tone over the trunk to the originating switch. The STP routes the ACM to the originating switch which connects the calling party's line to the trunk to complete the voice circuit from the calling party to the called party. The calling party hears the ringing tone on the voice trunk.
3.When the called party picks up the phone, the destination switch terminates the ringing tone and transmits an ISUP answer message (ANM) to the originating switch via its home STP. The STP routes the ANM to the originating switch which verifies that the calling party's line is connected to the reserved trunk and, if so, initiates billing.
4.If the calling party hangs-up first, the originating switch sends an ISUP release message (REL) to release the trunk circuit between the switches. The STP routes the REL to the destination switch. If the called party hangs up first, or if the line is busy, the destination switch sends an REL to the originating switch indicating the release cause (e.g., normal release or busy).
5.Upon receiving the REL, the destination switch disconnects the trunk
from the called party's line, sets the trunk state to idle, and transmits
an ISUP release complete message (RLC) to the originating switch to acknowledge
the release of the remote end of the trunk circuit. When the originating
switch receives (or generates) the RLC, it terminates the billing cycle
and sets the trunk state to idle in preparation for the next call.
ISUP messages may also be transmitted during the connection phase of
the call (i.e., between the ISUP Answer (ANM) and Release (REL) messages.
5.2 ISUP Message Format
ISUP information is carried in the Signaling Information Field (SIF)
of an MSU. The SIF contains the routing label followed by a 14-bit (ANSI)
or 12-bit (ITU) circuit identification code (CIC). The CIC indicates the
trunk circuit reserved by the originating switch to carry the call. The
CIC is followed by the message type field (e.g., IAM, ACM, ANM, REL, RLC)
which defines the contents of the remainder of the message.
Each ISUP message contains a mandatory fixed part containing mandatory
fixed-length parameters. Sometimes the mandatory fixed part is comprised
only of the message type field. The mandatory fixed part may be followed
by the mandatory variable part and/or the optional part.The mandatory variable
part contains mandatory variable-length parameters. The optional part contains
optional parameters which are identified by a one-octet parameter code
followed by length indicator field.
5.2.1 Initial Address Message
An Initial Address Message (IAM) is sent in the "forward" direction by each switch needed to complete the circuit between the calling party and called party until the circuit connects to the destination switch. An IAM contains the called party number in the mandatory variable part and may contain the calling party name and number in the optional part.
5.2.2 Address Complete Message
An Address Complete Message (ACM) is sent in the "backward" direction to indicate that the remote end of a trunk circuit has been reserved. The originating switch responds to an ACM message by connecting the calling party's line to the trunk to complete the voice circuit from the calling party to the called party. The calling party hears the ringing tone on the voice trunk generated by the destination switch.
5.2.3 Answer Message
When the called party answers, the destination switch terminates the ringing tone and sends an Answer Message (ANM) to the originating switch. The originating switch initiates billing after verifying that the calling party's line is connected to the reserved trunk.
5.2.4 Release Message
A Release Message (REL) is sent in either direction indicating that the circuit is being released due to the cause indicator specified. An REL is sent when either the calling or called party "hangs up" the call. An REL is also sent in the backward direction if the called party line is busy.
5.2.5 Release Complete Message
A Release Complete Message (RLC) is sent in the opposite direction of the REL to acknowledge the release of the remote end of a trunk circuit and end the billing cycle as appropriate.
5.2.6 Telephone User Part
In some parts of the world (e.g., China), the Telephone User Part (TUP) supports basic call processing. TUP handles analog circuits only; digital circuits and data transmission capabilities are provided by the Data User Part.
6 Transaction Capabilities Application Part (TCAP)[1][2][3]
The TCAP is an SS7 application protocol which can be used by various
applications. TCAP provides non-circuit related information transfer capabilities
and generic services to applications, yet remains independent of application.
TCAP maps into the OSI Layer 7.
TCAP enables the deployment of advanced intelligent network services
by supporting non-circuit related information exchange between signaling
points using the SCCP connectionless service. An SSP uses TCAP to query
an SCP to determine the routing number(s) associated with a dialed 800,
888, or 900 number. The SCP uses TCAP to return a response containing the
routing number(s) (or an error or reject component) back to the SSP. Calling
card calls are also validated using TCAP query and response messages. When
a mobile subscriber roams into a new mobile switching center (MSC) area,
the integrated visitor location register requests service profile information
from the subscriber's home location register (HLR) using mobile application
part (MAP) information carried within TCAP messages.
TCAP messages are contained within the SCCP portion of an MSU. A TCAP
message is comprised of a transaction portion and a component portion.
6.1 Transaction Portion
The transaction portion contains the package type identifier. There are seven package types:
6.2 Component Portion
The component portion contains components. There are six kinds of components:
7. Operations, Maintaince and Administration Part (OMAP)[4]
The OMAP provides procedures related to operations and maintaince functions. OMAP corresponds to the OSI model's Application Layer.
7.1 Management Dodel
Two OMAP ASEs are defined: the Message Routing Verification Test, and the Circuit Validation Test. The OMAP ASEs use the services of TCAP to perform their functions.
The ISDN-UP and TCAP have already covered some features and enhanced services made possible with SS7 signaling. From a broader, end-user perspective, SS7 and adunct services will enhance telecommunications capabilities by making features available on a network-wide basis.
8.1 Voice Message Integration
Calling an associate in another city may result in the call being answered by a voice messaging system. You may be prompted by one of the voice recordings:
"Press 1 to leave a message."
"Press 2 to activate ring
again."
"Press 3 to speak to an
operator."
Press "1" might store the calling line ID in the voice messging system. When your busy associate checks her voice messaging system and hears your message, she may enter a one-digit code to transfer her call back to your number, simplying the return call process. Precess "2" would be a convenient way to activate Network Ring again, while pressing "3" would provide access to an operator.
