الجمعة، 8 أغسطس، 2008

IEEE 802.11n

IEEE 802.11n is a proposed amendment to the IEEE 802.11-2007 wireless networking standard to significantly improve network throughput over previous standards, such as 802.11b and 802.11g, with a significant increase in raw (PHY) data rate from 54 Mbit/s to a maximum of 600 Mbit/s. Most devices today support a PHY rate of 300 Mbit/s, with the use of 2 Spatial Streams at 40 MHz. Depending on the environment, this may translate into a user throughput (TCP/IP) of 100 Mbit/s[1].

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[edit] Description

IEEE 802.11n builds on previous 802.11 standards by adding multiple-input multiple-output (MIMO) and Channel-bonding/40 MHz operation to the physical (PHY) layer, and frame aggregation to the MAC layer.

MIMO uses multiple transmitter and receiver antennas to improve the system performance. MIMO is a technology which uses multiple antennas to coherently resolve more information than possible using a single antenna. Two important benefits it provides to 802.11n are antenna diversity and spatial multiplexing.

MIMO technology relies on multipath signals. Multipath signals are the reflected signals arriving at the receiver some time after the line of sight (LOS) signal transmission has been received. In a non-MIMO based 802.11a/b/g network, multipath signals were perceived as interference degrading a receiver's ability to recover the message information in the signal. MIMO uses the multipath signal's diversity to increase a receiver's ability to recover the message information from the signal.

Another ability MIMO technology provides is Spatial Division Multiplexing (SDM). SDM spatially multiplexes multiple independent data streams, transferred simultaneously within one spectral channel of bandwidth. MIMO SDM can significantly increase data throughput as the number of resolved spatial data streams is increased. Each spatial stream requires a discrete antenna at both the transmitter and the receiver. In addition, MIMO technology requires a separate radio frequency chain and analog-to-digital converter for each MIMO antenna which translates to higher implementation costs compared to non-MIMO systems.

Channel Bonding aka. 40 MHz is a second technology incorporated into 802.11n which can simultaneously use two separate non-overlapping channels to transmit data. Channel bonding increases the amount of data that can be transmitted. 40 MHz mode of operation uses 2 adjacent 20 MHz bands. This allows direct doubling of the PHY data rate from a single 20 MHz channel. (Note however that the MAC and user level throughput will not double.)

Coupling MIMO architecture with wider bandwidth channels offers the opportunity of creating very powerful yet cost-effective approaches for increasing the physical transfer rate.

[edit] Data encoding

The transmitter and receiver use precoding and postcoding techniques, respectively, to achieve the capacity of a MIMO link. Precoding includes spatial beamforming and spatial coding, where spatial beamforming improves the received signal quality at the decoding stage. Spatial coding can increase data throughput via spatial multiplexing and increase range by exploiting the spatial diversity, through techniques such as Alamouti coding.

[edit] Number of antennas

The number of simultaneous data streams is limited by the minimum number of antennas in use on both sides of the link. However, the individual radios often further limit the number of spatial streams that may carry unique data. The a \times b \colon c moniker helps identify what a given radio is capable of.[original research?] The first number (a) is the maximum number of transmit antennas or RF chains that can be used by the radio. The second number (b) is the maximum number of receive antennas or RF chains that can be used by the radio. The third number (c) is the maximum number of data spatial streams the radio can use. For example, a radio that can transmit on two antennas and receive on three, but can only send or receive two data streams would be 2 \times 3 \colon 2.

The 802.11n draft allows up to 4 \times 4 \colon 4. Common configurations of 11n devices are 2 \times 2 \colon 2, 2 \times 3 \colon 2, and 3 \times 3 \colon 2. All three configurations have the same maximum throughputs and features, and differ only in the amount of diversity the antenna systems provide.

