Signaling Information Components
Signaling is defined as exchange of information between call components which is required to provide and maintain services.
All PSTN users exchange signaling with the network elements all the time. Signaling between a telephone user and telephone network includes dialing digits, providing dial tone, accessing voice mailbox, and sending call waiting tone
Definition Of Signaling System 7 (SS7)
SS7 is defined or known as elements of the telephone network exchange information.SS7 messages are carried in the form of messages.SS7 is characterized by high-speed packet data and out-of-band signaling.
Signaling system 7 is an architecture created to perform out-of-band signaling in support of call establishment, billing, routing, and information-exchange function of the public switched telephone network (PSTN).It identifies functions to be performed by a signaling-system network and a protocol to enable their performance.
Typical Figure of SS7 signaling system can be referred as below:
Signaling Points SSP = Service Switching Point STP = Signal Transfer Point, will be deployed usualy in pairs in case of failure SCP = Service Control Point, will be deployed usualy in pairs in case of failure.
SS7 Signaling Architecture
It is build using three essential components, which are interconnected via signaling links.
| Abbreviation | Name | Symbol |
| SSP | Signal's Switching Point or known as Service Switching Point | |
| STP | Signal's Transfer Point | |
| SCP | Signal's Control Point |
Signal's Switching Point
SSP's are switches which are installed with SS7 software and terminating signaling links. SSP's can be in the form of voice/SS7 combination switch or an adjunct computer system connected to a voice switch.
SSPs will create packets and send the created messages to other SSPs, it also sends queries to remote shared databases to find out how to route calls. SSPs can originate calls, terminate calls, or switch calls.
SSPs communicate with the voice switch through the use of primitives. It also has the capability to send messages using ISUP (call setup and teardown) and TCAP (database lookup) protocols.
SSP uses the dialed digits on determining how to route the call. It will look for the dialed digits in the SSPs routing table which will then be directed to the corresponding trunk circuit and terminating exchange. Then the SSP sends an SS7 message out to the adjacent exchange requesting a circuit connection on the trunk which was specified in the routing table.
After that, the adjacent exchange sends back an acknowledgement in giving permission to use that trunk. By using the calling party information which is contained in the setup info, the adjacent exchange will determine on how to connect to the final destination. This process might require several trunks to be set up between several different exchanges.
SSP will manage all of these connections until the destination is reached.
Signal's Transfer Point
STPs are known as packet switches, and it acts like routers in the SS7 network. Messages are not usually originated by an STP. Depending on the situation, an STP can also act like a firewall which functions in screening messages with other networks.
STPs will route SS7 messages to the outgoing signaling links through the SS7 network. STPs are categorized as the most versatile of all the SS7 entities, and play the role as a major component in the network.
The 3 levels of STPs are mentioned below:
- National Signal Transfer Point
- International Signal Transfer Point
- Gateway Signal Transfer Point
National Signal Transfer Point
It exists within the national network which varies according to the country. It is used to transfer messages that use the same national standard of protocol.
Messages through National STP can be passed to an International STP, but they can't be converted by the National STP. While conversion process, Protocol converters often interconnect a National and an International STP by converting from ANSI to ITU-TS.
International Signal Transfer Point
International Signal Transfer Point functions within an international network.
It is used to provide interconnection of all countries for SS7, using the ITU-TS standard protocol.
All nodes which are connecting to an International STP must use the ITU-TS protocol standard.
Gateway Signal Transfer Point
It is used in conversions of signaling data from one protocol to another. Gateway Signal Transfer Point often used as an access point to the international network.
Depending on the location, the Gateway STP must always be able to use both the International and National protocol standards.
Gateway STP must also be able to serve as an interface into another network's databases, such as from an interexchange carrier (IXC) to an end office.
The Gateway STP can also be configured and be used to screen for authorized users of the network.
It can also provide measurement of traffic and usage through the following means:
- Traffic - Process of Measuring the peg counts of the type of messages entering or leaving the network.
- Network events - Process of tracking events such as link out-of-service or local processor outage, for maintenance purposes
- Usage - Process of providing peg counts of the record number of messages by message type.
Signal's Control Point
Signal's Control Point normally regards to a computer which is used as a front end of a database system. It is an interface to telco databases, not usually to other, application-specific databases.
Telco database is normally linked to SCPs through X.25 links. The SCP should be able to provide protocol conversions from X.25 to SS7, or should be able to provide direct access to the database through the primitive's usage which can support access from one level of protocol to another.
Address of an SCP is known as a point code, while the address of the database which it interfaces with is called a subsystem number.
The database is an application entity which is accessed through the TCAP protocol.
