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| A Note from the Publisher:
Our inaugural issue of Rural Signals was well received, and
we appreciate the positive feedback that we received from our
readers. Our team of technical consultants strives to bring
you understandable information regarding current issues facing
the wireless telecommunications industry. In this second issue
of Rural Signals, our team writes about three areas of significant
interest to wireless operators today: broadband, 800 MHz interference,
and E911 technology.
There is little doubt that broadband data services will continue
to grow as perhaps the most significant source of revenue for
wireless operators. Rural Signals offers a pair of articles
that track advances in both WiFi and WiMax technologies. In
the opener, Judy Deng walks the reader through the standards
of WiFi and WiMax, looking at the growth of data mobility. As
an example of the possibilities, David Fritz reports in a related
article on his participation in a trial of an application using
WiMax technology to monitor the progress of athletes at the
Ironman Arizona triathlon event, which David previewed in the
inaugural issue of Rural Signals.
With the 800 MHz rebanding processing getting under way, Len
Garavalia provides Rural Signals readers with a timely introduction
to how 800 MHz band interference cases involving public safety
systems are being handled. In a subsequent issue, Rural Signals
will discuss how cellular and ESMR carriers can investigate
and respond to such interference claims.
While the industry, the press, and governmental agencies continue
to focus on controversial Enhanced 911 accuracy compliance issues,
Rural Signals editor Jim Egyud offers a reminder that not all
problems with E911 calls have to do with location accuracy.
The article, the first in a series, offers an overview of how
E911 technology works, and why some calls do not go where we
think they should.
Enjoy Rural Signals and, as always, your feedback is priceless.
Bennet & Bennet, PLLC
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| Surfing
the Wireless Broadband Evolution: WiFi to WiMax |
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By Judy Deng
WiFi is established
You might have already gotten used to surfing the Internet
wirelessly from one room to another at home, or checking your
e-mail and downloading some news wirelessly in a coffee bar
or airport, all using broadband. We owe all of these conveniences
to the standard of IEEE 802.11. The 802.11 standard allows all
WLAN (Wireless Local Area Network) products to communicate
with each other using radio frequencies in the 2.4 and 5 GHz
range to communicate between devices for indoor applications.
This article will discuss the evolution of the standards governing
WiFi and the ongoing evolution to the next step, WiMax.
As WiFi (Wireless Fidelity) left the starting blocks, IEEE’s
802.11a and 802.11b standards were the first specifications
into the marketplace. The 802.11b standard, with which the WiFi
term was first associated, defines how wireless devices can
communicate with speeds up to 11 Mbps using the 2.4 GHz frequency
band at a range of about 300 feet, depending on the environmental
conditions. Soon after, 802.11a was released to allow faster
communication (54 Mbps) in the 5 GHz frequency band, but this
band inherently came with the drawback of shorter range (about
75 feet). While consumers liked the faster throughput of 802.11a,
they were limited by its short effective range.
To meet consumer demand and overcome the limitations of both
802.11a and 802.11b, IEEE developed the 802.11g specifications.
802.11g was designed to have fast throughput (54 Mbps) in the
2.4 GHz wireless frequency band with a longer range (about 300
feet) and backwards compatibility with 802.11b. With further
optimization and the addition of compression, 802.11g devices
can transmit and receive at 108 Mbps. Taking things further,
the WiFi Alliance tests and certifies different manufacturers’
products for interoperability with each other.
Not surprisingly, 802.11g has become the most popular standard
used by WLANs. Now, most of the laptops and PDAs are integrated
with IEEE 802.11g devices that make it possible to wirelessly
access the Internet.
WiMax is on the move
All right, now imagine if you keep that connection as you leave
the coffee bar and jump on a bus or hop into the back seat of
your carpool for the commute to the office. Is it possible?
The answer is “yes”. WiMax makes it possible.
