Traffic.(Dimensions Details) Good

8/11/2019 Traffic.(Dimensions Details) Good

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a [email protected] 11Traffic&Switchin , Au ust, 2002

TRAFFIC THEORY

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What is Traffic ?

TRAFFICThis is the amount of calls of known usage length to be handled by the telecomsystem in use.

BUSY HOUR TRAFFICThis is the amount of call traffic handled by a group of RESOURCES during thebusiest hour of the busiest day for the system.

GRADE OF SERVICE OR BLOCKINGGOS or Blocking is a percentage that refers to the calls that get a busy signalbecause all lines are in use.

ERLANG MODELSTraffic is defined in Erlangs. Knowing the traffic and blocking factor, number ofresources needed can be calculated using ERLANG MODELS.


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What is Erlang ?

The unit that defines the traffic is the Erlang. (Danish Mathematician)

The Erlang:

Traffic Intensity, E = X Th Erlangs

= call arrival rate (calls/hour)

t h = mean holding time (hours/call)

1 Erlang is one resource (e.g. one voice channel) which is used Continuously.

Traffic of one resource:

Example: a subscriber who makes 2 phone calls of 90s per hour:

Traffic = (2 x 90) / 3600 = 0.05 Erlang

Resource usage duration

Total duration

T =


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Example: Assume 100 subscribers with the following traffic profile:

20 make 1 call/hour for 6 min. 20x1x(6/60) = 2 E

20 make 3 calls/hour for min. 20x3x(/60) = E

60 make 1 call/hour for 1 min. 60x1x(1/60) = 1 E

100 3.5 E

100 subscribers use 3.5 E... = 35 mE persubscriber

Traffic Example


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Mobile Subscribers traditionally use an average of 15-35 mEduring busy hour

Busy hour traditionally was 10am-noon the word hour in this context means a period, and not necessarily 60 minutes

Subscriber characteristics are changing however

Traditional subscribers are no longer employees on the move, making callsduring working hours

Mobiles are becoming much more popular for personal reasons

Busy Hour shifted to commuting hours (4-7pm is best)

Some carriers are seeing another shift in Busy Hour, due to reduced longdistance and free evenings/weekends.

Erlangs per subscriber have increased a little

Mobile Traffic Characteristics


6/51a [email protected] 61Traffic&Switchin , Au ust, 2002

Why Do We Need to Know the Traffic

The amount of traffic during peak hours allows us todimension our wireless system for a certain grade of

service If the system is not dimensioned to support the traffic, subscribers will beblocked from making a call

The Grade of Service (GOS) is the probability of having acall blocked during busy hour

In a wireless system, the design target is typically 2% (0.02),or less.

- This GOS definition applies when using the Erlang B traffic formula

Traffic Tables tell us how many channels are required for aminimum GOS.



Queuing systems

A queuing system may be with or without loss.

Example of queuing system with one server:

Arrival process Departure process

Service

time

Queuing Time

Queue length

A one server queuing system without any loss is a server with

an infinite queue size (theoretical only).

We call loss systems systems that have the same number of

servers that the queue length (no waiting time)



- 3 types of tables could be used in mobile applications

- The tables are drafted from probability equations

Poisson Table : Blocked calls are held in the queue for atime equal to the mean holding time of a call

Erlang B Table : Blocked calls are not held

Erlang C Table : Blocked calls are held in the queueindefinitely

Traffic Tables



Erlang laws

Erlang B:

Calls arrive randomly.

Arrival process is Poissonian with rate

Call Service time is either fixed length or exponentially distributed.Departure process is Poissonian with rate

Blocked Calls are not retried immediately.

N server loss system: when N servers are occupied, arrivingcustomer is thrown (no call reattempt).

System model with transitions at infinitesimal time intervals

Steady state: Number of departure=Number of arrival

k0 1 n-1 n

(k+1)

k

n



Erlang B

3 parameters are used in the Erlang formulas:

Offered Traffic (T)

Number of circuits (n) Blocking probability (Pblock)

With 2 of these parameters, one can calculate the third:

Examples:

On the air interface: (No Of Circuits,Pblock)-> Offered Traffic

On the A interface: (Offered Traffic,Pblock)-> Number Of Circuits



rlang B

Notation:

= 1/T, T is the mean inter departure time, ie the mean holding time

/ =T is the offered traffic to the system

Probability of arriving customer being blocked = probability of ncustomers in the system, ie P(n):

P block T n

T

nT

i

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Erlang B

Example on the air interface (2% blocking rate)

Erlang law: Offered Traffic=f(n) with 2% blocking rate

0

10

20

30

40

50

60

0 10 20 30 40 50 60

Number of channels

OfferedTraffic



Erlang B

Example: channel efficiency (2% blocking rate)

Channel Efficiency=Offered Traffic / n

0

20

40

60

80

100

0 10 20 30 40 50 60

Number of channels

Efficiency(%)

The Erlang law is not linear !!!4TRX (21.9Erl) > 2x2TRX (16.4Erl)



Other Erlang law

Extended Erlang B: Same assumptions as Erlang B buttakes into account of retried calls.

