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E-Lins Industrial router applications Industrial-grade routers as Internet network layer communication equipment application in all walks of life, brought a lot of convenience for our industry. "E-Lins" introduce the application of industrial router scenario analysis.   1 The self-service terminal network   E-lins industrial router networking...

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Classification of 4G industrial routers There are many types of 4G industrial routers, which can be divided into different categories from different perspectives. Different types of 4G industrial routers can be used in different environments. The following sections classify the 4G industrial routers from different perspectives. According to the performance From...

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Dual SIM Router vs. Dual Radio Router Projects are looking to save their enterprises time and money ask us this very often: “When would I need to use dual SIMs, and in what situations should I consider dual radio dual sim router?” In order to make this clear, let’s take a quick look at the dual SIM and dual SIM dual radio module functionality. Dual...

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工业路由器NBMA网络转化为点到点的链路 当我们使用点到点子接口将NBMA网络转化为点到点的链路时,整个NBMA网络将产生过多的PVC部分互联或全互联的网状结构,但这将产生一定的负面影响,会使网络中产生大量的LSP泛洪流量。我们都知道,运行IS-IS的工业路由器当接收到一个LSP报文后,会将此LSP从除接收接口以外的所有启用了IS-IS协议的接口泛洪出去,以使网络中的其他工业路由器都可以接收到此LSP。但是这种泛洪机制对于存在大量部分互联或全互联的网络将产生过多冗余的LSP扩散。 所谓全互联或全网状网络拓扑,是指所有工业路由器都与其他工业级无线路由器向连接(通常是点到点子接口)。在这样的一个网络中,当一台路由器从某接口收到邻居泛洪过来的LSP后,由于它并不知道这个LSP是否已经被其他邻居工业4g路由器收到,所以会再从其他接口泛洪出去,即使其他工业级4g路由器的链路状态数据库中已经存在这个LSP。如果网络中有n个全网路由器的话,那么网络中的每台工业级LTE路由器都会扩散n-2条冗余的LSP,这样总共被泛洪的多余的LSP将为(n-1)x(n-2),条而这些LSP的扩散是多余。如果每台工业全网通路由器都刷新一条LSP的话,那么这个数量还将会成倍数的增长,导致了大量带宽资源的浪费。 为了解这种在全互联或高度互联的网络中出现的LSP泛洪的冗余现象,IS-IS提供了一种解决方案——IS-IS全通组,也称作Mesh组。IS-IS全通组在RFC2973中进行了定义。所谓全通组,就是假设所有工业3G路由器之间都是完全互联的,每个工业级全网通路由器都会直接收到其他全网通工业级路由器泛洪的原始的LSP的拷贝。 可以将全网工业路由器的接口加入到某个全通组中,一个全网通工业路由器上可以存在多个全通组,全通组内接口之间的LSP泛洪是受限制的,全通组之间的LSP泛洪是正常的操作,未加入全通组的工业级3G路由器接口与全通组之间也是正常的LSP泛洪操作。全网通路由器 ...

