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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
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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.
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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)
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- 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
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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
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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
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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
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n
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
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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)
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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)
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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
leaving ModulationRF
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
leaving ModulationRF
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
leaving ModulationRF
system
BTS13 kbps 22.8 kbps 270.8 kbps