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GPRS (LLC) - 14

Logical Link Control (LLC) [Under GPRS]
» MS-SGSN; LLC - 04.64 «

LLC acts like a data link layer at network level between MS and SGSN. Like LAPD it comes under HDLC family with same types of frames and usage - refer Data Link layer article.

RLC works as data link layer at access level between MS and BTS. Also application data will mostly be of type TCP/IP packets which also has similar ack/unack type of operation. So LLC is not needed per se. The main purpose of LLC is to hide access related details (like radio access parameters, cell-to-cell, RA-to-RA handovers etc.) from core network. We may change RLC/MAC procedures, operations etc., LLC will take care of its impact on core network. Because of same reasons, LLC is right place for ciphering.

Interesting part here is which mode of operation LLC to set to. For GMM/SM PDUs, RLC is always in Ack mode (one-phase access). This in a way mean, we may use LLC Unack mode for signaling PDUs. For SNDCP PDUs, we may use any one of it. Reliability will improve if both RLC and LLC use Ack mode.

Based on combination of modes for RLC, LLC, and GTP, five reliability classes have been defined as below:

1 - LLC Ack & Protected, RLC Ack, GTP Ack
2 - LLC Ack & Protected, RLC Ack, GTP Unack
3 - LLC Unack & Protected, RLC Ack, GTP Unack
4 - LLC Unack & Protected, RLC Unack, GTP Unack
5 - LLC Unack & Un-protected, RLC Unack, GTP Unack

» SM procedures - 24.008 «
» QoS concepts - 23.107 «

Ciphering is done by LLC, the keys comes from GMM. Ciphering related communication between MS and network comes under GMM procedures. LLC frame header has two bits: PM (Protection Mode) bit - 1 indicate FCS cover header and information field, 0 indicate FCS cover header and first N202 octets (making part of information fields unprotected if exceed N202). Encryption bit indicate whether information field (and FCS field of UI frame) is encrypted (1 if yes).

LLC frame header has similar fields as used by HDLC family except few additional bits like PM and E described above.

In addition to frames mentioned in Data Link layer article, LLC has two additional supervisory frames (which may have information), Ack and SAck.

Ack frames is used to send an acknowledgement for received frames when received=sent - 1 which mean all sent received except last one. SAck is for Selective acknowledgement. It has bitmap to selectively indicate acknowledgement.

LLC header also has A(cknowledgement) bit for forcing receiver to send acknowledgement for received frames.

References: GPRS in Practice by McGuiggan

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