This document summarizes a project to deploy a wireless VoIP network across two rural sites in Bangladesh, serving approximately 80 customers. Key aspects of the project included using commodity wireless networking equipment to build a wireless backbone and last-mile connections, evaluating performance of the wireless links and VoIP service, and exploring providing internet access to a school. The network was designed to use 802.11b wireless technology between repeater stations over distances up to 15km to deliver voice and data services in a cost-effective manner to rural areas that previously lacked affordable communication options.

1. VoIP Report
DEPLOYMENT
WIRELESS VOIP
NETWORK IN
RURAL AREA IN
BANGLADESH
By
MD ENAMUL HAQUE
ID: 062656056
COURSE: ETE-605
SPRING-08
NORTH SOUTH UNIVERSITY
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2. VoIP Report
Background
Good communication services and universal access are necessary for a higher standard of living and economic growth.
However the high cost of legacy PSTN equipment may not be affordable to some developing nations, especially in rural
areas which have a much lower subscriber density, or areas with geographic challenges such as large bodies of water,
jungles, mountainous terrain etc.
Voice Communications
There are several paradigm shifts happening in todays telephony markets which are driving costs down by orders of
magnitude. First legacy telephony systems are based on Circuit Switched Networks or (CSNs) This means a telephone
call is allocated a dedicated circuit from end to end. In the old days this meant a physical pair of wires for the audio to
travel over. Today this typically means two 64Kbps channels one in each direction which are dedicated to that call even if
no one is talking, and since usually only one person is talking at a time about half of the bandwidth is wasted. For
example, a typical small PSTN trunk can carry 24 or 30 simultaneous calls. If the bandwidth were used more effectively
the circuit could carry much more if not almost twice as many calls. On the positive side CSN technology is very robust
and predictable which made it easier to build reliable telephone networks in the early years of the industry. Because these
PSTN switching systems were very big and centralized due to the state of the art at that time, they were very expensive
and relatively few were sold to big companies like AT&T. So the market never developed to a point where the prices could
drop significantly.
When computer networking technology was developed it was based on Packet Switched Networks (PSNs). Instead of
dedicating a circuit of a predetermined bandwidth to two endpoints, packets are sent with little messages inside as each
party has something to convey. This utilizes the bandwidth much more effectively. Instead of slicing it up into little pieces
that are reserved but not being used half of the time, it is all consolidated in one big pipe that is only used when data
actually needs to be sent. As the computer revolution evolved and the Internet grew exponentially so did the market for
PSN technology products. This caused prices to fall by orders of magnitude.
Another Paradigm shift that is in progress now is sending audio on a data network rather than sending data on an audio
network. Using modems over a Legacy PSTN is an example of the latter. This is a very unfavorable combination
because the modems at both ends usually send a carrier signal even when they have no data to send, and even though
the PSTN eventually digitizes the audio it knows nothing about the data encoded by the modem so both 64Kbps
channels are constantly in use even though the modems are not sending any packets. The phone companies saw that
this wasn't working very well and that there was a demand for lower cost data circuits, and started providing services like
ISDN, Frame relay, and eventually DSL, and ADSL. These services were designed to let the PSTN handle the
communications as data and not audio. But ISDN still used a dedicated 64Kbps or 128Kbps channel so this approach did
not capture a large portion of the achievable efficiency. The others had a quality of service metric known as Committed
Information Rate or (CIR) which was usually set lower than the maximum bit rate of the Circuit, and paved the way for the
consolidation of circuits into one pipe. These were some of the first steps taken in the transition from CSNs to PSNs in
Telecom networks.
As high speed wide area networks (WANs) became more affordable and Voice over IP technology developed to become
a commercial product thanks to standards organizations like the ITU and IETF, more and more organizations started
buying high speed data connections between their offices and providing data and inter-office phone service over these
links. Also many Internet Telephony Service Providers (ITSPs) started selling low cost long distance service over the
Internet. One limitation of this technology that may slow down the complete conversion to an audio over data network is
that there needs to be power at the subscribers’ site for the terminal equipment. Legacy telephones are powered only by
the PSTN so they will still work if there is a power failure, and this is often when it's needed the most. The PSTN is able
to provide this by having a battery bank and generator at each switching site. To provide a reliable VoIP system it is
usually necessary to have battery backup at each subscriber site. Telephony equipment manufacturers could no longer
ignore the compelling nature of these new communications paradigms, and now no one is building big switches anymore.
