Mobile QoS Primer

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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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3/333Q 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

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


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


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


SS7 Monitoring Profiles Subscribers . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

Security & Billing

Security & Billing Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

QoS Considerations for the Marketing Department . . . . . . . . . . . . . . . . . . .26


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.


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.


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.


12/33

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.


Operations &Maintenance


13/33

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.


14/33

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.


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


15/33

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


16/33

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.


Figure 15: E.422 Report


17/33

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


18/33

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.


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.


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


19/33

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


20/33

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.


Figure 19: The Drive Test System Provides A Map View

Figure 20: Conformity Measurement Result


21/33

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.


22/33

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


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.


Quality


23/33

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

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