Telecommunication Systems 1

TELECOMMUNICATION SYSTEMS

By

Syed Bakhtawar Shah Abid

Lecturer in Computer Science

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SS7 Network Architecture

SS7 can employ different types of signaling network

structures.

The worldwide signaling network has two functionally

independent levels:

– National

– International

This structure makes possible a clear division of

– Responsibility for signaling network management

– Numbering plans of SS7 nodes belonging to the international and

national networks.

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SS7 Network Architecture

SS7 network nodes are called signaling points (SPs).

– Each SP is addressed by an integer called a point code (PC).

– The international network uses a 14-bit PC.

– The national networks also use a 14-bit PC—except North America and

China, which use an incompatible 24-bit PC, and Japan, which uses a

16-bit PC.

– The national PC is unique only within a particular operator's national

network.

– International PCs are unique only within the international network.

– Other operator networks (if they exist) within a country also could have

the same PC and also might share the same PC as that used on the

international network.

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Signaling Links and Linksets

SPs are connected to each other by signaling links over which

signaling takes place.

The bandwidth of a signaling link is normally 64 or 56 kbps.

High-speed links have been introduced that use an entire 1.544

Mbps T1 carrier for signaling.

Links are typically engineered to carry only 25 to 40 percent of

their capacity so that in case of a failure, one link can carry the

load of two.

To provide more bandwidth and/or for redundancy, up to 16

links between two SPs can be used.

Links between two SPs are logically grouped for administrative

and load-sharing reasons.

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Signaling Links and Linksets

A logical group of links between two SP is called a linkset.

A number of linksets that may be used to reach a particular

destination can be grouped logically to form a combined linkset.

A group of links within a linkset that have the same characteristics

(data rate, terrestrial/satellite, and so on) are called a link group.

Normally the links in a linkset have the same characteristics, so the

term link group can be synonymous with linkset.

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Routes and Routesets

SS7 routes are statically provisioned at each SP. There are no

mechanisms for route discovery.

A route is defined as a preprovisioned path between source and

destination for a particular relation.

The following figure shows a route from SP A to SP C.

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Routes and Routesets

All the preprovisioned routes to a particular SP destination are called

the routeset.

The following Figure shows a routeset for SSP C consisting of two

routes.

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SS7 Node Types

There are three different types of SPs

The following figure graphically represents these nodes

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Signal Switching Point

SSPs are voice switches which exists in the embedded telephone switch circuit, that have SS7 software and terminating signaling links.

An SSP can originate and terminate messages

An SSP voice switch have the ability to send messages using ISDN User Part (ISUP) (call setup and teardown) & TCAP (database lookup) SS7 protocols.

The Transaction Capabilities Application Part (TCAP) of the SS7 protocol allows services at network nodes to communicate with each other using an agreed-upon set of data elements.

SSPs create packets (signal units) and send those messages to other SSPs, as well as queries to remote shared databases to find out how to route calls.

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Signal Transfer Point

STPs are packet switches, and act like routers in the SS7 network.

Routes each incoming message to an outgoing signaling link, based on

routing information contained in the SS7 message and a pre-defined

route table.

Messages are not usually originated by an STP.

Does not offer termination services

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Levels of STP

There are three levels of STPs.

– National STP

National STP exists within the national network

It can transfer messages that use the same national standards of protocol.

Protocol converters often interconnect a National and an International STP by converting from ANSI to ITU-TS.

International STP

– An International STP functions within an international network.

– It provides for SS7 interconnection of all countries, using the ITU-TS protocol.

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Levels of STP

Gateway STP – A Gateway STP converts signaling data

from one protocol to another.

– Gateway STPs are often used as an access point to the international network

– Gateway STP also serves as an interface into another network's databases

Example

– When International STP & National STP calling to each others, if International STP is in US and National STP is in Europe, for protocol translation, the STP used is called Gateway STP.

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Signal Control Point

An SCP is usually a computer used as a front end to a database system.

A Service Control Point (SCP) acts as an interface between telecommunications databases and the SS7 network. or It is an interface to Telco databases and other application-specific databases.

– Call management DB – call processing, routing, 800, 900

– Home / Visitor location register – cell phones

– Line information – billing

– Local number portability

A popular example is freephone calling (known as toll-free).

The SCP provides the routing number (translates the toll-free number to a routable number) to the SSP to allow the call to be completed.

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Databases Accessible via SCP

HLR

Home Location

Register

Used in cellular networks to store subscriber

information.

LNP

Local Number

Portability

Allows people to change service providers

but keep their same telephone number

OSS

Operation Support

System

Associated with remote maintenance center

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).

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SS7 Signaling Architecture

STP

transfer STP

SCP

control

SSP

Switching

SSP

STP STP

SCP

SS7 link

typical speed

56K or 64K

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SS7 Links

An SS7 link is the physical transmission line (serial 56/64 Kbps or DS0 channel) that connects the individual nodes in an SS7 network.

SS7 Link Types

– Access links (A links)

– Crossover links (C links)

– Bridge links (B links)

– Diagonal links (D links)

– Extended links (E links)

– Fully associated links (F links)

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A-Links

Access links (A links) provide

access to the network.

They connect "outer" SPs (SSPs

or SCPs) to the STP backbone.

Always two A links, one to each of

the home STP pairs

A links connect SSPs and SCPs to

their serving STP or STP mated

pair.

Maximum of 16 A links into one

STP

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B-Links

Bridge links (B links) are used to connect

mated pairs of STPs to each other across

different regions within a network at the same

hierarchical level.

These links help form the backbone of the

SS7 network.

The following figure shows two sets of mated

pairs of B links.

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D-Links

Diagonal links (D links) are the same as B links in that they connect mated STP pairs.

The difference is that they connect mated STP pairs that belong to different hierarchical levels or to different networks altogether.

For example, they may connect an interexchange carrier (IXC) STP pair to a local exchange carrier (LEC) STP pair or a cellular regional STP pair to a cellular metro STP pair.

Max. of 8 D links between mated STP pairs

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C-Links

Cross links (C links) are used to connect two STPs to form a mated pair—that is, a pair linked such that if one fails, the other takes the load of both.

Under normal conditions, they are used only to carry network management messages.

Not used for routing, up to 8 C links between paired STPs

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E & 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

E links connect to a non-home STP pair

Fully associated links (F-links) directly connect two signaling end points (SSPs and/or SCPs).

These links are not usually used in networks with STPs because they allow associated signaling only.

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SS7 Network Segments

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SS7 Network Segments

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Thank You

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