GSM Components

Each network component is designed to communicate over an interface specified by the GSM standards. This provides flexibility and enables a network provider to utilize system components from different manufacturers. For example Motorola Base Station System (BSS) equipment may be coupled with an Ericsson Network Switching System. The principle component groups of a GSM network are:

• home location register (HLR) —The HLR is a database used for storage and management of subscriptions. The HLR is considered the most important database, as it stores permanent data about subscribers, including a subscriber's service profile, location information, and activity status. When an individual buys a subscription from one of the PCS operators, he or she is registered in the HLR of that operator.
• mobile services switching center (MSC) —The MSC performs the telephony switching functions of the system. It controls calls to and from other telephone and data systems. It also performs such functions as toll ticketing, network interfacing, common channel signaling, and others.
• visitor location register (VLR) —The VLR is a database that contains temporary information about subscribers that is needed by the MSC in order to service visiting subscribers. The VLR is always integrated with the MSC. When a mobile station roams into a new MSC area, the VLR connected to that MSC will request data about the mobile station from the HLR. Later, if the mobile station makes a call, the VLR will have the information needed for call setup without having to interrogate the HLR each time.
• authentication center (AUC) —A unit called the AUC provides authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call. The AUC protects network operators from different types of fraud found in today's cellular world.
• equipment identity register (EIR) —The EIR is a database that contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations. The AUC and EIR are implemented as stand-alone nodes or as a combined AUC/EIR node.

The Base Station System (BSS):

All radio-related functions are performed in the BSS, which consists of base station controllers (BSCs) and the base transceiver stations (BTSs).

• BSC —The BSC provides all the control functions and physical links between the MSC and BTS. It is a high-capacity switch that provides functions such as handover, cell configuration data, and control of radio frequency (RF) power levels in base transceiver stations. A number of BSCs are served by an MSC.
  • BTS —The BTS handles the radio interface to the mobile station. The BTS is the radio equipment (transceivers and antennas) needed to service each cell in the network. A group of BTSs are controlled by a BSC.
  
  
  The Operation and Support System:
  The operations and maintenance center (OMC) is connected to all equipment in the switching system and to the BSC. The implementation of OMC is called the operation and support system (OSS). The OSS is the functional entity from which the network operator monitors and controls the system. The purpose of OSS is to offer the customer cost-effective support for centralized, regional, and local operational and maintenance activities that are required for a GSM network. An important function of OSS is to provide a network overview and support the maintenance activities of different operation and maintenance organizations.
  Additional Functional Elements:
  Other functional elements shown in Figure 2 are as follows:
  • message center (MXE) —The MXE is a node that provides integrated voice, fax, and data messaging. Specifically, the MXE handles short message service, cell broadcast, voice mail, fax mail, e-mail, and notification.
  • mobile service node (MSN) —The MSN is the node that handles the mobile intelligent network (IN) services.
  • gateway mobile services switching center (GMSC) —A gateway is a node used to interconnect two networks. The gateway is often implemented in an MSC. The MSC is then referred to as the GMSC.
  • GSM interworking unit (GIWU) —The GIWU consists of both hardware and software that provides an interface to various networks for data communications. Through the GIWU, users can alternate between speech and data during the same call. The GIWU hardware equipment is physically located at the MSC/VLR.
    GSM technology has helped revolutionize foreign telecom, especially in emerging markets as Afghan Wireless has demonstrated.
  
  

Service control point

A service control point (SCP) is a standard component of the Intelligent Network (IN) telephone system which is used to control the service. Standard SCPs in the telecom industry today are deployed using SS7, Sigtran or SIP technologies. The SCP queries the service data point (SDP) which holds the actual database and directory. SCP, using the database from the SDP, identifies the geographical number to which the call is to be routed. This is the same mechanism that is used to route 800 numbers.

SCP may also communicate with an intelligent peripheral (IP) to play voice messages, or prompt for information to the user, such as prepaid long distance using account codes. This is done by implementing telephone feature codes like "#", which can be used to terminate the input for a user name or password or can be used for call forwarding. These are realized using Intelligent Network Application Part (INAP) that sits above Transaction Capabilities Application Part (TCAP) on the SS7 protocol stack. The TCAP is part of the top or 7th layer of the SS7 layer breakdown.

SCPs are connected with either SSPs or STPs. This is dependent upon the network architecture that the network service provider wants. The most common implementation uses STPs.

SCP and SDP split is becoming a common industry practice. This is known generally in the industry by split architecture. Reason is that operators want to decouple the dependency between the two functionality to facilitate upgrades and possibly rely on different vendors.