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工业路由器使用CSNP来保证链路状态数据库的完整性 在广播网络中,工业路由器使用CSNP来保证链路状态数据库的完整性,并且只有DIS才会发送工业全网通路由器CSNP报文,DIS发送CSNP报文的间隔为10s。CSNP报文中描述了DIS的链路状态数据库中所有工业级3G路由器LSP的摘要信息。当其他工业级路由器收到DIS发送的CSNP后,会使用CSNP中的LSP摘要信息与与本地的链路状态数据库中的LSP进行比较,进行比较的目的是确定本地链路状态数据库中的信息是否已经同步和完整。如果工业级4g路由器发现本地数据库中缺少某个LSP条目,那么它将使用PSNP向DIS请求这个缺少的LSP条目。这个PSNP报文中包含就是请求的LSP条目的摘要信息。当DIS收到其他全网路由器发送的PSNP报文后,将会发送一个完整的LSP报文,这个LSP就是其他工业无线路由器所缺少的LSP条目。在广播网络中,DIS使用周期性的CSNP报文向网络中发送同步链路状态数据库的信号,而其他工业4g路由器使用PSNP报文来请求缺少的LSP条目。 在IS-IS的点到点类型的网络中,链路状态数据库同步的操作与广播网络中略有不同,而且工业级全网通路由器发送CSNP与PSNP报文的方式和其作用也有一些差别。 在点到点网络中不存在DIS,工业3G路由器不会周期性的发送CSNP报文,CSNP报文只在链路链路被激活时发送一次,而且链路两端的工业级4g路由器都会发送CSNP报文以描述本地链路状态数据库中所有LSP的摘要信息。当工业路由器发送对端发送的CSNP中含有本地缺少的LSP信息时,也会使用PSNP报文向对端请求LSP。当对端收到PSNP报文后,将向请求方发送包含完整LSP信息的LSP报文,这点上与广播网络中的操作是相同的。但是在点到点链路上,收到LSP报文的工业4g路由器还会向对方再次发送一个PSNP报文以对之前收到的LSP进行确认。可以说,在点到点网络中的LSP交换是可靠的。这点与广播网络不同,在广播网络中工业级无线路由器不使用PSNP报文对收到的LSP进行确认,而是通过DIS周期性地发送CSNP报文以弥补广播网络中不可靠的LSP交换。 在点到点链路上,工业路由器使用PSNP对收到的LSP报文进行确认,所以在点到点链路上是可靠的泛洪机制。 IS-IS路由协议支持两种网络类型:广播链路和点到点链路。默认情况下,全网通工业级路由器IS-IS将广播网络和NBMA网络看作是广播类型。对于封装了PPP或HDCL等协议的链路看作是点到点类型。对于NBMA网络中的主接口和点到多点子接口,IS-IS将其看作是广播类型;对于NBMA网络中的点到点子接口,将其看作是点到点类型。IS-IS不像OSPF那样,提供了对NBMA网络(例如Frame-Relay、ATM)的专门支持。对于NBMA网络,全网通工业路由器IS-IS认为其网络拓扑是PVC全互联(mesh)的,就是把它看作广播网络。但如果实际网络拓扑中并不是PVC全互联的结构时,例如部分互联的结构和Hub-Spoke结构,推荐使用点到点类型网络,即使用点到点子接口,以免造成NBMA网络中的链路状态数据库同步出现问题。无线路由器

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Talking about Long Distance Wi-Fi

文章目录 : 其他, 技术相关

Introduction
Since the development of the IEEE 802.11 radio standard (marketed under the Wi-Fi brand name), the technology has become markedly less expensive and achieved higher bit rates. Long range Wi-Fi especially in the 2.4 GHz band (as the shorter range higher bit rate 5.8 GHz bands become popular alternatives to wired LAN connections) have proliferated with specialist devices. While Wi-Fi hotspots are ubiquitous in urban areas, some rural areas use more powerful longer range transceivers as alternatives to cell (GSM, CDMA) or fixed wireless (Motorola Canopy and other 900 MHz) applications. The main drawbacks of 2.4 GHz vs. these lower-frequency options are:

poor signal penetration – 2.4 GHz connections are effectively limited to line of sight or soft obstacles
far less range – GSM or CDMA cell phones can connect reliably at > 16 km (9.9 mi) distances. The range of GSM, imposed by the parameters of Time division multiple access, is set at 35 km.
few service providers commercially support long distance Wi-Fi connections
Despite a lack of commercial service providers, applications for long range Wi-Fi have cropped up around the world. It has also been used in experimental trials in the developing world to link communities separated by difficult geography with few or no other connectivity options. Some benefits of using long range Wi-Fi for these applications include:

unlicensed spectrum – avoiding negotiations with incumbent telecom providers, governments or others
smaller, simpler, cheaper antennas – 2.4 GHz antennas are less than half the size of comparable strength 900 MHz antennas and require less lightning protection
availability of proven free software like OpenWrt, DD-WRT, Tomato that works even on old routers (WRT54G for instance) and makes modes like WDS, OLSR, etc., available to anyone. Including revenue sharing models for hotspots.
Nonprofit organizations operating widespread installations, such as forest services, also make extensive use of long-range Wi-Fi to augment or replace older communications technologies such as shortwave or microwave transceivers in licensed bands.

