Channel Allocation for the GPRSDesign and Performance Study

Huei-Wen Ferng, Ph.D.

Assistant ProfessorDepartment of Computer Science and Information Engineering (CSIE)Nation Taiwan University of Science and Technology (NTUST)Wireless Communications and Networking Engineering (WCANE) LabURL: mail.ntust.edu.tw/~hwferngE-mail: hwferng@mail.ntust.edu.tw

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Outline

Introduction Channel allocation schemes System model and assumptions Performance study and numerical examples Conclusions

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GPRS Architecture

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Service Requirements

Blocking vs. forced termination From the viewpoint of users, one may feel more

uncomfortable when an on-going call is abruptly terminated than directly getting blocked before his service.

Generally speaking, less forced termination than blocking.

Delay-sensitive vs. non-delay-sensitive Voice is more sensitive than data.

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Design Principles

Channel reservation It privileges handoff calls.

Priority Priority among buffers

Differentiation between voice and data requests Service priority between new voice calls and handoff voice calls

Differentiation between new calls and handoff calls Buffering strategy

Allows more net input rates Threshold control

Throttles different rates of new calls and handoff calls

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Channel Allocation Schemes

An example of dynamic allocation of the uplink data transfer

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Basic Dynamic Channel Allocation (DCA): For a data request, DCA allocates at most n channels to the request. For a voice call, only one channel is allocated.

Five DCAs are proposed based on service priority, threshold control, channel reservation, and buffering strategies.

Channel Allocation Schemes

: no priority for voice and data buffers, no threshold control.

: higher priority for voice buffer, no threshold control.

: higher priority for voice buffer with threshold control.

: similar to the 2n

d

scheme with handoff the highest pri.

: similar to the 2n

d

scheme with channel reservation.

NCQPDCA

NCNQPDCA

TCQPDCA

NCQHPDCA

NCQPGDCA

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CAS1 Scheme

1

C

1dB

vB 1

Voice buffer

Data buffer

New voice (handoff) calls

New data packets

FIFO

This scheme is proposed for reference.

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1

C

1dB

vB 1

Voice buffer

Data buffer

New voice (handoff) calls

New data packets

High priority

Low priority

When buffer is empty.

CAS2 Scheme

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New voice (handoff) calls

New data packets

CAS3 Scheme

When buffer is emptyHandoff call first.Then, new voice call.

1

C

1dB

vB 1

Voice buffer

Data buffer

High priority

Low priority

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1

C

1dB

vB 1

Voice buffer

Data buffer

High priority

Low priority

New data packets

New voice (handoff) calls

CAS4 Scheme

vT

When buffer is empty

Handoff voice calls

Blocked when exceeding the threshold

New voice calls

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New voice (handoff) calls

New data packets

New call and handoff call

Handoff call

When buffer is empty.

CAS5 Scheme

1

C

GCC

1dB

vB 1

Voice buffer

Data buffer

High priority

Low priority

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GSM (new) voice call and GPRS (new) data packet arrive according to Poisson processes with ratesλv and λd, respectively.

GSM (new and handoff) voice call holding time, GPRS packet transmission time, and GSM user dwelling time follow exponential distributions with mean 1/μv , 1/μd and 1/η, respectively..

Static data users are assumed for simplicity.

Each GSM user moves to any adjacent cell in a uniform manner; same traffic load as well as same number of channels are assumed to any cell. (resulting in homogenous cells).

System Model and Assumptions

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Simulation Environment

Square cell structure 6x6 wrapped mesh cells Homogeneous cells

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Define state space Write balance equations based on the state

