Good Day. My name is Bill Curtice. I represent the Miami Valley Mesh Alliance a group of ham radio operators working together to develop mesh

Transcription

1 Good Day. My name is Bill Curtice. I represent the Miami Valley Mesh Alliance a group of ham radio operators working together to develop mesh networks across Ohio ARES District 3. the nine counties surrounding Dayton, Ohio.

2 When I present this, I always feel compelled to make it clear that I am certainly NOT an expert in mesh. We started looking at mesh four years ago, and have gradually built a mesh network in the Dayton area. We have learned a lot. And we still have a lot to learn.

3 Today I will very briefly describe mesh.. What it is how it works.. How we deployed it..and, what it s good for. I will then focus on some implications for disaster communications support, public service, and support of youth involvement in Ham Radio. We will close with information on how to get involved. And. If you are seeing this as a live presentation perhaps a demonstration of mesh capabilities.

4 Simply put, a Mesh Network is a wireless digital network, made up of repeaters, we call Nodes. It runs on microwave frequencies, and it s job is to transport digital data at high speeds, just like any other digital network you may use in your home, work, factory, school, etc. In this example, data originating at the Laptop computer at the top of the page, can be received by the computer at the bottom of the page.

5 Each node fielded is based on using a re-purposed Wireless Access Point, as a Software Defined Radio. We currently use commercial gear, some of which is shown here, manufactured by Ubiquiti Networks. The Linksys WRT54 router shown on the far right is no longer used, as it cannot meet current performance and operating standards. When we Flash these devices with new mesh firmware uploads, they become Digital Ham Repeaters. Each repeater is capable of passing digital data along the network, and each may connect various clients to the mesh network, such as computers, IP telephones, or IP TV cameras. Each node has a DHCP capability that may assign network IP addresses to each hardwire-connected client.

6 Each mesh node then becomes a licensed Ham Radio Station. Each node is self-advertising, meaning that when you plug it in it is broadcasting your call letters, and advertising its availability to connect with another mesh node. Each node is self-discovering, meaning that when two nodes come within RF range they will automatically bind together to create a network link. Each node is self-configuring, meaning that the nodes negotiate among themselves the specific digital operating mode to be used to match the signal and propagation conditions. The network created is fault tolerant in that if traffic is moving across the net, and a node goes down, traffic is automatically routed around the faulty node to its destination. The magic that makes this happen is OLSR. Optimized Link State Routing Protocol embedded in the firmware on each node. It is significant to note that: Mesh nodes only talk to other mesh nodes; you cannot interact directly with the mesh network using the WiFi on your cell phone or computer. If that capability is needed, it is necessary to direct hardwire connect a general purpose WiFi access point to the node, and then control the WiFi access to the mesh. All nodes on the mesh may be controlled or modified from other nodes on the mesh under password control. Lastly, mesh nodes consume very little power.. 3 to 7 watts, making them ideal for powering with batteries or solar power.

7 The Mesh Network is a Wireless. Digital.. Wide-Area Network, operating in the Microwave ham bands shown here. Two of the bands have segments which overlap standard WiFi channels. Our entire allocation on 900 MHz is shared with Industrial, Scientific, Medical, Government, and consumer electronics applications. The curse of using shared spectrum, is that we are vulnerable to QRM from stations operating on our frequencies, which cause loss and retransmission of packets, resulting in a slowing of digital traffic. For this reason, we avoid operating on shared consumer channels. However the propagation properties of 900 MHz make that band VERY useful none the less. We use a broadband signal between 5MHz and 40 MHz wide. Compare that to using FLDigi on HF, where the maximum bandwidth is under 3 KHz. The wide bandwidth allows us to run high speed digital traffic at speeds between 1 and 100 Mbps using TCP/IP Protocol.

8 Shown here are the are representations of the RF spectrum used in the 2.4 GHz and 5.8 GHz ham bands. Looking at the 2.4 GHz illustration, you will see the standard consumer WiFi Channels shown in Pink. Amateurs may operate under Part 97 rules in the spectrum shown, meaning that we can use higher transmit power, and higher antenna gain, than is permitted for commercial and consumer WiFi applications. Hams are actually the Primary Users of the spectrum from 2390 to 2417 MHz, meaning that we may transmit, within the rules, without concern for the QRM our transmissions may cause other consumer or Unlicensed commercial users. However, that said, we must ALSO tolerate the in-band interference those commercial and consumer users cause us. The only Un-Shared part of the spectrum available to us is shown in the orange colored areas. We have two 5 MHz channels, centered on 2397 MHz; we may operate with transmission bandwidths of 5 MHz, or 10 MHz, in this area. The story on 5.8 GHz is similar, except that there, we have 7 channels that are shared only with satellite services.

