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Optimizing Quality of Service in Mobile Net works
Why, Wh en, Wh ere and How
Quality of Service in Mobile Networks
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Preface: How to Use This Document . . . . . . . . . . . . . . . . . . .4
Introduction
Life as A Network Operator: To Stand Still is to Fall Behind . . . . . . . . . . . . . .4
Quality of Service: Todays Key to Competitiveness . . . . . . . . . . . . . . . . . . . .5A Picture of the Business Case for QoS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
The Profit Impact of QoS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .6
Installation
Installation Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
QoS Considerations for Installation Personnel . . . . . . . . . . . . . . . . . . . . . . . .7
Measurement Challenges and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
BTS Transmit Tests are the Starting Point . . . . . . . . . . . . . . . . . . . . . . . . .8
Looking at Modulation Accuracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Protocol Analysis Supports Installation of New Features . . . . . . . . . . . .10
Protocol Analysis Solution Requirements . . . . . . . . . . . . . . . . . . . . . . . .11
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .11
Operations & MaintenanceOperations & Maintenance Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
QoS Considerations for the O&M Department . . . . . . . . . . . . . . . . . . . . . . .12
Measurement Challenges and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . .12
Portable Testers Provide Cost-effective Monitoring . . . . . . . . . . . . . . . . .13
New Features Mean Added Responsibility for Protocol Testing Tools . .14
SS7 Signaling Network is the Connection for Network-wide Monitoring .14
Call Generator/Analyzer is a Demanding Subscriber . . . . . . . . . . . . .16
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .17
Planning & Engineering
Planning & Engineering Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
QoS Considerations for the P&E Department . . . . . . . . . . . . . . . . . . . . . . . .18
Measurement Challenges and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . .18
Drive Tests Provide an End-to-End View of the Network . . . . . . . . . . . .18Test Plant Debugs Problems in a Neutral Environment . . . . . . . . . . . . .20
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21
Quality
Quality Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
QoS Considerations for Quality Personnel . . . . . . . . . . . . . . . . . . . . . . . . . .22
Measurement Challenges and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . .22
Drive Tests Deliver Objective Measurements . . . . . . . . . . . . . . . . . . . . .22
Non-intrusive Measurements View the Whole Network . . . . . . . . . . . . .23
Call Generators Duplicate Real-world Calling Situations . . . . . . . . . . . .23
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
Marketing
Marketing Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24QoS Considerations for the Marketing Department . . . . . . . . . . . . . . . . . . .24
Measurement Challenges and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
SS7 Monitoring Profiles Subscribers . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25
Security & Billing
Security & Billing Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26
QoS Considerations for the Marketing Department . . . . . . . . . . . . . . . . . . .26
Measurement Challenges and Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . .27
SS7 Monitoring System Guards Against Fraud Throughout the Network .27
Alarms, Fraud Reports Help Detect Offenders . . . . . . . . . . . . . . . . . . . .28
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
Appendix A: About Tektronix Role in Communications Test . . . . . . . . . . . .30Appendix B: Mobile Telephony Standards . . . . . . . . . . . . . . . . . . . . . . . . . . .30
Table ofContents
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The mobile communications marketplace has experienced
tremendous subscriber growth rates during the past
decade. Hundreds of millions of mobile subscribers are
today using digital mobile phones, and growth is expectedto continue unabated. For example, GSM network
subscribership alone is growing by up to 10 million
customers per month, with totals forecasted to exceed
250 million GSM users as of the end of 1999. 1These digital
phones interact with a network infrastructure made up
of varied base station types and switching equipment.
In turn, each individual network must interact with every
other, delivering the subscribers calls as though they were
traversing one seamless worldwide network.
Today most mobile networks are implemented with
2nd-Generation (as distinguished from 1st-Generation
analog cellular) technology. Widely referred to as 2G,current mobile technology encompasses standards such
as GSM in Europe and cdmaONE and IS-136 in the U.S.A.
2G digital technology was prompted mostly by the need to
provide more voice telephony capacity and better coverage
in urban and high-density geographic areas. 2G technology
came to market with improved Quality of Service (QoS) and
broader roaming capability.
But todays best 2G mobile performance is just a stepping
stone for the next round of subscriber demands. Mobile
users want enhanced data delivery, Internet access, email,
and more. The evolving solution is called, aptly enough,
3rd-Generation, or 3G, mobile telephony. Standards such
as Wideband-CDMA are being defined and refined even as
this document is written.
Ultimately all these trends say one thing about the mobile
network market: to stay competitive, mobile network opera-
tors must continuously expand and improve their services,
meeting subscriber demands while controlling operating
costs. And as we will see, Quality of Service plays a major
role in network competitiveness and business success.
Life as a Network Operator:
To Stand Still is to Fall Behind
Today there are huge pressures confronting mobile network
operators: business pressures, technical pressures, competi-
tive pressures the list goes on.
Hundreds of millions of mobile phone users around the
world rely on digital mobile telephony to keep in touch
with friends, family, and business associates. Increasingly,
this contact involves services above and beyond simple voice
connections. The business of running a mobile network is
a juggling act that involves keeping up with subscribers
coverage and service demandsbut not outpacing them.
Technical pressures add cost and complexity to the
equation. Standards are evolving and hardware capabilities
are advancing. Hardware and software compatibility issues
are becoming more difficult to handleor even to define
as new technology arrives almost daily.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
Ultimately all these
trends say one thing
about the mobile
network market:
to stay competitive,
mobile network
operators must
continuously expand
and improve their
services, meeting
subscriber demands
while controlling
operating costs.
Introduction
How to usethis document
This Mobile QoS Primer is designed to provide an overview of the problems, solutions, and benefits of a coherent network
QoS test and measurement strategy.
The Primer is not intended to be a cookbook to provide step-by-step solutions for specific measurement problems, nor is it
an authoritative reference for standards compliance. It will emphasize the real application needs of todays mobile networks,
and it will discuss solutions in general terms. For the sake of simplicity in this primer, we will focus on one or two solutions for
each type of network application. Bear in mind that most solutions have overlapping applications, and may find use in virtually
every division of a network.
Fundamentally, the Primer is divided into chapters that follow the organizational lines of the typical successful mobile
network business. In each chapter we will look at the unique responsibilities of a single activity.
The chapters will cover the entire range of organizational responsibilities and activities within a network, namely:
Installation Planning and Engineering Quality
Operations and Maintenance Security and Billing Marketing
Note here that not all network operators have an organization specifically titled the Quality Department. In many cases the
quality organization is simply part of the planning and engineering group. Note also that QoS is a broad concept that touches
every aspect of network operation, not just those who have quality as their primary responsibility.
Each organizations chapter is made up of four major subtopics:Overview Measurement Challenges and Solutions
QoS Considerations Conclusion
Most likely you will want to go directly to the chapter that pertains to your own area of responsibility. After youve finished
reading that, though, we recommend that you peruse the other chapters to learn about the similarities and differences in
QoS issues as they cut laterally across every network activity.
1 GSMWorld Web site; http://www.gsmworld.com/news/press_releases_23.html
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Moreover, mobile users are always seeking improved
service quality and reliability at lower cost. A mobile
subscriber tends to think only in terms of his or her latest
attempt to make a call. If there is service degradation of any
kind, the subscribers perception of the networks product(connectivity and services) suffers.
Competitive pressures are in some ways the most onerous
of all. The mobile market is dynamic, with new competitors
displacing old, mergers creating mega-networks, and every-
one striving to gain a bigger share of the market pie.
Competition has brought prices down. Every day, consumers
are barraged with special deals, signing bonuses and low-
price offers. The cost of marketing is rising, profit margins
are eroding, and end-users are becoming more sophisticated.
Today they look at value rather than price alone.
