| JPL's Wireless Communication Reference WebsiteChapter: Network Concepts and Standards |
With contributions by Torben Rune, Netplan.
Today's
typical LAN environment require costly planning and investment to build and
maintain. Radio-based LANs might soon become a more flexible and even less expensive
option in dynamic environments, than today's cable based systems. (See also:
business aspects of Wireless LANs).
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To spread or not to spread
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The European ETS 300 328 standard stipulates that frequency spreading must be used. Similarly, the U.S. FCC required spreading for equipment operating in the ISM bands. Frequency spreading is a technique in which the communications equipment is configured to use several frequencies to communicate or to intentionally use a wider bandwidth than absolutely needed. The spreading techniques initially used in wireless LAN products can be divided into a two families: Frequency Hopping Spread Spectrum (FH-SS) and Direct Sequence Spread Spectrum (DS-SS). The first approach resists interference by jumping rapidly from frequency to frequency in a pseudorandom way. The receiving system has the same pseudorandom algorithm as the sender, and jumps simultaneously. The second approach resists interference by mixing in a series of pseudorandom bits with the actual data. The receiver, using the same pseudorandom algorithms, strips out the extra bits. In a spread spectrum system, there is a possibility to accommodate signals from multiple users by assigning them different spreading keys. Such a system is called a Code Division Multiple Access (CDMA) system. In most wireless LAN products however, users belonging to the same wireless LAN utilize the same spreading key. Conflicts between transmissions from different users are resolved in time using nearly the same Carrier Sense Multiple Access (CSMA) protocol as in the Ethernet. |
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| Not all theoretical performance evaluations show that direct sequence CDMA is indeed a favorable solution for burst transmission of packets. See for instance our discussion on the use of spread spectrum in random access packet data networks and on frequency reuse in packet data nets.) | ||
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OFDM
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The IEEE 802.11 a / g/ n standards apply OFDM as the modulation method. Initially OFDM was defended as being some kind of spreading method as well. Yet currently, it is commonly accepted that OFDM is an appropriate modulation method for WLANs. |
The need for bandwidth in LAN based communication equipment has been constantly rising. New user-friendly technology such as graphics based documents and program- interfaces have strained many existing LAN infrastructures and the emergence of real time high definition video transmissions are sure to strain these infrastructures further. Standards for wireless communication have to address these challenges. The new technology had to be not only exciting but also able to meet tomorrow’s needs for bandwidth. For example, the emerging standard on compressed progressive digital video (MPEG3) currently requires individual data streams of 18 Mb pr. Second. This cannot be accommodated in any commercially available wireless equipment.
With the need for bandwidth in wireless communications also come the need to address the problems of quality of the transmission service. Especially video signals are very prone to drastic reductions of quality once the bandwidth drops even if only momentarily. Once the bandwidth drops, the video frame can freeze and the sound and the video signals might loose its synchronization.
One of the major problems concerning this technology is standardization. The problems concerning standardization are among other things making the best use of a scarce resource - the radio spectrum while at the same time making the standard attractive enough to use so the market can grow.
A number of global and national standardization initiatives have taken place in wireless technology.
WLAN largely use the Industrial, Medical & Scientific (ISM) band between 2400 and 2483.5 MHz. To use this frequency band, equipment must be compliant to the European Telecommunication Standard ETS 300 328. This standard defines the technical requirements on equipment using the 2400-2483.5 MHz frequency band. Since the 2400-2483.5 MHz frequency band is used by other types of equipment, such as micr wave ovens, techniques to avoid interference have to be used. The ETS 300 328 standard stipulates that frequency spreading must be used.
The spreading techniques initially used in wireless LAN products can be divided into two families: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). For very short range communication, also Ultra Wide Band, or pulse radio techniques have been proposed and tested.
The FH approach resists interference by jumping rapidly from frequency to frequency in a pseudorandom way. The receiving system has the same pseudorandom sequence as the sender, and jumps simultaneously. The second approach resists interference by mixing in a series of pseudorandom bits with the actual data. The receiver, using the same pseudorandom sequence, strips out the extra bits.
In a spread spectrum system, there is a possibility to multiplex users by assigning them different spreading keys. Such a system is called a Code Division Multiple Access (CDMA) system. However, most wireless LAN products are not CDMA systems since users belonging to the same wireless LAN utilize the same spreading key. Instead users are separated in time using nearly the same Carrier Sense Multiple Access (CSMA) protocol as in Ethernet.
The raw bit rate of equipment using the 2400-2483.5 MHz frequency band is normally 2 Mbit/s giving a net throughput of typically 600-800 kbit/s. In Europe, the effective radiated power from the antenna must not exceed 100 mW which is significantly lower than allowed by the Federal Communications Commission (FCC) in USA. With 100 mW effective radiated power from the antenna, the radio range is normally 20-50 meters indoor and a few hundred meter outdoor.
| DECT | Initially Digital European Cordless Telecommunications (DECT) was believed to become an imporant WLAN standard. This standard, ETS 300 175. DECT was developed by ETSI for a wide range of high-density wireless applications, both private and public, throughout Europe. The standard is supported by a range of pan-European regulatory instruments, including a pan-European frequency allocation in the 1880-1900 MHz-band now available in all member states, and Common Technical Regulations (CTR), covering type approvals in the European Union. | |
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Within the frequency allocation for DECT there are 10 channels, each of which uses Time Division Multiple Access (TDMA) and Time Division Duplex (TDD) to support 12 duplex speech channels or 24 simplex data channels. This gives a total of 120 duplex speech channels or 240 simplex data channels that can be accessed by any DECT system. A DECT system automatically chooses which of the channels to use utilizing a Dynamic Channel Allocation (DCA) algorithm. This allows multiple, independent DECT systems to operate in the same radio environment without interfering with each other. |
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| DECT works with a raw bit rate of 1152 kbit/s. When DECT is used to convey data it gives a net throughput of 552 kbit/s at a bit error rate not exceeding 10-8 and when using a single radio. Higher system capacities can be achieved by adding several base stations within the same cell. As DECT was designed to operate in residential, business and public environments, it also supports full authentication and encryption, thus ensuring that it is a suitable medium for confidential information. | ||
| One important difference between DECT and wireless LAN products using the 2400-2483.5 MHz frequency band is that higher output power and directive antennae are allowed in DECT. The DECT standard allows 250 mW plus the gain offered by directive antennae. This can give more cost-effective solutions when building larger wireless LAN infrastructures where premises can be covered from the outside by a single base station. The radio range of a DECT system is normally 30-100 meters indoors and up to 3 kilometers outdoors. | ||
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The addition of data services to a wireless PABX can also use the same installed set of base stations that already provide speech services. In particular, services such as fax, e-mail and access to files stored on a LAN can be provided. In practice, however, the market for WLAN appeared predominatly PC driven, and the packet switched IEEE 802.11 standards initiated to WLAN growth. The role of DECT is now best visible in the Bluetooth standard, which adopted very similar design choices. |
