introduction to evolved packet core: protocols and procedures

Kamakshi Sridhar, PhD

Distinguished Member of Technical Staff

Director Wireless CTO organization

August 2012

Introduction to Evolved Packet Core (EPC): EPC Elements, protocols and procedures

© 2009 Alcatel-Lucent. All rights reserved.

Agenda

1. Introduction to Evolved Packet Core (EPC) and Evolved Packet System (EPS)

2. LTE and all-IP: What is new?

3. EPC components Serving Gateway (SGW), PDN Gateway (PGW)

Mobility Management Entity (MME), Policy and Charging Control Function (PCRF)

4. LTE core functions and service procedures Core network functions

Network attachment, service requests, paging, IP addressing, handover

© 2009 Alcatel-Lucent. All rights reserved. 3 | Technical Sales Forum | May 2008

1 Introduction to Evolved Packet Core and

Evolved Packet System


LTE: All-IP, simplified network architecture

New, all-IP mobile core network introduced with LTE End-to-end IP (All-IP)

Clear delineation of control plane and data plane

Simplified architecture: flat-IP architecture with a single core

EPC was previously called SAE (System Architecture Evolution)

eNodeB is also called E-UTRAN

Evolved Packet System = EPC + E-UTRAN

What is EPC ?

LTE+EPC

eNode B

IP channel Evolved Packet Core

(All-IP)

Transport (backhaul and backbone)

“The EPC is a multi-access core network based on the Internet Protocol (IP) that enables operators to deploy and operate

one common packet core network for 3GPP radio access (LTE, 3G, and 2G), non-3GPP radio access (HRPD, WLAN, and

WiMAX), and fixed access (Ethernet, DSL, cable, and fiber).

The EPC is defined around the three important paradigms of mobility, policy management, and security.”

Source: IEEE Communications Magazine V47 N2 February 2009 REF: http://www.comsoc.org/livepubs//ci1/public/2009/feb/pdf/ciguest_bogineni.pdf

4 | Introduction to EPC | July 2010 | v6

http://www.comsoc.org/livepubs//ci1/public/2009/feb/pdf/ciguest_bogineni.pdf


Mobile core in 2G/3G



2 LTE and EPC – what is new?


EPC: new all-IP core, new network elements (functions)

EPC elements

LTE/EPC

eNode B

IP channel MME PCRF

PDN GW SGW

GSM

GPRS

EDGE

UMTS

HSPA

Evolved Packet Core

IP channel

Packet Switched Core

PSTN

Other mobile

networks

VPN

Internet

Voice

Channels

GGSN SGSN

MGW

MSC

BSC / RNC

Circuit Switched Core (Voice)

BTS

Node B

Softswitch GMSC

2G/3G

Serving Gateway (SGW)

Packet Data Network (PDN) Gateway (PGW)

Mobility Management Element (MME)

Policy and Charging Rules Function (PCRF)



EPC elements

EPC elements

LTE/EPC

eNode B

IP channel MME PCRF

PDN GW SGW Evolved Packet Core

Serving Gateway

Serving a large number of eNodeBs, focus on scalability and security

Packet Data Network (PDN) Gateway

IP management (“IP anchor”), connection to external data networks; focus on highly scalable data connectivity and QoS enforcement

Mobility Management Element (MME)

Control-plane element, responsible for high volume mobility management and connection management (thousands of eNodeBs)

Policy and Charging Rules Function (PCRF)

Network-wide control of flows: detection, gating, QoS and flow-based charging, authorizes network-wide use of QoS resources (manages millions on service data flows)



LTE + EPC elements and interfaces

MME PCRF

SGW

S5/S8

Gx S11

eNodeB

eNodeB

S1-U

S1-MME

S1-U

X2

HSS S6a

S10

PGW

SGi

Rx External networks

Operator Services

Applications

IMS

Internet

ACPs

EPC

IP connectivity layer (Evolved Packet System) = E-UTRAN + EPC

UE

Service Connectivity Layer

CONTROL PLANE (CP)

USER PLANE (UP)



