Resurrection of GSM Core Network at WCDMA Age

The developing mobile markets and the coming of age of 3G technologies have resulted in operators focusing on how to utilize these new technologies to optimize their 2G networks and ensure a smooth migration to 3G. Generally, at the core network end, there are two approaches for upgrade: one is to build a brand-new WCDMA core network with a 2G-radio network, while the other is to replace the legacy GSM MSC (Mobile Switching Center) with WCDMA MSCS (Mobile Switch Center Server) & MGW (Media Gateway) to optimize the core network hierarchy.

This paper focuses on current MSC upgrade solutions along with their pros and cons.

1 WCDMA Hierarchy and  Core Network Elements

1.1 WCDMA Network Hierarchy
WCDMA solutions feature highly advanced design and end-to-end service solution that can meet operator requirements for mobile NGN network construction and GSM upgrade. WCDMA network elements include Node B, RNS (Radio Network Subsystem), CN (Core Network), CG (Charging Gateway), GMLC (Gateway Mobile Location Center), MMS (Multimedia Service), etc. So far, all the commercially used network elements are based on 3GPP R99 and R4 specifications built with the maturing of technologies (as shown in Figure1).

1.2 CN Network Elements
CN network elements consist of circuit domain and packet domain. Based on TDM/ATM/IP backbone network, the circuit domain comprises MSCS and MGW. Based on IP backbone network, the packet domain includes SGSN (Serving GPRS Support Node), GGSN (Gateway GPRS Support Node) and CG, and fully inherits the features of the GSM/GPRS networks.

  The important feature of the core network lies in the separation of its control and bearer parts. MSC is divided into two parts: MGW and MSC Server. As a media gateway, MGW implements bearing functions such as 2G/3G wireless access as well as transmission and media flow conversion. MSC Server  is the core of the whole network and provides call connection and control.

  Compared with GSM MSC, MSCS and MGW feature high capacity and integration, which greatly simplifies the network planning and follows the network development trends.

  If we take ZTE’s products as an example, with 2 racks, MSCS capacity can reach 2 million, while 6 racks can support 2 million subscribers. For ZTE’s R99 CN CS (Circuit Switch) products, ZTE’s solutions adopt the hardware structure and functions that are the same as R4 and are integrated in MSC (as shown in Figure 2). Since the first sketch of WCDMA design, ZTE has attached great importance to the smooth migration from GSM to R99, R4, R5 and R6.

2 Optimization Solutions for GSM Network with Mobile  NGN

2.1 Technology Selection

2.1.1 R99 Solutions
Phase I: Replace VMSC of GSM with WCDMA MSC. For ZTE’s WCDMA structure, WCDMA R99 MSC is actually based on Server-GW structure, as shown in Figure 3.

  In this phase, the transmission network is still the legacy network due to the slow maturity rate for IP network technologies or lack of ATM network.

  Phase II: Upgrade the transmission network to IP/ATM and change the network to R4 hierarchy, as shown in Figure 4.

  (1) Advantages of R99 Upgrade Solutions:

• Decrease 3G network’s impact on the operation of existing GSM/GPRS network;
• Make the most of current network resources and protect the new investment on the migration from 2G to 3G;
• Ensure a smooth evolution to an all IP network;
• The end user services can smoothly migrate from GSM to WCDMA;
• Less network structure changes eliminate the need for continuous future investment.
  

  (2) Disadvantages of R99 Upgrade Solutions:

• Resulting impact on network services when upgrading to R4 and R5 network hierarchy;
• Cannot take the advantage of Tfo & Trfo (Tandem Free Operation & Transcode Free Operation);
• Traditional network hierarchy and high OPEX (Operational Expense).

2.1.2 R4 Solutions
According to WCDMA R4 Standard, circuit domain can be constructed with the means of IP, TDM and ATM.

  IP technology: Due to its inherent problems of security and real-time, IP technology cannot meet circuit domain’s requirement for QoS traffic, consequently, it won’t be adopted for present network construction.

  ATM technology: Since the complicated ATM technology is part of the trend for the future network, it won’t be used in large-scale network construction either. 
TDM technology: Featuring great credibility and maturity, TDM technology can meet the real-time requirement of circuit domain traffic. Therefore, it will be a good choice to maintain QoS (as shown in Figure 5).

  Phase Ⅰ: Overlay 3G CN over current 2G CN, introduce MSC server and MGWs to serve the emerging 3G users, and minimize impact on the existing 2G CN system.

  Phase Ⅱ: Keep the legacy BSSs connected with the existing GSM MSCs while connecting newly added BSSs to 3G MGW & MSC servers.

  Phase Ⅲ: Gradually migrate the GSM MSCs (connected to the existing BSSs) to UMTS (Universal Mobile Telecommunications System) MSCs due to the greater capacity of UMTS MSCs and natural aging of GSM MSCs (See Figure 6).

  (1) Advantages of R4 Upgrade Solutions:

• Simplified network structure
• Less impact on services when upgrading to R5
• Voice quality improved by Tfo&Trfo
  

  (2) Disadvantages of R4 Upgrade Solutions:

• Less O&M experience with R4
• Problematical interconnection between different vendors’ equipment
• QoS and security cannot be guaranteed for IP
• Interconnection protocols need to be improved

2.2 Advantages of ZTE’s Solutions
The implementation of basic mobile services, supplementary/IN services, MAP (Mobile Application Part) interface and ISUP/TUP (Integrated Services Digital Network User Part/ Telephone User Part) interface, is common for the UMTS and GSM CS CN. ZTE’s 3G MSC products strictly complies with the 3GPP standard specifications series, which are basically compatible with those of GSM CS CN.

  By fully supporting different RAN/CN interfaces for 3G & 2G CS CN (RANAP/BSSAP), and clearly identifying all differences between 3G & 2G CS CN common functionalities (security mechanism, phase of CAMEL, MAP operation, etc.), ZTE’s 3G MSC can invoke corresponding service logics as per user types, thus ensuring simultaneous service provision to GSM ane UMTS users.

  For GSM/UMTS dual-mode terminal users, ZTE’s MSC supports secure data conversion between 2G and 3G when inter-system roaming occurs.

  For GSM/UMTS dual-mode terminal users, ZTE’s MSC products also support smooth handover between GSM and UMTS.

  For operators, the evolution capability is very important; it can reduce operators’ investment on network upgrade.

  ZTE’s large-capacity UMSC can be easily upgraded to R5 and R6 versions.
  ZTE’s UMSC is the element that integrates MSC and SGSN. It is a unified switching platform of circuit and packet, where all the narrow band voice services and broadband packet services can be processed. Therefore, it can support simultaneous access for both GSM and 3G radio network.

3 MSC Upgrade Strategies

3.1 Diverting Traffic from MSC to MGW
Replace the small-capacity GSM MSC with the 3G MSC (including MSC server and MGW), to improve the network architecture and enhance the QoS (as shown in Figure 7).

3.2 Fully Converting GSM Core Network  to Mobile NGN
Replace the MSC completely with 3G MSC. Once ATM or IP interface hardware module is added, the MSC server can support 3G R99/R4 RAN access, with the new network without affecting the existing service (as shown in Figure 8).

3.3 Upgrading to all WCDMA Core Network
When the services are stable and well established, operators can replace the remaining four MSCs and at the same time increase the capacity of the 3G MSCs. This is followed, in effect, by the evolution of the GSM MSC to 3G (as shown in Figure 9).

Manuscript received: 2004-12-10