SHDSL (Single-pair High-speed Digital Subscriber Line) was developed as a meltdown of several symmetric DSL technologies (HDSL, SDSL, HDSL-2) producing, as a result, one single internationally recognized industry standard.

Unlike ADSL which is fine-tuned to the needs of a home user and can download enormous amounts of data but is limited when you need to upload any large amount of data, SHDSL has no problems with “two-way” Internet traffic. With the transfer rates of up to 2.3 Mb/s, this makes it a perfect highspeed solution for medium to big enterprises, branch offices, as well as high-end residential users.

Q1: What is SHDSL?

A1: SHDSL stands for “Single-pair High-speed Digital Subscriber Line.” It was developed to be the convergence of symmetric DSL technologies (HDSL, SDSL, HDSL-2) into a single internationally recognized industry standard.

SHDSL has been designed to improve reach performance and Power Spectral compatibility with other DSL technologies (ADSL, etc.).

User (payload) data rates are identical in both upstream and downstream directions (symmetric technology) and vary from:

  • 192 Kb/s to 2.3 Mb/s in two-wire (one pair) mode,
  • 384 Kb/s to 4.6 Mb/s in four-wire (two pair) mode.

Q2: Is SHDSL a standardized technology?

A2: Yes, SHDSL is an International Telecommunication Union (ITU) Recommendation. It is described in G.991.2. It relies on G.994.1 (Handshake procedures for Digital Subscriber Line transceivers) for the pre-activation phase.

Q3: What about POTS, ISDN, splitters, etc.?

A3: In ADSL part of the bandwidth is reserved for POTS or ISDN.
In SHDSL, this is not the case; all the bandwidth is used, so the line is totally dedicated to the SHDSL service. There is, therefore, no need for splitters.

Q4: What is “EOC”?

A4: EOC = Embedded Operations Channel.
The SHDSL DSLAM, the CPE, and the repeaters can communicate through this channel.

For instance, the DSLAM can send the following types of EOC requests:

  • inventory, to get information (Manufacturer, release, etc.), about the remote CPE,
  • enable/disable network loopbacks on the CPE,
  • ask the CPE to report statistics or failure information (SNR Margin, loop attenuation, number of CRC errors, etc.).

The EOC messages are defined in G.991.2 to provide interoperability.

Q5: What is SHDSL “auto-adaptive rate” mode?

A5: Normally in SHDSL, the line rate is fixed by configuration at the DSLAM side. For instance : 2048 Kbits/s. So it links at that configured rate, or it doesn’t link at all. This is because SHDSL’s target is more business services, replacement of E1 and T1 lines, and so on… where the rate is supposed to be known and guaranteed.

However, due to the strong “ADSL habits” in the field, some manufacturers propose an “auto-adaptive rate” mode for SHDSL. In this case, the DSLAM and the CPE try to link up at (quite) the best possible rate for the current line conditions.