9 We also have privileges on the 900 MHz band. While there may be unlicensed and unregulated transmissions across this band that may cause QRM, 900 MHz is very useful because of it s ability to propagate within buildings, and through foliage and trees. Many regard the 3.4 GHz band as the pot of gold which remains to be mined and used. We have 24 Channels, which are shared only with the US Military. I am advised that some Air Guard units have radar approach control units which operate in this spectrum; hence, In a REAL emergency, if the Guard came to town, and set up a landing zone. We may be denied use of some of these frequencies. Experiments are ongoing on using 3.4 GHz.

10 When mesh was initially introduced to the Amateur Radio community, the only frequency that could be practically used, was WiFi Channel 1. That is shown as the area in pink on this slide. Mesh operators experienced significant QRM when using Channel 1. Firmware was later developed that allowed us to use Ubiquiti gear on Channel -2 (2397 MHz); the area shown in green. That change SIGNIFICANTLY improved mesh performance. This spectrum may be used operating either 5 MHz wide, or 10 MHz wide. The choice is a tradeoff of signal to noise ratio and data transmission speed.

11 Shown here are the utilization possibilities for the 7 Un-Shared channels in the 5.8 GHz band.

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13 Given that mesh nodes operate on Microwave frequencies, and given that the BEST coax available has significant loss at Microwave Frequencies, we seek to avoid using coax of any kind. We typically locate our mesh transceivers as close to the antenna as possible; most often, they are connected directly to the antenna; in many instances, the transceivers are built into the antenna structure (called an all-in-one unit). These transceiver/antenna combination units are typically very cost effective. Mesh nodes operate using Power over Ethernet (PoE). Hence, we run Cat5e or Cat6 Ethernet cable from the transceiver to the shack. That allows us to provide power to the node, AND control the node over hard wire. Such hard-wire control is not necessary, as the node at the top of the tower can be controlled from another node in the shack, coupled by RF. However, most hams take advantage of the hard-wire for absolute control. We typically deploy lightning surge suppressors on the Ethernet cable to protect down-stream gear (Ethernet switches, clients, etc.).

14 Shown here, are WiFi access points, or routers, typically used as mesh transceivers. At first, we all used the old Linksys WRT54GS series routers; they were inexpensive, and available. However, they also had limited output power (70mw), limited memory, very slow processors, no weather protection, and. were confined to operating the commercial WiFi channels (Typically Channel 1). Use of the Linksys units is all but gone. On the right of the slide, are shown some of the WiFi Access Points offered by Ubiquiti Networks. They typically have six to ten times the output power of the Linksys, have much more sensitive receivers, and are built to be installed outside. Many are sold as antenna and transceiver in one unit. They support the Un-Shared frequency spectrum available to hams, and.. perhaps most significantly, are affordable for many hams.

15 Shown here are typical connections for Ubiquiti node hardware. Devices on the left are radios-only. On the right, the package includes the antenna and the radio. All Ubiquiti units run with Power Over Ethernet (PoE); hence, a PoE adapter must be inserted in the Ethernet line connected to the node. A typical PoE adaptor is shown at the lower left. Note that some Ubiquiti devices offer only one Ethernet jack, which must be connected to the PoE injector. The PoE injector provides a single connection for a client device, such as a computer. A simple Ethernet Switch (also called a hub) may be used to break out the single connection, providing multiple client connections.

16 The following series of slides illustrate the growth of a typical ham mesh station. First we start with a mesh node (antenna and transceiver), shown on the right, with its PoE injector. That node is then connected to a hub or Ethernet switch. That enables us then connected additional clients to the node.

17 Shown here, are several clients connected to our node, through the switch. In this example, we have connected a laptop computer, an IP telephone, a file server, and an IP television camera. Clients may include Computers of all kinds including Raspberry Pi, IP cameras, IP telephones, WiFi Access Points, and servers for web pages and file storage. You may connect almost any TCP/IP device directly using an Ethernet cable, or by using a WiFi Access Point (convenient for cell phones, tablets, etc.)

