Components of SS7

Signal Switching Points (SSP’s). SSP’s are telephone switches (end offices or tandems) equipped with SS7 capable software and terminating signalling links. An SSP sends signalling message to other SSP’s to setup, manage, and release voice circuits required to complete a call. An SSP may also send a query message to a centralized database (an SCP) to determine how to route a call. Sometimes SSP also referred  to as service switching point.

Signal Transfer Point (STPS). STPS are the packet switches of the SS7 network. An STP routes each incoming message to an outgoing signalling link based on routing information contained in the SS7 message. Because it acts as a network hub, an STP provides improved utilization of the SS7 network by eliminating the need for direct links between signalling points.  An STP may perform global title translation, a procedure by which the destination signalling point is determined from digit present in the signalling message. An STP can also act as a ‘‘fire wall’’ to screen SS7 messages exchanged with other networks.

Signal Control Points (SCP’s). SCP’s are databases that provide information necessary for advanced call processing capabilities. 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 prime number is busy or the call is unanswered within a specified time.

SCP’s and STP’s are generally deployed in mated pair configurations in separate physical locations to ensure network wide service in the event of isolated failure.

SS7 signalling link types. Signalling links are logically organized by link type (A through F)
according to their use in the SS7 signalling network. SS7 signalling links are characterized
according to their use in the signalling network. Virtually all links are identical in that they are 56 kbps or 64 kbps bidirectional data links that support the same lower layers of the protocol

A links. ‘A’ links interconnect an STP and either an SSP or SCP which are collectively referred to as signalling end points. ’A’ stands for access, because only messages originating from or destined to the signalling end point are transmitted on an ‘A’ link. Examples of ‘A’ links are 1–9, 3–10, 3–4, 3–8, 4–7, 5–9, 7–8 and 7–10 in Fig. 7.13.
Signalling that SSP or SCP switches to send to any other node is sent on either of it’s a links to its home STP, which in turn, processes or routes the messages. Examples are 4–7–10 (or) 8–7–10 in Fig. 7.13. Similarly, message intended for an SSP or SCP will be routed to one of its home STP which will forward them to the addressed node over its ‘A’ links.

Features of SS7

Introduction of this form of signaling is essential for the development of telecom networks in any country because of the following features:

1. Internationally standardised by the ITU.

2. SS7 is suitable for any transmission medium i.e., can be operated over both terrestrial and satellite links.

3. Even though SS7 is optimised to work with digital SPC exchanges utilising 64 kbps digital channels, it is suitable for operation over analog channels.

4. SS7 is suitable for various communication services such as telephony, text, data, images and video.

5. Transport mechanism is application independent.

6. High performance and flexibility along with a future oriented concept which meets new requirements.

7. High reliability for message transfer.

8. Processor friendly structure of the messages (signal units in multiples of 8 bits).

9. SS7 networks are much faster, efficient in the call setup and teardown process.

10. Flexible structure which accommodates all the technical advancement in telecommunication systems.

Purpose of SS7 Network

Purpose of SS7 Network

1. An interesting feature of SS7 is that, it is a prerequisite for introduction of the following

new services :

(a) The SS7 provides the internal control and network intelligence essential to an ISDN.

(b) Intelligent Network (IN) which uses network elements such as SSP, SCP and intelligent

peripheral (IP), for achieving improved subscriber substation.

(c) Personal Communication Systems (PCS) which uses SS7 to provide personalised voice,

data, image and video communication services that can be accessed regardless of

location, network and time utilizing advanced microcell technology. It enables personalised

billing, personalised numbering (one number for one person at any place at

any time) and time and location independent charging.

2. The primary purpose of SS7 was to access remote database to lookup. This lookup

process has several benefits because the carriers do not have to maintain a full database at

each switching node, but they know how to get the remote data base and find the information

quickly.

3. SS7 achieves, increased revenue generation, additional call charging capabilities,

variety of calling-card services.

SS7 - Purpose

4. SS7, in integration with SPC systems enables quick and efficient call setup and teardown across the network in less than one second. Also, this integration provides for better supervision, monitoring and billing systems integration.

5. SS7 enables full use of the channel for the talk path because the signalling is done out of band on its own separate channel.

6. SS7 achieves enhanced call features such as call forwarding, calling party name/ number display and three way calling. Also with SS7, efficient and secured world wide communication is possible. It also provides, toll free and toll wireline services.

7. It handles the rerouting of network by using automatic protection switching services such as those found in SONET.

8. SS7 uses packet switching concept. Hence SS7 network is capable of preventing the misrouted calls, duplication of call requests and lost packets (requests for service).

Forms of Signalling

(i) Supervisory signals or line signals. These are the signals necessary to initiate a call setup and to supervise it, once it has been established. It is also referred as subscriber loop signalling. Line signals can be transmitted by the use of a single control channel in each direction line.

Forms of Signalling

(i) Supervisory signals or line signals. These are the signals necessary to initiate a call setup and to supervise it, once it has been established. It is also referred as subscriber loop signalling. Line signals can be transmitted by the use of a single control channel in each direction line.

(ii) Routing signals or register signals. Information transfer related to call setup is usually referred to as register signals. The basic information is the dialed code which indicates to the subsequent switching centres the required routing. In addition to the basic information, signals such as route  information, terminal information, register control signals, acknowledgement signals, status of called terminal etc are also involved.

(iii) Management signals or interregister signalling. These signals are used to convey information or control between exchanges. This signalling also referred as inter exchange signalling. This signalling involves remote switching of private circuits, routing plans, modification of routing plans, traffic over load, priority of the call, class of service etc. The signalling may be performed by link-by-link basis, which passes signals exchange to exchange or end-to-end signalling which is between originating and terminating exchange referred as line signalling.

In PCM systems, signalling and speech are sampled, coded and transmitted within the frame of PCM channels. Thus, with PCM, a convenient way of transmission is possible. The signalling information and speech information carried in the same time slot is referred as inslot signalling. The signalling information carried in a separate time slot is referred as outslot signalling