Tuesday, July 3, 2007
First, the main purpose of this weblog is to introduce the developing history and the future of 802.11 standards which is illustrated in the beginning of this blog. As a student who did not know any thing about 802.11 standards before, my acquaintance procedure is quite similar to the arrangement of this weblog. After finishing this weblog, I get understand on the framework of 802.11 standards no matter from the technology aspect or the market aspect.
The first part of this weblog is to introduce the 802.11 standards briefly and globally.
So the first and the second posts are explaining the definition of 802.11 and the various amendments, respectively.
The second part of this weblog concentrates on three standards which are utilized most frequently recently (i.e. 802.11a/b/g). Among these articles, the prosperity of every amemdemts is depicted. Besides some general idea about each standard, the stating on the technology aspect including the comparison among them was also introduced.
The third part pay more attention to the application and market level of 802.11 standards. Analysis of the wifi market in China and my point of view on 802.11 market were the subtopics.
The forth part aimed at exploring the future of 802.11, and a new standard 802.11n becomes the researching object. I analysis the attributes and the peformance of 802.11n. Then I concentrating on the future application of this standard in the market.
After taking such an overview on 802.11 standard, I can conclude that this is a very successful standard. It brought in convenience and fulfilled the dream of wireless communication. When taking a look at the future of wi-fi, I can predict that it will still play an irreplaceable role in the wireless communication domain. Also the market behind this standard is huge and profitable.
Sunday, July 1, 2007
As I know, the certificate procedure has already started since June 2007. The alliance declare that they would like to certificate it to meet with the 802.11n draft standards. There are 11 lab carrying out this procedure all over the world. And the sencond step is to meet the former 820.11(a/b/g) standards.
As there are two years since the standards was proposed, experts predict that the final standards will be finished at the end of 2007. While in fact, this regulation could not be utilized until the end of 2008 according to the situation. Many company started to use the 802.11n draft as the standards for their products, which aimed at occupy the profitable market 802.11n bring in. Of course this will also introduce some risk, and the vendor will have to modify their standards according to the final version published by Wi-Fi Alliance.
The picutre below is the sticker on the 802.11n products nowadays, we can clearly see that there is a draft signature beside the 802.11n to indicate that it is still in draft.
With its increased coverage and throughput, 802.11n enables HD video and audio-visual (AV) multimedia applications in the home environment. Increased throughput and WMM capabilities enable more reliable transport of simultaneous voice and multimedia sessions.WMM certification helps ensure high quality of Wi-Fi calls, while the increased throughput and coverage of 802.11n provides sufficient bandwidth to transport multiple video streams to Wi-Fi enabled set-top boxes or TV sets around the house.
The high bandwidth and QoS of Wi-Fi CERTIFIED 802.11n draft 2.0 systems helps ensure that an internet connection can be reliably shared by the increasing number and type of Wi-Fi enabled devices in the home without degradation of service. Higher data rates of 802.11n also increase the throughput capacity of overlapping Wi-Fi networks.The increased range of 802.11n provides coverage of the entire house, reaching farther than the legacy technology and reducing “dead spots” or low-rate areas in the home. Even single-antenna mobile devices, such as Wi-Fi phones, will enjoy the benefits of increased range and throughput of Wi-Fi CERTIFIED 802.11n by virtue of the transmit diversity capabilities, such as Space Time Block Coding (STBC), Cyclic Shift Diversity (CSD) and Transmit Beamforming.Most network transactions, including voice and data services, will benefit significantly from the 802.11n frame aggregation technology. Printing files from PCs to printers, transferring files between PCs and network drives and sharing files between PCs, laptops and other devices on the network becomes more efficient with 802.11n thanks to frame aggregation.
802.11n is enterprise-grade technology that will provide IT managers with nearly the same reliable service they have come to expect from their Ethernet networks. Mission-critical enterprise applications, such as Customer Relationship Management (CRM) and Enterprise Resource Planning (ERP) access, collaboration tools, voice and video conferencing, will all benefit from the increased throughput and range of 802.11n.The new efficiencies and enhancements of 802.11n on the MAC and PHY layers, combined with the WMM QoS capabilities, serve to improve the quality of VoIP and to increase the number of simultaneous calls on the airlink. Physical layer transmission enhancements of 802.11n, such as Space Time Block Coding (STBC), improve reception even for single receiver Wi-Fi phones by virtue of transmitting multiple copies of a data stream via multiple antennas. Multiple versions of the signal reaching the phone are received and processed with specialized decoding techniques to provide redundancy of signaling and optimize reception.Of significant benefit to the Enterprise is lower density of APs, made possible by the improved efficiencies in the MAC, enhancements in the PHY operation and longer reach. Due to faster physical layer transmissions, stations get on and off the air faster, improving the airlink efficiency. The MAC layer mechanisms such as block ACK and frame aggregation also improve the airlink efficiency by reducing the overhead of packet headers, inter-frame gaps and ACK transmissions.Legacy stations in the 802.11n network can benefit from better coverage provided by the 802.11n APs’ CSD and MRC techniques and they can also gain increased access to the airlink as the new 802.11n devices get on and off the air faster.