[edit] Frame aggregation

PHY level data rate improvements do not increase user level throughput beyond a point because of 802.11 protocol overheads, like the contention process, interframe spacing, PHY level headers (Preamble + PLCP) and acknowledgment frames. The main medium access controller (MAC) feature that provides a performance improvement is aggregation. Two types of aggregation are defined:

  1. Aggregation of MAC Service Data Units (MSDUs) at the top of the MAC (referred to as MSDU aggregation or A-MSDU)
  2. Aggregation of MAC Protocol Data Units (MPDUs) at the bottom of the MAC (referred to as MPDU aggregation or A-MPDU)

Aggregation is a process of packing multiple MSDUs or MPDUs together to reduce the overheads and average them over multiple frames, thus increasing the user level data rate. A-MPDU aggregation requires the use of Block Acknowledgement or BlockAck, which was introduced in 802.11e and has been optimized in 802.11n.

[edit] Backward compatibility

When 802.11g was released to share the band with existing 802.11b devices, it provided ways of ensuring coexistence between the legacy and the new devices. 802.11n extends the coexistence management to protect its transmissions from legacy devices, which include 802.11g, 802.11b and 802.11a. There are MAC and PHY level protection mechanisms as listed below:

  1. PHY level protection: Mixed Mode Format protection (also known as L-SIG TXOP Protection): In mixed mode, each 802.11n transmission is always embedded in an 802.11a or 802.11g transmission. For 20 MHz transmissions, this embedding takes care of the protection with 802.11a and 802.11g. However, 802.11b devices still need CTS protection.
  2. PHY level protection: Transmissions at 40 MHz in the presence of 802.11a or 802.11g clients require using CTS protection on both 20 MHz halves of the 40 MHz channel, to prevent interference with legacy devices.
  3. MAC level protection: An RTS/CTS frame exchange or CTS frame transmission at legacy rates can be used to protect subsequent 11n transmission.

Even with protection, large discrepancies can exist between the throughput a 802.11n device can achieve in a greenfield network, compared to a mixed-mode network, when legacy devices are present. This is an extension of the 802.11b/802.11g coexistence problem.

[edit] Deployment Strategies

To achieve maximum throughput a pure 802.11n 5 GHz network is recommended. The 5 GHz band has substantial capacity due to many non-overlapping radio channels and less radio interference as compared to the 2.4 GHz band.[2] An all-802.11n network may be impractical, however, as existing laptops generally have 802.11b/g radios which must be replaced if they are to operate on the network. Consequently, it may be more practical to operate a mixed 802.11b/g/n network until 802.11n hardware becomes more prevalent. In a mixed-mode system, it’s generally best to utilize a dual-radio access point and place the 802.11b/g traffic on the 2.4 GHz radio and the 802.11n traffic on the 5 GHz radio.[3]

[edit] Status

Work on the 802.11n standard dates back to 2004. The draft is expected to be finalized in March 2009 with publication in December 2009,[4] but major manufacturers are now releasing 'pre-N', 'draft n' or 'MIMO-based' products based on early specs. These vendors anticipate the final version will not be significantly different from the draft, and in a bid to get the early mover advantage, are pushing ahead with the technology. Depending on the manufacturer, a firmware update may eventually be able to make current "Draft-N" hardware compatible with the final version.

[edit] Wi-Fi Alliance

As of mid-2007, the Wi-Fi Alliance has started certifying products based on IEEE 802.11n Draft 2.0.[5] This certification program established a set of features and a level of interoperability across vendors supporting those features, thus providing one definition of 'draft n'. The Baseline certification covers both 20 MHz and 40 MHz wide channels, and up to two spatial streams, for maximum throughputs of 144.4 Mbit/s for 20 MHz and 300 Mbit/s for 40 MHz (with Short Guard interval). A number of vendors in both the consumer and enterprise spaces, have built products that have achieved this certification.[6] The Wi-Fi Alliance certification program subsumed the previous industry consortium efforts to define 802.11n, such as the now dormant Enhanced Wireless Consortium (EWC). The Wi-Fi Alliance is investigating further work on certification of additional features of 802.11n not covered by the Baseline certification, including higher numbers of spatial streams (3 or 4), Greenfield Format, PSMP, Implicit & Explicit Beamforming and and Space-Time Block Coding.

