Why,Synchronous,for,Wireless,W communication Why Synchronous for Wireless?

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Why Not Asynchronous?To understand the benefits of a synchronous protocol, it helps to firstlook at the disadvantages of an asynchronous protocol. When a nodeusing an asynchronous protocol such as 802.11 wants to transmit aframe, it normally will simply transmit the frame after it senses thechannel is idle for a period of time (which is called Carrier SenseMultiple Access, or CSMA). If a collision is determined, due to thelack of an acknowledgment frame, the frame is re-transmitted afterwaiting an amount of time that increases exponentially for eachretransmission. In order to minimize the impact of a collision and tomaximize the chance of a successful reception of the data frame, 802.11includes an optional collision avoidance (CA) function where a shortRequest-To-Send/Clear-To-Send (RTS/CTS) exchange is first performed,which causes devices overhearing those frames to not access the channelfor a period of time. This collision avoidance function may bebeneficial in some situations, but it comes with a large overhead andintroduces problems of its own, and the impact of these problems isgreatly increased in a long-range outdoor system.Some of the problems associated with carrier sensing (CSMA) and collision avoidance (CA) protocols include:• Acknowledgment Overhead: This is compounded over long distance links due to propagation time. • Exponential Back-off:This is compounded in outdoor networks, where re-transmissions arecommon due to interference, which causes latency to increaseexponentially.• "Hidden Nodes": This is a classicproblem with 802.11 CSMA, where carrier sensing at the transmitter doesnot sense interference at the receiver. This is greatly compounded inoutdoor networks, where obstructions and long distances between thetransmitters normally results in them not being able to hear each other.• "Exposed Nodes":This is a classic problem with 802.11 CA, where the RTS message betweena transmitter and receiver causes other potential transmitters tobecome idle when they could have transmitted successfully to adifferent receiver. This is greatly compounded in a mesh network, wherethere are normally many active receivers.• CA Overhead:The collision avoidance overhead due to the RTS-CTS-Data-ACK exchangerequires 4 propagation times, which results in large overhead onlong-distance links. • CSMA Failures: In a smalloffice or cafe, all stations can normally hear each, which allow themto properly carrier sense and avoid collisions. In an outdoor wirelessnetwork, many stations can not normally hear each other, resulting incollisions that cause nodes to experience exponential back-off.• Ad-hoc Architecture:When connecting to an access point in a small office or cafe, allcommunications occur between the stations and the access point (whichis called infrastructure mode) and not directly between stations. Thismeans that most of the transmissions will never collide since alldownlink transmissions are from a single device, the access point. In awireless mesh networkusing either ad-hoc mode or infrastructure mode there are manysimultaneous transmitters and receivers, and all transmissions maycollide.• Unfairness: Another classic problem with802.11 is MAC layer unfairness, and the problem greatly increases inoutdoor networks. Due to the increasing back-off duringretransmissions, nodes with fewer retransmissions are more likely togain access to the channel than nodes that are retransmitting.Additionally, nodes that sense the channel becoming idle earlier aremore likely to get access to the channel, and over long distances thisresults in unfairness to some nodes due to their location. These problems are basic issues with asynchronous protocols such as802.11, and all of these problems are drastically increased in outdoorwireless networks. Most people have experienced performance problemsrelated to these issues in offices or cafes, but in outdoor meshnetworks the impact of these problems is greatly increased, sometimesresulting in a complete collapse of the MAC layer.Why Synchronous? The most obvious reason to choose a synchronous protocol for an outdoorwireless network is to coordinate communications over large coverageareas. Scheduling transmissions not only enhances the efficiency ofspectral utilization but also enhances quality of service (QoS) throughlatency controls, rate control, and traffic prioritization. There aresome crude ways to implement scheduled transmissions without beingsynchronous, such as by simple polling. In fact, 802.11 includes anoptional Point Coordination Function (PCF) that uses polling (and802.11e extends this functionality in its optional Hybrid CoordinationFunction). Additionally, 802.11 even includes some synchronous featuresin its base specification, specifically its Time SynchronizationFunction (TSF), which allows devices to periodically align their clocksthat can then be used by functions such as power-save where a sleepingdevice can periodically wake up at the right moment to see if there isdata for it.However, there are many reasons that 802.11 is not considered asynchronous protocol. Some features traditionally associated withsynchronous protocols, such as WiMAX or SkyPilot’s SyncMesh™ protocol,include:• Contention-less Data Transmissions: 802.11’sbase Distributed Coordination Function (DCF) normally puts data incontention, meaning that multiple nodes may transmit simultaneously.WiMAX and SyncMesh schedule data transmissions within time slots,avoiding the contention of data, allowing more bounded latency.