Most of the new telephony products are based on PC platforms with Compact PCI cards at this time.
Data Communications
Data rates on wired networks have been increasing by powers of ten over the years, and more recently wireless rates
have been catching up. This is due to many factors. Among them are the commercializations of spread spectrum
technology, improvements in IC manufacturing processes to fit these radios on small cards, and the allocation of radio
spectrum in the Gigahertz range for licensed and unlicensed use of these devices.
The advantages of wireless networking are hard to ignore and the market for wireless Network Interface Cards (NICs)
grew rapidly. Soon they too became commodity items. Initially they were targeted at networks within office buildings and
homes, but many users found that they could also be used for long distance communications if the systems were
designed properly. This use also became popular and another market grew which provided low cost high quality antennas
and amplifiers to increase the range.
This became a low cost alternative to the Microwave links used by the Telecom and broadcast industries, though at not
quite the same level of performance. Currently there are products available that work at 11, 45, 100, and 1000 Megabits
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3. VoIP Report
per second. Though one should note that the expected throughput will be about half of the data rate, and generally the
higher the speed the shorter the usable range.
Combining Wireless and VoIP
Wireless telephony is nothing new, there are microwave links for the trunk lines and Wireless Local Loop (WLL) for the
subscriber terminal equipment. But it's mostly CSN based technology and is therefore quite expensive. If one combines a
network built out of commodity wireless cards with Voice over IP equipment it is a low cost delivery infrastructure that
makes efficient use of the bandwidth it provides. Additionally one gets a high speed data network that can also provide
Internet access. Of course its not quite that easy. Each radio repeater needs battery backup, and as mentioned before so
does each subscriber. Also because of the change from the CSN model to PSN it will be necessary to manage the
bandwidth usage so that priority is given to voice traffic and that too many calls are not allowed to be placed
simultaneously. This was not an issue with the legacy CSN systems because there were only a finite number of slots on
the trunks for calls and when they were out of slots one got an all circuits are busy message. Overall rapid growth in this
area is expected, driven by fierce competition in long distance rates, and the large populations of people currently without
good communications services.
Goals
Objectives
To deploy such a wireless VoIP network at two sites serving a total of about 80 customers.
To evaluate the performance of a wireless point to point backbone link.
To evaluate the performance of the point to multipoint last mile links.
To evaluate the performance of the VoIP service over the links.
To evaluate the overall usability of the wireless network for VoIP.
To evaluate some low cost routing hardware used for the E1 data connections.
To explore the possibility of providing a school with Internet access over the network
Expected outputs
To gain experience in deploying these systems.
To understand better all the issues involved.
To fine tune the radio network to increase it's reliability and usability.
To fine tune the VoIP equipment to allow it to work effectively over the radio network and the PSTN
To eventually have a usable system that can provide service to the two rural areas.
Preliminary work
In the spring of 2001 the Consultant came to Bhutan as a UNV specialist attached to the Department of Information
Technology (DIT). The Consultant demonstrated some wireless gear with a usable range of about 8km from a PCMCIA
card in a laptop to a repeater and a similar range for some VoIP wireless phones. A proposal for this pilot project was
drafted which can be found here. Some more research was done on the equipment list and pricing, and in August the
funding for the project came through. The Consultant returned to Bhutan in the spring of 2002 to complete the project.
Project Sites
The project can be done anywhere in the rural area as well as remote area where deployments of PSTN and other
services pretty hard to deploy .The E1 link terminates at the PSTN switching room in somewhere near in town. The
Microwave tower there was used to mount the equipment to cover the surrounding areas. Because the design was
very conservative two dishes were used to reach the outlining areas and one omni to reach closer CPEs. Happily it turns
out that the omni was able to also reach one of the outlying areas. In each of these three areas there is a repeater to
serve the local CPEs.
General guidelines for site layout
The constraints involved designing a high speed data radio system are very similar to ones in a WLL or cellular system.
Everything needs to be Line Of Sight (LOS). This actually means a path which is also free of nearby obstructions such as
corners of buildings and rooftops, not simply being able to see the other antenna. This is because there is something
called the Fresnel zone around the centerline of the LOS path. Objects in this zone are likely to refract some of the signal
toward the antenna and cause it to be attenuated. Of course sometimes one can get by with things in the way like a few
wispy trees etc. But it's not a
good idea and when the leaves get wet they will adsorb even more of the signal.
