IEEE n Enterprise Class Wireless LAN?

Transcription

1 Introduction Over the last decade Wi-Fi has advanced from a technology offering a maximum 2Mbps over-theair data rate, to 11Mbps and now to 54Mbps. The technology has been improved to include additions such as quality of service, data security, centralised network management, and so on. Wi-Fi is now pervasive, in the home, to provide untethered access to broadband Internet routers, in shopping malls and public hot-spots, and in healthcare, manufacturing and other vertical markets. Despite all this hype and quoted data rates, Wi-Fi technology is severely hampered by protocol overheads and the fact that it is a shared medium. The actual throughput is about half the quoted data rate and then this has to be shared amongst all the users in the network. Today s consumers, with their smart phones and tablet PCs, are running data hungry multimedia applications such as video streaming and mobile TV. Hospitals, manufacturing facilities, emergency services and other markets have now become wireless savvy and are uploading and downloading video and images that can make the wireless networks strain for more bandwidth. All in all, for Wi-Fi to keep up with the expectations placed upon it, the performance had to improve dramatically. In September 2009, the IEEE ratified the standard IEEE n-2009, a new wireless LAN standard that will offer (at a minimum) five times as much throughput as any existing wireless LAN, together with improved range, more predictable coverage, and the opportunity to increase data throughput to over 300Mbps. This document describes the IEEE n technology. In order to put n into context, we first present a brief history of wireless LAN technologies. From that base, we describe the advances made in both the Physical and MAC layers of n, in order to produce these new levels of performance. Having described the technology behind n, we highlight the marketplaces that may be served by this new Wi-Fi technology and the applications that can be supported. The History of Wireless LAN Technologies Since the release of the original Wireless LAN Standard (IEEE ) in 1997, there have been many amendments to the standard, describing advances such as performance enhancement, security, quality of service, and management protocols, amongst others. For the purposes of this document, we will consider four amendments to the standard that describe progressive performance enhancements in Wireless LAN: b, a, g and n.

2 The development of WLAN standards and certifications may be charted through the activities of two main bodies, The IEEE, who set the standards for all WLAN releases and The Wi-Fi Alliance, who provide a Wi-Fi Certification Program, to certify the interoperability and quality of Wireless LAN products based on the IEEE standards. Both of these organisations, in turn, must work within the radio spectrum regulations laid down by the regulatory authorities within a particular country, The FCC or Ofcom. The relevant Wireless LAN standard and subsequent amendments leading up to n are as follows: IEEE was the original Wireless LAN standard. Released in 1997, it described both Direct Sequence and Frequency Hopping Spread Spectrum systems, operating at 1Mbps or 2Mbps in the 2.4GHz band and an Infrared Phy operating at 1Mbps. Low consumer acceptance of the original WLAN was probably due to a combination of the low bit rates and the expense of this new technology. IEEE b was ratified in 1999, operating in the 2.4GHz ISM band and offering data rates up to 11Mbps b was the first WLAN technology to be branded Wi-Fi by the Wi-Fi Alliance. A data rate of 11Mbps, combined with the convenience of mobility and the certification of the Wi-Fi Alliance, meant that b became accepted as the Wireless LAN technology of choice for the consumer and SOHO alike. IEEE a was ratified in 1999, operating in the 5GHz band and using OFDM technology, it offers data rates up to 54Mbps. The higher frequency of operation means the signal will not travel as far as 2.4GHz and signals and will be more readily absorbed by walls and other objects in the environment. The frequency of operation also means there will be no compatibility with b. IEEE a was not widely accepted at the time of its release, but recently has seen an upsurge in popularity, particularly for Voice over WLAN. IEEE g was ratified in Operating at 2.4GHz, it uses OFDM technology to provide data rates up to 54Mbps. and is backward compatible with b. IEEE g gained traction and has been widely adopted in homes, businesses and in Wi-Fi hot spots. A major drawback of an g network is that the mechanism for coexistence with b networks drastically reduces the throughput of the 11g network. The three amendments to the standard, b, a, and g, described above, all address improvements in performance. When a WLAN technology is described as above, the data rate quoted is normally the over- the-air data rate. In fact, the data throughput seen by the user is normally around half of this figure, due to protocol overheads, acknowledgements etc. The relative performance figures for the standards described above are highlighted in Figure 1. IEEE Standard Over the air Data Rate ~ Data Rate at MAC Layer SAP Comments Mbps 1 Mbps Original IEEE standard, no longer used in b 11 Mbps 5 Mbps Original Wi-Fi standard still pervasive a 54 Mbps 26 Mbps Choice for Voice over Wi-Fi before 11n appeared g 54 Mbps 26 Mbps Much slower when b present in network n 200 Mbps 100Mbps Minimum throughput requirements Up to 600 Mbps Up to 400 Mbps With 40MHz Channels + MIMO + MAC Enhancements Figure 1: Typical Over-the-air (OTA) and MAC layer throughput for given IEEE standard Copyright: Cellular Asset Management Ltd Page 2

