UMTS Core Network Planning Model and Comparison of Vendor Product Performance R. Shalak, Dr. K. Sandrasegaran, Dr. J. Agbinya, S. Subenthiran University of Technology, Sydney (UTS) Faculty of Engineering 1 Broadway, Ultimo Sydney, N.S.W, Australia Email: rshalak, kumbes, agbinya,
[email protected] Abstract - Third Generation mobile networks are currently being deployed worldwide. For the service provider, upgrading from their existing second generation network or deploying a new third generation network brings with it many considerations which will prove vital to their ultimate success. In these formative years, perhaps the most contentious issue is UMTS network planning and optimisation. In this article, planning considerations are discussed for the core network. A model is generated and proposals are made on how the operator can maximise efficiency as well as provide the user with the required QoS. Keywords - Universal Mobile Telecommunication Service, core network, Serving GPRS Support Node, Gateway GPRS Support Node, build-ahead.
I. Introduction Rapid growth in traffic volume combined with a multitude of new services has begun to alter the structure of wireless networks. To cope with increased demands, service providers must upgrade to flexible networks with high data rate capabilities and QoS guarantees. In short they must adopt Third Generation technologies. The UMTS network can be segregated into two main subsections, the Core Network and the Radio Access Network (RAN). The Core Network, which is the primary concern of this article, comprises of Mobile Switching Centres, Serving GPRS Support Nodes (SGSN), Gateway GPRS Support Nodes (GGSN), Home Location Registers and Visitor Location Registers. This nucleus is responsible for the main network functions including switching of traffic, providing of QoS, mobility management, network security and billing. Consequently, its meticulous planning and careful dimensioning are imperative in ensuring overall efficiency and reliability. To date, almost all planning research, publications and literature concentrate on the Radio Access Network and overlook the Core Network. Our ongoing work, aims to shift
this balance by exposing how core network planning strategies can effectively increase profits and efficiency and reduce wasteful expenditure. Although, this paper essentially focuses on UMTS core network planning, it is important to recognise that the RAN has a direct bearing on core network design. With this in mind, it would be neglectful not to overview various RAN configurations and their impact on the core network. An integral part of the UMTS network development process is network deployment and evolution planning. By deployment, we are referring to a carrier who is rolling out a ‘greenfield’ UMTS network, whilst evolution refers to a 2G operator who is upgrading to UMTS. Both cases primarily involve the evaluation and consequent purchase of new network hardware, followed by the integration of that equipment into the existing network. This paper recognises that on most occasions this selection of vendor equipment is rarely done on a merit basis but more often due to a service provider’s strategic alliance and ties with a particular supplier. These long built relationships can often be counter-productive, as they breed an air of automatic acceptance of a vendor’s product devoid of sound judgement. Furthermore, it is important to recognise that 3GPP standards were devised to produce global compatibility of multi-vendor equipment. These international standards enable service providers to utilise equipment produced by a multitude of vendors subsequently reaping benefits such as greater bargaining powers and competitively priced equipment. Initially it may seem wasteful to allocate time and resources to the process of determining which supplier can provide the most suitable equipment to satisfy a carrier’s current and future needs. However, considering that the decision’s subsequent effects are often long lasting and very costly to reverse adds further weight to the process of selection. This paper addresses this issue by comparing and assessing the strengths and weaknesses of several current market UMTS core network components developed by a multitude of vendors. The research presented in this paper, is condensed into a core network planning model. This model utilises several key planning inputs to produce an optimised configuration output. The inputs include the breakdown of traffic forecasts into voice,
data and subscriber usage, excess dimensioning based on buildahead requirements and interface dimensioning based on several factors including RAN configuration. In addition, the paper evaluates and underlines various considerations taken in dimensioning an efficient UMTS core network. Section II gives a brief outline of UMTS network components and architecture. The functionality of each element is reviewed including a description of how these components function collectively. Section III outlines the differences in network structure between GSM and UMTS. The migration from 2G to 3G is outlined with special attention given to the new packet handling components. Section IV discusses core network planning considerations, highlighting the dimensioning limitations of Core Network elements. Our novel core network planning model is presented. This model aims to increase overall network efficiency by targeting and improving core network structure. It goes further to underline the impact of the radio access network on interface dimensioning and core network. Section V compares different vendor product offerings, providing insight into current capacity limitations for the various core network components. Finally, proposals are made on how an operator can maximise efficiency as well as provide a subscriber with the required QoS.