Applications
Business
Provide coverage to a large office or business complex or campus.
Establish point-to-point link between large skyscrapers or other office buildings.
Bring Internet to remote construction sites or research labs.
Simplify networking technologies by coalescing around a small number of Internet related widely used technologies, limiting or eliminating legacy technologies such as shortwave radio so these can be dedicated to uses where they actually are needed.
Bring Internet to a home if regular cable/DSL cannot be hooked up at the location.
Bring Internet to a vacation home or cottage on a remote mountain or on a lake.
Bring Internet to a yacht or large seafaring vessel.
Share a neighborhood Wi-Fi network.
Nonprofit and Government
Connect widespread physical guard posts, e.g. for foresters, that guard a physical area, without any new wiring
In tourist regions, fill in cell dead zones with Wi-Fi coverage, and ensure connectivity for local tourist trade operators
Reduce costs of dedicated network infrastructure and improve security by applying modern encryption and authentication.
Military
Connect critical opinion leaders, infrastructure such as schools and police stations, in a network local authorities can maintain
Build resilient infrastructure with cheaper equipment that an impoverished war-torn region can afford, i.e. using commercial grade, rather than military-class network technology, which may then be left with the developed-world military
Reduce costs and simplify/protect supply chains by using cheaper simpler equipment that draws less fuel and battery power; In general these are high priorities for commercial technologies like Wi-Fi especially as they are used in mobile devices.
Scientific research
See also: Wireless sensor network
A long range seismic sensor network was used during the Andean Seismic Project in Peru. A multi-hop span with a total length of 320 kilometres was crossed with some segments around 30 to 50 kilometers. The goal was to connect to outlying stations to UCLA in order to receive seismic data in real time.
Large-scale deployments
The Technology and Infrastructure for Emerging Regions (TIER) project at University of California at Berkeley in collaboration with Intel, uses a modified Wi-Fi setup to create long-distance point-to-point links for several of its projects in the developing world. This technique, dubbed Wi-Fi over Long Distance (WiLD), is used to connect the Aravind Eye Hospital with several outlying clinics in Tamil Nadu state, India. Distances range from five to over fifteen kilometres (3–10 miles) with stations placed in line of sight of each other. These links allow specialists at the hospital to communicate with nurses and patients at the clinics through video conferencing. If the patient needs further examination or care, a hospital appointment can then be scheduled. Another network in Ghana links the University of Ghana, Legon campus to its remote campuses at the Korle bu Medical School and the City campus; a further extension will feature links up to 80 km (50 mi) apart.

The Tegola project of the University of Edinburgh is developing new technologies to bring high-speed, affordable broadband to rural areas beyond the reach of fibre. A 5-link ring connects Knoydart, the N. shore of Loch Hourne, and a remote community at Kilbeg to backhaul from the Gaelic College on Skye. All links pass over tidal waters; they range in length from 2.5 km to 19 km.

Increasing range in other ways
Further information: 802.11 non-standard equipment and Radio propagation
Specialized Wi-Fi channels
For more details on this topic, see List of WLAN channels.
In most standard Wi-Fi routers, the three standards, a, b and g, are enough. But in long-range Wi-Fi, special technologies are used to get the most out of a Wi-Fi connection. The 802.11-2007 standard adds 10 MHz and 5 MHz OFDM modes to the 802.11a standard, and extend the time of cyclic prefix protection from 0.8 µs to 3.2 µs, quadrupling the multipath distortion protection. Some commonly available 802.11a/g chipsets support the OFDM ‘half-clocking’ and ‘quarter-clocking’ that is in the 2007 standard, and 4.9 GHz and 5.0 GHz products are available with 10 MHz and 5 MHz channel bandwidths. It is likely that some 802.11n D.20 chipsets will also support ‘half-clocking’ for use in 10 MHz channel bandwidths, and at double the range of the 802.11n standard.

802.11n and MIMO
Preliminary 802.11n working became available in many routers in 2008. This technology can use multiple antennas to target one or more sources to increase speed. This is known as MIMO, Multiple Input Multiple Output. In tests, the speed increase was said to only occur over short distances rather than the long range needed for most point to point setups. On the other hand, using dual antennas with orthogonal polarities along with a 2×2 MIMO chipset effectively enable two independent carrier signals to be sent and received along the same long distance path.

Power increase or receiver sensitivity boosting

A rooftop 1 watt Wi-Fi amp, feeding a simple vertical antenna on the left.
Another way of adding range uses a power amplifier. Commonly known as “range extender amplifiers” these small devices usually supply around ½ watt of power to the antenna. Such amplifiers may give more than five times the range to an existing network. Every 6 dB gain doubles range. The alternative techniques of selecting a more sensitive WLAN adapter and more directive antenna should also be considered.