SS7 Architecture

SS7 Protocol layers:

The SS7 network is an interconnected set of network elements that is used to exchange messages in support of telecommunications functions. The SS7 protocol is designed to both facilitate these functions and to maintain the network over which they are provided. Like most modern protocols, the SS7 protocol is layered.

Physical Layer (MTP-1) This defines the physical and electrical characteristics of the signaling links of the SS7 network. Signaling links utilize DS–0 channels and carry raw signaling data at a rate of 56 kbps or 64 kbps (56 kbps is the more common implementation). Message Transfer Part—Level 2 (MTP-2) The level 2 portion of the message transfer part (MTP Level 2) provides link-layer functionality. It ensures that the two end points of a signaling link can reliably exchange signaling messages. It incorporates such capabilities as error checking, flow control, and sequence checking. Message Transfer Part—Level 3 (MTP-3) The level 3 portion of the message transfer part (MTP Level 3) extends the functionality provided by MTP level 2 to provide network layer functionality. It ensures that messages can be delivered between signaling points across the SS7 network regardless of whether they are directly connected. It includes such capabilities as node addressing, routing, alternate routing, and congestion control.

Signaling Connection Control Part (SCCP)
The signaling connection control part (SCCP) provides two major functions that are lacking in the MTP. The first of these is the capability to address applications within a signaling point. The MTP can only receive and deliver messages from a node as a whole; it does not deal with software applications within a node.
While MTP network-management messages and basic call-setup messages are addressed to a node as a whole, other messages are used by separate applications (referred to as subsystems) within a node. Examples of subsystems are 800 call processing, calling-card processing, advanced intelligent network (AIN), and custom local-area signaling services (CLASS) services (e.g., repeat dialing and call return). The SCCP allows these subsystems to be addressed explicitly.
ISDN User Part (ISUP)
ISUP user part defines the messages and protocol used in the establishment and tear down of voice and data calls over the public switched network (PSN), and to manage the trunk network on which they rely. Despite its name, ISUP is used for both ISDN and non–ISDN calls. In the North American version of SS7, ISUP messages rely exclusively on MTP to transport messages between concerned nodes.
Transaction Capabilities Application Part (TCAP)
TCAP defines the messages and protocol used to communicate between applications (deployed as subsystems) in nodes. It is used for database services such as calling card, 800, and AIN as well as switch-to-switch services including repeat dialing and call return. Because TCAP messages must be delivered to individual applications within the nodes they address, they use the SCCP for transport.
Operations, Maintenance, and Administration Part (OMAP)
OMAP defines messages and protocol designed to assist administrators of the SS7 network. To date, the most fully developed and deployed of these capabilities are procedures for validating network routing tables and for diagnosing link troubles. OMAP includes messages that use both the MTP and SCCP for routing.



More Info on each layer:::

Message Transfer Part (MTP)

The Message Transfer Part (MTP) layer of the SS7 protocol provides the routing and network interface capabilities that support SCCP, TCAP, and ISUP. Message Transfer part (MTP) is divided into three levels.

MTP Level 1 (Physical layer) defines the physical, electrical, and functional characteristics of the digital signaling link. Physical interfaces defined include E-1 (2048 kb/s; 32 64 kb/s channels), DS-1 (1544 kb/s; 24 64 kp/s channels), V.35 (64 kb/s), DS-0 (64 kb/s), and DS-0A (56 kb/s).

MTP Level 2 provides the reliability aspects of MTP including error monitoring and recovery. (MTP-2) is a signalling link which together with MTP-3 provides reliable transfer of signalling messages between two directly connected signalling points.

MTP Level 3 provides the link, route, and traffic management aspects of MTP. MTP 3, thus ensures reliable transfer of the signalling messages, even in the case of the failure of the signalling links and signalling transfer points. The protocol therefore includes the appropriate functions and procedures necessary both to inform the remote parts of the signalling network of the consequences of a fault, and appropriately reconfigure the routing of messages through the signalling network. 

Signaling Connection Control Part (SCCP)

The Signaling Connection Control Part (SCCP) layer of the SS7 stack provides provides connectionless and connection-oriented network services and global title translation (GTT) capabilities above MTP Level 3. SCCP is used as the transport layer for TCAP-based services.  It offers both Class 0 (Basic) and Class 1 (Sequenced) connectionless services. SCCP also provides Class 2 (connection oriented) services, which are typically used by Base Station System Application Part, Location Services Extension (BSSAP-LE). In addition, SCCP provides Global Title Translation (GTT) functionality.

The signaling connection control part (SCCP) provides two major functions that are lacking in the MTP. The first of these is the capability to address applications within a signaling point. The MTP can only receive and deliver messages from a node as a whole; it does not deal with software applications within a node.

