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Sunday, 18 August 2013

LTE : S1 Interface

S1 Interface is the interface that connect eNB to EPC. The Interface is devided into two part :

S1 - MME Interface
  • Control interface between eNB and MME
  • Signalling protocol is S1AP : S1 Application Protocol
  • Signaling transport based on SCTP
  • MME and UE will exchange non-access stratum signaling via eNB through this interface ( i.e. authentication, tracking area updates)
  •  the interface is more similar to Radio Access Network Application Part (RANAP), with some simplifications and changes due to the different functional split and mobility within EPS.

S1 - U Interface
  • User plane interface between eNB and serving gateway
  • Pure user data interface (U=User plane)
  • The interface will be based on GTP User Data Tunnelling (GTP-U) (similar to today’s Iu and Gn interface)

Packet Data Network Gateway (PGW)


The Packet Data Network Gateway (PGW) provides the connection between EPC and a number of external data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PDN GW for accessing multiple PDNs. Thus it is comparable to GGSN in 2G/3G networks. A major functionality provided by a PDN gateway is the QoS coordination between the external PDN and EPC. Therefore the PDN gateway can be connected via S7 or Gx to a PCRF (Policy and Charging Rule Function).

The main function of PGW are :
  •  External IP point of interconnect
  •  IP address allocation
  •  Packet routing & forwarding between SGW to external Data Networks
  •  charging & Lawful intercept support
  •  Policy enforcement
  •  In home or visited network
  • Packet screening (firewall functionality)

Serving Gateway (SGW)

The serving gateway is a network element that manages the user data path (SAE bearers) within EPC. It therefore connects via the S1-U interface towards eNB and receives uplink packet data from here and transmits downlink packet data on it.  Thus the serving gateway is some kind of distribution and packet data anchoring function within EPC. It relays the packet data within EPC via the S5/S8 interface to or from the PDN gateway. A serving gateway is controlled by one or more MMEs via S11 interface.

The main function of SGW are :
  • In visited network in case of roaming
  • Local mobility anchor point : Switching the user plane path to a new eNB in case of Handover
  • IRAT mobility anchor point :Mobility anchoring for inter-3GPP mobility. This is sometimes referred  to as the 3GPP Anchor function
  • Packet routing & forwarding between eNB, PDN GW and SGSN
  • Lawful intercept
  • LTE idle mode Packet buffering and notification to MME 
  • Charging per UE, PDN and QCI
  • Bearer bindings for PMIP S5/S8

Mobility Management Entity (MME)



The Mobility Management Entity (MME) is the key control-node for the LTE access- network. It is controll all the signaling and  responsible for idle mode UE tracking and paging procedure including retransmissions.

Summary of MME function are describe as below :
 1. Authentication
MME responsible for authenticating the user by interacting with HSS through S6a Interface

2. Bearer management & SGW selection
MME is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation

3. Non -Access Stratum (NAS) Signaling
The Non- Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs.

4. Roaming (S6a to Home HSS)
 It checks the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforces UE roaming restrictions.

5. NAS Chipering & Integrity Protection
The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management.Lawful interception of signaling is also supported by the MME.

6. Inter MME and IRAT mobility
The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN.

7. Idle Mode tracking

8. Paging


MME hardware solution can be re-utilised the existing SGSN network element, most all vendors already have SGSN extended capability to be MME function.

Evolved Packet Core (EPC)

Evolved Packet Core (EPC) is the main component of System Architecture Evolution based on 3GPP standard. It's evolution of GPRS Core Network, which consist of 5 main elements :
  • Mobility Management Entity (MME)
  • Serving Gateway (SGW)
  • Packet Data Network Gateway (P-GW)
  • Home Subscriber Server ( HSS)
  • Policy and Charging Rule Function (PCRF) 

E-UTRAN (eNB)

Evolve - UTRAN reponsible for RAN part of EPS architecture. It only consists of one node which well known as eNode B (eNB). Its main role is  for radio transmission to and reception from UEs in one or more cells.

