Core
networking
CS/PS/EPS/IMS Overview
Omar wahed Embarek
Index
Overview on PS/CS/EPS/IMS
1
Overview Arhchitecture of
PS/CS/EPS/IMS
2
main node and device
3
3GPP overall architecture and
interfaces
4
Libyana/ZTE overall architecture
5
main protocol at PS/CS/EPS/IMS
6
Call scenario at CS
7
Data flow session at PS/EPS
8
Calls at 4G (CSFB/IMS)
9
IMS: (IP Multimedia Subsystem)
10
Overview on PS/CS/EPS/IMS
In telecommunications, core networks are responsible for managing and directing voice, data,
and signaling traffic between access networks (like mobile networks) and the broader internet
or public telephone systems.
CS
(Circuit-Switched)
is mainly used for voice services.
use circuit-switched technology.
used for Traditional voice calls in
2G (GSM) and 3G (UMTS)
networks
PS
EPS
(Packet-Switched)
data is transmitted in small
packets
used for Data services (internet,
messaging) in 2G (GPRS/EDGE)
and 3G (UMTS) networks
(Evolved Packet System)
offers high-speed data services
and all-IP-based communication,
used in 4G LTE (Long-Term
Evolution) networks
Back to Agenda
IMS
IMS is an architectural framework
used to deliver IP-based
multimedia services
It works with LTE and 5G
networks to provide voice over IP
(VoIP) services, such as VoLTE
Overview Arhchitecture
CS (Circuit-Switched) core network
traffic
signaling
MSC
GMSC
GE-RAN
PSTN
HLR
UTRAN
MGW
GMGW
(Equipment Identity
Register )
(mobile services switching
center)
MSC
CS main
node
The MSC is a central component
responsible for call switching, call
setup, and call release
connects calls between mobile
users
EIR
(Authentication Center )
(Visitor Location Register)
VLR
The VLR is a temporary database that
stores information about subscribers
currently within the coverage area of a
particular MSC.
The EIR is responsible for maintaining
a list of valid mobile device identities
and identifying stolen or unauthorized
devices.
AUC
The AUC is responsible for
authenticating the identity of mobile
subscribers.
(Circuit switched media
gateway)
(Home Location Register)
HLR
The HLR is a database that stores
subscriber information, including
user profiles, subscription details,
and current locations.
MGW
A CS-MGW is a device that enables
the conversion of circuit-switched
traffic into packet-switched traffic and
vice versa. It is used to connect
traditional circuit-switched networks to
IP-based networks
Overview Arhchitecture
PS (Packet-Switched) core network
GE-RAN
SGSN
UTRAN
HLR
GGSN
PDN
(Serving GPRS Support Node)
SGSN
PS main
node
is responsible for managing the data
sessions and mobility of mobile users within
its coverage area in a PS domain.
(Visitor Location Register)
GGSN
The GGSN acts as the gateway between the mobile
network and external packet data networks like the
internet or private company networks. It routes data
to and from mobile devices, ensuring that they can
access external networks and services.
Overview Arhchitecture
EPS (Packet-Switched) core network
MME
HSS
E-UTRAN
PDN
S-GW
P-GW
Serving Gateway
The Mobile Management
Entity
MME
is a core component of (EPS),
It plays a crucial role in handling
the control plane functions,
including mobility management,
session management, and
authentication
SGW
Packet Data Network
Gateway
Home Subscriber Server
EPS main
node
HSS
The Serving Gateway (SGW) is the
intermediary node between the
eNodeB (the base station) and the
PGW It handles the routing and
forwarding of user data packets within
the EPC
is a core database in the EPC that
is the gateway between the EPC core
manages subscriber profiles,
network and external packet data
authentication, mobility, and
networks (PDNs) It handles IP address
authorization.
allocation, packet routing, and provides
It stores critical data about
various services to the UE for
subscribers, including service
connecting to external networks
subscriptions and location
information.
Policy and Charging Rules
PGW
Function
PCRF
is a key element in the Evolved Packet
Core (EPC) network, responsible for
managing policy control and charging
rules for data sessions in 4G LTE and
IMS (IP Multimedia Subsystem)
environments.