Small wonder, then, that life in the network operator busi-ness seems like a marathon with a constantly accelerating
pace. Business, technology, and competitive factors are
driving network operators to seek product differentiators
that will allow them to lead the race, recruiting and
retaining loyal subscribers.
Quality of Service:
Todays Key to Competitiveness
What can a mobile network operator do to protect existing
businessand perhaps even grow faster than the market
as a whole? Increasingly, Quality of Service (QoS) is thedifferentiator that mobile service providers are using to
define their place in the market. Lets take a look at why
and how this trend is progressing.
Most mobile subscribers seem to understand that they
must look past mere price considerations when choosing
a mobile service provider. Complex rate structures and
discount programs can make it difficult to compare vendors,
and such comparisons often reveal that actual costs are
fairly similar no matter which vendor provides the service.
But the quality of the mobile service the subscriber receives
is a daily reminder of the vendors competenceand compet-itiveness. The value of mobile service lies in its convenience,
and convenience is drastically compromised when there is
poor reception, dropped calls, or erratic service.
Mobile subscribers are a notoriously fickle bunch, willing
to change to a new network provider for the tiniest of
advantages. Little does it matter whether the subscriber
can actually gain much by churning from vendor to
vendor; all that counts is the perception that he or she
might get better service or save some money by switching
to the network next door. Changing providers has become
so easy that mobile subscribers quickly forget the incentives
they were so happy about when they signed up.
Complicating matters even more, this churning scenario
plays out against a backdrop of network compatibility
issues. For example, number portability makes it simple for
users to change service providers, even while it presents
new support challenges to all providers. Or, the subscriberslocal provider may be unjustly blamed for a roaming
problem that is actually the fault of another network. Lastly,
administrative complexities such as discounts and billing
procedures grow more cumbersome with each exciting
new rate plan, and security breaches, including large-scale
organized fraud, are an increasing threat.
The solution lies in ever-improving Quality of Service. But
QoS is more than guaranteed bandwidth, a clear and
continuous signal, and reliable roaming access. Certainly
these are the key benefits the consumer of mobile services
will receive. But the astute businessperson will see that
well-implemented QoS also serves the best interests of thenetwork provider.
QoS initiatives can:
Attract new, more sophisticated subscribers
Increase subscriber satisfaction, reducing churn
Motivate subscribers to adopt new, billable services faster
and to use these services more frequently
Improve industry relationships among diversenetwork operators, service providers and carriers
Provide a better understanding of subscribers
network usage, thus the subscribers themselves
Protect against fraudulent network use and piracy
Ensure compliance with standards andgovernmental regulations
Many network business have come to these same
conclusions, and have begun carrying out QoS programs
within their organizations. As used in this document, the
term QoS program has a broad definition that encom-
passes all of the efforts that support network quality. These
are not limited to test and measurement issues; they may
range from improvements in BTS installation procedures
to integrating a network-wide SS7 monitoring system.
A Picture of the Business Case for QoSThe success of any business rests on its subscriber loyalty.
And that is the crux of the problem in the mobile network
business. Loyal subscribers are cost-effective subscribers;
they are users whose continued business does not depend
on constant infusions of costly marketing energy or new
discount incentives with each passing month. Loyal, repeat
subscribers produce the reliable revenue stream that a
network needs to fund modernization programs, design
new service offerings, and support expansion.
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As we explained earlier, Quality of Service is the key to
competitiveness and therefore business success. Thats
because it is the path to subscriber loyalty. Figure 1 summa-
rizes the concept. There is an endlessly recirculating loop
of dependencies. No single link in the loop can be ignored,so the challenge for mobile network providers is to find the
right place to step aboard. It is the mission of this Primer
to show you how QoS programs can help a network break
out of the cycle of price wars and churning, and into the
quality improvement cycle shown in Figure 1.
The Profit Impact of QoS
Promoting subscriber loyalty is just one of the benefits of
a conscientious QoS effort. In fact, there is an even more
direct, measurable benefit of particular interest to shareholders
and executives. It is the immediate profit that accrues whenmobile providers deliver clean, reliable connections.
Simply stated,good service quality stimulates increased
service usage. And simple math tells us that more usage
means more billable minutes of air time. It has been shown
that mobile users will use their phones more often, and
stay on the phone up to 20% longer, when their calls are
not hampered by noise, echoes, or other interruptions.
A term known as the Quality Index has been devised as
a measure of signal quality. It is a composite figure of merit
that encompasses variables such as Echo Loss, Echo Path
Delay, and other components that affect the users percep-
tion of voice quality. Normally the Quality Index is not
measured directly; rather, it is based on the analysis of
many discretely measured parameters of traffic activity
and signal degradation.
Figure 2 introduces the concept graphically. It plots the
number of calls (vertical axis) vs. Quality Index (horizontal
axis). The two graphed areas represent Quality Indices
determined by measurements taken on calls originated
from a specific area, terminated at a specific destination,
and carried on two separate trunks. Here, Trunk B (Red)
achieved a mean Quality Index of about 40 out of a
possible 100. Trunk A (blue) performed somewhat better,
reaching a mean of 60.
Now look at Figure 3 to understand the impact of these
quality differences. Its plain to see that users of Trunk B
used their phones less. These are call duration graphs for
the same two trunks monitored in Figure 2. For the
purposes of this discussion we will ignore the vertical scales
in Figure 3, which denotes number of calls (this variable is
not significant unless service is so bad that subscribers stop
using their mobile phones altogether!).
Looking at the call duration, the mean duration for Trunk A
users was more than two minutes longerthan those on
Trunk B. The subscribers on Trunk A stayed on the phone
longer. They were getting consistently better connection
quality (as Figure 2 shows), which implies they were
relaxed and able to take their time conversing with the
connected party. As a result, they happily generated higher
billings for the network provider!Quality Index is
not measured
directly; rather,
it is based on the
analysis of many
discretely measured
parameters of
traffic activity and
signal degradation.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
250
150
50
050 100
Numberof
Calls
Quality Index
100
200
Trunk B
Trunk A
Figure 3: Call Duration
Figure 2: Quality Index
QualityofSer
vic
e
Custo
merLoyalty
ReliableRevenu
e
Serv
iceEn
hancements
Figure 1: Quality of Service Cycle
NumberofCalls
Call Duration
400 800 1200 1600
Trunk BMean Durati on: 367 sec.
500 1500 2500 3500
Trunk AMean Duratio n: 508 sec.
NumberofCalls
Call Duration
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Installation Overview
The networks field installation technicians are the first to
touch new equipment as it goes into service. Organizationally,
the installation team may be made up of technicians
gathered from other groups within the network, particularly
the O&M group. Often these individuals work closely with
a team from the equipment manufacturer, or they may be
assigned to double-check an installation the manufacturer
has completed.
The classic installation task is installing and turning up new
base stations that add capacity or coverage to the network.
The installers work can make the difference between a
trouble-free BTS turn-up with continuing reliability, and an
elusive noise or dropout problem that costs time and
money to correct.
Other installation responsibilities include the addition ofnew software-based features in existing facilities. A case in
point is the General Packet Radio System (GPRS), a set of
enhancements that improves the GSM networks ability to
deliver data. This capability currently is being added to GSM
networks in many regions of the world.
QoS is an obligation of every department within the network.
Some groups (such as the Operations & Maintenance
organization) are likely to include job titles and duties
pertaining to QoS implementation. While installers may not
have the same specific job descriptions, that doesnt exempt
them from ensuring the highest possible quality in their area
of responsibility. A conscientious installation effort is crucialto QoS throughout the network.
QoS Considerations
for Installation Personnel
An installers thoroughness in applying and verifying high
QoS standards when a BTS or new software is installed can
prevent problems at that particular site and over a much
larger area as well. After all, subscribers cant pinpoint the
origin of a weak or distorted voice signal to one newly-
installed BTS; they will tend to blame the networks service
as a whole. Therefore new installations must be broughtonline at the same high QoS as the existing sites.
