Signaling System 7 (SS7)
Definition
Signaling System 7 (SS7) is an architecture for performing out-of-band signaling
in 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.
Topics
1. What Is Signaling?
2. What Is Out-of-Band Signaling?
3. Signaling Network Architechture
4. The North American Signaling Architecture
5. Basic Signaling Architecture
6. SS7 Link Types
7. Basic Call Setup Example
8. Database Query Example
9. Layers of the SS7 Protocol
10. What Goes Over the Signaling Link
11. Addressing in the SS7 Network
12. Signal Unit Structure
13. What Are the Functions of the Different Signaling Units?
14. Message Signal Unit Structure
1. 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.
2. 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.
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.
3. 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.
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.
4. 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.
5. Basic Signaling Architecture
The following points should be noted:
1. 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.
2. 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.
3. The STPs of a mated pair are joined by a link (or set of links).
4. Two mated pairs of STPs are interconnected by four links (or sets of
links). These links are referred to as a quad.
5. 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.
6. Signaling architectures such as this, which provide indirect signaling
paths between network elements, are referred to as providing quasi-
associated signaling.
6. SS7 Link Types
SS7 signaling links are characterized according to their use in the signaling
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; what is
different is their use within a signaling network. The defined link types are shown
A Links
A links interconnect an STP and either an SSP or an SCP, which are collectively
referred to as signaling end points ("A" stands for access). A links are used for the
sole purpose of delivering signaling to or from the signaling end points (they
could just as well be referred to as signaling beginning points). Examples of A
links are 2−8, 3−7, and 5−12 in Figure 5.
Signaling that an SSP or SCP wishes to send to any other node is sent on either of
its A links to its home STP, which, in turn, processes or routes the messages.
Similarly, messages intended for an SSP or SCP will be routed to one of its home
STPs, which will forward them to the addressed node over its A links.
C Links
C links are links that interconnect mated STPs. As will be seen later, they are used
to enhance the reliability of the signaling network in instances where one or
several links are unavailable. "C" stands for cross (7−8, 9−10, and 11−12 are C
links). B links, D links, and B/D links interconnecting two mated pairs of STPs
are referred to as either B links, D links, or B/D links. Regardless of their name,
their function is to carry signaling messages beyond their initial point of entry to
the signaling network towards their destination. The "B" stands for bridge and
describes the quad of links interconnecting peer pairs of STPs. The "D" denotes
diagonal and describes the quad of links interconnecting mated pairs of STPs at
different hierarchical levels. Because there is no clear hierarchy associated with a
connection between networks, interconnecting links are referred to as either B,
D, or B/D links (7−11 and 7−12 are examples of B links; 8−9 and 7−10 are
examples of D links; 10−13 and 9−14 are examples of interconnecting links and
can be referred to as B, D, or B/D links).
E Links
While an SSP is connected to its home STP pair by a set of A links, enhanced
reliability can be provided by deploying an additional set of links to a second STP
pair. These links, called E (extended) links provide backup connectivity to the SS7
network in the event that the home STPs cannot be reached via the A links. While
all SS7 networks include A, B/D, and C links, E links may or may not be deployed
at the discretion of the network provider. The decision of whether or not to
deploy E links can be made by comparing the cost of deployment with the
improvement in reliability