The range one can get out of each link depends on several things. The most important is probably the chosen frequency
band. Because this project focused on a solution using commodity wireless hardware, this meant one of the ISM bands.
Namely 2.4GHz or 5.8GHz. The trade off here is the higher frequency allow for higher data rates but shorter ranges, and
more rain fade. So more power will be necessary to make up for the loss of signal strength when it rains or snows. At the
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4. VoIP Report
time of this report 802.11b devices worked in the 2.4GHz band and provided data rates up to 11Mbps, and 802.11a and
others worked in the 5.8GHz band and provided rates up to 50 and 100Mbps. 802.11b equipment was chosen because of
the lower cost and higher availability. For a quick introduction to
802.11b.
Once the band has been selected, the other factors influencing range can be adjusted: output power, receiver sensitivity,
antenna gain, and data rate. Increasing any of the first three or decreasing the data rate will cause the expected range to
increase. It is important to also allow some margin for rain fade. The Project chose to put amplifiers on the repeaters and
slightly higher gain antennas on the CPEs. A conservative rule of thumb for range is to try to limit the distance between
repeaters and CPEs to about some km. It was beyond the scope of the Project to explore the maximum distance between
the repeaters but most manufacturers publish sample performance data for different configurations. A conservative
estimate would be about 12-15km between 8dBi omnis and about 25-30km between 24dBi dishes using one watt amps.
Of course the regulations concerning transmitter power, EIRP, and antenna gain will vary from country to country.
Backbone
To build a full network a backbone is needed to deliver the bandwidth to the clusters of customer sites. Ideally the
backbone should be much faster than the last mile delivery system so that many sites can be aggregated onto it for
transshipment around hither and yon. Also because the backbone is a point to point system, one could take advantage of
this and design it to be full duplex. This would more than double it's capacity, and lessen delayed packets due to collisions.
This in turn would allow the maximum transit time of packets from one end of the network to the other to be much more
predictable which is a consideration for VoIP and other real time data.
In this phase of the project there wasn't enough time or budget to explore a higher speed or full duplex backbone. It's
interesting to note that as the number of calls in progress went up so did the collisions and retries. This is very
understandable because as was mentioned before a call sends data in both directions and on a half-duplex link this
means the two ends have to take turns sending on their shared frequency. Because 802.11b provides so much more
bandwidth than is used for a moderate number of simultaneous calls, the collision rate is acceptable and the voice quality
should be unaffected. On a system intended to run at near capacity one should seriously consider a full duplex high
speed backbone. The most likely candidates seem to be 5.8GHz equipment with amps, perhaps on non-overlapping
frequencies using horizontal and vertical polarizations for further isolation.
Last mile
The last mile delivery is typically structured with one or more repeaters serving the surrounding customers which need
LOS or Near LOS (NLOS) to a repeater. In order to account for rain fade and get better range each repeater in the system
has a one watt amp to boost the transmit and receive signals. For the repeater antennas 8dBi omnis were chosen. These
seemed to be a good balance between gain and a radiation pattern which wasn't too flat. This also provided service to
customers who were below the antenna at about a 30 or 40 degree angle. For the Customer sites 13dBi Yagi antennas
were used since they were always served by one repeater and it wouldn't have been cost effective to put amps at each
customer site. In cases where there is one CPE site way off by itself, it would be preferable to use an existing repeater if
possible. In this situation adding an amp to a CPE site would be a viable solution. There are variations on this scheme
where the CPE sites all talk to each other using 802.11b "ad hoc" mode and or a meshing protocol, but the available
bandwidth typically goes down quite a bit and eventually a repeater will be needed somewhere to get back to the
backbone. One should also consider how much bandwidth a community needs when choosing the last mile delivery
technology. 802.11a and others can provide upwards of 25Mbps but as mentioned before the range is less. Since there
are usually several non-overlapping frequencies available , either technology can be scaled up to easily triple the
aggregate bandwidth in an area. Most customers who need a telephone line and perhaps an Internet connection can
easily be served by 802.11b. Some organizations like hospitals and large government offices might require the higher
rates available in the 5.8GHz band and the two delivery systems could compliment each other in these areas.
