Introduction to Mobile Core Network Elements and Call Flow

Introduction into Mobile Core Network
WebEx Sunday Session
24 March 2013

About this presentation
• Goal
– Introduction into 2G/3G and 4G core
network element in Telco
technology, particular for non-Telco
person.
– Overview of call establishment process
– Experience sharing of 4G EPC project
delivery from existing Mobile Network
Operator.

• What is not covered here…
– Wireless and radio related part.
– Detail and ‘deep dive’ of each network
element.
– Detail procedure for call establishment.

• Disclaimer
– All material presented here is solely for non-profit
educational purpose.

Node and Interface in Telco
• What is a Node ?
Node is a network component that is capable of
receiving, sending or forwarding electronic information over
the designated communication link. For example
BSC, RNC, SGSN, MSC, HLR.
Known as Network Element too..

• What is an Interface ?
In telecommunication, an interface is a logical link connection
or a reference point, between two components in
telecommunication infrastructure, for example, Gn interface
is a reference point between SGSN and a GGSN.

However, Telecommunication also use term ‘interface’ as a
device or program of that enabling a communication between
user and a computing device.

2G and 3G Network Architecture
Circuit Core Domain
Nc
MSS/VLR MSS/VLR

MS
BTS Mc
Mc

Nb
MGW MGW PSTN
Abis Other
PLMN

BSC A
SMSC RADIUS Charging Centre
IuCS
HLR DNS NMS / OSS

PCRF DHCP
RNC Gb
Service & subscriber management

MS
Iub
IuPS Gz/Gs/Gr/Gf/Gd
Packet Data Network
Gx/Gz e..g Internet
NodeB
Gi
Gn
SGSN GGSN
Packet Core Domain
Control plane
User plane

Acronym used in Telco..
MS Mobile Subscriber A mobile phone with SIM card
BTS Base Transceiver Station Air interface toward MS in 2G
BSC Base Station Controller Controlling BTS
NodeB n/a Air interface toward MS in 3G
RNC Radio Network Controller Controlling NodeB
MSS Mobile Switching Server Mobility Management for CS Domain
VLR Visitor Location Register Manage temporary subscriber
information within an MSS
HLR Home Location Register Manage permanent subscriber
Database
MGW Media Gateway Manage user plane traffic
SGSN Support GPRS Service Node Mobility Management for PS Domain
GGSN Gateway GPRS Service Node Session Management , IP address
allocation, QoS enforcement, Routing
toward internet.

Circuit Switching vs. Packet Switching

• Circuit Switching
–Mostly used in the voice call.
–Whenever a call is being made, a dedicated
logical link needs to be established.
–Network resource is allocated and reserved for
the subscriber for entire duration of the call.

• Packet Switching
–Mostly used in the Packet Data Connection.. .
e.g. internet browsing
–Whenever an MS sends or receives data, data
is organized into packet and each packet has
source / destination ID.

Signaling Plane vs. User Plane
• User Plane is the user traffic that contain the
conversation between 2 or more user peers.
– Carried over the core network as a bearer
– Example. Voice call or Browsing data traffic through GPRS

• Control Plane is signaling communication used
to control the bearer traffic.
– Involving of sending and receiving messages between 2 nodes.
– Example. Communication between SGSN and HLR.

A Simplified 2G/3G PS Call Flow…(1/3)
Reference :
TS 23.060
1 Attach
request..
Herewith the
info
TS 24.008
3
TS 29.060
HLR Gn DNS
MS
BTS / Nodeb

Abis / Iub

I am attached 5 Get info for this Packet Data Network
to the network BSC 2 subscriber
e..g Internet
and want to RNC
browse, by
activating PDP Gb IuPS
Gn
context SGSN GGSN

Attach 4 Internet
accepted.. DNS

Note :
All procedures here are simplified for overview only.
Reader should refer to above reference for detail
procedures.

Reference :
Herewith the IP address TS 23.060
of GGSN
7 TS 24.008
TS 29.060
HLR Gn DNS
MS
BTS / Nodeb

Abis - Iub

I am attached 5 Gb -IuPS Packet Data Network
to the network BSC e..g Internet
and want to RNC
browse , by
activating PDP Gn
context SGSN GGSN Gi

9 Internet
8
11 6 Create PDP Context DNS
PDP context activation received Create PDP accepted
from MS, Context
PDP context activation What is the IP address of the Request
accepted, now browse to GGSN that serving this APN ? ?
www.google.com.au
10
Create PDP context
accepted, send this
Note : info to MS

procedures.

Reference :
TS 23.060
TS 24.008
TS 29.060
HLR Gn DNS
MS
BTS / Nodeb

Abis - Iub 12
Gb -IuPS
Packet Data Network
BSC e..g Internet
11 RNC
Gn
PDP context activation
SGSN GGSN Gi
accepted, now browse to
www.google.com.au
13

Internet
DNS

Note :
procedures.

