Wireless Internet Access in Latvia

Guntis Barzdins (main contact person)

Institute of mathematics and Computer Science, University of Latvia

Rainis blvd.29, Riga LV1459, LATVIA

Phone +371 9206943, Fax +371 7 820153, E-mail guntis@mii.lu.lv

John Tully

Acasia Research

Aizkraukles 23-514, Riga LV1006, LATVIA

Phone +371 9204836, Fax +371 7 820467, E-mail tully@mii.lu.lv

Keywords: Expanding and Enhancing Internet Access, Wireless IP, Developing countries experience

One Page Abstract

The Latvian Research Network LATNET was established in 1992, shortly after breaking of the USSR. The limited resources and poor communications infrastructure in the country (only analogue lines available) have forced LATNET to search for alternative ways to establish high speed data links for Internet access to the numerous University of Latvia departments scattered around the capital city Riga. The liberal radio frequency licensing policy and newly appeared inexpensive and easy to use wireless LAN products, made it possible to develop a high speed spread spectrum wireless Internet access network. There are currently over 30 academic, research, government, and other sites - spread all over the city - using these high-speed wireless links for Internet connection as an alternative to lower speed leased telephone lines. The many inquiries from fellow internetworkers of how this is done have lead to the composition of this paper to which, besides authors, actually have contributed people from various countries building similar wireless networks elsewhere.

Wireless networking still is a new technology and has not yet become common. Setting up a wireless network requires knowledge in many fields - networking, computer hardware, radio communication, wireless licensing regulations, and even geography. The purpose of this paper is to describe all basic things one might need to start developing a wireless Internet access network.

What spread spectrum radio is? This section of the paper briefly explains the principle of spread spectrum data transmission and why in most countries it can be used license free without interfering with the conventional radio transmission or other users of spread spectrum in the same area. Licensing terms for various countries are provided.

What You can buy off-the-shelf? The specifications of the most popular spread spectrum wireless products on the market are given here. For simple installations there is no need to "invent a bicycle" - just buy a complete set of hardware+software and plug it in. For bigger installations (like the one in Riga), or budget tight installations, the hardware described here is the building blocks of your network.

Wireless IP access network in Latvia. In this section is described the actual experience and solutions used in the wireless network developed by LATNET. Actually LATNET has developed two independent wireless networks: one based on 915MHz WaveLAN equipment, and another based on 2.4GHz ARLAN equipment. The networking specifics of both technologies and at both frequencies are described in detail. The specifics of the networks developed in Riga is that they are point-to-multipoint networks, where the central antenna provides Internet access to many (around 30) client sites spread in the radius of 15km. The point-to-multipoint setup significantly saves the resources, because connection of additional client site requires only one wireless adapter and a router.

A separate subsections briefly introduces JNOS - an easy to use public domain IP routing software for MS DOS based PC. This software is used for IP routing between the wired and wireless networks. JNOS is a descendent of the famous KA9Q software.

Another subsection provides the basic facts about radio antennas, cables, connectors, and amplifiers. Various factors influencing the maximum communication distance are considered: line of sight, attenuation in cabling, antenna gain, bilateral power amplifiers.

Despite itís relative complexity, the wireless IP networking has proved to work reliable in LATNET and elsewhere. It has become a viable and cost effective alternative to wired IP networking.

 

Wireless Internet Access in Latvia

Guntis Barzdins

Institute of mathematics and Computer Science, University of Latvia

Rainis blvd.29, Riga LV1459, LATVIA

Phone +371 9206943, Fax +371 7 820153, E-mail guntis@mii.lu.lv

John Tully

Acasia Research

Aizkraukles 23-514, Riga LV1006, LATVIA

Phone +371 9204836, Fax +371 7 820467, E-mail tully@mii.lu.lv

 

Keywords: Expanding and Enhancing Internet Access, Wireless IP, Developing countries experience

 

