4 Overview description of LLC functions and procedures

04.643GPPGeneral Packet Radio Service (GPRS)Mobile Station - Serving GPRS Support Node (MS-SGSN) Logical Link Control (LLC) layer specificationTS

The requirements of the LLC layer can be summarised as follows:

– LLC shall provide a highly reliable logical link between the MS and the SGSN.

– LLC shall be independent of the underlying radio interface protocols in order to allow introduction of alternative GPRS radio solutions with minimal change to the NSS.

– LLC shall support variable-length information frames.

– LLC shall support peer-to-peer data transfers.

– LLC shall support both acknowledged and unacknowledged data transfers.

– LLC shall permit information transfer between the SGSN and one or more MSs using the same physical (e.g., radio) resources. Thus each LLC frame shall uniquely identify the MS sending (uplink) or receiving (downlink) the information.

– LLC shall allow information transfer with different service criteria, such that high-priority data transfers may take precedence over lower-priority transfers to the same MS.

– LLC shall provide user data confidentiality by means of a ciphering function.

– LLC shall support user identity confidentiality.

4.1 Reference model

A model of layering the protocols in GPRS is illustrated in Error: Reference source not foundFigure 1.

Figure 1: Protocol layering in GPRS

The LLC layer operates above the RLC and BSSGP layers in the reference architecture to provide logical links between an MS and its SGSN.

Above the LLC layer is located the SubNetwork Dependent Convergence (SNDC) layer, that controls the transfer of user data network layer PDUs (N‑PDUs) between the MS and SGSN. The SNDC functionality is described in 3GPP TS 03.60 and specified in 3GPP TS 04.65 [11].

The logical link control layer Service Access Points (SAPs) are the points at which the LLC layer provides services to the layer‑3 protocols in Figure 1. In addition to the SNDC protocol, LLC provides service to the GPRS Mobility Management (GMM) protocol, and to the SMS protocol.

An LLC layer connection is identified by the DLCI consisting of the SAP Identifier (SAPI) and the MS’s Temporary Logical Link Identifier (TLLI).

Each LLC frame consists of the header, trailer, and information field. The header and trailer fields contain information such as SAPI, frame number and checksum, that are used to identify the frame and to provide reliable transmission. The information field is variable length. Both transmission and retransmission of each frame are controlled by the LLC layer.

Many of the formats and procedures are similar to the reference protocols, and differences are introduced only where needed to reflect the unique aspects of the GPRS architecture and requirements.

4.2 General description of the LLC protocol

LLC is considered to be a sublayer of layer 2 in the ISO 7-layer model. The purpose of LLC is to convey information between layer‑3 entities in the MS and SGSN. Specifically, LLC shall support:

– multiple MSs at the Um interface;

– multiple layer‑3 entities within an MS.

LLC includes functions for:

– the provision of one or more logical link connections discriminated between by means of a DLCI;

– sequence control, to maintain the sequential order of frames across a logical link connection;

– detection of transmission, format and operational errors on a logical link connection;

– recovery from detected transmission, format, and operational errors;

– notification of unrecoverable errors;

– flow control; and

– ciphering.

LLC layer functions provide the means for information transfer via peer-to-peer logical link connections between an MS and SGSN pair.

4.2.1 Services required by the lower layers

LLC requires the following services from the layers below:

– LLC PDU delimitation to allow the LLC layer to determine the first octet and the last octet in each LLC PDU; and

– transport of the MS address (a TLLI) of each LLC PDU between the MS and the SGSN.

To "transmit a frame" and "send a frame" refers to the delivery of a frame by the LLC layer to the layer below.

4.3 Unacknowledged operation

With this type of operation, layer‑3 information is transmitted in numbered Unconfirmed Information (UI) frames. The UI frames are not acknowledged at the LLC layer. Neither error recovery nor reordering mechanisms are defined, but transmission and format errors are detected. Duplicate UI frames are discarded.

Flow control procedures are not defined.

Two modes of unacknowledged operation are defined:

– protected mode in which the FCS field protects the frame header and information field; and

– unprotected mode in which the FCS field protects the frame header and only the first octets of the information field.

Unacknowledged operation is allowed for all SAPIs that are not reserved (see Table 2).

4.4 Acknowledged operation

With this type of operation, layer‑3 information is transmitted in order in numbered Information (I) frames. The I frames are acknowledged at the LLC layer. Error recovery and reordering procedures based on retransmission of unacknowledged I frames are specified. Several I frames may be unacknowledged at the same time. In the case of errors that cannot be corrected by the logical link control layer, a report to GPRS mobility management shall be made.

Flow control procedures are defined.

