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

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

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

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

ELQ 30A 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 30A+ provides ESEL, MUS and data rate dependent templates for ITU-T specified ADSL 2+ and VDSL 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 30A+ provides special test method for ESEL measurement up to 120 dB ELEKTR NIKA

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