Abstract

This paper describes the experiences accumulated by the Latvian Research Network (LATNET) where wireless solutions for connecting to the Internet, have been used routinely since 1993. The transmission technology used in LATNET is based on spread spectrum wireless LAN adapters adjusted for Internet interconnection - the original antennas are replaced with high gain antennas installed on tall buildings around the city and PC based IP routing software (JNOS) is used to provide efficient point-to-multipoint connections and error correction. Besides describing LATNETís experiences, this paper also contains a brief introduction to the underlying wireless technology, licensing terms in different countries, and overview of several wireless products. The paper should be of interest to networkers of developing countries where communications infrastructure is weak and wireless communications are not over-regulated. Wireless IP networking might be a cost effective solution also in industrial areas of the world.

 

1 Introduction

Sending data over the wireless links has been a dream for many networkers over the years, especially in the countries where appropriate communications infrastructure is not available. Until recent time wireless data links really were only a dream for most of us, because available radio transmission technology was:

Only a few years ago a completely new wireless data link technology arrived which has made building private wireless data links as simple as plugging a radio adapter card into PC and connecting this card to an antenna. The new technology operates at LAN speeds (up to 2Mb/s) at distances up to 45km (with direct line of sight), and costs around 800 USD for the necessary PC card and high gain antenna. And what is most attractive about the new technology - in many countries (USA, most European countries) it is that it does not require a licence for operation at low power and short distances (5 -10 km). What is this miracle technology? It is called simply "spread spectrum radio".

Since its introduction to the market in early 1990ís, the popularity of spread spectrum wireless data transmission has growing dynamically. Today wireless LANs are used in many university campuses to interconnect distant buildings, in banks, hospitals, warehouses, shops etc. In these places it has become as common as cordless telephones.

In this paper we will describe in detail the usage of wireless technology for providing Internet access. The description is based on the experience gained in LATNET while setting up a big wireless Internet access network in Riga, the capital city of Latvia. The network started in 1993 when five University of Latvia departments scattered all around the city were connected by wireless links to the Internet. Since then the network has evolved significantly: now it provides Internet connectivity at aggregate speed of 2Mb/s to more than 30 buildings in Riga and its suburbs. Four backbone network antennas are installed on two high buildings in the central part of city, each antenna capable to serve more than 20 connections in the radius of 15km. The distinct property of the Riga network is that radio links are set up in the point-to-multipoint mode which significantly reduces the cost of individual connections. The remarkable feature of the network is that it has been built using only standard wireless LAN equipment operating at both 900MHz and 2.4GHz frequencies. This means that the solutions used in Riga can easily be repeated anywhere else.

The paper is organised as follows. Section two briefly describes the spread spectrum radio transmission principle and explains why it usually does not interfere with other radio transmissions. The licensing terms for various countries are considered. Section three lists popular spread spectrum wireless data communication products which can be purchased "off the shelf." For small installations these off-the-shelf products might be all that you need. Section four gives a more complete example, based on LATNETís experience in the city of Riga, of how the wireless data communication products can be used to build an Internet access network in a city environment.

 

2 What Spread Spectrum Radio is?

Spread spectrum technology was developed for use by U.S. military to overcome the problem of intentional interference by hostile jamming and eavesdropping (espionage). The term "spread spectrum" arose from the characteristic broad spectral shape of the transmitted signal.

The spreading techniques normally used can be divided into two families: Frequency Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS). The first approach resists interference by jumping rapidly from frequency to frequency in a pseudorandom way. The receiving system has the same pseudorandom algorithm as the sender, and jumps simultaneously. The second approach (most famous) resists interference by mixing the data signal with a Pseudo Random Noise Code (PN-code). A PN-code is a noise-like sequence of chips valued 0 and 1. The number of chips within one code is called the period of code. In the most simple case a complete PN-code is multiplied with a single data bit (see Figure 1) and thus the resulting signal bandwidth becomes much larger. Receiving system applies to coded signal the same PN-code and retrieves the data signal.

