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Chapter 48. Available Bandwidth Measurements: Blixt

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48. Available Bandwidth

Measurements: Blixt

This chapter describes Ascom Network Testings Blixt technology for

available bandwidth measurements, or ABM for short.

For the testing setup, see section 12.20.3.10. Devices supporting ABM are

listed in the Device Configuration Guide, section 2.6. ABM servers are hostedby Ascom.

48.1. Background

Mobile networks are in the process of becoming the worlds leading medium

for data traffic. As ever faster data rates are offered by mobile network

technologies, the use of real-time applications such as media streaming in

such networks is becoming increasingly commonplace.

Now, as is well known, mobile network performance depends crucially on the

radio environment, which is subject to very rapid fluctuations. For example,

Rayleigh fading conditions change on a millisecond basis, as do scheduling

and cross-traffic (such as data from other users). Nonetheless, mobile

network operators are expected to be able to maintain uniform bandwidth

availability to all customers who are paying for a given service level (or class,

or experience). Accomplishing this requires metrics and measurement tools

designed specifically for the wireless environment.

As such measurements are performed in live commercial networks with

paying subscribers, it is important to prevent the measurements from

affecting the subscribers quality of experience. Ascoms approach to

Available Bandwidth Measurements (ABM), trademarked as Blixt, solves

this problem by keeping the level of test and measurement intrusiveness to

an absolute minimum. ABM identifies the throughput that can be delivered

over the measured wireless link at a given place and at a given point in time.


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48.2. Aspects of LTE and HSPA Networks ThatMust Inform ABM Design

Methods traditionally used to measure available bandwidth in wirelessnetworks have been comparatively simple and have involved the

downloading and uploading of files via FTP. While by no means ideal

having only limited mechanisms for adapting to changes in the radio

environment, for one thing these methods have been sufficient for

technologies such as WCDMA Release 99 and older.

Technologies such as LTE and HSPA, on the other hand, have a number of

features that render traditional ABM methods inadequate. The most salient of

these features are as follows:

In LTE and HSPA, the radio channel is a shared resourcebetween allusers in a cell. An FTP file transfer to one user in a cell (for example, the

testing device) will significantly affect other users in that cell, as will any

other traditional drive test activity.

It is also possible for multiple operators (carriers) to share the same

radio access network. This puts requirements on parallel testing, as

subscribers of different network operators might, for example, share the

radio network but use separate core networks.

High data rates.To pick a typical state-of-the-art configuration, using a

Category 3 user equipment (UE) in an optimal, unloaded LTE network

with 20 MHz system bandwidth, it is theoretically possible to attain

transfer rates of up to 100 Mbit/s. Just filling up such a large channel with

data in order to measure the channels true bandwidth can be a challenge;

every part of the system, all the way from the server to the FTP client,

must be carefully tuned to manage such transfer rates. UE-based

performance testing applications, especially, will have problems handling

all the data and filling the bit-pipe due to the UEs limited CPU

performance, which in turn is constrained chiefly by the performance of

the UE battery. Rich configuration possibilities.An LTE network can employ a large

array of different MIMO configurations, and the scheduler used in this

technology has very powerful and flexible mechanisms for maximum

utilization of the radio path (both uplink and downlink). Traditional ABM

techniques do not adapt to such rapid variations in the link capacity.


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48.3. Requirements on ABM for LTE/HSPA

Taken together, the points in section 48.2boil down to the following essential

requirements on an ABM method fit for use in an LTE or HSPA network:

1 Maximum network load with minimum intrusion.To be able to probe

the limits of bandwidth availability, the method must be capable of

loading the bit-pipe up to the maximum. At the same time, however, it

must have low intrusiveness meaning that it must keep down the

time-averaged network load as far as possible to minimize interference

with regular network users.

2 Fast adaptation in time domain. The method must take into account

the properties of a radio link with Rayleigh fading conditions varying on

a millisecond time scale.3 Adaptation to network and user equipment configuration.The

method must take into account different MIMO configurations, channel

bandwidths, and UE capability categories.

4 Adaptation to scheduling.The method must take into account the

network schedulers mechanisms for maximizing the utilization of the

radio path. The network scheduler adapts the resource allocation to

traffic patterns, quality of service settings, and load.

