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UNIT 1
1.2 EVOLUTION OF PUBLIC MOBILE SERVICES
The dominant services over public wide-area wireless networks have been circuit- switched mobile voice services. Today, many mobile operators still generate most of their revenues from circuit-switched mobile voice services. However, a fundamental shift has been occurring rapidly throughout the world
since 2G wireless networks became commercially available. The spectacular growth of 2G wireless networks over the past 10 years helped to push mobile voice services to reach saturation in many regions of the world. For example, the penetrations of mobile voice services (i.e., subscribers as percentage of the population in a given region) have reached 70% or higher in many western European and Asian countries. Mobile data and multimedia services have become the main drivers for the future growth of mobile services throughout the world. Mobile data and multimedia services have been growing more rapidly than mobile voice services worldwide. Mobile data and multimedia services have matured signicantly, evolving rapidly from text-based instant messaging services to low-speed mobile Internet services based on proprietary technologies, to higher speed and broader range of mobile Internet services based on open and standard Internet protocols, and to high-speed and multimedia Internet services. Mobile data and multimedia services are poised to become the dominant mobile services in the near future. Given the importance of mobile data and multimedia services, we next take a closer look at how these services have been evolving.
1.2.1 First Wave of Mobile Data Services: Text-Based Instant Messaging
The rst globally successful mobile data service is SMS (Short Message Services), which was rst introduced in Europe over GSM networks. SMS allows a mobile user to send and receive short text messages (up to 160 text characters) instantly. Supporting SMS does not require a packet core network. Instead, SMS messages
are delivered using the signaling protocol—Mobile Application Part (MAP)—that was originally designed to support mobility in GSM networks. This allowed SMS services to be provided over the completely circuit-switched 2G GSM networks long before packet core networks were introduced into wireless networks. SMS services grew rapidly rst in Europe. Today, SMS services are booming throughout the world.
. In the United Kingdom, the number of transmitted SMS messages more than doubled in the two-year period from 2001 to 2002. Based on statistics from the Mobile Data Association, an average of 52 million SMS messages were transmitted every day in the United Kingdom in December 2002, which translates into about 2.2 million messages per hour on the average. Figure 1. shows the SMS subscriber growth in the United Kingdom from 1998 to June 2003. . In Europe, 186 billion SMS messages were transmitted in 2002.
. In China, the revenues of SMS and value-added services (VAS) totaled 750 million in 2002. During an eight-day period around the Chinese New Year in 2003 (early February), approximately 7 billion SMS messages were transmitted. . In the United States, SMS is experiencing an explosive growth, although its usage levels have not reached the levels in Western Europe and the Asia
Fig. 1.6 Growth of SMS message transmissions in the United Kingdom
Pacic region. InphoMatch, a company that handles intercarrier SMS services for AT&T Wireless, Verizon, and T-Mobile, delivered 100 million SMS messages in August 2002 and saw the intercarrier SMS trafc doubling every two months.
In addition to providing a highly valuable data service to mobile users, SMS allowed mobile users to become familiar and comfortable with mobile data services and to appreciate the value of mobile data services. This helped pave the road for mobile users to adopt the more advanced data services that arrived next.
1.2.2 Second Wave of Mobile Data Services: Low-Speed Mobile Internet Services
Interactive and information-based mobile Internet services emerged as the second wave of widespread mobile data services. An example of successful mobile Internet
. Directory services, e.g., dictionary, restaurant guides, and phone directory. . Daily information, e.g., news, weather reports, road conditions, and trafc information. . Entertainment, e.g., Karaoke, network games, and horoscope.
