How To Make Wifi Links Over 1Km Range
We notice many of our site visitors are looking for wireless solutions for Internet access in remote, less accessible locations where normal wired infrastructure is not available or is considered to costly to implement. Even the rise of mobile Internet access is not an option from the point of view of coverage, cost or both in many locations and countries. We want to address this issue and give general advise on how to go about understanding what’s involved in finding wireless solutions.
Sharing Existing Internet Resources
There are many situations where existing Internet access is available in some locations but not in other nearby but less accessible locations. But there is a desire to share the Internet access and other resources with others. Because the costs involved in getting normal wired or cable access may not be a viable option in semi-remote locations, wireless Internet sharing can be made cost effective by cost splitting with friends, between homes or small communities. Wifi solutions are an obvious choice for these circumstances.
Wireless Solutions – How To Make Wifi Links Over 1Km Range
It is technically very feasible for ordinary people to provide a DIY Wifi solution with reasonable Internet speeds to achieve Internet and resource sharing over Wifi links of several kilometers.
The essential concept of a wireless link is that it has the same and very useful property of a physical wired link, that its digital signalling capacity can be shared, but without the physical infrastructure that wired links require.
A wireless link has its own infrastructure but can be implemented at a much reduced cost, by comparison, in many applications. There are many cases where wireless links are joined to wired networks for this reason. Sharing a links digital signalling capacity is what counts wether the link is a wired or wireless link. This moves us on to the idea of how links are shared. There are different and some common network structures for both wired and wireless networks.
Links and Network Characteristics
Each link type offers its own individual characteristics and technical challenges for example how to make an open wireless link secure. It is the sharing and management of the links and the network capacity created by these links and network server nodes they connect to that is the thing to grasp here. The networks are managed and shared resources. Fortunately for most of us these underlying managed network issues are transparent to us and we simply connect, like we all do when we access the Internet for example without worrying about how it is managed and shared. All we know is that when we access it we get a piece of it and we don’t worry about all the other pieces other people may have.
When you create private wireless links we have to be more aware of the management of the resources the link offers. A lot of the technical resource sharing management is provided for you by dedicated systems you can buy. But when planning what link characteristics you need, you will want to consider how many concurrent users the link will support and what kind of applications will be used to create traffic on the link.
Wireless links are often characterised as Point to Point (PTP) or point to multi-point (PTMP) wireless links among other network types like Mesh networks. In this article we will concentrate on a single PTP link and how to create one.
Point to Point Links
The simple idea of a PTP link is that at one end of the communication link we have access to useful and shareable resources like an Internet gateway and other network resources like printers, file systems etc. that can be shared by other people located at the other end point of the link. This is also a way to wirelessly extend local networks as if they were physically located in the same physical space. In this way resources at either end of the link can be shared as a local area network (LAN). The PTP wireless link is a relatively inexpensive way to do this.
You can view the link as providing a means to support a number of concurrent data traffic streams in both directions at the same time. The amount of traffic a link can support at the same time is influenced by the channel “bandwidth” which is characterised by a block of adjacent frequencies in the EM spectrum. This block of frequencies encompasses the signals used to transfer data. The wider the block the greater the data rate capacity of the channel using it. In computer networks, bandwidth is often used as a synonym for data transfer rate because of this relationship. Data rate is the amount of data that can be carried from one point to another in a given time. We will also use the term “bandwidth” to describe a channels capacity. In wireless networks bandwidth is often measured as the number of data bits per second (For example n Mbit/s – e.g. 20 mega bits per second). If you think of a link as a pipeline carrying water then it will support a number of open taps each delivering a finite number of litres of water per second. The number of open taps is like the number of concurrent users using the link at one time. The amount of water passing per tap per second is like the effective data rate per user. The link capacity or bandwidth is then analogous to the number of concurrent users x the planned data capacity per user.
Link Quality and Link Capacity
The ability of a link to support high data rates and therefore the number of concurrent users with a given data rate each, depends upon the channel “bandwidth” and the signal quality in relation to channel noise introduced by several factors on the link. In this case we are using the term “bandwidth” in the sense of a block of frequencies used to convey the signal in question. The higher the signal level in relation to channel noise the higher the data rate the channel can support. So when we consider setting up a wifi Link we have to be aware of these basic facts.
It is quite normal in all radio communication for signals to become weaker the further from the radio transmitter you get. In fact if you double the distance the power will fall by a factor of 4 just in free unobstructed space. This is quite apart from other losses. Signals can be blocked by obstacles, reflections, diffraction, absorption and have multiple path fading losses at the receiver.
What Makes a Wireless Link
The first thing to realise is that a Wifi link is created by the use of wireless radios combining radio transmitters and radio receivers. The transmitters are used to transmit digital signals and the wireless receivers are designed to receive those signals. The original digital signals are transferred by changing the properties of the waves used to send the signal. These include the wave amplitude, frequency, phase and polarisation of the transmitted waves. This process is called modulation. The process of recovering the original digital signal is called demodulation. The everyday term “modem” derives from the terms “modulate” and “demodulate”. The radios used operate on preselected frequency channels contained within pre-agreed frequency bands. In general Wifi operates on a number of channels (up to 13 depending on your world location).
These radios have a certain amount of signal transmit power (Tx power) which is limited by statutory regulations and this varies by country. The radios have to be capable of receiving relatively weak signals and their ability to do this is measured by their receiver sensitivity. The better the sensitivity the longer the physical link distance that is possible.