Figure 1. Direct Sequence Spread Spectrum signal components

The total signal power doesnít change during spreading, the Power Spectral Density decreases. The result is a signal that is essentially "buried" in the noise floor of the radio band, is extremely hard to detect, does not interfere with other services (like conventional AM and FM radios), and still passes a great bandwidth of data (due to much higher than usual bandwidth occupancy). However the Gaussian Noise level is increasing.

The receiver examines the bandwidth of the spread signal, and correlates the data (despreads it). The process of correlation is symmetric to spreading and causes any other signals received to be spread as the wanted signal is de-spread (Figure 2). This causes unwanted signals to be reduced to noise - allowing the same frequency to be reused many times within a given area. The possibility to multiplex users in the same frequency band by assigning them different spreading keys is called Code Division Multiple Access (CDMA).

Figure 2. Interference reduction in Spread spectrum transmission

The U.S. military de-classified spread spectrum technology in the mid-1980s. The FCC in 1985 approved (47 Sec.15.257) three so called Industry, Medical & Science (ISM) bands for licence free use: 902-928MHz, 2.400-2.4835GHz, 5.725-5.850GHz. These bands are used by microwave ovens, garage door openers, and also spread spectrum radios. FCC allows maximum 1 Watt of radio power, with up to 6 dB antenna for spread spectrum devices operating at ISM bands. Higher gain antennas may be used, with a corresponding reduction in transmit power. With appropriate outdoor antennas, and line of sight conditions, 40 mile paths are possible, with some types of spread spectrum radios. As these devices do use radio signal the devices themselves must be approved by the US Federal Communications Commission.

The present situation in most of Europe is that 2.4-2.4835GHz use needs no license with ETSI (European Telecommunications Standards Institute) certified equipment. According to ETSI standard ETS 300 328, the effective radiated power from the antenna must not exceed 100mW which is significantly lower than allowed by FCC in U.S. The 902-928MHz band generally is not permitted, because 890-915MHz band in Europe has been allocated for GSM mobile telephony.

In the developing world the situation will go country by country. Our experience has been that one way is to obtain a secondary license for use of the spectrum you desire to use (2.4GHz will be the easiest), with a stipulation that you will not interfere with the primary license holders transmissions. An example of this situation is in Latvia, where LATNET has a secondary licence for 902-928MHz band, while primary licence for 890-915MHz band belongs to GSM operator. Practice shows that low power spread spectrum wireless LAN adapters at 902-928MHz band do not interfere with GSM.

Those who would like to learn more about spread spectrum radio, can check Spread spectrum home page on http://www.tapr.org/ss/other.html.

 

3 What You can buy off the shelf?

The wireless industry now offers a wide range of spread spectrum devices operating in the licence free ISM bands. Currently data rates from 1200 baud through 2Mb/s are available. Many hardware interfaces are available, including V.35, RS-530, Ethernet, T1, and RS-232. Here we will list the most popular series of these devices.

All above mentioned hardware manufacturers provide also necessary software and manuals for installation and operation of the devices, as well as they sell separately high gain antennas and RF cables. Meanwhile many other companies provide better accessories - less expensive (but good) antennas and custom made cables. There are a number of companies which sell also their own LAN bridging software+hardware based on WaveLAN adapters: Persoft, C-SPEC, Solectek, KarlNet. For CYLINK Airlink modems the LAN bridges are sold by CYLINK, while specialised IP routers are made by TAL Inc. Bilateral amplifiers for 915MHz and 2.4GHz bands sell Hyperamp Inc.

For simple installations with point-to-point or few point-to-multipoint connections there is nothing easier than selecting one of the above mentioned "off the shelf" LAN bridging products (they come complete with antennas, cables, software, and hardware.) A very good overview of LAN bridging products by "Network Computing" can be found at http://192.216.191.71/techweb/nc/613/613f2.html.