48.4. Description of Ascoms Blixt ABM Algorithm

48.4.1. Algorithm Overview

Heres a summary of how the Blixt algorithm for available bandwidth

measurement addresses the requirements stated in section 48.3:

Data is sent in short, intense bursts(chirps) with much longer pauses

in between. The peak load is high enough to reach the networks

theoretical maximum, while the average load is kept low. This scheme

allows us to sound out the available bandwidth while still making minimum

use of network resources.

Using short bursts also meets the requirement of a high temporal

resolution. That is to say: at least once in a while, we can expect optimal

radio conditions to prevail throughout a data burst (provided that the

network configuration and the devices position permit this in the first

place).


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The algorithm adapts to network configuration parameters: the amount of

data sent is adjustedaccording to the networks maximum throughput

while keeping the level of intrusiveness to a minimum at all times.

The packet train transmissions are designed to make full useof themaximum bandwidth, without the throughput rate being limited by slow-

start or low-load scheduling mechanisms.

The whole design is based on a device communicating with an ABM

server, where the server reflects the packets back to the device, including

timestamps and other information included in the packets. The device can

then easily be configured to test the performance of different parts of the

network by accessing different servers.

48.4.2. Blixt Measurement Procedure

Data bursts are sent at one-second intervals. In between these bursts, whose

duration is always a small fraction of a second, nothing is sent.

Each data burst consists of a number of packets sent back-to-back,

collectively referred to as a packet train.

ABM data bursts (symbolic representation).

48.4.3. Example: LTE

Suppose we want to measure available bandwidth in an LTE network with

20 MHz bandwidth using a Category 3 device, whose maximum achievable

downlink throughput in optimal radio conditions is 100 Mbit/s on the physical

layer.

In order to fully load the bit-pipe and be able to attain this maximum

throughput rate, we need to transmit 100 kbit in each Transmission Time

Interval (TTI), since the TTI length in LTE is 1 ms. For the sake of obtaining a

reliable measurement, as further discussed in section 48.4.5, we want to


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current System Information parameters, UE capabilities, and other settings

(see section 48.4.6below).

To safeguard measurement accuracy, it is necessary to send not just a single

packet but a sequence of packets that are contiguous in time. The reason forthis is that if only one packet were sent, it would most likely not fill up one TTI,

or it would be scheduled across two TTIs, meaning that the full available

bandwidth would not be utilized in any TTI. On the other hand, with multiple

packets sent back-to-back and scheduled in consecutive TTIs, it is ensured

that the ABM service has the networks full available capacity allotted to it at

least for some TTIs in the middle of the burst.

Assuming that one TTI can accommodate 100,000 bits, the maximum size of

one IP packet is 1,500 bytes (= 12,000 bits). So in this case it takes at least

8.3 packets (100,000 / 12,000) to fill one TTI. It is important to transmit atleast a few times this number of packets to ensure that a reasonable number

of TTIs are filled with ABM traffic. However, note the trade-off here: the level

of intrusiveness of the measuring activity rises in direct proportion to the

number of packets sent.

Distribution of one ABM data burst across TTIs. The bandwidth allocated to other users

is not represented in this figure; furthermore, optimal radio conditions are assumed.

The point illustrated here is that at the beginning and end of the burst, the ABM trans-

mission is not competing for the whole of a TTI.

48.4.6. Adaptation of Blixt ABM to Network

Configuration and UE Capabilities

The amount of data sent in performing ABM must be adapted to the

fundamental network capacity (radio access technology). In the technical

paper A New Approach to Available Bandwidth Measurements for Wireless

Networks, doc. no. NT13-16812, a number of representative use cases are


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described along with their associated ABM setups, designed to achieve a

good trade-off between level of intrusiveness and measurement accuracy as

discussed in section 48.4.5.

The ABM packet train properties (packet size and interval) are selected to suitthe particular radio bearer configuration. Consequently, different ABM setups

will typically be used for different networks/operators. Likewise, as a testing

session proceeds, the ABM setup will frequently vary over time as the UE

moves between cells, or to another carrier, or switches to a different radio

access technology (for example, between a WCDMA and an LTE network).

