Date: 25/07/2004
Introduction
Customised Applications for Mobile network Enhanced Logic also known as CAMEL was developed as a standard for mobile intelligence across different vendor equipments for GSM network. What this means is that the end user should be able to roam between different networks (maybe in different countries) and be reachable at the same number and should receive only one bill from the original service provider (Home Operator).
Before CAMEL, the GSM networks used INAP (Intelligent Network Application Part) for injecting intelligence in GSM networks. The limitations of INAP was that it did not support mobility management. CAMEL solved the mobility management problem and provided with much more functionality. With the evolution of 3G standards, CAMEL has evolved to provide new functionalities that will be discussed later.
CAMEL has been divided into Phases. The last section in this primer lists the functionality of each phases. The latest Phase is Phase 4. If an IPLMN (Interrogating PLMN) or VPLMN (Visiting PLMN) supports CAMEL Phase 4, it shall also provide the functionality of all previous CAMEL phases.
The first commercial implementation of CAMEL (phase 1) was in August 2000 by France Telecom Mobiles (France), Mobistar (Belgium) and Dutchtone (The Netherlands).
NOTE: CAMEL feature is a network feature and not a supplementary service. It is a tool to help the network operator
Applicability of CAMEL procedures
- The CAMEL feature is applicable to Mobile Originated and Mobile Terminated Call Related Activities. CAMEL procedures are applicable to all circuit switched basic services without distinction (except Emergency calls).
- The CAMEL feature is applicable to Supplementary Services Invocation
- CAMEL procedures are applicable to GPRS sessions and PDP contexts
- CAMEL procedures are applicable to Mobile Originating/Terminating short message service through both circuit switched and packet switched serving network entities
- CAMEL procedures are applicable to IP multimedia services (except Emergency calls) to support legacy services
- CAMEL shall support IPMM sessions which are based on the same charging paradigm as CS/PS calls. This applies most probably to VoIP and Video over IP.
- CAMEL procedures are applicable to IP multimedia sessions addressed by either E.164 numbers or SIP URLs.
Example of CAMEL procedure
Before we proceed further and use more technical terms, it would be wise to understand CAMEL procedure with the help of some examples.
Take a simple scenario of a voice call being made. When a subscriber starts to make a call, this request is received by the network's Mobile Switching Centre (MSC). The MSC then sends a message that 'queries' the SCP's database. Note that the essential element of any CAMEL solution is a Service Control Point (SCP). This unit effectively hosts a database which holds the instructions needed for an intelligent application.
The SCP processes that query, comes up with an appropriate response and then sends a message back to the MSC telling what action it should take with the subscriber’s request for a specific service. The call is then connected in the most appropriate manner, a process which is transparent to the customer. A very good example of this process in action is short code dialling over a VPN (Virtual Private Network) where the user calls a colleague’s internal extension telephone number but is, in fact, routed to that person’s mobile phone which is roaming abroad.
The main addition in CAMEL phase 2 which phase 1 omitted is support for a Specialised Resource Function (SRF) a component most often found in Voice Response Units (VRUs). For example, when an account balance reaches zero for a pre-paid customer under phase 1, the customer will simply be cut off. With phase 2 thanks to support for SRF, the customer will hear automatically generated messages from the Voice Response Unit warning that the balance is dangerously low before a call and even during the call. Naturally this leads to greater customer satisfaction.
Technical Terms
For any further discussions we would need to use some technical terms. They are discussed in this section.
- Interrogating PLMN (IPLMN): The PLMN that interrogates the Home PLMN (HPLMN) for information to handle mobile terminating call.
- CAMEL Service Environment (CSE): A CSE is a logical entity which processes activities related to Operator Specific Services (OSS).
- CAMEL Subscription Information (CSI): Identifies that CAMEL support is required for the subscriber and the identities of the CSEs to be used for that support.
- Service Control Function (SCF): SCF contains the actual independent service logic to apply to the call.
- Basic Call State Model (BCSM): BSCM represents an abstract view of call processing, seen from the perspective of service feature control performed by the SCF. The BCSM consists of two sets of call processing logic, Originating BCSM (O-BCSM) and Terminating BCSM (T-BCSM).
- Points in Call (PIC): PIC are defined by standards to represent those points in which action might be taken. They provide a view of a state or event in which call processing logic may initiate an action, such as suspension of call processing while a database is queried.
- Detection Points (DP): DP represent transitional events that occur between some PICs.
- Triggers: A trigger is the term used to define specific call-processing logic associated with a given point in call. Triggers are nothing more than software logic that is loaded in a network element to carry out instructions to initiate an intelligent network process based on analysis of conditions at a detection point.
- Events: Unlike a trigger, which depends on some form of input criteria, an event is simply a call occurrence such as no answer, busy signal or call termination.
- Arming of detection points: Detection points have two classifications: Trigger Detection Points (TDP) and Event Detection Point (EDP). A detection point is armed if control logic is established to initiate service control based on a trigger or event.
- GSM Service Control Function (gsmSCF): functional entity that contains the CAMEL service logic to implement OSS. It interfaces with the gsmSSF, the gsmSRF, the GMLC and the HLR.
- GSM Service Switching Function (gsmSSF): functional entity that interfaces the MSC or GMSC to the gsmSCF. The concept of the gsmSSF is derived from the IN SSF, but uses different triggering mechanisms because of the nature of the mobile network.
- GSM Specialised Resource Function (gsmSRF): functional entity which provides various specialized resources. It interfaces with the gsmSCF and with the MSC.
Examples of Signalling procedures
[13] gives example of Signalling procedures using CAMEL for originating and terminating calls and has been described below.
Mobile Originating Calls:
If an active originating CAMEL Subscription Information (CSI) is found in the VLR during the call set up of a MS, the Visited Service Switching Function (VSSF) sends an InitialDetectionPoint message to the gsmSCF and the VMSC suspends the call processing. The InitialDetectionPoint shall always contain the service key, called and calling party number, calling party’s category, location number, bearer capability, event type Basic Call State Model (BCSM), location information and the International Mobile Station Identity (IMSI). After the service logic processing CAMEL specific handling is initiated from the gsmSCF, see figure above.
Mobile Terminating Calls:
In the case of mobile terminating call, the Gateway MSC (GMSC) in the interrogating PLMN identifies the HLR of the called party with the help of the MSISDN. Then the GMSC sends a RoutingInformation-Request to the HLR. The HLR checks the CSI of the called party and sends the information stored in the subscriber record back to the GMSC. Now, the GMSC acts according to CSI. If the terminating CSI is active the trigger criteria of a Detection Point (DP) is fulfilled and the call processing is suspended. An InitialDP message, which shall always contain the service key, called party number, event type BCSM and the IMSI is sent to the CSE and the service logic execution is started. Thereafter CAMEL specific handling is initiated, see figure above.