A major feature of all radio communication is the use of specifically designed radio antennas. These antennas are used to focus the radio signals in a way rather like optical lenses are used to focus light. In the case of PTP links they are used to focus the radio signals into narrow beams. These narrow beams offer greater concentration of signal in the desired direction at greater distances, so more signal is received by the radio receiver antenna located at these end points. A measure of an antenna’s ability to convey a signal in a directed path at a distance is called “Antenna Gain”. The greater the gain the more the signal power is received at a prescribed point.
The combination of good radios and powerful directional antennas leads to the possibility to get wireless links working over a few kilometers. It is now quite common to combine a radio with an antenna in one complete package although connecting them together as discrete components is also common.
Hardware and Software Required in a Wifi Point to Point Link
The radios and associated hardware in a Wifi link these days are controlled by a mixture of embedded systems and programmed hardware. Quite often at the heart of this programable hardware are dedicated digital signal processing chips created to support the international wireless protocols standards, which for Wifi are know as the 802.11 standards.
Dedicated embedded systems which are completely self contained are now available at reasonable prices. In these systems there is a on board computer and operating system which commonly are built on Linux kernels. These systems can be programmed in a variety of ways to fulfil different roles in a Wifi link with minimal dependency on other devices.
Other more simple systems have programable devices which have device drivers running on computers like simple wireless adapters which may be connected by USB or POE applied to slots on the mother board or integrated on mother boards. Most devices computers, laptops, tablets and mobile phones incorporate some Wifi functionality.
Wifi Access Points and CPE’s
A Wifi access point (AP) is a familiar item found in many public places where they are called “Wifi Hotspots”. A number of people connect their Wifi enabled devices to the AP which then act as wireless clients to the AP. They connect to the Internet and use popular applications like Email, Facebook, Twitter and chat to keep in touch. The AP broadcasts its “SSID” which is the access points name. This name is often protected by “pre-shared” key or pass phrase which when used by wireless clients gives secure access to the AP encrypting your data over the wireless signals. In many public places like a Shopping Malls, wireless access is open and there is no password required to gain access. But in this case tight data security may not be required.
A dedicated Wifi access point in a PTP link has the role of being an end point to a single Wifi client station at the other end of the Wifi link. It is a One-to-One connection only. The wireless client in this case is called a CPE (customer premises equipment – for historical reasons) or a wireless station. Its role is to share the link capacity provided by the AP with local users at a remote location. Like dedicated AP’s, CPE’s when used in a PTP situation are often self-contained embedded systems which are connected to a local router like a AP’s are at the other end of the wireless link. They can often be the same systems programmed to act in the role of either an AP or a CPE.
Dedicated Wifi systems used for PTP links are generally more powerful, flexible systems and offer higher speeds, higher quality, more control, durability and more features and configurability than simpler domestic Wifi equipment.
WAN and LAN
In a normal network there is normally a router device which routes data packets in the internal LAN and also routes data packets to locations outside the local network. The networks accessible outside of the routers internal LAN is accessed through the wide area network interface called a WAN interface. In normal domestic situations our home routers connect us to an Internet service via modem connected to the routers WAN port. The modems are there to convert the digital signals present on the LAN into a form that is compatible with and will pass through the physical medium the wider network is made of. This could be telephone lines, optical fibres, terrestrial wireless or satellite links.
In a PTP link the aim should be to create it to operate at just one speed and that this speed should be the highest speed possible consistent with the application needs of the link. Bear in mind that given the pace of technology change this normally equates to the best speed available. Normally a PTP link provided by an embeded system is connected locally to the WAN side of the main router at an end point of the PTP link.
Quite apart from the 802.11 standards a number of proprietary wireless link solutions exist. These solutions offer unique link protocols which offer considerable link speeds and special features. Take note that the link protocols and speeds on the link can be independent of the protocols and speeds employed by the network the link is connected to.
We have mentioned radio transmit power, receiver sensitivity and signal losses earlier but there are other losses to consider in making a practical Wifi link. Cable and connector losses are also a factor in working out how much useful signal survives for practical purposes. Cable losses increase with the applied frequency and even with special low loss cables there can be considerable losses when cable lengths are used to feed antennas.
In general the gains and losses in a link should be calculated for effective planning and this concept is generally known as the “Link Budget” and elsewhere on this blog you will find calculators to do the link budget calculations complete with explanations of the concepts involved.
Bridged Versus Routed Access Points and Stations
There are two primary modes of operation for wireless devices; bridged and routed. Although the actual configuration of their wireless components is very similar, there are differences that need to be explained and understood.
Routed Wireless Device
A routed device consists of multiple interfaces, In the case of our PTP link we have an AP and a CPE both of which have a wired and a wireless interface. Data must pass between these interfaces for effective communication. If a device is configured as a routed device then the data packets transferred between the interfaces must follow routing rules to determine packet flow. All packets must follow the routing rules, firewall rules, various queue and other system specific rules.
Bridged Wireless Devices
A bridged device consists again of multiple interfaces, but one or all of these physical interfaces are joined together into a logical interface called a bridge. The bridge interface joins the physical devices at Layer 2, such that any packets entering one interface pass freely out the other interface on the bridge (except in the case of a bridge firewall). In the configuration of a wireless interface joined to an Ethernet interface, think of a bridged device as a media adapter, joining two dissimilar media together.
Bridges are a useful configuration, especially with respect to wireless, however they must be used with caution because it is very easy to create a network with bridges that is not scalable and quickly outgrows itself. As a general rule of thumb, never connect two bridged devices together, either directly or with a switch. Instead, separate bridged links with routers. Routers block broadcast traffic and thereby allow you to use bridged devices while building a scalable network.
The examples that followed can be each configured in succession to build a complete wireless AP solution.