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5. VoIP Report
IEEE 802
Wireless WGs
802.11 802.15 802.16 802.XY
Spectrum Unlicensed Unlicensed Licensed
Licensed
Unlicens
ed
Freq Bands 2 Ghz Various 10-66 450 Mhz - 3Ghz
depending Ghz
on application 2-11
Ghz
Range Local Area Personal Space Metropolitan
Metropolitan Area Access
Area
Access
Mobility Support Portability Personal Fixed Vehicular Speed
Local Space Mobility
Roaming Connector Inter-
Avoidance Metro
Roami
ng
Station Power Battery Battery Mains Battery
LOS/NLOS NLOS NLOS LOS (10-66 NLOS
Ghz) NLOS (2-
11 Ghz)
Group Charter PHY and PHY and PHY and MAC PHY and
MAC MAC for Fixed Pt.- MAC for
for LAN for PAN Mpt. Wireless Vehicular
Access Speed Mobile
Access
Networks
A quick comparison of existing and proposed 802 wireless standards
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6. VoIP Report
Network layout
Fig. Original network diagram for the NOC and outward links
At the NOC in remote area were the VocalTec servers, and Flytech routers all on one subnet.There is only one tower at each
site even though there may be multiple tower icons. The network manager centralizes the management functions of for the VoIP
network. The Gatekeeper controls the placing, routing, and logging of calls. The Real Time Server (RTS) logs the calls in real
time, also known as Call Data Records (CDRs), and updates customer accounts. The billing server takes the account
information and generates statements. The RAID array stores the Data Bases. The PSTN gateway connects via an E1 line to
the PSTN using the R2 protocol. An E1 can carry 30 calls at one time so 30 of the approximately 80 customers could call
numbers on the PSTN side of things at once. Additionally almost any number of calls to other VoIP phones could be happening
simultaneously. Of course statistically only about 10% to 20% of a population will be using their phones at any given time.
Required Equipments
Radio
APs/Bridges
Wireless Ethernet converter
Amplifiers, Antennas
Mounting hardware
It's helpful if all the mounting hardware uses readily available nuts and washers because one will lose more than one cares to
admit by dropping them off of towers or roof tops. Also u-bolts can be easily made from long pieces of threaded rod sometimes
called all-thread. It's good to keep a supply of several sizes handy along with a box or two of nuts and washers. Also it can be
used for mounting to concrete footings bolts can be used for anchors as well as u-bolts.
Supplies
It's important to waterproof cable connections properly. One way is with special rubber tape that self seals to it once applied. It's
very important to cover the rubber tape with a UV and weather resistant electrical tape because the rubber tapes will eventually
crack and breakdown with prolonged exposure to sunlight.One can also use a brush applied waterproofing compound such as
ScotchKote
Linux Routers
The thought here was to demonstrate the viability of open systems and especially open source software solutions as alternatives
to more expensive commercial products. Briefly, the Consultant put together very small computers with E1 cards running Linux
which served as routers at the ends of the microwave E1 links. For more details on their configuration and maintenance please
see the Flytech training manual.
The Flytech boxes did meet most of these expectations. They excelled at being flexible and provided a work around for an
apparent problem with the Cisco Bridges. Also they were much more capable than a mere router needed to be, which a double
edged sword was. On one hand it allowed network monitoring and logging to be done from one box, on the other hand the
system image was quite large and this made it a bit awkward to back them up. Because they were essentially small PCs they
suffered from some of the same limitations, such as a risk of file system corruption when there was a power failure, and the
possibility of hard drive failure. Because they are so flexible they may require a little more skill to administer if one is
contemplating adding features. Though for the usual additions to the network, typically only one file needs to be edited.
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7. VoIP Report
If one agrees that this is a good direction to go and that the flexibility gained was a factor in the success of this project then most
of these issues can be addressed. The possibility of file system corruption can be almost completely eliminated by using a
journaling file system. The Consultant has done this on other Fly techs and no longer has to worry about power interruptions. The
Hard drive can be replaced with a compact flash card since the Flytech's have a CF slot. The tradeoff here is that there is limited
space for additional applications and no space for logging large amounts of data. On the positive side back up is a breeze, and
administration tends to be simpler because the system is smaller and more focused on one task. For sites with battery power there
is also a D supply option for the Fly techs. All in all the Flytechs did their job well, but they were probably overkill. The cost for a
Flytech was about $2000 total, comparable to a Cisco router which is much less capable. Though if the system monitoring is to be
done from another server then the routers could be scaled down somewhat with a similar reduction in cost. The Soekris SBCs
might be able to replace the Flytechs someday when an E1 card is available for their Mini PCI slot. Soekris has plans to come out
with such a card but the timing isn't certain.
