2.2 WLANs and Cellular Networks: Comparison and Contrast
2.2 WLANs and Cellular Networks: Comparison and Contrast
Any practical communications system represents a compromise between a variety of technology and cost criteria. Some of the principal figures of merit for wireless communications systems are bit rate, mobility of terminals, signal quality, coverage area, service price, and demands on the power supplies of portable terminals.
Goals for third-generation wireless communication, enunciated in the early 1990s by the International Telecommunications Union Task Group IMT-2000, focused on the first two criteria, bit rate and mobility. Third-generation systems should deliver 2 Mbps to stationary or slowly moving terminals, and at least 144 kbps to terminals moving at vehicular speeds. Meanwhile, WLAN development has confined itself to communications with low-mobility (stationary or slowly moving) terminals, and focused on high-speed data transmission. The relationship of bit rate to mobility in cellular and WLAN systems has been commonly represented in two dimensions by diagrams resembling Figure 2.1. The principal goal of succeeding generations of cellular technology has been to move to the right in the bit rate/mobility plane. Coverage, the geographical area that a signal can reach, is a third figure of merit. The relationship between bit rate and coverage is similar to the relationship between bit rate and mobility. Cellular systems provide wide area ubiquitous coverage, while WLANs, as the name implies, cover only local areas, with large gaps between coverage areas. With respect to signal quality, a fourth figure of merit, cellular networks employ elaborate radio resources management technology to maintain high signal quality for the highest possible user population. Moreover, cellular network operators own expensive licenses, granting them exclusive use of radio spectrum in their service areas. By contrast, WLANs, operating in unlicensed spectrum bands, are vulnerable to interference from various sources, including other WLANs, cordless telephones, microwave ovens, and Bluetooth personal area networks.
Figure 2.1: Bit rate and mobility in WLAN and cellular systems.
In addition to the criteria of mobility, bit rate, coverage, and signal quality, Table 2.1 indicates that cellular terminals make greater demands on their batteries than WLAN modems. The radiated power in a cell phone can be as high as several hundred milliwatts, while WLANs transmit at a maximum of 100 milliwatts. In addition, cellular networks are far more expensive to establish and maintain than WLAN access points. As a consequence, WLAN service prices are considerably lower (in many situations, they are free) than cellular prices. Consider the fact that in a cellular network, a 1-MB file transfer uses comparable transmission resources to 500 seconds of a phone conversation (at 16 kbps speech transmission) and a few thousand short messages (at 200 characters per message). Consumers are accustomed to paying much more per bit for phone calls and short messages than for Internet access. It is a challenge to the cellular industry to establish prices for broadband services at a level high enough to compensate them for the radio resources consumed and simultaneously low enough to attract a large number of customers. The other cellular advantage in Table 2.1 is the network infrastructure of base stations, switches, routers, and databases that regulate access to a network and facilitate mobility.
| Cellular | WLAN | |
|---|---|---|
| Strong | Ubiquitous coverage | High bit rate |
| High mobility | Low power | |
| Controlled signal quality | Low cost | |
| Infrastructure | ||
| Weak | Low bit rate | Isolated coverage |
| High power | Low mobility | |
| High cost | Vulnerable to interference |
All in all, we observe in Table 2.1 that, with respect to the figures of merit for wireless communications, cellular systems and WLANs are complementary; each one is strong where the other is weak. This suggests that both technologies will play important roles in a wireless Internet. As discussed in Section 2.2.3, coordinating WLAN and cellular access to a wireless Internet is a major task for industry and the research community. Meanwhile, the WLAN and cellular industries are moving ahead with technology advances in their own domains, as described in the following sections.
2.2.1 WLAN Trends
Although WLANs have been available commercially for more than a decade, their popularity as business and consumer devices dates from around 1999, when manufacturers converged on a technology referred to as 802.11b, published by the Institute of Electrical and Electronic Engineers (IEEE). The industry committed itself to interoperability, setting up the Wireless Ethernet Compatibility Alliance (http://www.wirelessethernet.org) to ensure that equipment produced by one company will communicate with equipment produced by other companies. An 802.11b WLAN operates in the 2.4-GHz unlicensed frequency band. The signaling rate is 11 Mbps, and terminals employ CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) to share the available radio spectrum.
At any particular time, the WLAN communicates at one of four possible bit rates (1 Mbps, 2 Mbps, 5.5 Mbps, and 11 Mbps), depending on the type of information it carries and the current channel conditions. The appropriate bit rate depends on channel quality, which can be measured as carrier-to-noise ratio (CNR). In a WLAN operating environment, the distance between transmitter and receiver has the greatest influence on CNR, which decreases with increasing distance. Accordingly, in order to maximize throughput, terminals transmit information at lower bit rates when they are far from the receiver and at higher bit rates when they are near the receiver. Figure 2.2, the result of a theoretical study, [1] predicts the relationship between user throughput and distance for the four transmission rates in 802.11b. The figure indicates that a terminal can achieve maximum throughput transmitting at 11 Mbps when it is within 28 meters of the receiver. Between 28 and 38 meters, the throughput is highest at 5.5 Mbps, while 2 Mbps is preferred between 38 and 55 meters. At distances greater than 55 meters, transmission at 1 Mbps maximizes throughput.
