PDCH

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Peter Stuckmann

Radio Network Dimensioning for GSM/GPRS

Supporting Circuit- and Packet-Switched Services

Peter Stuckmann

www.comnets.rwth-aachen.de/~pst

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Peter Stuckmann 2nd Wrzburg-Workshop on IP 2

Structure

the problem

coexisting circuit-switched traffic

dimensioning rules for fixed PDCH configurations

dimensioning rules for on-demand PDCH configurations

configurations with fixed and on-demand PDCHs

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Motivation

radio network dimensioning demands the relationship between

? offered traffic predicted by operator

? QoS desired by operator for his clients

? radio resources needed (number of PDCHs and TRX)

analytical complexity caused by

? radio channel attributes

? bursty traffic (no Erlang-B-formula like in CS)

system complexity no test bench or field trials with complete protocol

software and hardware components available

solution: computer simulation of the system that models

? system components and their protocols

? traffic sources

? radio channel

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Capacity planning and radio network dimensioning

relationship between

? offered traffic

? QoS desired

? radio resources needed

Traffic QoS

Capacity model

Simulation model Analytical model

Erlang table Erlang Formula

Radio network dimensioning

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Measures and methodology

term circuit-switched packet-switchedtraffic offered traffic in Erlang offered amount of data per

time in kbit/s

QoS parameter blocking probability (GoS) throughput, delay,...

resources traffic channels packet data channels

tool simple formula or table dimensioning graphs or

tables

methodology Erlang-B formula simulation results,

analytical / algorithmic

techniques

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Process

Arrival

Session

Management

Channel

LLC(SDL)

management)(session

SGSN

CAC

SNDCP

BSSGP

Frame

RelayRelay

Frame

BSSGP

BS

LLC

Relay

RLC/MAC

(SDL)

Transceiver

SNDCP

LLC

(SDL)

MS

(SDL)

RLC/MAC

Transceiver

Generator

SMTPHTTP WAPFTP RTTele-

matic

TCP/UDP

IP

Generator

Generator

Switched

Circuit Internet Load Generator (SDL)

Railway

Funet

Mobitex

Packet

Channel/Error

Model

bGUm

UplinkbG

DownlinkGb

0

0.20.3

0.4

0.60.7

0.80.9

1

01000

2000 5000 7000

8000

900010000

BlockingRate

56RA Slots24RA Slots88RA Slots

0.1

0.5

4000 6000

Offered Load [byte/s]3000

00.1

0.20.3

0.40.5

0.60.7

0.80.9

1

0 0.20.40.60.8 1 1.21.41.6

1.8 2

Railway

Funet

Mobitex

Offered Load (G)

Throughput(S)

GIST Statistical EvaluationWeb Interface

Simulation Environment (E)GPRSim

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Dimensioning rules for fixed-PDCH scenarios (I)

0

5

10

15

20

25

5 10 15 20 25 30

DownlinkIP

thro

ughput[kbit/s]

Offered IP traffic [kbit/s]

downlink IP throughput over offered traffic

3 fixed PDCH4 fixed PDCH5 fixed PDCH6 fixed PDCH

0

5

10

15

20

25

0 1 2 3 4 5 6 7

DownlinkIP

throughput[kbit/s]

Offered IP traffic / PDCH [kbit/s]

Dimensioning Graph

4 fixed PDCH5 fixed PDCH6 fixed PDCH

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Dimensioning rules for fixed-PDCH scenarios (II)

define the desired QoS

estimate the number of users per cell

estimate the offered traffic per user

calculate the total offered traffic per cell

determine the acceptable traffic per PDCH with the desired QoS fromthe dimensioning graph

calculate the needed number of PDCHs

PDCH = estimated offered traffic / acceptable traffic per PDCH

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Example

define the desired QoS

? example value: 12.5 kbit/s is desired

estimate the number of users per cell

? example value: 10 users with 540 kbyte/h per user

calculate the offered traffic per user and the offered traffic per cell

? offered traffic per user = 540 kbyte/h = 1.2 kbit/s

? total offered traffic per cell = 12 kbit/s

take the acceptable traffic per PDCH from the reference graph

? acceptable traffic per PDCH = 3.5 kbit/s/PDCH

? PDCH = 3.4

?