VoIP
For the VoIP component of the system there are a lot of considerations but due to time constraints the Consultant feels that
some were left unaddressed. Still a reasonable choice was made that worked adequately. Initially there was interest in wireless
VoIP products, and at the time of the research there were only one or two on the market. Primarily the search was for a fixed
wireless VoIP residential gateway rather than mobile wireless VoIP phone. E-tel provides the former in their GW210 model, and
Symbol's NetVision phone is an example of the latter.
The Etel GW210, even though it was only a two port unit, was felt to be cost effective because it combined the VoIP and wireless
components of a CPE into one unit. However it was difficult to confirm that it would interoperate with any of the gatekeepers
(GK) that the project was looking at. In hindsight E-tel's list of GKs that were compatible with the GW210 could also have been
checked into and then billing systems that worked with those could have been found.
Eventually VocalTec was chosen as the supplier of the VoIP equipment for several reasons. A complete solution with a billing
system was needed. They were very helpful, and in fact they were one of the few vendors that returned phone calls. Also the
project had the usual time constraints and they seemed to be the only choice when it was time to make one. VocalTec was
unable to interoperate with third party gateways (GWs) so other products could no longer be considered, but this also had it's
advantages because the two viable models had four and eight ports, compared to most (but not all) of the others that had been
looked at which were one or two port units. This allowed the CPEs to be consolidated so they served a small cluster of buildings
with one antenna. This is preferable to many CPEs and antennas because there is less contention for the radio channel.
Billing
VocalTec recommended the Mind billing system, a third party product which seems to be working well. Again if there had been
more time there was already an in house billing system in use by the ISP branch of Bhutan Telecom (BT) and it might have been
possible to adapt that one to the VoIP project.
Power
As noted above, one of the other differences between a legacy telephone system and a VoIP system is that the CPE must be
powered locally at the Customers site. This is because there are no wires to carry the power. Now if one were considering a
wired VoIP system using 10/100BT then they could take advantage of a new development called Power over Ethernet (PoE)
which runs power over the Cat 5 Ethernet cable. In this case though the power had to be provided locally, so instead of a
centralized and easily manageable bank of batteries at the Switching center one needs batteries at each customer site along
with a charger and Low Voltage Disconnect (LVD).
Commercial
At sites that had commercial power a battery charger that had a built in LVD was used . Apparently there weren't a lot of choices
locally for this product and the one that was available was not a great match for this application. Also it seemed impossible to get
documentation for it, and it appeared to be malfunctioning. Eventually an unlabeled LVD adjustment was discovered and, that it
was not set properly at the factory. In fact they seemed to be set randomly on each unit. So trips had to be made back out to the
CPE sites and readjust the LVD set points. This was after a couple of batteries were damaged.
Solar
For sites where commercial power was unavailable solar panels and charge controllers were used. There were both repeater
and CPE sites which used solar power. The panels were purchased from Tata BP solar and were 70 watts each. The Consultant
recommended charge controllers with integrated LVDs, and suggested several such solar chargers during the selection process.
But apparently there was a lot of pressure to get the project underway and not enough time to evaluate them all. An outside
vendor recommended the Trace C35 and C40 thinking that they also had LVD options; this is an understandable mistake
because they can indeed be used as an LVD but not when they are configured as solar charge controllers. Also the
specifications can sometimes be ambiguous, when in doubt it's best to contact the manufacturer. This was still an excellent
choice because trace produces some of the highest quality alternative energy products on the market. Eventually a standalone
LVD was found to work with the C35 but it's quality was suspect. As these examples show it can sometimes be difficult to find
units correctly sized for a site with all the desired features. In this case building one's system out of several smaller units which
better fit the constraints is probably advisable. In this instance the high cost of quality LVDs , or the low quality of locally
produced units seemed to be an issue. The Consultant eventually did further research and designed a very low cost (VLC) LVD
which could be used for an repeater or CPE site.