3GPP overall architecture and interfaces
Back to Agenda
Libyana/ZTE overall Core architecture
GERAN
USPP
(HSS/HLR/
ELR/AUC)
MSC
(VMSC/GMSC/
TMSC)
OCS
UTRAN
VDSC
EXTERNL
LINK
VMAC
(MME/SGSN)
XGW
(SGW/GGSN/
PGW/MGW)
E-UTRAN
Back to Agenda
PDN/
PSTN
MAP
PROTOCOL
(Mobile Application Part)
1. is a protocol used in the core network It enables communication
between various network entities and is a key component for
managing subscriber information, mobility, and services such as
SMS, roaming, and location updates
2. MAP is used to manage subscriber data between core network
components like the Home Location Register (HLR), Visitor Location
Register (VLR), Equipment Identity Register (EIR), and Authentication
Center (AUC), used in C,D,F,G,E interface.
3. MAP plays a crucial role in international roaming, where it enables
the serving network (VLR) to query the home network (HLR) to obtain
subscriber information and authorize services when the user is
roaming outside their home network.
4. MAP facilitates the authentication of users when they connect to the
network by querying the AUC for authentication vectors and
generating cryptographic keys to ensure secure communication.
CAP
PROTOCOL
(CAMEL Application Part)
1. s a protocol used in the core network of GSM (2G) and UMTS (3G)
mobile systems.
2. It allows for the deployment of intelligent network (IN) services like
prepaid billing, number translation, and call routing control.
3. CAP plays a major role in prepaid billing systems. When a call or data
session is initiated, CAP messages are exchanged between the core
network and the Service Control Point (SCP), which holds the user’s
account information. CAP ensures that a user’s prepaid balance is
checked before, during, and after a call or session, and it can
terminate the session if the balance runs out.
4. CAP enables advanced call routing capabilities, such as rerouting
calls based on time-of-day, user preferences, or special numbering
plans (e.g., shortcodes for customer service).
DIAMETER
PROTOCOL
1. is an advanced protocol used in modern telecommunications core
networks, primarily in 4G LTE and 5G systems.
2. DIAMETER ensures that users are properly authenticated before
accessing network services (authentication), determines what services
they are allowed to use (authorization), and tracks their resource
usage for billing purposes (accounting).
3. DIAMETER supports IPsec and TLS for secure communication between
network nodes, improving security over its predecessor,
4. DIAMETER uses reliable transport protocols like TCP or SCTP to ensure
message delivery,
5. DIAMETER is used in multiple interfaces such as :S6a,S13,GX,GY.
GTP
PROTOCOL
GTP (GPRS Tunneling
Protocol)
1. is a critical protocol used enabling communication between
different network elements to support data transmission and
signaling
2. GTP is primarily used in packet-switched networks to carry user
data and control messages for services like mobile interne
3. There are two main versions of GTP, each with a different
function GTP-C and GTP-U
4. GTP Interfaces in Core Networks are: S1-U , S5/S8,S4 , Gn/Gp
Interface
calls scenario#1
Location update
calls scenario#2
Call flow (1/2)
calls scenario#2
Call flow (2/2)
session scenario#3
attach procedure in PS
session scenario#4
PDP procedure in PS
session scenario#4
attach procedure in EPS (1/4)
session scenario#4
attach procedure in EPS (2/4)
session scenario#4
attach procedure in EPS (3/4)
session scenario#4
attach procedure in EPS (4/4)
CSFB: (Circuit-Switched
Fallback)
Calls at 4G
(CSFB/IMS)
CSFB (Circuit-Switched Fallback) is a technique used in 4G LTE
networks to handle voice calls by falling back to a 3G (UMTS) or
2G (GSM) network for voice communication while continuing to
provide LTE data services. This is done because LTE is packetswitched (PS) only, and cannot natively handle circuit-switched
(CS) voice calls like legacy networks (2G/3G).
IMS: (IP Multimedia
Subsystem)
IMS (IP Multimedia Subsystem) is a core network architecture that
provides multimedia services (like voice, video calls, messaging,
and presence) over IP-based networks, such as 4G LTE. Unlike
CSFB, which relies on older circuit-switched networks, IMS is
designed to deliver voice and other multimedia services over IP
using EPS networks.
