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International Regulatory BodiesITU-T
Is to fulfil the purposes of the Union relating to telecommunication standardization by studying
technical, operating and tariff questions and adopting Recommendations on them with a view to
standardizing telecommunications on a world-wide basis.
Performance Objectives and availability objectivesDimensioning of network connection is based on the required availability objective
and performance
Dimension a network must meet the standard requirements recommendations by ITU
The performance objectives are separated from availability objectives
Factors to be considered
radio wave propagation
hardware failure
Resetting time after repair
Frequency dependant interference problems
ITU-T Recs for Transmission in GSM Net
All BTS, BSC and MSC connections in GSM network are defined as multiples of the
primary rate if 2 Mbps,
ITU-T Rec G.821 applies as the overall standard for GSM network.
ITU-T Rec G.826 applies for SDH.
The ITU-T Recommendations (Standards)
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The ITU-T target standard are based on two
recommendations:
ITU-T Recommendation G.821,intended for digital connection with
a bit rate of 64 kBit/s. Even used for digital connection with bit rates
higher than 64kBit/s. G.821 will successively be replaced by G.826.
ITU- T Recommendation G.826, used for digital connection with bit
rates of or higher than 2,048 kBit/s (European standard) or 1,544
kBit/s (USA standard).
The main difference between G.826 and G.821 is that
G.826 uses Blocks instead of bits in G.821
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ITU-T G.821 some definitions
HRX : hypothetical Reference Connection
This a model for long international connection, 27,500 km
Includes transmission systems, multiplexing equipment and switching
HRDP: Hypothetical Reference Digital Path
The HRDP for high grade digital relay systems is 2500 km
Doesnt include switching
HRDS: Hypothetical Reference Digital Section
It represents section lengths likely to be encountered in real networks
Doesn't include digital equipments, such as multiplexers/demultiplexers.
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ITU-T G.821 some definitions (course contents)
SES : Severely Errored Seconds
A bit error rate (BER) of 10-3 is measured with an integration time of 1 second.
DM : Degraded Minutes
A bit error rate (BER) of 10-6 is measured with an integration time of 1 minute.
ES : Errored Seconds
Is the second that contains at least one error
RBER: Residual Bit Error Rate
The RBER on a system is found by taking BER measurements for one month
using a 15 min integration time, discarding the 50 % of 15 min intervals whichcontain the worst BER measurements, and taking the worst of the remaining
measurements
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Digital Systems and BER
Performance of digital transmission system can be
evaluated by BER, Bit Error Rate
Telephony BER degradation versus audible degradation:
10-6: Noise not audible
10-5: Barely audible
10-4: audible, understandable
10-3: disturbing
More than 10-3: sync loss, link loss
Data and in particular multimedia media application
require a very low BER
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ITU-T G.821 some definitions
The system is considered unavailable when one or both of the
following conditions occur for more than 10 consecutive seconds
The digital signal is interrupted
The BER in each second is worse than 103
Unavailable Time (UAT)
Begins when one or both of the above mentioned conditions occur for 10
consecutive seconds
Available Time (AT)
A period of available time begins with the first second of a period of 10
consecutive seconds of which each second has a bit error ratio (BER) better than
10-3
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ITU-T G.821 Availability
Route availability equals the sum of single link
availabilities forming the route.
Unavailability might be due to
Propagation effect
Equipment effect
Note: Commonly used division is to allocate 2/3 of the allowed total
unavailability to equipment failure and 1/3 to propagation related
unavailability
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ITU-T G.821 Performance Objectives
SES : Severely Errored Seconds
BER should not exceed 103 for more than 0.2% of one second intervals in any
month
The total allocation of 0.2% is divided as: 0.1% for the three classifications
The remaining 0.1% is a block allowance to the high grade and the medium grade
portions
DM : Degraded Minutes
BER should not exceed 106 for more than 10% of one minute intervals in any
month
The allocations of the 10% to the three classes
ES : Errored Seconds
Less than 8% of one second intervals should have errors
The allocations of the 8% to the three classes
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