Karim Rabie No Comments

Core Network Architecture in 3G Mobile Networks – Most of the Mobile Broadband Networks worldwide are currently delivering Data Services based on 3G & 4G Technologies. Although 4G/LTE is widely deployed globally but still there are countries and operators running 2G/3G with relatively convenient Data rates to the market demands.

In this article, I am focusing on the Packet Switched Network Architecture in 3G, the corresponding interfaces, & the relevant protocols relying on the terms and principles explained in the first article.

Mobile Broadband Ecosystem

The Basic Core Network Architecture in 3G mobile network is shown in the below figure :

Blank Flowchart - Main PS Core

I’d say that “Basic” comes from the fact that these reference points out of many other reference points are mandatory for any MBB Network to be launched. So let’s take a look on these reference points focusing on the Core Interfaces but before that, let’s review the basic functionalities of the main PS Core nodes (SGSN & GGSN)

Serving GPRS Support Node (SGSN)

  • Mobility Management – Controls all procedures related to Subscribers attachment to network and to Subscribers Mobility in general maintaining MM contexts for all Subscribers.
  • Session Management – Controls all procedures related to Subscribers requests to access services  (e.g. Internet Service) including modification adn deactivation procedures.
  • Subscriber Authentication & Registration – Responsible of the authentication and Authorization of Subscribers trying to attach to the Network
  • Routing and Transfer – Routing Traffic in both Uplink and Downlink.
  • Charging – Generate CDR to account for Subscribers Sessions and Data Usage.
  • Lawful Interception

Gateway GPRS Support Node (GGSN)

  • Session Management – Responsible of Services (APNs) management and controls all related procedures together with SGSN.
  • Dynamic IP Addresses Allocation – Takes control of the allocation of IP Addresses to end users.
  • Routing and Transfer – Routing Traffic in both Uplink and Downlink.
  • Service Management (APNs) – hosts the APNs/Services Management Configuration.
  • Charging – Generate CDR to account for Subscribers Sessions and Data Usage.
  • Policy Enforcement – Interacts with PCRF for Policy rules enforcment.
  • Lawful Interception

The reference points for Basic PS Core Network Architecture are

Gb Interface

The Gb interface is the logical interface between the SGSN and BSC. Gb interface can be deployed both over IP and Frame Relay (Old Legacy Solution)

Iu-PS Interface

The Iu-PS interface is the logical interface between the SGSN and an RNC. Iu-PS interface is deployed both over IP and ATM (Old Legacy Solution). Iu-PS is commonly a SIGTRAN Interface (SS7 over IP)

Gn interface

The Gn interface supports user data and signalling transmission between the SGSN and the backbone (GGSN). A GPRS-specific protocol, GPRS Tunnelling Protocol (GTP) has been specified for the Gn interface and is used to create logical tunnels to the backbone in order to carry user data packets between the GSN elements.

Gp interface

From SGSN/GGSN perspective, the Gp interface is exactly the same as the Gn interface. However, instead of providing an access to operator’s internal backbone networks, the Gp interface leads to other operators’ GPRS networks, or to a backbone network connecting several GPRS operators. As with the Gn interface, the Gp interface is based on GTP.

Gr interface

The SGSN uses the Gr interface for GMM functions. Subscription information is retrieved from the Home Location Register (HLR) via the Gr interface. In addition, users are authenticated and their locations are updated to the HLR using the Gr signaling interface.

One of the fundamental steps to understand PS Core Network is to get familiar with the reference points between the different Network Elements with respect to Protocol Stack, Functionalities, & Procedures.

How this can be realized? Let’s do a spot on the Iu-PS interface. One of the most important interfaces that connects the RAN to the Core Network, carries the signalling between the UE (User Equipment) and the CN on the Iu-PS Control Plane protocol stack , transports the User Plan on the Iu User Plane protocol stack,  & many other functions.

The Protocol stack of Iu-PS (Control Plane) where all signalling procedures are initiated and transported is shown below (Highlighted in Red)

Screen Shot 2016-07-09 at 3.37.17 PM

The observation here is that the protocol stack for a MBB interface in 3G is commonly having more than 8 layers resulted from the fact the IP Protocol stack is carrying the SS7 layers where every layer has its own settings and configuration.

For Iu-PS Interface, A standard deployment will have the following the protocol stack

Screen Shot 2016-07-09 at 3.53.07 PM

SCTP (Stream Control Transmission Protocol ) is a reliable, message oriented transport protocol that provides new services and features for IP communication.

The transport protocol commonly used in Telco environment is SCTP because of the attractive features such as Reliable transmission, Congestion Avoidance, Multi-Streaming, & Multi-homing.

M3UA (MTP Level 3 User Adaptation Layer) is a user adaption layer that is part of SS7 Signaling transport over IP (SIGTRAN) relying on Services from SCTP protocol

SCCP (Signaling Connection Control Part) identified as MTP3 user provides the connection-oriented and connectionless services for other applications such as RANAP with main function is to switch Calls.

RANAP (Radio Access Network Application Part) provides the signaling service between UTRAN and CN that is required to fulfill the RANAP functions. (3GPP TS 25.413)

A tip here for IP Planners, SCTP is commonly used in PS Core network so whenever SCTP is adopted in one of the interfaces, there is no need to perform HSRP, VRRP, or any other equivalent redundancy techniques

That’s because SCTP has a built-in redundancy mechanism such as SCTP Multihoming so for one Signalling point (e.g RNC Iu-PS Application), it will commonly have two paths (primary and secondary) to SGSN Iu-PS Application with two different IP Addresses and same port.

What is required from the Transport network is to avail two different paths for the primary and the secondary.

Have you ever heard someone saying that the association is down?! Below is a single SCTP multi-homed association between RNC & SGSN on the Iu-PS interface.

Screen Shot 2016-07-09 at 4.31.24 PMBlank Flowchart - SCTP Multi

 

 

 

 

Let’s view now the full setup of the PS Core Network with all supplementary reference points..

Blank Flowchart - PS Core Interfaces - All

Ga interface

The Ga interface connects the SGSN to the charging gateway. A GTP’ based Interface.

It carries the charging records generated by the SGSN to the charging gateway to be further delivered to the operators’ billing systems.

Bp interface

The Bp interface is used for transferring CDRs from the SGSN/GGSN to the billing domain (BD), commonly FTP

Ge interface

The SGSN has also the Ge interface to the SCP. The protocol in the SGSN-SCP interface is CAMEL

Application Protocol (CAP)

Gy Interface

Gy is the Interface between GGSN & OCS (Online Charging System) for Online Charging.

Gf interface

The Gf interface is similar to the Gr interface but it is used for terminal authentication (IMEI check) instead.

Gd interface

The Gd interface between the SGSN and the SMSC to SMS delivery via GPRS. Thus, the operators can select either the PS GPRS or the traditional CS GSM for sending SMs to and from the MS.

Gs interface

The Gs interface links the GMM of the SGSN to the equivalent GMM of MSC/VLR. The Gs interface enables a combined GPRS and IMSI attach the Gs interface makes it possible to page terminals for CS service via GPRS.

Lg Interface is a map-based interface between SGSN/MSC & GMLC for Location Services.

Gx interface is a diameter-based interface between PCRF & GGSN for Policy Control. Policy retrieval and rules activations are done through the communication on Gx Interface.

Starting from next article, I will discuss the main Concepts and Call Flows in PS Core Network with an objective to see the E2E service flow with the role of every interface and Network Element involved.

Thanks and see you in next article.

 

 
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