WiMax (Worldwide Interoperability for Microwave Access) is
a standard-based wireless technology that provides high throughput
broadband connections over long distances. WiMax can be used
for a number of applications, including “last mile”
broadband connections, hotspots, and high-speed connectivity
for business customers. It provides wireless Metropolitan Area
Network (MAN) connectivity at speeds up to 70 Mbps, and the
WiMax base station, on the average, can cover up to 10 miles.
The WiMax Forum is a non-profit corporation that was formed
in April 2001 by equipment and component suppliers to help promote
and certify the compatibility and interoperability of Broadband
Wireless Access (BWA) equipment. The WiMax Forum’s members,
which include Airspan, Alcatel, Alvarion, Fujitsu, Intel, the
OFDM Forum, Proxim, and Siemens, account for over 75% of sales
in the 2 to 11 GHz BWA markets. The WiMax Forum takes a similar
role to the WiFi Alliance in the WLAN arena, backing development
of wireless MAN products based on IEEE. The WiMax Forum believes
that a common standard for BWA will drive down the cost of equipment
and accelerate performance improvements. Besides, BWA operators
would not be locked into a single vendor since base stations
will interoperate with multiple vendors’ Customer Premise
Equipment (CPEs).
The standards development of BWA systems has taken place in
the U.S. by IEEE 802 MAN. The IEEE MAN systems have been focused
into the 802.16 task group, which has published standards for
frequency bands ranging from 2 to 66 GHz. In turn, the IEEE
802.16 Working Group is the IEEE group for wireless MANs. The
IEEE 802.16 standard published in April 2002 defines the wireless
MAN air interface. The IEEE designed 802.16 to operate in the
10-66 GHz spectrum, and it specifies the physical layer (PHY)
and medium access control layer (MAC) of the air interface for
BWA systems. As a limiting factor, transmission in the 10-66
GHz range requires Line-of-Sight (LOS), i.e., no physical obstructions.
While the 802.16 standard provides the foundation for a wireless
MAN industry, the physical layer is not suitable for lower frequency
applications where non-line-of-sight (NLOS) operation is required
and longer distances would be possible. For this reason, IEEE
published the 802.16a standard as an extension of 802.16 to
accommodate NLOS requirements in April 2003. NLOS is the preferable
technology for last-mile applications where obstacles like trees
and buildings are present. The standard applies to licensed
and unlicensed frequency bands between 2 GHz and 11 GHz, and
allows users to obtain broadband connectivity without needing
direct line of sight with the base station.
IEEE 802.16a specifies three air interface specifications,
and these options provide vendors with the opportunity to customize
their products for different types of deployments. The three
physical layer specifications in 802.16a are:
- Wireless MAN-SC: it uses a single carrier (SC) modulation format.
- Wireless MAN-OFDM: it uses orthogonal frequency division multiplexing
(OFDM) with 256-point Fast Fourier Transform (FFT). This modulation
is mandatory for license exempt bands.
- Wireless MAN-OFDMA: it uses orthogonal frequency division
multiple access (OFDMA) with a 2048-point FFT. Multiple access
is provided by addressing a subset of multiple carriers to individual
receivers.
WiMax is moving towards interoperability
One of the purposes of the WiMax Forum is to create a single
interoperable standard from the IEEE standards. The 802.16a
standard is intended to allow for multiple vendors to produce
interoperable equipment. However, the standard allows the vendors
to implement different modulation schemes and to customize their
equipment. Therefore most of the existing products differ from
vendor to vendor.
In order to create a single interoperable standard, the WiMax
Forum has decided to focus on the 256 FFT OFDM modulation scheme,
which is common in 802.16a. The Forum has developed system profiles
covering the popular license-exempted bands at 2.4 GHz and 5
GHz and other licensed bands at 2.3 GHz, 2.5 GHz and 3.5 GHz.
At the moment, the WiMax Forum will focus its conformance and
interoperability test procedures on equipment that operates
in the 2.5 GHz and 3.5 GHz licensed bands and the 5.8 GHz unlicensed
band using 256 FFT OFDM. It appears that the Forum will adopt
a flexible channel plan with bandwidths from 1.5 MHz to 20 MHz
per channel.
