Mobile 2G/3G Workshop

Mobility Workshop
2G/3G Network Architecture
October 5th, 2010

2G GSM
2 Copyright © 2009 Juniper Networks, Inc. www.juniper.net

2G GSM Network Architecture

SMSG
E

MSC EIR
F C

RAN
B D
BSS VLR
(TDM) E HLR
H
EIR
F
AuC
D
Um Abis A
MS BTS BSC B C
MSC VLR

NSS E
PSTN
GMSC

Abis interface is comprised of multiple T1s (TDM).
Signaling between elements is carried out over an SS7 or SIGTRAN (SS7oIP) network.

Slide # 3 Copyright © 2009 Juniper Networks, Inc. www.juniper.net

GSM Node Definitions

MS: Mobile Subscriber
 The subscriber and the mobile device.
BTS: Base Station Transceiver
 Mobile wireless transceiver used to aggregate discrete mobile systems in the BSS.
BSC: Base Station Controller
 BTS aggregator and controller. Termination point for the Abis interface.
BSS: Base Station Subsystem
 The MS, BTS and BSC.
MSC: Mobile Switching Center
 Main call processing element in the mobile wireless network. Equivalent to a Class 5 switch.
VLR: Visitor Location Register
 A database that stores information related to the MSs associated with the MSC to which it is attached.
HLR: Home Location Register
 A central database that stores the subscriber information for all mobile subscribers authorized to use a given
GSM network.
EIR: Equipment Identity Register
 A database that stores information regarding specific mobile subscriber equipment (used for blocking
access and tracking stolen equipment, etc.).
AuC: Authentication Center
 A central database used to authenticate each SIM that attempts network access.


GSM Interface Definitions

A Interface
 TDM interface connecting BSC to MSC. Carries all bearer and signaling traffic associated with GSM mobile
services.
Abis Interface
 The BSS-based TDM interface connecting the BTS to the BSC.
B Interface
 TDM signaling interface between the MSC and VLR. Used to access subscriber account information.
C Interface
 Interface between the HLR and a GMSC or SMSG. Every call originating from the PSTN has to go through
a gateway to obtain routing information using the MAP/C protocol.
D Interface
 TDM signaling interface between the VLR and HLR. Used to pass master subscriber account information
from the HLR to the VLR
E Interface
 TDM bearer interface connecting MSCs or MSC gateways (GMSC, SMS Gateway). Used to transport voice
bearer traffic.
F Interface
 TDM signaling interface between the MSC and EIR. Used to access equipment registration information.
G Interface
 Interconnects two VLRs that belong to different MSCs and uses the MAP/G protocol to transfer subscriber
information (such as during a location update procedure).
H Interface
 TDM signaling interface between the HLR and AuC. Used to authenticate subscribers.
Um Interface
 The air interface used for communication between the MS and a BSS. LAPDm (modified version of ISDN
LAPD), is used for signaling.

GPRS Overview

General Packet Radio Service (GPRS)
 Generally referred to as 2G
 Developed specifically for the transmission of data in mobile wireless
networks
 Maximum theoretical data rate of 171.2 kbps
 Uses the same underlying RF technology as GSM
Core Services
 Point-to-point IP applications
 Point-to-multipoint IP applications (relatively uncommon)
 Internet applications for smart devices through WAP and IP
 SMS
 MMS
 Push-to-talk over Cellular (PTT/PoC)
Copyright © 2009 Juniper Networks, Inc. www.juniper.net

EDGE Overview

Enhanced Data rates for GSM Evolution (EDGE)
 Also known as EGPRS
 Generally referred to as 2.5G
Characteristics:
 Uses the same base RF attributes as GPRS
 Enhances data rates by applying enhancement to carrier modulation
– GMSK used by GSM/GPRS
– GMSK + 8PSK used by EDGE
– EDGE produces 3-bit words for every change in phase
– Effectively triples the gross data rate
– Theoretical maximum data rates:
- 2 timeslots == 118.8 kbps
– Most carriers deploy 2 or 4 timeslots and reserve the remainder for voice


