General Packet Radio Service (GPRS); Mobile Station - Serving GPRS Support Node (MS-SGSN) Logical Link Control (LLC) layer specification
The present document defines the Logical Link Control (LLC) layer protocol to be used for packet data transfer between the Mobile Station (MS) and Serving GPRS Support Node (SGSN).
It defines the frame structure, elements of procedure, format of fields, and procedures for the proper operation of the logical link control layer. It is based on ideas contained in IS-130 [21], ISO 3309 [16], ISO 4335 [17], and ISO 7809 [18, 19, 20] (HDLC of ISO), as well ITU-T Q.920 [13] and Q.921 [14] (LAPD). The concepts, the overview description of LLC layer functions and procedures, and the relationship with other Technical Specifications are described in general terms in 3GPP TS 03.60 [5].
LLC spans from the Mobile Station (MS) to the Serving GPRS Support Node (SGSN). LLC is intended for use with both acknowledged and unacknowledged data transfer.
The frame formats defined for LLC are based on those defined for LAPD and RLP. However, there are important differences between LLC and other protocols, in particular with regard to frame delimitation methods and transparency mechanisms. These differences are necessary for independence from the radio path.
The LLC procedures are modelled upon the concepts of HDLC as outlined in ISO 4335. Data sequence integrity between the data source and data sink is effected by means of a cyclic numbering scheme. An independent numbering scheme is used for each logical data link, as identified by the a data link connection identifier. LLC supports two modes of operation:
- Unacknowledged peer-to-peer operation:
A logical link entity may initiate transmissions to a peer entity without prior establishment of a logical connection with the peer entity. LLC does not guarantee in-order delivery. LLC can detect errors in a received frame, and, depending on whether the frame is sent in protected mode or not, either discard or deliver the erroneous frame. No error recovery procedures are defined at the LLC layer. Higher-layer protocols can be used to provide reliability, if needed. This mode of operation is known as Asynchronous Disconnected Mode (ADM).
- Acknowledged peer-to-peer operation:
A balanced data link involves two participating entities, and each entity assumes responsibility for the organisation of its data flow and for error recovery procedures associated with the transmissions that it originates. Each entity operates as both a data source and data sink in a balanced link, allowing information to flow in both directions. This mode of operation is known as Asynchronous Balanced Mode (ABM), and provides a reliable service with in-order delivery.
The present document is organised as follows:
- An overview of the LLC layer functions is given in clause 4.
- The frame structure for peer-to-peer communication is given in clause 5.
- The elements of procedure and formats of fields are given in clause 6.
- The elements of layer-to-layer communication are contained in clause 7.
- The details of the peer-to-peer ABM procedures are given in clause 8.
- The details of LLC frame ciphering are given in annex A.
- The details of the TOM protocol layer are contained in annex B.
- An overview of the LLC layer states is provided in annex C.