Introduction to Evolved Packet Core: Protocols and Procedures

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

Kamakshi Sridhar, PhD Distinguished Member of Technical Staff Director Wireless CTO organization August 2012

Agenda 1.

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

2.

LTE and all-IP: What is new?

3.

EPC components  

4.

Serving Gateway (SGW), PDN Gateway (PGW) Mobility Management Entity (MME), Policy and Charging Control Function (PCRF)

LTE core functions and service procedures  

Core network functions Network attachment, service requests, paging, IP addressing, handover

© 2009 Alcatel-Lucent. All rights reserved.

1

Introduction to Evolved Packet Core and Evolved Packet System

3 | Technical Sales Forum | May 2008


LTE: All-IP, simplified network architecture LTE+EPC

Evolved Packet Core

IP channel

(All-IP)

eNode B

Transport (backhaul and backbone)

What is EPC ?

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  “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

4 | Introduction to EPC | July 2010 | v6

REF: 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) 2G/3G GSM

GMSC Voice Channels

UMTS

Other mobile networks

MSC IP channel

BTS

PSTN

MGW

Circuit Switched Core (Voice)

GPRS EDGE

Softswitch

BSC / RNC Node B

HSPA

SGSN

   

EPC elements

Internet

Packet Switched Core

GGSN

VPN

Serving Gateway (SGW) Packet Data Network (PDN) Gateway (PGW) Mobility Management Element (MME) Policy and Charging Rules Function (PCRF)

LTE/EPC

MME

IP channel SGW eNode B



PCRF

Evolved Packet Core

PDN GW

EPC elements LTE/EPC

MME

IP channel SGW eNode B

PCRF

Evolved Packet Core

PDN GW

 Serving Gateway  Serving a large number of eNodeBs, focus on scalability 

EPC elements 




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)


USER PLANE (UP)

LTE + EPC elements and interfaces

CONTROL PLANE (CP)

S6a

HSS

Rx

S10

External networks Operator Services

MME

S1-MME

PCRF

Applications

S11 Gx eNodeB

Internet

S1-U SGi

S5/S8

X2 S1-U SGW UE

IMS

eNodeB

PGW

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



ACPs

“Flat IP” = less hierarchy means lower latency GSM UMTS CDMA

control plane RNC BSC

Node B BTS

GGSN HA

direct tunnel

data plane RNC BSC

LTE

SGSN PDSN

SGSN PDSN

GGSN HA

control plane MME

eNode B

data plane SGW


S/P GW


PGW

Key implications on user plane (UP) and control plane (CP) Control plane gets new mobile-specific attributes

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

BSC

SGSN/GGSN

GSM/GPRS/EDGE

RNC

 Mobility across networks (and operator domains)  Distributed mobility management  Massive increase in scalability  Dynamic policy management

SGSN/GGSN

RNC

WCDMA/HSPA

PDSN

CDMA/EV-DO

Service Delivery Platforms

LTE IP channel

MME SGW eNode B



PCRF

Evolved Packet Core

PDN GW

Quick Reference: Overview of EPC components and functionality eNB

 eNodeB:  all radio access functions

Policy, Charging & Rules Function

Inter Cell RRM

 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

RB Control

 Radio admission control

 Scheduling of UL and DL data

Connection Mobility Cont.

 Scheduling and transmission of paging and system broadcast

Radio Admission Control

 IP header compression (PDCP)  Outer-ARQ (RLC)

MME NAS Security eNB Measurement Configuration & Provision

PCRF

Idle State Mobility Handling

Dynamic Resource Allocation (Scheduler)

Policy EPS Bearer Control

Decisions

RRC PDCP S-GW

P-GW

RLC Mobility Anchoring

MAC

UE IP address allocation

PDN Gateway    

IP anchor point for bearers UE IP address allocation Per-user based packet filtering Connectivity to packet data network

S1 PHY

Packet Filtering internet

E-UTRAN

EPC

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     


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

All-IP mobile transformation 2G/3G

CS Core Backhaul (TDM/ATM) PS Core Node B BTS BS

SGSN PDSN

RNC

GGSN HA

1

2

3

4

5

6

7

Radio intelligence moving to eNodeB

Backhaul transition to IP/Ethernet

RNC bearer mobility evolves to the SGW

MSC voice and packet data switching evolve into the SGW

CS and PS evolve into a unified all-IP domain

Best effort to e2e QoS

Internet browsing to Web 2.0+

RNC control distributed into the MME/eNB

Packet data control evolves into the MME

LTE Backhaul (IP/Ethernet)