18 Ethernet ports available through the switch also allow us to Direct Connect multiple nodes located at the same site. The switch enables digital traffic to move from node to node through the wire, vs. over the air. This can be used to connect cross-band nodes, or to connect nodes within the same band.

19 A Managed Switch is used to control, apply, and propagate Internet within the mesh network. Such devices create virtual local area networks (V-LANs) in compliance with IEEE Standard 802.1q, providing the firewalls and control required. Normally, we do not propagate the Internet across the mesh network due to FCC restrictions on encrypted communications and commercial use of amateur frequencies. However, during a REAL emergency, propagation of Internet may be necessary. However, we often DO want to make Internet access available to our direct-connected nodes and client devices, if only for downloading and installing software upgrades. Use of the managed switch: -- Enables Tunneling (creation of a Virtual Private Network, VPN) to interconnect distant nodes that can t be connected using an RF link -- Enables connected clients (including computers on a home LAN) to access both the Mesh and the Internet -- Prevents RF-connected Mesh nodes and users from seeing the Internet or your home LAN -- Prevents outside Internet users from seeing the mesh or your home LAN The most common Managed Switch used in mesh is the Netgear GS108E, which sells for ~$45

20 The Mesh Ham Station is a little different than what you may be accustomed to. The antennas and transceivers are all located on the tower or a mast, outside. Lightning surge suppressors are located at the base of the tower, or at the Cat5e cable point of entry. In the illustration above, all of the PoE Injectors are lined up under the counter, one under another. In front of the PoEs are two switches; a Managed Switch at the top, and an ordinary hub switch below it. A file server, running off of a USB stick memory, is shown below the PoE units. An Analog Telephone Adaptor is shown below the switches. Computers used to control and monitor the mesh are shown on the counter.

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22 Shown here is the Node Status Page. Shown are IP addresses for the node as seen over RF, the LAN address of the node (and the root IP of the DHCP addresses it will issue to connected client devices), and the WAN Address. Also shown is the strength of the received signal and noise. The Charts button brings up a page which allows users to see the signal and noise ratio, over time, for each RF connected node.

23 Shown here is a mesh status page. It shows each station sharing an RF link to this node (on the right), and all nodes which are indirectly connected (through other nodes) on the left.

24 The node normally runs continuously; it serves as a relay, and as a mesh connectionn for clients like cameras or telephones, without the use or connection of a computer.

25 Shown here are some of the common Ubiquiti radios, used as mesh nodes, which do not include a built-in antenna. Suitable antennas for these transceivers are shown on the bottom half of the slide. A wide choice of antennas is available, to include single polarized omnidirectional antennas, yagi antennas, and Multiple-Input, Multiple-Output (MIMO) antennas which have both a horizontally polarized and a vertically polarized antenna inside the same enclosure. The MIMO antennas are used with the Ubiquiti Rocket transceiver, which has dual receivers that optimize reception in multi-path and obstructed propagation conditions.

26 Shown here are some of the common Ubiquiti units that have both antenna and transceiver built into the same package. You may note that antenna beam width can vary from very narrow (3 degrees) to approximately 120 degrees. Typical power levels, and antenna gains are shown. Additional information on Ubiquiti devices may be found on the back-up slides at the end of this presentation.

27 Ubiquiti nodes are connected to your shack using Ethernet Wire. Typically Cat5e or Cat6 shielded wire, with shielded connectors. Where required, wire may be purchased that is approved for installation in ventilation ducts and ceiling plenums, or which is water proof (gel filled) for install on towers or under ground. We strongly suggest you never buy Copper Clad Aluminum wire, often labeled as just CCA. Buy the solid copper stuff! Chinese manufacturing standards are inconsistent, and hence, performance is sometimes unsatisfactory, especially for runs over 75 feet. When you see a 1000 foot spool of Cat5e shielded wire selling for under $50.. Beware!!! The good stuff will run 15 to 20 cents per foot. Surge suppressors are available from a number of manufacturers, costing between $20 and $50 to protect a single Ethernet line.