D
MSC Iu -CS
HLR
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RNC Iu-PS Iub
Gr SGSN
G Gn
Iur Node B
Packet Data Networks
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Key: Signalling Interface Data & Signalling interface Core Network Boundaries Figure 1: UMTS Core Network Model
II. UMTS Network Structure UMTS networks are designed to support user applications requiring high data rates. Consequently, the architecture of UMTS core networks is an adaptation of its GSM predecessor. The release’99 architecture depicted below contains all the traditional GSM core network components with the addition of two packet handling nodes – The SGSN and GGSN. These two nodes work collectively enabling UMTS networks to exchange information with external data networks. The SGSN is the central element in the packet switched network. The main SGSN functions are mobility management, traffic routing and user information and authorisation. In addition to this, it provides a number of functionalities such as ciphering and compression. The SGSN location register stores location information (eg. Current cell, current VLR) and user profiles (eg. IMSI and temporary identities) of all users registered with this SGSN. The GGSN acts as an interface between the UMTS backbone network and external packet data networks. It converts packets coming from the SGSN into the appropriate packet data protocol (PDP) format (IP or X.25) and sends them out on the corresponding packet data network. In the other direction, PDP addresses of incoming data packets are converted to the UMTS address of the destination user. The readdressed packets are sent to the responsible SGSN. For this purpose, the GGSN stores the current SGSN address of the user and their profile in its location register. The GGSN is also responsible for charging and authentication.
Generally, there is a many to many relationship between SGSNs and GGSNs. A GGSN is the interface to external packet data networks for several SGSNs and a SGSN may route its packets over different GGSNs to reach different packet data networks.
III. Evolution from GSM to UMTS Projections for future network utilisation point towards a significant mobile data usage increase. The predictions indicate that although “ simple voice” will remain a vital component of operators’ service portfolio, by 2010 voice will only generate $88bn compared to $237bn for all other services. It is also anticipated that in 2010, the average subscriber will spend about $30 per month on 3G data services [1]. These utilisation pattern forecasts are driving the push for a change from the current mobile standards to mobile networks with high data processing capabilities. During the upgrade from 1st generation to 2nd generation networks existing technologies were not retained. Service providers were forced to abandon old for new. The move from 2G to 3G promises to be different. New technologies are designed to function alongside existing technologies rendering the transition almost seamless. From a core network perspective, the transition from GSM to UMTS involves more than just a software upgrade. The actual configuration of the GSM network is dramatically altered. The
existing MSC/VLR and HLR are retained but upgraded. Two additional nodes have been added. The SGSN and GGSN provide the network with its packet handling capabilities. The SGSN is responsible for the delivery of data packets to and from mobile nodes within its service area, whilst the GGSN provides network connectivity to several external Packet Data Networks (PDN) including the Internet. UMTS’ ability to process high data rates is a direct result of its new W-CDMA radio access technology. However, in contrast to the core network, the existing GSM radio network will remain relatively unaltered. Instead, UMTS designers have opted to develop the UMTS Terrestrial Radio Access Network (UTRAN) to attach and run in parallel with existing Base Station Controllers (BSCs) and Base Transceiver Stations (BTSs). The strategy developed aims to allow the UTRAN to handle the delivery of packet switched data to and from the mobile nodes, whilst existing GSM infrastructure maintains control of the circuit switched traffic. Terminal manufacturers are also following “the seamless transition” and have committed to producing dual – mode UMTS/GSM terminals enabling users to roam freely concurrently utilising both 2G and 3G services. These dualmode devices are expected to cost up to 30% less than those of any other technology [2].
