Wireless Data Networks by Kostas Pentikousis, VTT

Wireless Data Networks by Kostas Pentikousis, VTT ost IPJ readers are familiar with Wireless Local-Area Net- works (WLANs; see, for example, IPJ Volume 5, No. 1). M Some may even be familiar with recent developments in Wireless Metropolitan-Area Networks (WMANs), such as WiMAX. Although nonproprietary WMAN technologies are still in the standard- ization phase, the IEEE 802.11 family of protocols has reached maturity and rendered inexpensive (and often free) WLAN access increasingly popular. Both WLANs and WMANs provide high-speed connectivity (in the order of tens of Mbps), but user mobility is restricted. In fact, it is probably more appropriate to talk about “portability” rather than “mobility”[1] when referring to WLANs and WMANs. Wireless wide-area networks (WWANs), on the other hand, allow full user mobility but at data rates typically in the order of tens of kbps. This will change to some extent when third-generation (3G) cellular networks are fully deployed. Still, 3G deployment is slower than originally anticipated, a development often attributed to the combination of high spectrum license costs, the recent economic downturn, and high equip- ment costs. As a result, both population and geographical coverage tend to be uneven. For example, in Finland, a forerunner in wireless communications, population coverage is well below the 35-percent level, and geographical coverage is even smaller This article introduces several wireless network technologies, perhaps not so widely known, which deserve attention when considering how to provide mobile connectivity to field personnel, introduce machine-to- machine (M2M) communication, or deploy applications that require always-on connectivity. The approach taken in this article is a bit different from the one typically followed in the literature: We focus more on higher-level issues, the information that is essential for applica- tion developers, instead of modulation, channel coding, and other low- level details. Unlike WLANs and WMANs, none of the networks sur- veyed provide data rates in the order of tens of Mbps. Nevertheless, successful applications can be built even with stringent bandwidth limi- tations. For example, online gambling and several gaming applications can be served by really “thin” networks (and possibly “thick” clients). Cellular Networks The Global System for Mobile Communications (GSM) specifies a cellular, wide-area, circuit-switched, digital mobile phone network architecture[2]. Circuit-switched networks such as GSM and IS-95, com- monly referred to as Code Division Multiple Access (CDMA) in the United States, can provide wireless data connectivity, cover a large area, and handle mobile host handovers efficiently[3]. Users can transfer data over, say, GSM, by establishing a “dialup” connection[4]. Mobile hosts can roam, even at high speeds, and remain connected throughout. The Internet Protocol Journal 6 Communication is full-duplex at a radio data rate of 9.6 kbps or 14.4 kbps in GSM Phase 2+[5]. User throughput is always smaller than the nominal radio data rate. While the user is connected using a wireless circuit-switched network, phone calls cannot be initiated or received whether data is being transferred or not. This is not much different from wire-line dialups over basic telephone service. The difference is that a dialup over a Public Switched Telephone Network (PSTN) takes up a resource, namely the wire-line local loop, which is dedicated to a single user, whereas a dialup over a cellular network such as GSM consumes a resource, the radio channel, which is shared among many users. Because of the burstiness that data traffic usually exhibits, circuit switching may lead to inefficient use of the network capacity. Establishing a GSM dialup connection usually takes several seconds, meaning that if the user has a small amount of data to send, a small e-mail message, for example, the overall experience is poor. Moreover, after the connection is estab- lished, the channel remains idle between traffic bursts and the allocated bandwidth is wasted. Packet switching is more efficient for bursty data transmission over a shared medium[6]. Another variable that favors packet-switching over circuit-switching, es- pecially over slow wireless networks, is billing. Users of circuit-switched networks are usually charged based on the duration of a connection re- gardless of the amount of traffic transmitted or received. On the other hand, users of packet-switched networks can be charged based solely on the amount of data transferred—not how long they remain attached to the network. In short, introducing packet switching to wireless networks can lead to better use of network resources and attract more users as data transfers become more economical. Two-way, packet-switched WWANs permit users to roam freely in- doors and outdoors, even at relatively high speeds[7]. Most WWANs employ a cellular architecture to take advantage of frequency reuse and increase capacity while covering a