What is Signaling System No.7 (SS7)

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   What is Signaling System No.7 (SS7)





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 Network (PSTN).

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.