2.5G is the realm of enhanced data services, and four primary techniques fall under this category: General Packet Radio Service (GPRS), High-Speed Circuit-Switched Data (HSCSD), Enhanced Data Rates for Global Evolution (EDGE), and the GSM EDGE Radio Access Network (GERAN). These standards, as well as standards for messaging services, continue to evolve within the various working groups of the standards organizations.
HSCSD is an enhancement to GSM that uses existing circuit-switched equipment with software upgrades. The main difference between HSCSD and the original GSM data transmission mechanism lies in the ability to use different coding methods as well as multiple time slots for a single user in order to increase data throughput. By using time slot aggregation, HSCSD supports data rates from 9.6Kbps to 57.6Kbps; also performing channel aggregation enables up to 100Kbps. However, those are the theoretical possibilities. In general practice, the operational speed is closer to 28.8Kbps.
HSCSD also provides for several levels of error correction, which can be deployed according to the quality of the radio link. This means that with HSCSD, assuming the best of conditions, a time slot that would normally carry 9.6Kbps (in GSM's original CSD mechanism) can carry 14.4Kbps. By using multiple time slots at the same time, it is possible to achieve an increase in maximum transfer rates up to 57.6Kbps. The time slots being used for such higher-speed data transfer must be fully reserved for a single user. Because the GSM network is configured such that normal voice calls take precedence over additional time slots for HSCSD users, at some point during a call, it might not be possible to satisfy an HSCSD user's full request. In order to maintain the user connection, the user will be charged at a higher rate, sometimes multiplied by the number of time slots allocated and based on the amount of time the user has an active connection. This means that HSCSD can be relatively expensive; therefore, GPRS (discussed in the following section) is becoming the more common choice.
HSCSD is designed for applications that require continuous availability of bandwidth, such as large file transfers, remote access to corporate facilities where a user will be working on a remote server for some time, videoconferencing, and multimedia transmission. HSCSD is offered to subscribers using either voice terminals that support the feature or a special PC card with a built-in GSM phone that turns a notebook computer or other portable device into a complete high-speed mobile office, with the ability to make hands-free voice calls and engage in data transfer. All operators that support HSCSD have implemented national roaming agreements.
GPRS, an enhancement to the GSM system, supports data packets. It is the world's most ubiquitous wireless data service, available now with almost every GSM network. This IP-based connectivity supports a wide range of enterprise and consumer applications and provides users with throughput rates of up to 40Kbps, with the convenience of being able to connect from anywhere. GPRS customers enjoy advanced, feature-rich data services such as color Internet browsing, e-mail on the move, powerful visual communications such as video streaming, multimedia messages, and location-based services.
For operators, the adoption of GPRS is a fast and cost-effective strategy that not only supports the real first wave of mobile Internet services but also represents a big step toward 3GSM (i.e., W-CDMA) networks and services. GPRS has been integrated into GSM standards releases since Release 97. At the end of 2005, there were a total of 290 live GPRS networks.
GPRS is a packet-switched solution that works by overlaying a packet-based air interface on the eight time slots used for GSM transmissions. GPRS is an always-on service; it enables continuous flows of IP data packets over the system for applications such as Web browsing and file transfer. In theory, GPRS supports transmission speeds of up to 172.2Kbps. In practice, due to various interference issues that affect wireless communications, the experienced speeds are substantially lower than this.
Figure 14.5 shows the main elements of the GPRS network. Basically, two core modules are required: a gateway GPRS support node (GGSN) and a serving GPRS support node (SGSN). The GGSN acts as a gateway between the GPRS network and a public data network such as an IP network or perhaps even an X.25 network. The GGSN also connects to other GPRS networks to facilitate GPRS roaming. The SGSN provides packet routing to and from the SGSN service area for all users in that service area. It provides the packet-switched link to mobile stations. A few additional changes need to take place, including the addition of the packet control units, which, for the example shown in Figure 14.5, are hosted at the base station subsystem. There is a requirement for some mobility management to locate the GPRS mobile station, as well as a new air interface for packet traffic (i.e., the GPRS handsets). Additional security features also need to be implemented, including firewalls and encryption. Finally, there is a requirement for GPRS-specific signaling.
Figure 14.5. GPRS components
All in all, GPRS is a migration strategy. It allows you to add packet-based data services to the existing network with minimal changes. It does not require new frequency allocations or licensing of new spectrum, and it does not require a whole new infrastructure to be put in place.
The key applications that GPRS supports include corporate e-mail, Internet e-mail, various information services, job dispatch, remote LAN access, file transfers, Web browsing, still image transfer, and, to some extent, moving images and video and audio. The data rates may be a bit low to support video and multimedia applications with the performance that users want to see, so GPRS is largely a tool for a variety of less complex data sessions.
The GSM Association (GSMA; www.gsmworld.com) has established a technical GPRS roaming platform that it calls Roamfest. Roamfest enables executives from GSMA member operators to meet, discuss, and reach bilateral agreements for GSM, GPRS, WLAN, and MMS interworking roaming. This program has been successful as a one-stop-shop to fast-track GSM roaming agreements.