larger area. Furthermore, because the coverage area of a single cell is generally large (cell diameters are typically in the order of dozens of kilometers), mobile hosts do not have to go through frequent and lengthy handovers. Hosts remain connected throughout after they attach to the network, permitting users to receive and transmit data on demand without having to dial up. The following sections survey some of the most widely deployed packet-switched wireless data networks. Mobitex Mobitex is the first digital data-only WWAN developed by Ericsson and Swedish Telecom. Not based on IP, Mobitex was introduced in Sweden in 1986 for emergency communications[8]. It uses a cellular architecture with cell diameters of up to 30 km. Each service area can operate 10–30 channels[9] and each base station is usually allocated 1 to 4 channels. Each channel is composed of a frequency pair: different fre- quencies are used for the uplink and the downlink. The Internet Protocol Journal 7 Wireless Data Networks: continued Communication between the base station and a single mobile host is, nevertheless, effectively half-duplex. Although base stations can transmit and receive simultaneously, mobile nodes are unable to do so[10]. The Mobitex Maximum Transmission Unit (MTU) is 545 bytes, with up to 512 bytes of user data. Although the system has undergone several revisions, the raw transfer rate remains only 8 kbps. Effective user throughputs range from 4 kbps (for 125-byte packets) to 4.6 kbps (for 512-byte packets)[11], and round-trip times can be up to 10 seconds. Mobitex deals with network lapses using a store-and-forward proce- dure: Packets destined for a mobile node outside the network coverage area are stored while awaiting delivery. When the mobile node recon- nects, the stored packets are delivered. Mobitex uses a hierarchical routing architecture that prevents local traffic from being injected into the backbone network. In other words, packets destined for a node in the range of the same base station are switched locally[8]. Besides sup- porting unicast addressing, Mobitex allows hosts to send one packet to several recipients[10]. According to the Mobitex Association (www.mobitex.org), the technology features “true push functionality,” whereby data can be pushed to both a single mobile node and a predefined group of nodes, a feature that can be very useful when trying to send an ur- gent message to field personnel. And, because the mobile host does not have to keep querying for pending data, network traffic can be kept to a minimum. All these features can also significantly boost battery life. According to the Yankee Group, despite the limited data rates, a vari- ety of applications have been developed based on Mobitex, including: burglar and fire alarm systems; paging, interactive messaging, e-mail, form-based applications, and access to databases; telemetry; credit card authorizations; field service; and fleet management. Virtually all of them require small and bursty transfers. Mobitex does not lend itself to large file transfers, e-mail with large attachments, or video transmission. In fact, file transfers of more than 20 KB used to be discouraged[8]. On the other hand, by using a slotted ALOHA[12] variation for channel access, Mobitex can provide message delivery delay guarantees and support hundreds of users within the same cell. Parsa[13] calculated that Mobi- tex can accommodate 2,000 users per channel, assuming two uplink and two downlink messages per hour. Other networks simply cannot provide tight delay bounds for such a large number of users. For example, the Mobile Data Magazine (No. 1, 2002) reported that a Korean operator launched real-time stock trading and horse gambling mobile applications with great commercial success, by guaranteeing delay bounds notwithstanding the low data rates. DataTAC DataTAC (also known as ARDIS in the United States) was developed by Motorola in the mid-1980s. DataTAC is also a non-IP based, wide- area, data-only message-oriented network. A single base station can cover an area exceeding 20 km in diameter[14]. Like Mobitex, communication between the base station and a single DataTAC mobile node is half-duplex, and mobile hosts have to compete to get access to transmit and receive data. The Internet Protocol Journal 8 Unlike Mobitex, DataTAC was designed to provide optimal in-build- ing coverage, and it uses a cellular architecture that does not take advantage of frequency reuse. Instead, a single frequency is used, in- creasing the probability that a packet transmission is successful (because the same transmission can be picked up by more than one base station), but at the expense of network capacity[8]. Bodsky notes that the U.S. DataTAC operator formerly recommended refraining from trans- ferring files larger than 10 KB. Although neither Mobitex nor DataTAC provides native IP support, middleware can take care of protocol translation and allow un- modified, off-the-shelf applications to communicate.

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