DSL Advances

Home radio LANs may connect one or more information appliances to a home gateway and can also enable direct communication between the information appliances. Radio LANs are distinct from public wireless WANs, such as those using microwave multipoint distribution service (MMDS), local multipoint distribution service (LMDS), and satellite technology. In comparison to wireless WANs, the radio LANs provide higher throughput performance and lower cost. Wireless LANs are primarily targeted at computer- related nodes, although audio and video applications may also be supported (see Figure 12.9). Although home radio LANS are slightly more expensive than other home LAN solutions such as HomePNA, radio LANs provide attractive benefits:

• No wiring to install; LAN installation is quick and easy, and additional terminals are easily added later.

• Information appliances may be located anywhere in any room.

• Information appliances may be located in the nearby out-of-doors (e.g., poolside).

• Local mobility with no tether is especially important for the laptop and handheld computers.

• Standards permit the same wireless LAN modem to be used at other sites such as the business office, hotel, and home.

Wireless LANs have their limits due to several impairments:

• Signal loss increases with distance much more rapidly than the 1/ R ² rate for free-space radiation. Walls, furniture, plants, and even people attenuate the radio signals. Metal and reinforced concrete walls and floors are particularly limiting. Glass, wood, and plaster cause little attenuation.

• Radio frequency noise from sources in the same frequency band such as microwave ovens (2.4 to 2.45 GHz), intrusion detection alarm systems, some types of electrical motors, cordless phones, and other types of wireless LANs. For example, operating a Bluetooth wireless LAN nearby an 802.11 wireless LAN can reduce 802.11 throughput by up to 33 percent depending on the traffic patterns. Effects of the noise may be spatially localized; thus moving the radio transceiver a few feet may avoid the noise.

• Multipath distortion caused by delayed echoes of a signal being received. This effect is similar to the effect of bridged tap on DSL transmission and is illustrated in Figure 12.10. The effect of multipath distortion is often most severe in narrow portions of the frequency band.

  Figure 12.10. Multipath distortion.

• Intersymbol interference (ISI) : differences in propagation delay at different frequencies causes the received symbols to spread out in the time domain. The effects of ISI increase with symbol rate.

Another problem for wireless LANs is called a hidden node ; this occurs when node A can receive the transmissions from nodes B and C, but nodes B and C are too far from each other to receive the other's transmissions. As a result, transmissions from nodes B and C can collide without detecting the collision. To prevent this, some wireless LAN specifications, such as 802.11, provide for request-to-send (RTS) and clear-to-send (CTS) messages that are relayed through the access point. Although node B does not receive the RTS message from node C, it will receive the CTS message from the access point (node A in the example above), and thus node B will know to wait before transmitting.

Wireless LANs employ several countermeasures to overcome impairments:

• Spread spectrum transmission of a lower signal power across a wider frequency band than traditional single-carrier radio transmission. This permits reception despite strong narrowband impairments such as noise and multipath distortion. Spread spectrum transmitted signals have the additional benefit of causing less interference into other radio systems.

• Antenna diversity : since the effects of multipath distortion change greatly with position, receiving signals from multiple antennas separated by a few inches often will overcome the effects of multipath distortion.

• Forward error control (FEC) coding can correct for the effects of short term noise.

• Frequency equalization can reduce the effects of noise, signal loss, and multipath distortion.

Regional regulatory agencies, such as the FCC in the United States, set requirements for each portion of the radio frequency band. Rules are defined for each range of frequencies, including the characteristics of radio transmissions (e.g., signal power), the appropriate applications, and whether the user must be licensed. Radio LANs use RF bands that do not require user licenses. The use of an unlicensed band allows anyone to quickly and easily set-up a radio LAN, but does not provide statutory protection against RF interference from other unlicensed systems operating in the same RF band.

Figure 12.11 shows two wireless LAN configurations. The nodes in the independent basic service set (IBSS, aka an ad hoc network) connect only to each other with no wired infrastructure used other than the optional addition of a DSL or other external network connection via one of the nodes. The infrastructure based wireless LAN (also known as an extended service set, ESS) adds access points that are connected to each other via a wired infrastructure. The access point functions as a LAN hub and a gateway between the LAN and the WAN. The distributed access points permit the wireless LAN to cover a greater area, and logical segmentation of access points assigned to different radio frequencies also permits greater aggregate network capacity. As shown in the figure, one of the access points may also contain a gateway function with a link to an external network. Wireless LAN specifications such as 802.11 permit nodes to roam among many access points.