transition diagram Use the recursive approach to obtain results

Analysis of Channel Allocation Scheme

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ddvhvni

n

iidhnn

CAS

BbBbBbi

Cy

CxCiyxbbbyyx

S

0 ,0 ,0 ,0

0 ,0|,,,,,,0

1

3

Define state space

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State transition diagram

for otherwise 0,

if 1,

11for otherwise 0,

if 1,

1

1

njnCiyx

n-jjCiyx

n

ii

pi

n

ii

pi

ji

jiji if ,0

if ,1

For convenience, let us define two sets of indicator functions

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State transition diagram

dhnknnkkkk

dhninniiii

dhninniiii

dhnndhnn

dhnn

bslbsbyyylx

n

k

k

l

l

sdk

bbbyyyx

n

idi

bbbyyyx

n

idpi

bbbyyxbbbyyx

bbbyyx

n

i

n

idpidi

Pyk

Pyi

P

PxP

Piyx

,,,,,,,,2

1

1 0

,,,,,,,,1

,,,,,,,,11

,,,,,,1,,,,,,1

,,,,,1 1

11

11

11

11

1

)1(

)1(

)1(

)(

0,0 when : 1 1

dhnd

n

ii bbbCiyxCase

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State transition diagram

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Blocking probabilities

fttftfftfft

vbtvbfvbfvb

PPPP

PPPP

)1(

)1(

)]([),,,,,(,,,,,,

3,1

1

vhvCASdhnn

dhvnBbbSbbbyyx

bbbyyxftfvbf PPP

)1(3,1

1),,,,,(

,,,,,,

ftfh

Sbbbyyxbbbyyxh

ftt P

Pb

P CASdhnn

dhvn

)1(3,1

1),,,,,(

,,,,,,

vbfv

Sbbbyyxbbbyyxn

vbt P

Pb

P CASdhnn

dhvn

)]([),,,,,(

,,,,,,

3,1

1

ddCASdhnn

dhvnBbSbbbyyx

bbbyyxdd PP

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Delay times

)1(

)1(

)1(

31

,1

31

,1

31

,1

),,,,,,(,,,,,

),,,,,,(,,,,,

),,,,,,(,,,,,

ddd

Sbbbyyxbbbyyxd

d

ftfh

Sbbbyyxbbbyyxh

hv

vbfv

Sbbbyyxbbbyyxn

nv

P

Pb

W

P

Pb

W

P

Pb

W

CASdhnn

dhnn

CASdhnn

dhnn

CASdhnn

dhnn

Using Little’s formula:

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Number of channels (C ) 7

Voice buffer size 7

Maximum available number of channels for data packets (n)

3

Voice holding time

Data transmission time

Data arrival rate

Parameters Setting

v100/1

v25

1/1 v

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Performance Measures

Blocking probability for a new voice call Forced termination probability for a handoff voice

call Data packet dropping probability Delays Cost comparisons among different schemes

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The Effect of Data Buffering (on new voice blocking probability)

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The Effect of Data Buffering (on forced termination probability)

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The Effect of Data Buffering (on data dropping probability)

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The Effect of Data Buffering (on delays of data packet)

Delays of new voice calls and handoff voice calls are similar to probabilities.

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The Effect of Data Buffering

Data buffer size affects little to new call blocking probability and handoff call forced termination probability, except CAS1.

Increasing data buffer size greatly improves data dropping probability.

The effects on delays of new voice calls and handoff calls are similar to blocking probability and forced termination probability, respectively. Increasing data buffer size raises data packet delay time.

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The Effect of Threshold Control(on blocking/termination probability)

Decreasing Tv makes new voice call blocking increase and improves forced termination probability.

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The Effect of Threshold Control(on data dropping probability)

Because lower value of Tv permits fewer queued new voice calls in the system;therefore, data packets have a better chance to be served, thus data dropping probability decreases.

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The Effect of Threshold Control(on delays of new voice calls and handoff voice calls)

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The Effect of Threshold Control(on delays of data packets)

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Schemes Comparison (new voice blocking probability for various traffic load)

The best to worst schemes are CAS2, CAS3, CAS4, and CAS5.

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Schemes Comparison (forced termination probability for various traffic load)

The best to worst schemes are CAS5, CAS4, CAS3 and, CAS2.

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Schemes Comparison (data dropping probability for various traffic load)

CAS4 performs best in terms of data dropping probability.

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Schemes Comparison (delays of new voice calls for various traffic load)

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Schemes Comparison (delays of handoff voice calls for various traffic load)

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Schemes Comparison (delays of data packets for various traffic load)

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Effect of data user mobility and computation illustration on channel reservation

The performance of voice calls is not sensitive to variation of ηd because voicecalls have higher precedence over data packets.

When CG increases, performance of new voice calls and data packets becomes poor.

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Effect of different data traffic models

The exponential distribution may not be appropriate in modeling data traffic.

Instead, the Pareto distribution can be used to capture the nature of data traffic.

tt

tf ,)(1

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Cost comparisons(fixedτ3)

ddvbftf PPPc 321 Cost function:

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Cost comparisons(fixedτ2)

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Cost comparisons (fixedτ1)

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Conclusions

We conclude:Buffering for both voice calls and data packets reduces blocking probability, forced termination probability, and data dropping probability but it increases delay times. The threshold control is an effective approach to reduce forced termination and data dropping probabilities. But it enlarges new voice call blocking probability.

We have examined and compared the improvement of channel allocation schemes using four techniques.

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Conclusions

Finally, we suggest scheme CAS3 and CAS4 to be used in the GPRS system.

Although reservation greatly improves forced termination probability and delay time for handoff voice call, it causes the performance of new voice call and data service poor.

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The EndThank You!