The i-Mode services has experienced a rapid and steady growth ever since its launch. Figure 1.7 shows the growth of i-Mode subscribers for the three-year period from 2001 to 2003 (Source: NTT DoCoMo). By the summer of 2003, i- Mode was supporting over 38 million subscribers. i-Mode represents a signicant milestone in the evolution of mobile services. It was the rst major success in bringing Internet-based services to a large population of mobile subscribers. It demonstrated the values and the potentials of the mobile Internet to the world. Today, however, the i-Mode services are suffering from two major limitations:
. i-Mode services are limited by the low data rate of the PDC radio networks. . i-Mode users rely on proprietary protocols developed by NTT DoCoMo, rather than on standard IP-based protocols, to access i-Mode services. The i-Mode services are provided by World-Wide Web (WWW) sites specically designed for mobile users. Mobile devices use a set of proprietary protocols developed by NTT DoCoMo to communicate with these WWW sites via a gateway. The gateway converts between the protocols over the radio access network and the
Fig. 1.7 Growth of i-Mode subscribers
Many of the services described above are already available over 2.5G or early forms of 3G wireless networks and are changing the ways people communicate in a fundamental manner. Camera phones, for example, enabled the rst multimedia mobile applications that are truly useful to and accepted by large populations of mobile users worldwide. The world’s rst commercial camera phones that allow users to take pictures and videos and allow them to send the pictures and videos over wireless networks to other users emerged in late 2001. Ever since their introduction, camera phones have been experiencing a spectacular growth. According to Strategy Analytics (January 2003), 10 million camera phones were sold worldwide in the rst nine months of
- The number of camera phones sold in 2002 was estimated to be 16 million, which is approximately the same level of worldwide sales of PDAs in 2002. Within merely around a year of camera phones’ existence, their annual worldwide sales in 2002 were already close to the approximately 22 million digital cameras sold in the same year (Strategy Analytics). Today, camera phones are improving rapidly. For example, picture display
screens are larger, picture resolutions are higher, and user applications for handling pictures and videos are richer (e.g., applications for editing pictures and videos, taking snapshot pictures from videos, and sending pictures and videos as instant messages or email attachments over wireless networks to other users). Although the functionalities of camera phones are improving, their prices are declining due to intensive competition as all major mobile phone manufacturers around the world are competing in the camera phone market now. For example, camera phones priced at
Fig. 1.8 Evolution of mobile services
around US $100 became available in late 2002, whereas the average camera phone prices were around US $300 in early 2002. The improving functionalities and the falling prices of the camera phones, with the higher data rates of 2.5G and 3G wireless networks, make camera phones more useful and affordable to the consumers and will therefore further accelerate the growth of camera phones. Strategy Analytics expects that camera phones will outsell digital cameras by 2004. The widespread use of camera phones is not only changing the ways people communicate, but also it is changing the mix of mobile services and the nature of the network trafc generated by mobile services. In particular, camera phones help increase multimedia trafc over the wireless networks signicantly. The evolution of mobile services is illustrated in Figure 1.8.
1.3 MOTIVATIONS FOR IP-BASED WIRELESS NETWORKS
Wireless networks are evolving into IP-based mobile networks. Is this worldwide march toward IP-based wireless networks a short-term phenomenon or a long-term trend? Several fundamental reasons suggest that IP-based wireless networks are more promising choices than circuit-switched wireless networks for the future. IP-based wireless networks are better suited for supporting the rapidly growing mobile data and multimedia services. As mobile data and multimedia services continue to grow more rapidly than mobile voice services, they will overtake mobile voice services to become the dominant mobile services in the near future. First, we will see trafc volume of mobile data services surpass that of mobile voice services. As mobile data services become increasingly important to consu- mers, the revenues generated by network operators from mobile data services will also surpass the revenues from mobile voice services. Figure 1.9 illustrates the estimated and the forecast (by Analysis Research Limited) Average Revenue Per User (ARPU) for mobile voice and non-voice services in Western Europe, 2000 – 2007. It shows that non-voice mobile services will grow signicantly and steadily over the next few years to account for over 35% of the total revenue from mobile services in 2007. In the US, the penetration of mobile Internet services is expected to be even higher than in Western Europe, suggesting that non-voice mobile services could account for an even higher percentage of mobile operators’ total revenue. Figure 1.10 shows the growth of mobile services projected by the Universal Mobile Telecommunications System (UMTS) Forum. Here, non-voice mobile services are projected to grow even faster than the projection for Western Europe shown in Figure 1.9. Wireless networks should evolve to support predominately mobile data and multimedia services and trafc, rather than circuit-switched voice services and trafc. IP technologies, which are already universal over wireline data networks, are the most promising solutions available today for supporting data and multimedia applications over wireless networks.