Like so many tasks within the modern communications
network, installation procedures must be carried out quickly
and at minimum cost. When a new base station site is
chosen and acquired, it is in the networks best interest to
put in the BTS hardware as soon as possible. A new site
means more capacity or better coverage, which are key
competitive advantages.
Naturally this time and cost pressure is in conflict with the
need for the thorough verification measurements and
compliance tests that support QoS. But that is a manageable
conflict if the network equips its installers with the tools to
do the job efficiently. That means instrumentation thatensures that the technicians can go to the BTS site, make
consistent measurements without a lot of costly setup time,
and quickly identify problems if they should arise.
Measurement Challenges and Solutions
Base station installation involves a variety of different tasks.
Frequently, the network operator and the equipment
vendor work closely together for the deployment period.
But ultimately the responsibility is in the hands of the
network personnel.
Among the tasks are:
Clearing the transmit and receive bands of existing usersIn some regions (especially in the case of new PCS networks
in North America), the operator must find and relocate exist-ing private microwave systems already using the frequencies
they need. This is a substantial cost that rests entirely on the
shoulders of the network operator.
Baseline performance checksAs sites are selected and base
stations installed, technicians perform a baseline perfor-
mance check on the BTS. This procedure includes antennaline performance measurements, interference tests, and a
wealth of transmitter performance tests.
Final offline compliance testsInstallation time is a goldenopportunity to perform full compliance tests on a base
station operating in a network setting, but not carryingtraffic. It is the last chance to verify these characteristicswithout impacting subscribers use of the network.
The other kind of installation, adding GPRS and other new
features to existing base stations, is usually a matter of
loading new software into the BTS, BSC (Base Station
Controller), and MSC (Mobile Switching Center) system
and verifying that the base stations are responding correctly.
While this is less of a hands-on job than hardware
installation, its no less important to do a thorough job in
minimal time.
One of the challenges in performing all these tests and
measurements is doing the job cost-effectively. Field techni-
cians obviously cant carry a lab full of equipment to every
BTS site for an installation job. Installation needs have
spawned a generation of compact, high-performance test
and monitoring instruments, the best of which offer ease of
use, accuracy, integration of multiple measurement functions,
and automation. This makes it possible to affordably equip
field teams with the flexible measurement capability they
need to support QoS goals at installation time.
7I N S T A L L A T I O N
Installation
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Looking at Modulation Accuracy
There is a second category of base station measurements
that requires the facilities of a more full-featured spectrum
analyzer. These are tests such as Error Vector Magnitude
orin the case of CDMA systemsRho (waveform quality),
that are designed to determine the modulation accuracy of
the transmitter. To carry out such tests, as well as the all-
important code domain measurements for CDMA and
W-CDMA systems, it is necessary to use an instrument that
includes modulation analysis capability.
In recent years, optimized instruments containing both
a spectrum analyzer and a modulation analyzer have
emerged to address the most critical base station installa-
tion needs. These tools can of course perform the essential
spectrum measurements described earlier, and they can
also provide a detailed transmitter analysis that suffices
for initial compliance verification.
Here again, the combination of two major test functionali-
tiesspectrum and modulation analysisin one instrument
gives the field installation technician greater convenience
and mobility at lower cost, and requires training on only
one instrument rather than two. And with two tools in one,
the technician is more likely to have the right tool on hand
to solve problems quickly.
Imagine a technician who installs a BTS and makes the
necessary RF spectrum measurements. Having determined
that everything is within specifications to this point, he or
she must still verify transmitter parameters such as errorvector magnitude, phase error, magnitude error, GSM ramp
up/down power, and more. These can be complex measure-
ments, and it is easy to imagine the technician wishing for
some automated measurement and analysis capability!
Fortunately the state of the art in spectrum/modulation
instruments has advanced to the point where just such capa-
bilities are available. Figure 6 shows a single measurement
on a GSM transmitter. In this case, the transmitter happens
to be frequency hopping. The instrument automatically
completes a full range of measurements on the frequency
agile signal from the transmitter. This includes spectrum
performance, EVM (Error Vector Magnitude), and phase andfrequency errors to help diagnose any transmitter anomalies.
Clearly, this level of automation allows technicians and
engineers to quickly investigate service-affecting problems
with a minimum of training.
Some spectrum/modulation instruments take the process
a step further, automating the full suite of conformance tests
on BTS transmitters. An internal program automates every
common measurement, with an on-screen display that
lets the user select the tests by means of simple, on/off
keystrokes. An example is shown in Figure 7. Such programs
offer the added benefit of enhancing repeatability among
widespread field teams.
9
If a field installation
team can be equipped
with an instrument
versatile enough to
bridge several test
needs, then the team
is freed from learning,
carrying, setting up
and maintaining a
set of discrete single-
purpose tools.
I N S T A L L A T I O N
Figure 6: A sample measurement on a hopping GSM signal.The cursor in each window represents the same s ignal sample.
Figure 7: Automated Test Control Screen
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Protocol Analysis Supports
Installation of New Features
Currently, one of the most urgent network needs is to
add capability and capacity for data transactions over the
mobile network. Subscribers want to send and receiveinternet content and other data on their mobile phones.
In addition, a new generation of mobile data appliances is
emerging. It all adds up to intense pressure to add data
services to mobile networks. A clear competitive advantage
goes to network providers who come to the market first
with reliable data services.
Pending acceptance of new 3G protocol specifications such
as UMTS Wideband-CDMA, many networks are installing a
transitional protocol known as General Packet Radio system
(GPRS). The new protocol uses existing BTS and other
network elements as much as possible. To the Installation
department, installing GRPS is a matter of loading new
application software into certain network elements, running
a series of tests, and documenting the result. As mentioned
earlier, there is an entirely separate class of instruments
known as protocol analyzers that supports protocol testing
and verification during the installation of software-based
network features and to a lesser extent, new BTS equipment.
The protocol analyzer is another tool that has made great
strides in the past few years. Todays leading field-portable
analysis solutions can, under user-programmed control,
address a wealth of protocol types, monitor several test
points at once, and present a coherent, human-comprehen-
sible picture of network activity in the protocol domain.
The protocol analyzer is the cornerstone of QoS measure-
ments during GPRS installation. Because GPRS installation
is such a key issue at this time, we will use it to illustrate
the broad range of tasks the protocol analyzer can perform
(note that other tools such as call generators may be
needed as well).
As explained elsewhere in this document, GPRS is a
packet-switched technology added in parallel to the existing
networks circuit-switched architecture.
Consequently there are four areas that must be verified
at the time of GPRS installation:
Physical layerWhile this is not explicitly a protocol test,physical layer access is essential because layer 2, which
must be tested, uses the physical layer.
Circuit-switched testingThis step includes viewing thesignaling protocols, and checking the switched circuits.
A comprehensive procedure might require call generation,
speech quality checks, and more. Other tests involve cover-age analysis with a call generator system, and testing of fax
and data functionality.
Packet-switched testingThis step involves testing of thenetwork layers up to and including layer 3 in the ISO OSI
model. These network layers are handled by network nodesor network entities. The layers above are normally handled by
user equipment such as workstations and mobile stations.
In addition, packet switched testing can involve testing withreal applications at the user level. This might include check-
ing functionality with a Web browser using the HTTP
protocol, or checking file transfer using file-transfer protocol(FTP). It also applies to some mobile applications using the
newly-developed Wireless Application Protocol (WAP).
Inter-operationThe operation of circuit-switched andpacket-switched networks as they interact must be checked.
Looking at the list above, it is clear that a simple GPRS
retrofit procedure can be every bit as challenging as a full
BTS install.