“Flat IP” = less hierarchy means lower latency

eNode B

Node B BTS data plane

control plane

data plane

control plane

SGSN

PDSN

RNC

BSC GGSN

HA

SGSN

PDSN

RNC

BSC GGSN

HA

GSM

UMTS

CDMA

LTE

MME S/P GW

PGW SGW

direct tunnel



Key implications on user plane (UP) and control plane (CP)

User plane has many common attributes

with fixed broadband

Broadband capacity

QoS for multi-service delivery

Per-user and per-application policies

Highly available network elements

Control plane gets new mobile-specific attributes

Mobility across networks (and operator domains)

Distributed mobility management

Massive increase in scalability

Dynamic policy management

WCDMA/HSPA GSM/GPRS/EDGE CDMA/EV-DO

PDSN RNC RNC BSC SGSN/GGSN SGSN/GGSN

eNode B

IP channel

LTE MME PCRF


Service Delivery Platforms



Serving Gateway Local mobility anchor for inter-eNB handovers

Mobility anchoring for inter-3GPP handovers

Idle mode DL packet buffering

Lawful interception

Packet routing and forwarding

Policy, Charging & Rules Function Network control of Service Data Flow (SDF)

detection, gating, QoS & flow based charging

Dynamic policy decision on service data flow

treatment in the PCEF (xGW)

Authorizes QoS resources

Quick Reference:

Overview of EPC components and functionality

internet

eNB

RB Control

Connection Mobility Cont.

eNB Measurement

Configuration & Provision

Dynamic Resource

Allocation (Scheduler)

PDCP

PHY

MME

S-GW

S1

MAC

Inter Cell RRM

Radio Admission Control

RLC

E-UTRAN EPC

RRC

Mobility

Anchoring

EPS Bearer Control

Idle State Mobility

Handling

NAS Security

P-GW

UE IP address

allocation

Packet Filtering

eNodeB: all radio access functions

Radio admission control

Scheduling of UL and DL data

Scheduling and transmission of

paging and system broadcast

IP header compression (PDCP)

Outer-ARQ (RLC)

Mobility Management Entity Authentication

Tracking area list management

Idle mode UE reachability

S-GW/PDN-GW selection

Inter core network node signaling for

mobility between 2G/3G and LTE

Bearer management functions

PDN Gateway IP anchor point for bearers

UE IP address allocation

Per-user based packet filtering

Connectivity to packet data network

Policy

PCRF

Decisions



All-IP mobile transformation

RNC

Radio intelligence moving to eNodeB

1 2 4

Node B BTS BS SGSN

PDSN

Backhaul (TDM/ATM)

RNC bearer mobility

evolves to the SGW

3

Backhaul transition

to IP/Ethernet

Backhaul (IP/Ethernet)

MSC voice and packet data switching

evolve into the SGW

RNC control distributed

into the MME/eNB

Packet data control

evolves into the MME

CS Core

5

CS and PS evolve into a unified all-IP

domain

Service and mobile aware all-IP network

Evolved Packet Core

MME

PCRF

PDN GW SGW eNodeB

PS Core

GGSN HA

Best effort to e2e QoS

6 7

Internet browsing

to Web 2.0+

2G/3G

LTE



LTE: more than an evolution for the packet core

Existing paradigm (3G) LTE

Voice Circuit switched (CS)

No (CS) core in LTE

- e2e IP: VoIP (IMS), OneVoice

- Through EPC: OTT, SR-VCC

-Alternatives: CS fallback, VOLGA

Broadband

services Best effort,

Limited expensive “broadband”

Real-time, interactive,

low latency, true broadband QoS

Multisession

data - Rudimentary in 3G (none in 2G/2.5G)

- On request

Based on service data flows (IP flows)

- user-initiated sessions

- network-initiated sessions

QoS

- Driven by UE

-Control-plane intensive setup

- theory: up to 8 CoS, practice: 2 – 4

(voice/control, best effort data)

-Driven by policy management, not UE

-Faster setup through EPC

--9 QoS classes

- End-to-end, associated with bearers

Policy

Management - PCRF introduced in 3GPP R7

- Not widely adopted (static policy mgt used)

Network-wide, dynamic

policy charging and control (PCC)