4 PDCHs have to be allocated for GPRS

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Radio resource management

X

X X X X X X X X

X

maximum numberof on-demand PDCHs

TCHs allocatedfor GPRS

TCHs allocated for CS

X X X X X

maximum number

of fixed PDCHs

Transition

TCH releaseTCH shift

on-demand assignment of PDCHs for GPRS

voice connections are prioritized

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Circuit switched traffic

State probability vs no. of PDCHsState probability vs no. of TCHs for 1TRX

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 1 2 3 4 5 6 7 8

Probability

Number of channels

0

0.05

0.1

0.15

0.2

0.25

0.3

0 1 2 3 4 5 6

Probability

Number of channels

radio network dimensioned for voice services with blocking probability

of e.g. 1 % (blocking probability = Grade of Service (GoS))

unutilized resources can be used for GPRS

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GPRS performance with coexisting CS traffic (I)

downlink IP throughput per user

0

5

10

15

20

25

30

0 2 4 6 8 10 12

DownlinkIP

throughput[kbit/s]



0

0.5

1

1.5

2

0 2 4 6 8 10 12

DownlinkIP

delay[s]


downlink IP packet delay


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GPRS performance with coexisting CS traffic (II)

0

1

2

3

4

5

6

7

8

0 2 4 6 8 10 12

DownlinkPDCHsassig

ned(average)


downlink PDCHs assigned (average)


0

20

40

60

80

100

0 2 4 6 8 10 12

DownlinkPDCHu

tilization[percent]

Offered IP traffic offer [kbit/s]

downlink PDCH utilization


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Dimensioning rules for on-demand-PDCH scenarios

simulation results show that the shared use of GSM physical channelsboth for CS and GPRS makes sense

dimensioning rules for on-demand-PDCH scenarios are needed

the aim is to take an existing TRX scenario as the basis and find the

acceptable CS traffic so that the desired GPRS performance can be

achieved

if the predicted offered traffic is exceeded a new TRX module should

be added to the base station

in the following an example is given for a 3 TRX scenario

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Example: 3 TRX scenario (I)

0

5

10

15

20

25

5 10 15 20 25 30

DownlinkIP

throughput[kbit/s]


downlink IP throughput per user

0.5% GoS2% GoS

10% GoSExample values

? offered CS traffic with 1.5 % GoS

? 10 GPRS users per cell

? 540 kbyte/h offered per user

? desired QoS of 12.5 kbit/s

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Example: 3 TRX scenario (II)

calculate the offered traffic per cell

? total offered traffic per cell = 10 * 540 byte/h = 12 kbit/s

regard the operating point p defined by the desired user performance

on the y-axis and the offered traffic per cell on the x-axis and choose

the adequate GoS curve as the next that lies above the operating point

? p = (x = 12 kbit/s, y = 12.5 kbit/s)

if the offered CS traffic corresponding to the GoS is predicted to be

exceeded, a new TRX should be added

? the operating point p lies just below the 2 % GoS curve

? coexisting CS traffic up to 2 % GoS is acceptable

? since 1.5 % GoS was assumed an additional TRX is not necessary in this cell

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Mixed configurations

to be able to guarantee the availability of GPRS, operators mightprovide fixed PDCHs and the rest as on-demand PDCHs

in this case simple estimations can be done:

? scenarios with 1,2 or 3 fixed PDCHs: take pure on-demand configurations for

dimensioning (probability that the first PDCHs are used by CS is low)

? scenarios with more than 3 fixed PDCHs: take a pure fixed PDCH configuration fordimensioning (probability that the on-demand PDCHs are used by CS is high)

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Conclusions

fixed PDCH configurations can be dimensioned with one simpledimensioning graph

on-demand PDCH configurations are recommended, since not all

physical channels are highly utilized by CS

dimensioning of on-demand PDCHs can be done by taking

dimensioning graphs for the regarded TRX scenario as the basis

for mixed scenarios with both fixed and on-demand PDCHs simple

estimations can be done and pure fixed or pure on-demand

dimensioning rules can be applied

PDCH

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