From a specific protocol monitoring standpoint, the
measurement tool must first and foremost be capable
of providing a view of all the interfaces involved in any
transaction of interest, as shown in Figure 8. The instrument
can provide appropriate statistics to summarize the activity.
Other measurements might be Mean Packet Delay and
Mean Packet Size GPRS attach, transmit, and detach;
PDP context activation, transmission, deactivation, and
billing radius.
Packet testing is also comprehensive: TCP timing problems;
packet generation and verification; emulation of all lower
layers up to IP on Abis, Gb, Gn, Gi; generation and checking
of user layers, and more.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
Figure 8: Protocol Analysis on The Gb Interface
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A third category of tests, conformance tests, is challenging in
a different way. Conformance test suites normally consist of
hundreds of test cases grouped together to test the differentstates of a protocol. By including a parameter list adapted to
the specific implementation, one entire conformance test
suite can be run automatically, without human intervention.
Unlike other areas of protocol testing, there is no concrete
standard specification for conformance tests available at the
moment. Therefore, the common practice is to use confor-
mance tests developed cooperatively among providers of
GPRS functions. At this time certain conformance tests, i.e.
ATS and ETS, are available for the BSS site and SGSN site,
the NS, and the BSSGP layer. This is enough to ensure that
the connection between SGSN and the BSS equipment
from diverse manufacturers is working.
Figure 9 depicts a part of the results of a typical conformance
test cycle. It shows the interaction between the system under
test (SUT) and the protocol analyzer with verdicts indicating
whether a specific test case is successfully completed or not.
Protocol Analysis Solution Requirements
Selecting the right protocol analyzer for GPRS installation
is the key to doing the installation efficiently. Moreover,
the protocol analyzer enables the Installation department
to meet the networks QoS goals by ensuring a clean,
thoroughly-verified installation of the GPRS features.
To do this the analyzer must provide all of the physical
interfaces and layer 2 variants, and all protocols for GSM
and GPRS. It should also be able to monitor and simulate
multiple interfaces in parallel to truly capture the systems
complex behavior.
In addition to normal monitoring simulation, there is a
requirement for emulation and packet generation and
checking. A standard interface to user applications with IP
data is also helpful. Lastly, the analyzer must offer a means
of conformance testing.
Conclusion
Although a network Installation department is not expressly
chartered with the responsibility of QoS, it nevertheless plays
a key role in supporting the networks overall QoS goals.
As the group tasked with installing new equipment and
services, it falls upon the Installation department to make
sure that new base stations and services like GPRS join the
network in top operating condition. This is the foundation
of a continuing pattern of reliability that keeps network
quality at competitively high levels.
Given these expectations, the Installation group relies on
measurement tools that can assist the install process with
automated features and the versatility to handle a broad
range of installation measurements. Among these tools areRF spectrum analyzers and spectrum/modulation analyzers
that help bring up the RF aspects of a newly-installed BTS.
Another Installation department discipline is the addition of
new capabilities to existing base stations and network
elements. The best example is GPRS, a software-based
feature that requires thorough verification of protocol
behavior. Here, the Installation group calls on the versatile
protocol analyzer to capture and display transactions across
all layers of the protocol, as well as tests of the circuit-and
packet-switched network elements.
11I N S T A L L A T I O N
Figure 9: Conformance Test Results
Conformance test
suites normally consist
of hundreds of test
cases grouped together
to test the different
states of a protocol. By
including a parameter
list adapted to the
specific implementa-
tion, one entire
conformance test
suite can be run
automatically, without
human intervention.
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Operations & Maintenance Overview
In many respects, the Operations & Maintenance (O&M)
organization is the heart of the mobile network enterprise.
It is staffed by hands on technical personnel who have
the expertise to install, troubleshoot, monitor, and maintain
complex network elements such as switching and SS7
signaling equipment. O&M work goes on at far-flung field
sites as well as within the plant.
The O&M organization is responsible for running the
network day-in and day-out. O&M keeps the network
up-to-date and up to the subscribers expectations for
features, accessibility, and quality, as defined by Planning
and Engineering. O&M is responsible not for envisioning
the network of tomorrow, but for making todays network
a practical reality. The technical team faces the challenge
of debugging new equipment under intense time pressure.
It is the O&M activitys job to understand the network as a
whole; to evaluate network performance from a global
standpoint. Inevitably, some elements of the network will
be weaker and more error-prone than others. These the
O&M group must characterize and correct where possible.
In addition, the group must carry out the mechanics of the
network operators competitive QoS programs, that is, they
must implement the hardware and software procedures
that maintain predefined network quality levels.
We said it earlier: to stand still is to fall behind. The network
O&M organization plays a key role in helping the network
move ahead in meeting subscriber needs.
QoS Considerations for the O&M Department
Network O&M personnel face a paradox in executing
their maintenance duties. Subscribers want more coverage
and a growing variety of services. Yesterdays voice-only
subscriber wants paging or data services today. The only
way to deliver these improvements is to install new
equipment and software.
Yet these same subscribers also insist on uninterrupted
high-quality connections. Unfortunately the need to
constantly upgrade and enhance services can easily
degrade mobile traffic in some way, unless precautions
are taken. The impact may range from noise or distortion
to outright discontinuities.
Notwithstanding this challenge, the O&M organization also
has the responsibility to measure network QoS on a day-to-
day basis, and to respond to problems that have prompted
subscriber complaints. Note here that O&M must address
not only the networks provable quality problems, but also
the subscribers perceptions of quality.
Some examples of subscriber complaints are:
Interrupted calls
Calls requiring more than one dialing attempt
before connection
Inaccessible special services such as 800 numbersand the Short Message Service Center
And of course, roaming limitations, noise, and generally
poor audible voice quality
As always (in the network business) O&Ms job must be
done cost-effectively. In this context, the term cost-effective
has implications about the cost of tooling up for the job,
the cost of hiring and training skilled technical personnel,
and even the cost of maintaining spares and software
updates for the measurement and monitoring equipment!
Measurement Challenges and SolutionsIn keeping with the need to minimize network traffic
interruptions, O&M work is carried out on live network
elements with non-intrusive measurements wherever
possible. Typically this requires tools specifically designed
for passive measurements, that is, tools that connect to
the network but do not divert, delay, or degrade the
signals therein.
These instruments, which range in scale from fully
integrated SS7 signaling monitoring systems to small
portable protocol testers, observe key points of interest
throughout the network. Integrated systems provide a
global view of the network, while portable instruments
help technicians spot more localized problems.
Integrated SS7 signaling monitoring systems offer the
advantage of continuous monitoring across many network
test pointspotentially thousands of them. Because these
systems view SS7 signaling activity rather than in-band
events, they dont interfere with active network transactions,
yet they can track all aspects of network activity down to
the individual call level. Typically these SS7 monitoring
systems are scalable to meet the needs of diverse network
QoS strategies. They enable automated monitoring on a
network-wide scale.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
Operations &Maintenance
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Portable Testers Provide Cost-effective
Protocol Monitoring
The portable protocol tester is a valued QoS tool. Although
the protocol tester is designed for on-the-spot troubleshooting
and analysis, some of the more advanced protocol testerscan observe several interfaces simultaneously. This allows
the instrument to evaluate the interaction of multiple
network elements. This scheme is shown in Figure 10.
Here, the protocol tester is observing four test points (the
connections are shown encircling the line to symbolize
the non-intrusive nature of the measurement), including
the ultimate connection to the Public Switched Telephone
Network (PSTN). By this it means it is possible to view the
calls as they traverse all of the network elements following
the base station (BTS). Test setups like these are useful for
quickly assessing global network performance, comparing
the various elements, and locating problem areas.
