1. Downstream Power Back Off (DPBO)
The Problem is:
Service loss at mixed networks.
At mixed networks a part of subscriber lines are directly connected to the Exchange and others to the local DSLAM-s.
Serious crosstalk problem and service loss may occur if the local DSLAM-s work with full power
because in that case there is a considerable level difference between the directly connected and local lines at the flexibility point where the lines are adjoining.
Low power
Service Loss
Exchange
Exchange side cable
Remote side cable
Cabinet
FEXT
Full power
Flexibility point
The Solution is:
Transmitting power reduction at the local DSLAM-s in cabinet
The generally accepted method to protect the directly connected customers is the use of spectrum shaping or other words DPBO (Downstream Power Back Off)
ELEKTR NIKA

2. Exchange Side Electric Length (ESEL)
The amount of DPBO changes with the electric length of the exchange side cable and the frequency. Electric length=attenuation at 1 MHz. Here it is named as ESEL (Exchange Side Electric Length).
The effect of spectrum shaping is illustrated in the next figure in case of different ESEL-s
There is no power reduction beyond a given frequency. What is the reason of it?
In case of long Exchange Side cable the usable frequency range is limited for the directly connected customers because beyond a Maximal Usable Frequency (MUF) the level falls below the Minimum Usable Signal (MUS). For frequencies over MUF the Exchange cannot allocate any bits any more, there is no need for power reduction.
In this way higher data rate can be reached on lines connected to the local DSLAM
without disturbing the lines directly connected to the exchange
ELEKTR NIKA

3. Programming of Local DSLAM-s
The key points in local DSLAM programming is the prediction of the attenuated transmitter signal. The predicted attenuation should be calculated by means of a ITU-T recommended cable model consisting of four parameters:
■ ESEL (Exchange Side Electric Length)
■ A, B and C constants
The next figure shows the Power Spectral Density (PSD) Mask in case of DPBO
ELEKTR NIKA

4. Effect of DPBO on the Local Connections
Remote Side Electric Length (RSEL)
Effect of DPBO on the Local Connections
The value of RSEL is equal with the measured attenuation of remote side cable at 1 MHz
The maximum electric length of a remote side line depends on two parameters:
■ ESEL (As the PSD mask of Local DSLAM is determined by ESEL)
■ DATA RATE required
The maximum electrical length of a remote side line is shown in the next figure as a function of:
■ ESEL (in dB on the horizontal axis).
■ ADSL 2+ DATA RATE (In kbit/s marked by colors)
ELEKTR NIKA

5. ELQ 2+ Provides Excellent DPBO Tools
Qualification of Local Lines when DPBO is Used
The maximum achievable data rate of a local ADSL 2+ line depends on:
■ ESEL (As the PSD mask of Local DSLAM is determined by ESEL)
■ Attenuation of the remote side cable
ELQ 2+ provides ESEL, MUS and data rate dependent templates for ITU-T specified ADSL 2 systems.
ESEL Measurement for the Programming of DSLAM-s
According to the ITU-T recommendations the ESEL should be measured up to 120 dB.
Due to the noises the traditional loss measuring methods are not applicable over 80 dB.
ELQ 2+ provides special test method for ESEL measurement up to 120 dB
ELEKTR NIKA

6. THANK YOU FOR YOUR ATTENTION!