Higher gain antennas and adapter placement
Specially shaped directional antennas can increase the range of a Wi-Fi transmission without a drastic increase in transmission power. High gain antenna may be of many designs, but all allow transmitting a narrow signal beam over greater distance than a non-directional antenna, often nulling out nearby interference sources. A popular low-cost home made approach increases WiFi ranges by just placing standard USB WLAN hardware at the focal point of modified parabolic cookware. Such “WokFi” techniques typically yield gains more than 10 dB over the bare system; enough for line of sight (LOS) ranges of several kilometers and improvements in marginal locations. Although often low power, cheap USB WLAN adapters suit site auditing and location of local signal “sweet spots”. As USB leads incur none of the losses normally associated with costly microwave coax and SMA fittings, just extending a USB adapter (or AP, etc.) up to a window, or away from shielding metal work and vegetation, may dramatically improve the link.

Protocol hacking
The standard IEEE 802.11 protocol implementations can be modified to make them more suitable for long distance, point-to-point usage, at the risk of breaking interoperability with other Wi-Fi devices and suffering interference from transmitters located near the antenna. These approaches are used by the TIER project.

In addition to power levels, it is also important to know how the 802.11 protocol acknowledges each received frame. If the acknowledgement is not received, the frame is re-transmitted. By default, the maximum distance between transmitter and receiver is 1.6 km (1 mi). On longer distances the delay will force retransmissions. On standard firmware for some professional equipment such as the Cisco Aironet 1200, this parameter can be tuned for optimal throughput. OpenWrt, DD-WRT and all derivatives of it also enable such tweaking. In general, open source software is vastly superior to commercial firmware for all purposes involving protocol hacking, as the philosophy is to expose all radio chipset capabilities and let the user modify them. This strategy has been especially effective with low end routers such as the WRT54G which had excellent hardware features the commercial firmware did not support. As of 2011, many vendors still supported only a subset of chipset features that open source firmware unlocked, and most vendors actively encourage the use of open source firmware for protocol hacking, in part to avoid the difficulty of trying to support commercial firmware users attempting this.

Packet fragmentation can also be used to improve throughput in noisy/congested conditions. Although packet fragmentation is often thought of as something bad, and does indeed add a large overhead, reducing throughput, it is sometimes necessary. For example, in a congested situation, ping times of 30 byte packets can be excellent, while ping times of 1450 byte packets can be very poor with high packet loss. Dividing the packet in half, by setting the fragmentation threshold to 750, can vastly improve the throughput. The fragmentation threshold should be a division of the MTU, typically 1500, so should be 750, 500, 375, etc. However, excessive fragmentation can make the problem worse, since the increased overhead will increase congestion.

Obstacles to long-range Wi-Fi
Methods that increase the range of a Wi-Fi connection may also make it fragile and volatile, due to various factors including:

Landscape interference
Obstacles are among the biggest problems when setting up a long-range Wi-Fi. Trees and forests attenuate the microwave signal, and hills make it difficult to establish line-of-sight propagation.

In a city, buildings will impact integrity, speed and connectivity. Steel frames and Sheet metal in walls or roofs may partially or fully reflect radio signals, causing signal loss or multipath problems. Concrete or plaster walls absorb microwave signals significantly, reducing the total signal.

Tidal fading
When point-to-point wireless connections cross tidal estuaries or archipelagos, multipath interference from reflections over tidal water can be considerably destructive. The Tegola project uses a slow frequency-hopping technique to mitigate tidal fading.

2.4 GHz interference
Main article: Electromagnetic interference at 2.4 GHz
Microwave ovens in residences dominate the 2.4 GHz band and will cause “meal time perturbations” of the noise floor. There are many other sources of interference that aggregate into a formidable obstacle to enabling long range use in occupied areas. Residential wireless phones, baby monitors, wireless cameras, remote car starters, and Bluetooth products are all capable of transmitting in the 2.4 GHz band.

Due to the intended nature of the 2.4 GHz band, there are many users of this band, with potentially dozens of devices per household. By its very nature, “long range” connotes an antenna system which can see many of these devices, which when added together produce a very high noise floor, whereby no single signal is usable, but nonetheless are still received. The aim of a long range system is to produce a system which over-powers these signals and/or uses directional antennas to prevent the receiver “seeing” these devices, thereby reducing the noise floor.

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