Wireless networks: current status and prospects.

Wireless Networks: CURRENT STATE AND PROSPECTS

RadioEthernet wireless network equipment is becoming increasingly widespread in Russia and the CIS countries for organizing local networks, connecting to the Internet and connecting remote networks and individual computers over long distances.

Currently, the bulk of radio equipment is manufactured in accordance with the IEEE 802.11 «Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications» standard adopted in 1997.

The largest manufacturers of RadioEthemet equipment are united in the wireless network association WLANA Wireless LAN Alliance.

Its goal is to educate consumers, test equipment and disseminate experience in its use.

WLANA includes AMD and Harris Semiconductor, developers and suppliers of equipment components and a number of manufacturing companies.

These include Aironet Wireless Communications, BreezeCOM, Cabletron Systems, Intermec Technologies, Lucent Technologies, Bay Networks Wireless LAN Group, Proxim, Symbol Technologies, Raytheon Corporation, and 3Com, which has announced that it has begun work on RadioEthemet equipment.

Radio equipment manufactured by WLANA member companies complies with the IEEE 802.11 standard,

RadioEthcrnct IEEE 802.11 standard equipment

The IEEE 802.11 RadioEthemet standard defines the procedure for organizing wireless networks at the MAC level and the physical (PHY) level.

The standard defines one version of the MAC (Medium Access Control) level and three types of physical channels.

The MAC level defines the basic structures of the network architecture and the list of services provided by this level. There are two typical versions of network architecture:

In an independent configuration (adhoc), stations can communicate directly with each other, no infrastructure is required. The configuration is simple, but the service area and capabilities of such a network are limited.

In an extended configuration, stations communicate via an access point. The access point serves the stations of the base zone, the totality of which forms an extended service area. Access points are connected to each other either by cable network segments or by radio bridges,

The standard defines a protocol for using a single transmission medium called carrier sense multiple access collision avoidance (CSMA/CA).

The probability of collisions between wireless nodes is minimized by sending a short message called ready to send (RTS) in advance.

RTS informs other nodes of the duration of the upcoming transmission and the recipient. This allows other nodes to delay transmission for a time equal to the advertised duration of the message.

The receiving station must respond to RTS with a clear to send (CTS) message. This allows the transmitting node to know whether the medium is clear and whether the receiving node is ready to receive. After receiving a data packet, the receiving node must transmit an acknowledgement (ACK) of the fact that the packet was received without errors. If an ACK is not received, the attempt to transmit the data packet will be repeated.

The data packetization specification provided by the standard prescribes the division of data into packets supplied with control and address information, it occupies about 30 bytes, after which there follows a block up to 2048 bytes long, then a 4-byte CRC code of the information block.

The standard recommends using packets of 400 bytes for a physical channel of the FHSS type and 1500 or 2048 -#8212; for a physical channel of DSSS.

The standard provides for data security, which includes authentication of the node entering the network, as well as data encryption.

For portable computers, the standard provides for a power saving mode.

A characteristic feature of RadioEthernet is the roaming mode, which allows network clients to move between access points without losing connection to the network.

At the physical level, the standard provides for two types of radio channels and one infrared range.

Both types of radio channels use spectrum spreading technology, which allows reducing the average value of the spectral power density of the signal by distributing energy in a frequency band wider than necessary to ensure a given transmission rate.

This reduces the level of interference created by other users operating in this range and provides increased noise immunity.

Both radio technologies use the ISM frequency range of 2.4-2.4835 GHz, which is intended in most countries for unlicensed use in industry, science and medicine. In Russia, the use of this range requires permission from the State Communications Supervision Authority.

The first type of radio channel — Frequency Hopping Spread Spectrum (FHSS) Radio PHY — provides a transmission rate of 1 Mbit/s (optionally 2 Mbit/s). The 1 Mbit/s version uses two-level Gaussian frequency modulation (2GFSK), and the 2 Mbit/s version uses four-level (4GFSK).

At a rate of 1 Mbit/s, the signal frequency changes over the duration of a message symbol equal to 1 μs according to the Gaussian law from the nominal value to the value +G, and returns to the nominal value.

To transmit zero, the signal frequency changes similarly, but by the value -M. For a speed of 2 Mbit/s, four levels of frequency deviation are provided, so each symbol carries 2 bits of the message.

The signal spectrum width with such modulation is 1 MHz regardless of the transmission speed.

This makes it possible to use 79 frequency positions in the range 2402-2480 MHz with a step of 1 MHz for transmission.

To improve noise immunity, a pseudo-random change of frequency positions (frequency hopping) is provided at least once every 400 ms, while the standard specifies that the data packet must be completely transmitted on one frequency.