• Ranging:DOCSIS (the cable modem standard), WiMAX, and SyncMesh all include atime ranging function, which determines how far apart nodes are inorder to compensate for RF propagation at the speed of light. Thismaximizes efficiency, since inter-frame spaces then do not have toallow for the time of the RF propagation. Synchronous protocols that donot support ranging suffer from this overhead and polling protocols paythe propagation penalty twice. While the speed of light is normallyconsidered fast, on long distance links the 10s of microseconds startto add up, especially as frame transmission times decrease at higherbandwidths and modulations.• Periodic Time Slot Grants:SyncMesh’s synchronous nature enables the ability to grant recurringtime slots. This means that nodes can be granted extended rights tocommunicate on certain time slots, which increases efficiency.Asynchronous protocols do not provide this. Periodic time slot grantsare useful for providing higher classes of service for applicationslike Voice over IP (VoIP).• Clock Precision: Thefeatures of a synchronous protocol benefit from very precise clocks,which means continually adjusting for phase between time sync messages(or signals from an external clock source) or using very frequent syncmessages (SyncMesh performs the former since it is more efficient).These advanced MAC features are just some of the benefits of using asynchronous protocol, but there is another equally important, if notmore important, reason to use a synchronous protocol for broadband wireless mesh – to dynamically point antennas.One of the most effective tools an RF engineer uses to improve awireless link and to minimize a link’s impact on others is the use ofdirectional antennas. The benefits of directional antennas include:• Increased link budget (both on transmit and receive), resulting in higher modulation and longer range.• Decreased interference susceptibility from external sources• Decreased interference to other systems• Increased power due to point-to-point regulations in many countriesHowever, the challenge with using directional antennas is just that –they are directional, which requires manual pointing and alignment. Inmesh networks, it’s advantageous to have 360° omnidirectional coverage.360° coverage from every node provides easy installation, maximizesredundancy, and avoids expensive and time-consuming system engineeringof the mesh.To provide a node with 360° coverage using directional antennas,multiple antennas are needed, and as the gain of the antennas increasesthe number of antennas needed to provide 360° coverage also increases.This basic relationship applies no matter what antenna technology isused, from fixed sectors to beam-forming arrays – each of these antennadesigns focuses RF energy, and as the antenna gain increases, the RFenergy is more focused, decreasing the coverage angle. And while someadvanced beam-forming techniques do not use fixed antenna sectors, theRF energy is still focused in a particular direction, so the beamdirection needs to be varied in order to provide 360° coverage.So, most 802.11 mesh networks with directional antennas use manualpointing, where 360° coverage is not provided, and the network must beengineered link-by-link. There has been some research arounddynamically pointing antennas with 802.11, but its asynchronous natureprohibits antenna pointing coordination. One challenge with anasynchronous protocol is that some of the transmissions need to be madewith omnidirectional antennas (such as omnidirectional Request-To-Sendmessages), since transmissions are not naturally pre-coordinated. Whilesuch a method may allow for higher modulation transmission of theactual data frames, it suffers from decreased range, increasedinterference, and increased overhead due to the coordination (thelatter can be very significant in an outdoor wireless system due tohigh modulations and the speed-of-light propagation). Alternatively, anasynchronous system could simply use a directional antenna only fortransmissions, and use a separate omnidirectional antenna forreceptions. The challenge here is that interference is an issue withthe receiver, and an omnidirectional receive antenna neither increasesthe desired signal nor decreases the interference or noise. So, rangeand link modulation are limited due to the lack of receive antennagain. Additionally, when only a single side of a link uses adirectional antenna, it is not normally classified as a point to point communications link, and many regions limit the effective output power of the link.By using a fully synchronous protocol, such as SyncMesh, where everycommunication is coordinated (even bandwidth request opportunities andnetwork entry points), antennas can be pointed on both transmit andreceive. This provides all of the benefits of a system consistingentirely of point-to-point links, while still providing the redundancyand simple installation of an omnidirectional system. While thesebenefits are significant, there are some challenges around creating afully synchronous mesh protocol. To summarize so far, there are two primary reasons to use asynchronous, scheduled protocol within a mesh network: MAC layercoordination and to point directional antennas.Regarding the latter, to avoid the challenges of dynamically pointingantennas, some multi-antenna systems use a separate radio for eachantenna (or subset of