The above mentioned products are not specifically aimed for providing Internet connectivity - they are general purpose data links or wireless LAN bridges (although C-SPEC, KarlNet and TAL products provide also IP routing functionality). If wireless links are used solely for providing IP connectivity between the distant sites, then there is less expensive and more flexible solution available, described in the Section 4.

Finally it has to be mentioned that in near future we can expect new products entering the wireless market, as the technologies like IEEE 802.11 standard and ETSI HIPERLAN standard for wireless LAN will be finalised. These standards will permit development of competitive wireless LAN market, currently dominated by the proprietary products and protocols (this is why none of the above mentioned products will interoperate with others).

 

4 Wireless IP Access in Latvia

The goal of wireless Internet access network set up in capital of Latvia, Riga by LATNET is to provide Internet connectivity to various University of Latvia departments and other establishments scattered around the one million inhabitant city with a diameter of approximately 25km. Riga is on a plain without hills with several high (25 store) buildings in the central part (hotels, office buildings). Since international Internet connection for LATNET is over single 128kb/s line, then 64kb/s to individual sites is considered sufficient.

From various spread spectrum hardware options (listed in Section 3) we chose to use wireless LAN, because:

The wireless network for Internet access in Riga has been in operation three years now, since starting in 1993. Actually two independent radio networks have been built (for comparison purposes) based on different hardware: WaveLAN at 915MHz and ARLAN at 2.4GHz.

Figure 3. Point-to-multipoint wireless IP access network

The WaveLAN 915MHz installation historically was the first. We have found it necessary to add link level error correction (LAP-B) in router software for WaveLAN installation, because in case of marginal radio signal quality at long distances, multipoint configuration, and heavy traffic load, a significant packet loss (>30%) is encountered. The random 30% packet loss deteriorates TCP/IP performance catastrophically. Meanwhile, after adding link level error correction the 30% packet loss causes only some speed reduction.

The ARLAN 2.4GHz network installation was started in early 1995 because of two reasons: ARLAN 2.4GHz is ETSI compliant and therefore will not cause licensing problems in long term, and ARLAN has built-in link level error correction, thus avoiding the need for error correction done by routers. ARLAN also has more sophisticated management and security features.

As PC routing software was chosen JNOS - freeware software developed by ham packet radio community and freely available on the Internet. JNOS has originated from the famous KA9Q package developed by Phil Karn in 1990-1991. Actually there are many IP routing software options available for PC: any UNIX running on PC, Windows NT, Novell server, KA9Q, PCroute, etc. The main reasons for selecting JNOS were:

Therefore in this paper we will consider only JNOS as a PC based router software.

Finally we will give also short overview about cables, connectors, antennas, and amplifiers used with 900MHz and 2.4GHz spread spectrum radio systems. The typical working distances for different setups are given.

 

4.1 Using JNOS for building a PC based router

JNOS (Johan Reinaldaís Network Operating System) is one of the descendants of the famous KA9Q software developed by Phil Karn primarily for ham packet radio use, but afterwards widely used also for IP routing in other circumstances. Ham packet radio networks were initially low-speed (1.2kb/s) and very congested and unreliable. This lead to the reliable link AX.25 protocol (a derivative of X.25) for the encapsulated support of IP in wireless links (we will use this feature in advanced setup of WaveLAN network). Compared to original KA9Q software, JNOS has many more features and also is more reliable. You can download the latest version of JNOS from ftp://ftp.ucsd.edu/hamradio/packet/tcpip/jnos. You must download three files (version numbers may change):

With JNOS being very well documented, there is no need to repeat it in this paper. Moreover, you can subscribe to JNOS mailing list nos-bbs@hydra.carleton.ca. We will only mention that JNOS has incredibly lot of features, including ping, traceroute (called Ďhop checkí), telnet, ftp, SLIP, PPP, SMTP, POP, NNTP, RIP, IP tunneling, IP filtering, remote configuration and rebooting, lot of amateur packet radio specific stuff, etc. Meanwhile you can recompile JNOS only with the features which you really need, and get a small and efficient executable file. Here, to sense the flavour of JNOS, we will show only tiny example of how to configure JNOS as IP router between multiple packet driver interfaces (for example, wired Ethernet LAN adapter and wireless LAN adapter). To start an IP routing process on the PC, it is necessary to:

    1. Load the FTP Specification compliant packet drivers for both LAN interface cards. These drivers usually are supplied with LAN adapters on the diskette. Assume that the drivers are loaded at interrupts 0x60 and 0x62.
    2. Create the JNOS configuration file autoexec.net. In the configuration file must be configured interfaces (e1, e2 - name of interface, 8 - output buffer size in packets, 1500 - MTU of interface) and IP routing table:

      attach packet 0x60 e1 8 1500
      ifconfig e1 ipaddress 10.5.5.1
      ifconfig e1 netmask 0xffffff00

      attach packet 0x62 e2 8 1500
      ifconfig e2 ipaddress 10.5.6.1
      ifconfig e2 netmask 0xffffff00

      route add 10.5.8.0/24 e2 10.5.6.3
      route add default e1 10.5.5.2
    3. Start JNOS with this configuration file:
      jnos110k.exe autoexec.net

This is all - now JNOS is routing the IP packets between the two interfaces. After starting JNOS, your PC screen gets into the monitor mode from where you can issue numerous JNOS commands to monitor the network (type "?" for the list of commands). Later you can always return to the monitor mode by pressing F10.

There are a few things to remember about JNOS: the PC on which you run JNOS preferably must be dedicated for that, you may remove keyboard and screen from this PC when it is configured. No hard disk required in this PC - DOS, JNOS and configuration file will fit on floppy disk. JNOS will work on any PC, including 8086 with 640K of memory, while at least 386SX 40MHz is needed for full speed. JNOS can be configured with a maximum of 3 packet driver interfaces.

 

4.2 ARLAN Network Setup

Setting up IP router network using ARLAN wireless LAN is very simple: ARLAN completely hides the radio "visibility/invisibility" and packet loss problems from the user, therefore regardless of the actual topology of radio links, the whole wireless network to the user appears as one long Ethernet cable. This property is achieved by employing already in the ARLAN hardware the link level cell routing between repeaters, and cell acknowledging and retransmission. The only price for this simplicity of use is higher cost of ARLAN (compared to WaveLAN) and lower (but guaranteed!) throughput.

To set up the ARLAN network, you must use ARLAN 630 Access Point for the central site (Access Point is a "transparent" device that connects two medias: wireless LAN and wired Ethernet),

and ARLAN 655 ISA adapters mounted in PC routers for the client sites (Figure 4). ARLAN 655 ISA adaters normally do not communicate directly, but all communication between ISA adaters goes through the central ARLAN 630 Access Point. The distinct property of ARLAN is that several Access Points can be used to cover larger area; the Access Points themself can be interconnected through wired Ethernet interfaces, or over radio, if one Access Point can "see" another Access Point. In this way it is possible to use an Access Point as a radio repeater to extend the range of wireless communication.

Figure 4. ARLAN 630 Access Point and ARLAN 655 ISA adapter

All ARLAN devices come complete with good manuals and utilities for set-up and testing of radio connectivity. To set up ARLAN devices, in the simplest case you have to configure only three parameters: bitrate, frequency, and network-id. ARLAN 630 Access Point can be configured locally through terminal, or remotely over telnet. ARLAN 655 ISA cards are configured through pkt.ini file used by packet driver. Once radio connectivity between the devices has been established, you can forget about ARLAN being a wireless network. In the PC router containing ARLAN 655 ISA and ordinary wired Ethernet adapter, load the packet drivers for both adapters and run the JNOS with a similar setup as shown in the previous section. Adjust the IP routing as necessary.

There is one thing to be careful about ARLAN - there are ARLAN modifications for Europe, USA, Japan etc., which have slightly different frequency sets - modifications for different regions generally will not interoperate!