The second type of radio channel is Direct Sequence Spread Spectrum (DSSS) Radio PHY.

This option provides for transmission at rates of 1 and 2 Mbit/s with signal spectrum expansion according to the law of the II-element Barker sequence.

At a transmission rate of 1 Mbit/s, binary phase shift keying (BPSK) is used in combination with DSSS (Fig. 1).

A one bit is represented by a II-element Barker code, and a zero bit is represented by an inverse.

The Barker code manipulates the phase of the carrier frequency oscillation. It is important to understand that the elementary symbols of the Barker code do not carry information; the bits are transmitted by the entire code at once, direct or inverse. This allows the signal to be given noise properties, which is necessary to ensure noise immunity in the complex interference environment of the ISM range. The spectrum width of such a signal is 2/Te, where Te is the duration of an elementary symbol, and at a transmission rate of 1 Mbit/s is 22 MHz.

To increase the transmission rate to 2 Mbit/s, the standard provides for the use of quadrature phase manipulation — QPSK in combination with DSSS. In the time interval used to transmit 1 bit, in this case, a dibit is transmitted — i.e. 2 message bits. This requires not 2, but 4 different signals. The order of signal formation is as follows.

Together with the main carrier oscillation (in-phase), a second one is used, shifted relative to it by 90 (quadrature). Each of these oscillations is manipulated in phase by a direct or inverse Barker sequence, and then the in-phase and quadrature oscillations are added. Thus, the signal receives four degrees of freedom, allowing the transmission of a dibit. In this case, the transmission rate is doubled while maintaining the same spectrum width as in binary transmission. Fig. 2 shows the procedure for forming two symbols of a QPSK-DSSS message.

To transmit a DSSS signal, one of 13 overlapping frequency bands defined by the standard in a total frequency band of 83.5 MHz is used.

For the infrared channel (Infrared PHY), the standard provides a rate of 1 Mbit/s (optionally 2 Mbit/s). The 1 Mbit/s version uses pulse-position modulation with 4 positions (4-PPM), and the 2 Mbit/s version uses 16-PPM.

Radio equipment of the IEEE 802.11 standard

The series of radio equipment produced by WLANA members in accordance with the IEEE 802.11 standard includes an access point and a network PC card, and often other devices.

As an analysis carried out by Diamond Communications shows, the equipment of different manufacturers has approximately the same characteristics. This is explained by the need to comply with the requirements of the standard and approximately the same level of technology development. The main difference is in the availability and functional completeness of the series.

The composition of the equipment series of the main manufacturers, manufactured using DSSS and FHSS technology, is presented in Table. 1.

The most complete series of RadioEthernet IEEE 802.11 equipment are produced by Aironet, BreezeCom, Lucent Technologies and Symbol Technologies, of which the first three are widely represented on the Russian market. The vast majority of wireless networks in Russia and the CIS countries are implemented on equipment from these manufacturers.

New speeds and ranges

No sooner had the IEEE 802.11 standard appeared than reports began to arrive about the creation of new high-speed radio equipment with a throughput of 10 or 11 Mbit/s, about the creation of personal radio networks of ultra-short range and the use of new frequency ranges.

A number of companies have begun to produce high-speed RadioEthernet equipment. Of greatest interest is the equipment of WLANA member companies: Aironet Wireless Communications and Lucent Technologies. This is due to the relatively low price and compatibility with equipment manufactured according to the IEEE 802.11 standard, at transmission rates of 1 and 2 Mbit/s.

The development of a modulation method that provides an increase in speed to 10/11 Mbit/s was carried out in compliance with the frequency limitations of the IEEE 802.11 standard; under these conditions, there is no other way to increase the transmission rate except by increasing the volume of the signal alphabet. Therefore, the developers were forced to add new degrees of freedom to the signal.

In this case, Aironet and Lucent Technologies took different paths.

Lucent Technologies proposed a modulation method called DS/PPM (Direct Sequence with Pulse Position Modulation). In-phase and quadrature signals are used to achieve a speed of 10 Mbps.

Each signal uses 8 time positions of the 11-bit Barker code, obtained by cyclically shifting the original code, 2 degrees of freedom — due to the direct or inverse Barker code and 2 levels of signal amplitude.

As a result, each of the signals has 8x2x2==2^5 degrees of freedom, and the total signal obtained by adding the in-phase and quadrature components is 2^10 degrees, which allows data to be transmitted in the same frequency band as at speeds of 1 and 2 Mbit/s, at a speed of 10 Mbit/s. When using one signal, a speed of 5 Mbit/s is ensured.