IV. Core Network Planning Under normal circumstances, careful planning of wireless networks is vital if operators wish to make full use of existing investments. However, during the transition phase from 2nd generation to 3rd generation networks, planning assumes a new importance. The increase in radio network capabilities means that even the well established 2G network operators will need to re-dimension their core networks to ensure service quality and network efficiency is maintained. The integration of new packet handling components including the Serving GPRS Support Node (SGSN), the Gateway GPRS Support Node (GGSN) and the Radio Network Controller (RNC) necessitate an entire review of current running planning policies. The UMTS network planning model illustrated in this section highlights the various factors affecting core network design and planning. These factors include traffic usage projections, node capacity limitations, build-ahead policy and RAN configuration. Networks must be dimensioned to support user demands. Consequently, traffic projection figures are vital for planners. Traffic Projection is a blanket term used to denote the volume and nature of traffic processed by network nodes. The volume of traffic received determines the number of nodes used and capacity provisioned between nodes, whilst the nature of traffic has a bearing on the type of nodes deployed as well as allowing the planner to forecast traffic trends [3]. For example, if subscribers are accessing the Internet, then the quantity of downlink traffic would be sufficiently greater than the uplink traffic. Planners can use this trend to efficiently dimension network capacity.
Traffic forecasts are separated into two main categories: data and voice. Voice usage forecasts are further disseminated into mobile to land, land to mobile, mobile to mobile and mobile to voice mail usages. In evaluating data usage forecasts, planners must dimension the network according to the nature of data traffic and its intended destination. For example, the volume of Internet traffic passing to and from mobile users has a direct bearing on the dimensioning of SGSN and GGSN interfaces. Alternatively, the volume of data traffic terminating on an email server inside the network directly affects the interface dimensioning of that server as well as SGSN. This process should be reiterated for all types of data traffic passing through the network. In calculating the required number of each type of core network node, it is important to initially understand the capacity limitations of each node. By contrasting node capacity limitations with expected traffic volumes, it is possible to clearly determine the basic network node requirements. It is imperative to recognise that these calculations represent the minimal network operating requirements. To meet any expected growth in demand, networks must be over-dimensioned. This is referred to as build-ahead [4]. Build-ahead accounts for traffic growth over a period of time. In its infancy, the operator’s network will be dimensioned according to traffic forecasts alone, therefore it is wise to dimension for a longer build-ahead period of around 12 months. This ensures that the network will cope with any excess demand as well as allow the operator’s traffic to grow without needing to constantly re-dimension their network. As the operator’s understanding of traffic trends increases, it will become possible to safely dimension for a much shorter build-ahead period increasing overall network efficiency. To determine projected growth in traffic, carriers rely on widely available government statistics involving population types, incomes, distribution of wealth, taxation and spending habits. There is also a need for statistics depicting the existing penetration of mobile voice services and average Internet usage in the market [5]. The radio access network has a direct bearing on core network dimensioning. The Iu-CS and Iu-PS links are dimensioned to reflect expected RNC loading. Taking the case where a particular RNC is expected to process 40Mbps of data traffic, then the bandwidth of the Iu-PS should be 20% -30% more to allow for GTP and ATM overhead. Iu-CS dimensioning can be similarly determined by estimating the amount of Erlangs processed by the RNC. A SGSN with a maximum throughput of around 250Mbps can support a maximum of five RNCs carrying 40Mbps. If more than five RNCs are required in a particular service area then another SGSN will need to be deployed.
All other network interfaces can be dimensioned according to expected user rates with a little extra capacity for overhead and bursty periods.
V. Comparison of Vendor Equipment As previously mentioned, each node in the UMTS network has design limitations. The exact limitations differ from vendor to vendor. Node limits coupled with traffic projections are used to determine the quantity of a particular node element used in the network.
Only Ericsson, Nokia and Siemens specified the Quality of Service of their SGSN. Ericsson's SGSN are compliant with the GSM 3.60 class. It supports the reliability classes 2 and 3, Delay classes 1 to 4 for subscriber data. Nokia's QoS was based on priority, interactive and background classes while Siemens’ QoS are configurable to meet the needs of different types of users. Most of the vendors collect charging information in relation to the time and date, QoS, duration and volume of the packet data transferred. Nokia also collect information in regards to the radio resources used for re-transmitting.
In this section , functionality and limitations of multi-vendor SGSNs and GGSNs are compared. All data has been collected from the respective vendor websites.