Mobility

Management - Historically very much aligned (part of) with

RAN

- no RNCs - radio mgt. by eNodeB

- Mobility and session management important

functions of the core



Example of UMTS QoS mapping to IP (transport perspective)

Conversational

Streaming

Background

Interactive

Mapping UMTS traffic types to IP QoS (DiffServ Code Points)

End-to-end QoS in UMTS



“Flat-IP” also implies need for a sound QoS mechanism

2G/R99 3G Access

CS PS

IP TDM

LTE (and HSPA)

EPC

IP TDM

IMS

Dedicated radio resource allocation per user Shared radio resource allocation for all users

PS re

sourc

es

CS re

sourc

es

Share

d

reso

urc

es

By nature, 2G and Rel99 3G legacy network

architecture provides dedicated CS resources

ensuring:

Low latency (optimized for voice service)

A guaranteed bit rate for the whole duration of the CS call (even in case of congestion)

Without QoS control in flat-IP mobile networks,

the end-user would experience (e.g. for

voice/video service):

High latency when cell/network is congested

High voice packet loss when cell/network is congested

Degraded perception for the end-user

QoS control becomes mandatory to offer real-time services (Voice, Video or

Gaming) over flat-IP mobile networks



LTE QoS terms

Service Data Flow = IP flow

SDFs are mapped to bearers by IP routing elements (gateways)

QoS Class Identifier (QCI)

A scalar that is used as a reference to node specific parameters that control packet forwarding treatment (e.g.,

scheduling weights, admission thresholds, queue management thresholds, link layer protocol configuration, etc.),

and that have been pre-configured by the operator owning the access node

Allocation and Retention Priority (ARP)

The primary purpose or ARP is to decide if a bearer establishment/modification request can be accepted or

rejected in case or resource limitation

Guaranteed Bit Rate (GBR)

Maximum Bit Rate (MBR)

Aggregate Maximum Bit Rate (AMBR) (for non-GBR bearers)

QCI + ARP + GBR + MBR + AMBR

bearers



LTE QCI (QoS Class Identifier), as defined by 3GPP TS23.203

QCI Resource Type Priority Packet Delay

Budget

Packet Error Loss Rate

Example Services

1

Guaranteed Bit Rate (GBR)

2 100 ms 10-2

Conversational voice

2 4 150 ms 10-3

Conversational video (live streaming)

3 3 50 ms 10-3

Real-time gaming

4 5 300 ms 10-6

Non-conversational video (buffered streaming)

5

Non-GBR

1 100 ms 10-6

IMS signalling

6

6

300 ms

10-6

Video (buffered streaming) TCP-based (e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc.)

7 7 100 ms 10-3

Voice, video (live streaming), interactive gaming

8

8 300 ms 10-6

“Premium bearer” for video (buffered streaming), TCP-based (e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc) for premium subscribers

9 9 300 ms 10-6 “Default bearer” for video, TCP-based services, etc. for non-privileged subscribers

From: 4 classes in UMTS and CDMA to: 9 classes in LTE

One of LTE standards goals: backward compatibility with UMTS QoS



EPC bearer management

Data plane needs to support fine-granularity of QoS and charging enforcement functions beyond transport / bearer level

Uplink (UL) and Downlink (DL) packet filters are defined for each bearer and QoS

enforcements (policing, shaping, scheduling, etc.) are applied

PGW acts as the Policy and Charging Enforcement Function (PCEF) point to maintain

QoS / SLA for each of the bearers (and SDFs)

eNodeB SGW PGW peer UE

LTE-Uu S1 S5/S8 SGi

End-to-end service

EPS bearer External

bearer

Radio

bearer

S1

bearer

S5/S8

bearer

E-UTRAN EPC Internet



3 EPC elements


eNode B

MME SGW

eNodeB (E-UTRAN) (not a part of the EPC), but let’s look at…

Interactions with other functional elements

eNode B

MME SGW

UE

eNode B

Pool of SGWs Pool of MMEs

Other eNodeBs

• Mobility Management

• Bearer handling

• Security settings

• Radio Resource

Management

• Mobility management

• Bearer handling

• User plane data delivery

• Securing and optimizing

radio interface delivery

• User plane tunnels for

UL and DL data delivery

• Inter eNodeB handovers

• Forwarding of DL data

during handovers

User Equipment

CONTROL PLANE (CP)

