Asynchronous Digital Subscriber Line (ADSL) modems


ATU-C stands for ADSL Termination Unit - Central (Office) also called the Digital Subscriber Line Access Module (DSLAM). ATU-R stands for ADSL Termination Unit - Remote better known as the ADSL Modem. These two devices are the ADSL devices that sit on either side of the telco copper loop. In other words, the ATU-C/R are fancy words for ADSL modems. The ATU-R is a self-contained box suitable for customer use, including an AC adapter for powering. The ATU-C is packaged into Central Office DSLAM equipment which combines the Voice Channel and the Network data into the modulated ADSL stream.

Typical ADSL modem installation

Central Office DSLAM


Shares the local loop with the voice channel.

  • upstream: 25.875 khz to 138 kHz
  • downstream: 138 kHz - 1104 kHz


Uses Discrete Multi-tone (DMT) Modulation

  • 256 channels of 4 khz QAM:
  • Channels start at 0 Hz
  • channel 0 is not used
  • channels 1-6 (in the 4 to 25 kHz range) are used as a guard band between the voice channel and ADSL

Uses QAM2 to QAM32768. QAM2 allows 2 bits per tone. QAM32768 allows 15 bits per tone. The Signal to Noise Ratio (SNR) Determines which modulation scheme will be used. Has a BER Bit Error Rate 1 in 10^7 = 10^7

ADSL Upstream bandwidth: divided into 25 x 4.3125 kHz subchannels

Downstream Bandwidth divided into 224 x 4.3125 kHz subchannels

Channel Capacity

Bit-loading is adaptive, varies from 2 to 15 bits per tone depending on the relative noise of each carrier. When high noise levels are detected in a given subchannel, the DMT modem can shut down a particular subchannel altogether.

Two modes (set in modem):

  • Mode 1: fixed transfer rate
  • Mode 2: rate adaptive

Initial Startup

  1. The DSL Transceiver does a self-test.
  2. The DSL Transceiver checks the Ethernet (Firewire or USB) connection between the DSL Transceiver and the computer.
  3. The DSL Transceiver then synchronizes with the DSLAM.
  4. Tests the connection to optimize the performance according to the characteristics of the line in use.

External, or stand-alone modem units have an indicator labeled "CD", "DSL", or "LINK", which can be used to tell if the modem is synchronized. During synchronization the light flashes; when synchronized, the light stays lit, usually with a green color.

ADSL Filters

Since both the network data and voice are shared on the line from the Central Office (CO), a low pass filter is used to separate the voice component from the ADSL modulation. If you connected a phone to an unfiltered ADSL connection, you would hear a lot of background noise.

ADSL filter for POTS phone


PC interface: Ethernet, USB, Firewire.

DSLAM side: Digital Subscriber Line Access Module, PPPoE (Ethernet over PPP), TCP/IP Encapsulated in PPP (Point to Point Protocol)



  • Up to 12 Mbps downstream, 1 Mbps upstream
  • Reach Extended ADSL2
  • Up to 6 Mbps downstream, 500 kbps upstream
  • Longer distance


  • Up to 24 Mbps downstream, 1 Mbps upstream
  • 2.2 Mhz bandwidth

  • Up to 52 Mbps downstream, 3 Mbps upstream
  • Or 13-25 Mbps symmetrical
  • Up to 1000 m only

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Copyright July 2013 Eugene Blanchard