12.7.1 IEEE 802.11, 802.11b, and Other 802.11 Wireless LANs

The 802.11 wireless LAN standard for operation at 1 and 2 Mb/s was approved in 1997 by the IEEE and ISO/IEC. It was followed in 1999 by the IEEE approval of the 11 Mb/s 802.11b standard that has quickly become one of the most attractive business and home LAN solutions. Many equipment vendors adopted 802.11b, and its success is making 802.11 obsolete. IEEE 802.11b products are sometimes marketed under the Wi-Fi name , and typical 2002 prices were $90 per "access point" wireless hub plus $63 per PCMCIA adapter. Prices are expected to drop lower as the market grows toward the forecasted 3 million units per year by 2003. The original 802.11 standard consisted of three PHY specifications: infrared operating at wavelengths of 850 to 950 nm with up to 2 Watts of peak transmitted power, DSSS radio, and FHSS radio. The DSSS is the most often used PHY for 802.11 products. The radio transmissions for North America and most of Europe are in the 2.412 to 2.472 GHz frequency range; this is known as the ISM band. Japan, Spain, and France use different frequency ranges.

The 802.11 direct sequence spread spectrum (DSSS) transmission at 1 Mb/s uses differential binary phase shift keying (DBPSK) modulation and at 2 Mb/s uses differential quadrature phase shift keying (DQPSK) modulation. DSSS spreads the signal evenly across the transmission band by using an eleven-chip pseudonoise code similar to that used by code division multiple access (CDMA) mobile cellular telephone systems and global positioning system satellites . However, unlike cellular telephone systems, all 802.11 stations use the same pseudonoise code, and it operates with less processing gain (10.4 dB) to permit higher bit rates. In the United States, 802.11 transmits up to 1 Watt of RF power.

The 802.11 frequency- hopping spread spectrum (FHSS) transmission at 1 Mb/s uses 2-level Gaussian frequency shift keying (2-GFSK) modulation, and at 2 Mb/s uses 4-GFSK. FHSS transmits a relatively high amplitude signal in a small portion of the usable frequency band for 400 ms (or less), and then the transmission moves to a different sub-band. All stations hop simultaneously using the same sequence of sub-bands.

The 802.11 and 802.11b media access control (MAC) uses CSMA/CA that is similar to the CSMA/CD algorithm specified for 802.3 Ethernet. The maximum 802.11 packet size is 1,500 bytes. To improve throughput, the 802.11 and 802.11b MAC assigns highest priority to message acknowledgments (ACKs). The 802.11 standard specifies optional security features called wired equivalent privacy (WEP), including encryption and authentication. The encryption encodes messages using the RC4 algorithm with a 40-bit key plus a 24-bit node identifier (hence WEP is often said to use 64-bit encryption). Authentication assures that messages are from known nodes. This provides security appropriate for applications needing protection against casual interception, but WEP can be quickly circumvented by widely available methods . IEEE 802 is developing further security measures for applications where privacy and authentication is critical.

Furthermore, the administration of security may become difficult for installations with a very large number of stations. WEP's security is reduced by the use of a modest- sized key and the use of the same encryption key for all nodes connected to an access point. Some vendors provide proprietary security enhancements, including 128-bit encryption. Because 802.11 and 802.11b reach at most a few hundred feet, an eavesdropper must be in close proximity.

The 802.11 MAC specifies CSMA/CA for asynchronous packet-oriented distributed coordination function (DCF), and a point coordination function (PCF) for isochronous information such as voice and video. The PCF function is not suitable for LANs with a large number of stations. The 802.11's provisions for voice applications are not as complete as HomeRF.

The IEEE 802.11b standard specifies DQPSK modulation with an 8-chip DSSS code known as complementary code keying (CCK) in the 2.4 GHz ISM frequency band. 802.11b provides for backward compatibility with 802.11 DSSS systems, but the FHSS and infrared PHYs are not included in 802.11b. The 802.11b marketing makes claims of 11 Mb/s at a distance of 500 ft, and 1 Mb/s at a distance of 1700 ft. Actual user data throughput and real-world distances are far less than the marketing claims. Due to protocol overhead, the effective data throughput of 802.11b is up to 7 Mb/s for large packets and distances up to 80 ft with ideal conditions; for smaller packets the effective data rate can be less than 4 Mb/s. In typical indoor conditions beyond about 40 ft, the bit rate reduces in several steps (11, 5.5, 2, and 1 Mb/s) toward a cut-off distance of 80 to 250 ft depending on the environment. Throughput can be greatly reduced while a microwave oven is operating within 15 ft of the 802.11b device. Despite the misleading marketing claims, 802.11b serves most wireless LAN applications well and sales of 802.11b products exceed all other types of wireless LANs. Proponents of 802.11b claim that the prices for 802.11b products will become one of the lowest cost home LAN solutions.

