DSL Interference

DSL Interference

ADSL, POTS (Plain old telephony service), and HomePNA shared the same copper wire coming into your home (through a Frequency Division Multiplexed (FDM) topology).  The ARRL EMC Committee now describes some of the various flavors of DSL and the frequency spectrum they use.

ADSL initially existed in two flavors- named CAP (Carrierless amplitude phase modulation- which is a non-standard variation of  QAM) and Discrete MultiTone (DMT). CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs at the time. However, DMT was chosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called G.dmt and G.lite respectively). Therefore all modern installations of ADSL are based on the DMT modulation scheme.

ADSL and POTS (Plain old telephony service) share the same copper wire coming into the home. The same inside telephone wiring may simultaneously carry POTS, ISDN or ADSL services, and even “in-home” networking signals [HomePNA] whereby the signals for each of these services occupy spectrum that is different in frequency than any of those services. The figure below assumes a system sharing the phone-line media in a Frequency Division Multiplexed (FDM) topology. In this arrangement Plain Old Telephone Service (“POTS”) exists in the 0-4 kHz region, the xDSL service is present from 25kHz to approximately 2.2MHz (depending on the definition of “x”), and the HPN spectrum occupies 4.75-9.25 MHz for HomePNA (Version2) technology. VDSL services, which use the 26 kHz to 10 MHz region, are also shown.

There are several flavors of xDSL (fill in the letter of the day for “x”). ADSL access utilizes the 1.1 MHz band, while ADSL2+ utilizes the 2.2 MHz band.  Discrete Multi-Tone (DMT), the most widely used modulation method, separates the ADSL signal into 255 carriers (bins) centred on multiples of 4.3125 kHz. DMT has 224 downstream frequency bins and up to 31 upstream bins. Bin 0 is at DC and is not used. When voice (POTS) is used on the same line, then bin 7 is the lowest bin used for ADSL.

The center frequency of bin N is (N x 4.3125) kHz. The spectrum of each bin overlaps that of its neighbors: it is not confined to a 4.3125 kHz wide channel. Up to 15 bits per symbol can be encoded on each bin on a good quality line.

With standard ADSL the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz – 1104 kHz is used for downstream communication. Each of these is further divided into smaller frequency channels of 4.3125 kHz. During initial training, the ADSL modem tests which of the available channels have an acceptable ability to carry data (a lot of reason why it couldn’t but….) and allocates or de-allocates these sub-channels based on that analysis. De-allocation reduces the effective usable bandwidth and reduces throughput of the link.

The frequency layout can be summarized as:

  • 0-4 kHz, voice.
  • 4-25 kHz, unused guard band.
  • 25-138 kHz, 25 upstream bins (7-31).
  • 138-1107 kHz, 224 downstream bins (32-255)

Typically, a few bins in ADSL around 31-32 are not used in order to prevent interference (“guard band”) between upstream and downstream bins either side of 138 kHz.