48.4.7. TWAMP as Time-stamping Protocol

Ascoms ABM technique relies on a time-stamping protocol commonly knownas Two-Way Active Measurement Protocol or TWAMP. Other time-stamping

protocols could have been used; our reason for selecting TWAMP was that it

is a standard protocol in the field which has a simple implementation and is

easily extendable. See IETF RFC 5357 for more details.

48.5. Comparison with Traditional ABM

48.5.1. Data Rate Ramp-upBelow, one feature of traditional ABM methods is described which is not used

in the Blixt ABM algorithm.

Traditional ABM methods used in fixed-line networks often start out by

probing the bit-pipe between the server and the client at a low data rate, then

ramp up the data rate until the bottleneck (the maximum bandwidth or data

transfer rate) of the bit-pipe is reached. The load is kept at that threshold level

for a short time so that the connection is just about overloaded and the

available bandwidth is sampled. Finally, the load is released until the next

measurement is made (which may be, for example, once every second).

When this procedure is iterated and its output filtered, a reliable estimate of

the available bandwidth is obtained.

By contrast, in Ascoms implementation, there is no ramping up of the amount

of data until the knee is encountered. Rather, the bit pipe is loaded to its

maximum just as in an FTP session but for a much shorter time, down to

a few milliseconds. In other words, the data rate always stays above the

knee.


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48.5.2. Use of FTP for ABM

Traditionally, ABM in mobile networks has been conducted by running FTP

sessions. Throughput is then typically averaged over one-second intervals

and reported once every second at the application layer. There is no way toobtain higher-resolution performance metrics from the application layer; that

is, without drilling down into RF data.

Now it is highly unlikely that a one-second throughput average will ever

reflect the full available bandwidth, since that would require perfect radio

conditions to have prevailed throughout the one-second interval. As the radio

environment typically undergoes substantial change on a millisecond time

scale, such a scenario is highly improbable.

ABM as implemented in Blixt, by contrast, samples much shorter timeintervals (down to 8 ms for LTE, as described in section 48.4.3) and is

therefore able to hit the maximum bandwidth, or somewhere very close to it.

For this reason, ABM as implemented by Ascom can be expected to give a

more accurate (though also more varying) estimate of the available

bandwidth than an FTP-based method.

Comparison of approaches to ABM. The black line curveindicates the true available

bandwidth as a function of time. The red barsrepresent TEMS ABM data bursts. Near-

maximum bandwidth is attained for the second ABM data burst. The blue arearepre-sents ABM performed by means of an FTP data transfer (1 s segment). The average

throughput over this one-second period is substantially below the maximum throughput

reached.

There is, in fact, an additional and grave shortcoming to using FTP with

currently available UEs: it has proven impossible during LTE network testing

to reach bit rates higher than about 60 Mbit/s (one-second average) even in

perfect radio conditions and with no other users present. The bottleneck here

is the UE processor, whose performance is hampered by the tasks imposed


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on it by the UE operating system (running applications, background

processes, etc.). Since the packet trains used in Ascoms ABM approach

minimize the load on the UE processor, measuring and reporting on the

networks full bandwidth is now possible.


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2.3.4. New Supported Scanner: PCTel SeeGull CX

PCTel SeeGull CX is a high-performance, cost-efficient RF scannerfor

worldwide testing of WCDMA/HSPA(+), GSM, and TD-SCDMA cellular

networks. The SeeGull CX scanner is available in single-technology as wellas dual-technology WCDMA/GSM and TD-SCDMA/GSM models.

2.4. What Was New in TEMS Investigation 15.0

2.4.1. Available Bandwidth Measurements: Blixt

Ascom has devised a method of available bandwidth measurement (ABM) in

state-of-the-art wireless networks such as LTE. The patent-pending ABMalgorithm, trademarked as Blixt, helps operators track the bandwidth

being offeredto their subscribers with great precision and millisecond

resolution. It has also been carefully designed for minimum intrusiveness,

that is to say, to have the smallest possible impact on the quality-of-

experience of paying network users.

What creates the need for a novel ABM algorithm is the rapid evolution of

recent mobile telecom technologies such as LTE and HSPA, with their vastly

higher data rates and complex configuration options. For these technologies,

traditional ABM methods are no longer adequate; what is required are metricsand measurement techniques designed specifically for the wireless

environment.