In the interest of long-term cost-effectiveness, the protocol
tester should be able to evolve and grow with the
networks measurement needs. It should be designed
around a powerful, flexible, and extensible operating
system such as Windows NT. Its application software must
adapt to many different network interfaces (far more than
the four shown in Figure 10). Moreover, the instrument
must offer hardware configurability and expandability. If it
is to be used afield, it should be easily transportable and
controllable via remote connection.
Figure 11 depicts the results of a test (viewing just one
test point) using a protocol analyzer as illustrated above.
It shows a detailed analysis of messages taken between
a BTS and a BSC (the Abis interface). The upper part shows
the message flow itself (one message per line is shown),
along with the most important parameters. These can
be selected by the user. In this example only the IMSI
is selected as a special parameter. The lower part of the
window decodes every bit of the selected messages
and helps pinpoint wrong values and settings.
13
Here, the protocol
tester is observing
four test points (the
connections are
shown encircling
the line to symbolize
the non-intrusive
nature of the
measurement),
including the
ultimate connection
to the Public
Switched Telephone
Network (PSTN).
O P E R A T I O N S & M A I N T E N A N C E
HLR
BTS BSC MSC
PSTN
Protoc0l Tester
Figure 10: Network Monitoring With a Portable Protocol Analyzer
Figure 11: Protocol Analyzer Results Screen
The scenario below assumes a straightforward network
connection made up of one BTS connected to one BSC
connected to one MSC, and so on. When it is possible toisolate ones troubleshooting or monitoring efforts to this
level, the protocol analyzer is in its comfort zone. But
what about more complex arrangements?
One answer is to use a switch matrix that connects the
protocol analyzers basic complement of channels to many
more network monitoring points. Through the switch, the
analyzers resources can be shared among many groups
of test points with no compromise in performance. By this
means it is possible to monitor different network segments
at different times of day, a feature that simplifies, for
example, the analysis of changing traffic patterns during
the business day.
Another approach is to use an integrated, network-wide
SS7 monitoring system, as explained later.
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New Features Mean Added Responsibility
for Protocol Testing Tools
One network interface that is of particular interest to
network operators today is GPRSthe General Packet Radio
System. GPRS is a transitional technology that enables
current-generation GSM networks to better support data
transmission and reception in addition to the normal voice
activity. As such, GPRS is gaining wide acceptance and
creating new test challenges for the O&M group.
Existing GSM networks are circuit-switched structures with
a packet-switched component for signaling. GPRS is a
purely packet-switched network added in parallel to the
GSM system. Some of the critical shared resources between
the two are the base station system (especially the BTS),
the visitor and home location registers, and the equipment
identity register. GPRS base station systems have an addi-
tional packet control unit (PCU) plus Serving GPRS Support
Nodes (SGSN) and Gateway GPRS Support Nodes (GGSN).
The GPRS system is normally connected to packet data
networks like the Internet, X.25 network, or other GPRS
networks, providing the needed data connections for
mobile service.
SS7 Signaling Network is the Connection
for Network-wide Monitoring
Consider a typical modern digital mobile network. It probably
grew up gradually over the past 5-8 years, during which
time the network operator installed several successive
generationsoften from diverse manufacturersof BTS, BSC,
and MSC equipment. Certainly this made business sense
at the time, since each new round of improvements gave
the networks more functionality at lower cost. But now the
O&M organization is confronted with testing and trouble-
shooting all these various makes and models, and theirinteractions as a network. It is a daunting responsibility.
The key to meeting this challenge lies in one fundamental
element of todays network architecture. Underlying the
networks in-band functions is a supporting network of SS7
signaling functions. The scale of the SS7 apparatus is all-
encompassing; it is involved in every transaction; and it has
a profound effect on subscriber-perceived Quality of Service
issues such as dropped calls.
Thus SS7 network measurements can provide a microscopic
view into how subscribers use the network, what services
they use, how often, and what problems they encounter.
This kind of monitoring problem cries out for a solution that
can be everywhere at once. No single fixed test site will
provide all the information that is needed to monitor SS7
network quality. Once it is understood that there must be
many test points, where should they be? Ideally the moni-
toring tool should be deployed in every link of the mobile
network, although this is not really feasible for reasons of
cost and complexity.
Increasingly, O&M organizations are turning toward the
integrated SS7 signaling monitoring system as a means of
supporting QoS efforts. With its scalability, its widespreadprobing points, and its ability to capture network activity down
to the individual call level, the SS7 monitoring system is the
most effective permanent QoS monitoring solution available.
Probably the most strategic SS7 test points are located at
the Mobile Switching Centers (MSC). At these locations,
there is a concentration of links to Base Station Controllers
(BSC), networks (PSTN as well as other PLMN mobile
networks), network databases (specifically the Home
Location Register or HLR), and other MSCs. With monitoring
instrumentation permanently based at MSC sites, a broad
and comprehensive view of SS7 network activity is available
for collection by a centralized system element.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
OtherPLMN
SMS-GMSCSMS-IWMSC
SM-SC
GGSN
PDN TEGGSN
MSC/VLR
E
Gs
CGd
Gp
D
Gr
GiGn
Gf
Gb
Gn
UmR
HLR
GSM
Signalling Interface
Signalling and DataTransfer Interface
GRPS
Other
TE MT BSS
GcA
EIR
SGSN
SGSN
Figure 12: GPRS Network Interfaces
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As stated earlier, the SS7 signaling structure is involved
in every single network transaction. That makes it difficult
to interrupt or intrude upon SS7 activity for the purpose
of monitoring and troubleshooting. Intrusive monitoring
techniques are inherently limited to connections at theperiphery of the network; they use controlled test calls
between two test sets at opposite ends of the communica-
tion path to characterize quality.
A better approach is to use non-intrusive monitoring, in
which instruments are woven integrally into the network.
Note that this isnt the same as the built-in QoS monitoring
features included in certain types of network equipment
(such as MSCs). The performance of these built-in tools
depends to some degree on the performance of the very
same network they are observing. In contrast, integrated
non-intrusive QoS tools are independent of these
environmental problems.
The combined need for integral, non-intrusive monitoring,
a multiplicity of test points at various MSCs throughout the
network, and a central unit connected via a WAN says one
word to an experienced observer: system. Clearly the only
acceptable solution for a measurement/monitoring problem
of this scale is a carefully planned system made up of
compatible elements designed to work as one. This is not
the place for off-the-shelf instrument clusters or for small
general-purpose tools.
Figure 13 shows a non-intrusive QoS monitoring system
integrated into a network environment as described above.Here, the QoS probes, which reside at the MSC installa-
tions, connect through a Wide Area Network (WAN) using
a TCP/IP connection. This is in turn connected to a Central
Unit that is responsible for system-wide data collection,
correlation and storage. In addition, the Central Unit is
the focal point for management and control of the entire
monitoring process.
A network-wide non-intrusive measurement system can
provide a wealth of critical data about network quality and
traffic. The Call Detail Record (CDR) is a key deliverable
of the network analysis system. Todays state-of-the-art
network analysis systems can generate Call Detail Recordsfor every call or call attempt and store all this information
in a relational database. An example of a Call Detail Record
report is shown in Figure 14.
Each row in the report represents a single CDR. The infor-
mation can be filtered and displayed based on the values
or ranges in one or more fields. For example, a filter might
be used to view only the calls originating from one specificmobile subscriber (identified by the calling mobile number)
in order to track a suspected quality problem with that
subscribers connection.
A CDR contains information about a specific call or
attempt. This data ranges from calling and called telephone
numbers, to duration (holding time, conversation time)
to the outcome of the call (whether the call was successful
and, if not, the reason for call failure).
15
The QoS probes,
which reside at the
MSC installations,
connect through a
Wide Area Network
(WAN) using a TCP/IP
connection. This is in
turn connected to a
Central Unit that is
responsible for
system-wide data
collection, correlation
and storage.
