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Wireless Nirvana
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October 21, 2002
By Peter Rysavy
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It all seems so sexy on TV--the image of a luscious plate of pasta on their buddy's cell phone gets the other guys drooling. You know you want it. Now you can have it--if not the spaghetti, the data service.
After years of anticipation, next-generation cellular-data services are being deployed worldwide. In the United States, AT&T Wireless, Cingular Wireless and T-Mobile (formerly VoiceStream) are deploying GPRS (General Packet Radio Service) data technology in their GSM (Global System for Mobile Communications) networks, while Alltel, Sprint PCS, U.S. Cellular and Verizon Wireless are deploying CDMA2000 1XRTT. These new services give a big boost to networking options for remote-access solutions.
Both IP packet networking technologies offer substantial speed improvements over previous cellular technologies, including CDPD or circuit-switched data services. Is this wireless heaven? Yes and no. GPRS and 1XRTT (which stands for "one-carrier radio-transmission technology") are a huge step forward and will enable a broad range of new wireless applications, but in terms of performance, they are more like dial-up than broadband and require some care to fully exploit their capabilities. In particular, you'll need to consider throughput, latency, security, usage costs, service coverage, IP-address and data-session management, and intranet interconnections.
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Network Details
Although they're different in almost every other detail, both GPRS and 1XRTT are packet-switched--the most important improvement over earlier technologies. Data services for GSM and CDMA were circuit-switched and emulated modem connections. With packet switching, a user's modem (whether a data-capable mobile telephone, PC Card modem or PDA with integrated modem) uses only the radio channel when sending and receiving packets. Thus, packet switching uses scarce radio resources more efficiently than does GSM or CDMA circuit-switched data services, and theoretically provides users with an always-on, always-virtually-connected experience. Because cellular networks, even digital ones, were designed for circuit-switched voice communications, packet switching requires new a infrastructure to handle authentication for data services, IP-address management and network interconnections.
To the user or IT manager, GPRS and 1XRTT provide very similar services, namely the transport of IP packets to and from mobile terminals. The connection to the packet data service is functionally equivalent to a connection to an ISP. The connection begins at the mobile terminal, such as a notebook computer. A mobile application interfaces to a TCP/IP protocol stack, which interfaces to the wireless modem, which is inserted in the computer or, in the case of a data-capable cell phone, connected by a serial cable, USB cable, infrared or Bluetooth. The modem presents a PPP interface or NDIS interface. NDIS, normally used by Ethernet devices in a Microsoft Windows environment, offers better plug-and-play capabilities and is typical for PC Card devices; PPP suits tethered devices, such as mobile telephones. In the case of GPRS, the PPP connection terminates in the device; in 1XRTT, the PPP connection terminates in the network. Although this difference illustrates CDMA's greater use of IP protocols within its infrastructure, it hardly matters to users.
Across the air, GPRS and 1XRTT use different approaches, though end users probably will never know the difference. GPRS takes the GSM time-division approach: Each 200-KHz radio channel is divided into eight time slots. Normally, one time slot supports one voice user, but for packet data, GPRS can combine up to four time slots for effective throughputs of about 40 Kbps. 1XRTT, in comparison, is a DSSS (direct-sequence spread-spectrum) system that uses a much wider, 1.25-MHz radio channel and different codes to designate channels. Compared to prior CDMA versions, 1XRTT introduces high-speed supplemental channels that can operate at 16 times the fundamental voice-call channel rate. This technology allows burst rates of 144 Kbps, though typical user rates are generally 40 Kbps to 60 Kbps. In both GPRS and 1XRTT, a medium-access protocol controls which user gets to send packets at what time; data users must contend for radio resources with other data users and voice users. Operators can specify how much of their system capacity to allocate to data and to voice. Both technologies can take advantage of idle voice capacity to boost data capacity, though the converse also holds true--in a cell busy with voice traffic, only a small amount of capacity may be left for data.
Unlike Ethernet network nodes, which can monitor network activity directly and instantly detect collisions, cellular networks' mobile terminals listen to and communicate only with the base station, not with one another. This increases latency relative to wireline networks as the network must coordinate all communications. For packet communications, the mechanism to transmit data packets occurs in two stages. First, the mobile terminal uses a control channel to request a traffic channel. The control channel is a random-access channel and collisions can occur from other terminals, in which case the terminal must repeat the request. The network responds by assigning a traffic channel (specified time slots in the case of GPRS and a specified code in the case of 1XRTT) for data transmission. The complexity of managing radio access results in wireless-network latencies higher than those of a wireline network. Round-trip times of .5 seconds to 1.5 seconds are typical. Latency can slow down some applications more than throughput, especially if the application shuttles a lot of messages. Delays also can restrict what applications are feasible. Some applications, such as packetized voice, are highly sensitive to delay. On the other hand, streaming applications that are not interactive should not have a latency problem.
With both GPRS and 1XRTT, the network separates packet data from circuit data at the base-station controller and connects it to a separate packet infrastructure. The core packet nodes in GPRS are SGSN (Serving GPRS Support Node), which tracks user locations, and GGSN (Gateway GPRS Support Node), which handles IP-address management and gateways to external networks, such as the Internet. In the case of 1XRTT, the core packet node is PDSN (Packet Data Serving Node); it performs much of the same functions the SGSN and GGSN do. This internal architecture is of little consequence to users, other than the fact that server-side connections are via packet networks, and not telephone networks.
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