Campus and Municipal Networks
Campus and municipal networks typically operate in challenging environments where range is the biggest issue. 802.11n is well-equipped to improve the operating range even for single-antenna handheld devices used in such outdoors networks. Increased range of handheld devices is achieved through AP transmit and receive diversity mechanisms. Transmit diversity of APs, including STBC and transmit Beamforming, improves the downlink range performance. Receive diversity of APs, including MRC, reciprocate the transmit diversity and thus maintain the range for both the uplink and the downlink directions.
Improvements in Voice Performance
The handheld devices operating in the enterprise, campus and municipal networks enjoy improvements in VoIP performance and efficiency through protocols like, block ACK and frame aggregation. The required WMM part of the certification ensures that voice streams get priority over other classes of traffic and thus further enhances the voice service.
The developing IEEE 802.11n standard is based on MIMO (multiple-input multiple-output) air interface technology. MIMO is a significant innovation and a technology that is being adapted for use by several non-802.11 wireless data communications standards, including 4G cellular. MIMO employs a technique called spatial multiplexing to transport two or more data streams simultaneously in the same frequency channel. Spatial multiplexing is central to 802.11n and has the potential of doubling the throughput of a wireless channel when two spatial streams are transmitted. Generating multiple spatial streams requires multiple transmitters, multiple receivers and distinct, uncorrelated paths for each stream through the medium. Multiple paths can be achieved using antenna polarization or multipath in the channel.
Saturday, June 30, 2007
"The essential rationale for deployment of 3G networks -- gaining spectrum efficiencies, easing network capacity constraints, lowering operating costs, and expanding revenue opportunities through provisioning of data services -- remains intact," says Dr. Shiv K. Bakhshi, research manager for the IDC's Wireless and Mobile Network Infrastructure program. He believes that the rising popularity of MMS and picture messaging will "legitimize the culture of data consumption in a mobile environment and spur deployment of network infrastructure." But, he notes, it's not just 3G driving these developments; "public WLANs and hotspots" will also help in this development.
"The WLAN industry will continue to experience stellar growth as deployments in several key markets take place," predicts Allied Business Intelligence (ABI) analyst John W. Chang, senior analyst, and some of that growth will come at 3G's expense.
ABI reports in its Worldwide Deployments, Drivers, Players and Forecasts for 802.11x, that "Some of the leading wireless carriers worldwide, including T-Mobile (Quote), AT&T (Quote), and Verizon (Quote), have made announcements of deploying WLAN services as their 3G plans are delayed. WLAN is easier to install and costs far less than setting up a 3G network. In addition, 3G's data rate of 144 kbps, a portable data rate of 384 kbps, and an in-building fixed rate of 2 Mbps are slow, compared to that of WLAN. As WLAN moves toward 54 Mbps, it is apparent that 3G cannot compete with the data rate of WLAN. Though 3G will be deployed worldwide due to its voice capacity benefits, telecom carriers are seeing WLAN hotspots as the immediate revenue generator for data services." This view is not just that of an analyst looking at plans. On January 29, British Telecomm (BT) announced that it would be deploying 802.11b--and 802.11a soon--hotspots with three business partners. BT plans to have 4,000 hotspots in place by the summer of 2005. According to David Hughes, BT director of mobility, its BT Openzone hotspot customers will pay 10% of the price to download 1MB of data compared to a 3G user at four times the speed. In short, he declares, "At the moment, it looks like Wi-Fi is one-tenth of the price of 3G, and four times as fast." Even with 3G's much better range, which would you rather have? Some analysts, like ABI's director of automotive electronics Frank Viquez, think that, "802.11 promises to have the most potential, given its minimum raw bandwidth of 10 Mbps and dramatic growth outside the vehicle industry," even when a wireless data user is traveling at speed. Can the two technologies get along? Some experts think they can, but given the stalled economy and 802.11's lower price, deployment costs alone may cause 3G to flounder. Who knows? Instead of 3G laptops in 2007, perhaps we'll have 802.11 mobile phones.
In theory, 3G wireless networks are capable of throughput up to 384Kbps, which still puts them at the bottom end of 802.11b's range. In practice, though, 3G isn't available in the United States at all except in experimental deployments.
Instead, we have telecomms using the "3G" name for what's actually, at best, 2.5G. This is a middle step between what we currently have, 2G, basic digital service, and the science fiction speeds of 3G. With 2.5G networks, you can transfer data at rates of up to 114Kbps generally using General Packet Radio Service (GPRS) (define).
So how good is GPRS, really? David Ferris, CEO and analyst for Ferris Research, has "been testing out GPRS connections with mobile phones in major metropolitan areas in the UK and US. These are now being brought on-stream by a wide variety of mobile carriers. In a nutshell, GPRS provides an always-on connection to the Internet. To be precise, GPRS enables per-handset data rates of 9.05-107.2 Kbit/sec depending upon the coding scheme employed and time slots (from 1-8) allocated to a data packet. In practice, we're finding that transfer speeds of 400 to 1000 bytes/sec are the norm."