Gateway GPRS Support Node (GGSN)
Serving GPRS Support Node (SGSN)
The GGSN by Cisco, Ericsson, Nokia, Nortel and Siemens has the following functions:
The SGSN produced by Alcatel, Ericsson, Lucent, Nokia, Nortel and Siemens have the same functionality as listed in the standards GSM 3.60, these include: • • • • •
•
Authentication and authorization, Mobility management, admission control relay and routing, Address translation and mapping Encapsulation tunnelling, Compression, ciphering logical management Path management
• • • • • •
Authentication and authorisation Ciphering Relay and routeing Address translation and mapping Encapsulation and tunnelling Mobility management
Only the GGSN by Ericsson, Lucent, Motorola, Nokia and Nortel have the charging information collection function.
Ericsson have additional performance measurements and event recording functions built into their SGSN. Nokia have prepaid functions and Nortel possess an additional SGSN Accounting Server (SAS), responsible for billing functionality.
The number of subscribers supported ranges from 400,000 to 15 000. Lucent's GGSN supports the most while Ericsson's combined platform of SGSN/GGSN supports the least. Ericsson’s platform can be scaled to support more users.
The interfaces supported by all the vendor's SGSNs are the Gb (SGSN-BSS) and the Gn (GSN-GSN in same PLMN) interfaces. Alcatel's SGSN supports the most interfaces while Lucent support the least.
The number of PDP contexts varies from the 1 Million supported by Nortel to the 180,000 supported by Cisco.
The number of subscribers supported by SGSNs range from 50,000 up to 900,000. Alcatel's SGSN can support the largest number, whilst the 'Entry Level' SGSN/GGSN by Lucent supports the least. However, it is worth noting that Lucent’s platform can be scaled to support 300 000 attached users. There is also great variation in the number of PDP contexts supported by different SGSN vendors. Ericsson's SGSN can support 200,000 simultaneous PDP contexts while Nokia's Release 2 SGSN can support up to 960,000. The data rates of the SGSN vary around the 200Mbps. Lucent’s Flexent is highly scalable offering data rates ranging from 200Mbps to a maximum of 1.2Gbps. The Siemens’ @dvange and Ericsson SGSN platform offer rates of 250Mbps and 350 Mbps respectively.
The supported data throughput varies from 2Gbps by Nortel’s Univity platform to 100 Mbps by Siemens’ @vantage. Most vendors allocate IP addresses statically via the HLR and dynamically using a RADIUS server. The Cisco GGSN supports QoS negotiation and handling and its QoS classes are mapped to Internet Differentiated Service Classes. Nortel's GGSN supports traffic management functions such as queuing, policing and shaping of the traffic while Ericsson's model complies with the reliability and delay classes of the GSM 3.60 standard. The charging information collected includes time taken between the commencement and termination of PDP contexts, volume of data exchanged, and packet network destinations. It should be noted that the above-mentioned comparisons are void of pricing considerations. Equipment pricing and support play a vital role in vendor product selection. On most occasions,
vendors are hesitant to provide equipment pricing unless a nondisclosure contract is signed. Conclusion Increased services and subscriber numbers are pushing service providers to acquire Third Generation wireless technologies. As the 2G network structure begins to change, current planning strategies become dated and new strategies must be adopted. The aim of these strategies is to maintain network efficiency throughout the transition period and beyond. This paper outlines core network changes experienced by service providers upgrading from GSM to UMTS. Modifications, mainly involve the addition of packet handling nodes and a requirement for increased interface dimensioning due to the extra capabilities of the W-CDMA radio network. In addition, the paper presents a core network planning model yielding long term planning strategies as well as performing a comparison of current market vendor products.
REFERENCES [1] K. Van der Spiegel, “Accessing the development of UMTS and 3G overseas”, UMTS Forum, Institute for International Research Conference, Sydney, May 2001. [2] Siemens URL, “UMTS Overview”, http://www.siemens.com. [3] Clint Smith P.E, Daniel Collins, “3G Wireless Networks”, McGraw-Hill, 2000. [4] J.P. Castro, “The UMTS Network and Radio Access Technology”, published by Wiley, New York, NY, 2001. [5] UMTS World, “ Planning Basics”, URL http://www.umtsworld.com.
