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Guntis BARZDINS <guntis@latnet.lv>
John TULLY <tully@latnet.lv>
LATNET Academic Network
Latvia
Arnis RIEKSTINS <rieks@pmi.lv>
Polymer Mechanics Institute
Latvia
Wireless technology has become a primary system for delivering high-speed Internet access in Latvia. Because of poor-quality telephone infrastructure, developing countries face the difficult task of connecting locations (located in the same area/city) with dedicated high-bandwidth needs (from 256 kbps to 4 Mbps). The Eastern European countries of Latvia and Moldova are given as examples of where poor telephone infrastructure has been overcome by providing high-speed wireless Internet links to universities, schools, and government agencies.
In 1993, the Latvian academic network LATNET began experimenting with low-cost 2 Mbps wireless local area network (LAN) personal computer adapters for use in a citywide university network. Currently the LATNET wireless system in Riga is the most important part of the educational network, which connects more than 200 sites including university departments, institutes, high schools, and government agencies.
The country of Moldova received its first dedicated Internet connection in 1996, when the capital Kishinev was connected to the Internet by a 256 K VSAT (very small aperture terminal) link to Norway. Virtually overnight, the city-government-sponsored project was connecting schools, government agencies, and nonprofit, nongovernmental organizations with full-time, high-speed dedicated Internet links. Since then the wireless network in Moldova has expanded considerably through local efforts.
This paper gives a practical account of the development of the two wireless systems, including regulatory issues, technical solutions, educational and social benefits, and highlights upcoming wireless solutions for developing countries.
Sending data over high-speed wireless links has been a dream for many networkers over the years, especially in countries where appropriate communications infrastructure is not available. Until recently, wireless data links were impractical for most Internet providers because available radio transmission technology was very expensive, licensing was required for use of radio frequencies, and much professional knowledge was needed to assemble an operational microwave link (modulators, transceivers, antennas, etc.). The situation changed in the early 1990s when a completely new wireless data technology arrived. Direct sequence spread spectrum (DSSS) wireless local area network (LAN) interfaces have made building private wireless data links as simple as plugging a wireless LAN adapter into a personal computer (PC), configuring basic radio and Internet protocol (IP) parameters, and pointing an antenna. Wireless LAN technology was originally intended for in-building use, and like cordless telephones did not require a license for operation in most countries. But it soon turned out that by attaching a wireless LAN adapter to a high-gain antenna located on the rooftop (as is done for microwave links), LAN speed connectivity (typically 2 Mbps) can be extended 40 km or more depending on the antennas used. And it all costs less than U.S.$1,000 for the wireless LAN adapter, high-gain antenna, and cable! These are the key features that have promoted widespread use of this technology in developing countries around the world.
Figure 1. Locations where University of Latvia has consulted or assisted
with installation of wireless IP networks since 1994
The University of Latvia installed its first citywide wireless LAN Internet access link in 1993, which was then probably one of the first large systems of its kind. Figure 1 shows the locations around the world where the University of Latvia has since helped to install such low-cost, high-speed wireless IP systems -- either by providing consulting or even helping to install the first links. Moldova is one such country. To give some idea of what it takes to start a wireless network we have included this undertaking as a case study in our paper. This technology (at various degrees of sophistication and penetration) now is used in many developing countries -- except those with totalitarian regimes not permitting any private radio transmission. The Soros foundation and United Nations Development Programme have sponsored initial development of many of these wireless networks.
It should be noted that this technology is not widely used for Internet access in Western countries. One reason is the availability of conventional communications infrastructure. Other reasons are the nature of the very high bandwidth needs (for inter-company links) and the high density of businesses in Western industrialized cities. Because of interference from other users, unlicensed wireless technologies are less reliable and the distance is limited. Nevertheless there are individual success stories about the use of this technology for Internet access in many European countries and the United States.