antennas). This has several problems, with themost obvious problem being cost. Even though there is now theavailability of inexpensive 802.11 radios, these radios have manyhidden costs due to:• amplifiers • increased processing power and processor interconnect • increased node size• increased power consumptionHowever, there is a bigger problem with using multiple radios –self-interference. Even if the radios each use separate frequencies andemploy guard bands (which is impractical due to the limited number ofchannels in many frequency bands), all radios interfere on some level.This can be seen by looking at an 802.11 radio’s published adjacentchannel rejection values, which is basically the amount of interferencefrom communications on an adjacent non-overlapping channel. Theproblems due to this self-interference are magnified by thecharacteristics of outdoor wireless, such as high levels of externalinterference and weak signal reception due to long links and highamounts of obstruction.To address the issues of cost and limited channel availability, areduced number of radios is sometimes used. For instance, some systemsuse 2 or 3 radios per node. However, a reduced number of radios means areduced number of antennas, which means either very low gain antennasare used, or 360° coverage is not provided. Both of these restrictionsare a large problem for an outdoor mesh system.To mitigate the interference issues, the most obvious solution is toprovide high levels of isolation between the radios and between theantennas. Traditionally, this would mean expensive filters and largeamounts of physical shielding which is expensive and increases nodesize. However, it is impractical to cost effectively provide asufficient amount of isolation in a mesh node, given typical outdoorwireless scenarios where the received signal may be under -90 dBm whilethe transmissions might be at +30 dBm. Adjacent, or even alternate,channel rejection along with filters and physical isolation are notenough to provide anywhere near the level of isolation required. So,interference between the radios is not addressed, and results indecreased link modulation and reduction in link range, which are thetwo main reasons one would use a directional antenna in the firstplace. Another general technological issue with using a radio per directionalantenna is that such a system can’t take advantage of steerable(adaptive beam-forming) antennas. Steerable antenna technology allowsan antenna’s pattern to be electronically adjusted, so a radio per beamcannot be used since there are no fixed beams. All of these issues can be addressed by using a synchronous protocol tocoordinate all transmissions so that a single radio can be switchedamong many antennas (or between beam-steering weights). And even thougha single radio architecture may not seem to have the capacity of amultiple radio architecture, a multiple radio system cannot takeadvantage of additional radio capacity due to self-interference. And,the real bottleneck of a mesh network is almost always at the bandwidthinjection point (gateway), which means the use of multiple radios inthe majority of nodes in a mesh network is wasted money. Why Not Synchronous? We've analyzed the benefits of synchronous protocols and thedisadvantages of asynchronous protocols in outdoor wireless networks,but what are the disadvantages of using a synchronous protocol? Hereare a few disadvantages, and potential solutions:• Clocks need to be synchronized:Devices participating in a synchronous protocol obviously neededsynchronized clocks. This can be provided in several ways, includingexternal clock sources such as GPS or over-the-air clocksynchronization. SyncMesh uses a combination of the two, whichleverages the accuracy of GPS clocks with the low cost of over-the-airsynchronization.• Clocks need to be very accurate:This usually requires expensive clock crystals that are accurate over awide temperature range. SyncMesh provides an extremely accurate clocksource by utilizing an over-the-air calibration protocol along with aninternal calibration algorithm that maintains accuracy even withinexpensive crystals.• Inefficiencies: Manysynchronous, slotted protocols are inefficient due to their simple TimeDivision Multiple Access (TDMA) MAC layers, which assigns fixed slotsto each user. To overcome this, SyncMesh uses a dynamic slot allocationscheme which assigns all slots in real time.• Lack of interoperability with other systems.Since many outdoor wireless systems leverage unlicensed frequencies,multiple systems may need to share the spectrum. Carrier sensingsystems may be able to (in theory) share the spectrum by avoidingsimultaneous use, while more complex synchronous systems will notunderstand each other. However, we've already seen that carrier sensinghas issues, and many systems 'tweak' the carrier sensing and back-offprotocols to get an unfair advantage over other users of the spectrum.SyncMesh handles multiple users of the spectrum by pointing antennas -the high link budget point-to-point link can avoid interference fromother systems, while its directional nature avoids interfering withother systems.• Complexity: WiMAX-like synchronous systems are much more complex thatasynchronous 802.11 systems. That is why WiMAX CPEs are more expensivethan 802.11 clients, and why WiMAX base stations are significantly moreexpensive than 802.11 access points. SyncMesh has been developed over aperiod of 6 years and runs on top of off-the-shelf 802.11 silicon,which lowers cost.