SS7 protocol

SS7 protocol suite
Layer
Protocols
Application
INAP, MAP, IS-41...
TCAP, CAP, ISUP, ...
Transport
SCCP
Network
MTP Level 3
Data link
MTP Level 2
...
Physical
MTP Level 1
...
The SS7 protocol stack borrows partially from the OSI Model of a packetized digital protocol stack. OSI layers 1 to 3 are provided by the Message Transfer Part (MTP) of the SS7 protocol; for circuit related signalling, such as the Telephone User Part (TUP) or the ISDN User Part (ISUP), the User Part provides layers 4 to 7, whereas for non-circuit related signalling the Signalling Connection and Control Part (SCCP) provides layer 4 capabilities to the SCCP user. The Transaction Capabilities Application Part (TCAP) is the primary SCCP User in the Core Network, using SCCP in connectionless mode. SCCP in connection oriented mode provides the transport layer for air interface protocols such as BSSAP and RANAP. TCAP provides transaction capabilities to its Users (TC-Users), such as the Mobile Application Part, the Intelligent Network Application Part and the CAMEL Application Part.
The MTP covers the transport protocols including network interface, information transfer, message handling and routing to the higher levels. SCCP is a sub-part of other L4 protocols, together with MTP 3 it can be called the Network Service Part (NSP), it provides end-to-end addressing and routing, connectionless messages (UDTs), and management services for the other L4 user parts. TUP is a link-by-link signaling system used to connect calls. ISUP is the key user part, providing a circuit-based protocol to establish, maintain, and end the connections for calls. TCAP is used to create database queries and invoke advanced network functionality, or links to intelligent networks (INAP), mobile services (MAP).

[edit] MAP Signaling
In mobile cellular telephony networks like GSM and UMTS the SS7 application MAP is used. Voice connections are Circuit Switched (CS) and data connections are Packet Switched (PS) applications.
Some of the GSM/UMTS Core Switched interfaces in the Mobile Switching Center (MSC) transported over SS7 include the following:
B -> VLR (uses MAP/B). Most MSCs are associated with a Visitor Location Register (VLR), making the B interface "internal".
D -> HLR (uses MAP/D) for attaching to the CS network and location update
E -> MSC (uses MAP/E) for inter-MSC handover
F -> EIR (uses MAP/F) for equipment identity check
H -> SMS-G (uses MAP/H) for Short Message Service (SMS) over CS
There are also several GSM/UMTS PS interfaces in the Serving GPRS Support Node (SGSN) transported over SS7:
Gr -> HLR for attaching to the PS network and location update
Gd -> SMS-C for SMS over PS
Gs -> MSC for combined CS+PS signaling over PS
Ge -> Charging for Customised Applications for Mobile networks Enhanced Logic (CAMEL) prepaid charging
Gf -> EIR for equipment identity check

[edit] SS7 in the IMS Future

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Users invested heavily in SS7 architecture in the late 20th century, and the evolution of SS7-based signaling network infrastructure to Session Initiation Protocol-based (SIP-based) signaling infrastructure, or IMS networking, does not involve just changing from SS7 to SIP protocols and procedures, it requires a fundamental shift in the network's design to accommodate more types of services, devices and customer preferences. This expensive change is difficult for investors to accept in a short time. Here are some important considerations as operators are making the transition:
SS7 is likely to remain the principal signaling technology for years to come. Operators continue to invest billions of dollars each year to maintain and expand their existing SS7 networks and to develop enhanced voice and data services that protect against commoditization of POTS voice service. In fact, almost all of the past decade's value-added services including mobility, voicemail, wireless prepaid services, Short Message Service (SMS), and number portability, exist because of SS7 signaling. As a result, at least a subset of carriers will continue to squeeze as much as they can out of their existing signaling networks and investments.
Operators are at different stages in their IP migration paths and require tailored solutions. These typically combine several "bridging technologies," which help transition between SIP-based, next-generation networks and existing networks.
The first step in the transition is to simply replace the SS7 transport with IP transport using SIGTRAN protocols. This protects, to the maximum extent possible, the existing investment in the SS7 technology base and huge cost savings can be reaped in certain situations due to the vast difference in transport cost of IP compared to the traditional SS7 TDM. Network elements such as Signaling Gateways (SG), Signaling Transfer Points (STP), or Edge STPs (ESTP) provide the signaling translation from SS7 to SIGTRAN. For more information on SIGTRAN, refer to RFC 2719: Architectural Framework for Signaling Transport.
Beyond the simple replacement of transport, we enter the realm of gateways that convert network addresses, protocol content, and even one kind of protocol to another. One example solution, a SIP-SS7 gateway, bridges the 2G and 3G networks at the signaling layer. It expands the services and applications available to SS7 and SIP subscribers and enables a larger subscriber base to access those services, driving additional revenue for the carrier.
The signaling control layer is a cornerstone of IMS, largely because of the value SS7 signaling has provided to generic dial-tone networks over the past 20 years. Looking ahead, the SIP signaling intensity of multimedia networks will increase dramatically, and experts predict the number and size of SIP signaling messages will increase over SS7 by an order of magnitude.
Another bridging solution, an SMS gateway, integrates messaging capabilities between SIP networks and existing mobile networks. It enables transferring of SS7-based SMS messages to SIP and short message peer-to-peer protocol (SMPP) networks, expanding the potential carrier subscriber base significantly.