1.3 MOTIVATIONS FOR IP-BASED WIRELESS NETWORKS 21
Fig. 1.10 Growth of mobile voice and non-voice services
Many mobile network operators also operate wireline networks. They have already built out IP core networks to support wireline IP services or as a backbone network for transporting circuit-switched voice trafc. Mobile network operators could leverage their existing IP core networks to support radio access networks and provide services to mobile users. IP-based radio access systems are becoming important components of public wireless networks. IP-based radio access systems, e.g., IEEE 802.11 WLANs, are becoming increasingly important parts of public wireless networks worldwide. WLANs, which generally assume IP as the network-layer protocol for supporting user applications, are best supported by IP-based core networks rather than circuit- switched core networks. Public WLANs could become “pico-cells” used to provide high system capacities and data rates to target geographical areas. Before public WLANs became available, pico-cells in public wireless networks are implemented using cellular radio technologies. Such a pico-cell is implemented using a pico-cellular radio base station to cover a small area. Alternatively, a wireless base station may use smart antennas to implement a pico-cell by shaping one of its radio beams to cover a small geographical area. Implementing a large number of pico-cells using cellular radio technologies are typically expensive—a key reason that pico-cells are not widely
available today. Public WLANs offer a new way to provide such pico-cells at much lower costs. IP technologies provide a better solution for making different radio technologies transparently to users. Different radio technologies will continue to coexist in public wireless networks. These radio technologies include not only different wide-area radio technologies but also the fast growing IP-based public WLANs. One radio technology (e.g., public WLANs) may meet communications needs other radio technologies (e.g., cellular radio systems) may not be able to meet easily. Therefore, heterogeneous radio systems are expected to coexist in the long run. Mobile users typically do not want to be bothered with the specics of each radio technology. They want to receive services not technologies. They want the technologies to be made transparent to them. Therefore, there is a long-term need to interconnect radio systems that use different radio technologies, to support roaming between different radio systems, to provide mobile services over different radio systems in a seamless manner, and to support global roaming between different mobile providers and different countries. IP-based protocols, which are independent of the underlying radio technologies, are better suited than circuit-switched network technologies for achieving these goals. With IP as the common network-layer protocol, a terminal with multiple radio interfaces (or a single radio interface capable of accessing different types of radio systems) could roam between different radio systems. IP-based network services and applications could be provided to all users in a seamless manner, regardless of which specic radio systems or mobile devices (e.g., PDAs, laptops, phones, or any other special-purpose devices) they are using.