O P E R A T I O N S & M A I N T E N A N C E
Figure 14: Call Detail Record Report
HLR
To Central Unit
PLMN
PSTN Y
PSTN X
BSC
MSC
MSC
WAN
BSC
MSC
Figure 13: SS7 Monitoring Probes Acquire Signaling Data Via the MSC
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All this information provides valuable insight into
subscribers behavior, leading to important inferences about
the subscribers perception of service quality. For example,
a very low average conversation time points toward poor
transmission quality, which often causes users to abandontheir calls prematurely.
Lets look at an example scenario demonstrating how an
SS7 monitoring system can resolve a subscriber complaint
quickly and easily.
Suppose a subscriber complains about a call completion
problem. He is having difficulty reaching his friends
across town.
Because the subscribers network provider has wisely
chosen to install a comprehensive SS7 signaling monitoring
system, the data needed to investigate the problem is
already in the CDR database. Thats convenient! The O&Mgroup need only browse through the particular subscribers
CDRs to spot trends like congestion or radio interface
failures. The powerful search features of the relational
database can accelerate this process. In addition, the
operator can set up a user call trace to get a close-up view
of protocol-related issues that may be causing the problem.
Given all this detailed information, plus the map-based
network views that the system provides, the problem is
quickly localized. The intervention can be fast, focused,
and cost-effective.
The network analysis system can also deliver reports suchas the E.422 Report, an ITU-T standardized report for quality
assessment at the interface between network operators,
whether the networks are local, national, international,
fixed, or mobile. The E.422 Report makes it easy to resolve
any disputes between network providers who have
reciprocal agreements to put through calls for one another.
An example of the E.422 form produced by an advanced
network monitoring system is show in Figure 15.
The E.422 report takes information from the Call Detail
Records and provides an assessment of the Quality of
Service that calling subscribers obtain. Every call attempt is
classified based on the outcome of the call. In particular,the E.422 report details the calls successfully put through,
and specifies, for the unsuccessful calls, how many are
due to the customer behavior and how many are due to
the network. Thus it is possible to determine whether call
failures are due to interworking problems or to protocol
failures among calls to a specific destination.
Lastly, the SS7 monitoring system makes available a host of
day-in, day-out statistical views such as the Gauge display
in Figure 16. The gauge display provides a continuous
near-real-time view of the health of the network. Significant
statistical counters are regularly updated and ranked inorder to provide worst-case values of measurements all
across the network. This and other SS7 monitoring system
displays are designed to simplify the comprehension of
complex measurement data, ensuring efficient, error-free
interpretation of the networks behavior. By this means, it
is possible to focus and prioritize network interventions to
resolve critical performance issues that can impact Quality
of Service.
Call Generator/Analyzer
is a Demanding Subscriber
Of course, the O&M groups tasks are not limited to simply
viewing network activity and reacting whenever there is a
problem. A comprehensive QoS strategy usually includes
active stress testing of network elements in simulated
real-world situations. Significantly, the most realistic stress
situations are not the normally initiated and terminated
calls; rather, they are abnormal circumstances that occur
all too often in networks.
Examples include:
Ping-pong handoffswherein a call bounces back andforth erratically between two adjacent cells
Overloadingwhich can cause delays in voice mail prompts,
in turn causing callers to disconnect before they are awareof being connected
To create these and other aberrant network situations,
a test system that can actually generate mobile traffic is
needed. The system must interact with the mobile network
through the air interface, and with fixed networks through
common PSTN or ISDN lines. It must not only generate
traffic; it must also acquire and analyze the resulting
network behavior in order to perform true end-to-end tests.
To create aberrant
network situations, a
test system that can
actually generate
mobile traffic is
needed. It must not
only generate traffic;
it must also acquire
and analyze the
resulting network
behavior in order
to perform true
end-to-end tests.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
Figure 15: E.422 Report
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This call generator/analyzer can be installed as part of the
network itself, managed under the auspices of the O&Morganization. Whats important to note here is that the
system is a surrogate for all the different elements in
the networkBTS, BSC, and MSC alike. The instrument
emulates the behavior of mobile subscribers who use any
and all of the available network services. Among these are
speech and data, Short Message Services, supplementary
services such as call waiting and call barring, and voice
mail. The system also can produce heavy traffic, a matter
of great interest to the O&M organization. Lastly, the most
advanced mobile call generation systems can perform
a host of RF measurements and reporting.
To be truly effective, such a system must be easy to use.
How does the O&M engineer design calls that are both
realistic and thorough? Some systems have graphical
Call Modeling tools that simplify the process. With these
instruments, the user selects from a library of pre-writtenbuilding blocks that include such terms as Cell Select, Wait
on Hook, Generate DTMF, and many others, as shown in
Figure 17. The blocks are concatenated to form a full call.
Calls can be combined to build up an exhaustive test that
challenges the network units capacity and services.
Conclusion
Much of the responsibility for a mobile networks quality
of service rests on the shoulders of the Operations &
Maintenance organization. In turn, the O&M organization
places its trust in expert personnel using the latest test,measurement and monitoring equipment.
In this chapter we have seen how the protocol analyzer can
be used to observe and analyze the quality of live network
traffic without disturbing the networks most valuable
asset, its subscribers. We have also discussed the scalable,
network-wide capabilities of the SS7 signaling monitoring
system. In addition, we have discussed the mobile call
generation system, which can be used to troubleshoot or
certify myriad mobile network elements under worst-case
call and traffic conditions. This is a reliable way to ensure
(again, without interrupting subscriber traffic) that newly-
installed or upgraded BTS, BSC, and MSC units will deliverexcellent Quality of Service.
17O P E R A T I O N S & M A I N T E N A N C E
Figure 16: Gauge Display
Figure 17: Call Generator/Analyzer Setup Screen
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Planning & Engineering Overview
In the Planning & Engineering (P&E) organization, the
networks future takes shape. Here, nothing is more constant
than change itself. Operators pressured by competition must
steadily introduce new services and enhance the old ones.
P&E personnel work hard to provide new network features
in a timely and cost-effective manner. Its a process that
involves some new design work, some upgrading of existing
infrastructure elements, and inevitably some concessions
to the limitations of older installed equipment. Networks
become ever more complex as the mixture of old and new
equipment, varying brands, and diverse software revision
levels begins to grow.
New and improved services are essential for competitive
reasons, but bigger is also better. That is, successful networks
are constantly expanding their capacityadding more base
stations, more mobile switching centers, and so on.
From the viewpoint of an engineer working in the P&E orga-
nization, this creates an environment thats always growing
and is in migration from one technology to another, or
transitional. Frankly its not a stable platform to stand on,
yet the P&E engineer must somehow ensure that all of the
variables come together in a network system that works
reliably and smoothly. Moreover, it is the responsibility of
the P&E organization to deliver network elements (both
hardware and software) that are supportable by Operations
& Maintenance personnel without requiring lengthy (and
costly) new training cycles.
Functionally, P&E is charged with:
Planning of the network topology
Evaluating and selecting network elements (BTS, MSC, andso on) that will provide the best performance and QoS at
the least cost
Increasing the networks capacity
Planning and designing the SS7 signaling network
Helping to resolve network problems such as coverage gaps
and dropped calls
Operating and managing the test plant
QoS Considerations for the P&E Department
In a sense the networks ability to compete in its market
begins with the P&E group. After all, this is the organization
that determines what equipment will be sited where.
Therefore QoS is a mission that stands or falls on the
choices made in Planning & Engineering.
Choosing a base station site is among the first big decisions
that must be made when adding network coverage or
capacity. Planning for base station siting is commonly done
with special software planning tools. These applications
weigh volumes of geographical and morphology data to
predict the coverage that a given BTS site will achieve.