Handover
Action of switching a call in
progress from one radio
channel to another to
secure the continuity of an
established call.

The purpose of handover control is to
manage the mobility aspect of the
Subscriber. This
means keeping track of mobile stations
(MS) as they move around in the
network, and
ensuring that their connections are
uninterrupted and meet the negotiated
Quality of
Service requirements.

Move to 4G..???
• Wait…first, let see the widely
involvement of IP protocol. 
• Protocol used in the control
plane messaging has evolved.
– Changes in Transport Layer
protocol while keeping all the
Application Layer remain
unchanged.
– Completely changed at Transport
and Application Layer.
• Transport Layer has changed
dramatically from legacy
protocol toward IP.

General LTE Architecture
BTS/NodeB SGSN
BSC/RNC
Pre R8
Gb PCRF
2G/3G
Iu

R7 DT Gn Gn
BTS BSC 2G SGSN Gx
R8 2G Gb SGi
Internet

SGi Operator services
S5
S12 DT SGW PGW
S4
NodeB RNC
R8 3G SGi
Iu

S11 HSS
SGSN

S1-U S3 S6a
eNodeB
LTE S1-MME
MSC
SGs

MME
S10

Control plane Basic EPS architecture defined in 3GPP TS 23.401
User plane

Acronym used in 4G

E-NodeB Air interface toward MS in 4G

MME Mobility Management Entity Mobility Management for 4G, Control
Plane only

S-GW Serving Gateway Mobility Management for 4G for User
Plane, Handover anchoring .

P-GW Packet Data Network Gateway Routing to Packet Data Network,
Session Management, IP address
Allocation. Function of P-GW is similar
to GGSN in the 2G/3G network

HSS Home Subscriber Server Manage Permanent 4G subscriber
data. Evolution of HLR.

Internet has driven operation model…(1/3)
Operator Network

TDM HLR

SGSN IP / MPLS
GGSN

Internet

ATM RNC

Multiple network type exist on Mobile Operator Network

Operator Network

TDM HLR

SGSN IP / MPLS
GGSN

Internet

ATM RNC

Driven by internet grows, Telco vendor is developing their product using
IP as the transport media.

Operator Network

HLR

SGSN IP / MPLS
GGSN

Internet

RNC

Driven by internet grows, Telco vendor is developing their product using
IP as the transport media.
Mobile Operator needs to ‘simplify’ the O&M for many network type.
New challenges and opportunities are on-board for IP (IT) and Telco engineer.

Protocol has been changed.. (1/2)
INAP/CAP MAP BSSAP
ISUP TUP
TCAP

SCCP M3UA

SCTP
MTP Layer 3
IP
MTP Layer 2
Data Link Layer

MTP Layer 1 Physical Layer

Legacy Gr Interface Gr Interface
Between SGSN – HLR Between SGSN – HLR
Using MAP Application Using MAP Application
SS7 protocol SS7 protocol transported
over IP

Protocol has been changed… (2/2)

Diameter Application

M3UA

SCTP SCTP

IP IP

Data Link Layer Data Link Layer

Physical Layer Physical Layer

Gr Interface
Between SGSN – HLR S6a Interface
Using MAP Application Between MME – HSS
SS7 protocol transported Using Diameter Application
over IP RFC 3588
TS 29.272

IP is here…
UE eNB S-GW P-GW

PDCP PDCP GTP-U GTP-U GTP-U/GRE GTP-U/GRE
User Plane

UDP UDP UDP UDP
RLC RLC
IP IP IP IP
MAC MAC L2 L2 L2 L2
PHY PHY L1 L1 L1 L1

Uu S1-U S5/S8

UE eNB MME S-GW P-GW

NAS NAS GTP-Cv2
Control Plane

GTP-Cv2 GTP-Cv2
RRC RRC S1AP S1AP PMIP PMIP

PDCP PDCP SCTP SCTP UDP UDP UDP
RLC RLC IP IP IP IP IP
MAC MAC L2 L2 L2 L2 L2
PHY PHY L1 L1 L1 L1 L1

Uu S1-MME S11 S5/S8

…and here…
SGSN / MME MME SGSN S-GW
Control Plane

GTP-Cv2 GTP-Cv2 GTP-U GTP-U

User Plane
UDP UDP UDP UDP
IP IP IP IP
L2 L2 L2 L2
L1 L1 L1 L1

S3/S4/S10/S16 S4/S12

MME HSS/EIR
Control Plane

Diameter Diameter

SCTP/TCP SCTP/TCP

IP IP Reference :
L2 L2 TS 29.281
L1 L1 TS 23.401

S6a/S6d/S13

…and everywhere…
Example : VRF Concept

VRF A
•IP addressing scheme ? Private/public ?
VRF B •Running OSPF between Node and PE
VRF C •Area 0 or non-backbone ?
VRF D
•Stub or totally stub ? NSSA ?
•or should BGP being used ??