. Protocol stacks for packet data network (Section 2.1.11) . How to use a 3GPP packet network to access other IP networks (Section 2.1.2)
2.1.1 Network Architecture
A public network administrated by a single network operator for providing land mobile services is referred to as a Public Land Mobile Network (PLMN). The conceptual architecture of a 3GPP PLMN is illustrated in Figure 2.1. It consists of one or more Radio Access Networks (RANs) interconnected via a Core Network (CN). A RAN provides radio resources (e.g., radio channels, bandwidth) for users to access the CN. Release 5 currently supports GSM/EDGE RAN (GERAN) and UMTS Terrestrial RAN (UTRAN). Work is underway on 3GPP to specify how to
Fig. 2.1 3GPP conceptual network architecture (Release 5)
84 WIRELESS IP NETWORK ARCHITECTURES
- 2.1 3GPP PACKET DATA NETWORKS
Fig. 2.2 Functional architecture of a user equipment (UE)
Module (USIM) [13]. USIM is developed based on the Subscriber Identity Module (SIM) used in GSM systems. A ME, consisting of Mobile Termination (MT) and Terminal Equipment (TE), is the device a user uses to access the network services. TE provides functions for the operations of the access protocols. MT, on the other hand, supports radio transmission and channel management. Depending on applications, an MT may have a combination of different Terminal Adapters (TA). In realization, MT could also be a mobile handset and TE could be a laptop computer. It is also possible to integrate MT and TE in the same device. Each MT is identied by a globally unique International Mobile Station Equipment Identity (IMEI) [15]. A mobile station may be congured to access the PS domain only, the CS domain only, or both the CS and the PS domains. Each subscriber to 3GPP network services is assigned a globally unique International Mobile Subscriber Identity (IMSI) as its permanent identier. A subscriber uses its IMSI as its common identier for accessing PS services, CS services, or both PS and CS services at the same time. A subscriber’s IMSI is stored on a USIM on a mobile station. A subscriber can move its USIM from one mobile station to another so that the subscriber can use different mobile stations to access the network while being identied by the network as the same subscriber. The network uses the IMSI to identify a subscriber and to identify the network services and resources used by a subscriber for billing purpose. A mobile’s IMSI may be used as the mobile’s identier at multiple protocol layers in 3GPP, e.g., at the physical layer, link layer, and the network layer. An IMSI can consist of only numerical characters 0 through 9. It contains three parts as shown in Figure 2.3 [15]:
Fig. 2.3 Structure of International Mobile Subscriber Identity (IMSI)
. Mobile Country Code (MCC): The MCC uniquely identies a mobile subscriber’s home country. . Mobile Network Code (MNC): The MNC uniquely identies a mobile subscriber’s home PLMN in the mobile subscriber’s home country. The MNC can be two or three digits in length, depending on the value of the MCC. . Mobile Subscriber Identication Number (MSIN): The MSIN uniquely identies a mobile subscriber within one PLMN.
Allocation of MCCs is administrated by the ITU-T according to ITU-T Blue Book Recommendation E.212. MNC þ MSIN is commonly referred to as the National Mobile Subscriber Identity (NMSI). The NMSIs are allocated by the numbering administrations in each country. When more than one PLMN exists in a country, a unique MNC is assigned to each of these PLMNs. To reduce the need to transmit IMSI, which uniquely identies a mobile subscriber, over the air, 3GPP uses a Temporary Mobile Subscriber Identity (TMSI) to identify a mobile whenever possible. A TMSI is a four-octet number assigned to a mobile temporarily by an MSC/VLR for circuit-switched services or by an SGSN for packet-switched services. The two most signicant bits in a TMSI indicates whether the TMSI is for packet-switched services. A TMSI for packet-switched services is referred to as a Packet TMSI or P-TMSI. An MSC or SGSN uses a TMSI to uniquely identify a mobile. The TMSI will only be allocated in ciphered form. Furthermore, measures will be taken to ensure that the mapping between a mobile’s IMSI and TMSI is known only by the mobile and the network node (MSC or SGSN) that assigned the TMSI (Section 2.1.8). A mobile’s TMSI, instead of its IMSI, will then be used as the mobile’s identity whenever possible in signaling messages transmitted over the air. As only the mobile and the MSC or SGSN that assigned the TMSI to the mobile know the mapping between the mobile’s IMSI and TMSI and that a TMSI is valid only when the user is served by the MSC or SGSN that assigned the TMSI, the security impact of transmitting unencrypted TMSI over the air is lower than transmitting unencrypted IMSI. To send and receive IP packets over the PS CN, a mobile also needs to be congured with at least one IP address. The mobile may use multiple IP addresses simultaneously. However, a mobile is not required to have a valid IP address at all
times while it is attached to the PS domain. Instead, a mobile may acquire an IP address only when it needs to activate packet data services over the PS CN.