Because much of the 3.5 GHz spectrum within the U.S. is occupied
by the Federal Government, the unlicensed 5 GHz and licensed
2.5 GHz bands appear to be the most viable options for WiMax
deployment in the U.S. The licensed 2.5 GHz spectrum has been
apportioned for use in the Broadband Radio Service (BRS) and
Educational Broadband Service (EBS), which were previously known
as Multichannel Multipoint Distribution Service (MMDS) and the
Instructional Television Fixed Service (ITFS), respectively
(for an explanation, please see the March 31, 2005 issue of
Rural Signals). This spectrum could represent the greatest opportunity
for broadband wireless in the near future. This licensed band
has advantages of better quality of service and better NLOS
reception at lower frequencies, but with higher barriers for
entrance. By contrast, the unlicensed 5 GHz band has the advantages
of faster rollout, lower initial costs, and commonality in much
of the world. However, with its higher frequency and unlicensed
nature, it is more limited in range and subject to interference
and crowding.
Look for WiMax technology to deploy in three phases:
- The first phase of WiMax technology will provide fixed wireless
connections via outdoor antennas in the first half of 2005.
Outdoor fixed wireless can be used for T1 services, cellular
network backhaul, hotspots and other commercial services.
- In the second half of 2005, WiMax should be available for
indoor installation with smaller antennas similar to a WiFi
access point. In this fixed indoor model, WiMax would be available
for use in residential broadband deployments.
- By 2006, expect the WiMax technology to be integrated into
mobile computers to support roaming between WiMax service areas.
It will allow people to access the Internet virtually everywhere.
WiMax is one of the most talked about developments for next-generation
wireless broadband service. A recent report issued to Congress
by the Federal Communications Commission concluded that WiMax
“has the potential to alter and further accelerate the
evolution of broadband services.” As the next evolutionary
step of its WiFi predecessor, WiMax is being touted as an easily
deployable “third pipe” that will deliver both flexible
and affordable last-mile broadband access to millions. Many
believe that WiMax will do for broadband access what cellular
phones did for voice: connect users directly to the Internet
from just about anywhere, most likely starting in the metropolitan
areas.
WiMax holds a great deal of promise. Since wireless technologies
are easier to install than wired infrastructure, providers might
use WiMax to provide broadband services in previously unserved
and underserved areas quickly and cost-effectively. Its low
cost of deployment and remarkable throughput could make WiMax
a valuable tool for achieving increased rural broadband deployment.
As a final note, on May 5, 2005, Sprint and Intel announced
an agreement to work together to develop the WiMax mobile technology
to provide high-capacity wireless broadband coverage and services
through metro areas and an enriched multimedia user experience.
According to the release, Sprint is exploring the technology’s
use with some of its MMDS licenses in the 2.5 GHz band.
******
If you would like to learn more about the various aspects of
WiMax and its development, please do not hesitate to contact
Judy Deng.
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| WiMax
Technology Takes A Test Run at Ironman Arizona |
|
By David Fritz
Editor’s Note: David Fritz recently had the opportunity
to participate in a test of wireless broadband equipment and
applications using pre-WiMax technology over unlicensed spectrum
(frequency bands where the equipment must comply with FCC rules
but which otherwise do not require FCC licensing for their use).
The setting was the Ironman Arizona competition, where the equipment
provided video coverage over the Internet and timing data to
officials and fans over wireless links. Our technical consultants
have extensive experience designing radio paths used to communicate
in these frequency bands.
From the start of the 2.4 mile swim at 6:45 AM, through the
112 mile bike ride, to the last finisher at midnight after the
26.2 mile run in the Ironman Arizona triathlon on April 9, 2005,
unlicensed broadband wireless equipment was used to demonstrate
the many potentials for developing WiMax technology.
Throughout the 17-hour grueling endurance event, I participated
with Airspan Networks in a test application of its WipLL (pre-WiMax
point-to-multipoint broadband solution) and AS3030 product lines.
