Signaling System 7

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The correct title of this article is Signaling System #7. The substitution or omission of a # sign is because of technical restrictions.
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 ...

Signaling System #7 (SS7) is a set of telephony signaling protocols which are used to set up the vast majority of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls. Other uses include number translation, prepaid billing mechanisms, short message services, and a variety of mass market services.

It is usually abbreviated to SS7 though in North America it is often referred to as CCSS7, an acronym for "Common Channel Signaling System 7". In some European countries, specifically the United Kingdom, it is sometimes called C7 (CCITT number 7) and is also known as number 7 and CCIS7. (ITU-T was formerly known as CCITT.)

There is only one international SS7 protocol defined by ITU-T in its Q.700-series recommendations.[1] There are however, many national variants of the SS7 protocols. Most national variants are based on two widely deployed national variants as standardized by ANSI and ETSI, which are in turn based on the international protocol defined by ITU-T. Each national variant has its own unique characteristics. Some national variants with rather striking characteristics are the China (PRC) and Japan (TTC) national variants.

The Internet Engineering Task Force (IETF) has also defined level 2, 3, and 4 protocols that are compatible with SS7 MTP2 (M2UA and M2PA) MTP3 (M3UA) and SCCP (SUA), but use an SCTP transport mechanism. This suite of protocols is called SIGTRAN.

Contents

[edit] History

Common Channel Signaling protocols have been developed by AT&T, BT and the ITU-T since 1975 and the first international Common Channel Signaling protocol was defined by the ITU-T as Signalling System No. 6 in 1977.[2] Signalling System No. 7 was defined as an international standard by ITU-T in its 1980 (Yellow Book) Q.7XX-series recommendations.[1] SS7 was designed to replace Signalling System No. 6, which had a restricted 28-bit signal unit that was both limited in function and not amenable to digital systems.[3] SS7 has substantially replaced SS6, SS5, R1 and R2, with the exception that R1 and R2 variants are still used in numerous nations.

SS5 and earlier used in-band signaling, where the call-setup information was sent by playing special multi-frequency tones into the telephone lines (known as bearer channels in the parlance of the telecom industry). This led to security problems with blue boxes. Modern designs of telephone equipment that implement out-of-band signaling protocols explicitly keep the end-user's audio path—the so-called speech path—separate from the signaling phase to eliminate the possibility that end users may introduce tones that would be mistaken for those used for signaling. See falsing.

SS6 and SS7 moved to a system in which the signaling information was out-of-band, carried in a separate signaling channel.[4] This avoided the security problems earlier systems had, as the end user had no connection to these channels. SS6 and SS7 are referred to as so-called Common Channel Interoffice Signalling Systems (CCIS) or Common Channel Signaling (CCS) due to their hard separation of signaling and bearer channels. This required a separate channel dedicated solely to signaling, but the greater speed of signaling decreased the holding time of the bearer channels, and the number of available channels was rapidly increasing anyway at the time SS7 was implemented.

The common channel signaling paradigm was translated to IP via the SIGTRAN protocols as defined by the IETF. While running on a transport based upon IP, the SIGTRAN protocols are not an SS7 variant, but simply transport existing national and international variants of SS7.[5]

[edit] Functionality

The term signalling, when used in telephony, refers to the exchange of control information associated with the establishment of a telephone call on a telecommunications circuit. An example of this control information is the digits dialed by the caller, the caller's billing number, and other call-related information.

When the signalling is performed on the same circuit that will ultimately carry the conversation of the call, it is termed Circuit-Associated Signalling (CAS). This is the case for earlier analogue trunks, MF and R2 digital trunks, and ISDN PRI or DSS1/DASS PBX trunks.

In stark contrast, SS7 signalling is termed Non-Circuit-Associated Signalling (NCAS) in that the path and facility used by the signalling is separate and distinct from the telecommunications channels that will ultimately carry the telephone conversation. With Non-Circuit-Associated Signalling, it becomes possible to exchange signalling without first siezing a facility, leading to significant savings and performance increases in both signalling and facility usage.

Because of the mechanisms used by signalling methods prior to SS7 (battery reversal, multi-frequency digit outpulsing, A- and B-bit signalling), these older methods could not communicate much signalling information. Usually only the dialed digits were signalled, and only during call setup. For charged calls, dialed digits and charge number digits were outpulsed. SS7, being a high-speed and high-performance packet-based communications protocol, can communicate significant amounts of information when setting up a call, during the call, and at the end of the call. This permits rich call-related services to be developed. Some of the first such services were call management related services that we take for granted today: call forwarding (busy and no answer), voice mail, call waiting, conference calling, calling name and number display, called name and number display, call blocking, call screening, malicious call identification, busy callback.