USER PLANE (UP)



Mobility Management Entity

MME controls how UE interacts with the network via non-access stratum (NAS) signalling

Authenticates UEs and controls access to network connections

Controls attributes of established access (e.g., assignment of network resources)

Maintains EPS Mobility Management (EMM) states for all UE’s to support paging, roaming and handover

Manages ECM (EPS Connection Management) states

eNode B

IP channel

MME is control plane element that manages network access and mobility

MME PCRF




MME SGW

MME:


MME SGW

UE

eNode B

SGWs Other MMEs

Other eNodeBs

• Handovers between MMEs

• Idle state mobility between MMEs

• Authentication and Security

•Location management

• User profiles

• Control of user plane tunnels

• Inter eNodeB handovers

• State transitions

• Bearer management

• Paging

User Equipment

MME

HSS

eNode B

CONTROL PLANE (CP)

USER PLANE (UP)



Serving Gateway and Packet Data Network (PDN) Gateway

SGW is local mobility anchor

Terminates (S1-U) interface towards E-UTRAN

Local anchor point for inter-eNB handover and inter-3GPP mobility

Support ECM-idle mode DL packet buffering and network-initiated service request

IP routing and forwarding functions

PGW is IP anchor for bearers

Terminates (SGi) interface towards the PDN

Provides UE IP address management (allocation)

Provide Policy and Charging Enforcement Function (PCEF)

Per-SDF based packet filtering

Interface to Online and Offline Charging Systems

eNode B

IP channel

eNode B

MME PCRF




MME PGW

SGW:


MME PGW

eNode B

PGWs MMEs

Other SGWs

• Control of GTP tunnels and IP service flows

• SGW Mobility control

PMIP S5/S8

• IP service flow <-> GTP tunnel

mapping information

GTP S5/S8

• Control of GTP tunnels

• GTP tunnels for UL and DL

data delivery

PMIP

• IP service flows

• User Plane tunnels for

DL and UL data delivery

SGW

eNode B

SGW SGW

•Indirect forwarding of DL data

during handovers (in S1-U)

when direct (X2) inter-eNodeB

connection is not available

PCRF

eNodeBs

PCRF

CONTROL PLANE (CP)

USER PLANE (UP)



PGW:


SGWs

• Policy and Charging Control requests

• PCC rules

• IP flows of user data

PGW

SGW SGW

• Control of User Plane tunnels

• UP tunnels for UL and DL data

delivery

PCRF

External networks

PCRFs

Online Charging

Systems

Offline Charging

Systems

CONTROL PLANE (CP)

USER PLANE (UP)



End-to-end protocol stack (User Plane)

MAC

RLC

PDCP

L1

S1-U LTE-Uu

eNodeB UE

L2

UDP/IP

GTP-U

L1

S5/S8

L2

UDP/IP

L1

L2

UDP/IP

L1

L2

UDP/IP

L1

MAC

RLC

L1

PDCP

SGW PGW

SGi

IP

applications

services

* S5/S8 reference point between S-GW and PDN-GW can also be GTP based

Key role of S-GWs and PDN-GWs = to manage the user plane (bearer traffic)

user traffic = end-to-end IP

eNode B

IP channel MME

PCRF

PDN GW

SGW

Evolved Packet Core

GTP-U

RELAY IP

GTP-U GTP-U

RELAY



PCRF:


SGWs


PCRF

SGW SGW

PGW

External networks

PGWs

AF

PGW


• PCC rules

• QoS rules when S5/S8 is PMIP

• QoS rules when S5/S8 is PMIP

• QoS rules for mapping IP service flows

and GTP tunnel in S1 when S5/S8 is

PMIP

CONTROL PLANE (CP)

USER PLANE (UP)