GPRS vs. EDGE Comparison

GPRS EDGE

Modulation GMSK 8PSK/GMSK

Symbol Rate 270 ksym/s 270 ksym/s

Modulation Bitrate 270 kbps 810 kbps

Radio Data Rate Per Timeslot 22.8 kbps 69.2 kbps

User Data Rate Per Timeslot 20 kbps (CS4) 59.2 (MCS9)

160 kbps 473.6 kbps
User Data Rate (8 Timeslots)
(182.4 kbps) (553.6 kbps)


2.5G GPRS/EDGE Network Architecture

Gi
Remote
Internet
GTP PLMN

GGSN
GRX
Gp
SGSN
Gn Gi
Local
PSN Internet
RAN PLMN

BSS Gb Gs GGSN
(TDM) GTP

PCU EIR Gr
F
D
Um Abis A HLR
MS BTS BSC B H
MSC VLR
AuC

E C

PSTN
GMSC


Serving GPRS Support Node (SGSN)

The SGSN is responsible for delivery of packets to and from mobile
stations within a geographical service area.
Common SGSN Functions:
 Detunnel GTP Packets from the GGSN (downlink).
 Tunnel IP Packets in GTP towards the GGSN (uplink).
 Carry out Mobility Management as standby mode mobile moves from
one Routing Area to another Routing Area.
 Billing user data.
 Lawful intercept.
 Encrypt down-link data, decrypt up-link data to/from mobiles.
 Logical Link management
 Authentication
 Stores temporary data such as location information and user profiles
in the serving Visiting Location Registrar (VLR).


Gateway GPRS Support Node (GGSN)

The GGSN is the main component of the GPRS core network.
 GGSN is the IP anchor point for mobile data traffic
 Responsible for interworking between the mobile network and
external networks such as the Internet.
 From the external network’s point of view, the GGSN is a router to a
subnetwork.
Common GGSN Functions:
 Tunnels/de-tunnels GTP protocol packets from the SGSN.
 Manages PDP Contexts.
 Responsible for IP address assignment.
 Responsible for quality of service.
 Acts as default gateway for the mobile.


GPRS/EDGE Node Definitions

PCU: Packet Control Unit
 An addition to the BSC that provides packet processing capabilities between the RAN and the
SGSN.
SGSN: Serving GPRS Support Node
 An interworking element responsible for providing packet access between the RAN and the
Packet Switched Network (PSN). Encapsulates subscriber traffic in GTP for transmission across
the Gn interface.
GGSN: Gateway GPRS Support Node
 An IP element responsible for de-encapsulating GTP traffic from the Gn interface and interfacing
with external networks (such as the Internet).
GTP: GPRS Tunneling Protocol
 The protocol used to tunnel user traffic across the PSN on the Gn interface.
AP: Access Point
 A distinct closed user group or VPN. Represents a group of users with some common criteria.
APN: Access Point Name
 The name used to designate a particular AP.
PDP Context: Packet Data Protocol Context
 The PDP Context describes the active session of an MS connected to a particular APN. The
PDP Context is active on the SGSN and the GGSN.