PCRF MME

Service and mobile aware all-IP network eNodeB


SGW

Evolved Packet Core


PDN GW

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) Mapping UMTS traffic types to IP QoS (DiffServ Code Points) Conversational Streaming Interactive Background

End-to-end QoS in UMTS



“Flat-IP” also implies need for a sound QoS mechanism Shared radio resource allocation for all users

Dedicated radio resource allocation per user

TDM TDM

IP

IP IMS

CS

PS EPC

2G/R99 3G Access

Shared resources

PS resources

CS resources

LTE (and HSPA)

 By nature, 2G and Rel99 3G legacy network architecture provides dedicated CS resources ensuring:

 Without QoS control in flat-IP mobile networks, the end-user would experience (e.g. for voice/video service):

 Low latency (optimized for voice service)

 High latency when cell/network is congested

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

 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 17 | Introduction to EPC | July 2010 | v6


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 From: 4 classes in UMTS and CDMA to: 9 classes in LTE One of LTE standards goals: backward compatibility with UMTS QoS

Priority

Packet Delay Budget

Packet Error Loss Rate

2 4 3

100 ms 150 ms 50 ms

10 -3 10 -3 10

5

300 ms

5

1

6 7

QCI

1 2 3 4

Resource Type

Guaranteed Bit Rate (GBR)

-2

Conversational voice Conversational video (live streaming) Real-time gaming

10

-6

Non-conversational video (buffered streaming)

100 ms

10

-6

IMS signalling Video (buffered streaming)

6

300 ms

10

-6

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

7

100 ms

10

-3

8

300 ms

10-6

Voice, video (live streaming), interactive gaming “Premium bearer” for video (buffered streaming),

9

300 ms

10-6

Non-GBR 8 9


Example Services

TCP-based (e.g., www, e-mail, chat, ftp, p2p file sharing, progressive video, etc) for premium subscribers

“Default bearer” for video, TCP-based services, etc. for non-privileged subscribers


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)

E-UTRAN UE

EPC eNodeB

SGW

Internet PGW

peer

End-to-end service External bearer

EPS bearer


Radio bearer

S1 bearer

S5/S8 bearer

LTE-Uu

S1

S5/S8


SGi

3

EPC elements



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

USER PLANE (UP) CONTROL PLANE (CP)

Pool of MMEs • Mobility Management • Bearer handling • Security settings

Pool of SGWs MME

SGW MME

SGW

• User plane tunnels for UL and DL data delivery

eNode B

• Radio Resource Management • Mobility management • Bearer handling • User plane data delivery • Securing and optimizing radio interface delivery

• Inter eNodeB handovers • Forwarding of DL data during handovers

eNode B

UE

User Equipment


eNode B


Other eNodeBs

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

IP channel

MME SGW eNode B

PCRF

Evolved Packet Core

PDN GW

MME is control plane element that manages network access and mobility 23 | Introduction to EPC | July 2010 | v6


MME: Interactions with other functional elements

Other MMEs

HSS


• Authentication and Security •Location management • User profiles

SGWs

MME MME

SGW

• Handovers between MMEs • Idle state mobility between MMEs

SGW

• Control of user plane tunnels

MME

• • • •

User Equipment

eNode B

UE

eNode B


Inter eNodeB handovers State transitions Bearer management Paging


Other eNodeBs

Serving Gateway and Packet Data Network (PDN) Gateway SGW is local mobility anchor

PGW is IP anchor for bearers

 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

 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

IP channel

MME SGW eNode B

eNode B 25 | Introduction to EPC | July 2010 | v6


PCRF

Evolved Packet Core

PDN GW

SGW: Interactions with other functional elements PCRF MMEs

PCRF


PMIP S5/S8 • IP service flow <-> GTP tunnel mapping information

PGWs

MME MME

PGW

• Control of GTP tunnels and IP service flows • SGW Mobility control

PGW

GTP S5/S8 • Control of GTP tunnels • GTP tunnels for UL and DL data delivery PMIP • IP service flows SGW