Figure 2.2: Relationship of throughput to distance between transmitter and receiver in a WLAN.
Most WLANs transmit signals between an access point connected to an Ethernet and a laptop computer with a built-in WLAN modem or a modem contained in a plug-in card. WLANs also are capable of direct (peer-to-peer) communication between two terminals.
While the overwhelming majority of WLANs in operation conform to 802.11b, more-advanced technologies were on the drawing boards in 2002, and to a small extent marketed commercially. Two organizations guide the standardization of new WLAN technology, the IEEE (http://ieee802.org/11) and ETSI (European Telecommunications Standards Institute) (http://portal.etsi.org/bran/kta/Hiperlan/hiperlan2.asp). IEEE efforts take place within the 802.11 Working Group, which consists of a number of task groups, each labeled with a lower case letter. ETSI activity, referred to as HiperLAN2 (high performance LAN), focuses on WLAN technology operating in the 5-GHz band at a bit rate of 54 Mbps. Table 2.2, a summary of bit rates and spectrum bands for existing and emerging WLANs, shows that 802.11a technology operates at the same bit rate and in the same part of the electromagnetic spectrum as HiperLAN2. This congruence has been the stimulus for discussions on harmonizing the two technologies. [2]
| Spectrum Band (CHz) | Maximum Bit Rate (Mbps) | |
|---|---|---|
| 802.1 la | 5 | 54 |
| 802.11b | 2.4 | 11 |
| 802.11g | 2.4 | 20 |
| HiperLAN1 | 5 | 20 |
| HiperLAN2 | 5 | 54 |
2.2.2 Cellular Trends
Progress in cellular communications technology has been measured by "generations." The principal characteristics of first-generation systems, introduced in the early 1980s, were analog speech transmission over radio channels and limited built-in roaming capability. Second-generation systems, transmitting digital speech signals, were introduced in the early 1990s and today account for the overwhelming majority of cellular telephone communications. Starting with a wide array of incompatible first-generation radio transmission technologies deployed throughout the world, the number converged to four in the second generation. GSM (Global System for Mobile Telecommunications), standardized by ETSI, has by far the largest subscriber base and the most-widespread adoption geographically. The most-salient characteristic of GSM radio transmission is its TDMA (time division multiple access) technique. A GSM signal occupies a bandwidth of 200 kHz. The transmission bit rate is 270 kbps with eight digital signals sharing the same carrier. The CDMA (code division multiple access) system, conforming to Interim Standard 95 published by the TIA (Telecommunications Industry Association), has the second-largest subscriber base. It is deployed throughout North America and in several Asian countries. CDMA signals occupy a bandwidth of 1.25 MHz with a binary signaling rate of 1,228,800 chips per second. The two other digital systems are similar to one another. NA-TDMA, the North American time division multiple access system conforming to TIA Interim Standard 136, operates with a bandwidth of 30 kHz per channel and a signaling rate of 48,600 bits per second. It is deployed throughout North America and in a few countries in Latin America. PDC (Personal Digital Cellular), with a signal bandwidth of 25 kHz, is a Japanese standard similar to NA-TDMA.
In 2002, the introduction of new radio technology proceeds in two streams, one based on GSM and the other on CDMA. Both streams contain 2.5G (advanced second generation) systems, with signals confined to the existing 2G bands (200 kHz for GSM and 1.25 MHz for CDMA) and 3G systems, with signals occupying 4 or 5 MHz bandwidth. Table 2.3 is a catalog of the systems in the GSM and CDMA streams. In North America, NA-TDMA operating companies have announced technology migration paths to the GSM stream. In Japan, PDC operating companies have introduced 3G systems based on W-CDMA (wideband code division multiple access).
| Generation | Channel BW (Hz) | Channel Rate (bps) | Principal Information Format | |
|---|---|---|---|---|
| GSM | ||||
| GSM | 2 | 200 k | 271 k | Voice and circuit data |
| EDGE | 2.5 | 200 k | 813 k | Voice and circuit data |
| GPRS | 2.5 | 200 k | 271 k | Packet data |
| E-GPRS | 2.5 | 200 k | 813 k | Packet data |
| W-CDMA/FDD | 3 | 5 M | 3.84 M | Multimedia |
| W-CDMA/TDD | 3 | 5 M | 3.84 M | Multimedia |
| CDMA | ||||
| CDMA1 | 2 | 1.25 M | 1.2288 M | Voice and circuit data |
| 1XRTT | 2.5 | 1.25 M | 1.2288 M | Voice and circuit data |
| HDR | 2.5 | 1.25 M | Uplink 2.4 M Downlink 153 k | Packet data |
| CDMA2000 | 3 | 3.75 M | 3.6864 M | Multimedia |
The original second generation systems were designed to carry voice conversations, for the most part. They also carry circuit-switched data. Their enhancements (2.5G) are segregated in two categories: EDGE and 1XRTT carry voice and circuit-switched data at higher bit rates than GSM and CDMA1, respectively. On the other hand, GPRS (General Packet Radio Service), E-GPRS, and HDR are designed for packet-switched data. A principal characteristic of the 3G systems is their ability to carry a variety of traffic types. While 2G and 2.5G systems classify information as either circuit or packet oriented, 3G systems' planners classify information according to latency requirements within four categories: background, interactive, streaming, and conversational.
