5.3 High-Level Functions Required for GPRS

03.603GPPGeneral Packet Radio Service (GPRS)Release 1998Service descriptionStage 2TS

The following list gives the logical functions performed within the GPRS network. Several functional groupings (meta-functions) are defined which each encompasses a number of individual functions:

– Network Access Control Functions;

– Packet Routeing and Transfer Functions;

– Mobility Management Functions;

– Logical Link Management Functions;

– Radio Resource Management Functions;

– Network Management Functions.

5.3.1 Network Access Control Functions

Network access is the means by which a user is connected to a telecommunication network in order to use the services and/or facilities of that network. An access protocol is a defined set of procedures that enables the user to employ the services and/or facilities of the network.

User network access may occur from either the mobile side or the fixed side of the GPRS network. The fixed network interface may support multiple access protocols to external data networks, for example X.25 or IP. The set of access protocols to be supported is determined by the PLMN operator.

Individual PLMN administrations may require specific access-control procedures in order to limit the set of users permitted to access the network, or to restrict the capabilities of individual users, for example by limiting the type of service available to an individual subscriber. Such access control procedures are beyond the scope of the GPRS specifications.

In addition to the standard data transfer, GPRS may support anonymous access to the network. The service allows an MS to exchange data packets with a predefined host that can be addressed by the supported interworking protocols. Only a limited number of destination PDP addresses can be used within this service. IMSI or IMEI shall not be used when accessing the network thus guaranteeing a high level of anonymity. Therefore, no authentication and ciphering functionalities are foreseen for anonymous access.

5.3.1.1 Registration Function

Registration is the means by which a user’s Mobile Id is associated with the user’s packet data protocol(s) and address(es) within the PLMN, and with the user’s access point(s) to the external PDP network. The association can be static, i.e., stored in an HLR, or dynamic, i.e., allocated on a per need basis.

5.3.1.2 Authentication and Authorisation Function

This function performs the identification and authentication of the service requester, and the validation of the service request type to ensure that the user is authorised to use the particular network services. The authentication function is performed in association with the Mobility Management functions.

5.3.1.3 Admission Control Function

The purpose of admission control is to calculate which network resources are required to provide the quality of service (QoS) requested, determine if those resources are available, and then reserve those resources. Admission control is performed in association with the Radio Resource Management functions in order to estimate the radio resource requirements within each cell.

5.3.1.4 Message Screening Function

A screening function concerned with filtering out unauthorised or unsolicited messages is required. This should be supported through packet filtering functions. Network-controlled screening is supported in the first phase of GPRS. Subscription-controlled and user-controlled screening may be provided in a later phase.

5.3.1.5 Packet Terminal Adaptation Function

This function adapts data packets received / transmitted from / to terminal equipment to a form suitable for transmission across the GPRS network.

5.3.1.6 Charging Data Collection Function

This function collects data necessary to support subscription and/or traffic fees.

5.3.2 Packet Routeing and Transfer Functions

A route is an ordered list of nodes used for the transfer of messages within and between the PLMN(s). Each route consists of the originating node, zero or more relay nodes and the destination node. Routeing is the process of determining and using, in accordance with a set of rules, the route for transmission of a message within and between the PLMN(s).

5.3.2.1 Relay Function

The relay function is the means by which a node forwards data received from one node to the next node in the route.

5.3.2.2 Routeing Function

The routeing function determines the network node to which a message should be forwarded and the underlying service(s) used to reach that GPRS Support Node (GSN), using the destination address of the message. The routeing function selects the transmission path for the "next hop" in the route.

Data transmission between GSNs may occur across external data networks that provide their own internal routeing functions, for example X.25, Frame Relay or ATM networks.

5.3.2.3 Address Translation and Mapping Function

Address translation is the conversion of one address to another address of a different type. Address translation may be used to convert an external network protocol address into an internal network address that can be used for routeing packets within and between the PLMN(s).

Address mapping is used to map a network address to another network address of the same type for the routeing and relaying of messages within and between the PLMN(s), for example to forward packets from one network node to another.

5.3.2.4 Encapsulation Function

Encapsulation is the addition of address and control information to a data unit for routeing packets within and between the PLMN(s). Decapsulation is the removal of the addressing and control information from a packet to reveal the original data unit.

Encapsulation and decapsulation are performed between the support nodes of the GPRS PLMN(s), and between the serving support node and the MS.

5.3.2.5 Tunnelling Function

Tunnelling is the transfer of encapsulated data units within and between the PLMN(s) from the point of encapsulation to the point of decapsulation. A tunnel is a two-way point-to-point path. Only the tunnel endpoints are identified.

5.3.2.6 Compression Function

The compression function optimises use of radio path capacity by transmitting as little of the SDU (i.e., the exterior PDP PDU) as possible while at the same time as preserving the information contained within it.

5.3.2.7 Ciphering Function

The ciphering function preserves the confidentiality of user data and signalling across the radio channels and inherently protects the PLMN from intruders.

5.3.2.8 Domain Name Server Function

The Domain Name Server function resolves logical GSN names to GSN addresses. This function is standard Internet functionality according to RFC 1034 [43], which allows to resolve any name for GSNs and other GPRS nodes within the GPRS backbone networks.

5.3.3 Mobility Management Functions

The mobility management functions are used to keep track of the current location of an MS within the PLMN or within another PLMN.

5.3.4 Logical Link Management Functions

Logical link management functions are concerned with the maintenance of a communication channel between an individual MS and the PLMN across the radio interface. These functions involve the co-ordination of link state information between the MS and the PLMN as well as the supervision of data transfer activity over the logical link.

Refer to GSM 04.64 [13] for further information.

5.3.4.1 Logical Link Establishment Function

Logical link establishment is performed when the MS attaches to the GPRS service.

5.3.4.2 Logical Link Maintenance Functions

Logical link maintenance functions supervise the logical link status and control link state changes.

5.3.4.3 Logical Link Release Function

The logical link release function is used to de-allocate resources associated with the logical link connection.

5.3.5 Radio Resource Management Functions

Radio resource management functions are concerned with the allocation and maintenance of radio communication paths. GSM radio resources is shared between the circuit mode (voice and data) services and the GPRS.

Refer to GSM 03.64 for further information.

5.3.5.1 Um Management Function

This function manages the set of physical channels used in each cell and determines the amount of radio resources to be allocated for GPRS use. The amount of radio resources allocated for GPRS may vary from cell to cell depending upon local user demand or other policies established by the PLMN operator.

5.3.5.2 Cell Selection Function

This function enables the MS to select the optimal cell for use in establishing a communication path with the PLMN. This involves the measurement and evaluation of signal quality from nearby cells as well as the detection and avoidance of congestion within candidate cells.

Refer to GSM 03.22 [7] and GSM 03.64 for further information.

5.3.5.3 Um-tranx Function

The Um-tranx function provides packet data transfer capability across the radio interface between the MS and the BSS. This function includes procedures that:

– Provide medium access control over radio channels;

– Provide packet multiplexing over common physical radio channels;

– Provide packet discrimination within the MS;

– Provide error detection and correction;

– Provide flow control procedures.

5.3.5.4 Path Management Function

This function manages the packet data communication paths between the BSS and the serving GSN nodes. The establishment and release of these paths may be dynamic based upon the amount of data traffic or may be static based upon the maximum expected load within each cell.

5.3.6 Network Management Functions

Network management functions provide mechanisms to support O&M functions related to GPRS.