Wireless LAN equipment uses spread spectrum transmission technology and operates in the ISM (industrial, science, and medical) license-free bands initiated by the U.S. Federal Communication Commission (FCC): 902-928 MHz, 2.4-2.484 GHz, and 5.725-5.875 GHz. Unfortunately, ISM bands are not the same in different parts of the world. For example, the 902-928 MHz band is not available in Europe because it is partially occupied by GSM (Global System for Mobile Communications) mobile phones. Only the 2.4 GHz ISM band is nearly universally available worldwide and thus is the most popular. In various countries there are different limits on the license-free radiation power in the 2.4 GHz band, with the United States being the least restrictive and Europe being the most restrictive. According to European Telecommunications Standards Institute (ETSI) recommendations, the maximum transmit power is 50 mW and max antenna gain is 6 dBi. Nevertheless, this is sufficient for point-to-point links up to 5-10 km. In the developing world the licensing situation varies by country. In our experience, one way to start is to obtain a secondary license for use of the spectrum you desire to use (2.4 GHz will be the easiest), with a stipulation that you will not interfere with the primary license holder's transmissions. An example of this situation is in Latvia, where LATNET for a long time had a secondary license for the 902-928s MHz band, while primary license for the 890-915 MHz band belongs to GSM operator.
Manufacturers offer a wide range of wireless LAN products. In 1997 an IEEE 802.11 industry standard was adopted to provide inter-operability between various wireless LAN vendor products. This is a real advantage for dense in-building networks. But for long-range links, where sensitivity and throughput matter most, use of equipment from the same vendor is crucial since it is optimized for inter-operation with a like device (usually through proprietary protocols). Lucent WaveLAN, Aironet ARLAN, and Breezecom Breezelink are the most popular names for long-range wireless LAN equipment.
This paper is based on the experience gained through development of the wireless Internet access network in Latvia. The wireless system was started in 1993 when the University of Latvia (LATNET) installed the first citywide wireless LAN link to connect a remote university campus to the central building, located 5 km away. At that time spread spectrum wireless LAN technology was little known. Inspiration to start these tests came from a Cylink demonstration of spread spectrum wireless data link during the INET'93 developing countries workshop in San Francisco, and from Stockholm KTH network operations center staff who brought the first wireless LAN cards to Riga in autumn 1993.
Figure 2. Point-to-multipoint wireless IP access network
Since then two generations of wireless Internet access systems have been built in Riga which today connect more than 200 locations to the Internet. Initial connections were made to buildings of the University of Latvia, Riga Technical University, Academy of Sciences, Academy of Arts, Academy of Agriculture, 20 secondary schools, and Riga municipality.
The first-generation network was based on AT&T WaveLAN equipment operating in the 902-928 MHz band and operated from 1993 until 1997. When the network was started this was nearly the only operational wireless LAN equipment available. The frequency band 902-928 MHz was chosen instead of the 2.4-2.485 GHz band for several reasons: It offered cheaper equipment, longer distances, simpler antenna and cabling requirements, and better penetration of obstacles. By the time we began to change to 2.4 GHz, about 50 902-928 MHz sites were linked to the Internet. Three omnidirectional "central point" antennas were installed on high buildings in the center of the city to provide "wireless Internet feed" to the user sites in a radius up to 15 km (operational distance depends on the antenna gain at the user site). PC-based routers were used both at central and user sites to provide connectivity between wireless and wired networks (see figure 2). This first wireless installation has been described in detail in a separate paper (Barzdins). The system had to be migrated to second-generation wireless Internet access network in 1997 for two reasons:
Figure 3. Wireless Internet access in Latvia and the capital city Riga
The red dots indicate an access point (AP) (at some sites there are multiple AP units)