There were several solar powered CPE sites which required much less power than the repeaters, and it seems there are quite a
few small charge controller with LVDs available. Though the Specifications are not completely clear on this matter the Trace C12
seems to be LVD capable when it is configured as a charge controller unlike the C35 and C40. In any case the reliability of future
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8. VoIP Report
CPE sites would benefit by having integrated Charge controllers, and LVDs. Also note that when the sun is shining that no part
of a solar panel should be shaded. This is because a great number of cells in a panel are connected in series, and that when
shaded, a cell will act like a resistor. This will greatly reduce the output of a panel even if only a corner is shaded. Some
situations one could save some money by installing a Maximum Power Point Tracker (MPPT) which allows the panels to supply
current at their most effective operating voltage rather than whatever voltage the batteries happen to be at. There are claims that
these devices can get an additional 25% to 30% out of the panels but this is often not realizable. Still in certain cases they are
warranted. If one used an MPPT as a charge controller then the above calculations would be more accurate if they were
converted to use Watts and Watt hours instead of Amps and Amp hours. One can get a hint of the difference this might make if
the solar panel wattage is divided by the nominal battery voltage (12V) instead of the solar panel working voltage (17V) in step
10 above. One MPPT on the market that seems to be sized right for the CPEs is the B.Z. products MPPT200. It should be
seriously looked into as an alternative to the Trace C12 since it also has a built in LVD and is competitively priced.
Batteries
Lead acid batteries still appear to be the most cost effective solution for most situations though one should take into account the
cost of an environmentally sound way of disposing of worn out batteries when making comparisons. Again in legacy Telecom
systems all the batteries are at the local switching center where they are kept in a controlled environment and monitored
regularly. This makes it feasible to use ordinary flooded cell lead acid batteries with Catalyti recombiners. On the other hand a
VoIP system requires many batteries in the field where there is little control over operating conditions. Also they often need to be
transported and handled by less experienced people. In this situation sealed lead acid batteries are the better choice even
though they are more expensive. There are several types of sealed lead acid batteries, Valve Regulated (VRLA), and Gel-Cells
are two examples. Gel-Cells won't spill if tipped over and can often be mounted in any position. Proper venting is also important,
especially for flooded cell lead acid batteries. There have been accounts of roofs being blown off by hydrogen explosions.
In any case there will be a sizable investment in batteries for the system. Therefore one would want to maximize the lifetime of
the batteries and minimize their maintenance requirements In order to lower the cost of ownership. This means extra care should
be taken when choosing a charging / monitoring system. Again all things being equal it is desirable to buy locally but carefully
research the quality of all products before making a choice.
There is a wide spectrum of chargers available for lead acid batteries. Generally the higher end three stage chargers are
required here, and ideally one with an integrated LVD. The LVD protects the battery(s) from being over discharged which would
seriously damage them. In fact if a Lead Acid battery is only discharged to 80% of it's capacity then that will almost double its
lifetime.
Also it's interesting to note that it gets harder and harder to put energy back into a battery as it gets fuller and fuller. This is called
it's charging efficiency, and it varies nonlinearly with the battery's state of charge (SOC). The Consultant's hypothesis is as the
battery is charged the chemical reactants are converted to the charged state. As the battery nears full charge less and less
reactants are available for conversion, and the internal resistance of the battery goes up. In the end about half of the power
going into charging the battery is wasted as heat. If this is not taken into account when designing solar systems the panels may
be under sized and may not be able to keep the battery bank fully charged. Most likely this is not an issue here because of the
aggressive recharging time that was required for these solar systems. There are several good papers on the appropriate design
of battery systems for solar sites at Sandia Labs.
Timers
As a bit of added insurance for the repeaters the Consultant felt that it wouldn't hurt to add a timer that would power cycle the
unit once each day at four in the morning. Sometimes computer based devices (also known as embedded systems) can lock up
for unknown reasons and then need to be rebooted. A timer is a low cost solution that can save one a trip into the field. If one
had more control over the specification of the embedded systems then a watch dog timer is the preferable way to protect against
these type of problems. Watch dog timers are built into a lot of modern microcontrollers. They act like a dead man switch and
once activated they need to be "touched" by the software every half second or so or they cause the system to reboot. If properly
implemented they add a good fai safe component to one's embedded system. For this project low cost digital timers were
purchased from Amazon.com and modified to work on 12V.There are also ready made 12V digital timers but they seem to be
quite a bit more expensive, probably because there is not as much demand
Site preparation
The site surveys were carried out by Bhutan Telecom before the Consultant arrived and were ready for the equipment once the
project started. As mentioned before, some of the relevant questions to ask are:
Will this location serve a good number of customers?