Even with these powerful predictive tools, QoS issues can
make site selection a complex equation. For example, the
most accessible real estate for the base station might be
compromised by conflicting signals from other providers,
which would very likely impact QoS. The site with the bestelevation might be hampered by proximity to tall build-
ingsa one-way ticket to fading and interference. The reality
is that predicted base station coverage and actual coverage
can differ significantly. And gap between prediction and
reality is expected to grow even wider as new high-speed
data services are introduced. Therefore a thorough evalua-
tion of the site environment, both before and after BTS
installation, is a necessity.
Measurement Challenges and Solutions
After sites have been selected and base stations installed,the P&E groups work still is not finished. P&E is responsible
for an ongoing process known as network tuning and
optimization. This involves continued monitoring of site
and network performance long after the initial turn-up.
Why is this necessary? There are a host of reasons: new
structures or competitors base stations may be erected
near a BTS, causing interference; an antenna may get
damaged by wind or snow; transmission lines can become
impaired by moisture ingress or mechanical damage. Any
and all of these problems will affect QoS in both measured
and perceived terms.
Drive Tests Provide an End-to-End View of the Network
The cornerstone of network optimization is the drive test.
The drive test is exactly what its name implies: a series of
quality measurements conducted from the vantage point of
a moving vehicle. This job is the province of an integrated
instrument designed to take these moving performance
snapshots. As always, there is a variety of solutions in this
instrument category. Until recently, most of these have
offered rather basic measurement capabilities: Rx (receive)
quality and level measurements, certain call statistics, and
downlink information correlated to the receivers position.
Increasing utilization and the presence of competingnetworks in almost every region of the industrialized world
is changing the nature of the drive test. As complex
network services become more common, and as QoS
concerns (and therefore, subscriber churning) continue to
mount, older 1st-Generation drive test methods simply
cant provide enough data. They dont take readings on
both the downlink (in the direction of the mobile
subscriber) and the uplink (in the direction of PSTN
subscribers and other mobiles) simultaneously. Typically
they lack capability to monitor more than one network at a
time, even though there may be several networks operating
in the same area. Lastly, P&E needs extensive details about
network performance as it relates to the callers physical
location. That means more information recorded and corre-
lated accurately to geographic coordinates.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
In a sense the
networks ability
to compete in its
market begins with
the P&E group.
After all, this is the
organization that
determines what
equipment will be
sited where. Therefore
QoS is a mission that
stands or falls on the
choices made in P&E.
Planning &Engineering
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The solution of choice for comprehensive drive testing is a
tool optimized for end-to-end testing of the network. This
type of analyzer offers a wealth of important features that
help the drive testing process deliver results more quickly
and economically, with less margin for error.
Among these characteristics are:
Automatic generation of wireless-to-wireless, wireless-to-POTS,
and POTS-to-POTS calls, including voice, fax, and data calls
Automatic generation and receipt of SMS messages
Configuration and automatic execution of complex
test sessions
Analysis of the information exchanged with protocollayers 1 and 2 of the mobile network.
Uplink and downlink transmission quality tests
Simultaneous management of multiple mobilenetwork interfaces
And more
A typical end-to-end analyzer system consists of two units
a fixed base station unit that connects to the PSTN, and a
mobile station with up to four links, as shown in Figure 18.
These components can automatically call one another and
exchange voice and data content. The analyzer takes
measurements on the call process during these calls,
including transmission quality measurements in the voice
band. The test system also acquires the data obtained by
decoding the messages exchanged between the mobile
unit and the mobile network. Once the drive test is
concluded, the mobile unit can transfer its stored results
via the mobile network connection it has just finished
testingto the fixed unit, which gathers all the relevantinformation in one place for analysis.
Looking at Figure 18, note that the analyzer system uses the
signal from the Global Positioning System (GPS) to track
the exact position, moment by moment, of the mobile
station. This information, too, is stored in the instrument,
tightly correlated with the measurements occurring at the
same instant.
19
P&E is responsible
for an ongoing
process known as
network tuning and
optimization. This
involves continued
monitoring of site
and network perfor-
mance long after
the initial turn-up.
P L A N N I N G & E N G I N E E R I N G
Figure 18: End-to-End Analyzer Setup
PSTNGPS
NetworkY
Mobile Stations
BTS
Drive TestSystem-stationaryunit
BTS BTS
NetworkX
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From the standpoint of the P&E organization, the next step
provides the true benefit of using an advanced analysis
system. The system has, built in, a powerful software appli-
cation that displays an actual map of the drive test and its
results. Figure 19 is an example of the systems output.
With all the foregoing talk about measurements, maps, and
stored results, weve begun to see exactly what the SS7
monitoring system can do to characterize network coverage,
as well as quality in general terms. But what about its
capacity to objectivelymeasure the Quality of Service?
Its essential to take the opinionating out of the discussion
about Quality of Service. Anybody can place a call and
describe its quality as poor or good. But until you can
support words like poor, acceptable, and excellent
with actual measurements, you simply cant build a QoS
program. Quantification gives you a starting point and a
way to track the success of improvements.
Quantification begins with accurate measurement
of the following key parameters:
AccessibilityThe ratio of successful calls to those
terminated abnormally or routed incorrectly.
ConformityThe extent to which a call provides aclean, clear, uninterrupted connection. The conformity
measurement produces a Mean Opinion Score basedon a hypothetical subscriber profile, see Figure 20.
ContinuityThe percentage of calls that fail during a
simulated conversation between the mobile and fixedunits of the monitoring tool.
As we have said elsewhere in this document, virtually
all networks are made up of elements both new and old,
from diverse equipment manufacturers. Even though most
vendors of such equipment strive to test their new products
thoroughly, it is not feasible to test for the innumerable
combinations of base stations, MSC, base station controllers,
and so on. Consequently the network operator faces the
risk of unpredictable interactions and malfunctions once
the equipment is installed.
Test Plant Debugs Problems in a Neutral Environment
The accepted solution for this problem is the test plant,
a self-contained miniature network designed for the
purpose of testing and debugging network elements. The
Planning & Engineering group is normally the organization
responsible for the test plant. The test plant operates under
controlled conditions and is a focal point of measurements
that help predict the QoS impact that new network
elements might have. The test and verification proceduresuse many of the same measurement tools deployed in the
actual operating network.
In the test plant, the goal is to find one tool that represents
the widest range of equipment in the field, including
elements that the future may bring. Increasingly the
solution is a call generation system that interfaces to the
network through the normal air interface and with the fixed
network through conventional PSTN or ISDN lines.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
Figure 19: The Drive Test System Provides A Map View
Figure 20: Conformity Measurement Result
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The RF connection is usually made through coaxial cables
connected (via splitters or branching units) to one or morebase stations under test, as shown in Figure 21. The
adjustable RF attenuator immediately preceding the BTS is
used to simulate varying radio propagation conditions.
With a call generation system as the heart of the test plant,
its possible to create a set of tireless, demanding auto-
matic subscribers who use every imaginable service the
network can offer. The call generator creates controlled,
lab quality traffic on mobile and fixed networks and
reports detailed call analyses, including failure causes
and QoS levels.
Conclusion
Planning & Engineering organizations have a huge
responsibility in the typical networks business, where
services are always in flux and the size and complexity of
the network are constantly changing. The P&E group must
oversee network expansion, the introduction of new
features, and evaluation of new hardware and software
elements. No wonder, then, that P&E engineers are seeking
QoS measurement solutions that can get the job done
quickly and efficiently.
Drive test systems support wide-ranging network expansion
efforts, and can do double-duty as QoS evaluation tools for
the existing network. Their ability to pinpoint geographical
locations and correlate them with network behavior at
those locations makes them indispensable P&E tools.
Similarly, the call generation system is key to test plant
activities in which there is a need to simulate real users
demands on the network. This system creates controlled
yet exhaustive traffic for evaluating network elements and
predicting how newly-installed equipment will interact
with that which is already in place.