The equipment, running on unlicensed 2.4 GHz spectrum, provided
wireless backhaul and remote control functions between the Ironmanlive.com
web cast production facility and remote video cameras positioned
throughout the course to enable live online access for race
fans. Additional equipment using unlicensed 900 MHz spectrum
provided Internet access and real time updating from RFID timing
mats at the finish line to the Ironman information desk.
ON THE COURSE
 Course
equipment consisted of a 10 foot movable light stand, Sony IP
camera, Airspan’s WipLL 2.4 GHz subscriber unit, 18 dBi
patch panel antenna, 10/100 5-port workgroup switch, and a portable
generator. Two full camera assemblies were moved around the
course throughout the day, with subscriber unit antennas pointed
back to base station units located on a bluff next to the Arizona
State University Stadium. Camera locations with working equipment
were easily set up or broken down within 30 minutes and relocated
to other venues as the triathlon progressed from the swim to
the bike to the end with live coverage of the emotional racers
and fans at the finish line.Each data link provided up to 3.2
Mbps of throughput, with the day’s average on the order
of 2.8 Mbps for each video feed. The wireless interface also
provided remote control of each IP camera from the production
location, allowing video producers to pan up to 270 degrees
along with zoom/tilt functionality.
The WipLL subscriber units come with 90 degree, 8 dBi internal
antennas, and transmitter output power of 27 dBm, at a size
comparable to a desktop cable modem. The equipment uses an LED
lighting sequence to assist customers in orienting units without
a computer connection. Also, as demonstrated on the course,
subscriber units can accommodate an external antenna for additional
gain and directionality. All connections other than the external
antenna use CAT5 cabling and seamless “plug-in-play”
integration with any existing 10BaseT, 100BaseTX or 10/100 networks.
ON THE BLUFF
 The
bluff next to the Arizona State University Stadium served as
a backhaul collection point for all of the remote video cameras.
Airspan utilized three different configurations of its 2.4 GHz
base station units to provide coverage to different parts of
the swim, run, and bike courses.Configurations included two
different types of external antennas and an internal 90 degree,
8 dBi antenna. All units have a maximum transmit power of 27
dBm using a frequency hopped CDMA (FH–CDMA) radio technology
and a Time Division Duplex (TDD) mode for single channel operation.
Base station to subscriber unit distances ranged from 0.5 to
2.2 miles, consisting mainly of line-of-site operation. Base
station electronics and the internal antenna are packaged into
a single weatherproof housing about the size of a laptop and
include a mounting bracket for tower top deployments. All connections
other than the external antenna use CAT5 cabling and seamless
“plug-in-play” integration with any existing 10BaseT,
100BaseTX or 10/100 networks.
Camera data streams were switched together at the top of the
bluff and transmitted to the Ironmanlive.com production facilities
using the AS3030 wireless IP high capacity point-to-point system
at 5.8 GHz. Supporting up to 72 Mbps over the air, this radio
provided ample backhaul over the 0.46-mile distance to the production
facilities. The AS3030 system utilized 5.8 GHz external patch
panel antennas at each end of the path and was deployed along
with the WipLL base station in a one-day setup window.
AT THE PRODUCTION FACILITES
 Video
feeds and control of the remote cameras were delivered to the
production facilities and then produced into a live streaming
feed for Internet broadcast over Ironmanlive.com (www.ironmanlive.com).
Also, Airspan WipLL equipment provided Internet access and real-time
data updates from RFID timing mats at the finish line to the
Ironman information desk. A mix of subscriber units provided
access to the timing officials at the end of each race stage
with a 900 MHz base station radio using a Cushcraft 9 dBi yagi
antenna mounted on the one-story awning of the production facility.
The 900 MHz system was slated for use along the bike course
for camera operations, but since existing facilities on the
bluff were using the unlicensed spectrum, it was replaced with
the 2.4 GHz equipment and was easily deployed in the short spaced
co-channel environment in the transition area below the bluff.