The second-generation wireless Internet access network in Riga was started in 1995 and today is connecting more than 200 sites not only in Riga but also in other towns around the country (see figure 3). This network is based on Aironet Arlan equipment operating in the ISM 2.4-2.485 GHz frequency band. A distinctive feature of Arlan equipment is that it features hardware error correction which dramatically improves the reliability of the network and also increases the operational distance at the expense of a slight degradation in speed. The Arlan equipment has proven to be very well suited for wireless Internet access; therefore today it is used in most wireless Internet installations in developing countries. In the central points Arlan BR2000 or AP2000 stand-alone wireless units are installed, while user sites are equipped with Arlan IC2200 ISA cards installed in PC-based routers (more detailed description of network setup is provided in section 5). The network started with just one omnidirectional "central point" antenna on a high building in the center of the city which provided connectivity to users in a radius of approximately 10 km. As the number of the users was growing, additional "central point" antennas had to be installed around the city for two reasons:
In the meantime wireless Internet access has spread also to smaller towns outside the capital city Riga, as shown in figure 3. Usually a single central antenna is sufficient for smaller towns. As more "cells" are being installed, the wireless Internet access network starts to resemble cellular mobile telephone network infrastructure: Central point antennas for each cell have to be linked over high-speed trunk lines (usually 2 Mbps) to the Internet backbone. Although Aironet technology allows us to provide such feed over the same Arlan bridges (when used as a repeater), in highly loaded systems this is not a good solution: In this case each IP packet has to go over two wireless hops, reducing throughput of the cell by half and creating extra noise in the precious 2.4 GHz band. Therefore leased lines, fiber, point-to-point wireless links, and dedicated microwave links in other frequencies are used in Latvia to connect cells to the Internet backbone. International connectivity is provided over two mutually redundant satellite links: a 3 Mbps link to Atlanta (Crawford Communications) and a 2 Mbps link to Oslo (Taide Network).
The know-how gained through development of the network in Riga was actively spread around afterward. In 1996 a detailed technical description (Barzdins) of the wireless network in Riga was published on the Web, which since then has generated much interest from around the world. Engineers from the University of Latvia have also traveled to other countries to help start wireless networks (Moldova is one such example, described in the next section).
In early 1996, the city government of the Moldovian capital Kishinev began planning Moldova's first dedicated link to the Internet (moldova.net). The local-government-sponsored project, the nonprofit organization Apriori, considered connecting schools, government agencies, and nonprofit nongovernmental organizations with full-time, high-speed dedicated Internet links.
One of the organizers of this project, Ruslan Buyukly, met Guntis Barzdins, coauthor of this paper, at a Cisco training seminar in Stockholm. The poor condition of Moldova's local telephone infrastructure convinced Ruslan that Moldova should use LATNET's example and install a wireless system. The Internet backbone would be a 256 Kbps VSAT (very small aperture terminal) link. Uniquely, this plan did not have any reliance on the telephone system. This system would be completely independent of the local infrastructure except for the electrical supply.
In February, engineers from Taide VSAT networks installed a 256 Kbps earth station on the roof of a 12-story office building located approximately 4 km from the center of Kishinev.
A wireless system using LATNET's experience was planned to initially connect five sites:
Two weeks after the VSAT installation, the wireless equipment and two engineers from the University of Latvia arrived. Preparation for antenna mounts and electrical connections had been made by Apriori engineers. The first full day, LATNET engineers with Apriori networkers installed the satellite modem and Cisco router. That was the first day of Moldova's connection to the Internet.
Basic wireless training was organized the morning of the second day. The radio system configuration was learned quickly by network engineers already familiar with IP networking. The equipment for each client site consisted of 2.4 GHz DSSS Aironet BR2000, Conifer high-gain antenna, and a two-port Ethernet Cisco router. The 12-story building with the VSAT earthstation was deemed suitable to mount an access point (AP). It took approximately four hours for the omni antenna mount to be made AP-installed.
On the third morning, the links to the Apriori NOC and the White House were installed and tests were performed. In three days, installation, routing, and testing were completed for three sites. The locally trained engineers completed the other two sites in the next week. Since then, some of the first client nodes have been converted from "wireless bridge + Ethernet router" setup to just a wireless PC router with Ethernet interface. Thus, the wireless bridges are now mainly used only for APs to cover areas of the city where hills had blocked line of sight. This has reduced the expansion cost of the system to approximately US$1,000 plus PC cost per client site.