28 Current Mesh Firmware is provided by AREDN Free of charge. History: Amateur Radio Mesh was started by a group of volunteer Hams who formed an organization called Broadband Hamnet. They developed the mesh firmware, and trademarked the terms HSMM-MESH (High Speed Multi Media) and Broadband-Hamnet. Their firmware was based on OpenWRT software, commonly used with Linksys routers. Over time, conflict arose among the BBHN software developers. Most of the firmware developers left BBHN to form a new organization called AREDN Amateur Radio Emergency Data Network. Since that split, we have seen no activity from the BBHN group, and no new firmware releases. The AREDN organization continues to develop and improve the Mesh Firmware. Features introduced by AREDN over the last three years include use of non-shared operating frequencies, compatibility with an expanded list of hardware models, built-in analytics allowing users to measure node performance, and remote node update. The Miami Valley Mesh Alliance uses the AREDN firmware.

29 We strongly recommend that you check with MVMA or AREDN, before you purchase hardware, and before installing mesh firmware. We can advise on best models for a given application, and guide you in selection of hardware that is compatible with mesh firmware currently available. We can also alert you if special installation procedures are required to successfully install the mesh firmware. Failure to use compatible firmware, or to follow specific installation instructions, can brick the transceiver when firmware is uploaded. We speak from experience. Please check with us FIRST

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31 The maximum spacing between the nodes depends on many things; the transmitted power, receiver sensitivity, antenna gain, the terrain between nodes, atmospherics (rain, snow), and the noise level presented by interfering stations. At very close ranges, some penetration of trees is possible. However, for longer distances, a clear line of sight is required AND we need more than that. The blue ellipse area surrounding the line of sight shown above is called the Fresnel (pronounced frenell) Zone. Most of the transmitted energy is contained within the Fresnel Zone. The maximum radius of the Fresnel Zone, in the center, depends on the length of the path, and the frequency. Any structure or tree which extends into the Fresnel Zone will reduce signal strength. Trees can eat between 10 and 20 db of signal strength if they fully block the zone. The only exception to these constraints, lies in the 900 MHz band which penetrates trees fairly well. All of the above factors determine Link Quality. Link Quality is defined as the percentage of packets passed, that do not have to be retransmitted. Multiple-hop mesh connections work successfully ONLY when the Link Quality is very high, connecting all adjacent nodes. Mesh is NOT a good technology to cover long distances; hence, we use point-to-point backbone links to join mesh node clusters.

32 These measures are typically used to determine Mesh link performance. Each performance parameter Is displayed on the AREDN node control pages, which you can view with a computer/web browser connected to your node. While it may appear that we can span great distances (more than 10 miles) with multiple mesh nodes, daisy chained from point to point, this is usually NOT THE CASE!! If Link Quality is moderate, the cascaded performance of multiple nodes quickly degrade the total end-to-end performance to an unacceptable level. If you have four mesh nodes daisy-chained, and the first link moves its packets with 60% LQ, the second with 40% LQ, and the third with 50% LQ.. You have a total connection that is the product of these three.. 50% of 40% of 60% = 12%... Which is useless.

33 Shown here is an example of a successful 2.4 GHz link between nodes spaced at seven miles. This link is successful because: 1) It is over primarily farm land, with little QRM, 2) The terrain is favorable to protecting the Fresnel Zone, and 3) There are few intrusions into the Fresnel zone.

34 Software is available on-line to perform mesh link propagation analysis. Hey what s That is a free, on-line analysis tool that will show lineof-sight and the related Fresnel Zone between two points, given the frequency, antenna height, and distance between the two nodes. This software is easy to use, and can be mastered in as little as an hour. Highly recommended. More comprehensive propagation analysis tools are also available. One common one is Radio Mobile. However, the learning curve for Radio Mobile is high; it requires a long time to learn the software. and frequent use is needed to keep analysis skills current.

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36 Locally, the Miami Valley Mesh Alliance has identified four Types of Mesh Deployments. Each will be explained on the slides that follow.

37 A Tactical deployment is done ad hoc, in response to a served agency need or an emergency, using mesh gear from Go Kits or other sources, stored and kept ready for such deployments. Equipment is installed on the fly. As needed, where needed. Mesh networks can be installed inside existing facilities, such as Emergency Operations Centers, Shelters, public offices, etc. when a total power down situation exists to distribute mesh capabilities and services. Typically, these mesh nets cover the last 300 feet of the service area. Outside the facility. We typically do what ever is required to connect the inbuilding mesh to the Local Area mesh, available in that neighborhood. Ad Hoc roof top mounts, mast poles, tripods, duct tape, bailing wire, etc are used to mount nodes as required to relay the mesh signal to the supported facility. When possible, we seek to install mesh capabilities in Amateur Radio Emergency Support vehicles.