AT THE FINISH LINE
With a day’s worth of setup on the AU bluff, an early
morning start on race day, a group of dedicated volunteers,
one disappearing generator, and an emotional midnight finish,
the Ironman Arizona demonstration showed a huge range of possibilities
for WiMax at a time when consumer demand for broadband access
is surging. For WiMax, IEEE 802.16e standards should be ratified
in late 2005, with new consumer products and services pushing
the marketplace in 2006 and 2007. With backing and development
by companies like Intel and Airspan, it is evident that WiMax
technology is poised to be an effective and popular solution
for broadband data delivery.
******
The possibilities appear endless for WiMax and similar technologies.
Additionally, Bennet & Bennet’s technical staff has
extensive experience in backhaul path design in the unlicensed
frequency bands to support these applications. If you should
have any questions about the technology, the demonstration,
path design, or other possible applications, please contact
David Fritz.
Additional Information:
Ironman Arizona Triathlon - http://www.ironmanarizona.com/
Intel WIMAX - http://www.intel.com/netcomms/technologies/wimax/?iid=search&
Airspan WipLL Product - http://www.airspan.com/products_bwa_aswipll.htm
Ironmanlive.com - http://vnews.ironmanlive.com/
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800 MHz Interference Notification System for Public Safety and Critical Infrastructure
In the face of an interference
claim, can Cellular and ESMR carriers respond in time? |
| By Leonard Garavalia
As Bennet & Bennet’s legal team has previously reported,
the FCC released a Supplemental Order and Order on Reconsideration
to clarify and modify its decision to resolve the growing problem
of interference experienced by public safety communications
systems operating in the 800 MHz Band. The primary focus of
these orders was the FCC’s long-term solution to reconfigure
the 800 MHz band to separate incompatible technologies, i.e.,
systems that use cellular architecture (“low-site”
technologies) from public safety and private wireless systems
that do not (“high-site” technologies), otherwise
known as “rebanding”. In the meantime, the Commission’s
800 MHz Report and Order addressed the ongoing interference
problem over the short term by adopting technical standards,
defining unacceptable interference, and setting up procedures
for abating this interference. This article is intended to provide
an understanding of those standards and procedures, and to assist
our readers in preparing to deal with potential interference
claims.
In between the cellular transmit and receive bands, the 800
MHz spectrum is a tangled web of interleaved channels utilized
by SMR, ESMR (such as Nextel), Public Safety, Critical Infrastructure
(CI), and B/ILT (Business, Industrial or Land Transportation)
licensees. While the cellular A-band frequencies lie closer
to the public safety/CI bands, cellular B-band carriers are
not exempt from these rules. To understand the congested nature
of the spectrum, a chart showing the divisions of the 800 MHz
band can be viewed here.
The FCC’s procedures for abating this interference required
cellular and ESMR (such as Nextel) carriers to establish an
Internet-based single point of contact for Public Safety officials
to contact in the event of an interference complaint. The procedure
also required those carriers to respond to interference complaints
within 24 hours if the complaining party is a Public
Safety or CI licensee and within 48 hours if the complaining
party is a B/ILT or a traditional high-site SMR licensee. In
conjunction with the “800 MHz Notification System Working
Group”, Ventera developed a web-based notification system
(See http://www.PublicSafety800MhzInterference.com). As of March
of this year, all ESMR and cellular carriers were to have reported
to Ventera with the County and Zip Code pairings within which
those carriers provide service. Per Ventera and CTIA, as of
April 25, 2005, 51 carriers were participating. If you are a
cellular or ESMR carrier, you should ready yourself for what
may happen next.