While MTP network-management messages and basic call-setup messages are addressed to a node as a whole, other messages are used by separate applications (referred to as subsystems) within a node. Examples of subsystems are 800 call processing, calling-card processing, advanced intelligent network (AIN), and custom local-area signaling services (CLASS) services (e.g., repeat dialing and call return). The SCCP allows these subsystems to be addressed explicitly.

The signaling connection control part (SCCP) provides two major functions that are lacking in the MTP. The first of these is the capability to address applications within a signaling point. The MTP can only receive and deliver messages from a node as a whole; it does not deal with software applications within a node.

While MTP network-management messages and basic call-setup messages are addressed to a node as a whole, other messages are used by separate applications (referred to as subsystems) within a node. Examples of subsystems are 800 call processing, calling-card processing, advanced intelligent network (AIN), and custom local-area signaling services (CLASS) services (e.g., repeat dialing and call return). The SCCP allows these subsystems to be addressed explicitly.

The second function provided by the SCCP is Global Title translation, the ability to perform incremental routing using a capability called global title translation (GTT). GTT frees originating signaling points from the burden of having to know every potential destination to which they might have to route a message. A switch can originate a query, for example, and address it to an STP along with a request for GTT. The receiving STP can then examine a portion of the message, make a determination as to where the message should be routed, and then route it.

For example, calling-card queries (used to verify that a call can be properly billed to a calling card) must be routed to an SCP designated by the company that issued the calling card. Rather than maintaining a nationwide database of where such queries should be routed (based on the calling-card number), switches generate queries addressed to their local STPs, which, using GTT, select the correct destination to which the message should be routed. Note that there is no magic here; STPs must maintain a database that enables them to determine where a query should be routed. GTT effectively centralizes the problem and places it in a node (the STP) that has been designed to perform this function.
In performing GTT, an STP does not need to know the exact final destination of a message. It can, instead, perform intermediate GTT, in which it uses its tables to find another STP further along the route to the destination. That STP, in turn, can perform final GTT, routing the message to its actual destination.
Intermediate GTT minimizes the need for STPs to maintain extensive information about nodes that are far removed from them. GTT also is used at the STP to share load among mated SCPs in both normal and failure scenarios. In these instances, when messages arrive at an STP for final GTT and routing to a database, the STP can select from among available redundant SCPs. It can select an SCP on either a priority basis (referred to as primary backup) or so as to equalize the load across all available SCPs (referred to as load sharing).

Transaction Capabilities Application Part (TCAP)

Transaction Capabilities Application Part (TCAP) defines the messages and protocol used to communicate between applications (deployed as subsystems) in nodes. It is used for database services such as calling card, 800, and AIN as well as switch-to-switch services including repeat dialing and call return. Because TCAP messages must be delivered to individual applications within the nodes they address, they use the SCCP for transport.

TCAP enables the deployment of advanced intelligent network services by supporting non-circuit related information exchange between signalling points using the SCCP connectionless service. TCAP messages are contained within the SCCP portion of an MSU. A TCAP message is comprised of a transaction portion and a component portion.

TCAP supports the exchange of non-circuit related data between applications across the SS7 network using the SCCP connectionless service. Queries and responses sent between SSPs and SCPs are carried in TCAP messages. For example, an SSP sends a TCAP query to determine the routing number associated with a dialed 800/888 number and to to check the personal identification number (PIN) of a calling card user. In mobile networks (IS-41 and GSM), TCAP carries Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming.

ISDN User Part (ISUP)

ISUP (ISDN User Part) defines the messages and protocol used in the establishment and tear down of voice and data calls over the public switched telephone network (PSTN), and to manage the trunk network on which they rely. Despite its name, ISUP is used for both ISDN and non–ISDN calls. In the North American version of SS7, ISUP messages rely exclusively on MTP to transport messages between concerned nodes.

ISUP controls the circuits used to carry either voice or data traffic. In addition, the state of circuits can be verified and managed using ISUP. The management of the circuit infrastructure can occur both at the individual circuit level and for groups of circuits.

Services that can be defined using ISUP include: Switching, Voice mail, Internet offload. ISUP is ideal for applications such as switching and voice mail in which calls are routed between endpoints.

When used in conjunction with TCAP and SIGTRAN, ISUP becomes an enabler for Internet offload solutions in which Internet sessions of relatively long duration can be isolated from relatively brief phone conversations.

A simple call flow using ISUP signaling is as follows:

Call set up: When a call is placed to an out-of-switch number, the originating SSP transmits an ISUP initial address message (IAM) to reserve an idle trunk circuit from the originating switch to the destination switch. The destination switch rings the called party line if the line is available and transmits an ISUP address complete message (ACM) to the originating switch to indicate that the remote end of the trunk circuit has been reserved. The STP routes the ACM to the originating switch which rings the calling party's line and connects it to the trunk to complete the voice circuit from the calling party to the called party.