Or can several repeaters and CPEs link to this location?
Is there LOS to the repeater for this area?
Is the commercial power reliable enough to keep the batteries charged or should it be a solar site?
Can arrangements be made with the owners to install the equipment here?
Installation.
Most installations went smoothly. Sometimes the appropriate fasteners were not available so alternatives were used
Bridging vs. Routing
The Cisco radios use Spanning Tree Protocol (STP) to act as network bridges. This will work even if the SSIDS are different if
they have a wired connection to each other. They also will allow one to enter routes and act like routers but the Consultant had
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9. VoIP Report
difficulty making this feature work. This was one of several occasions where the Cisco product didn't perform as documented,
however if used in the most common configurations it worked flawlessly. The lesson learned here seemed to be, let bridges be
bridges and routers be routers. Instead of giving each repeater a address in it's own subnet and making that the default gateway
for it's CPEs, all repeaters in a site were given IP addresses in one subnet which included an interface on the Linux router. This
became their default route. Next, virtual interfaces were added for each default route that the CPEs needed, being in different
subnets as they were. Since the Cisco radios would bridge these subnets back to the Linux router, as far as the CPEs were
concerned they had a local router for each of their subnets. One small advantage of this was that one could experiment with
other radios that behaved as routers instead of bridges without changing the CPEs configurations. Of course bridging is a nice
feature, it allows many sub-sites to appear as one seamless network, and probably would be a good choice for other projects.
Having different subnets for each repeater was mostly an arbitrary way of organizing the project as much as anything else.
Radio
Mutual interference
The microwave towers that some of the repeaters were on had a preexisting service known as Digital Radio Multiple Access
Subscriber System (DRMASS), which was also in the 2.4 GHz band. The channel used by DRMASS was below the ones
available to the 802.11b radios, but close enough that mutual interference was an issue. These problems manifested themselves
differently at each site. At Remote initially there was noise on the DRMASS voice circuits, and the backbone link would go down
periodically.
Various steps were taken to correct the problem at remote. First the backbone dishes were changed from vertically polarized to
horizontally polarized and different channels were tried. At one point intermod was suspected at one site because the only
frequency that would work was the center one available to the 802.11b radios. Note that 802.11b is a compromise between a
narrow band signal and full spread spectrum, also known as wide band. The DRMASS channels are even narrower. This
indicates that intermod might be possible between them. It was inconvenient and difficult to make trips to the repeater sites bu
eventually the test for intermod was made and this proved not to be the case. Another related common problem is that the
amplifier distorts the signal when over driven, and the distortion can interfere with adjacent channels. Another common problem
that can affect closely spaced antennas is side lobe radiation. The radiation pattern does not steadily drop off at the the edge of
the usable area, but has smaller lobes at the edges. If antennas are spaced too close together, or are transmitting at a relatively
high power then it's possible for the side lobes from one to radiate into another and desensitize it to it's intended signal. For these
reasons it's much better to run one's equipment at the lowest power setting that still allows for some rain fade.
A large portion of the interference to the 802.11b radios at the remote hill top was caused by the other 802.11b radio. The
Consultant had recommended moving the backbone dish halfway down the tower to help correct this mutual interference and to
protect it more from lightning strikes. Eventually it was possible to make another trip to remote to do so. At the same time it was
also desirable to move the omni antenna to get it a little farther away from the lightning rod though it was in relatively the same
placement as the repeaters which were mounted on telephone poles. This proved difficult because it had to be near the top of
the tower to hit all the CPEs and the Talo repeater, and the other possibilities put it very close to the DRMASS antennas which
caused them unacceptable levels of interference. So the 802.11b omni had to be left where it was. The Talo repeater which
provided service to some CPEs that couldn't see remote directly seemed to be having problems making itself heard. This was
probably because the remote omni was in a somewhat noisy environment. So next the omni was moved onto another pole
mounted a short distance away from the tower but it didn't seem to help much, and it might have even increased the interference
between the 802.11b radios. Then Talo was upgraded to two radios with one using a dish antenna to talk back to remote. This
did seem to help. Because the Talo link has since become intermittent the Consultant recommends moving the omni back onto
the tower. Additionally it might be possible to lower the DRMASS antennas a bit to lessen the side lobe interference between
them and the omni.