3 In addition to this apparent reduction in our useable data rate, the throughput rates of 5Mbps and 26Mbps are actually shared between the numbers of users on the network at any given time. Therefore, in a crowded environment we could well end up with less than 1Mbps per user. Despite these limitations, Wi-Fi has become a pervasive technology. The convenience of mobility has taken precedence over outright performance. Applications such as internet access in the home, for multiple users sharing one broadband connection, internet access in hot-spots within shopping malls and commercial mobile-data collection facilities, have all driven the demand for Wi-Fi technologies. However the demand for today s multimedia applications, Voice over IP, video streaming and other high data rate applications requires more performance than we can get from the technologies descried thus far. The next step in performance enhancement is the development of n, (the subject of this paper), a major advance in wireless LAN technology, which may see WLAN become a viable replacement for wired ethernet networks In September 2003 the Task Group (TGn) was formed to define modifications to the PHY and MAC layers to enable throughputs of at least 100 Mbps measured at the MAC data service access point. The process of designing, certifying and ratifying the n standard took almost seven years to complete. During that time an interim specification was released by the IEEE, called Draft 2.0, against which the Wi-Fi Alliance produced a certification program Wi-Fi CERTIFIED n draft 2.0 In September 2009, the standard IEEE n-2009 was finally ratified. Following the ratification of the standard in 2009, the Wi-Fi Alliance, began the Wi-Fi CERTIFIED n interoperability test program One of the major concerns of potential end users of n technology, particularly in the enterprise space was that the final ratified standard may differ from the earlier Draft 2.0 release, rendering any equipment purchased before September 2009 non-interoperable. However, in the release document for the Wi-Fi CERTIFIED n interoperability test program, the Wi-Fi Alliance made the following statement... all products certified under the draft-n program meet the core requirements of, and interoperate with, the updated program, they are eligible to use the Wi-Fi CERTIFIED n logo without retesting. (Wi-Fi Alliance 2009) This opened the door for n to become the new Wi-Fi standard of choice for extremely high bandwidth, low latency and efficient communications. Since the release of the standard and certification programs n is being considered as a viable option across most enterprise platforms Copyright: Cellular Asset Management Ltd Page 3

4 IEEE n The n amendment introduces a range of enhancements at both the Physical and MAC layers that will improve the range, reliability and throughput of WLAN systems compared with current networks: The radio interface can comprise multiple spatial streams, transmitted over MIMO antenna systems, using spatial diversity to improve range and reliability, and spatial multiplexing to increase throughput n can be deployed using 40 MHz channel bandwidth to increase throughput and efficiency. Mechanisms including data frame aggregation and block acknowledgements are used in the MAC layer to reduce protocol overhead and effectively increase throughput. Copyright: Cellular Asset Management Ltd Page 4

5 MIMO Technology and Beamforming MIMO Multiple-In-Multiple-Out is a signal processing technique that uses multiple antennas at the transmitter and at the receiver (in the client and the access point). Whereas in traditional WLAN systems, multipath signal propagation between the transmitter and the receiver can cause destructive interference, MIMO technology can actually use the multipath to achieve gains in reliability and operating range. In a MIMO system, data is arranged into spatial streams that are transmitted between the multiple antennas on the transmitter and the receiver, Figure2. Depending on the particular MIMO technique used, we can increase the data rate, increase the reliability of decoding the signal at the receiver (increase SNR), or focus an RF beam on a particular receiver to increase the distance of reliable transmission. Figure 2. Multipath reflections (spatial streams) between MIMO Access Point and Client Spatial multiplexing is a method of increasing link capacity, whereby the information to be transmitted is divided into independent and separately encoded data streams and each stream is sent simultaneously from a separate antenna (multiple spatial streams). If we have N transmit antennas then we must also have N receive antennas. If the resultant data streams are sufficiently uncorrelated to be distinguishable from one another, then the total throughput of the link will be N times the single data stream. By using spatial diversity we can increase the range and/or reliability of a link by transmitting the data over multiple independent fading signal paths and making use of the low probability of these signal paths experiencing deep fades simultaneously. By placing the antennas at the both the transmitter and the receiver 1 2 wavelength apart, we can combine the incoming signals to increase SNR, hence improve link reliability and range. Copyright: Cellular Asset Management Ltd Page 5

6 Transmit beamforming (TxBf) can be used to improve signal reception and increase the range to a particular receiving device, by using multiple transmit antennas (multiple streams) to focus the RF beam on that device. TxBf requires knowledge of the channel conditions to ensure the transmitted beams will arrive at the receiver such that they will combine constructively. TxBf is not used y all vendors and TxBf is specified in the n standard but NOT in the Wi-Fi Alliance Certification. 20MHz and 40MHz Channels Within the n standard we have the option to double the useable channel bandwidth from 20MHz to 40 MHz, which would effectively double the data rate. Actually, because adjacent channels need a slight gap between them (to separate them in the frequency band) a single 40 MHz channel has slightly more than twice the bandwidth of two adjacent 20 MHz channels. This is due to the inter-channel frequency gap now being part of the actual channel space. As a result, we have slightly more than double the data rate. Figure 3: g channels 1-14 in the 2.4 GHz band Considering the 2.4GHz channel model (Figure 3), we only have a total of 3 non-overlapping 22MHz channels available. If we were to deploy n in the 2.4GHz band, using a 40GHz channel, we would be restricted as to the positioning of this and would only leave one nonoverlapping channel for legacy (802.11b/g) use. To deploy n in 2.4GHz using a 20MHz channel would clearly be a waste of resource, therefore it is not common practice to deploy n in 2.4GHz spectrum. Figure 4: 5GHz UNII Bands and ETSI Band (Diagram courtesy Fluke Networks) Copyright: Cellular Asset Management Ltd Page 6