38 A Ham Terminal Node. Is what a ham would install at his home, to connect to the local neighborhood mesh. It is a permanent installation, but assumes that there is no tower available, and mounting options are limited. It represents the Ham s best effort to connect to the local mesh. Mesh nodes may be located on roof-tops, chimneys, mast poles, in trees, in attics, or in second story windows. Whatever works! Typically. Directional antennas are used. The objective here is NOT to provide mesh to the neighborhood, but rather, to connect to a node that does provide neighborhood coverage.

39 Neighborhood nodes are usually placed on towers; they can provide coverage for an area one to three miles from the tower. Usually, neighborhood nodes are linked to each other, and serve to propagate the mesh net across the community.

40 High Profile Nodes, support connections between neighborhoods, and among communities. They are typically mounted on commercial towers, or on buildings at heights of 150 feet or more. They may also support Backbone links, which will be described on the following slide.

41 A mesh node System might look something like this. High power 2.4 GHz omni-directional nodes at the top of each tower support the cluster of mesh nodes in the immediate area below. Typical hardware is an L-Com 16 dbi omni-directional antenna with a Ubiquiti Bullet M2 HP. The area supported will depend on tree cover and height of the node. A coverage radius of several thousand feet is normal with moderate tree cover. The 2.4 GHz omni-directional nodes at the top of each tower will see each other, and bind as a mesh; however, low signal to noise ratios may prevent passage of high bit-rate traffic. So. 5.8 GHz directional nodes are installed to create backbone links between each tower set. The 2.4 GHz and 5.8 GHz tower nodes are direct, hardwire connected, through an Ethernet switch, enabling free flow of mesh traffic across both the 2.4 and 5.8 links. We find that 5.8 GHz links provide very high speed, reliable service between towers, given a clear line of sight, and a protected Fresnel zone. Typical 5.8 GHz hardware is a Ubiquiti Airgrid model AG-HP-5G27, which has a beam width of approximately 3 degrees.

42 AREDN firmware includes a tunneling feature. Tunneling allows us to connect mesh nodes via the Internet. In a sense, you might call it cheating, given that most will want their mesh systems to function independent of grid power and Internet connectivity. However, tunneling can be very useful in connecting far distant mesh clusters when it is not possible to make a connection using RF links. Such connections allow distant hams to use a common mesh, to enjoy common services on the mesh, to share training, etc. In our local example, it would allow us to connect mesh clusters located in the Dayton, Cincinnati, Cleveland, and Columbus areas. Given that many disasters are localized, leaving surrounding infrastructure intact, such connections could prove useful for some Emergency Communications scenarios. However, we recommend you avoid any use if Internet-dependent services for EmComm support.

43 Tunneling is easy to implement, but DOES require use of a Managed Switch capable of supporting V-LANs.

44 We believe that the Mesh Network must function when disaster strikes, and the area is total power down. We seek to restore essential network services under such conditions. Most commercial communications provided by TWC, AT&T, and most cell providers will fail within six hours of a total power down. Neighborhood-based telephone switches and light-wave relay boxes will fail within two hours. Only traditional AT&T Central Telephone Offices, with copper wire connections, large generators, and fuel delivery contracts, can be expected to survive. Hence, we seek to locater nodes at sites with reliable emergency power. Solar or Battery Backup power is required where reliable backup generators are not available. Mesh nodes are ideally suited to battery/solar operation, in that they consume less than 7 watts power, at 12 VDC.

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46 Shown here is the original MVMA mesh network. Operating on WiFi Channel 1, we had a few useful links, and some not so useful links.

47 When the AREDN firmware was introduced, we realized a SIGNIFICANT improvement in mesh performance. Suddenly, most of our nodes were seeing each other, and the links between many of our nodes could bear significant digital traffic.

48 When the AREDN firmware was introduced, we realized a SIGNIFICANT improvement in mesh performance. Suddenly, most of our nodes were seeing each other, and the links between many of our nodes could bear significant digital traffic.