IMS overall architecture and interfaces
(Proxy Call Session Control
Function)
P-CSCF
IMS main
node
S-CSCF
The P-CSCF is the first point of contact
between the user device (UE, User
Equipment) and the IMS network. It
acts as a proxy for signaling and
forwards SIP (Session Initiation
Protocol) messages between the user
and the IMS core network
The S-CSCF is the central component of
the IMS network. It is responsible for
session control, user authentication, and
handling the routing of SIP messages. It
maintains the state of active sessions for
registered users and enforces the service
logic defined for each user.
The I-CSCF is primarily responsible for
routing and interfacing with other IMS
networks or external networks. It plays a
key role in finding the correct S-CSCF
for a given user during the registration or
call setup process.
Responible for choosing where a breakout
to the CS domain occurs. select MGCF
which interworking with PSST/CS and
forward sip massage to it.
(Media Gateway Control
Function)
(Serving Call Session
Control Function)
(Interrogating Call
Session Control Function)
I-CSCF
BGCF
(Breakout Gateway
Control Function)
MGCF
responsible for controlling the Media
Gateway (MGW) and handling the signaling
required to bridge communications between
IP-based (packet-switched) IMS networks
and circuit-switched (CS) legacy networks,
such as the PSTN (Public Switched
Telephone Network) or PLMN (Public Land
Mobile Network).
(Application Server)
AS
provides value-added services and
functionalities beyond basic call control.
The AS hosts and executes various
applications and services that enhance
user experience, such as call forwarding,
voicemail, multimedia messaging,
conferencing, and presence services.
SIP
PROTOCOL
(Session Initiation Protocol)
1. a signaling protocol used for initiating, maintaining, modifying,
and terminating real-time communication sessions between
endpoints over IP networks.
2. SIP is responsible for setting up, managing, and terminating
communication sessions. These sessions can involve voice,
video, text, or other multimedia content.
3. SIP works in a peer-to-peer model, meaning that two devices or
applications communicate directly with each other.
4. It supports a variety of transport protocols like TCP, UDP.
IMS Call flow
#1 UE Registration
The UE registers with the IMS core network via SIP. This process
involves:
a. The UE establishes an IP connection with the LTE network by
sending an Attach Request to the MME, which forwards it to the
SGW/PGW.
b. The MME performs an authentication procedure using HSS.
c. Once the IP connection is established, the UE initiates the SIP
registration by sending a REGISTER message to the P-CSCF,
which forwards it to the S-CSCF.
d. The S-CSCF queries the HSS for user authentication and profile
information.
IMS Call flow
#2 SIP INVITE (Session Initiation)
1. When a user initiates a call, the UE sends a SIP INVITE message to the
P-CSCF, which forwards it to the S-CSCF in the IMS core.
2. The S-CSCF checks the user’s profile (via HSS) and applies callrouting logic. The SIP INVITE contains session parameters, such as
codec information for voice media.
IMS Call flow
#3 SIP Routing
1. The S-CSCF routes the SIP INVITE to the destination. If the call is to
another VoLTE user within the same IMS network, the S-CSCF
forwards the SIP message to the P-CSCF of the recipient.
2. If the destination is outside the VoLTE/IMS domain (such as a PSTN
number), the S-CSCF may route the call to an MGCF to connect to a
circuit-switched network.
IMS Call flow
#4 Bearer Setup
1. Simultaneously, the SIP INVITE triggers the setup of a dedicated
bearer in the LTE network with QoS (Quality of Service) guaranteed
for voice.
2. The P-CSCF contacts the PCRF, which instructs the PGW to create a
dedicated bearer for voice traffic, ensuring low latency and high
priority for the call.
IMS Call flow
#5 Ringing and Call Acceptance
1. The called party (destination UE) receives the SIP INVITE and
responds with a 180 Ringing message, which is forwarded to the
originating UE to indicate that the phone is ringing.
2. Once the called party answers the call, the destination UE sends a
200 OK response back to the originating UE, indicating that the call is
accepted
3. The originating UE sends an ACK to confirm the call setup.
IMS Call flow
#5 Media Session (Voice) and Call Termination
1. With the session established, RTP (Real-Time Transport Protocol) is
used for the actual voice media exchange between the two UEs. The
voice packets are routed through the established dedicated bearer
in the LTE network.
2. The voice call is now live, and the media is flowing between the two
devices.
When the call is ended, the UE sends a BYE message via SIP to
terminate the session.
The S-CSCF handles the call termination and informs all involved
network entities.
The LTE network releases the dedicated bearer, and the call session
is closed.