Acknowledged operation requires that ABM operation has been initiated by an establishment procedure using the Set Asynchronous Balanced Mode (SABM) command.

Acknowledged operation is allowed for all SAPIs that are not reserved (see Table 2) except SAPIs 1 and 7.

4.5 Establishment of information transfer modes

4.5.1 Data link connection identification

A logical link connection is identified by a DLCI consisting of two identifiers: a SAPI and a TLLI.

The SAPI is used to identify the service access point on the SGSN side and the MS side of the LLC interface. SAPI is carried in the address field of each LLC frame.

The TLLI is used to identify a specific MS. TLLI assignment is controlled by GMM. TLLI is not carried in LLC frames, but in BSSGP messages as defined in 3GPP TS 08.18 [12], and in RLC/MAC blocks as defined in 3GPP TS 04.08 [8].

4.5.2 Logical link states

A logical link entity may be in one of three basic states:

– TLLI Unassigned state: information transfer shall not be possible with the following exception: the SGSN shall be able to receive UI and XID frames for SAPI = 1;

– TLLI Assigned / ADM state: in this state a TLLI has been assigned. Unacknowledged information transfer and XID negotiation shall be possible on SAPIs that are assigned to a layer‑3 entity; or

– ABM state: this state shall be established by means of an ABM establishment procedure. Both acknowledged and unacknowledged information transfer shall be possible.

The basic states and additional states are shown in annex B.

4.5.3 TLLI assignment

TLLI assignment is controlled by GMM. TLLIs are assigned, changed, and unassigned with the LLGMM-ASSIGN-REQ primitive, as described in subclause 7.2.1.1.

4.5.4 Establishment of ABM operation

Before peer-to-peer acknowledged information transfer can start, an exchange of a SABM frame and an Unnumbered Acknowledgement (UA) frame shall take place. The ABM establishment procedure is specified in clause 8.

4.6 Data confidentiality

The LLC layer shall provide data confidentiality by ciphering the information and FCS fields of data frames:

– The information and FCS fields of I frames shall be ciphered whenever ciphering information has been assigned to the TLLI.

– The information and FCS fields of UI frames shall be ciphered whenever layer 3 indicates that the UI frame shall be ciphered and ciphering information has been assigned to the TLLI.

4.7 LLC layer structure

The LLC layer structure is shown in Figure 2. This figure is a model shown for illustrative purposes only, and does not constrain implementations.

Figure 2: Functional model of the LLC layer

4.7.1 Logical Link Entity

The logical link procedures consist of multiple Logical Link Entities (LLEs) that control the information flow of individual connections. There may be multiple LLEs per TLLI. Functions provided by each LLE are:

– unacknowledged information transfer;

– acknowledged information transfer;

– flow control in ABM operation; and

– frame error detection.

The LLE analyses the control field of the received frame (see subclause 6.3) and provides appropriate responses and layer-to-layer indications. In addition, LLE analyses the LLC layer service primitives and transmits the appropriate command and response frames. There is one logical link entity for each DLCI.

4.7.2 Multiplex procedure

On frame transmission, the multiplex procedure generates and inserts the FCS, performs the frame ciphering function, and provides SAPI-based logical link control layer contention resolution between the various LLEs.

On frame reception, the multiplex procedure performs the frame decipher function and checks the FCS. If the frame passes the FCS check, the multiplex procedure distributes the frame to the appropriate logical link entity based on the DLCI.

3GPP TS 01.61 [2] contains the requirements for the GPRS ciphering algorithm.

4.7.3 Logical Link Management

The Logical Link Management Entity (LLME) manages the resources that have an impact on individual connections. There is one LLME per TLLI. Functions provided by the LLME are:

– parameter initialisation;

– error processing; and

– connection flow control invocation.

The RLC/MAC layer functions are described in 3GPP TS 03.64 [6]. BSSGP is specified in 3GPP TS 08.18. SNDCP is specified in 3GPP TS 04.65.

4.8 GPRS Mobility Management

GPRS Mobility Management (GMM) uses the services of the LLC layer to transfer messages between the MS and the SGSN. GMM includes functions such as attach and authentication, and transport of session management messages for functions such as PDP context activation and deactivation. GMM procedures are defined in 3GPP TS 04.08 and are beyond the scope of the LLC layer. Interaction between GMM and LLC is defined in terms of service primitives, see clause 7.

4.9 Short Message Service

The Short Message Service (SMS) uses the services of the LLC layer to transfer short messages between the MS and the SGSN. SMS procedures are defined in 3GPP TS 03.40 [4] and 3GPP TS 04.11 [9] and are beyond of the scope of the LLC layer. Interaction between SMS and LLC is defined in terms of service primitives, see clause 7.