Call connection: When the called party picks up the phone, the destination switch terminates the ringing tone and transmits an ISUP answer message (ANM) to the originating switch via its home STP. The STP routes the ANM to the originating switch which verifies that the calling party's line is connected to the reserved trunk and, if so, initiates billing.

Call tear down: If the calling party hangs-up first, the originating switch sends an ISUP release message (REL) to release the trunk circuit between the switches. The STP routes the REL to the destination switch. If the called party hangs up first, or if the line is busy, the destination switch sends an REL to the originating switch indicating the release cause (e.g., normal release or busy). Upon receiving the REL, the destination switch disconnects the trunk from the called party's line, sets the trunk state to idle, and transmits an ISUP release complete message (RLC) to the originating switch to acknowledge the release of the remote end of the trunk circuit. When the originating switch receives (or generates) the RLC, it terminates the billing cycle and sets the trunk state to idle in preparation for the next call. 

Mobile Application Part (MAP)

Mobile Application Part (MAP) messages sent between mobile switches and databases to support user authentication, equipment identification, and roaming are carried by TCAP. In mobile networks (IS-41 and GSM) when a mobile subscriber roams into a new mobile switching center (MSC) area, the integrated visitor location register requests service profile information from the subscriber's home location register (HLR) using MAP (mobile application part) information carried within TCAP messages.
The Mobile Application Part (MAP), one of protocols in the SS7 suite, allows for the implementation of mobile network (GSM) signaling infrastructure. The premise behind MAP is to connect the distributed switching elements, called mobile switching centers (MSCs) with a master database called the Home Location Register (HLR). The HLR dynamically stores the current location and profile of a mobile network subscriber. The HLR is consulted during the processing of an incoming call. Conversely, the HLR is updated as the subscriber moves about the network and is thus serviced by different switches within the network.
MAP has been evolving as wireless networks grow, from supporting strictly voice, to supporting packet data services as well. The fact that MAP is used to connect NexGen elements such as the Gateway GPRS Support node (GGSN) and Serving Gateway Support Node (SGSN) is a testament to the sound design of the GSM signaling system.
MAP has several basic functions:
* Mechanism for a Gateway-MSC (GMSC) to obtain a routing number for an incoming call
* Mechanism for an MSC via integrated Visitor Location Register (VLR) to update subscriber status and routing number.
* Subscriber CAMEL trigger data to switching elements via the VLR
* Subscriber supplementary service profile and data to switching elements via the VLR.

Intelligent Network Application Part (INAP)

Intelligent Network Application Part (INAP) is the signaling protocol used in Intelligent Networking. Developed by the International Telecommunications Union (ITU), IN is recognized as a global standard. Within the International Telecommunications Union, a total functionality of the IN has been defined and implemented in digestible segments called capability sets. The first version to be released was Capability Set 1 (CS-1). Currently CS-2 is defined and available. The CAMEL Application Part (CAP) is a derivative of INAP and enables the use of INAP in mobile GSM networks.
INAP is a signaling protocol between a service switching point (SSP), network media resources (intelligent peripherals), and a centralized network database called a service control point (SCP). The SCP consists of operator or 3rd party derived service logic programs and data.

• Service Switching Point (SSP) is a physical entity in the Intelligent Network that provides the switching functionality. SSP the point of subscription for the service user, and is responsible for detecting special conditions during call processing that cause a query for instructions to be issued to the SCP.
  The SSP contains Detection Capability to detect requests for IN services. It also contains capabilities to communicate with other physical entities containing SCF, such as SCP, and to respond to instructions from the other physical entities. Functionally, an SSP contains a Call Control Function, a Service Switching Function, and, if the SSP is a local exchange, a Call Control Agent Function. It also may optionally contain Service Control Function, and/or a Specialized Resource Function, and/or a Service Data Function. The SSP may provide IN services to users connected to subtending Network Access Points.
  The SSP is usually provided by the traditional switch manufacturers. These switches are programmable and they can be implemented using multipurpose processors. The main difference of SSP from an ordinary switch is in the software where the service control of IN is separated from the basic call control.
  