Virtually overnight the Moldovian capital Kishinev obtained a high-speed wireless data network and satellite Internet backbone connection. Without the independent characteristics of the wireless system (both satellite and DSSS), Moldova's connection to the Internet would have been delayed for months or even longer. Since then the wireless network in Moldova has expanded considerably through local efforts. More client nodes have been added to it, and additional base stations have been installed.
The best explanation of the latest wireless solutions can be given by a description of the wireless equipment and architecture that LATNET is installing in Latvian cities that do not have any current wireless systems. The following system might be installed in a city of 50,000 people which had potentially 50 buildings that needed a dedicated wireless link.
Figure 4. Picture of AP with omnidirectional; picture of wireless client with parabolic antenna.
The AP is either an Aironet BR2000 or AP2000 (the AP2000 provides all the needed features and the BR2000 has some extended features that are usually not needed) which is connected to a Procom 2.4 GHz 8 dBi omnidirectional antenna with five meters of Times Microwave LMR400 coaxial cable (comparable to Belden 9913) (see figure 4). It is possible to serve clients in a 12 km radius of the AP. The AP has wired cable connection for UTP (unshielded twisted pair) or 10Base2 or AUI (attachment unit interface) cabling. The AP uses standard 802.3 Ethernet protocol so that all wireless units on the network appear as Ethernet devices at the network level. In the 2.4 GHz band there are three nonoverlapping frequencies that are each 22 MHz wide. The radio protocol offers retransmission of packets similar to Ethernet. The radio data rate can be set up to 2 Mbps. This does not correspond to the actual IP data rate. Tests with a UDP bandwidth tester indicate that the maximum IP rate is 1.4 Mbps with 1500-byte packets. Considering higher level protocols, retransmissions, and varying packet sizes, the effective data rate decreases to around 1 to 1.2 Mbps. Other important physical characteristics are as follows: The power input supports 110v/220v, the units can operate in the temperature range -20c to 50c, and lights give information on connection status (Aironet).
None of the standalone wireless devices by Aironet or other manufacturers provide the routing features needed for the client site, such as firewall filters, masquerading, and traffic logging. This leaves the expensive option of installing the entire network with the Aironet BR2000 bridge units and routers such as Ciscos at each site or using a lower costing PC-based routing solution with a wireless interface card and standard Ethernet cards. We have developed a PC-based router software system, called MikroTik Tik, which performs all basic routing, supports remote management, and has specific features designed to assist with installation and management of the wireless part of the system. The router supports wired interfaces: UTP/10Base2 (NE2000 compatible), 10/100 Intel Etherexpress Pro, 100 Mbps fiber, HP 100VG, multiport modem pool asynchronous serial solutions with RADIUS and TACACS + authentication, and even a synchronous V.35 interface with support for Cisco HDLC up to 5 Mbps. The standard PC router has 8 MB memory, 486 100 MHz or higher processor, standard motherboard, hard disk drive (only 10 MB is needed), standard case, and VGA (variable graphics array) card. Only the wireless card, multiport asynchronous, and synchronous card are not available from the local PC parts suppliers; The wireless cards and equipment can be purchased through an Aironet dealer. The Aironet wireless interface is the IC2200 ISA card. The radio unit on the card is the same one that comes with the AP unit. The wireless client unit can be configured to connect to the AP or point-to-point with another wireless client. The cabling is the same Times Microwave LMR400 used with the AP. The antenna is a 2.4 GHz ,24 dBi parabolic dish. Two manufacturers -- Conifer and California Amplifier of the United States -- offer similar designs and prices. The software is available from MikroTikls.