If a Public Safety or Critical Infrastructure licensee is the
complainant in an interference report, the cellular or ESMR
carrier might be hard-pressed to respond knowledgably within
24 hours. If the complainant utilizes Ventera’s Interference
Notification Site, the information transmitted to the carrier’s
designated contact may only include the following:
- Contact Name
- Frequency Affected
- Geographic Area
- Type of License the affected licensee holds
If it uses the Interference Notification Site, the complainant
will set up a meeting time and place which, along with the minimal
background information listed above, will be e-mailed to the
carrier’s designated point of contact. The web-based scheduler
is geared towards default meeting times to be held during typical
middle-of-the-night maintenance windows, where the carrier’s
site can be taken out of service to determine whether or not
it really is the cause of the suspected interference. To keep
the carrier’s initial response time within the required
24 hours, the scheduler is set up so that if the complainant
submits its complaint between midnight and 6:00 AM, the default
meeting time is 11:00 PM THAT DAY. While the complainant can
schedule an alternate time, it cannot set a meeting prior to
11:00 PM that day through the scheduler. If the complaint is
submitted after 6:00 AM, the meeting time can be set any time
within the 24 hours as required. Even so, it could be difficult
for a carrier’s technician or engineer to attend such
a meeting on such short notice, making it all the more important
for the complainant to include as much detail as possible. Please
note that if the meeting time is set outside of the maintenance
window, then on/off testing cannot be set through the web-based
scheduler prior to 11:00 PM the following day.
While the web-based reporting mechanism allows fields for the
complainant to input specific coordinates and the affected receiver’s
make, model, signal level and performance level, these are optional
pieces of information that may or may not be provided. The FCC
stated, “In sum, we will not burden interfered-with parties
with information collection requirements as a prerequisite to
abating interference to what oftentimes are mission critical
communications.” Unfortunately, it is this very information
that could assist a CMRS carrier in identifying the potential
interference source and get a quicker start in abating the interference.
(Public Safety officials and consultants reading this issue
of Rural Signals are encouraged
to include as much of this information as possible for their
own benefit. – Ed.)
While it may take some time to isolate potential causes of
interference, the FCC has defined certain thresholds that must
be met with respect to the complainant’s system. These
include interference margins, minimum signal strengths, and
criteria for receiver performance.
INTERFERENCE DEFINED - The FCC’s
rules defines interference in terms of a parameter known as
the carrier to interference plus noise ratio [C/(I+N)] of a
receiver. The FCC’s order recommends a minimum acceptable
C/(I+N) ratio for Public Safety, CI, and B/ILT voice systems:
- 17 dB before rebanding
- 20 dB after rebanding
For non-voice public safety communications systems, the FCC
suggests that the equipment manufacturer supply the minimum
“information value”, which may be listed in bit
error rates.
MINIMUM SIGNAL STRENGTH DEFINED
- The FCC specified that the public safety, CI or other 800
MHz non-cellular signal (in the 806816 MHz/851-861 MHz
band segment) will be entitled to protection only if the median
power threshold of the [protected] received signal is greater
than or equal to the following:
Portable or Hand Held Units
- -85 dBm before rebanding
- -101 dBm after rebanding
Mobile/Vehicle Mounted Units
- -88 dBm before rebanding
- -104 dBm after rebanding
In the 816-817 MHz/861-862 MHz band segment, measured median signal
power for interference abatement increases as a function of frequency.
To demonstrate, the minimum median received power level required
for interference protection increases as shown below in Figure
1 from the FCC:
MINIMUM RECEIVER PERFORMANCE CRITERIA
DEFINED – The FCC has found that certain interference
definitions and measurement procedures contained in the record
allow it to establish a reasonable standard for determining
when public safety and other non-cellular systems can expect
to operate free from unacceptable interference. The operational
parameters and system characteristics identified below are minimums
that Public Safety and CI providers must meet to get full protection:
Portable or Hand Held Units
- 70 dB intermodulation rejection ratio
- 70 dB adjacent channel rejection ratio
- -116 dBm reference sensitivity
Mobile/Vehicle Mounted Units
- 75 dB intermodulation rejection ratio
- 75 dB adjacent channel rejection ratio
- -116 dBm reference sensitivity
Given the short response time imposed by the FCC upon cellular
and ESMR carriers, and in the interests of public safety, all
parties that may be involved in an interference claim are encouraged
to become intimately familiar with these technical thresholds
and benchmarks, not to mention efficient procedures for addressing
such claims. Isolating and abating the cause of interference
can be a time-consuming and iterative undertaking, especially
if multi-source intermodulation is involved, as opposed to the
simpler case of spurious out-of-band emissions. It is important
to have the right equipment and expertise available.