• Service Control Point (SCP) validates and authenticates information from the service user, processing requests from the SSP and issuing responses.The SCP stores the service provider instructions and data that direct switch processing and provide call control. At predefined points during processing an incoming or outgoing call, the switch suspends what it is doing, packages up information it has regarding the processing of the call, and queries the SCP for further instruction. The SCP executes user-defined programs that analyze the current state of the call and the information received from the switch. The programs can then modify or create the call data that is sent back to the switch. The switch then analyzes the information received from the SCP and follows the provided instruction to further process the call.
  Functionally, an SCP contains Service Control Function (SCF) and optionally also Service Data Function (SDF). The SCF is implemented in Service Logic Programs (SLP). The SCP is connected to SSPs by a signalling network. Multiple SCPs may contain the same SLPs and data to improve service reliability and to facilitate load sharing between SCPs. In case of external Service Data Point (SDP) the SCF can access data through a signalling network. The SDP may be in the same network as the SCP, or in another network. The SCP can be connected to SSPs, and optionally to IPs, through the signalling network. The SCP can also be connected to an IP via an SSP relay function. The SCP comprises the SCP node, the SCP platform, and applications. The node performs functions common to applications, or independent of any application; it provides all functions for handling service-related, administrative, and network messages. These functions include message discrimination, distribution, routing, and network management and testing. For example, when the SCP node receives a service-related message, it distributes the incoming message to the proper application. In turn, the application issues a response message to the node, which routes it to the appropriate network elements. The SCP node gathers data on all incoming and outgoing messages to assist in network administration and cost allocation. This data is collected at the node, and transmitted to an administrative system for processing.
  
• Intelligent Peripheral (IP) provides resources such as customized and concatenated voice announcements, voice recognition, and Dual Tone Multi-Frequencies (DTMF) digit collection, and contains switching matrix to connect users to these resources. The IP supports flexible information interactions between a user and the network. Functionally, the IP contains the Special Resource Function. The IP may directly connect to one or more SSPs, and/or may connect to the signalling network.

• Service Management Point (SMP) performs service management control, service provision control, and service deployment control. Examples of functions it can perform are database administration, network surveillance and testing, network traffic management, and network data collection. Functionally, the SMP contains the Service Management Function and, optionally, the Service Management Access Function and the Service Creation Environment
  Function. The SMP can access all other Physical Entities.

Conceptual model of the Intelligent Network :
The IN standards present a conceptual model of the Intelligent Network that model and abstract the IN functionality in four planes:

• The Service Plane (SP): This plane is of primary interest to service users and providers. It describes services and service features from a user perspective, and is not concerned with how the services are implemented within the network.
• The  Global Functional Plane (GFP): The GFP is of primary interest to the service designer. It describes units of functionality, known as service independent building blocks (SIBs) and it is not concerned with how the functionality is distributed in the network. Services and service features can be realised in the service plane by combining SIBs in the GFP.
• The Distributed Functional Plane (DFP): This plane is of primary interest to network providers and designers. It defines the functional architecture of an IN-structured network in terms of network functionality, known as functional entities (FEs). SIBs in the GFP are realised in the DFP by a sequence of functional entity actions (FEAs) and their resulting information flows.
• The  Physical Plane (PP): Real view of the physical network.The PP is of primary interest to equipment providers. It describes the physical architecture for an IN-structured network in terms of physical entities (PEs) and the interfaces between them. The functional entities from the DFP are realised by physical entities in the physical plane.

Services that can be defined with INAP include:

• Single number service: one number reaches a local number associated with the service
• Personal access service: provide end user management of incoming calls
• Disaster recovery service: define backup call destinations in case of disaster
• Do not disturb service: call forward
• Virtual private network short digit extension dialing service

Advantages created by the IN architecture:

• extensive use of information processing techniques;
• efficient use of network resources;
• modularization of network functions;
• integrated service creation and implementation by means of reusable standard network functions;
• flexible allocation of network functions to physical entities;
• portability of network functions among physical entities;
• standardised communication between network functions via service independent interfaces;
• customer control over their specific service attributes;
• standardised management of service logic.

SS7 Over IP

Service providers can cut costs with SS7oIP by offloading data traffic from SS7 networks onto IP networks. For example, Short Message Service (SMS) data is saturating GSM service providers' SS7 networks. SS7 Over IP enables wireless service providers to rapidly deploy emerging IP-based services for the mobile Internet that freely interact with the legacy mobile infrastructure.

SIGTRAN is the name given to an IETF working group that produced specifications for a family of protocols that provide reliable datagram service and user layer adaptations for SS7 and ISDN communications protocols . The most significant protocol defined by the SIGTRAN group was the Stream Control Transmission Protocol ( SCTP ) which uses the Internet Protocol (IP) as its network protocol.

SCTP is a reliable transport protocol operating on top of a potentially unreliable connectionless packet service such as IP. It offers acknowledged error-free non-duplicated transfer of datagrams (messages). Detection of data corruption, loss of data and duplication of data is achieved by using checksums and sequence numbers. A selective retransmission mechanism is applied to correct loss or corruption of data.