Translated, what this means is that 2.5G is in no way competition for 802.11 for moving data. As Ferris explains, performance like this "means that communications need to be kept short, and that, in turn, means most of them will be text-based. E-mails with attachments will usually take much too long to transfer."Still, he thinks, that "applications like instant messaging, or distributing appointment information, can be run successfully." However, instant messaging or Web browsing on 2.5G or 3G phones isn't what 802.11-enabled laptops users think of as IM or the Web. On digital phones you must use Short Messaging Service (SMS) (define) or Multimedia Messaging Service (MMS) (define). Without a special gateway between the SMS/MMS servers and consumer IM clients like AOL Instant Messenger (AIM), or business-class IM clients such as Lotus Sametime or NetLert, you can't send messages from IM to someone using MMS or SMS on a digital phone. On the Web side, for a Web page to be viewed effectively on a digital phone, the signal must be sent in Wireless Application Protocol (WAP) (define) and the page should be written, not in the usual HyperText Makrup Language (HTML) (define) used for most Web pages , but in Wireless Markup Language (WML) (define). In short, viewing Web pages with on 2.5G and 3G is inherently more problematic.3G is also much more troublesome for telecom carriers to install. To deploy it you must overhaul your wireless infrastructure and replace it. Of course, you must do the same thing with 802.11 hotspots, but while hotspots have far less range, a business class hotspot with advanced antennas also can be deployed for about $1500, while all but the smallest (pico range) 3G base stations start around six figures and move up from there. Anyone can set up a hotspot; only a telephone carrier or corporation can afford 3G base station.
Friday, June 29, 2007
The graph below illustrates the market occupation percentage of the China network operators.
Thursday, June 28, 2007
Telecom Operators Market
Home Internet Service and Provisioning of Wi-Fi Equipment: Telecom operators provide Wi-Fi internet access service for home users for a monthly fee.
Hotspot Application: Telecom operators set up a publicly-available Wi-Fi network for use by any person with an account. The fees, generally calculated by the minute of use, are paid according to the stipulations of individual operators.
Enterprise Network (Customer Premises Network)
Campus/Educational Application: Wi-Fi network is constructed by the educational institution for use by students and teachers.
Corporate Application: Wireless LAN access to a corporate network or for specialized use such as wireless data collection in an industrial environment (factory or warehouse).
Consumer/Home Use Market
Individual users purchase Wi-Fi equipment and set up their own Wi-Fi network, using the telecommunication provider’s cable network for access.
The Wi-Fi Alliance is planning its first annual China Wi-Fi Summit on September 26, 2007. 'China is very important to the Wi-Fi industry, due to increasing adoption of the technology in Chinese homes and enterprises, as well as the key role that Chinese firms play in the development of Wi-Fi equipment and services,' said Wi-Fi Alliance managing director Frank Hanzlik. 'We believe the time is right for a world-class event such as the China Wi-Fi Summit, so that Chinese and multinational firms can come together for learning and networking. 'The China Wi-Fi Summit will feature a two-day conference featuring keynote addresses and panel discussions on a wide range of technical and business topics. In addition, the conference will feature a vendor expo with product demonstrations and interoperability testing events. The Wi-Fi Alliance and BII group expect that Chinese firms and multinational corporations alike will participate in the event as demonstrators, speakers, and sponsors. This event further signals the significant growth of the Wi-Fi market, with an installed base of three hundred million users and a worldwide growth rate of 25% per year. Chinese firms are increasingly developing equipment for the world Wi-Fi market, as well as the Chinese market. According to Analysis, the size of the China market for networking equipment is expected to exceed RMB10.3 billion next year. (10RMB= 1Euro)
Wednesday, June 27, 2007
Tuesday, June 26, 2007
Monday, June 25, 2007
The table below compare the three standards from many aspects:
Sunday, June 24, 2007
Prior to the recent selection of the IEEE 802.11g draft standard for wireless local area networks (WLANs) operating up to54 megabits per second (Mbps) in the 2.4 gigahertz (GHz) band, the market was served by two non-compatible specifications, 802.11b and 802.11a. Faced with market availability of both products in late 2001, some end users were potentially confused about which technology would evolve to meet their future needs, and some networking manufacturers were unsure about which specification would be best to direct their developmental efforts. There is much to be understood about the new 802.11g draft standard, including its history, specifications and implications in the WLAN market, but this much is certain: it combines the best of the existing 802.11b and 802.11a standards, and promises a harmonized future that will encourage continued and rapid market development in 802.11 WLANs. In addition, users will benefit from higher data rates, extended range and compatibility with already installed Wi-Fi? devices.
