|In telephone network, the signaling method that is
used to provide control and management functions
is called Common Channel Signaling (CCS). CCS includes
addressing, call information and supervisory functions.
It also determines the status of the network and control
the amount of traffic.
To carry signaling messages CCS uses a separate out-of band
signaling network. Signaling between a Personal Communication
Services (PCS) also known as Wireless services and the Signaling
System No.7 (SS7) achieves the Public Switch Telephone
Signaling System No.7 (SS7) is a CCS system. It is design
and developed to improve the earlier signaling system
and it satisfy the requirements of the telephone companies.
Earlier signaling systems was not much sophisticated.
Their service quality and coverage range was also not good.
So to solve these problems we go towards SS7 network.
SS7 can support both voice and data services at a much good rate.
Architecture of SS7 Network
The architecture of SS7 consists of three different elements
and these network elements are used for interconnection
between a PCS network and the PSTN.
Service Switching Point (SSP)
Signal Transfer Point (STP)
Service Control Point (SCP)
1.Service Switching Point (SSP):
SS7 links have nodes and telephone switch is interconnected by SS7 links.
All call processing on calls that originates or terminates at that node
is performed by SSPs. In PSTN a local SSP is called a Central
Office or End Office (EO) while in PCS network an SSP is called
a Mobile Switching Center (MSC).
2.Signal Transfer Point (STP):
STP transfers SS7 messages between network switches databases.
The messages, which are routed by STPs to the correct out-going
signaling links, are based on the address fields of the SS7 messages.
3.Service Control Point (SCP):
To provide enhanced services, the required databases are contained
in SCPs. The SSP send request or queries to SCP. An SCP accepts
or receives queries from SSP and returns the requested information to SSP.
In mobile applications the Home Location Register (HLR) and
Visitor Location Register (VLR) are contained in SCP.
SS7 Protocol Model
The SS7 protocol has layered model and these layers are
interrelated with Open System Interconnection (OSI) layer model.
SS7 network protocol consists of following parts:
.Message Transfer Part (MTP)
.Signaling Connection Control Part (SCCP)
.Transaction Capabilities Application Part (TCAP)
.Integrated Services Digital Network User Part (ISUP)
.Operations, Maintenance, and Administration Part (OMAP)
.Mobile Application Part (MAP)
1.Message Transfer Part (MTP):
MTP consists of three levels, which are interrelated to the OSI
Physical layer, Data link layer and Network layer respectively.
MTP level 1:The Physical, electrical and functional
characteristics of the signaling links connecting SS7
components are defined by MTP level 1.
MTP level 2:Reliable transferring of messages between
two directly connected signaling points is provided by MTP level 2.
MTP level 3:The functions and procedures, which are
related to message routing and network management,
are provided by MTP level 3.
2.Signaling Connection Control Part (SCCP):
To transfer no circuit-related signaling information such as PCS
registration and cancellation, additional functions are provided to
MTP by SCCP.
3.Transaction Capabilities Application Part (TCAP):
To exchange information such as no circuit related data e.g. credit
card validation, between applications, capabilities or links are
provided by TCAP.
4.Integrated Services Digital Network User Part (ISUP):
To establish a call, necessary messages for call set up and tear down
are send by ISUP. Messages are sending from a switch to the switch
where the next circuit connection is required.
5.Operations, Maintenance, and Administration Part (OMAP):
OMAP is an application of TCAP. All functions for network
management are provided by OMAP.
6.Mobile Application Part (MAP):
MAP is also an application of TCAP. All network management
protocols are implemented at this layer