The IEEE also developed the 802.11a standard, which operates at a gross bit rate of 54 Mb/s and has effective throughout of 24 Mb/s in the single channel mode. Bit rates of 72 to 108 Mb/s are possible in the multi-channel "turbo" mode. 802.11a uses orthogonal frequency division modulation (OFDM) in the 5 GHz band and operates over distances of up to 100 feet. 802.11a products became available during 2002, and future versions are expected to provide backward compatibility with 802.11 and 802.11b. The IEEE 802.11 committee is also considering the development of a 22 Mb/s wireless LAN standard.

The Wireless Ethernet Compatibility Alliance (WECA) is an industry association dedicated to interoperability between all 802.11 products. Information regarding wireless ethernet and interoperability programs may be found at www.standards.ieee, www.wirelessethernet.org, and www.wi-fi.net.

12.7.2 HIPERLAN

ETSI has standardized HIPERLAN that operates at gross bit rates up to 23.5 Mb/s using Gaussian minimum shift keying (GMSK) in the 5.15 to 5.3 GHz band. HIPERLAN was first implemented in 1999.

12.7.3 HomeRF

HomeRF operates at a gross bit rate of 1.6 Mb/s using a frequency shift keyed FHSS transmission defined in the shared wireless access protocol (SWAP) specification. As the name implies, HomeRF is primarily targeted at networking within the home. The key distinctions of HomeRF is its low cost: about $90 per PC and its support of both packet mode data (using a CSMA/CA MAC) and isochronous transport of up to four voice channels using 32 kb/s ADPCM coding. A portion of the capacity is reserved for time division multiple access (TDMA) for high-quality digital enhanced cordless telephony (DECT). The maximum effective data throughput is about 800 kb/s. HomeRF operates in the 2.4 GHz ISM frequency band with a nominal 100 mW of transmitted RF power, and has an effective range up to 150 ft indoors. HomeRF includes a low-complexity encryption algorithm and optional data compression to improve data throughput.

The first HomeRF products were available late in 1999, shortly after the SWAP 1.1 specification was published. The planned introduction of a new 10 Mb/s HomeRF-2 is expected. Both HomeRF-1 and HomeRF-2 products are expected to be lower cost than 802.11b products. Advocates claim that HomeRF's support of high-quality voice, video, and data is an advantage over 802.11b; however, it is likely that there will be ways for 802.11b do support these applications as well. Further information may be found at www.homerf.org.

12.7.4 Bluetooth

Named for a tenth-century Danish king, Bluetooth is designed to provide a very low-cost one Mb/s personal area network (PAN) within a radius of 30 ft. Bluetooth can easily cover a room, but not a typical house. With a transmitted RF power of 2.5 mW, Class 2 Bluetooth devices can be very compact. For example, Bluetooth devices fit within cellular phones, personal digital assistants (PDAs), and even a wristwatch. Somewhat larger Class 1 Bluetooth devices can reach about 300 ft by transmitting up to 100 mW. Bluetooth is primarily targeted at interconnecting cellular phones, PDAs, PCs, and PC peripherals without the inconvenience of wires. This permits collaboration among the electronic devices within the vicinity of a person.

Bluetooth uses Gaussian binary frequency shift keying with FHSS at 1600 hops per second within the 2.4 GHz to 2.4835 GHz ISM frequency band. The effective data throughput is 721 kb/s in one direction with 57.6 kb/s in the opposite direction, or 432.6 kb/s simultaneously in both directions. Less throughput may be realized in noisy environments (e.g., noise from 802.11b or microwave ovens nearby). Bluetooth's frequency hopping permits it to avoid narrowband noise, but the low-transmitted power makes Bluetooth highly susceptible to wideband noise. Both asynchronous (packet) and isochronous (circuit) type information transport is supported. Encryption and authentication are included for secure communications.

Bit rates up to 20 Mb/s are being considered for a future version of Bluetooth. Further information regarding Bluetooth may be found at www.bluetooth.com.