As one might guess, 802.11g is a third wireless standard related to 802.11a and 802.11b. The 802.11g standard, which is nearing ratification by the IEEE, can be considered a mixture of both existing standards. It will use the same 2.4-GHzradio spectrum as current 802.11b equipment, but with the higher data rates, packet structure, and modulation technology of 802.11a. As such, 802.11g is sometimes viewed as an upgrade path for existing 802.11b networks, although certain technical limitations may not ideally suit it for this role. All three wireless network standards, 802.11a, b, and g, can be used together in the same local installation. In some cases, total network throughput will increase as more client devices come on line. However, some combinations will actually reduce throughput because of interference between these technologies. The total performance of these networks is sometimes less than the sum of their parts. In brief, 802.11g will be a compatible faster alter-native to 802.11b. It achieves this by operating in the same 2.4-GHzfrequency band as 802.11b, but with the faster data rates of 802.11a devices. That means that 802.11b and 802.11gdevices will communicate on the same network. An 802.11b client can talk to an 802.11g access point, and vice versa. New 802.11g devices can be introduced into an existing 802.11b network at any time. In such cases, the newer 802.11gdevices will reduce their speed and act just like 802.11b devices transmitting at 11 Mbps. Part of the 802.11g standard defines the complex negotiations that allow these two wireless networks to operate together, but these details are handled automatically by the hardware and software/firmware.
Saturday, June 23, 2007
Friday, June 22, 2007
How does 802.11a differ from 802.11b?
Both IEEE 802.11a and IEEE 802.11b are wireless LAN technology standards.
• Like Ethernet and Fast Ethernet, 802.11b and 802.11a use an identical MAC. However, while Fast Ethernet uses the same physical-layer encoding scheme as Ethernet-only faster-802.11a uses an entirely different modulation scheme called orthogonal frequency division multiplexing (OFDM).
• Because 802.11a has a range approximately half that of 802.11b, more access points are required to cover the same area in a building.
Will 802.11a replace 802.11b?
No. It's believed that the emerging IEEE 802.11a standard for wireless LANs will complement and co-exist rather than compete with the 802.11b standard. The higher data rate will prove beneficial when wireless video and multimedia applications become widespread. If you need to increase bandwidth, you can begin by deploying pockets of 802.11agear right alongside your 802.11b installation. Wi-Fi's greater range and sustainable 11 Mbps data rate complement802.11a's shorter range and 54 Mbps data rate. Because the two standards can coexist without interference risk, products could even be deployed that use both standards simultaneously, such as dual-radio access points.
Are 802.11a products backward compatible with 802.11b products?
No. Short of replacing the radios, there is currently no defined upgrade path between 2.4 GHz and 5 GHz technologies. This could prove to be a difficult selling point for 802.11a-only vendors.
What are the likely applications for 802.11a?
It's expected that 802.11a equipment makers will market products to home and SOHO users. This market segment is likely to deploy Wi-Fi for shared Internet access, and a higher bandwidth standard like 802.11a for video streaming and video sharing applications. This is because of the higher data rate and the fact that the shorter range limitations would be less of a factor for these users. Equipment using this standard could network gaming applications, devices like high definition televisions, and multiple streaming audio and video devices. The enterprise market segment will likely have deployments of both 802.11a and 802.11b for a number of years. As Wi-Fi is a much further developed standard, the following trends will persist:
• Its popularity will continue to drive down costs
• Wi-Fi certified interoperability will continue to be a catalyst for wide-spread adoption
• The risk-averse enterprise segment will continue to focus on cost savings and increasing the return on investment in Wi-Fi
• As all public access wireless deployments today are based on 802.11b, mobile professionals will continue to support Wi-Fi, as 802.11a cards won't offer connectivity.802.11a technology will likely be limited to these types of applications:
• Building-to-building connections
• Video and audio conferencing/streaming video and audio
• Data mining• Large file transfers, such as engineering CAD drawings
• Faster web access and browsing
• High worker density or high throughput scenarios, such as a trading floor with multiple net-works and numerous PCs running graphics-intensive applications
Thursday, June 21, 2007
• Ultrahigh spectrum efficiency: More data can travel over a smaller amount of bandwidth than competing technologies
• High resistance to multi-path: Reflected multi-path signals are less likely to cancel the main signal, making it much more suitable for indoor wireless networking
• Relative immunity to interference: If interference happens to block one data pathway, the other carrier waves remain unaffected
The disadvantages of OFDM include:
• Expense: Components are typically more expensive to produce due to their added complexity
• Higher Power Consumption: OFDM-based systems draw more power than 802.11b-based systems. This is a problem for notebook users.
OFDM was developed specifically for indoor wireless use and offers performance much superior to that of spread spectrum solutions. OFDM works by breaking one high speed data carrier into several lower speed sub-carriers, which are then transmitted in parallel in a DMT way, the same that is used by the ADSL modems. Each high-speed carrier is 20MHz wide and is broken up into 52 sub-channels, each approximately 300 KHz wide. OFDM uses 48 of these sub channels for data.
The advantages of IEEE 802.11a are:
• Operating speeds up to 54 Mbps.
• This difference is primarily a result of 802.11a's modulation scheme.
• The larger bandwidth allocation in the 5 GHz range can be exploited for greater data rates.
• Less interference in the 5 GHz frequency range. The crowded 2.4 GHz band is shared by cordless phones, microwave ovens, Bluetooth, and WLANs.
• Greater potential to handle more users, as a result of more radio frequency channels and increased operating bandwidth.