 

4.3 WaveLAN Network Setup (Simple Case)

Setting up WaveLAN based wireless network is quite different from that described for ARLAN. The basic difference is that WaveLAN has no link level protocol, except CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance, similar to CSMA/CD used by wired Ethernet) to avoid too many collisions and packet losses due to simultaneous transmissions. WaveLAN has no packet forwarding, or acknowledging/retransmission functionality. This means that the WaveLAN based wireless network behaves as one long Ethernet cable on which some stations "see" each other, but others donít (depending on the radio "visibility" between the stations); significant packet loss can occur in case of weak signal, radio interference, or network congestion.

In WaveLAN based network, WaveLAN ISA adapter equipped stations communicate peer-to-peer. Although there is available also WaveLAN Access Point which connects transparently wired network to wireless network, it passes packets only from wired network to wireless network and vice versa. WaveLAN Access Point never passes packets from one wireless station to another wireless station (if there is such need, the packets must be routed by the JNOS software). This also means that WaveLAN Access Point cannot be used as radio repeater to increase the range of wireless communication (as it was in case of ARLAN). In most cases WaveLAN Access Point has no use for providing wireless IP connectivity.

Figure 5. WaveLAN ISA wireless LAN adapter

Despite all of the above mentioned limitations, the WaveLAN still is a highly useful device for providing IP connectivity, if used properly. Besides other things, WaveLAN is twice less expensive than ARLAN and at short distances delivers higher speeds (2Mb/s).

Setting up IP routing in WaveLAN wireless network is similar to ARLAN case, except that there is no central Access Point - all WaveLAN nodes communicate peer-to-peer. First, using utilities supplied with WaveLAN ISA adapter, you must make sure that radio connectivity between the necessary sites has been established (for WaveLAN there is only one parameter to configure - network-id). One should use the test utility supplied with WaveLAN, which provides a realtime graphic display of signal level, packet loss, and link reliability. After performing these end to end tests, in each PC router containing WaveLAN ISA adapter and wired Ethernet adapter, load the packet drivers for both and run the JNOS as described previously.

If you have a portable PC, the PCMCIA WaveLAN adapter is a very valuable tool for surveying proposed sites and troubleshooting links and software configurations. Unfortunately, you cannot connect an external antenna to this adapter.

There are several things to remember while setting up WaveLAN wireless IP network:

 

4.4 WaveLAN Network Setup (Advanced Case)

Advanced WaveLAN setup deals with two problems (which in case of ARLAN are solved already in the ARLAN hardware):

The features necessary for solving the problem of packet acknowledgement and retransmission, are present in the JNOS software. JNOS besides IP protocol supports also AX.25 protocol which is an amateur packet radio variation of classic X.25 protocol. For link level transmission control X.25 (and AX.25) relies on LAP-B protocol which is effectively a sliding windows protocol with packet acknowledgement and retransmission. JNOS permits tunnelling of AX.25 protocol over IP network and vice versa. The solution used in Riga relies on double tunnellinng: on the WaveLAN wireless network natively is run closed transport IP network with internal IP addressing and reduced MTU (MTU has to be reduced, because in noisy environment shorter packets have higher probability to get through uncorrupted). The wireless transport IP network is used to create AX.25 virtual circuits between the PC routers on the network; these circuits internally use error correcting LAP-B protocol. Finally, the real Internet IP is tunnelled through error free AX.25 virtual circuits (Figure 6).

Figure 6. Packet acknowledgement and retransmission requires double tunnelling in JNOS

Such double tunnelling, of course, reduces the throughput - the speed of individual IP links drops to approximately 64kb/s. Meanwhile, this mechanism preserves the radio network from being overloaded by the single user. The radio network itself can carry simultaneously approximately 20 such reliable 64kb/s IP links.

