Signaling system 7 (ss7)

 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.
 SS7 is a means by which elements of the
telephone network exchange information.

 Signaling System 7 (SS7) is an international
telecommunications standard that defines
how network elements in a public switched
telephone network (PSTN) exchange
information over a digital signaling network.
 Nodes in an SS7 network are called signaling
points.

 SS7 uses out-of-band signaling, which means that
signaling (control) information travels on a
separate, dedicated 56 or 64 Kbps channel
rather than within the same channel as the
telephone call.
 Historically, the signaling for a telephone call
has used the same voice circuit that the
telephone call traveled on (this is known as in-
band signaling).
 Using SS7, telephone calls can be set up more
efficiently and special services such as call
forwarding and wireless roaming service are
easier to add and manage.

 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.

 SS7 is used for these and other services:
 Setting up and managing the connection for a call
 Tearing down the connection when the call is
complete
 Billing
 Managing call forwarding, calling party name and
number display, three-way calling, and other
Intelligent Network (IN) services
 Toll-free (800 and 888) and toll (900) calls
 Wireless as well as wireline call service including
mobile telephone subscriber authentication, personal
communication service (PCS), and roaming

 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 consists of a set of reserved or dedicated
channels known as signaling links.
 There are three kinds of network points
signaling points
 Service Switching Points (SSPs)
 Signal Transfer Points (STPs)
 Service Control Points (SCPs)

 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

 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, as shown below.

 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.
 Signaling architectures such as this, which provide indirect
signaling paths between network elements, are referred to
as providing quasi-associated signaling.

 A link (access)
 Connects signaling end point (SCP or SSP) to STP
 B link (bridge)
 Connects an STP to another STP(STPs from a network connect to STPs
of another network)
 C link (cross)
 Connects STPs performing identical functions, forming a mated pair
(for greater reliability)
 D link (diagonal)
 Connects a secondary (local or regional) STP pair to a primary (inter-
network gateway) STP pair in a quad-link configuration; the
distinction between B and D links is arbitrary
 E link (extended)
 Connects an SSP to an alternate STP
 F link
(fully associated)
 Connects two signaling end points (SSPs and SCPs) in the same local
network

 the SS7 network is an interconnected set of
network elements that is used to exchange
messages in support of telecommunications
functions.
 The SS7 protocol isdesigned to bothfacilitate
these functions and to maintain the network
over which they are provided. Like most
modern protocols, the SS7 protocol is
layered.

 Massege Transfer part 1
 Signaling Connection
Control Part
 Transaction Capabilities
Application Part
 Mobile Application Part
 IN Application part

 MTP1 is equivalent to the physical layer of
the OSI model
 MTP1 defines the physical and electrical
characteristics of the signaling link.
 The physical interfaces defined for SS7
include E-1 (2048kb/s; 3264kb/s channels),
DS-1(1544kb/s; 24 64kb/s channels),
V.35(64kb/s), DS-0(64kb/s) and DS-
0A(56kb/s)

 MTP2 ensures reliable end-to-end transmission of
a message across a signaling link that directly
connects 2 nodes.
 Implements flow control, message sequence
validation, error checking and error recovery.

 ISUP defines one call control protocol used to
set-up, manage and release circuits that
carry voice and data calls in the PSTN.
 ISUP uses the MTP for routing messages from
one SSP to another
 SCCP provides connectionless and
connection-oriented network services via
MTP3 for the transfer of signaling messages
between SSP's.

 While MTP3 provides point codes to allow
messages to be addressed to specific
signaling points, SCCP provides Subsystem
Numbers (SSN) to let messages be addressed
to specific applications at these signaling
points. MTP transfers messages node-to-node
while SCCP transfers messages end-to-end.
 SCCP is used as the transport layer for TCAP
based services like, freephone(800/888),
local number portability and roaming.

 TCAP messages are destined for application
entities.
 TCAP is designed for non-circuit related messages.
 TCAP provides a means for SCP-to-SCP communication
via STP's.
 TCAP messages are used by SSPs to query an SCP (via
an STP) to determine the routing numbers associated
with a global title. The SCP uses TCAP to return a
response containing the routing numbers back to the
SSP via STP.
 TCAP messages are transfered end-to-end using the
services of SCCP.
 TCAP is not limited to database access, it is also used
to invoke other features from remote switches. TCAP
services include freephone, calling card, and wireless
roaming.

 Mobile Application Part messages sent
between mobile switches and databases to
support user authentication, equipment
identification, and roaming are carried by
TCAP

Signaling system 7 (ss7)

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