802.11a Implementation Barriers
• The total cost of ownership (TCO) for 802.11a must be close to that of 802.11b before wide-scale iimplementation takes place. Since the range of 802.11a (approximately 50 meters) is roughly half that of802.11b, this will be difficult.
• Unlike 802.11b, 802.11a is not accepted worldwide. For example, Japan only permits the use of a smaller band containing half the channels. And Europe is still holding onto the promise of High Performance Radio Local Area Network Type 2 (HiperLAN2). In fact, it's illegal to use 802.11a in Europe, as the standard doesn't
comply with various EU requirements.
• Furthermore, vendors are uncertain whether to deploy at 5.2 GHz or 5.8 GHz. Certain military and government installations use portions of the 5 GHz band for ground tracking stations and satellite communications, creating additional barriers to worldwide 802.11a deployment.
• OFDM is inherently less power-efficient than DSSS. This means a 54 Mbps OFDM transceiver operating at a given range will consume much more power than an 11 Mbps DSSS transceiver with the same range. This presents an extra burden on the battery life of notebook PCs.
• Currently, there is no interoperability certification available for 802.11a products. Wi-Fi certification (performed by WECA) ensures multi-vendor interoperability of 802.11b products.
• 802.11a is not compatible or inter-operable with the 802.11b protocol
• The 802.11a standard does not address growing concerns over wireless networking security
• Although there is less interference in the 5 GHz frequency range, signals at 5 GHz have a higher absorption rate, and are therefore blocked more easily by walls and other building structures.
Wednesday, June 20, 2007
The OFDM PHY layer consists of two protocol functions: first a PHY convergence function, which adapts the capabilities of the Physical Medium Dependent (PMD) system to the PHY service. This function is supported by the Physical Layer Convergence Procedure (PLCP), which defines a method of mapping the IEEE 802.11 PHY Sublayer Service Data Units (PSDU) into a framing format suitable for sending and receiving user data and management information between two or more stations using the associated PMD system. Second a PMD system whose function defines the characteristics and method of transmitting and receiving data through a wireless medium between two or more stations, each using the OFDM system.
802.11a utilizes 300 MHz of bandwidth in the 5 GHz Unlicensed National Information Infrastructure (U-NII) band. Though the lower 200 MHz is physically contiguous, the FCC has divided the total 300 MHz into three distinct 100 MHz domains, each with a different legal maximum power output. The "low" band operates from 5.15 - 5.25 GHz, and has a maximum of 50 mW. The "middle" band is located from 5.25 - 5.35 GHz, with a maximum of 250 mW. The "high" band utilizes5.725 - 5.825 GHz, with a maximum of 1 W. Because of the high power output, devices transmitting in the high band will tend to be building-to-building products. The low and medium bands are more suited to in-building wireless products. One requirement specific to the low band is that all devices must use integrated antennas.
Different regions of the world have allocated different amounts of spectrum, so geographic location will deter-mine how much of the 5 GHz band is available. In the United States, the FCC has allocated all 3 bands for unlicensed transmissions. In Europe, however, only the low and middle bands are free. Though 802.11a is not yet certifiable in Europe, efforts are currently underway between IEEE and the European Telecommunications Standards Institute (ETSI)to rectify this. In Japan, only the low band may be used. This will result in more contention for signal, but will still allow for very high performance.
The frequency range used currently for most enterprise-class unlicensed transmissions, including 802.11b, is the 2.4GHz Industrial, Scientific & Medical (ISM) band. This highly populated band offers only 83 MHz of spectrum for all wireless traffic, including cordless phones, building-to-building transmissions, and microwave ovens. In comparison, the 300 MHz offered in the U-NII band represents a nearly four-fold increase in spectrum; all the more impressive when considering there is limited wireless traffic in the band today.
802.11a represents the next generation of enterprise-class wireless LAN technology, with many advantages over current options. At speeds of 54 Mbps and greater, it is faster than any other unlicensed solution. 802.11a and 802.11b both have a similar range, but 802.11a provides higher speed throughout the entire coverage area. The 5 GHz band in which it operates is not highly populated, so there is less congestion to cause interference or signal contention. And, the 8 non overlapping channels allow for a highly scalable and flexible installation. 802.11a is the most reliable and efficient medium by which to accommodate high-bandwidth applications for numerous users.
Tuesday, June 19, 2007
It is a Very High-Speed, Highly Scalable Wireless LAN Standard.
The Institute of Electrical and Electronics Engineers [IEEE] has developed 802.11a, a new specification that represents the next generation of enterprise-class wireless LANs. Among the advantages it has over current technologies are greater scalability, better interference immunity, and significantly higher speed, up to 54 Mbps and beyond, which simultaneously allows for higher bandwidth applications and more users. This paper provides an overview, in basic terms, of how the 802.11a specification works, and its corresponding benefits.
