Telecom Tutorials by Samir Amberkar

LTE - Signaling stacks UE[Under LTE]
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In following articles, we would look at end-to-end protocols involved in control and user paths. We will start with signaling stacks at UE. Below digram show the signaling stacks at various nodes.

AT interface Adaptor ESM EMM RRC PDCP RLC MAC PHY		RRM/Relay RRC S1AP PDCP SCTP RLC IP MAC L2 PHY L1		MME AL ESM GTP-C EMM S1AP SCTP UDP IP IP L2 L2 L1 L1		SGW AL GTP-C UDP IP L2 L1	Towards PGW
UE	SRBs (RRC SRB0/1/2, MAC LCID 0/1/2)	eNB	S1 (S1AP ID)	MME	GTP-C (IMSI, EPS bearer ID)	SGW

LTE does not support circuit switched connections; it supports only packet switched connections or "bearers". Bearer capabilities are expressed in terms of QoS. Bearer management (setup/modify/release) functions are handled by ESM layer (located in UE and MME). We will cover more details on end-to-end data path/bearers, QoS etc. in later articles.

Below diagram shows the signaling entities at UE, one possible view of their internals, and their important functions.

AT Interface AT Command interpreter		..	27.007, Few related commands +CFUN Activate (limited or full) +COPS PLMN selection +CGATT PS attach +CGDCONT Define PDN connection +CGDSCONT Define bearer resource +CGACT Deactivate/Activate bearer resource
Adaptor ESM EMM Access stratum Data plane management		..	Implementation specific, usually functionality wise modules (e.g. EMM-Adaptor module to talk to NAS EMM module for EMM specific commands & indications), framework for data plane management, adopter may also be in charge of talking to MMI, additional drivers like bluetooth, COM port, USB etc.
ESM Bearer context1 .. Bearer context2 . . . Bearer context11		..	24.301, EPS bearer context management
EMM EMM state machine		..	24.301, mobility management like attach/detach, tracking area update, authentication, providing NAS signaling connection, NAS ciphering and integrity protection
RRC RRC state machine SRB0 SRB1 SRB2		..	36.331, signaling connection management/configuration, data radio bearer configuration, measurement reporting, configuring AS ciphering and integrity protection, handling broadcast system info, paging, handover
PDCP Control SRB1 SRB2 DRB1 . . DRB11*	RRC To RLC	..	36.323, integrity protection, ciphering
RLC Control SRB0 SRB1 SRB2 DRB1 . . DRB11*		..	36.322, buffering, segmentation, concatenation, reassembly, ARQ
MAC Logical channels CCCH DCCH DTCH PCCH BCCH Transport channels RACH PCH BCH SCH		..	36.321, channel mapping, multiplexing, channel prioritisation, HARQ
PHY Transport channels RACH PCH BCH SCH Physical channels PRACH PBCH PSCH PCCH Phy procedures		..	36.201 modulation, coding, power control, cell search, measurements, CRC insertion and other phy specific procedures

* as per RRC TS 36.331, sec 6.4, maxDRB is 11; but as per MAC TS 36.321, section 6.2.1, 11 LCIDs are possible, three of which goes for SRB 0-2, leaving maximum 8 for DRBs !

In next article, we will look at eNB from same angle.

References: 3G Evolution: HSPA and LTE by Dahlman, Parkvall, Sköld, and Beming, LTE - From Theory to Practice, Edited by Sesia, Toufik, and Baker, and SAE and the Evolved Packet Core: Driving the Mobile Broadband Revolution by Olsson, Sultana, Rommer, Frid, and Mulligan.

Copyright © Samir Amberkar 2010-11

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