GPRS Core Network Architecture

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What Is SGSN ?

A Serving GPRS Support Node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its geographical service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information (e.g., current cell, current VLR) and user profiles (e.g., IMSI, address(es) used in the packet data network) of all GPRS users registered with this SGSN.

What Is SMSC ?

A Short Message Service Center (SMSC) is a network element in the mobile telephone network which delivers SMS messages.

All SMS sent by a mobile subscriber are first submitted to SMSC of his mobile network. After submission SMSC find the location of destination mobile subscriber and deliver the SMSC to that subscriber. SMSC works on store and forward mechanism. If for some reasons SMSC is unable to deliver SMS to destination subscriber, SMSC can store this SMS for longer durations and keep retrying to deliver this SMS at regular interval.

What Is MSC ?

The Mobile Switching Center or MSC is the primary service delivery node for GSM, responsible for handling voice calls and SMS as well as other services (such as conference calls, FAX and circuit switched data). The MSC sets up and releases the end-to-end connection, handles mobility and hand-over requirements during the call and takes care of charging and real time pre-paid account monitoring.

In the GSM mobile phone system, in contrast with earlier analogue services, fax and data information is sent directly digitally encoded to the MSC. Only at the MSC is this re-coded into an "analogue" signal (although actually this will almost certainly mean sound encoded digitally as PCM signal in a 64-kbit/s timeslot, known as a DS0 in America).

There are various different names for MSCs in different contexts which reflects their complex role in the network, all of these terms though could refer to the same MSC, but doing different things at different times.

A Gateway MSC is the MSC that determines which visited MSC the subscriber who is being called is currently located. It also interfaces with the Public Switched Telephone Network. All mobile to mobile calls and PSTN to mobile calls are routed through a GMSC. The term is only valid in the context of one call since any MSC may provide both the gateway function and the Visited MSC function, however, some manufacturers design dedicated high capacity MSCs which do not have any BSSes connected to them. These MSCs will then be the Gateway MSC for many of the calls they handle.

The Visited MSC is the MSC where a customer is currently located. The VLR associated with this MSC will have the subscriber's data in it.

The Anchor MSC is the MSC from which a handover has been initiated. The Target MSC is the MSC toward which a Handover should take place. An MSC Server is a part of the redesigned MSC concept starting from 3GPP Release 5.

What Is VLR?

Visitor Location Register (VLR) is a database - part of the GSM mobile phone system - which stores information about all the mobiles that are currently under the jurisdiction of the MSC (Mobile Switching Center) which it serves. Of all the information it stores about each MS (Mobile Station), the most important is the current LAI (Location Area Identity). LAI identifies under which BSC (Base Station Controller) the MS is currently present. This information is vital in the call setup process.

Whenever an MSC detects a new MS in its network, in addition to creating a new record in the VLR, it also updates the HLR of the mobile subscriber, apprising it of the new location of that MS.

What Is HLR ?

The Home Location Register or HLR is a central database that contains details of each mobile phone subscriber that is authorized to use the GSM core network. There is one logical HLR per PLMN, although there may be multiple physical platforms.

The HLR stores details of every SIM card issued by the mobile phone operator. Each SIM has a unique identifier called an IMSI which is the primary key to each HLR record.

The next important items of data associated with the SIM are the MSISDNs, which are the telephone numbers used by mobile phones to make and receive calls. The primary MSISDN is the number used for making and receiving voice calls and SMS. Each MSISDN is also a primary key to the HLR record.

Examples of other data stored in the HLR against an IMSI record is:

• GSM services that the subscriber has requested or been given, like can he make call, can he receive call , can he make sms, can he roam etc
• GPRS settings to allow the subscriber to access packet services
• Current Location of subscriber (VLR and SGSN), i.e where is this subscriber currently.
  
• Call divert settings applicable for each associated MSISDN.

What Is SS7 ?

Brief Description :

Signaling System #7 (SS7) is a set of telephony signaling protocols which are used to set up most 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 service (SMS), and a variety of other 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. 

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.

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. Signalling System No. 7 was defined as an international standard by ITU-T in its 1980 (Yellow Book) Q.7XX-series recommendations. 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.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. 

SS6 and SS7 moved to a system in which the signaling information was out-of-band, carried in a separate signaling channel. 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.

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