IEEE 802.11 standard specifies a 2.4 GHz operating frequency with data rates of 1 and 2 Mbps using either Direct Sequence Spread Spectrum (DSSS) or Frequency Hopping Spread Spectrum (FHSS). The IEEE 802.11a standard specifies an OFDM physical layer (PHY) that splits an information signal across 52 separate sub-carriers to provide transmission of data at a rate of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps. In the 802.11a IEEE standard the 6, 12, and 24Mbps data rates are mandatory. Four of the sub-carriers are pilot sub-carriers that the system uses as a reference to disregard frequency or phase shifts of the signal during transmission.
In the 802.11a standard, a pseudo binary sequence is sent through the pilot sub-channels to prevent the generation of spectral lines. In the 802.11a, the remaining 48 sub-carriers provide separate wireless pathways for sending the information in a parallel fashion. The resulting subcarrier frequency spacing in the IEEE 802.11a standard is 0.3125 MHz(for a 20 MHz with 64 possible subcarrier frequency slots).
802.11a standard, the primary purpose of the OFDM PHY is to transmit Media Access Control (MAC) Protocol Data Units (MPDUs) as directed by the 802.11 MAC layer. The OFDM PHY of the 802.11a standard is divided into two elements: the Physical Layer Convergence Protocol (PLCP) and the Physical Medium Dependent (PMD) sublayers. The MAC layer of 802.11a standard communicates with the PLCP via specific primitives through a PHY service access point. When the MAC layer instructs, the PLCP prepares MPDUs for transmission. The PLCP also delivers incoming frames from the wireless medium to the MAC layer. The PLCP sublayer minimizes the dependence of the MAC layer on the PMD sublayer by mapping MPDUs into a frame format suitable for transmission by the PMD.
Under the direction of the PLCP, the PMD provides actual transmission and reception of PHY entities between two stations through the wireless medium. To provide this service, the PMD interfaces directly with the air medium and provides modulation and demodulation of the frame transmissions. The PLCP and PMD communicate using service primitives to govern the transmission and reception functions.With 802.11a OFDM modulation, the binary serial signal is divided into groups (symbols) of one, two, four, or six bits, depending on the data rate chosen, and converted into complex numbers representing applicable constellation points. Ifa data rate of 24 Mbps is chosen, for example, then the PLCP maps the data bits to a 16QAM constellation. After mapping, the PLCP normalizes the complex numbers in the 802.11a standard to achieve the same average power for all mappings. The PLCP assigns each symbol, having duration of 4 microseconds, to a particular subcarrier. An Inverse Fast Fourier transform (IFFT) combines the sub-carriers before transmission.
As with other 802.11 based PHYs, in the 802.11a the PLCP implements a clear channel assessment protocol by reporting a medium busy or clear to the MAC layer via a primitive through the service access point. The MAC layer uses this information to determine whether to issue instructions to actually transmit an MPDU. The 802.11a standard requires receivers to have a minimum sensitivity ranging from -82 to -65 dBm, depending on the chosen data rate.
Monday, June 18, 2007
The next 48 bits are collectively known as the PLCP header. The header contains four fields: signal, service, length and HEC (header error check). The signal field indicates how fast the payload will be transmitted (1, 2, 5.5 or 11 Mbps). The service field is reserved for future use. The length field indicates the length of the ensuing payload, and the HEC is a 16bits CRC of the 48 bits header.In a wireless environment, the PLCP is always transmitted at 1 Mbps. Thus, 24 bytes of each packet are sent at 1 Mbps. The PLCP introduces 24 bytes of overhead into each wireless Ethernet packet before we even start talking about where the packet is going. Ethernet introduces only 8 bytes of data. Because the 192 bits header payload is transmitted at 1Mbps, 802.11b is at best only 85 percent efficient at the physical layer.
The IEEE 802.11b is a Direct Sequence Spread Spectrum (DSSS) system very similar in concept to the CDMA Wireless, using a spread spectrum chip sequence. In the 802.11b the transmission medium is wireless and the operating frequency band is 2.4 GHz. 802.11b provides 5.5 and 11 Mbps payload data rates in addition to the 1 and 2 Mbps rates provided by 802.11. To provide the higher rates, 8 chip Complementary Code Keying (CCK) is employed as the modulation scheme. The CCK uses 6 bits to encode the code sent, this increase the speed of the 802.11 by 6.Thechipping rate is 11 MHz, which is the same as the DSSS system as described in 802.11, thus providing the same occupied channel bandwidth. 802.11b describes an optional mode replacing the CCK modulation with packet binary convolutional coding (HR/DSSS/PBCC).
Another optional mode of 802.11b allows data throughput at the higher rates (2, 5.5, and 11 Mbps) to be significantly increased by using a shorter PLCP preamble. This mode is called HR/DSSS/short. This Short Preamble mode can coexist with DSSS, HR/DSSS under limited circumstances, such as on different channels or with appropriate CCA mechanisms. The High Rate PHY contains three functional entities: the PMD function, the physical layer convergence function, and the layer management function. For the purposes of MAC and MAC Management when channel agility is both present and enabled, the High Rate PHY shall be interpreted to be both a High Rate and a frequency hopping physical layer. The High Rate PHY service shall be provided to the MAC through the PHY service primitives. To allow the MAC to operate with minimum dependence on the PMD sub layer, a physical layer convergence procedure (PLCP)sub layer is defined. This function simplifies the PHY service interface to the MAC services. The PMD sub layer provides a means and method of transmitting and receiving data through a wireless medium (WM) between two or more STAs each using the High Rate system. The PLME performs management of the local PHY functions in conjunction with the MAC management entity.