Aironet used in its equipment the element base of Harris Semiconductor, which refused to use the Barker code and replaced it with orthogonal modified Walsh codes. The ensemble of eight 8-element Walsh sequences used is presented in Table 2.

Any direct or inverse Walsh sequence can be used on each of the quadrature components, which gives 8×2=2″ degrees of freedom.

This modulation method allows transmitting 8 bits in the total channel with one symbol.

At the clock rate specified by the standard (II MHz), the message symbol has a duration 11/8 = 1.375 times shorter, therefore the symbol transmission rate, compared to the standard version of 1 Mbit/s, increases by 8×1.375 = 11 times. When using one quadrature component, the transmission rate is 5.5 Mbit/s.

The efforts of high-speed equipment developers are reflected in the work of the IEEE 802.11 committee in the form of projects to supplement the standard.

Project 802. It is dedicated to high-speed transmission in the 5 GHz range. This opportunity arose due to the presence in the USA and Canada of three frequency bands in this range, each 100 MHz wide, allocated for unlicensed use. In Europe, the 5.15-5.25 GHz range is available for this purpose.

The project is based on a new type of physical channel with orthogonal frequency division multiplexing (OFDM).

The physical layer specification provides for a transmission rate of 6-54 Mbit/s with a frequency spacing between adjacent channels of 20 MHz. Support for rates of 6, 12 and 24 Mbit/s is mandatory. Optionally — 9, 18, 36, 48 and 54 Mbit/s. The multi-rate MAC protocol mechanism proposed in the project will ensure communication at the optimal speed for each radio network device.

The 802.11b project is intended to legitimize high-speed radio equipment in the 2.4 GHz range. Transmission technologies used by Lucent Technologies and Aironet are being considered as options.

The project provides for the possibility of automatic speed reduction to 2 and 1 Mbit/s as the distance increases or the quality of the connection deteriorates.

To facilitate the transition to the high-speed standard, manufacturers of FHSS equipment have introduced an additional Frequency Agility option, which allows the DSSS signal to change frequency according to predetermined rules. It is possible to transmit the packet header at a standard speed and the packet body at an increased speed.

The 802. lie project is developing extensions to the 802. Id standard to support radio networks using high-speed technologies.

Personal Radio Networks

The Bluetooth project, named after Harold Bluetooth, a Danish king who ruled in the 10th century and united Denmark, southern Sweden and southern Norway under the Danish crown, is supposed to unite various devices into a single wireless network.

Such devices can be: access point to a cable network, computer, laptop, mobile phone, mouse, headphones, printer, etc. The distance between devices is from 10 cm to 10 m.

To implement the project, in May 1998, Ericsson, IBM, Intel, Nokia and Toshiba founded the Bluetooth Special Interest Group (SIG) consortium. More than 200 large companies showed interest in the development. The result of the joint activity should be the appearance of a built-in small-sized cheap radio module by mid-1999.

At the network level, Bluetooth assumes a microcellular structure. A microcell must have a master station and no more than seven slave stations. Information exchange between stations of different microcells is possible.

It is proposed to use FHSS signals at a transmission rate of 1 Mbit/s. Instead of Radio Ethernet, it is proposed to use the Frequency-hop/time-division-duplex (FH/TDD) protocol, in which the system's time resource is divided into 625 μs intervals called slots.

Each slot uses its own frequency for transmission, which provides 1600 hops per second, for transmission and reception — different non-overlapping sequences of frequency hops. Synchronization of transmission within the microcell is provided by the master station.

Packets have a fixed format and contain a 72-bit access code, a 54-bit header and from 0 to 2745 bits in the information part.

A packet can occupy 1, 3 or 5 slots and is always transmitted on one frequency, corresponding to the frequency of the first slot. After the end of the packet, a transition to the frequency determined by the master station occurs.

Synchronous and asynchronous (symmetrical and unsymmetrical) transmission modes are provided. Synchronous mode is used to transmit voice at a rate of 64 kbps.

In asynchronous mode, each subscriber can transmit data at a rate of 108.8-432.6 kbps with symmetric duplex and 108.8-721 kbps with asymmetric duplex transmission.

A study group on personal radio networks, the Wireless Personal Area Network (WPAN), was created within the IEEE 802.11 committee to work in this area.

Its task is to prepare proposals for the standardization of equipment for wireless connection of peripheral devices to a computer, including those developed within the Bluetooth program.

Thus, RadioEthernet equipment has received new development prospects on the threshold of the third millennium.

Detailed information on new equipment samples can be found on the Diamond Communications website at diamond.ru.