Network Quality

Technology for executives, Moscow - 13 September 2004Author: Morten RaaumPresenter: Hans Myhre

Network Quality, as perceived by customer

Coverage Capacity Speech quality Services quality Customer care..

GSM in the field

BTS

BTS

BTS

BTS

To BSC / MSC

To BSC / MSC

To BSC / MSC

To BSC / MSC

Handover

Handover

Some practical experiences.. No network access (= no ‘Telenor’ in the display)

– Lack of coverage– Network overload situation (users barred from access)– Network failure.

3 beeps/3-tone beeps at call setup– Blocking/error situations somewhere in the network– User error (e.g. non-existent number)

Poor speech quality– Marginal coverage– Interference between base stations

Dropped call– Poor quality of radio link– Blocking in target cell (moving mobile, handover situation)– Network failure (transmission)

The mobile is the weakest link in the chain:Mass production units – Internal antenna – Increased software and processing complexity (WAP/GPRS/HSCSD/games&features)

GSM in the field

BTS

BTS

BTS

BTS

To BSC / MSC

To BSC / MSC

To BSC / MSC

To BSC / MSC

Handover

Dropped call

Poor speech quality

Handover to non-optimal cell

No traffic channels available

How is Network Quality monitored by the operator? Coverage

– Measurements by technical staff. Test telephone + laptop.

– Measurements by remotely controlled test systems (installed in bus, taxi etc).

– Customer complaints and performance stats as input.

Capacity– Network Maintenance Center / Operations center– The network counts # of events (pos./neg.)

Post processing in Performance Management tool.– Tools for prognosis on capacity needs vs. required

quality of the network (Grade of Service)

How is Network Quality monitored by the operator? Speech quality

– The network counts Dropped calls and to some extent report on speech quality

– Measurements by remotely controlled test systems / tech.staff.

Services quality– Services are built up by combining equipment and

software– External tools needed to ensure quality of delivery

• Correct content / Response time• 3rd party content delivery• 3rd party Service Level Agreements (SLA)

Capacity and quality – a trade off at a given funding

Capacity in the GSM radio network Base station with antennas connected in a mast

– BTS contains a number of TRX based on the need for capacity.

UPLINK

DOWNLINKTx

Rx

Tx

Rx

BTS cabinet

TRX TRX

TRX

Radio network capacity

Building blocks for capacity at the base stations: TRX– 8 channels that are used for signalling (SMS, suppl.services etc) and

speech/data.

Call duration -> Erlang– 1 Erlang = 1 hour of speech for 1 user– Call duration 90 sec: 90s / 3600s = 0,025 Erl = 25 mErl – I.e. 1 Erlang: 40 calls of 90 seconds each. (1 Erl = 40 x 25 mErl)

Theoretical capacity: Erlang’s model and chosen Grade of Service.

– GoS (%) Number of call attempts that may meet blocking in a busy hour.• Typical values used: 2-5%

– Example, GoS=2%– 1 TRX 7 speech channels 2.9 Erlang 41%– 2 TRX 15 speech channels 9.0 Erlang 60%– 3 TRX 22 speech channels 14.9 Erlang 68%– 4 TRX 30 speech channels 21.9 Erlang 73%

Radio network capacity

Base station configuration example:

3 TRXes, Omni directional cell– 1 cell with 3 TRX, GoS=2%:

• Capacity = 14.9 Erl• 90 sec call duration (25mErl)• 14.9 / 0.025 = 596 calls in a busy

hour (theoretical max!)

If we accept higher blocking figures we can carry more traffic with less customer satisfaction.5% GoS used in this example => 17.1 Erl capacity => 684 calls (+15%)

BTS

TRX TRX TRX

Base station configuration example:

3 TRXes, Sectorised 1/1/1.– 3 cells with 1 TRX each:

• Capacity = 3 x 2,9 Erl = 8,7 Erl• 90 sec call duration • 8,7 / 0,025 = 348 calls in a busy hour

If we accept higher blocking figures we can carry more traffic with less customer satisfaction.5% GoS used in this example => 11.1 Erl capacity => 444 calls (+28%)

BTS

TRX TRX TRX

Radio network capacity

Frequency planning

Re-use of GSM channels is necessary. In this example a frequency is re-used in every 12th cell => reuse factor 12

Resource: Frequency spectrum

Spectrum size strongly affect base station roll out. Re-use of GSM channels is necessary. 1 TRX use 1 GSM

channel Example, Malaysia:

– 24.8 MHz => 124 GSM channels– # of TRX per cell: >6. Good capacity per base station (xx Erlang).

Example, Russia:– 4 MHz => 20 GSM channels– Maximum ~1,5 TRX per cell = a denser base station network will be needed

for serving increasing traffic (xx Erlang)

Good radio/frequency planning and use of “smart” features enables a tight re-use of GSM channels

– Power control • Mobile and BTS adjust their output power as low as possible, adjustment 2 times per

sec.

– Discontinuous Transmission (DTX) • Mobile and BTS only transmit when user actually speak. (User benefit: less power

consumption)

– Frequency hopping • Mobile and BTS jump from GSM channel to GSM channel (480ms), less affected by

interference.

Area= 50km2Population= 1 M

Subscribers= 346k15 mErl/sub

Traffic= 5190 Erl

Spectrum vs. investmentsCapacity case

Malaysia case..– 6TRX/cell= 34,6 Erl/cell– 900TRX, 150 cells and 50 BTS

needed

Russia case..– 1,5 TRX/cell= 5,8 Erl/cell– 1343TRX, 895 cells and 299 BTS

needed

We need 6 times as many sites for capacity reason!

For illustration purpose only – the conclusions and calculations are simplified

Network Quality,some KPIs Minutes per dropped call

– Summary of all traffic minutes divided by number of dropped calls during a period of time (e.g. Week/month)

– Typical values: 50-150 minutes.

Blocking/Congestion– Call attempts that meet blocking, all resources are

occupied.– The network is dimensioned to meet a certain traffic level

in the busy hour. Typical dimensioning values (GoS): 2 - 5 %Typical values measured in the networks: 0 - 50+ %

Network Quality, some KPIsNetwork Utilisation

Cell utilization

0

20

40

60

80

100

120

140

160

Cell utilization distribution

Uti

liza

tio

n (

%)

About 22% of the cells in the network are utilised > theoretical max = customers experience more blocking than acceptable in these cells!

Technical KPIs

PSTN

Access network OSS Core network OSS

TechnicalKPIs Financial

MarketHRTechnical

BenchmarkingReport

..................

..................

..................

BTS BSC

MSC

MSC

MSC

MSC

Events& alarms

Events& alarms

Network Quality, challenges How to measure customer perceived quality?

– Unmanned active measurement units. New and advanced services

– Fast development and deployment of new services.– Increased complexity, both for network and mobile.

3.party content providers– Timeout... Not available... – Service Level Agreements. How do you know what is

delivered?– New and advanced measurement systems for

test&monitoring. Roaming

– How well are we able to keep foreign roamers in our network – and how are our roamers abroad doing?

– Advanced measurement systems (signalling) for test&monitoring.

Environment

Base station location:

Aim to minimise the effect of masts and antennas on the local environment.

Trøndelag theatre

Tree-mounted antenna..

Antenna

Vindern

Typical antenna-installation in a city environment

Mini-BTS mounted on a pole by the fjord