P-GW PE
VRF S5
VRF S1u
VRF OAM
VRF SGi

A wake up call for Telco Engineer !!!

Multiple OSPF process

Multihoming Concept
ISP A
Customer – Provider
Costumer
C

ISP B

…While in telecommunication has another view about multihoming..
Client Role Server Role

192.168.1.1/24 Primary Path 192.168.3.1/24

192.168.2.1/24 Secondary Path 192.168.4.1/24

• Based on IETF - RFC 4960
• Application always use 2 IPs from different subnet for each node.
• Whenever there is a failure in primary path, the client and server automatically
switch the traffic into secondary path.
• Applied for most signaling traffic, such diameter, Sigtran.
• Telco engineer to make sure that Client – Server communicate correctly.
• IP engineer to make sure that there is a path diversity within IP network and there is
NO unseen single point of failure may happen.

A Simplified PS Call Flow with 4G…(1/2)
Herewith the IP

Attach Herewith the
Addresses 5 Reference :
request.. info 3
1 TS 29.281
HSS DNS TS 23.401
MS
eNodeB
TS 29.274
Get info for this What is the S-GW IP Address ?
2 subscriber What is the P-GW IP Address ? 4
S1
Create Session Request with
I am attached to MME IP Address of S-GW and P-
the network and GW 6
S1u Packet Data Network
want to browse to S11
www.google.com.au e..g Internet
11
Create Session Request
response (Accept)
10
S-GW P-GW
SGi
S5/S8

8
Create Session Request with
IP Address of P-GW
7 Create Session Request
Create Session Request response (Accept)
response (Accept)
9
Note :
procedures.

A Simplified PS Call Flow with 4G… (2/2)
Reference :
TS 29.281
HSS DNS TS 23.401
MS
eNodeB
TS 29.274

S1

I am attached to MME
the network and Packet Data Network
want to browse to S1u
S11 e..g Internet
www.google.com.au
11 12

S-GW P-GW
SGi
S5/S8

13

Internet
DNS

Note :
procedures.

LTE QoS Concept Overview
• Admission Control
– Prioritize bearers based on QoS parameter
– Protect network element in case of congestion or overload.

• Traffic Control
– Traffic classification (queuing, scheduling)
– Traffic policing and shaping

EPC Project Delivery
• Client : one of live mobile operator
• Duration : Jan 2012- November 2012
• Involving 3 areas of expertise :
– PS Core
– CS Core
– IP Prepare

Optimization Plan

Operate Design

Implement

Network Assessment : Current 2G/3G Mobile Packet Core
Project Delivery: EPC Node Commissioning and Integration
Network Target : LTE & 2G/3G interworking
BTS/NodeB SGSN
BSC/RNC
Gb
Iu

R7 DT Gn Gi
Gn
Pre R8 GGSN
Gr
2G/3G
HLR/HSS Internet

SGW PGW
Operator services
S6a

S5 SGi
LTE
S11

S1-U
eNodeB
S1-MME
MSC
SGs

MME
S10

Control plane
User plane

Lessons learned and Challenges...
• It is a new technology..
– New network elements, protocols and
procedures.
– Everyone involve may have their own
interpretation during planning,
commissioning and integration.
– Limited reference from available
production network.
– Always refer to International Standard.
3GPP, RFC and GSMA.

It is all IP..
– A complexity of client MPLS network.
– Design and Planning is taking an
important step.
– Requires an intensive collaboration
work with IP team.
– Things worth to note…
• L1 implementation. Wrong cable positioning
• VLAN and IP addressing, public/private ??
• Pay attention into any node limitation : port
type, MTU, active/standby behavior, Load
Balance concept, Client-Server role

Diameter Routing Planning..
Without DRA
DRA=Diameter Routing Agent With DRA

HSS HSS HSS HSS EIR
HSS EIR

DRA
DRA Roaming
Partner /GRX

MME MME MME MME

Some screenshots from 4G handset
Some speed test in various
location.

Upcoming technical challenges with LTE

More Information
Technical Specification and Standard. Readable and downloadable for
public 
• www.3gpp.org/specification-numbering
• www.ietf.org
• http://www.gsma.com/newsroom/technical-documents

EPC Product Knowledge. Also readable and downloadable for public 
• http://www.cisco.com/en/US/products/ps11072/products_installatio
n_and_configuration_guides_list.html

Further discussion

• Parakontel
• Indocisco

…ready to ‘take-off’ now ?

…then please fasten your seatbelt…

Thank You….

Images Credits
http://wallpoper.com/wallpaper/board-circuit-253188
http://forzamotorsport.net/en-US/fm4/tracks.aspx?t=Circuit_de_Catalunya
http://www.certshelp.com/blog/wp-content/uploads/2012/10/ip-address.jpg
http://coachgiorgio.com/wp-content/uploads/2011/05/teamwork4.jpg

Further discussion.

Introduction to Mobile Core Network Elements and Call Flow

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