Table below shows some Application-specific databases:
| Telco Databases | Name | Description |
| BSDB | Business Services Database | Allows companies to create and store proprietary databases, as well as create private networks. |
| CMSDB | Call Management Services Database | Provides information relating to call processing, network management, call sampling, routing, billing and third-party billing |
| HLR | Home Location Register | Used in cellular networks to store subscriber information. |
| LIDB | Line Information Database | Provides billing instructions. |
| LNP | Local Number Portability | Allows people to change telco service providers but keep their same telephone number. |
| OSS | Operations Support Systems | Associated with remote maintenance centers for monitoring and managing SS7 and voice networks. |
| VLR | Visitor Location Register | Used when a cell phone is not recognized by the mobile switching center (MSC). |
SS7 Links
SS7 link is the physical transmission line (serial 56/64 Kbps or DS0 channel) that connects the individual nodes in an SS7 network.
SS7 networks are built to be highly reliable and redundant. Link diversity is built into the network design, providing multiple signaling paths, so that there is no single point of failure.
This ensures that redundant links have the capacity to handle all routed network traffic.
Link Types
A-Links
Access Links (A-links) interconnect an STP and either an SSP or and SCP. Their sole purpose is to deliver signaling to and from signaling end points. End points always have at least two A-links (also called signaling beginning points).
Any signaling that an SSP or SCP needs to send to any other node in the SS7 network is sent on one of its a-Links to its “home”STP, which processes and routes the message along its way. Messages addressed to an SSP or SCP is routed to its “home” STP, which forwards them to the addressed node over its A-links.
B and D-Links
Bridge links (B-links) are the quad of links interconnecting peer pairs of STPs. Diagonal links (D-links) are the quad of links interconnecting mated pairs of STPs at different hierarchical levels.
Since SS7 network has no clear hierarchy, this links are referred to as B-links, D-links, or B/D-links.
C-Links
Cross links (C-links) interconnect mated STPs and are used to enhance the reliability of the signaling network not regularly used by SS7 traffic. They are used only when there has been a link failure which causes an STP to have no other route.
E and F-Links
Extended links (E-links) connect an SSP to an alternate STP to provide backup connectivity to the network if the SSP's “home” STP cannot be reached on its A-link.
Fully associated links (F-links) directly connect two signaling end points (SSP sans/or SCP's).They are not usually used in networks with STPs because they allow associated signals only, thus bypassing the security features provided with an STP.
Linksets
Links which are put together in groups are known as linksets. Up to 16 links can be assigned to one linkset. All links in a linksets must have the same adjacent node.
Switches will alternate traffic across all links in a linkset to ensure equal usage of all facilities in the network.
Linkset Characteristics
Links should be terrestrial. Satellite links can be used but are not preferred because of the inherent delay.
Alternate linksets are set up to provide backup paths when congestion occurs in the network. When a link fails, all other links within the linksets must take over.
If any SS7 entity such as an STP fails, its mate assumes the full traffic load. For this reason, SS7 entities are designed to send less then 40 percent of the traffic on any given link.
If any entity fails at 40-percent capacity, there is still enough room on its mate for it to carry the entire traffic load of the mated pair.
Routes
The signal point must define linksets and routes in SS7 messaging. Following entities are used in SS7 messaging:
- Route - Collection of linksets to reach a particular destination. A linkset can belong to more than one route.
- Routeset - Collection of routes that are assigned to destinations and also provide alternate routes.
- Destination - Address entered into the routing table of a remote signaling point. Destination need not be adjacent to the signaling point, but must be a point code that can be reached by the signaling point.
Point Codes
In SS7, addresses are assigned using a three-level hierarchy
- Member - Signaling point within a cluster
- Cluster - collection of signaling points
- Network - Each cluster is defined as being part of network
Node in the SS7 network can be addressed by three-level number defined by its network, cluster, and member numbers. Each of these numbers is an 8-bit number assigned a value from 0-255. This three-level address is called the “point code” of the signaling point.
Network Numbers
Network numbers are assigned on a nationwide basis. The numbers are relatively scarce. Smaller networks can be assigned one or more clusters within network numbers 1, 2, 3, and 4.
The smallest networks are assigned point codes within number 5. The cluster to which they are assigned determines the state or province they are in.
Network number 0 is not available and 255 are reserved.
SS7 Protocol Layers:
SS7 network is an interconnected set of network elements that is used to exchange messages in support of telecommunications functions.
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)
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 more common implementation.
Message Transfer Part - Level 2 (MTP-2)
MTP level 2 provides link-layer functionality. It ensures that the two end points of a signaling link can reliably exchange signaling messages. It also incorporates such capabilities as error checking, flow control, and sequence checking.
Message Transfer Part - Level 3 (MTP-3)
MTP level 3 extends the functionality provided by MTP level 2 to provide network layer functionality. It ensures messages can be delivered between signaling points across the SS7 network regardless of whether they are connected directly. It also includes capabilities such as node addressing, routing, alternate routing, and congestion control.
Signaling Connection Control Part (SCCP)
SCCP provides two major functions that lacks in MTP. First is the capability to address applications within a signaling point. MTP can only receive and deliver messages from a node as a whole and it does not deal with software applications within a node.
While MTP network-management messages and 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).
The SCCP allows these subsystems to be addressed explicitly.
ISDN User Part (ISUP)
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.
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
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