LTE Protocol Stack

LTE Protocol Stack
http://beyond-3g-wireless.blogspot.com/

After a long break ....

By reading my earlier chapter LTE Architecture Overview,I hope everyone would be familiar with different network elements and their functionality to some extent.

Before we dive into LTE Protocol stack, we will have look at different interfaces connecting the LTE network elements

1) S1-MME:Reference point for the control plane protocol between E-UTRAN and MME.
2) S1-U:Reference point between E-UTRAN and Serving GW for the per bearer user plane tunnelling and inter eNodeB path switching during handover.
3) S3: It enables user and bearer information exchange for inter 3GPP access network mobility in idle and/or active state.(eg. GSM/UMTS network)
4) S4: It provides related control and mobility support between GPRS Core and the 3GPP Anchor function of Serving GW. In addition, if Direct Tunnel is not established, it provides the user plane tunnelling.
5) S5: It provides user plane tunnelling and tunnel management between Serving GW and PDN GW. It is used for Serving GW relocation due to UE mobility and if the Serving GW needs to connect to a non-collocated PDN GW for the required PDN connectivity.
6) S6a: It enables transfer of subscription and authentication data for authenticating/authorizing user access to the evolved system (AAA interface) between MME and HSS.

7) S8: Inter-PLMN reference point providing user and control plane between the Serving GW in the VPLMN and the PDN GW in the HPLMN. S8 is the inter PLMN variant of S5.
8) S9: It provides transfer of (QoS) policy and charging control information between the Home PCRF and the Visited PCRF in order to support local breakout function.
9) S10: Reference point between MMEs for MME relocation and MME to MME information transfer.
10) S11:Reference point between MME and Serving GW
11) S12:Reference point between UTRAN and Serving GW for user plane tunnelling when Direct Tunnel is established. It is based on the Iu-u/Gn-u reference point using the GTP-U protocol as defined between SGSN and UTRAN or respectively between SGSN and GGSN. Usage of S12 is an operator configuration option.
12) S13:It enables UE identity check procedure between MME and EIR.
SGi: It is the reference point between the PDN GW and the packet data network. Packet data network may be an operator external public or private packet data network or an intra operator packet data network, e.g. for provision of IMS services. This reference point corresponds to Gi for 3GPP accesses.
13) Rx: The Rx reference point resides between the AF and the PCRF in the TS 23.203 [6].
14) SBc:Reference point between CBC and MME for warning message delivery and control functions.
15) Gx: It provides transfer of (QoS) policy and charging rules from PCRF to Policy and Charging Enforcement Function (PCEF) in the PDN GW.

We would be discussing mostly on those highlighted in bold font.

This Interface information is taken from 3gpp Spec 23.401/4.2.3/Reference Points

LTE Protocol stack is split into two communication path: Control Plane & User Plane(carry actual user payload)

LTE Control Plane Stack


The control plane consists of protocols for control and support of the user plane functions:-

a) controlling the E-UTRA network access connections, such as attaching to and detaching from E-UTRAN;
b) controlling the attributes of an established network access connection, such as activation of an IP address;
c) controlling the routing path of an established network connection in order to support user mobility;
d) controlling the assignment of network resources to meet changing user demands.

When a mobile is turned ON,the UE communicates to Network by performing Attach Procedure. This where the control plane communication begins.
UE communicates to eNB via Radio Resource Control protocol.This is where all the attach request/response etc are created. RRC is way to communicate to with eNB.
The RRC performs broadcast, paging, RRC connection management, Radio Bearer control, Mobility functions, UE measurement reporting and control.

You can find more information from 3gpp Spec 36.331.
Above RRC we have something called Non Access Stratum Protocol which terminates at MME.

All NAS Messages are sent as a part of RRC message.

Main functions of NAS Protocol:
a) Support of mobility of the user equipment (UE); and
b) Support of session management procedures to establish and maintain IP connectivity between the UE and a packet data network gateway (PDN GW).
c) Also NAS security
e.g. integrity protection and ciphering of NAS signalling messages.

To know more about Non Access Stratum Refer Spec 24.301

LTE User Plane Stack


The Data from UE goes to eNB and eNB maps this data over GTP tunnel and sends it to SGW over S1_U.

MAC, RLC and PDCP are at Layer 2 in UE and eNB

Just below MAC is Physical Layer this where actual concepts of high speed comes into picture, because of sophisticated physical layer. Its no secrete on what technology used here? OFDMA.

Currently I dont have any knowledge over the lower layer(Phy,MAC,RLC,PDCP). So not much discussion on this right now


Below picture shows the interface with protocol used between network elements
Thatz it for now
bye from
Sree