GPRS/EDGE interface definitions

Gb Interface
 A Frame Relay interface between the BSC and the SGSN.
Gn Interface
 IP-based interface between the SGSN and internal GGSNs. Uses GTP protocol.
Gp Interface
 IP-based interface between the SGSN and external GGSNs. Also uses GTP protocol.
Ga Interface (not shown)
 Interface that serves the Call Data Records (CDRs) which are written in the SGSN and sent to
the Charging Gateway (CG). This interface uses a subset of the GTP protocol called GTP’ (GTP
Prime).
Gr Interface
 Interface between the SGSN and the HLR. Messages going through this interface use the MAP3
Protocol.
Gd Interface (not shown)
 Interface between the SGSN and the SMS Gateway (SMSG). Can use either MAP1, MAP2 or
MAP3.
Gs Interface
 Interface between the SGSN and the MSC/VLR. Uses BSSAP+ Protocol. This interface allows
paging and station availability when it performs data transfer. When the station is attached to the
GPRS network, the SGSN keeps track of which Routing Area (RA) and Location Area (LA) to
which the station is attached. When a station is paged this information is used to conserve
network resources. When the MS initiates a PDP Context, the SGSN knows which BTS the MS
is attached to.
Gi Interface
 The IP-based interface between the GPRS/EDGE network and the Internet.

GPRS Tunneling Protocol (GTP)

GPRS Tunneling Protocol (GTP) is a group of IP-based protocols
used to carry data traffic within GSM and UMTS networks.
GTP is really composed of 3 separate protocols:
 GTP-C
– The control portion of the GTP standard. When a subscriber requests a PDP Context, the
SGSN sends a Create PDP Context Request message to the GGSN. The GGSN will
respond with a Create PDP Context Response message.
– UDP port 2123
 GTP-U
– The user portion of the GTP standard that is used to tunnel IP traffic within and between
mobile operator networks. Each subscriber may have one or more tunnels to support
different connections or different quality of service requirements.
– UDP port 2152
 GTP’ (pronounced GTP Prime)
– The charging data portion of the GTP standard. Used to transfer charging data from the
SGSNs and GGSNs to the Charging Gateway Function (CGF).
– TCP or UDP port 3386


PDP Context

The Packet Data Protocol (PDP) Context is a data structure which contains
the subscriber’s session information when an active session exists on the
network. It usually contains the following:
 Subscriber’s IP address
 Subscriber’s IMSI (International Mobile Subscriber Identity)
 Subscriber’s Tunnel Endpoint ID (TEID) at the SGSN
 Subscriber’s Tunnel Endpoint ID (TEID) at the GGSN
The TEID is a random number allocated by the SGSN/GGSN which identifies
the tunneled data related to a particular PDP Context.
2 kinds of PDP Contexts:
 Primary
– Has a unique IP address associated with it.
 Secondary
– Shares an IP address with the Primary Context.
– Created based on the Primary Context.
– Secondary PDP contexts may have different quality of service settings.


Access point Name (APN)

3GPP has the concept of virtual private networks that represent
communities of interest. These VPNs are called Access Points
(APs) and are identified by Access Point Names (APNs).
 An APN defines a service description and routing for
GPRS/EDGE/UMTS data.
 All traffic within and APN is routed in a similar manner.
 When an SGSN sends user traffic in a GTP tunnel, it sends that
traffic to a GGSN that services the APN the subscriber’s traffic
belongs to.
Examples of APNs:
 wap.cingular
 epc.tmobile.com
 blackberry.net
 pp.vodafone.co.uk


SGSN-to-GGSN Routing

How does an SGSN know which GGSN serves a particular APN?
 APN names look suspiciously like DNS name… and this is precisely
what it is.
 When an SGSN wants to connect a subscriber to an APN, it performs a DNS
lookup on the APN name (e.g. wap.cingular)
 This query is sent to a special DNS infrastructure called Gn DNS.
 The IP address returned by the Gn DNS is the destination address of the
nearest serving GGSN.
– Note that in order to scale the network and provide geo-redundancy, the Gn DNS
may return different results depending on the source IP address of the SGSN.
– Examples:
- SGSNs from Region 1 have source IP addresses in the 172.16.1.0/24 range.
- SGSNs from Region 2 have source IP addresses in the 172.16.2.0/24 range.
 Gn DNS is a globally-connected infrastructure.
– More details on this will be covered in the section on Data Roaming.