49 Most recently, we have added some high profile nodes in Huber Heights.

50 When you put it all together, our network now looks like this.

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52 Basically, you can run any client device on a mesh network, that would run on any other TCP/IP network. That includes computers of all descriptions, peer-to-peer communications utilities like LinPhone and Homer (but not Skype. it must first accesses an Internet server to establishing a point-to-point connection). Mesh also supports remote control applications such as Ham Radio Deluxe, and servers supporting , texting, chat, file storage and web pages including replicated servers needed by served agencies.

53 Mesh also supports WinLink , IP (Internet Protocol) TV Cameras and laptops streaming video, IP telephones, GPS reporting devices, WiFi access points, and.. IF NEEDED.. An Internet Connection.

54 Mesh applications are endless. They include ham radio projects like remote control of Software Defined Radios, contest logging systems, group communications, and home automation systems. Mesh also offers a variety of public service opportunities; ranging from video coverage along race routes to video surveillance of wildlife linked back to school classrooms.

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56 This. Is my ONE political Slide. Over the history of Ham Radio, we have seen the introduction of many new technologies, which changed our hobby considerably. We went from Spark, to CW, to AM, to Sideband, to VHF, to UHF. And most recently to Digital Modes and Software Defined radios. Unfortunately it appears that we are using the LATEST Technology. To do the Same Old Thing.. We are still offering our the public and our served agencies Text Radiograms and the National Traffic System. Just like we did in 1955! Western Union stopped selling telegrams many years ago. But we continue. We are still offering repeater voice Communications. Hams with their HT portable units.. To back up police and fire communications.. Just like we did in We are supporting a customer (served agency).. Who has a smart phone. Who can place a video call to anywhere in the world. For free. Do you see anything wrong with this picture?

57 I believe EmComm is where Mesh shines. I think it is important to recognize that the agencies we serve (fire, police, Red Cross, government agencies, etc.) all rely upon the Internet to perform their daily business; they use on-line search, access stored data and databases, use Skype, , text messaging, social media, and most significantly.. mobile devices (smart phones). This is not lifestyle! This is the way they perform routine business. While our traditional ham services remain of value when everything else is down. What our agencies REALLY want. Is comm support that will back up the communications they routinely use. Telephone, and the Internet. Is it any wonder that the served agencies increasingly devalue our service? Mesh. can help close this gap, by restoring network services, providing high speed tactical communications, and restoring internet service to essential local agencies, delivered on their smart phones, if they desire.

58 Mesh is most useful. When it is used to leverage traditional Ham Radio. Applications like LinPhone can be used to provide Skype-Like communications, delivered to the cell phones of agency personnel. We can offer outbound in inbound service using systems such as WinLink. We can back up local voice telephone service, using Mesh IP phones coupled with modern day phone patches which tie the mesh to the MaBell public switched telephone networks. And we can do all of this, with total power down.

59 Mesh. IS HAM RADIO. But. For most hams, it is a new technology, requires new hardware, and new operational concepts. It offers vast new communications capabilities.and opens the door to many new ways to serve our communities and our served agencies. It also provides new opportunities to capture youth interest in Ham Radio. For most youth. Working DX on 20 meters just won t hack it any more.. They want to communicate with capabilities that are at least as good as their three-year-old cell phone. Mesh IS Ham Radio for the Next Generation!

60 So We Invite you to Join Us in Developing Mesh for the Miami Valley! It is a steep learning curve for all of us! To this point we have proved that MESH WORKS. Mesh is a Viable Communications Technology. And. It is Fun! C mon in. The Water s Fine!!

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69 Some have suggested operating Mesh on Channel -1. This is a bad idea.. Consumer/commercial Channel +1 WiFi traffic extends down band to the center frequency of Channel -1, 2402 MHz. There is no way you can operate on Channel - 1, at any bandwidth, without overlapping consumer WiFi which exists above 2402 MHz. Packet collisions will occur. Mesh users may operate on Channel -2 (2397 MHz), running either 5 MHz wide, or 10 MHz wide, without overlapping the Channel +1 WiFi signals. However, mesh cannot be run at both 5 MHz and 10 MHz, in the same air-space. IEEE TCP/IP signals will play nice and operate with little conflict ONLY if they are on the same channel frequency, AND if they are the same bandwidth. Otherwise, packet collisions will occur.

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