******
In a future issue of Rural Signals,
we will examine complaint response and tracking considerations.
In the meantime, if you have any questions, please contact Leonard
M. Garavalia. We would be glad to work with your organization
on a recommended plan for addressing interference issues.
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Understanding E911 Technology, Part I
Routing the Call is Half the Battle |
| By Jim Egyud
Several weeks ago, I was reading through some trade publications
and came across yet another story about wireless 911 calls not
getting handled properly. According to the article, a test 911
call on the west side of Manhattan got routed to a PSAP (Public
Safety Answering Point) across the Hudson River in New Jersey.
While the article linked this occurrence to limitations in triangulation-based
location technology, I speculated on what might have actually
happened in this and other examples reported by the media. In
the midst of all the media hype over wireless 911, I said to
myself, "My goodness, they probably don’t understand the
technology!"
Phase I Routing: An Inexact Science
Was it a Phase II location fix error as some might have thought?
Was it a telco error? A wireless switch routing error? Would
a network-based or handset-based Phase II solution have made
the difference? I bet it was nothing more than a wireless coverage
issue and a byproduct of industry-accepted E911 call routing.
The caller was probably located on the far west side of Manhattan,
perhaps along the east bank of the Hudson. Anyone that has visited
Manhattan realizes that the buildings form a wall of steel,
glass, and concrete, a fortress with street openings. With all
the clutter from those buildings, a cell site on the other side
of the river probably had the least obstructed path to the caller,
with signals stronger than nearby sites in Manhattan. Wireless
calls tend to take the path of least resistance.
For Phase I of Enhanced 911 (E911) services, the FCC requires
that a 911 call be delivered to the PSAP serving the area with
the identity of the cell site on which the call originated,
along with a call-back number. When a wireless user makes a
911 call, the switch must send an Origination Request (ORREQ)
to the Mobile Positioning Center (MPC) used by the wireless
company. The MPC, equivalent to a Gateway Mobile Location Center
(GMLC) in the GSM world, will identify the PSAP to which the
call gets routed, will tell the switch to route the voice portion
of the call to that PSAP, and will send the call information
to a real-time database used by the PSAP. This database, usually
hosted by a Local Exchange Carrier, will then enable the 911
dispatcher to see the caller’s data. Without knowing the
caller’s actual location, how does the MPC identify the
PSAP to which to route the call? It uses the identity of the
cell site. This works just fine so long as the cell site’s
coverage area does not extend beyond the PSAP’s boundaries.
Of course, radio signals traveling through the air do not pay
much attention to lines drawn on a map, such as the Hudson River.
The Phase II Routing Dilemma
For Phase II, the FCC requires that the call be delivered with
an estimate of the caller’s location. To do this, wireless
carriers utilize a Position Determining Entity (PDE), also known
as the Serving Mobile Location Center (SMLC) for those with
GSM networks. When alerted to a 911 call, the PDE typically
uses the identities of caller’s handset and the cell site,
provided by the switch or the MPC, to locate the caller in the
network. CDMA carriers in the U.S. use a “handset-based”
Phase II technology whereby the PDE must summon handset readings
of the GPS satellite constellation, enhanced by CDMA network-measured
timing data that provides an additional form of triangulation
to the handset. GSM and TDMA carriers use one of a handful of
“network-based” technologies, the most common of
which triangulate to the caller’s position by calculating
the differences in how long it took the handset’s signal
to reach different cell site antennas.
Once armed with a set of coordinates from the PDE, the MPC
provider can then match the location to a PSAP using a Coordinate
Routing Database (CRDB). In an ideal world, the MPC would have
the call routed to the correct PSAP based on this data. However,
there’s a catch: it takes time for the PDE to make that
location estimate, regardless of the Phase II technology. GPS-based
solutions need time to acquire satellite signals, and triangulation-based
solutions have to wade through the real-time call data to calculate
a location fix. None of the technologies can start the process
until triggered by the switch, after the call has been initiated.