Of the 290 live GPRS networks at the end of 2005, nearly 60% had committed to the EDGE upgrade. EDGE builds on enhancements provided by GPRS and HSCSD technologies. By using EDGE, operators can handle three times more subscribers than with GPRS, triple their data rate per subscriber, or add extra capacity to their voice communications. EDGE uses the same TDMA frame structure, logic channel, and 200KHz carrier bandwidth as today's GSM networks, which allows it to be overlaid directly onto an existing GSM network. For many existing GSM/GPRS networks, EDGE is a simple software upgrade. EDGE allows the delivery of advanced mobile services such as the downloading of video and music clips, full multimedia messaging, high-speed color Internet access, and e-mail on the move. Due to the very small incremental cost of including EDGE capability in GSM network deployment, virtually all new GSM infrastructure deployments are also EDGE capable, and nearly all new mid- to high-level GSM devices also include EDGE radio technology.
EDGE basically enables a greater data transmission speed to be achieved in good conditions, especially near the base stations. It does so by implementing an 8-PSK modulation scheme instead of Gaussian Minimum-Shift Keying (GMSK). 8-PSK enables each pulse to carry 3 bits of information versus GMSK's 1 bit per pulse rate. Thus, EDGE has the potential to increase the data rate of existing GSM systems by a factor of three. For EDGE to be effective, it should be installed along with the packet-switching upgrades used for GPRS. Again, this entails the addition of the GGSN and the SGSN. In addition, an EDGE transceiver unit must be added to each cell, and the base stations must receive remote software upgrades. EDGE can coexist with GSM traffic, switching to EDGE mode automatically.
EDGE ultimately allows the combination of digital TDMA and GSM and provides an enhanced version of GPRS. It supports 48Kbps to 69.2Kbps per time slot, and by aggregating time slots, it can support up to 384Kbps. GSM's 200KHz channel spacing is also maintained in EDGE, allowing the use of existing spectrum bands. EDGE is also sometimes referred to as a 2.75G system.
At the end of 2005, there were 121 commercial EDGE networks in 70 countries, and there were 174 GSM/EDGE terminal devices available. There were also 57 mobile operators deploying combined EDGE/W-CDMA networks and 26 combined EDGE/W-CDMA networks already in service. EDGE has become a standard in most new data-enabled phones.
GERAN is a second phase of EDGE that is planned to offer data rates of up to 1,920Kbps, to support packetized voice and real-time services. GERAN enables the common evolution of GSM and TDMA toward providing full 3G services. This includes introducing support for both generic real-time services and a spectrum-efficient service for packet-switched voice, as well as interfacing to an all-IP 3G core network common with UMTS.
GERAN is a common evolution path for GSM and TDMA that intends to provide a cost-efficient means to deliver 3G services within the existing frequency bands. The 3G Partnership Project (3GPP; www.3GPP.org), discussed later in this chapter, has completed a feasibility study to define the GERAN features to be standardized and incorporated into 3GPP Release 7.
Messaging Services Standards
Part of the GSM service spectrum involves messaging, and messaging services are evolving. The enormous success of SMS seems to indicate that person-to-person messaging will be the most likely driver of mobile data revenues for some time. Enhanced Messaging Service (EMS) and Multimedia Messaging Service (MMS), in combination with growth in the use of prepaid services, are likely to become critical drivers of the mobile Internet.
EMS is an extension of SMS that enables users to send a combination of simple melodies, images, sounds, animations, and formatted text as a message to another EMS-compatible phone. EMS supports three image formats: small, which is 16 x 16 pixels; large, which is 32 x 32 pixels; and variable, which depends on the formats the phone manufacturer supports and how each individual phone model is designed. EMS also supports animated images in two formats: small, which is 8 x 8 pixels, and large, which is 16 x 16 pixels. EMS phones used predefined animations that represent various emotional states, such as sadness, skepticism, happiness, anger, and playfulness.
MMS is a further extension of both SMS and EMS designed to operate over both 2.5G and 3G networks. It makes use of newer and quicker mobile transmission methods, such as GPRS, HSCSD, EDGE, and UMTS. MMS involves attaching to messages multimedia extensions such as video and sound. MMS requires a new network infrastructure as well as MMS-enabled handsets. Unlike SMS and EMS, MMS is not limited to 160 characters per message. MMS will soon become a standard feature and the default messaging mode for mobile phones. At the end of 2005, 278 operators were offering MMS service. Although at this time MMS roaming and intercarrier exchange are not widely available, that will certainly improve as more networks are deployed and more subscribers choose MMS over SMS.
MMS supports standard image formats such as GIF and JPEG, video formats such as MPEG-4, and audio formats such as MP3, MIDI, and WAV. MMS standardization over GSM, GPRS, and W-CDMA is being managed through 3GPP. The dramatic shift toward the use of phones equipped with cameras, illustrating the growing importance of imaging from high-end smart phones down to basic midrange and even low-end mobile phones, is creating a growing market for MMS services.
G Moving Toward Broadband Wireless
Part I: Communications Fundamentals
Telecommunications Technology Fundamentals
Traditional Transmission Media
Establishing Communications Channels
Part II: Data Networking and the Internet
Data Communications Basics
Local Area Networking
Wide Area Networking
The Internet and IP Infrastructures
Part III: The New Generation of Networks
Broadband Access Alternatives
Part IV: Wireless Communications
Wireless Communications Basics
WMANs, WLANs, and WPANs
Emerging Wireless Applications