The wireless radio generates a 2.4 GHz carrier wave (2.4 to 2.483 GHz) and modulates that wave using a variety of techniques. For 1 Mbps transmission, BPSK (Binary Phase Shift Keying) is used (one phase shift for each bit). To accomplish 2 Mbps transmission, QPSK (Quadrature Phase Shift Keying) is used. QPSK uses four rotations (0, 90, 180and 270 degrees) to encode 2 bits of information in the same space as BPSK encodes. The trade-off is increase power or decrease range to maintain signal quality. Because the FCC regulates output power of portable radios to 1 watt EIRP(equivalent isotropic radiated power), range is the only remaining factor that can change. On 802.11 devices, as the transceiver moves away from the radio, the radio adapts and uses a less complex (and slower) encoding mechanism to send data.
The MAC layer communicates with the PLCP via specific primitives through a PHY service access point. When the MAC layer instructs, the PLCP prepares MPDUs for transmission. The PLCP also delivers incoming frames from the wireless medium to the MAC layer. The PLCP sub layer minimizes the dependence of the MAC layer on the PMD sub layer by mapping MPDUs into a frame format suitable for transmission by the PMD.
Under the direction of the PLCP, the PMD provides actual transmission and reception of PHY entities between two stations through the wireless medium. To provide this service, the PMD interfaces directly with the air medium and provides modulation and demodulation of the frame transmissions. The PLCP and PMD communicate using service primitives to govern the transmission and reception functions. The CCK code word is modulated with the QPSK technology used in 2 Mbps wireless DSSS radios. This allows for an additional 2 bits of information to be encoded in each symbol. Eight chips are sent for each 6 bits, but each symbol encodes 8 bits because of the QPSK modulation. The spectrum math for 1 Mbps transmission works out as 11 Mchips per second times 2 MHz equals 22 MHz of spectrum. Likewise, at 2 Mbps, 2 bits per symbol are modulated with QPSK, 11 Mchips per second, and thus have 22 MHz of spectrum. To send 11 Mbps 22 MHz of frequency spectrum is needed. It is much more difficult to discern which of the 64 code words is coming across the airwaves, because of the complex encoding. Furthermore, the radio receiver design is significantly more difficult. In fact, while a 1 Mbps or 2 Mbps radio has one correlator (the device responsible for lining up the various signals bouncing around and turning them into a bit stream), the 11 Mbps radio must have 64 such devices.
Sunday, June 17, 2007
With FHSS, a transmitting and receiving station are synchronized to hop from channel to channel in a predetermined pseudorandom sequence. The prearranged hop sequence is known only to the transmitting and receiving station. In the U.S. and Europe, IEEE 802.11 specifies 79 channels and 78 different hop sequences. If one channel is jammed or noisy, the data is simply retransmitted when the transceiver hops to a clear channel. 802.11 networks using FHSS are limited to1- and 2-Mbps data rates.
Under DSSS, each bit to be transmitted is encoded with a redundant pattern called a chip, and the encoded bits are spread across the entire available frequency band. The chipping code used in a transmission is known only to the sending and receiving stations, making it difficult for an intruder to intercept and decipher wireless data encoded in this manner. The redundant pattern also makes it possible to recover data without retransmitting it if one or more bits are damaged or lost during transmission. DSSS is used in 802.11b networks.
Wi-Fi Certification Program
In a heterogeneous wireless network environment, it is important to select 802.11b standards-based wireless products that are interoperable. The main measure of 802.11b equipment interoperability is the Wireless Fidelity (Wi-Fi)certification program. (802.11b networks are sometimes referred to as Wi-Fi networks.) Administered by the industrygroup, Wireless Ethernet Compatibility Alliance (WECA), the Wi-Fi logo on a product certifies its interoperability with other products containing the logo. An independent lab, the Agilent/Silicon Valley Networking Labperforms the actual testing. The Wi-Fi interoperability program tests for association and roaming capabilities, throughput, and required features such as 64-bit encryption. WECA tracks standards developments and enhances the interoperability testing to reflect these advancements.
802.11b Options and Proprietary Extensions
Some vendors differentiate their 802.11 products with additional features. Some are options in the 802.11 standard such as 128-bit encryption, and some are proprietary features such as security/authentication schemes, roaming capabilities, key management, and "Power Over Ethernet." Using or enabling proprietary extensions usually requires that the wireless equipment, including APs and network cards, be supplied by a single vendor. Proprietary extensions are not suitable for heterogeneous environments with a mix of hardware. Although the extensions provide specific benefits, they limit future flexibility. Before choosing to implement these features, it is important to assess all the environments that must be supported in addition to the corporate wireless LAN, including home office and public (airports and hotels)wireless LANs.