Wireless point-to-point links have a maximum range of about 40 km depending on line of sight (though longer distances are possible depending on the terrain and tower height). When it is possible to get line of sight, two Aironet BR2000 (Ethernet only) or two wireless client routers with 24 dBi parabolic antennas can be used (see figure 4). The wireless client unit can use any of the above listed interfaces on either end, but usually an Ethernet UTP or 10Base2 is used. Because our wireless technology is limited to line of sight, most backbone links used are digital, leased telephone lines provided by the state telephone company. Improvements in the telephone system have recently brought digital data services online for most medium to large Latvian cities. Generally, cities with populations of 10,000 to 100,000 are connecting with 128 kbps to 256 kbps backbone links to Riga. The equipment used is a standard router with an Ethernet port and synchronous Cisco HDLC (high-level data link control) capability (V.35 port). Newbridge digital modems are provided by the telephone company.
If the backbone is made using wireless client systems, then it is possible to put multiple Aironet IC2200 ISA cards in one router. An additional antenna and cabling can be added to make another backbone link to another location or double the bandwidth for the current backbone link.
The AP2000 and BR2000 offer SNMP (simple network management protocol) management and statistics reporting. We use MRTG (multi router traffic grapher) to report on the bandwidth of each link. In some cases it is reasonable to use traffic management to guarantee a level of service. The wireless network does not have a fixed bandwidth for links as a synchronous leased line.
If the AP is located on top of the same building as the NOC then the simple solution is an Ethernet cable from an NOC router to the AP. In most situations the AP is not in reach of standard wired connections. For situations where the aggregate bandwidth needed for the city is 400 to 600 Kbps, the easiest and cheapest solution is to use the Access Point as a repeater. A wireless client is located at the NOC and is connected to the Access Point just as other wireless clients are. In this configuration all packets from the NOC to clients make two wireless hops. If you consider that a city might have a 256 kbps backbone, then the 400 to 600 kbps aggregate connection speed for the wireless client base means that the wireless system will not be a bottleneck. Of course intracity connections could be improved by using a dedicated backbone from the NOC to the AP. The preferred solution is to make a dedicated wireless backbone with wireless clients installed at the NOC and AP location. In a situation where there are multiple AP units on the tower and the three nonoverlapping frequencies are all in use, backbone wireless client antennas can be mounted in horizontal polarization with lower-power directional antennas; the effect will be minor degradation in AP bandwidth. A crossover UTP Ethernet cable or hub connection provides the connection between the AP and dedicated wireless client.
Other potential wireless backbone connections are licensed microwave point-to-point systems and the new 5.7 GHz unlicensed ISM equipment. We are currently testing RadioLAN 5.7 GHz links for use as a backbone. The benefits of the system are as follows: 5.7 GHz is a new and unused frequency that will not interfere with 2.4 GHz. Our tests show that half-duplex bandwidth for UDP packets of 1500 bytes is approximately 9 Mbps for up to 13 km, and the equipment cost is a little more than that of a wireless client using Aironet equipment. The difficulties include the following: 5.7 GHz does not have the same range as 2.4 GHz (increasing frequency means greater signal attenuation over distance), a parabolic antenna is required for distances of more than 2 km, and cable length is very limited because of high signal loss with coaxial cabling at 5.7 GHz.
The AP should always be installed on a tall building which gives line of sight to the greatest number of potential wireless client sites. To have the highest possible aggregate bandwidth available to the wireless clients, it is possible to install three APs that can be connected to a UTP hub or with a coaxial Ethernet segment, also connecting the backbone connection through the hub (Stallings). The approximate TCP (transmission control protocol)/IP bandwidth of the system will be 3.6 Mbps, although the raw advertised radio data rate is 2 Mbps per unit for a total of 6 Mbps. For the highest bandwidth, the system should connect to a backbone connection to the NOC and not use the repeating capability of the AP.
The APs can be configured by console, telnet, or an HTML (hypertext markup language) connection. Logs, diagnostics, radio setting, and many other controls are remotely configurable.
It is easy to increase bandwidth by adding APs on one of the three nonoverlapping channels.