Benefits:

• Ease of deployment: When using signaling gateways (such as access service group [ASG]), there is no need to disrupt the existing SS7 network, and future enhancements are transparent.
• Less costly equipment: There is no need for further expensive investments in the legacy signaling elements.
• Better efficiency: SIGTRAN over an IP network doesn't require the physical E1/T1 over synchronous digital hierarchy (SDH) rings. Using new technologies like IP over SDH and IP over fiber, for instance, can achieve much higher throughput.
• Higher bandwidth: SIGTRAN information over IP does not constrain to link capacity as it does in the SS7 network. The IP network is much more flexible than the TDM-based legacy network.
• Enhanced services: Implementing a core IP network facilitates a variety of new solutions and value-added services (VAS). 
  
  
  
  

GSM interfaces

The network structure is defined within the GSM standards. Additionally each interface between the different elements of the GSM network is also defined. This facilitates the information interchanges can take place. It also enables to a large degree that network elements from different manufacturers can be used. However as many of these interfaces were not fully defined until after many networks had been deployed, the level of standardisation may not be quite as high as many people might like.

1. Um interface   The "air" or radio interface standard that is used for exchanges between a mobile (ME) and a base station (BTS / BSC). For signalling, a modified version of the ISDN LAPD, known as LAPDm is used.
2. Abis interface   This is a BSS internal interface linking the BSC and a BTS, and it has not been totally standardised. The Abis interface allows control of the radio equipment and radio frequency allocation in the BTS.
3. A interface   The A interface is used to provide communication between the BSS and the MSC. The interface carries information to enable the channels, timeslots and the like to be allocated to the mobile equipments being serviced by the BSSs. The messaging required within the network to enable handover etc to be undertaken is carried over the interface.
4. B interface   The B interface exists between the MSC and the VLR . It uses a protocol known as the MAP/B protocol. As most VLRs are collocated with an MSC, this makes the interface purely an "internal" interface. The interface is used whenever the MSC needs access to data regarding a MS located in its area.
5. C interface   The C interface is located between the HLR and a GMSC or a SMS-G. When a call originates from outside the network, i.e. from the PSTN or another mobile network it ahs to pass through the gateway so that routing information required to complete the call may be gained. The protocol used for communication is MAP/C, the letter "C" indicating that the protocol is used for the "C" interface. In addition to this, the MSC may optionally forward billing information to the HLR after the call is completed and cleared down.
6. D interface   The D interface is situated between the VLR and HLR. It uses the MAP/D protocol to exchange the data related to the location of the ME and to the management of the subscriber.
7. E interface   The E interface provides communication between two MSCs. The E interface exchanges data related to handover between the anchor and relay MSCs using the MAP/E protocol.
8. F interface   The F interface is used between an MSC and EIR. It uses the MAP/F protocol. The communications along this interface are used to confirm the status of the IMEI of the ME gaining access to the network.
9. G interface   The G interface interconnects two VLRs of different MSCs and uses the MAP/G protocol to transfer subscriber information, during e.g. a location update procedure.
10. H interface   The H interface exists between the MSC the SMS-G. It transfers short messages and uses the MAP/H protocol.
11. I interface   The I interface can be found between the MSC and the ME. Messages exchanged over the I interface are relayed transparently through the BSS.

Although the interfaces for the GSM cellular system may not be as rigorouly defined as many might like, they do at least provide a large element of the definition required, enabling the functionality of GSM network entities to be defined sufficiently.

GSM

Following is the simple architecture diagram of GSM Network.
The added components of the GSM architecture include the functions of the databases and messaging systems:

• Home Location Register (HLR)
• Visitor Location Register (VLR)
• Equipment Identity Register (EIR)
• Authentication Center (AuC)
• SMS Serving Center (SMS SC)
• Gateway MSC (GMSC)
• Chargeback Center (CBC)
• Transcoder and Adaptation Unit (TRAU)

Following is the diagram of GSM Netwrok alongwith added elements.
The MS and the BSS communicate across the Um interface, also known as the air interface or radio link. The BSS communicates with the Network Service Switching center across the A interface.

GSM network areas:

In a GSM network, the following areas are defined:

• Cell: Cell is the basic service area: one BTS covers one cell. Each cell is given a Cell Global Identity (CGI), a number that uniquely identifies the cell.
• Location Area: A group of cells form a Location Area. This is the area that is paged when a subscriber gets an incoming call. Each Location Area is assigned a Location Area Identity (LAI). Each Location Area is served by one or more BSCs.
• MSC/VLR Service Area: The area covered by one MSC is called the MSC/VLR service area.
• PLMN: The area covered by one network operator is called PLMN. A PLMN can contain one or more MSCs.