Radio System monitoring
Because the Cisco Bridges were monitor able by SNMP the Cricket router monitoring and graphing package was used to
monitor them. The Bridges provided two useful types of data, Throughput, and RF link data. For example, in figure 19 one can
see graphs for the bandwidth used on the "backbone" link over a twenty four hour period. First one notices that the throughput is
asymmetrical. This could be caused by one person doing most of the talking on some of the calls. Second that the outgoing in
"a" (blue) matches the incoming (green) in "b" this is as it should be. Also if one knows the data rate for the default Codec then
the number of simultaneous calls at a given time can be estimated. As the bandwidth usage increases so do the RF errors. As
mentioned earlier this is somewhat pronounced because the link is simplex. There would still be a small increase in the error rate
on a full duplex link but it would be proportional to the throughput in one direction only and not dependent on how much data was
being sent back. In figure 20 a one can see that the retires roughly match the receive errors in 20 b. This is typical for a point to
point link. Also the red graph in 20 b (Hold offs) indicates that the remote dish is in a bit of a noisy environment. Holds off
Timeouts are where the radio has waited an unreasonable amount of time for a clear space to transmit a packet. They are also
graphed in red but would fill in the area under the graph like the green retires graph. Since this was never seen it means there
was no Hold off Timeouts. This shows that the interference from DRMASS was not severe. It is noticeably different from the
other two probably because it is talking on a different frequency and to several CPEs and a repeater. This hints at the limitation
of this type of monitoring when several factors are aggregated into one graph. It would be much more helpful to monitor each
CPE and look at their error rates because they tend to link back to only one repeater, and as seen in figure 19 each end of a link
has virtually a mirror image of the other's information. Unfortunately the Avaya ECs were not SNMP capable. However the
Soekris SBC are, and it will be interesting to see how much this information helps in trouble shooting.
9

10. VoIP Report
R&D
At the request of the Consultant an e-tel GW210, two Symbol NetVision phones and, a four port GW built from an old computer
were purchased and demonstrated to work with a free Linux based H323 software. Even though this very low cost system
performed fundamentally the same functions as the Commercial vendors system it was obviously not ready to be deployed in the
field without further development. The Consultant feels that in six months to a year such a system could give commercial
vendors like VocalTec a run for their money.
The Consultant also brought a Single Board Computer (SBC) and some wireless cards to experiment with. After quite a lot of
development He was able to demonstrate a diskless wireless router with these main features.
Web status page
SNMP monitorable
DHCP server
DNS Caching
Wireless stats including signal strength and quality
Separate interfaces for VoIP equip and classroom computers
Traffic Shaping
ssh, ftp
References
Wireless
Tutorial on 802.11b spectrum usage http://www.euro.dell.com/countries/eu/enu/gen/topics/vectors_2001-
wireless_deployment.htm
IEEE Call for Interest Session March 12, 2002 http://grouper.ieee.org/groups/802/16/docs/02/80216-02_17.pdf
Frequency Domain Equalization for 2-11 GHz Broadband Wireless Systems http://www.ieee802.org/16/tutorial/80216t-01_01.pdf
Tinkerers Say They've Found a Cheap Way to Broadband, By JOHN MARKOFF, June 10, 2002
This article can probably be found at one of these URLs:
http://www.bostonwireless.org/mail/msg00064.html
http://www.ieee802.org/secmail/msg02424.html
http://www.nytimes.com/auth/login?
URI=http://www.nytimes.com/2002/06/10/technology/10WIRE.html&OQ=exQ3D1025054574Q26eiQ3D1Q26enQ3Df9c7eb2d241
cc542
Assigning 802.11b Access Point Channels http://www.80211-planet.com/tutorials/article/0,,10724_972261,00.html
Regulations Affecting 802.11 Deployment http://www.lns.com/papers/part15/Regulations_Affecting_802_11.pdf
Avaya Ethernet Converter http://www.hyperlinktech.com/web/avaya/avaya_converter.php
Linksys client adapter http://www.linksys.com/products/product.asp?grid=22&prid=432
Introduction to 802.11b http://www.mobilecomms-technology.com/projects/ieee802/
Hyperlink Technologies http://www.hyperlinktech.com/
YDI http://www.ydi.com/
Cisco 350 Series 802.11b Radios http://www.cisco.com/en/US/products/hw/wireless/ps458/index.html
Orinoco/Wavelan http://www.orinocowireless.com/
10