Stepping Up to HSCSD

Third-generation mobile systems trials currently are under way, and initial commercial product is expected within 18 months. Third-generation promises twice the voice capacity and support for multimedia applications with data rates up to 2Mb/s. Nevertheless, there is a sense of urgency among equipment vendors to produce interim enhancements, referred to as 2G+, to respond to carriers' immediate need for higher speed and improved access.

Today, high-speed circuit-switched data (HSCSD) provides GSM carriers with the ability to offer advanced multimedia services to their subscribers. HSCSD provides a standardized approach for combining multiples of the existing 9.6kb/s or 14.4kb/s traffic channels (time slots) for a single connection. This enables carriers to offer data and low-speed video services with bit rates ranging from 9.6kb/s to 64kb/s.

The European Telecommunications Standards Institute (ETSI) approved the HSCSD standard in early 1997. HSCSD now is being deployed worldwide by carriers that need to provide their customers with high-bandwidth services.

INTERNATIONAL DEPLOYMENTIn addition to improved performance for existing services, HSCSD has extended the range of possible multimedia services to encompass: video telephony and video teleconferencing; video services such as network games and live video camera surveillance; and mobile information services.

However, the primary market push behind HSCSD has been the capability to offer data rates equivalent to the public switched telephone network (PSTN) modem connectivity, enabling carriers to market services to current users of PSTN modems and to capitalize on the recent growth in Internet/intranet traffic.

HSCSD provides circuit-switched connections, where transmission resources (up to four time slots) are dedicated to the user for the duration of the call. This requires both the available spectrum and the operational tools necessary to manage the changing capacity demands and minimize the impact on existing voice users.

MANAGING CAPACITYHSCSD supports both service prioritization and the ability to modify the allocated resources (number of time slots) dedicated to a user during a connection. This enables a carrier to slow down the high-bit-rate users effectively in periods of high use.

HSCSD also supports non-transparent operations where the network will allocate time slots according to availability. This is particularly important during handoff, as one free time slot in the terminating cell will be sufficient to support handoff.

However, for users requesting video services such as video conferencing, multiple dedicated time slots are required throughout the call to maintain desirable call quality. For these services, carriers must have the spectrum available to support multiple-time-slot allocation in a handoff situation.

HSCSD VS. GPRSHSCSD is an ideal way of connecting if there is a continuous data stream to be transmitted. For example, video telephony requires a constant bit rate and is sensitive to transmission delay.

Nonetheless, the demand for these services is expected to be low, and carriers are focusing on variable-bit-rate data services such as e-mail and Internet web browsing. However, this type of data communications is bursty in nature, which means that the traffic channel is not required all of the time. For such data, a packet-switched connection where multiple packets from multiple users transverse the same traffic channel provides a more spectrally efficient solution.

General packet radio service (GPRS), which recently was approved by ETSI, provides GSM carriers the capability to support packet-data communications. HSCSD and GPRS can be considered complementary offerings because each supports a different type of service connectivity. However, if Internet access is the driving service, GPRS is a more spectrally efficient solution.

HSCSD, however, potentially offers two significant advantages: It is commercially available today and could provide carriers with several early-deployment marketing advantages. It also can be implemented without having to replace equipment or redesign the network.

OPPORTUNITY OR OBSOLESCENCEDespite high expectations, the demand for wireless data services in the United States is low, with estimates varying between 1 percent and 3 percent market penetration. However, data growth on landline networks is growing exponentially, largely due to the Internet and intranets, and data traffic now exceeds voice traffic on many of these networks. The expectation is that it is just a matter of time before these trends are experienced in wireless networks.

Today, the dominant GSM data service is short messaging. With HSCSD available now, GPRS expected to be commercially available within six months and 3G expected within 18 to 36 months, the timeline for making capital-equipment decisions is compressed drastically. The crucial question in a competitive environment is assessing the benefits of early deployment vs. the risk of technology obsolescence.

U.S. IMPLEMENTATION?As HSCSD combines existing traffic channels into a single connection, it keeps the implementation of new services consistent with existing network infrastructure and limits the GSM modifications primarily to data protocols in the mobile and the interworking functions of the switch. This means that HSCSD can be implemented through software upgrades and does not require a hardware upgrade to the network. However, changes to the mobile handsets are required.

In the United States, the GSM systems still are early in the deployment cycle, with many operators still integrating their billing and other operational support systems. As such, U.S. carriers are unlikely to introduce HSCSD in the near future due to the complexity of dealing with new handsets and nationwide inter-operability issues. Given the focus on Internet access and the availability of GPRS in just a few months, HSCSD implementation in the United States will be delayed if not deferred indefinitely.

HSCSD provides an important step on the path to providing high-bandwidth services. For those GSM carriers needing to offer multimedia services today, HSCSD is a simple, easy-to-implement solution. In the event that the market for multimedia is slow to take off, then HSCSD also gives carriers the ability to provide multimedia services and obviates the huge investment required to migrate to 3G.

For carriers particularly focused on Internet access, the issue is one of time to market and spectrum availability. HSCSD can be implemented today, or a carrier can delay until the more spectrally efficient GPRS becomes available later this year.

With the emphasis on wireless Internet access and the need for new handsets, the focus in the United States will be on implementing GPRS. HSCSD still is a viable technology for those carriers needing to implement premi um video services today or not planning to move to 3G in the near term.