Erlang C

the Erlang C, is obtained from the same system without loss(infinite queue size, n servers):

0 1

k

n

n

n-1 n

n2 (n-1)



The Erlang B Table is typically used in mobile wireless

Most systems do not queue blocked calls, and except for some

users who make multiple re-try attempts, the traffic is best

approximated using Erlang B

Example: How many channels are required to support 100 users

with a GOS of 2% if the average traffic per user is 30 mE?

100x30mE = 3 Erlangs

3 Erlangs @ 2% GOS = 8 channels

Erlang B Table


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Queuing systems in a GSM call

Allocation of Air interface resources

Allocation of SDCCH channels to a MS: loss system with nSDCCHservers (immediate assignment procedure)

Allocation of TCH channel to a MS: loss or queuing system with nTCHservers (handover and assignment procedure)

Allocation of A interface resources

Allocation of a circuit to the call (assignment procedure)

Allocation of a SCCP (signalling Connection Control Part) connectionbuffer to the signaling transaction (SCCP connection establishmentprocedure)


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heoretical background Conclusion

The basis of the Erlang laws is the Poissonian Process

Good model for telecommunication systems

Memory less property:

no assumptions made on call repetition in case of blocking

still a good assumption if the blocking probability is low (no snowballeffect)

Erlang proven


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SWITCHING THEORY


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Analog and Digital

ANALOG represents continuous numeric value - WAVEFORM

Ex. Original waveforms of Voice or Video signals

Digital Means signal can be represented by 0s and 1s

Analog can be converted to Digital and vice versa.

Advantages of Digital transmission:

Noises caused in the transmission can be easily eliminatedSemiconductor devices are available and reliable

Secured transmission - Ciphering

Integrated Services - Audio, Video, Data, Fax..

Analog

A/D Conversion

Digital


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Digitisation

Is the Process of Converting an Analog Signal to Digital FormatA COder-DECoder performs this operation by applying Pulse

Code Modulation algorithm

The CODEC may be placed at any point

A logarithmic (com-panding) scale is used to map the amplitudeto its digital value

The PCM companding rules define:

255 amplitude levels, -law, in USA, Canadaand Japan

256 amplitude levels, A-law, almost rest ofthe world

t

Analogue signal

t

Sampling

Ts

t

Sa mpled signal


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Switch Board

Connects Input Channels to Output Channels


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Strowger Automatic Switch / Crossbar Switch


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me Switch

nals are temporarily stored in the memory

der of Signal is changed in the output01

2

3

31

01

23

31

Buffer Memory

Control Memory

0 7

7


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Group Switch

3

17

Internal Time SlotSwitching


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CONCENTRATION

01

2

3

127

Time Switch

01

127

Subscribers

32 Channel PCM System


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Space Switch

Control Memory

A

B

C

D

O

Multiplexer

B A

a b


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Time and Space Switches

Time Switch

7 8 9 78 9

Space Switch

7 8 97

8

9

6


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Normal TRA/TRH connections

SPM

64 kb/s GS

FR/EFR

HR

ETC

ETC

BTS

MSC

TRA

TRA

TRH

(Speech)

(LAP-D)

(Speech)

1+2

(LAP-D+Speech)

31(Speech)


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Subrate Switch

SPM

64 kb/s GS

SRS/TSMP

FR/EFR

HR

ETC

ETC

BTS

MSC

TRA

TRA

TRH

6+24

16/24

3+24

(Speech)

(LAP-D)

(Speech)

TRA inPool

LAP-DMultiplexi

2

(Speech+LAP-D)

31(Speech)


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MSCMSC/VLR Hardware/VLR Hardware

EC

IOG SP

RP

RP

RP

CP-A

GSS

ETC

ETC

ETC

EC

ETC

ETC

PCD-D ST-7

AST

CSK

CCD

ETC

PCD

PCD

PCD

GIWU

ETC

ISDN

VMAIL

BSC

HLR

RG RGSU

RSMECD

ECC

A-BL

CD or TCON

CANS

Digital BLBL

AnalogueBL

Testingequipment

APZ APT

RMOG/PML

12060 A


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HLR HardwareHLR Hardware

APZ

IOG 11B SP

RP

RP

CP-A

ST-7 MUX MSC

ST-7 MUX MSC

APT

RMOG/PML

12140 A


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BSC Hardware

IOG SP

RP

RP

RP

CP-A

GSS

ETC

ETC

STC

ETC

TRH

TRA

ETC

PCD-D

MSC

BTS

BTS

STCMUX

ST-7

APZ APT


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GSM System

ETC

GSS

ETC

TRH

TRA

PCD-D

DXU

ST-7

GSS

ETC

RPD

ETCGMSCTRU CDU

ETCHLR

ETCPSTN


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CDMAFull time use of the full spectral allocation