Data Roaming

Notice that Home PLMN
•IP Anchor (GGSN) is in the home network
•SGSN is in the visited network
•LI and Charging can be done in both networks HLR
Service
Center Internet

GGSN
SS7/SIGTRAN Gn DNS

GTP

3

PCU SGSN
Root
Gn DNS
MS BTS BSC 2
1
GRX

Gn DNS

Visited PLMN


3G UMTS
Copyright © 2009 Juniper Networks, Inc. www.juniper.net

UMTS Family Air Interface Technologies
UMTS - Universal Mobile Telecommunications System

Wideband Code Division Multiple Access (W-CDMA)
 Introduced in 3GPP R99
 Max theoretical data rate of 1.92 Mbit/s in the downlink
High-Speed Downlink Packet Access (HSDPA)
 Based on 3GPP R5
 Improves downlink data rate to speeds up to 14Mbit/s
 Majority of deployments provide up to 7.2 Mbit/s in the downlink
High-Speed Uplink Packet Access (HSUPA)
 Based on 3GPP R6
 Enhanced to increase the uplink data rate up to 5.8 Mbit/s
Evolved High Speed Packet Access (HSPA+)
 Based on 3GPP R7 & R8
 Data rates up to 42 Mbit/s in the downlink and 11 Mbit/s in the uplink (per
5 MHz carrier)
 Supports Multiple Input, Multiple Output (MIMO) technologies


3G UMTS network architecture (R4)

IuX Interfaces are ATM-based: Gi
Remote
IuPS = AAL5 GTP
Internet
PLMN
IuCS = AAL2 GGSN
GRX
Gp
SGSN
Gn Gi
Local
UTRAN PSN Internet
PLMN
IuPS Gs GGSN
GTP

EIR

Uu IuB IuCS(S) HLR
MS NodeB RNC MSCS VLR
AuC
IuCS(B)
NxT1 ATM IMA

MGW
PSN
PSTN
MGW


3G UMTS network architecture (R5+)

IuX Interfaces are IP-based Gi
• Changes the Backhaul Routing and Internet
Remote
GTP PLMN
Aggregation requirements
• MX supports ethernet but not the (older) GGSN
GRX
ATM interfaces
Gp
SGSN
Gn Gi
Local
UTRAN PSN Internet
PLMN
IuPS Gs GGSN
GTP

EIR

Uu IuB IuCS(S) HLR
AuC
IuCS(B)

MGW
PSN
PSTN
MGW


3G UMTS network architecture

Notes
1. The IP anchor can be either in the home
or visited network.
• Pay attention this will come up again in LTE
• Multiple concurrent APNs are generally not used in 3G
Gi
2. Later releases support “direct tunnel” which GTP
Internet
Remote
PLMN
allows the bearer traffic to bypass the SGSN GGSN
• Greatly reduces SGSN bearer load GRX
• Some bearer (e.g. LI and Roaming) still
must go through the SGSN. Gp
SGSN
Gn Gi
Local
UTRAN PSN Internet
PLMN
IuPS Gs GTP GGSN
Gn
EIR GTP

Uu IuB IuCS(S) HLR
AuC
IuCS(B)
NxT1 ATM IMA

PSN
MGW PSTN
MGW


UMTS Node Definitions

UE
 User Equipment (the mobile device).
NodeB
 Analogous to the BTS in the GSM/GPRS BSS, the NodeB is responsible for Ue
aggregation. Uses W-CDMA for the air interface.
Radio Access Controller (RNC)
 Analogous to the BSC in the GSM/GPRS BSS, the RNC aggregates Iub traffic from
the NodeBs in a particular region of the RAN.
 Responsible for air encryption, mobility management (handover), and voice/data
transmission to the circuit-switched and packet-switched core.
Call Server
 A cut-down version of the MSC that is disassociated with the bearer path to reduce
size and cost. Responsible for all voice signaling functions.
Media Gateway (MGW)
 The element responsible for interworking voice traffic between and among formats
(e.g. ATM AAL2 RTP/IP).


