There are many signaling systems use in the telecommunications network. There are two essential components to all telephone calls. The first, and most obvious, is the actual content—our voices, faxes, modem data, etc. The second is the information that instructs telephone exchanges to establish connections and route the “content” to an appropriate destination. Telephony signaling is concerned with the creation of standards for the latter to achieve the former. These standards are known as protocols.
SS7 or Signaling System Number 7 is simply another set of protocols that describe a means of communication between telephone switches in public telephone networks. Signaling sytem no. 7 (SS7/C7) is one of the most used system in telecommunication industry for fix line and cellular networks
They have been created and controlled by various bodies around the world, which leads to some specific local variations, but the principal organization with responsibility for their administration is the International Telecommunications Union or ITU-T.
The primary function of SS7 / C7 is to provide call control, remote network management, and maintenance capabilities for the inter- office telephone network. SS7 performs these functions by exchanging control messages between SS7 / C7 telephone exchanges (signalling points or SPs) and SS7 / C7 signalling transfer points (STPs). Basically, the SS7 / C7 control network tells the switching office which paths to establish over the circuit-switched network. The STPs route SS7 control packets across the signalling network. A switching office may or may not be an STP.
The SS7 network and protocol are used for providing intelligent network services such as:
• basic call setup, management, and tear down
• wireless services such as personal communications services (PCS), wireless roaming, and mobile subscriber authentication
• local number portability (LNP)
• toll-free (800/888) and toll (900) wireline services
• 911, 411 services
• enhanced call features such as call forwarding, caller ID display, and three-way calling
• efficient and secure worldwide telecommunications
The current SS7 / C7 network, one of the largest data network in the world, connects together local telcos, cellular, and long-distance networks nationwide and worldwide.
To understand SS7 we must first understand something of the basic inefficiency of previous signaling methods utilized in the Public Switched Telephone Network (PSTN). Earlier all telephone connections were managed by a variety of techniques centered on “in band” signaling.
A network utilizing common-channel signalling is actually two networks in one:
1. First there is the circuit-switched "user" network which actually carries the user voice and data traffic. It provides a physical path between the source and destination.
2. The second is the signalling network which carries the call control traffic. It is a packet-switched network using a common channel switching protocol.
Protocol Structure
SS7/C7 protocol suite covers all 7 layers of the OSI model as showing in the following diagram:
o ASP (Application Service Part)
ASP provides the functions of Layers 4 through 6 of the OSI model.td
o BICC (Bearer Independent Call Control protocol)
BICC is a call control protocol based on ISUP used between serving nodes to support the ISDN services independent of the bearer technology and signalling message transport technology used.td
o BISUP (B-ISDN User Part)
BISUP is an ATM protocol intended to support services such as high-definition television (HDTV), multilingual TV, voice and image storage and retrieval, video conferencing, high-speed LANs and multimedia
o DUP (Data User Part )
DUP defines the necessary call control, and facility registration and cancellation related elements for international common channel signalling by use of SS7 for circuit-switched data transmission services.
o ISUP (ISDN User Parttd)
ISUP supports basic telephone call connect/disconnect between end offices. ISUP was derived from TUP, but supports ISDN and intelligent networking functions. ISUP also links the cellular and PCS network to the PSTN.
o MAP (Mobile Application Parttd)
MAP is used to share cellular subscriber information among different networks.
o MTP (Message Transfer Parttd)
MTP acrosses physical, data link and network layers. It defines what interface to be used, provides the network with sequenced delivery of all SS7 message packets; and provides routing, message discrimination and message distribution functions.
o SCCP (Signalling Connection Control Parttd)
SCCP provides end-to-end routing. SCCP is required for routing TCAP messages to their proper database.
o TCAP (Transaction Capabilities Application Parttd )
TCAP facilitates connection to an external database
o TUP (Telephone User Part)
TUP is an analog protocol that performs basic telephone call connect and disconnect.
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