The eNode B is connected to EPC nodes by S1 interface.The S1 interface is devided into two part,  S1 MME interface connects eNB to MME and S1-U interface connects eNB to S-PGW. The eNB may also be connected to its neighbour by X2 interface.

Since the eNB connect to the EPC directly, thus RNC functionality have been moved to eNB. This configuration also known as flat architecture in LTE.

3GPP introduce around 36 series of standarisation as a guidance for all vendor to develop their eNB HW solution. Most of eNB HW solution consist of two main part which are :
  • Digital Unit or System Module
  • Radio Unit or Radio Module
Some big vendors such as Ericsson has RBS 6000 Ericsson  family solution where NSN has Flexi BTS family solution.

 

Below follows a description of the functionality provided by eNode B.

Cell control and MME pool support
eNode B owns and controls the radio resources of its own cells. Cell resources are requested by and granted to MMEs in an ordered fashion. This arrangement supports the MME pooling concept. S-GW pooling is managed by the MMEs and is not really seen in the eNode B.

Mobility control
The eNode B is responsible for controlling the mobility for terminals in active state. This is done by ordering the UE to perform measurement and then performing handover when necessary.

Control and User Plane security
The ciphering of user plane data over the radio interface is terminated in the eNode B.
Also the ciphering and integrity protection of RRC signalling is terminated in the eNode B.

Shared Channel handling
Since eNode B owns the cell resources, eNode B also handles the shared and random access channels used for signalling and initial access.

Segmentation/Concatenation
Radio Link Control (RLC) Service Data Units (SDUs) received from the Packet Data convergence

Protocol (PDCP) layer in the AGW consist of whole IP packets may be larger than the transport block size provided by the physical layer. Thus, the RLC layer must support segmentation and concatenation to adapt the payload to the transport block size.

HARQ
A Medium Access Control (MAC) Hybrid Automatic Repeat reQuest (HARQ) layer with fast feedback provides a means for quickly correcting most errors from the radio channel. To achieve low delay and efficient use of radio resources, the HARQ operates with a native error rate which is sufficient only for services with moderate error rate requirements such as for instance VoIP. Lower error rates are achieved by letting an outer Automatic Repeat reQuest (ARQ) layer in the eNode B handle the HARQ errors.

Scheduling
A scheduler with support for the QoS model provides efficient scheduling of UP and CP data.

Multiplexing and Mapping
The eNode B performs mapping of logical channels onto transport channels.

Physical layer functionality
The eNode B handles the physical layer such as scrambling, Tx diversity, beamforming processing, and OFDM modulation. The eNode B also handles L1 functions like link adaptation and power control.

Measurements and reporting
eNode B provides functions for configuring and making measurements on the radio environment and eNode B-internal variables and conditions. The collected data is used internally for RRM but can be reported for the purpose of multi-cell RRM.

Saturday, 17 August 2013

LTE Network Architecture


What is new in LTE ?

New Radio Transmission schemes
DL : OFDMA
UL : SC-FDMA (DFT OFDMA)
Multi layer Transmission Technology (MIMO)
New Radio Protocol Architecture
Complexity Reduction
Focus on shared channel operation, no dedicated channels anymore
New Network Architecture
More Functionality in Base Station (eNode B)
Focus on Packet Switch Domain
Flat Architecture

What is LTE Requirement ?


What is LTE ?


q3GPP Long Term Evolution

A project within 3GPP TSG-RAN

Evolution of air interface beyond UMTS & HSPA to stay competitive with other technologies (WiMAX, UMB)

Official title: Evolved UTRAN (E-UTRAN)


qWhat about SAE?

System Architecture Evolution – a related project in TSG-SA

Focus is on overall network architecture evolution, to support

LTE requirements,

All-IP network specification, and

Mobility between different Radio Access Technologies (incl. CDMA)

Official title: Evolved Packet Core (EPC)



TSG = Technical Specifications Group

SA = Service & Systems Aspects