What Is Signaling System 7? – Signaling System 7 (SS7) is an architecture for performing out-of-band signalingin support of the call-establishment, billing, routing, and information-exchange functions 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.
What Is Signaling System 7?
What Is Signaling?
Signaling refers to the exchange of information between call components required to provide and maintain service.
As users of the PSTN, we exchange signaling with network elements all the time. Examples of signaling between a telephone user and the telephone network include: dialing digits, providing dial tone, accessing a voice mailbox, sending a call-waiting tone, dialing *66 (to retry a busy number), etc.
SS7 is a means by which elements of the telephone network exchange information. Information is conveyed in the form of messages. SS7 messages can convey information such as:
- I’m forwarding to you a call placed from 212-555-1234 to 718-555-5678. Look for it on trunk 067.
- Someone just dialed 800-555-1212. Where do I route the call?
- The called subscriber for the call on trunk 11 is busy. Release the call and play a busy tone.
- The route to XXX is congested. Please don’t send any messages to XXX unless they are of priority 2 or higher.
- I’m taking trunk 143 out of service for maintenance.
SS7 is characterized by high-speed packet data and out-of-band signaling.
What Is Signaling System 7?
What Is Out-of-Band Signaling?
Out-of-band signaling is signaling that does not take place over the same path as the conversation.
We are used to thinking of signaling as being in-band. We hear dial tone, dial digits, and hear ringing over the same channel on the same pair of wires. When the call completes, we talk over the same path that was used for the signaling.
Traditional telephony used to work in this way as well. The signals to set up a call between one switch and another always took place over the same trunk that would eventually carry the call. Signaling took the form of a series of multifrequency (MF) tones, much like touch tone dialing between switches.
Out-of-band signaling establishes a separate digital channel for the exchange of signaling information. This channel is called a signaling link. Signaling links are used to carry all the necessary signaling messages between nodes. Thus, when a call is placed, the dialed digits, trunk selected, and other pertinent information are sent between switches using their signaling links, rather than the trunks which will ultimately carry the conversation.
Today, signaling links carry information at a rate of 56 or 64 kbps. It is interesting to note that while SS7 is used only for signaling between network elements, the ISDN D channel extends the concept of out-of-band signaling to the interface between the subscriber and the switch. With ISDN service, signaling that must be conveyed between the user station and the local switch is carried on a separate digital channel called the D channel. The voice or data which comprise the call is carried on one or more B channels.
What Is Signaling System 7?
Why Out-of-Band Signaling?
Out-of-band signaling has several advantages that make it more desirable than traditional in-band signaling.
- It allows for the transport of more data at higher speeds (56 kbps can carry data much faster than MF outpulsing).
- It allows for signaling at any time in the entire duration of the call, not only at the beginning.
- It enables signaling to network elements to which there is no direct trunk connection.
Signaling Network Architecture
If signaling is to be carried on a different path from the voice and data traffic it supports, then what should that path look like? The simplest design would be to allocate one of the paths between each interconnected pair of switches as the signaling link. Subject to capacity constraints, all signaling traffic between the two switches could traverse this link. This type of signaling is known as associated signaling
Associated signaling works well as long as a switch’s only signaling requirements are between itself and other switches to which it has trunks. If call setup and management was the only application of SS7, associated signaling would meet that need simply and efficiently. In fact, much of the out-of-band signaling deployed in Europe today uses associated mode.
The North American implementers of SS7, however, wanted to design a signaling network that would enable any node to exchange signaling with any other SS7−capable node. Clearly, associated signaling becomes much more complicated when it is used to exchange signaling between nodes which do not have a direct connection. From this need, the North American SS7 architecture was born.
The North American Signaling Architecture
The North American signaling architecture defines a completely new and separate signaling network. The network is built out of the following three essential components, interconnected by signaling link:
- Signal Switching Points (SSPs)—SSPs are telephone switches (end offices or tandems) equipped with SS7−capable software and terminating signaling links. They generally originate, terminate, or switch calls.
- Signal Transfer Points (STPs)—STPs are the packet switches of the SS7 network. They receive and route incoming signaling messages towards the proper destination. They also perform specialized routing functions.
- Signal Control Points (SCPs)—SCPs are databases that provide information necessary for advanced call-processing capabilities.
Once deployed, the availability of SS7 network is critical to call processing. Unless SSPs can exchange signaling, they cannot complete any interswitch calls. For this reason, the SS7 network is built using a highly redundant architecture. Each individual element also must meet exacting requirements for availability. Finally, protocol has been defined between interconnected elements to facilitate the routing of signaling traffic around any difficulties that may arise in the signaling network.
To enable signaling network architectures to be easily communicated and understood, a standard set of symbols was adopted for depicting SS7 networks.
STPs and SCPs are customarily deployed in pairs. While elements of a pair are not generally co-located, they work redundantly to perform the same logical function. When drawing complex network diagrams, these pairs may be depicted as a single element for simplicity,
Basic Signaling Architecture
Figure shows a small example of how the basic elements of an SS7 network are deployed to form two interconnected networks.
The following points should be noted:
- STPs W and X perform identical functions. They are redundant. Together, they are referred to as a mated pair of STPs. Similarly, STPs Y and Z form a mated pair.
- Each SSP has two links (or sets of links), one to each STP of a mated pair. All SS7 signaling to the rest of the world is sent out over these links. Because the STPs of a mated pair are redundant, messages sent over either link (to either STP) will be treated equivalently.
- The STPs of a mated pair are joined by a link (or set of links).
- Two mated pairs of STPs are interconnected by four links (or sets of links). These links are referred to as a quad.
- SCPs are usually (though not always) deployed in pairs. As with STPs, the SCPs of a pair are intended to function identically. Pairs of SCPs are also referred to as mated pairs of SCPs. Note that they are not directly joined by a pair of links.
- Signaling architectures such as this, which provide indirect signaling paths between network elements, are referred to as providing quasiassociated signaling.
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