The wireless client system is usually installed in an attic or upper-level office of a building so that the parabolic antenna can be mounted with line of sight to the AP. Generally around 10 m of cable is used. Due to the high attenuation of 2.4 GHz signals with standard coaxial cable, longer cable lengths should not be used for systems located more than 4 km from the AP (Straw). The PC routing software can be configured through the RS232 port which acts as a standard router console port. The software allows radio options to be set and connection tests to be performed. The PC router also acts a firewall for local uses. With the firewall set, the router can be securely connected to the local hub or router or directly to a workstation. The bandwidth of the AP is distributed on an as-needed packet basis. For example, if 20 client sites are connected to an AP and no traffic is being generated by 19 wireless clients, then the twentieth wireless client could use almost the total amount available -- approximately 1.2 Mbps. If another wireless client started generating bandwidth, the bandwidth might be split evenly depending on the TCP/IP session rates. The bandwidth is half-duplex, directly comparable to standard 10 Mbps Ethernet (which is half-duplex unless you have a full-duplex hub/switch installed).
All radio and router setting can be controlled by telnet or a Java box client. Generally the wireless network administrator is the only one who has access to this. Traffic and syslog options can also be set on the PC router.
As the PC router can accept a large number of interfaces, the network can be extended in many different variants. For example, it is possible add another wireless card and antenna to extend the wireless connection to a point that did not have line of sight to the AP; multiple Ethernet cards can be put in the PC router to provide for firewalls for different local area networks in the same building; a dial-up modem pool could be added using multiport asynchronous cards (up to 16 serial ports); and so on...
Product development news from major ISM band wireless manufacturers suggests that we will see a rapid increase in the speeds and capabilities of ISM wireless networks. Two new products available now are the Aironet PC4800 series of 11 Mbps 2.4 GHz spread spectrum PCMCIA (Personal Computer Memory Card International Association) card and the RadioLAN 10 Mbps, 5.7 GHz narrowband ISA card. Lucent WaveLAN is expected to release a 10 Mbps, 2.4 GHz product in the middle of 1999.
The new 11 Mbps Aironet wireless devices will soon enter the testing phase at LATNET. New software drivers have been written and will be tested soon. From conversations with Aironet technical staff, we believe that the new 11 Mbps system will actually increase our system capacity by approximately five times. In the future we plan to test RadioLAN products to see if they will support point-to-multipoint. This requires writing a driver and radio control system that will use polling features and organizing the highest-quality antennas and cabling systems.
With the installation of the new 11 Mbps equipment we will also put bandwidth (traffic) shaping in use. In the future we will implement dynamic bandwidth controls for allowing better quality of service. Our goals for management control include systemwide diagnostics to determine the link quality and bandwidth potential of each client site, a modeling system that will incorporate the diagnostics statistics with bandwidth allocations made by the administrator (to provide a stable quality of service environment), and implementation of selective compression of relatively slow links (where the delay for the compression will increase the overall link speed and free bandwidth on the wireless network).
Since 1994, wireless Internet has become the most important part of the Internet infrastructure for the Latvian Academic Network. The following characteristics of a DSSS wireless system make it an optimal solution in developing countries for distributing high-speed dedicated Internet.
Wireless systems have enabled LATNET to rapidly and cheaply expand Latvian Internet. The wireless Internet system in Latvia has had a major beneficial influence on this post-Soviet country.
Barzdins, Guntis, Wireless Internet Access in Latvia (Riga: LATNET Web, 1996).
Stallings, William. Data and Computer Communications - 4th Edition. Englewood Cliffs, NJ: Macmillan, 1991. pp. 342-364.
Straw, R. Dean. The ARRL Antenna Handbook, (Newington: ARRL, 1994), pp. 24-29.
OVUM Analyzes the Causes of the Slow Take-up of the WLL Market. Local Loop Report, Vol. 20, pp.12-13, June 1998.
Equipment Links:
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California Amplifier |
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Conifer |
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MikroTikls |
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RadioLAN |
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Times Microwave |
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WaveLAN |