7 Clearly there is more scope for deploying n with 40MHz channels in the 5GHz UNII band as can be seen in the channel plan above. Modulation and Coding Schemes (MCS) The over-the-air data rate that can be achieved from an n WLAN system depends on a combination of factors, which are: the number of spatial streams used, the type of RF modulation, the coding rate, the guard interval and the channel bandwidth (20 or 40 MHz). These variables give us a total of 77 options for the transmitted data rate. The range of data rates graduates from a minimum of 6.5Mbps to 600Mbps. Not all combinations are specified in the standard and not all are used by vendors, but the options are there if required. Each modulation and coding permutations is identified by a number, as can be seen in the table below: Figure 5: Modulation and Coding Schemes (MCS) for IEEE n (Table courtesy IEEE) Copyright: Cellular Asset Management Ltd Page 7

8 Frame Aggregation and Block Acknowledgement - Increasing MAC Layer Efficiency In a legacy Wireless LAN technology, such as b/ g/ a, each packet that is sent across the network is acknowledged individually, an extremely inefficient method which adds to the protocol overhead in WLAN systems. Within the n protocol are mechanisms to support Frame Aggregation, whereby multiple frames are combined and transmitted together as one packet across the network. In addition to frame aggregation, the n MAC supports Block Acknowledgements. A Block Ack will be sent in response to receipt of a stream of packets and will indicate just the frames that need to be retransmitted. (The process is similar to the windowing mechanism in TCP). Together, Frame Aggregation and Block Acknowledgements can increase MAC layer efficiency by up to 40%. However packet aggregation is more suited to the transfer of applications such as file transfers. Real- time applications such as voice still needs to be transported in smaller packets to avoid issues related to latency and jitter. There are three further issues to be considered when designing an n network:- When an Access Point is operating with two or more radios active, the power consumption will almost certainly exceed the maximum watts that can be delivered by 802.3af (PoE), Power over Ethernet. Network switches or power injectors compliant with the 802.3at specification will be required. The volume of traffic generated by multiple users accessing an n network will be considerably more than that generated over legacy WLAN technologies. This traffic (probably in the gigabytes) needs to be backhauled to the WLAN controller and beyond. Backhaul network dimensioning will become a key issue in network design. Wireless site surveys and network planning are key issues in the design of any WLAN network but they are more important when deploying a technology like n, which may need to co-exist with b/a/g networks, whilst working in a completely different manner to that with which we are familiar in complex multipath scenarios. Network planning is beyond the scope of this introductory document. Copyright: Cellular Asset Management Ltd Page 8

9 Conclusions The n standard makes use of several innovative technologies to improve the range, throughput, and reliability of wireless LANs. The three primary innovations are Multiple Input Multiple Output (MIMO) technology, packet aggregation and channel bonding. Together, these techniques allow n solutions to achieve an approximate fivefold performance increase over current a/b/g networks and allow for more robust connections at up to twice the range of these legacy technologies. The increases in throughput and capacity offered by n will allow for more users to access data hungry applications such as streaming video, without slowing down the network. New applications such as HDTV transmission between client devices in the home or office will become a viable proposition. In the healthcare environment for example, high definition images can be sent over the wireless network without the worry of capacity. Wi-Fi CERTIFIED n wireless routers or access points have a much better capability to blanket the whole home in a strong signal, delivering up to twice the range of previous-generation Wi-Fi networks. From the attic playroom, to the living room, to the kitchen, to the backyard, an n Wi-Fi signal can reach all the places a consumer might want to connect. Dead spots are dramatically reduced, and even applications that require a lot of bandwidth, such as high-definition video, can run effortlessly throughout the house. (Wi-Fi Alliance The IEEE n standard represents the beginning of the end for wired Ethernet as the dominant LAN access technology in the enterprise. Over the next few years, refinements in system silicon, radio design, network control, wireless security, and power management will improve n and its successor products to the point where they will begin to erode the switched Ethernet market. (Burton Group: Sept IEEE n will deliver on its promises, so long as the technology is understood and the network is designed, planned and rolled out with regard for all the complexities and potential pitfalls that may occur when deploying a complex technology such as this. Details of network design and planning are beyond the scope of this document but engineers at Cellular Asset Management will be happy to discuss the potential with any interested parties. Copyright: Cellular Asset Management Ltd Page 9