UMTS Interface definitions

Uu
 Air interface connecting the UE to the NodeB.
IuB
 An ATM interface (R4) or IP interface (R5+) between the NodeB and RNC
carrying all voice and data bearer and signaling traffic.
IuR
 An ATM interface (R4) or IP interface (R5+) connecting RNCs to one another.
AN RNC may operate as an S-RNC (Serving RNC), D-RNC (Drift RNC), or C-
RNC (Controlling RNC)
Iu-PS
 An ATM interface (R4) or IP interface (R5+) connecting the RNC with the SGSN.
Used to transmit UE data traffic to the packet core.
Iu-CS
 An ATM interface (R4) or IP interface (R5+) connecting the RNC with the Call
Server (Iu-cs(S)) and MGW (Iu-cs(B)). Used to transmit UE voice traffic
(signaling and bearer) to the circuit core.


Introduction to Handoff in 2G and 3G
Target cells and all that fun stuff…

Dang! These guys keep moving on me!
Why don't they stand still and take their calls
like a man!?


The Simple Stuff- User is moving but not connected
Re-selection (Packet Example)
HLR 1. Mobile is “camped” on cell 1
2. Mobile device collects data on signal
q Internet
strength of nearby cells.
GGSN

PSTN 3. Mobile sees that cell 2 has a better signal
MSC
o and meets reselection criteria.
SGSN
n 4. Mobile tells network it is relocating.
Rnc 5. Network accepts (or rejects)
m 6. SGSN is updated for paging if needed (if the
paging area changes).
j 7. If SGSN needs to change with the cell this
there is a proceedure.
k jl
k 8. IF needed HLR is updated so the user can
be found for paging. (I.E. if SGSN is
changes).
9. If this causes a change of SGSNs then the
old SGSN tells the new SGSN and the HLR
For more detail see:
3GPP TS 23.060 V10.0.0 Section 6.9 Page 80

Cell Update Examples This slide for Reference only

(23.060 section 6.9)
MS BSS new SGSN old SGSN GGSN HLR

1. Routeing Area Update Request
MS BSS SGSN 2. SGSN Context Request
1. Routeing Area Update Request
2. SGSN Context Response
2. Security Functions
3. Security Functions (A)
3. Routeing Area Update Accept
4. SGSN Context Acknowledge
C1
4. Routeing Area Update Complete
C1

Intra SGSN Routing Area Update Procedure 5. Forward Packets

6. Update PDP Context Request
(B)
6. Update PDP Context Response

7. Update Location

8. Cancel Location

8. Cancel Location Ack

9. Insert Subscriber Data

9. Insert Subscriber Data Ack
3GPP TS 23.060 V10.0 Section 6.9
10. Update Location Ack

C2

11. Routeing Area Update Accept

C3
12. Routeing Area Update Complete

Inter SGSN Routing Area Update Procedure

Hard Handoff
2G Example
Internet

GGSN Connection is broken with original cell
PSTN and established to a new (target) cell.
MSC SGSN
• Short interruption in bearer occurs
• Generally not obvious to voice user
BSC • Used for 2, 3 and 4G systems

Internet

GGSN

PSTN

MSC SGSN

1. Mobile device collects data on signal strength
of nearby Cells
2. BSC sees current cell getting weak and target cell BSC
getting strong.
3. BSC instructs mobile to retune to a new cell
4. Mobile breaks connection to old cell, retunes, and
announces its presence on the new cell.
5. Bearer stream is sent to new cell.


Soft Handoff
3G Example
Internet
Soft Handoff:
GGSN • UE may be able to communicate with two
cells
PSTN • Symbols (data) may be collected from both
cells
MSC SGSN • Downlink data is sent to both cells
• Frame selector chooses best symbol on a
symbol by symbol basis
• Eventually one path becomes clearly better
Frame Selector and the other path is dropped.
• Used for most handoffs in 3G. Not used in
RNC 2G or 4G


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