GSM System Architecture

In GSM system the mobile handset is called Mobile Station (MS). A cell is formed by the coverage area of a Base Transceiver Station (BTS) which serves the MS in its coverage area. Several BTS together are controlled by one Base Station Controller (BSC). The BTS and BSC together form Base Station Subsystem (BSS). The combined traffic of the mobile stations in their respective cells is routed through a switch called Mobile Switching Center (MSC). Connection originating or terminating from external telephone (PSTN) are handled by a dedicated gateway Gateway Mobile Switching Center (GMSC). The architecture of a GSM system is shown in the figure 2.1 below.

Figure 2.1: GSM Architecture (Source: Bettstetter et. all)

In addition to the above entities several databases are used for the purpose of call control and network management. These databases are Home Location Register (HLR), Visitor Location Register (VLR), the Authentication Center (AUC), and Equipment Identity Register (EIR).
Home Location Register (HLR) stores the permanent (such as user profile) as well as temporary (such as current location) information about all the users registered with the network. A VLR stores the data about the users who are being serviced currently. It includes the data stored in HLR for faster access as well as the temporary data like location of the user. The AUC stores the authentication information of the user such as the keys for encryption. The EIR stores stores data about the equipments and can be used to prevent calls from a stolen equipments.
All the mobile equipments in GSM system are assigned unique id called IMSI (International Mobile Equipment Identity) and is allocated by equipment manufacturer and registered by the service provider. This number is stored in the EIR. The users are identified by the IMSI (International Module Subscriber Identity) which is stored in the Subscriber Identity Module (SIM) of the user. A mobile station can be used only if a valid SIM is inserted into an equipment with valid IMSI. The ``real'' telephone number is different from the above ids and is stored in SIM.

Mobile Station

The mobile station (MS) consists of the physical equipment, such as the radio transceiver, display and digital signal processors, and a smart card called the Subscriber Identity Module (SIM).  The SIM provides personal mobility, so that the user can have access to all subscribed services irrespective of both the location of the terminal and the use of a specific terminal.  By inserting the SIM card into another GSM cellular phone, the user is able to receive calls at that phone, make calls from that phone, or receive other subscribed services. The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI).  The SIM card contains the International Mobile Subscriber Identity (IMSI), identifying the subscriber, a secret key for authentication, and other user information.  The IMEI and the IMSI are independent, thereby providing personal mobility.  The SIM card may be protected against unauthorized use by a password or personal identity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC).  These communicate across the specified A�bis interface, allowing (as in the rest of the system) operation between components made by different suppliers. The Base Transceiver Station houses the radio tranceivers that define a cell and handles the radio�link protocols with the Mobile Station.  In a large urban area, there will potentially be a large number of BTSs deployed.  The requirements for a BTS are ruggedness, reliability, portability, and minimum cost.
The Base Station Controller manages the radio resources for one or more BTSs.  It handles radio�channel setup, frequency hopping, and handovers, as described below.  The BSC is the connection between the mobile and the Mobile service Switching Center (MSC).  The BSC also translates the 13 kbps voice channel used over the radio link to the standard 64 kbps channel used by the Public Switched Telephone Network or ISDN.

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching Center (MSC).  It acts like a normal switching node of the PSTN or ISDN, and in addition provides all the functionality needed to handle a mobile subscriber, such as registration, authentication, location updating, handovers, and call routing to a roaming subscriber.  These services are provided in conjuction with several functional entities, which together form the Network Subsystem.  The MSC provides the connection to the public fixed network (PSTN or ISDN), and signalling between functional entities uses the ITU�T Signalling System Number 7 (SS7), used in ISDN and widely used in current public networks. The Home Location Register (HLR) and Visitor Location Register (VLR), together with the MSC, provide the call�routing and (possibly international) roaming capabilities of GSM.  The HLR contains all the administrative information of each subscriber registered in the corresponding GSM network, along with the current location of the mobile.  The current location of the mobile is in the form of a Mobile Station Roaming Number (MSRN) which is a regular ISDN number used to route a call to the MSC where the mobile is currently located.  There is logically one HLR per GSM network, although it may be implemented as a distributed database.
The Visitor Location Register contains selected administrative information from the HLR, necessary for call control and provision of the subscribed services, for each mobile currently located in the geographical area controlled by the VLR.  Although each functional entity can be implemented as an independent unit, most manufacturers of switching equipment implement one VLR together with one MSC, so that the geographical area controlled by the MSC corresponds to that controlled by the VLR, simplifying the signalling required.  Note that the MSC contains no information about particular mobile stations - this information is stored in the location registers.
The other two registers are used for authentication and security purposes.  The Equipment Identity Register (EIR) is a database that contains a list of all valid mobile equipment on the network, where each mobile station is identified by its International Mobile Equipment Identity (IMEI).  An IMEI is marked as invalid if it has been reported stolen or is not type approved.  The Authentication Center is a protected database that stores a copy of the secret key stored in each subscriber's SIM card, which is used for authentication and ciphering of the radio channel.