8.2 Network Automatic Call Distribution (NACD)
NACD is a natural fit for organizations such as airlines that have customer service centers located across the country over different time zones. NACD can improve customer service by pooling NACD agent at different locations into one large group. If one NACD location is very busy or out of service, incoming calls would be automatically routed to alternate sites. The same number of agents could be now handle more calls, with a reduced average answering time. The airline gains sales, since more customers get through to agents and fewer customers hang up and a call competitor. The airline would also have more flexibility in selecting customer service locations and setting staff levels at each site.
8.3 Virtual Private Networking (VPN)
VPN is a service which provides multi-site organization with the appearance
of a private network with dedicated inter-switch circuits and a private
dialing plan. However, in reality, the customer is part of a shared network
within other VPN customer, or the customer is using the public network.
VPNs provide users with a number of advantages of both private and public
networks.
Organizations with multiple sites which are not large enough to support
dedicatedconnections can use shared VPN facilities to connect the sites.However,
the use of the shared VPN facilities still provides the advantages of a
private network, such as access to long-haul facilities and services, private
numbering plan, and feature transparency.
8.4 Customer Area Signaling Services (CLASS)
CLASS provides residential subscribers with many of the enhanced features previously available only with PBXs or Centrex service. The flexibility offered by these services can provide substantial benefits to busy users. From a distinctive ring for the in-laws, to the rejection of a particular telemarketing firm, there is a useful service for all.
8.5 Enhanced 800 Service
Enhanced 800 service options provide organizations with considerable
flexibility in tailoring 800 services to their unique requirements. With
the time-of-day and day-of-week routing options, organizations can set
up full-time customer service centers backed up by part-time centers which
only operate during peak business hours. All calls after hours, or on week-ends,
would automatically be routed to the main business center.
Call routing based on location of caller provides additional flexibility.
With this option not only could the call be routed based on the ared code,
but also on the NXX (the office code) as well. An organization could select
an 800 number to be valid only in certain cities or regions within a state.
When combined, 800 service with these enhanced features provide a powerful
and flexible way to meet certain telecommunications requirements.
9 Summary
This essay has presented an overview of the SS7. ehanced services and
telephone network features once viewed as luxuries are quickly besoming
necessities in today's fast paced world. Residential telephone service
customers as well increasingly are demanding the efficiency and convenience
advanced telecommunications technology can provide. Naturally, telephone
operating companies want to justify the demands efficiently. Tranditional
signaling systems cannot provide the level of sophistication required to
deliver much more than basic telephone service. To deliver the enhanced
services, telecomminutions carriers need new, intelligent message-based
signalling systems. The solution is SS7.
This essay just presents a brief overview to SS7, and give a general
appreciation and understanding of its structure and impact. It covers the
necessarity and step by step action procedures that occur in the network
when SS7 functionality is being used.
References
[1] Denis Pierrot & Jean Pierre Allege, The HP OpenCall SS7 platform
allows
users to build computer-based signaling
applications connected to SS7 signaling
network, August, 1997 [referred 20.
11. 1998]
<
http://hpcc923.external.hp.com/hpj/97aug/au97a7.htm>
[2] MicroLegend, MicroLegend SS7 Tutorial, February 6, 1998 [referred
20.11.1998]
<
http://www.microlegend.com/aboutss7.htm>
[3] Sudeep K. Palat, CCITT Signalling System No 7, Apr 6, 1997 [referred
20 .11.1998]
<
http://www.item.ntnu.no/~palat/GSM/node8.html>
[4] Toni Beninger, SS7 Basics, Telephony Div., Intertec Publishing
Corp., 1991 [refferred 20.11.1998]
[5] Walt Mansell, Telecommunications Signaling and the SS7 Network,
1998 [referred
20.11.1998]
<
http://w3.mit.edu/org/t/telecom/www/Ss7/ss7.html>
Further Information
More information related to the topics of SS7 can be found in following link list.
Feature
Story
An SS7 troubleshooting guide
for LNP.
Integrated SS7
Description of the funcationality,
features, and applications of the integrated SS7.
Intelligent
Network (IN) Tutial
A tutorial discusses the
networks' evolution from switch-based service logic services
to service-independent Advanced
Intelligent Networks (AIN)
INTELLISS7
APIs: INAP
List of IntelliNet Technologies'
SS7 API functions.
Internet Telephony
Over Frame Relay
Information about frame
relay technology which enabled network performance to move
from kilobits per second
(Kbps) to megabits per second(Mbps).
Major Features of the DSC
INfusion STP C32
Describing the major features
of DSC COMMUNICATION's INfusion Signal Transfer
Point C32 (STP C32).
Major Features Of The MegaHub
STP
Information about the major
features of DSC COMMUNICATION's MegaHub Signal
Transfer Point family.
MicroLegend SS7/C7 - Protocol
Converters
Information of MicroLegend,
MicroLegend's SS7/C7- Protocol Converters
MicroLegend VSP - Versatile
Signaling Point
Information of MicroLegend,
MicroLegend's Versatile Signalling Point
Signaling
protocol ushers in intelligent nets
Discription of the model
of Intelligent networks whch offers the control and
invocation of services through
the network.
SS7 A-Link Concentration
Information about DTI’s
DXC Tandem Switch with SS7 which can be used to
concentrate the SS7 A-Links
in a network.
SS7
networks in a PCS World
Describing the functionality
of SS7 in the field of personal communication services.
SS7 Overview
An overview of different
SS7 vendors's SS7 hardware and software platforms
SS7 - Signaling System 7 Controller
The functionality of Excel's
Signaling System 7 controller hardware.
THE WORLD
SS7 MARKET
The table of contents of
The World SS7 Market.