Ascoms Blixt technology for ABM is characterized by:

High peak load low average load.Test data is sent in short, intense

bursts (chirps) with much longer pauses in between. The peak load is

high enough to hit the networks theoretical maximum, while the average

load is kept low. This scheme allows sounding out the available bandwidth

while still making minimum use of network resources.

Fast adaptation in time domain.The data bursts that probe the network

are short enough to track changes in radio conditions on a millisecond

time scale. In this way a high-resolution profile of the available bandwidth

is obtained.

Adaptation to network configuration.The amount of data sent is

adjusted according to the networks maximum throughput (for example,

when the UE moves between LTE and HSPA networks), while keeping

the level of intrusiveness to a minimum at all times.


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Server-based design.The device that is performing ABM communicates

with a server which reflects the packets back to the device, including

timestamps and other data in the packets. That means it is easy to test

different parts of the network by having the device access different

servers. A timestamping protocol called TWAMP is used.

2.4.2. New Supplied Device: Sony Xperia V LT25i

This is an Android smartphone operating on LTE, WCDMA, and GSM

networks. Equipped with TEMS software, it offers extensive control

functionalityfor all of these technologies, including LTE RAT lock and LTE

band lock.

The casing of the Xperia V LT25i is water-resistant, making the phone lesssusceptible to moisture damage in wet or damp environments.

Frequency bands:

LTE 2100 (Band 1), 1800 (B3), 850 (B5), 2600 (B7), 800 (B20)

WCDMA 850 (Band V), 900 (VIII), 2100 (I)

GSM 850, 900, 1800, 1900

Throughput categories:

LTE Category 3 (100/50 Mbit/s)

HSDPA Category 24 (42 Mbit/s), HSUPA Category 6 (5.8 Mbit/s)

GPRS/EDGE Class 12

Control capabilities:

RAT lock on LTE, WCDMA, GSM

Band lock on LTE, WCDMA, GSM

Both RAT and band lock are real-time functions. No reboot of the phone

required.

LTE carrier (EARFCN) lock

WCDMA cell lock (UARFCN/SC)

GSM cell lock/multi-lock, cell prevention

Voice codec control

Cell barred control

Access class control


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12.20.3.9. Stop IP Sniffing

Stops capture of IP packets initiated by the Start IP Sniffingactivity.

Configurationproperty:

12.20.3.10. Available Bandwidth Measurement

Calculates the available bandwidth in the cellular network at successive

moments in time.

This activity is performed by means of data transmissions between the TEMS

Investigation device and one or several ABM servers (which are hosted by

Ascom). Measurement can be performed against up to four ABM servers

concurrently. All ABM servers are addressed with a single script activity.

Available ABM servers:

162.13.38.90, port 15001 (London, UK: EMEA)

119.9.67.138, port 15001 (Hong Kong, China: APAC)

192.237.128.240, port 15001 (Chicago, IL, US: AM/CALA)

For a description of the ABM algorithm, please consult chapter48.

The devices supporting available bandwidth measurement are listed in the

Device Configuration Guide, section 2.6.

Configurationproperty:

Lists ofABM information elementsand events.

Client:

PC: The TEMS Investigation built-in IP sniffer is used.

ODM:An on-device IP sniffing service is used.

Duration:Duration of available bandwidth measurement.

For each of ABM servers 1, ..., 4, the following needs specifying:

Server Enabled:Governs whether or not this ABM server is used.

Server Address:IP address to the ABM server (cannot be given as an

URL).

Server Port:The port on which the ABM server listens for requests.


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2.6. Supported Devices with ABM Capability

Sony Xperia V LT25i

Sony Xperia T LT30a Samsung Galaxy S III GT-I9305

2.7. Supported Devices Capable of On-deviceMeasurement

Regarding on-device measurement generally, see the Users Manual,

chapter15.

2.7.1. ODM for Voice

ODM MTSI

Samsung Galaxy SCH-R820

ODM Call Control

Sony Xperia V LT25i

Sony Xperia T LT30a

LG Lucid 2 VS870

Samsung Galaxy S III GT-I9305

Samsung Stratosphere SCH-I405

2.7.2. ODM for AQM

ODM POLQA

Sony Xperia V LT25i (CS)

LG Lucid 2 VS870 (VoLTE)

2.7.3. ODM for IP Sniffing

Samsung Stratosphere SCH-I405

available bandwidth measurements

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