Although the channel signaling rates are fixed for each system, only 2G systems specify constant user throughput. All of the other systems contain "rate adaptation" technology that matches the transmission rate available at each terminal to the current channel quality, as determined by network congestion and location-specific radio propagation conditions. For example, EDGE defines 12 "modulation and coding schemes," with user bit rates ranging from 8.8 to 88.8 kbps per time slot. [3] An application can use from one to eight time slots to exchange information.
2.2.3 Uniting WLANs and Cellular
The complementary strengths and weaknesses of WLANs and cellular systems make it certain that a wireless Internet will contain both technologies. Recognizing this prospect, the technical community has turned its attention to coordination of cellular systems and WLANs. Short-term approaches to this coordination use existing network infrastructure, while more futuristic work anticipates new network architecture based on Internet protocols that inherently accommodate both types of radio access. One example based on existing infrastructure is an OWLAN (operator WLAN) [4] combining GSM subscriber management and billing mechanisms (authorization, authentication, and accounting) with WLAN radio access. A key aspect of the OWLAN is incorporation of a GSM SIM (subscriber identity module) in the subscriber equipment containing a WLAN modem. Another example uses a cellular data modem as a bridge linking the Internet with a cluster of laptop computers, all communicating with a WLAN access point. [5] The cellular modem relays data between the access point and the cellular network infrastructure operating a suite of Internet protocols.
In contrast to cellular-WLAN coordination using existing infrastructure, there is intense industry effort devoted to specification of a core network based on Internet protocols. Such a core network would serve terminals that communicate by means of WLAN, cellular, and a variety of other wired and wireless access technologies. Section 2.4.2 describes examples of work in progress on network architectures that address a broad range of technical challenges including roaming, handoff, security, and quality of service (QoS).
2.2.4 Personal Area Networks
Although cellular telephones and WLANs have attracted the greatest consumer acceptance to date, other wireless networks have a role to play in a wireless Internet. Among them personal area networks (PANs) using Bluetooth technology are the most prominent. [6] The original aim of Bluetooth was to provide low-cost, low-power connections between a variety of consumer products. One example is a Bluetooth link between a laptop computer and a 3G cell phone enabling the computer to gain access to the Internet by means of the 3G packet data infrastructure. Another example is a cordless headset linked to a cell phone or a personal stereo device. In the context of these applications, Bluetooth appears as a low-cost alternative to WLAN modems. In addition, Bluetooth also contains sophisticated ad hoc networking capabilities. These capabilities are contained in technologies built into the Bluetooth standard for creating piconets and scatternets that use Bluetooth modems to create networks linking a large number of wireless devices.
2.2.5 Technology Gaps
Each of the emerging advances in the cellular, WLAN, and PAN domains works within a region of the six-dimensional figure of merit volume (mobility, bit rate, coverage, signal quality, power, and price) described at the beginning of Section 2.2. All of them address the "last mile" or "last five meters" problem of linking devices to the Internet. An examination of the details of each of these technologies reveals that in sum they will remain inferior to wired connections consisting of Ethernets, digital subscriber lines, or cable modems connected to a 10 Gbps Internet backbone. The result will be
wireless Internet = Internet - some of the wires < Internet with wires
To get beyond these limitations, it will be necessary to create new communications paradigms that are matched directly to the requirements and constraints of the users, the information, and the operating environment of a wireless Internet. The next section adopts the theme of "geography" to formulate a framework for technology creation, and Section 2.4 describes current research within this framework.
[1]Fainberg, M. and Goodman, D., Maximizing performance of a wireless LAN in the presence of Bluetooth, Proc. 3rd IEEE Workshop on Wireless LANs, 2001.
[2]Grass, E. et al., On the single-chip implementation of a Hiperlan/2 and IEEE802.11a capable modem, IEEE Pers. Commun., 8 (6), 48–57, 2001.
[3]Eriksson, M. et al., System performance with higher level modulation in the GSM/EDGE radio access network, IEEE Globecom, 5, 3065–3069, 2001.
[4]Ala-Laurila, J., Mikkonen, J., and Rinnemaa, J., Wireless LAN access network architecture for mobile operators, IEEE Communications Magazine, 39(11), 82–89, 2001.
[5]Noerenberg, J., Bridging wireless protocols, IEEE Communications Magazine, 39(11), 90–97, 2001.
[6]Haartsen, J.C., The Bluetooth radio system, IEEE Pers. Commun., 7 (1), 28–36, 2000.
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