While the entire process might take but a few seconds, every
second counts. Because every second counts, many PSAPs do not
want to wait for that location fix before receiving the call.
So, how does the MPC identify the PSAP to which to route the
call? If requested by the PSAP, it sets aside the CRDB and .
. . . uses the identity of the cell site! This takes us right
back to the realities of Phase I routing, since radio signals
don’t take the time to look down while passing over PSAP
boundaries, such as the Hudson River.
In New York City, where cellular carriers limit microcell and
picocell coverage areas to a few blocks, a single building,
or a single floor, coverage from tens or hundreds of sites might
be contained within the bounds of a single PSAP. Our West Side
example would certainly be the exception to the rule for such
an urban situation where the PSAP might prefer the choice of
immediate routing based on the site location. However, consider
the situation of a rural cell site sitting near a county boundary
and covering a hundred square miles of farmland. If the PSAP
boundary follows the county boundary, then 50% of the calls
might occur within one PSAP’s area, and 50% within the
other PSAP’s area. If the PSAPs decide not to wait for
the first location fix (or if only Phase I services have been
enabled), then 100% of those calls must be routed to one PSAP.
What then? It is common practice for the receiving PSAP to transfer
the call to the other PSAP.
Using the site-based option, the vast majority of E911 calls
still get routed to the correct PSAP, with the Phase II location
data reaching the PSAP within a few seconds, usually before
the caller can finish saying, “I need assistance.”
As location-finding technology improves, we suspect that more
PSAPs will request routing based on the caller’s coordinates.
It is imperative for all stakeholders to understand the technologies,
including the different position determination technologies
applicable to various types of wireless networks.
Location, Location, Location
While the routing of the call on the west side of Manhattan
might not have necessarily been an issue of Phase II location
accuracy, the subject of accuracy compliance remains a lightning
rod for scrutiny. The Network Interoperability and Reliability
Council (NRIC) has issued several reports, among them a report
to the FCC with recommendations for E911 accuracy compliance
and testing. The report has generated its fair share of controversy
in light of recommendations for accuracy compliance on a statewide
basis, deployment benchmarks to trigger testing, and accuracy
levels for rural carriers. Given the numerous implications and
uncertainties for rural carriers, Bennet & Bennet, PLLC
has issued a memorandum that discusses these issues. If you
would like a copy of this memo, please do not hesitate to contact
Michael Bennet or
Jim Egyud and ask
about our memo subscription service.
******
Given the complexity of the issues, we encourage rural carriers
in particular to learn about the various avenues of E911 compliance
available to them before they receive PSAP requests for service.
In a subsequent issue of Rural Signals,
Part II of our Understanding E911 Technology series will examine
these Phase II location technologies. In the meantime, if you
should have any questions about E911 technology, or if you are
a carrier wanting to learn more about the options available
to you, please do not hesitate to contact Jim
Egyud.
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If you have come across Rural Signals on-line and do not already receive our free quarterly e-mailed version, simply e-mail the Editor, Jim Egyud, by clicking here. Thank you for your interest.
Questions??? Call Rural Signals Editor Jim Egyud [(202) 371-1500], and refer to Vol. 1, No. 2.
About Rural Signals
Rural Signals is a quarterly publication of Bennet & Bennet, PLLC's technical consulting service division. Rural Signals is delivered by e-mail four times a year and features technical discussions on current spectrum related happenings affecting rural America. For subscription information or to inquire about specific rural spectrum issues, please call/fax/e-mail Rural Signals Editor Jim Egyud at 202-371-1500 or 202-371-1558 (fax).
While it is our intention to provide valuable information to readers of Rural Signals, the transmission of this newsletter does not create an attorney-client relationship. You should not act upon any information contained in Rural Signals or at www.bennetlaw.com without first seeking the advice of an attorney.
Copyright 2005 Bennet & Bennet, PLLC
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