The earliest deployed upper layer protocol in the SS7 signalling suite were dedicated to the setup, maintenance, and release of telephone calls. The Telephone User Part (TUP) was adopted in Europe and the Integrated Services Digital Network (ISDN) User Part (ISUP) adapted for Public Switched Telephone Network (PSTN) calls was adopted in North America to process Plain Old Telephone System (POTS) telephone calls. ISUP was later used in Europe when the European networks upgraded to the ISDN. (North America never accomplished full upgrade to the ISDN and the predominant telephone service is still the older PSTN POTS service.) Due to its richness and the need for a completely separate signalling network for its operation, SS7 signalling is mostly used for signalling between telephone switches and not for signalling between local exchanges and Customer Premise Equipment (CPE).

Because SS7 signalling does not require seizure of a channel for a conversation prior to the exchange of control information, Non-Facility-Associated Signalling (NFAS) became possible. Non-Facility-Associated Signalling is signalling that is not directly associated with the path that a conversation will traverse and may concern other information located at a centralized database such as service subscription, feature activation, and service logic. This makes possible a set of network-based services that do not rely upon the call being routed to a particular subscription switch at which service logic would be executed, but permits service logic to be distributed throughout the telephone network and executed more expediently at originating switches far in advance of call routing. It also permits the subscriber increased mobility due to the decoupling of service logic from the subscription switch.

Also possible with SS7 is Non-Call-Associated Signalling. Non-Call-Associated Signalling is signalling that is not directly related to the establishment of a telephone call. An example of this is the exchange of the registration information used between a mobile telephone and a Home Location Register (HLR) database: a database that tracks the location of the mobile.

[edit] Physical network

SS7 clearly splits the signaling planes and voice circuits. An SS7 network has to be made up of SS7-capable equipment from end to end in order to provide its full functionality. The network is made up of several link types (A, B, C, D, E, and F) and three signaling nodes - Service switching point (SSPs), signal transfer point (STPs), and Service Control Point (SCPs). Each node is identified on the network by a number, a point code. Extended services are provided by a database interface at the SCP level using the SS7 network.

The links between nodes are full-duplex 56, 64, 1,536, or 1,984 kbit/s graded communications channels. In Europe they are usually one (64 kbit/s) or all (1,984 kbit/s) timeslots (DS0s) within an E1 facility; in North America one (56 or 64 kbit/s) or all (1,536 kbit/s) timeslots (DS0As or DS0s) within an T1 facility. One or more signaling links can be connected to the same two endpoints that together form a signaling link set. Signaling links are added to link sets to increase the signaling capacity of the link set.

In Europe, SS7 links normally are directly connected between switching exchanges using F-links. This direct connection is called fully-associated signaling. In North America, SS7 links are normally indirectly connected between switching exchanges using an intervening network of STPs. This indirect connection is call quasi-associated signaling. Quasi-associated signaling reduces the number of SS7 links necessary to interconnect all switching exchanges and SCPs in an SS7 signaling network.

SS7 links at higher signaling capacity (1.536 and 1.984 Mbit/s, simply referred to as the 1.5 Mbit/s and 2.0 Mbit/s rates) are called High Speed Links (HSL) in contrast to the low speed (56 and 64 kbit/s) links. High Speed Links (HSL) are specified in ITU-T Recommendation Q.703 for the 1.5 Mbit/s and 2.0 Mbit/s rages, and ANSI Standard T1.111.3 for the 1.536 Mbit/s rate. There are differences between the specifications for the 1.5 Mbit/s rate. High-Speed Links utilize the entire bandwidth of a T1 (1.536 Mbit/s) or E1 (1.984 Mbit/s) transmission facility for the transport of SS7 signaling messages.

SIGTRAN provides signaling using SCTP associations over the Internet Protocol. The protocols for SIGTRAN are M2PA, M2UA, M3UA and SUA.

[edit] SS7 protocol suite

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.

[edit] Notes

  1. ^ a b ITU-T Recommendation Q.700
  2. ^ (Ronayne 1986, p. 145).
  3. ^ (Ronayne 1986, p. 145).
  4. ^ (Ronayne 1986, p. 141).
  5. ^ RFC 2719 - Framework Architecture for Signaling Transport

[edit] References

  • Ronayne, John P. (1986). "The Digital Network", Introduction to Digital Communications Switching, 1st edition, Indianapolis: Howard W. Sams & Co., Inc.. ISBN 0-672-22498-4. 

[edit] External links