Policy Charging and Control (PCC) Architecture

BBERF

BBERF = Bearer Binding and Event Reporting Function

OCS = Online Charging System

OFCS = Offline Charging System

PCEF = Policy and Charging Enforcement Function

SPR = Subscription Profile repository

OCS SDF-based credit control

OFCS

AF

PCEF

Gy

Gz

PCRF

Rx

Gx Gxx

SGW PGW

SPR Sp



Service level policy control

The PGW needs to support fine-granularity of QoS and charging enforcement functions

beyond transport / bearer level

Multiple Service Data Flow (SDF) can be aggregated onto a single EPS bearer

Uplink and downlink packet filters are defined for each bearer, and QoS enforcements

are applied

PDN-GW

IP-Connectivity Access Network Session UE-IP1@

Dedicated bearer (GBR) UE-IP1@

UE

Default bearer

SDF-1

SDF-3

SDF-2

UE-IP1@

Service Data Flow (SDF)

• Packet filters

• QoS parameter: QCI, Guaranteed bit rate (UL/DL),

Maximum bit rate (UL/DL), Aggregate maximum bit rate



4 Core procedures


EPC: Core functions and service procedures

Core Functions

Charging

Subscriber management

Mobility management (new!)

Bearer management

Policy management (new!)

Interconnection

Core Procedures

Network attachment

Service requests (paging, buffering)

Handovers and (X2 routing)

Roaming (home/visiting PDN breakout)

Interworking with 3GPP ANs

Interworking with non 3GPP ANs

(EVDO/EHRPD treated as a special case)



Roaming – breakout through home PDN

SGi

GERAN

UTRAN

S11

S3

S8a

HSS

S4

S1-U

S1-MME

MME

S6a

SGSN

S12

HPLMN

VPLMN

X2

Gx Rx

H-PCRF

eNode B

PDN Gateway

Serving Gateway E-UTRAN

Home Operator’s IP Services

eNode B



Roaming – local breakout (through visiting PDN)

GERAN

UTRAN

S11

S3

HSS

S4

S1-U

S1-MME

MME

S6a

SGSN

S12

HPLMN

VPLMN

X2

Rx

H-PCRF

eNode B

Home Operator’s IP Services

SGi S5 PDN

Gateway Serving

Gateway IP Network

eUTRAN

eNode B

V-PCRF

Gx

S9

E-UTRAN



IP address assignment

Network attachment and IP address assignment

S7c

SGi S11

S5

E-UTRAN S1-U

S1-MME

MME

Serving Gateway

PDN Gateway

IP Network

S7

X2

PCRF

Always-on IP connection is established and anchored at

PDN-GW

eNode B

eNode B

IP

IPv4 direct

IPv4 via DHCP (after)

IPv6 /64 stateless

IPv6

IPv6 shorter



UE and service requests

S7c

SGi

S11

S5

E-UTRAN S1-U

S1-MME

MME

Serving Gateway

PDN Gateway

IP Network

S7

X2

PCRF

1. UE sends NAS Service Request

message towards MME

2. Update Bearer Request is sent to the S-GW to establish/modify

S1-bearer

3. Dedicated bearer established after

interaction with PCRF

eNode B

eNode B



eNode B

Handover and X2 routing

SGi

S11

S5

E-UTRAN S1-U

S1-MME

MME

Serving Gateway

PDN Gateway

IP Network

X2

S7c S7

PCRF

eNode B

eNode B

X2 = active mode mobility

- User Plane (UP) ensures lossless mobility

- Control Plane (CP) provides eNB relocation capability

UDP

IP

L2

L1

GTP-U

UDP

IP

L2

L1

GTP-U

eNB eNB

X2-U

SCTP

IP

L2

L1

X2-AP

SCTP

IP

L2

L1

X2-AP

eNB eNB

X2-C

X2 protocol stacks



4a SMS and legacy voice


SMS service for initial “data-only” devices

Data and SMS only

Handset uses LTE network where possible to achieve highest throughput

Handset served by an MSC in legacy network for voice and SMS

SMS delivered over SGs – without requiring inter-RAT handover

GERAN

UTRAN

SGSN

MSC

CS Network

PDN

E-UTRAN

eNode B

PGW SGW

MME

Data

Paging/SMS

New interface “SGs” from MSC to

MME

SMS-C



GERAN

UTRAN

SGSN

MSC

CS Network

PDN

E-UTRAN

eNode B PGW SGW

MME

GERAN

UTRAN

SGSN

MSC

CS Network

PDN

E-UTRAN

eNode B PGW SGW

MME

Voice support using “CS Fallback” (CSFB)