• User Plane tunnels for DL and UL data delivery •Indirect forwarding of DL data during handovers (in S1-U) when direct (X2) inter-eNodeB connection is not available

eNodeBs

eNode B eNode B

Other SGWs SGW



SGW

PGW: Interactions with other functional elements


PCRFs PCRF

• Policy and Charging Control requests • PCC rules

External networks • IP flows of user data

PGW

• Control of User Plane tunnels • UP tunnels for UL and DL data delivery

Online Charging Systems

Offline Charging Systems

SGWs SGW

SGW



End-to-end protocol stack (User Plane)

MME

IP channel

PCRF

SGW

eNode B

PDN GW

Evolved Packet Core

applications services

user traffic = end-to-end IP

IP

IP

RELAY

RELAY

PDCP

PDCP

GTP-U

GTP-U

GTP-U

GTP-U

RLC

RLC

UDP/IP

UDP/IP

UDP/IP

UDP/IP

MAC

MAC

L2

L2

L2

L2

L1

L1

L1

L1

L1

L1

S1-U

LTE-Uu

UE

eNodeB

S5/S8

SGW

SGi

PGW

* 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) 28 | Introduction to EPC | July 2010 | v6


PCRF: Interactions with other functional elements


AF

External networks • Policy and Charging Control requests

PCRF

• Policy and Charging Control requests • PCC rules • QoS rules when S5/S8 is PMIP

SGWs SGW

SGW


• QoS rules when S5/S8 is PMIP • QoS rules for mapping IP service flows and GTP tunnel in S1 when S5/S8 is PMIP


PGWs PGW

PGW

Policy Charging and Control (PCC) Architecture

SPR

AF

Rx

Sp

PCRF

Gxx

Gx

OCS Gy

BBERF

PCEF Gz

SGW

PGW

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 30 | Introduction to EPC | July 2010 | v6

SDF-based credit control


OFCS

Service level policy control Service Data Flow (SDF) • Packet filters • QoS parameter: QCI, Guaranteed bit rate (UL/DL), Maximum bit rate (UL/DL), Aggregate maximum bit rate PDN-GW

UE SDF-1

Default bearer SDF-2

Dedicated bearer (GBR)

UE-IP1@

SDF-3

UE-IP1@

IP-Connectivity Access Network Session  UE-IP1@

 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 31 | Introduction to EPC | July 2010 | v6


4

Core procedures



EPC: Core functions and service procedures

Core Functions

Core Procedures Charging

Network attachment

Subscriber management

Service requests (paging, buffering)

Mobility management (new!)

Handovers and (X2 routing)

Bearer management

Roaming (home/visiting PDN breakout)

Policy management (new!)

Interworking with 3GPP ANs Interworking with non 3GPP ANs (EVDO/EHRPD treated as a special case)

Interconnection



Roaming – breakout through home PDN Gx

HSS

PDN Gateway

SGi

HPLMN S6a

VPLMN UTRAN

SGSN GERAN S3 S8a

MME S1-MME

X2

S11

S4

S12

eNode B

E-UTRAN eNode B


S1-U

H-PCRF

Serving Gateway


Rx

Home Operator’s IP Services

Roaming – local breakout (through visiting PDN) Rx

HSS

H-PCRF

Home Operator’s IP Services

HPLMN S6a

VPLMN

S9

UTRAN

SGSN GERAN S3

V-PCRF MME Gx

S1-MME

X2

S11

S4

S12

eNode B

eUTRAN E-UTRAN

S1-U

Serving Gateway

S5

PDN Gateway

eNode B



SGi

IP Network

Network attachment and IP address assignment PCRF S7c

Always-on IP connection is established and anchored at PDN-GW

S7

MME S1-MME

X2

S11

SGi

eNode B

E-UTRAN

S1-U

Serving Gateway

S5

IP Network

PDN Gateway IPv4 direct

eNode B

IP

IPv6 shorter

IP address assignment IPv6



IPv4 via DHCP (after)