ISUP call flow

Normal prepaid IN call flow in GSM

Hi All,

      Prepaid Call Flow

The above flow describes a basic direct dial call scenario with normal call termination:

1. Mobile A starts a new call by dialing Mobile B. 

2. The MSC sends an IDP (Initial Detection Point) event, which notifies the IN-SCP of the new call.

  IDP Message Contains :- A-Party no, B-Party no., Service key=90, A Party Location, Time stamp.

3. The IN SCP processes the request and after authorizing the user, the IN SCP sends 3 IN messages to the MSC - 

• AC (Apply Charging) :-  Check the A-Party Balance / tariff related facilities & provides maximum granted time for a call.
• CIQ (Call Information Query) :- IN request from the MSC for call information, like CAET - Call Attempt Elapsed Time (time between call ringing & user picks the call), CCET - Call Connect Elapsed Time (Duration of a call), CST - Call Stop Time (exact time when call disconnect), RC - Release Cause (Exact release cause due to which call got disconnected).
• RRBCSM (Request Report Basic Call State Module) :- IN again request from MSC for detailed Release Cause.

4. Connect :- After getting everything OK i.e. all request from IN side have been done then IN sends a Connect message to MSC for further call processing & connect the call.

5. Activity Test :- After IN provides "connect" message to MSC, its a sort of ping message which is unidirectional, sent by IN to MSC, to know the progress of a call.

6. Once the connection is made, an event report (for answer event) is sent to SDP via IN-SCP.for preventing any revenue loss.

7. After conversation, when call gets disconnected, a new event report is sent to SDP via IN-SCP, which in turn instructs to release the call.

8. Once the call is released, a new Apply Charging Report (ACR) is sent to IN-SCP, which contains full time usage data of a call. This report is sent to SDP for accurate & final call charging.

9. CIR (Call Information Report) :- By using the details which was noted in CIQ message, MSC makes a report i.e. CIR (which contains CAET, CCET, CST, RC) & sends to SDP via IN-SCP.

10. ERB (Evert Report BCSM) :- Another report send by MSC to IN, which contains actual release cause in details, which may be - Abandon, B-Party Busy, B-Party no answer, B-Party not reachable, route selection failure, disconnect.

11. Finally signaling terminates, which ends the call.

STP ( Signaling Transfer Point)

STP (Signaling Transfer Point)

Hi,

There are different types of signaling points in SS7 Standards, like
Service Switching Point (SSP) - like MSC, Tandem Switch, etc.
Service Control Points (SCP) - like HLR, etc.
Signaling Transfer Point (STP).

These Signaling points provide access to the SS7 network, databases, & transfer messages to other signaling points.

STP :- It Ÿis a vital element in SS7 network serving as a Signaling hub for the transfer of digital data packets between network nodes.
  
It routes messages throughout the network using, call information & network addressing structured within SS7 data packets.

It serves as dynamic router, controlling traffic flow & access to variety of SS7 nodes & network.

Functions of STP :-

1. Receives the MSU's & direct them to appropriate destination.
2. Network Management.
3. (ANSI to ITU) or (ITU to ANSI) protocol conversion.
4. Global Title Translation (GTT).
5. Measurement of Data.
6. Gateway Function.
7. Gateway Screening (GWS)
8. Local Number Portability (LNP).

 Different Links used with STP :-

• A-Link - It provide STP/SSP or STP/SCP connectivity. Maximum of 16 links can be there in a link set or maximum of 32 links in combined set.

• B-Link - It connects one STP to other STP of same hierarchical level. Maximum of 8 links in quad configuration of link set.

• C-Link - Maximum of 16 links in a link set.
• D-Link - Maximumof 8 links in a link set.

 

• E Links - It connects a STP to other STP other then its Home STP & provides an alternate route for SS7 messages if congestion occurs at home STP.

•  F Links - It provides SSP to SSP connectivity. It provides only Call Setup/TearDown capability & it should be adjacent.
  

 Why STP ???

• It provides Centralized Network Management, facilitating the delivery of Intelligent Services throughout the network.
• Reduction in Signaling Terminal Hardware in SSPs / SCPs. (like MSC, IN, HLR, etc.)
• Central Database for GTT at STP, Minimizes Errors.
• Efficient Routing of Messages.
• Flexible SS7 Network Management.
• Fast Integeration of New Nodes in the network.
• Easy Migration to Next Generation Networks like VOIP, Soft Switch, etc.