Configuring JNOS for double tunnelling involves following tasks:

Following is an example of JNOS configuration file containing all these functions. Both ends of virtual circuit must have this kind of configuration.

ax25 blimit 2

ax25 irtt 100

ax25 maxframe 3 Adjusting AX.25 LAP-B parameters

ax25 maxwait 500 for optimal performance over WaveLAN

ax25 pthresh 1000

ax25 retry 5

ax25 timertype linear

attach packet 0x60 wave1 8 300 WaveLAN interface, closed transport IP

ifconfig et1 ipaddress 10.1.1.1 with private addresses and reduced MTU

ifconfig et1 netmask 0xffffff00

attach packet 0x61 et1 8 1500 Wired LAN interface, real Internet

ifconfig et2 ipaddress 159.148.37.254 addressing and MTU

ifconfig et2 netmask 0xffffff00

attach axip tun1 256 10.1.1.2

ifconfig tun1 mtu 1500 AX.25 tunnel interface, real Internet

ifconfig tun1 ipaddress 159.148.38.1 addressing and MTU

ifconfig tun1 netmask 0xffffff00

mode tun1 vc

route add default tun1 159.148.38.2 Uses AX.25 tunnel for real Internet packets

 

The second problem of establishing a central gateway node on the WaveLAN wireless network to avoid necessity for full peer-to-peer radio connectivity between the client nodes, has no simple solution.

If the wireless network has a gateway to Internet at the central node, without doing anything special all clients will have connectivity to the global Internet, but will not be able to communicate between each other (without direct radio connectivity). To deal with this problem, in Riga we are using AX.25 tunnels from the client nodes to the central node (central node handles AX.25 tunnel interfaces to all clients) and all real Internet routing is implemented only on the AX.25 tunnel interfaces. In this way everybody can communicate to everybody through the central node. JNOS supports also simple IP over IP tunnelling (encapsulation) without error correction. You can use this type of tunnelling to ensure full connectivity, if error correction is not an issue in your setup.

 

4.5 Antennas, cables, connectors, amplifiers

The distance a connection can be established at is a factor of the antenna gain, signal loss in cable and connectors, line of sight, transmitter output power, amplification of the signal going to the antenna, and pre-amplification of the signal going to the radio receiver.

Basic concepts:

Antennas are key components which can vary greatly for different needs. The manufacturers of radio cards generally include a small omni-directional antennas which can easily be mounted on walls. To cover large distances, it is necessary to install external high-gain omni and directional antennas.

Directional Antennas are used at the client sites or on very long point to point links. The common Loop Yagi directional antenna used by LATNET is one meter long with a height of 12cm and width of 12cm (Figure 7). It has a gain of 14.5dBi. Increments of plus or minus 3dBi indicate a doubling or halving of gain of the antenna (i.e. doubling or halving the communication distance.)

 

Figure 7. 16.5dBi Loop Yagi antenna for 915MHz

For common point to point connections at 915MHz, 14dBi antennas are suitable and economical at about $100. Connectors for antennas should be "N-Female."

 

Omni directional antennas with high gain are used at the central site. The quality and performance of this antenna is crucial for high performance in the whole system. They are more difficult to make and therefore more expensive. LATNET is currently using the custom made 12dBi 915MHz omni-directional (Figure 8). The cost of such antenna is around 800USD.

Figure 8. 12dBi 180 degree antenna for 915MHz

Antenna Cables. It has happened many times that when installing a system people will find no connection or a bad connection. The simple culprit in many cases is bad or poorly constructed (or planned) antenna cabling. Although for short distances you may use ordinary Ethernet cable with BNC connectors, for quality installations it is generally good to use at least Belden 9913 or similar type high quality low loss cable with "N-connectors" on both ends. If you have access to it, 2.5cm hardline (low loss rigid coax) is the perfect cabling (often available from cable TV and microwave companies.) Cable runs should generally be no more than 50 feet. If it is necessary for longer runs then an amplifier should be considered for replacing the power lost in the cable attenuation. Note that WaveLAN uses 75ohm cables. Here is some typical loss information for coax cables for 915MHz signals:

Belden 9913 -4.2dB/100ft

RG-59 -11dB/100ft

RG-8 -7.5dB/100ft

Copper Heliax 1 5/8" -0.7dB/100ft

Copper Heliax 7/8" -1.3dB/100ft

Copper Heliax Ĺ" -1.7dB/100ft

Line of Sight and Antenna Location. When attempting a connection beyond 200 meters, a line of sight between the antennas is necessary. When using directional antennas, reflections off of buildings and roofs can assist with a short connection where no line of sight installation is possible. Antenna location depends not only on line of sight but also possible interference. Generally tall buildings are a magnet for all wireless systems. LATNET had the unfortunate experience of burning itís amp/pre-amp system mounted near a police radio transmitter that was putting out a high powered signal near 460MHz. Their antenna produced a double harmonic frequency at 920MHz. Usually moving antennas at least five meters apart corrects such problems and allow all parties to use the same antenna tower.

The communications at 915MHz and 2.4GHz are not substantially affected by the weather conditions (rain, snow). Meanwhile the performance of antenna itself might deteriorate if it is covered by ice or snow. Therefore in very hard climate it is a good idea to mount antenna inside plastic tube of sufficient size (so called "radom".)

To calculate the possible line of sight for longer distances (more than 10km) it is necessary to take into account also curvature of the earth. If h1 and h2 are heights of the antennas (in meters), the maximum distance (in kilometres) for line of sight between the antennas in plain area can be calculated according to the formulae 3.55(sqrt(h1)+sqrt(h2)). For example, elevation of antennas 5m and 40m give line of sight 30.30km = 3.55 ( SQRT (5m) + SQRT(40m) ).

Amplifier and Pre-amplifier. The transmit power of wireless LAN adapters is very low (100mW for 2.4GHz models, 250mW and 1W for 915MHz WaveLAN and ARLAN models respectively). For longer distances, bilateral amplifiers - which include transmit and receive amplification - are being used by LATNET. The Hyperamp 5 watt 915MHz has shown positive results extending range up to 40 km with high gain directional antennas at each end.

LATNETís Antenna Setup. LATNETís central antenna site for WaveLAN network uses 12dBi 915MHz omni-directional antenna that covers most of Riga. Most connecting sites use a Loop Yagi 14.5 dBi 915MHz. Far off sites (15-25km) use a 5watt Hyperamp bi-lateral amplifier with an extended Loop Yagi 16.5dBi 915MHz.

 

5 Conclusion

We have tried to describe in this paper all the basic things you might need to start developing a wireless Internet access network. We have described the solutions which are tested to work well in relatively big network in the city of Riga.

All freeware software mentioned in this paper can be found also in ftp://ftp.latnet.lv/pub/radio/ directory. All equipment mentioned in LATNET setup (adapters, antennas, cables and amplifiers) in detail are described (including ordering information) in http://www.pmi.lza.lv/ACASIA.

Finally, we want to stress that wireless Internet connections are not a replacement for wired solutions - they should be used in applications where more traditional wired solutions are not possible or cost effective for the throughput required. Compared to wired connections, wireless network involves more components (PCrouters, radio adapters, cables, antennas) which all have to be properly installed and maintained to avoid network failures. Wireless is also subject to weather conditions - icing, lightning, extreme temperatures, etc. Wireless requires careful access configuration to protect your communications from intruders. And finally, the wireless solutions described here are not mobile solutions - mobility would require much higher transmission power to ensure stable communication also without direct line of sight, as well as roaming support between the radio cells.

Despite above mentioned limitations, wireless IP networking is here and it has proved to work reliably in LATNET and elsewhere. It has become a viable and cost effective alternative to wired IP networking.

We would like to express special thanks to our friends Johnny Ericsson, Peteris Tiss, Maris Neihards, Dave Olean, and Lew Shannon who have shared their wireless networking experience with us to make this paper happen.