2.1.1.2 Circuit-Switched Domain in Core Network The CS domain consists of all the CN entities for providing circuit-switched voice and data services to mobile users. The CS CN domain is built on the GSM core network technologies. Its main network entities are:
. Mobile-services Switching Center (MSC) . Gateway MSC . Visitor Location Register (VLR) . Home Subscriber Server (HSS), Equipment Identity Register (EIR), and Authentication Center (AuC)
The MSC performs switching and call control functions needed to provide basic circuit-switched services to mobile terminals. In addition, it also performs mobility management functions, including location registration and handoff functions for mobile terminals. The MSC interconnects RANs to the CS CN domain. One MSC may interface with multiple GSM BSSs or UTRAN RNSs. In 3GPP Release 5, the CS CN made a signicant improvement over the previous releases: It allows the switching and call control functions of an MSC to be separated and implemented on separate network entities:
. MSC Server for handling call control and mobility management. . CS Media Gateway (CS-MGW) for handling circuit switching, media conversion, and payload processing (e.g., echo canceller, codec) and payload transport over the circuits.
Separation of switching and call control allows switching and call control technologies to evolve independently. It also helps increase network scalability. For example, one MSC Server can support multiple CS-MGWs and new MSC Servers and/or CS-MGWs can be added to increase call control and/or switching capabilities. A dedicated MSC called Gateway MSC (GMSC) may be used to interface with
external circuit-switched networks. A GMSC is responsible for routing a circuit- switched call to its nal destination in external networks. The switching and call control functions of a GMSC can also be separated and implemented on separate network entities: CS-MGW for switching and media control and a GMSC Server for call control. A VLR maintains location and service subscription information for visiting mobiles temporarily while they are inside the part of a network controlled by the VLR. It tracks a visiting mobile’s location and informs the visiting mobile’s HLR of the mobile’s current location. It retrieves a visiting mobile’s service subscription
information from the mobile’s HLR, maintains a copy of the information while the visiting mobile is inside the part of the network controlled by the VLR, and uses the information to provide service control for the visiting mobile. A VLR is typically integrated with each MSC because no open standard interface has been dened between an MSC and a VLR. The Mobile Application Part (MAP) [12] protocol is used for signaling between a VLR and an HLR. The other information servers HSS, EIR, and AuC are shared by the CS and the PS domains and will be discussed in Section 2.1.1.5.
2.1.1.3 Packet-Switched Domain in the Core Network The PS CN domain provides the following main functions for supporting packet-switched services:
. Network access control: Determines which mobiles are allowed to use the PS domain. These functions include registration, authentication and authoriz- ation, admission control, message ltering, and usage data collection. . Packet routing and transport: Route user packets toward their destinations either inside the same PLMN or in external networks. . Mobility management: Provides network-layer mobility management func- tions. These functions include tracking the locations of mobile terminals, initiating paging to determine an idle mobile’s precise location when the network has data to send to the mobile, and maintaining up-to-date CN routes to mobiles as they move.
The PS domain is built on the GPRS network platform. As in GPRS, the 3GPP PS CN domain consists of two main types of network nodes:
. Serving GPRS Support Node (SGSN) . Gateway GPRS Support Node (GGSN)
An SGSN interconnects one or more RANs to a PS CN. A SGSN performs the following specic functions:
. Access control: The SGSN is responsible for the rst line of control over users’ access to the PS CN domain. The GGSN provides an additional line of control for access to the PS CN domain. . Location management: The SGSN tracks the locations of mobiles that use packet-switched services. It may report the location information to the HLR so that the location information may be used, for example, by the GGSN to perform network-initiated procedures to set up connections to mobiles. . Route Management: The SGSN is responsible for maintaining a route to a GGSN for each mobile and to relay user trafc between the mobile and the GGSN.