[edit] Timeline

January 2004
IEEE announced that it had formed a new 802.11 Task Group (TGn) to develop a new amendment to the 802.11 standard for wireless local-area networks. The real data throughput will reach a theoretical 270 Mbit/s for the required dual stream MIMO device. (which may require an even higher raw data rate at the physical layer), and should be up to 20 times faster than 802.11b, up to 3 times faster than 802.11a, and up to 4 times faster than 802.11g.
July 2005
Previous competitors TGn Sync, WWiSE, and a third group, MITMOT, said that they would merge their respective proposals as a draft. The standardization process is expected to be completed by the second quarter of 2009.
19 January 2006
The IEEE 802.11n Task Group approved the Joint Proposal's specification, based on EWC's draft specification.
March 2006
The IEEE 802.11 Working Group sent the 802.11n Draft to its first letter ballot, allowing the 500+ 802.11 voters to review the document and suggest bug fixes, changes, and improvements.
2 May 2006
The IEEE 802.11 Working Group voted not to forward Draft 1.0 of the proposed 802.11n standard. Only 46.6% voted to approve the ballot. To proceed to the next step in the IEEE standards process, a majority vote of 75% is required. This letter ballot also generated approximately 12,000 comments—much more than anticipated.
November 2006
TGn voted to accept draft version 1.06, incorporating all accepted technical and editorial comment resolutions prior to this meeting. An additional 800 comment resolutions were approved during the November session which will be incorporated into the next revision of the draft. As of this meeting, three of the 18 comment topic ad hoc groups chartered in May have had completed their work and 88% of the technical comments had been resolved with approximately 370 remaining.
19 January 2007
The IEEE 802.11 Working Group unanimously (100 yes, 0 no, 5 abstaining) approved a request by the 802.11n Task Group to issue a new Draft 2.0 of the proposed standard. Draft 2.0 was based on the Task Group's working draft version 1.10. Draft 2.0 was at this point in time the cumulative result of thousands of changes to the 11n document as based on all previous comments.
7 February 2007
The results of Letter Ballot 95, a 15-day Procedural vote, passed with 97.99% approval and 2.01% disapproval. On the same day, 802.11 Working Group announced the opening of Letter Ballot 97. It invited detailed technical comments to closed on 9 March 2007.
9 March 2007
Letter Ballot 97, the 30-day Technical vote to approve Draft 2.0, closed. They were announced by IEEE 802 leadership during the Orlando Plenary on 12 March 2007. The ballot passed with an 83.4% approval, above the 75% minimum approval threshold. There were still approximately 3,076 unique comments, which will be individually examined for incorporation into the next revision of Draft 2.
25 June 2007
The Wi-Fi Alliance announces its official certification program for devices based on Draft 2.0.
7 September 2007
Task Group agrees on all outstanding issues for Draft 2.07. Draft 3.0 is authorized, which possibly may go to a sponsor ballot in November 2007.
November 2007
Draft 3.0 approved (240 voted affirmative, 43 negative, and 27 abstained). The editor was authorized to produce draft 3.01.[7]
January 2008
Draft 3.02 approved. This version incorporates previously approved technical and editorial comments. There remain 127 unresolved technical comments. It is expected that all remaining comments will be resolved and that TGn and WG11 will subsequently release Draft 4.0 for working group recirculation ballot following the March meeting.[7]
May 2008
Draft 4.0 approved.[7]

[edit] CSIRO Patent Issues

In late November 2007, work on the 802.11n standard slowed due to patent issues. The Australian Commonwealth Scientific and Industrial Research Organisation (CSIRO) holds the patent to a component of the 802.11n standard. This component is also part of 802.11a and 802.11g. The IEEE requested from the CSIRO a Letter of Assurance (LoA) that no lawsuits would be filed for anyone implementing the standard. In Sep 2007, CSIRO responded that they would not be able to comply with this request since litigation was involved.[8]

[edit] Comparison chart

Wireless networking standards view talk edit
802.11
Protocol
Release[4] Freq.
(GHz)
Thru.
(Mbit/s)
Data
(Mbit/s)
Mod. rin.
(m)
rout.
(m)
1997 2.4 00.9 002
~20 ~100
a 1999 5 23 054 OFDM ~35 ~120
b 1999 2.4 04.3 011 DSSS ~38 ~140
g 2003 2.4 19 054 OFDM ~38 ~140
n 2009 2.4, 5 74 248
~70 ~250[9]
y 2008 3.7 23 054
~50 ~5000

[edit] See also

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