Time

Freq

Code

Freq

Time

Code

TDMAEach user has part time use of the spallocation

ireless Access Technologie

Freq

Time

Code

FDMAEach user defined fulltime use of thespectral allocation


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Multiplexing Techniques

Time Division Multiplexing (TDM)

Conventional

Bit-Interleaved

Byte-Interleaved

Statistical (STDM)

T S - 1

t

f

T S - 2 T S - 3 T S - 4 T S - 1 T S - 2 T S - 3 T S - 4 T S - 1 T S - 2 T S - 3 T S - 4TDM


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Multiplexing Techniques

Frequency Division Multiplexing (FDM)(CATV is a good example)

Wavelength Division Multiplexing (WDM)

(often used in optical data transmission)

t

f

F C - 1

F C - 2

F C - 3

F C - 4

FDM


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Communication Modes

Simplex

data is transmitted in one direction only

Half DuplexData can be transmitted in both directions, but only in one direction at

any given time

Full Duplex

Data is transmitted in both directions simultaneously


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CIRCUIT SWITCHING

- Used in conventional telephone Switches

- Communication channel is used continuously and exclusively

until call is disconnected


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PACKET SWITCHING

- Used only when required to transmit

- Information divided into appropriate sizes and sent

- Control and Header added to information and then transferred

T i i M d


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Transmission Modes

SYN character Bit stream of many characters

Asynchronous

Synchronous

SYN character

Stop bit Character Start bit

E C t l


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Error Control

Parity Bit Method

an additional bit is added to each tansmitted character to detect

single bit errorsEven / Odd parity

Block sum check algorithms

two additional bits are added (row / column) to detect errorstwo bit errors that escape the row parity checking, will be detected by

this method

E C t l


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Error Control

Frame to be transmitted Calculated CRC value

fInput data Output data

Inputp

olynomia

l


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ata Compression

Packed Decimal

Reduce the number of transmitted data

Relative EncodingData that has only small differences between successive values, (send

only the magnitude)

Character Suppression

Used for more general caseHuffman Coding

Statistical coding


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Speech

Decoding

Channel

Decoding

De Inter

leavingEqualisation/

Demodulation

RF

system

SpeechCoding

ChannelCoding

Interleaving Modulation

RFsystem

BT

GSM Speech Transmission


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Speech Coding

To transmit speech with best quality at smallest rate

Two categories of CodersWaveform Coders ( PCM, ADPCM etc )

Relatively High bit rate with very good quality

Vocoders ( LPC etc )Vocoders are complex but can use much less transmission rate

Hybrid Coders ( CELP, RELP, RPE-LTP etc )

Mostly Used in Cellular Systems

Speech

Coding

Channel

Coding

Inter

leaving ModulationRF

system

BTS

GSM Speech Coders


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Band

Pass

Filter

A/D RPE-LTP

Coder

ACELP

300-3400 Hz 8 KHzX 13 Bits

104 kbps13 kbps

To

Channel Coder

Full Rate(13kbps), RPE-LTP

(Regular Pulse Excitation- Long Term Prediction Coding)

Half Rate(6.5kbps),

Not Used because of Poor Quality

Enhanced Full Rate(12.2kbps), ACELP

Algebraic Code Excited Linear Prediction Code

Adaptive Multi Rate, AMR

VAD Voice Activity Detection

VAD

GSM Speech Coders

Ch l C di


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Channel Coding

To minimise errors in the transmission

Adds redundant bits to protect the user information bits

Block Coders ( Error Detection Ex. Parity bits)

Check bits are added depending on the information bits

Convolutional Coders ( For Error Correction )

Speech

Coding

Channel

Coding

Inter


system

BTS


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Interleaving

Errors often occour in Bursts

Channel coding is effective for single errors

Interleaving is adding time diversity without adding any redundant bits

Speech

Coding

Channel

Coding

Inter


system

BTS

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

Message 1 Message 2 Message 3 Message 4

1 1 1 1 2 2 2 2 3 3 3 3 4 4 4 4

1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4

XXXX

XXXX

Message Blocks

Interleaving

De interleaved


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Modulation

Baseband signals are not suitable for transmission

Baseband signal is carried over a high frequency carrier (900 MHz)

Modulation Techniques used: FSK, PSK, GMSK, QPSK etc

GMSK Gaussian Minimum Shift Keying

Baseband Signal is filtered with a gaussian passband

GMSK offers much smaller bandwidth compared to ordinary MSK

GMSK is less resistant to noise compared to MSK

Speech

Coding

Channel

Coding

Inter


system

BTS13 kbps 22.8 kbps 270.8 kbps

Documents

Traffic.(Dimensions Details) Good