IPv6 /64 stateless

UE and service requests

PCRF S7c

1. UE sends NAS Service Request message towards MME

S7

2. Update Bearer Request is sent to the S-GW to establish/modify S1-bearer

MME S1-MME

X2

3. Dedicated bearer established after interaction with PCRF

S11

eNode B

E-UTRAN

S1-U

Serving Gateway

S5

PDN Gateway

eNode B



SGi

IP Network

Handover and X2 routing

PCRF S7c

S7

eNB

eNB

eNB

eNB

X2-AP

X2-AP

GTP-U

GTP-U

SCTP

SCTP

UDP

UDP

IP

IP

IP

IP

L2

L2

L2

L2

L1

L1

L1

L1

MME S1-MME

X2

X2-C

X2-U

X2 protocol stacks

S11

eNode B

E-UTRAN

S1-U

Serving Gateway

S5

PDN Gateway

SGi

IP Network

eNode B

X2 = active mode mobility - User Plane (UP) ensures lossless mobility eNode B 39 | Introduction to EPC | July 2010 | v6

- Control Plane (CP) provides eNB relocation capability © 2009 Alcatel-Lucent. All rights reserved.

4a 40 | Technical Sales Forum | May 2008

SMS and legacy voice


SMS service for initial “data-only” devices MSC GERAN UTRAN

SMS-C

CS Network

New interface “SGs” from MSC to MME

SGSN

MME

E-UTRAN

PDN SGW

PGW

Data

eNode B

Paging/SMS

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



Voice support using “CS Fallback” (CSFB) New interface “SGs” from MSC to MME

MSC GERAN UTRAN

MSC GERAN UTRAN

CS Network

SGSN

SGSN MME

E-UTRAN

CS Network

MME

E-UTRAN PDN

eNode B

SGW

PGW

PDN

Data

eNode B

SGW

Paging/SMS

PGW

Circuit Voice Data

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



Voice via IMS MSC

GERAN UTRAN

CS Network

3

SGSN

2

MME

E-UTRAN

TAS

IMS SCC AS

PDN eNode B

GERAN UTRAN

PGW

SGW

MSC CS Network

SGSN

3 2

MME

E-UTRAN

TAS

IMS

PDN eNode B

1

SGW

Circuit Voice


PGW

Circuit signaling

SCC AS

1

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

Packet Voice


IMS Signaling

Packet Data

Alcatel-Lucent EPC Solution 8650 SDM HSS S6a

9271 eRNC

UTRAN S101

7500 SGSN

5780 DSC (PCRF)

S3

Gxc

AFs 8615 IeCCF OFCS

Gx

9471 MME X2

S1-MME

eUTRAN UE

9326 eNB

S1-U

Control Plane

HSGW

GERAN

9326 eNB

Data Plane

CDMA/EVDO

Ro

Rf S11

S4

7750 SR Serving Gateway

S12

Gn Gp

S5/S8

8610 ICC OCS

S2a

7750 SR PDN Gateway

IP Network SGi

End-to-end IP management (incl. services) 5620 SAM



User Plane Scalability

Full UP and CP Management

First mobile gateway to deliver over 100 Gbps

Full GUI management of bearers (UP and CP)

Deployment Universality

Deployment Flexibility As SGW, PGW/GGSN or combo

e2e wireless IP management: RAN, core and backhaul

Performance/QoS

Integration in OSS/BSS

Per-UE, per-app, per-flow hierarchical QoS

Part of full NM portfolio Full OSS/BSS integration

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

7750 Service Router-based Architecture Optimized split of router and gateway functions

Reliability

7750 Service Router Mobile Gateway

5620 SAM Service Aware Manager

Geo-redundancy

Scalability/Architecture

Alcatel-Lucent Ultimate Wireless Packet Core

Suited for Tier X to Tier1 operator environments

Control Plane Scalability

Mobile Core Business Engine

Millions of subscribers Thousands of eNodeBs

Policy Convergence Monetization and Personalization

Deployment Agility

Deployment Flexibility

Flexi rules engine with wizards Up and running in minutes Add new rules easily

As SGSN, MME or SGSN/MME combo

Integration with NM

Performance Superior paging capabilities High-signallng loads

Reliability

9471 Wireless Mobility Manager

5780 Dynamic Services Controller

Part of full NM portfolio Same NM/GUI paradigm

Reliability

Geo-redundancy, pooling No single point of failure

Geo-redundancy No single point of failure

Platform/Architecture

Platform/Architecture

ATCAv2 platform for all CP functions


ATCAv2 platform for all CP functions

www.alcatel-lucent.com


Introduction to Evolved Packet Core: Protocols and Procedures

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