21P L A N N I N G & E N G I N E E R I N G
PC/Laptop
BTS Under TestTo PSTN
Test Plant System
BranchUnit
Attenuator
Figure 21: Test Plant
The RF connection
is usually made
through coaxial
cables connected
(via splitters or
branching units) to
one or more base
stations under test,
as shown in Figure 21.
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Quality Overview
To achieve the best possible QoS while remaining competi-
tive in terms of cost and services, quality efforts must bridge
organizational boundaries within the network. Seamless
cooperation among P&E, O&M, Marketing, and all the other
entities is essential. To foster this level of cooperation,
many networks assign dedicated personnel to the task of
overseeing QoS across network departmental lines. Others
establish a fully-constituted quality department with a
charter to develop, implement, and support QoS programs.
Subscriber satisfaction is the path to increased market
share, higher revenues, and rapid subscriber growth for
the network. But quality programs cannot be pursued
blindly without regard to cost. A pragmatic investment in
technical infrastructure can increase ROI, reduce costs,
and boost profits. Conversely, an excessive investment can
drive up costs (and therefore rates), yet may not provide a
perceptible improvement from the subscribers viewpoint.
Moreover, constantly upgrading and enhancing services is
very likely to interrupt mobile traffic in some way. And thats
all it takes to prompt the dreaded churn phenomenon.
The Quality activity supervises QoS programs that maintain
this equilibrium. A good QoS program acts as a virtual
subscriber who can be replicated throughout the system to
artificially stress it and identify the lapses in service that
would be perceptible by real subscribers. If the QoS program
discovers deficiencies that will remain safely beneath the
subscribers threshold, they can be tolerated in the interest of
holding investment costs and operating costs to a minimum,
and keeping profits up. If the program reveals deficiencies
that produce perceptible lapses in service, then technical
investments can be stepped up proportionally.
The Quality activity supports the networks business in other
ways, as well. For example, QoS is an article often specified
in Service Level Agreements (SLAs) between network oper-
ators and between operators and carriers. And regulatory
bodies in some regions require network providers to
publish QoS data. The Quality activity is the focal point
of compliance with these provisions.
QoS Considerations for Quality Personnel
From the perspective of the typical network user, that is,
the individual who simply wants the convenience of a
mobile phone for person-to-person conversations, voice
quality is a key criterion of his/her network providers QoS.
Problems with voice transmissions are easy to hear and are
very distracting to the par ties on either end of the conversa-
tion. The subscriber doesnt know about base stations and
protocols; only that its hard to understand the voice
coming through the mobile set. Unresolved complaints
about voice quality quickly lead the subscriber away from
the network.
As a result, voice quality is a major point of emphasis for
network quality efforts. Several methodologies are accepted
for managing voice quality in the network:
Drive tests
INMD (In-service Non-intrusive Measurement Devices)
Traffic generation and analysis
Measurement Challenges and Solutions
Many network operators choose to manage voice quality by
conducting tests that simulate real voice calls, and duplicate
as closely as possible the circumstances that impact voice
qualityproblems such as interference, fading, and others.
Given these techniques, it is no surprise that drive tests are
a widely-used method of checking voice quality, as well as
other phenomena such as dropped calls, handover failure,and more. Normally the drive test system includes a fixed
station connected to a PSTN network, and mobile station
that (as the name implies) is driven around in a car or
truck. The mobile station has access to a GSM network.
With this combination of equipment, it is possible to set
up PSTN-GSM or GSM-GSM connections.
Drive Tests Deliver Objective Measurements
Drive tests are a powerful tool for monitoring voice quality.
Innately, they perform end-to-end testing of the whole
transmission/reception path. They deliver call-based analysis,
so specific calls can be analyzed. The drive test system
provides geographical details about the location of the
mobile unit at any given time, which means voice quality
issues can be correlated with physical obstructions that
might be the source of the problem. A further benefit of
drive testing is that it makes it possible to compare
competing networks performance.
Using the drive test system to conduct GSM-GSM tests,
both the uplink and downlink path are measured. To further
localize the source of the problem, the system can be set
up to run PSTN-GSM measurements. In this case only the
uplink or the downlink is measured.
To achieve the best
possible QoS while
remaining competitive
in terms of cost and
services, quality
efforts must bridge
organizational
boundaries within
the network.
Seamless cooperation
among P&E, O&M,
Marketing, and
all the other entities
is essential.
Q U A L I T Y O F S E R V I C E I N M O B I L E N E T W O R K S
Quality
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Drive testing has a few limitations, however. It is not fully
automated. Its two-station architecture requires a live
user at either end, which is costly in terms of personnel.
Moreover, drive testing is considered an intrusive
methodology, since it actually uses the channels under testrather than simply monitoring them.
Some modern drive test solutions eliminate the need for
a skilled technician at the mobile end; the operation at the
end is simplified to the point where unskilled personnel
can be used.
The most advanced drive testing tools produce an objective
quality measurement known as the Mean Opinion Score
(MOS). This is based on a model (known as the E-model)
defined by ETSI that accounts for all possible impairments
in a mobile network. The MOS takes the opinion out of
drive testing and presents a figure based on analysis of real
acoustic phenomena such as clipping, echo, noise, etc.
Non-intrusive Measurements
View the Whole Network
In-service non-intrusive Measurement Devices (INMD) are
a more sophisticated way of getting the same jobvoice
quality analysiscarried out effectively. The INMD measure-
ment systems name tells it all: the solution monitors active
(in-service) network elements, yet it does so without itself
sending and receiving information through those elements.
Importantly, the INMD does not require any call simulation
or generation; it simply tracks the network during actual
use by subscribers.
The INMD system is installed integrally with the network
itself. It is made up of a central management/analysis
element linked via WAN to remote site monitoring units
connected at key points in the network, especially access
trunks to subscriber premise and interconnection trunks
between networks. The INMD system captures and analyzes
key voice band transmission parameters such as echo,
noise, clipping, and more. In addition, it makes measure-
ments in both directions (incoming and outgoing) on
answered calls.
The embedded INMD system performs automatedmeasurements constantly, with no incremental cost of opera-
tion other than routine management and maintenance of
the monitoring system. And importantly, the system delivers
measurement results that correlate closely with actual mobile
use perceptions. For example, when the INMD system
detects a poor echo reading, it almost always corresponds
to an objectionable artifact that mobile users can hear.
The INMD approach does not provide a means of
comparing one networks QoS performance with that of its
competitors, however. That sort of activity is better handled
by drive tests or by traffic generation and analysis.
Call Generators Duplicate
Real-world Calling Situations
Traffic generation is another powerful tool to support
network voice quality control. The solution architecture
involves call generator nodes monitoring strategic pointsin the network, or even the signals from other networks.
Like the INMD system, these probes are managed from
a central system linked to them via WAN. Tests can be
automated, minimizing the personnel resources needed
to oversee this class of QoS measurements.
The traffic, or call generator is designed to simulate real-
world conditions. It is uniquely capable of reproducing
complex phenomena such as ping-pong handovers,
allowing the operator to characterize the response of the
network under these circumstances. By generating many
calls simultaneously, the system can stress the network
and observe the results. It is also the best tool for testing
billing (e.g., rates changes when crossing boundaries or
time zones).
The call generator is the right solution when in-line network
elements must be tested from end to end. It allows the
operator to simulate very complex and challenging call
procedures, and it faithfully models realistic user calling
patterns. The call generator is, however, an intrusive tool.
By definition, using the call generator consumes some of
the very network capacity it is testing for.
Conclusion
A network operators quality function, whether it is
organized as a full-fledged Quality Department or as a
company-wide floating resource, exists, ultimately, to
ensure subscriber satisfaction. As such, the Quality function
has to become a surrogate network user who expects
perfect conne