Infrastructure Mode vs. Ad hoc Mode
802.11b networks can be implemented in "infrastructure" mode or "ad hoc" mode. In infrastructure mode-referred to inthe IEEE specification as the basic service set-each wireless client computer "associates" with an access point (AP) via a radio link. The AP connects to the 10/100-megabits per second (Mbps) Ethernet enterprise network using a standardEthernet cable, and provides the wireless client computer with access to the wired Ethernet network. Ad hoc mode isthe peer-to-peer network mode, which is suitable for very small installations. Ad hoc mode is referred to in the 802.11b specification as the independent basic service set.
Saturday, June 16, 2007
Friday, June 15, 2007
IEEE 802.11 - THE WLAN STANDARD was original 1 Mbit/s and 2 Mbit/s, 2.4 GHz RF and IR standard (1997), all the others listed below are Amendments to this standard, except for Recommended Practices 802.11F and 802.11T.
IEEE 802.11a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001)
IEEE 802.11b - Enhancements to 802.11 to support 5.5 and 11 Mbit/s (1999)
IEEE 802.11c - Bridge operation procedures; included in the IEEE 802.1D standard (2001)
IEEE 802.11d - International (country-to-country) roaming extensions (2001)
IEEE 802.11e - Enhancements: QoS, including packet bursting (2005)
IEEE 802.11F - Inter-Access Point Protocol (2003) Withdrawn February 2006
IEEE 802.11g - 54 Mbit/s, 2.4 GHz standard (backwards compatible with b) (2003)
IEEE 802.11h - Spectrum Managed 802.11a (5 GHz) for European compatibility (2004)
IEEE 802.11i - Enhanced security (2004)
IEEE 802.11j - Extensions for Japan (2004)
IEEE 802.11k - Radio resource measurement enhancements (proposed - 2007?)
IEEE 802.11l - (reserved and will not be used)
IEEE 802.11m - Maintenance of the standard; odds and ends. (ongoing)
IEEE 802.11n - Higher throughput improvements using MIMO (multiple input, multiple output antennas) (September 2008)
IEEE 802.11o - (reserved and will not be used)
IEEE 802.11p - WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (working - 2009?)
IEEE 802.11q - (reserved and will not be used, can be confused with 802.1Q VLAN trunking)
IEEE 802.11r - Fast roaming Working "Task Group r" - 2007?
IEEE 802.11s - ESS Extended Service Set Mesh Networking (working - 2008?)
IEEE 802.11T - Wireless Performance Prediction (WPP) - test methods and metrics Recommendation (working - 2008?)
IEEE 802.11u - Interworking with non-802 networks (for example, cellular) (proposal evaluation - ?)
IEEE 802.11v - Wireless network management (early proposal stages - ?)
IEEE 802.11w - Protected Management Frames (early proposal stages - 2008?)
IEEE 802.11x - (reserved and will not be used, can be confused with 802.1x Network Access Control)
IEEE 802.11y - 3650-3700 Operation in the U.S. (early proposal stages - ?)
There is no standard or task group named "802.11x". Rather, this term is used informally to denote any current or future 802.11 amendment, in cases where further precision is not necessary. (The IEEE 802.1x standard for port-based network access control, is often mistakenly called "802.11x" when used in the context of wireless networks.)
802.11F and 802.11T are stand-alone documents, rather than amendments to the 802.11 standard and are capitalized as such.
Tuesday, June 12, 2007
A Linksys Residential gateway with a 802.11b radio and a 4-port ethernet switch. A Compaq 802.11b PCI cardThe 802.11 family currently includes multiple over-the-air modulation techniques that all use the same basic protocol. The most popular techniques are those defined by the b/g and are amendments to the original standard; security was originally purposefully weak due to multi-governmental meddling on export requirements and was later enhanced via the 802.11i amendment after governmental and legislative changes. 802.11n is a new multi-streaming modulation technique that has recently been developed; the standard is still under draft development, although products based on proprietary pre-draft versions of the standard are being sold. Other standards in the family (c–f, h, j) are service amendments and extensions or corrections to previous specifications. 802.11b was the first widely accepted wireless networking standard, followed by 802.11g and then 802.11n.
802.11b and 802.11g standards use the 2.4 GHz (gigahertz) band, operating (in the United States) under Part 15 of the FCC Rules and Regulations. Because of this choice of frequency band, 802.11b and 802.11g equipment will suffer interference from microwave ovens, cordless telephones, Bluetooth devices, baby and security monitors, amateur radio and other appliances using this same band. The 802.11a standard uses a different 5 GHz band, which is clean by comparison. 802.11a devices are not affected by products operating on the 2.4 GHz band.
The segment of the radio frequency spectrum used varies between countries. While it is true that in the U.S. 802.11a and g devices may be legally operated without a licence. Unlicensed (legal) operation of 802.11 a & g is covered under Part 15 of the FCC Rules and Regulations. Frequencies used by channels one (1) through six (6) (802.11b) fall within the range of the 2.4 gigahertz amateur radio band. Licensed amateur radio operators may operate 802.11b/g devices under Part 97 of the FCC Rules and Regulations, allowing increased power output but not allowing any commercial content or encryption.