Simultaneous Voice + Data

Handset falls back to legacy circuit coverage for voice

Incoming calls to MSC trigger paging over SGs and delivered via MME

Data sessions handover to SGSN if possible

Tradeoff:

Re-uses legacy circuit infrastructure

But at the cost of Inter-RAT handover per voice call, and reduced capacity (3G) or

suspended (2G) data sessions

Data

Data

Circuit Voice

Paging/SMS

New interface “SGs” from MSC to

MME



GERAN

UTRAN

SGSN

MSC

CS Network

PDN

E-UTRAN

eNode B PGW SGW

MME IMS TAS

SCC AS

GERAN

UTRAN

SGSN

MSC

CS Network

PDN

E-UTRAN

eNode B PGW SGW

MME IMS TAS

SCC AS

Voice via IMS

Simultaneous Voice and Data on LTE

Handset has concurrent access to:

1. Data services including internet access

2. IMS Services including VoIP end-end calling

3. IMS interworking towards legacy

PSTN/PLMN networks

Uses IMS nodes “Telephony Application

Server” (TAS) and “Service Centralization and

Continuity Application Server” (SCC AS)

IMS Services outside of LTE coverage

For service transparency, IMS Centralized

Services (ICS) provides IMS services even

when the handset is out of LTE coverage

Handset has concurrent access to:

1. Data Services including internet access

2. IMS Services including circuit-mode

transport of voice path

3. Calls to-from the PSTN/PLMN legacy

network as well as calls to VoIP end users

in IMS

1

2

3

1

2

3

Circuit Voice Packet Voice IMS Signaling Packet Data Circuit signaling



Alcatel-Lucent EPC Solution

Gxc

SGi

GERAN

UTRAN

S11

S3

S5/S8

8650 SDM

HSS

S4

S1-U

S1-MME

S6a

7500

SGSN

IP Network

Gx

X2

AFs 5780 DSC

(PCRF)

7750 SR

Serving

Gateway

S101

S12

7750 SR

PDN

Gateway

CDMA/EVDO 9271

eRNC

HSGW

S2a

9471

MME

9326 eNB

Data Plane

Control Plane

Rf Ro

8615

IeCCF

OFCS 8610

ICC

OCS Gn Gp

UE

eUTRAN

9326

eNB

5620 SAM

End-to-end IP management (incl. services)



Alcatel-Lucent

Ultimate Wireless Packet Core

7750 Service Router

Mobile Gateway

9471 Wireless

Mobility Manager 5780 Dynamic

Services Controller

5620 SAM

Service Aware Manager

User Plane Scalability First mobile gateway

to deliver over 100 Gbps

Deployment Flexibility As SGW, PGW/GGSN

or combo

Performance/QoS Per-UE, per-app, per-flow

hierarchical QoS

Reliability 99.999+ % field proven 48,000+ units shipped

7750 Service Router-based Architecture

Optimized split of router and gateway functions

Control Plane Scalability Millions of subscribers Thousands of eNodeBs

Deployment Flexibility As SGSN, MME

or SGSN/MME combo

Performance Superior paging capabilities

High-signallng loads

Reliability Geo-redundancy, pooling No single point of failure

Platform/Architecture ATCAv2 platform

for all CP functions

Full UP and CP Management Full GUI management

of bearers (UP and CP)

Deployment Universality e2e wireless IP management:

RAN, core and backhaul

Integration in OSS/BSS Part of full NM portfolio Full OSS/BSS integration

Reliability Geo-redundancy

Scalability/Architecture Suited for Tier X to Tier1 operator environments

Mobile Core Business Engine Policy Convergence

Monetization and Personalization

Deployment Agility Flexi rules engine with wizards

Up and running in minutes Add new rules easily

Integration with NM Part of full NM portfolio Same NM/GUI paradigm

Reliability Geo-redundancy

No single point of failure

Platform/Architecture ATCAv2 platform

for all CP functions


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