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Hybrids explained.

The hybrid numbers reported in the log for the 5100b/4100 modems, »192.168.0.1/log.htm, indicate which hybrid of four choices the modem has chosen to complete its sync. A hybrid is an electrical circuit that acts as an analog echo canceller. It subtracts the locally transmitted signal (the upstream side) from the receive path. By using multiple hybrids that can be switched in or out, or combined, the modem can more closely match the impedance of various in-home wiring topologies and bridged tap conditions resulting in improved echo rejection.

The modem makes its signal to noise measurements for each of the four hybrids during the sync training period to determine the optimum hybrid for the loop conditions. Within a fraction of a second the best hybrid is chosen.

Although the switchable hybrid technology can significantly improve performance under bridged tap conditions, it cannot completely cancel out the adverse affects of bridged taps. The reason is that bridged taps also can cause impedance nulls in the middle of the downstream spectrum where the hybrid circuitry is not effective. These impedance nulls result in reduced bit loadings in those tone bins and greatly reduce the maximum attainable sync speeds.

The loop conditions detected by the hybrids.

The Texas Instruments AR5 was the multi-chip predecessor to the AR7 chipset that is used in the Siemens 5100b/4100 modems. The AR7 chipset has the four hybrids directly integrated into the chipset circuitry unlike the AR5 in which they had to be added to the modem motherboard as separate chips. The following direct quote is from a white paper on the Westell 2200 modem which uses the AR5 chipset:
The 4 hybrids optimize performance on the following loop topologies:

Hybrid #1 - Standard Hybrid: Straight Cable without bridged taps
Hybrid #2 - 0-1000' Bridged taps or in home wiring tap located 0 to 50' from modem
Hybrid #3 - 0-1000' Bridged taps or in home wiring tap located 50'-250' from modem
Hybrid #4 - 0-1000' Bridged taps or in home wiring tap located 250'-1000' from modem

Note that the explanations for hybrid numbers 3 and 4 have been swapped from their original order as published by Westell. Several lines of evidence including this thread: »[Line Problem] Cant upgrade but im close enough suggests that they were erroneously transposed in the white paper. It is unlikely that inside wiring could separate an in-home bridged tap 250'-1000' from a modem, but separation of 50'-250' seems possible.

So what could this mean about my line?

Note: The following information is not valid for the 4100b modem. The revision of the printed circuit board and components in the 4100b and/or the change to a7.00.02.00 firmware in the 4100b has modified the operation of the various hybrids with respect to the line conditions and impairments. Hybrid #3 seems to indicate normal line conditions on a 4100b modem.

The interpretation of hybrids depends on whether the modem is plugged inside or at the NID jack, so if your modem is reporting:

Hybrid #1 - The line is a straight loop and has no detectable bridge taps. This is the normal hybrid.

Hybrid #2 - Usually due to in-home wiring. This hybrid reading is commonly seen in newer homes with star topology wiring (multiple CAT5 wiring runs connected to the NID) without a POTS splitter. It can also be caused by older, daisy-chained, inside wiring if the modem is connected to a jack in the middle of the chain.

Hybrid #3 - May be caused by a bridge tap(s) outside of the building; however, there have been instances of a hybrid #3 bridged tap located within in-home wiring. Note: Inside hybrid #3 readings have also been caused by microfilters, POTS splitters, and alarm filters in the NID.

Hybrid #4 - May be caused by a bridge tap(s) on the loop.

Hybrid numbers 3 and 4 are relatively rare conditions, and if caused by a bridged tap, the line's attenuation would be increased and its maximum attainable speeds severely reduced. In one instance, an inside hybrid #3 bridged tap reduced the loop's potential speed down to 35% or less of its maximum attainable level.

What should you do if you have a DSL line problem?

The hybrid used by the modem should only be a concern if there are problems such as excessive CRC error rates, abnormally low maximum sync capacity, or low noise margins (below 8.0 dB). If the DSL line is problem free, then the hybrid used really doesn't matter.

If you are having a line problem (low sync speeds, sync instability, high CRC rates, etc.) AND the modem is reading any hybrid other than #1 in the log, you MUST run a NID test with your modem connected directly to the test jack to see if the hybrid number changes from the inside readings. See the following FAQ for additional information concerning NID tests: »AT&T Southeast Forum FAQ »How to check modem stats, synch rates & speeds from test jack at the NID.. A change in hybrid numbers indicates that the bridged tap condition is due to in-home wiring complexity and may need to be fixed by a POTS splitter-home run installation if the inside wiring is reducing sync speeds or causing line errors.

If the hybrid number is the same at the NID as inside, then the line condition is outside the residence. In some rare instances, the modem will report a hybrid #3 at the NID due to other line conditions (wire gauge changes, splices?) and not due to bridged taps. These non-bridged tap, hybrid #3 readings are not accompanied by any other line problems.

Once you have completed the NID test, the techs in the AT&T Direct forum should be contacted about your line problems. You can post your hybrid numbers and line stats for both the inside and NID tests there. They will verify the modem's readings and possibly schedule a tech visit.

If the modem reports alternating use of Hybrid #1 and #2 between repeated sync drops, this would indicate that your line has an intermittently failing POTS splitter: »[General] HYBRID 2 and HYBRID 1. When a POTS splitter fails, the modem will see the home run as an in-home bridged tap and report Hybrid #2 if it is even capable of syncing.

Feedback received on this FAQ entry:
  • Quote: " If the modem reports alternating use of Hybrid #1 and #2 between repeated sync drops, this would indicate that your line has an intermittently failing POTS splitter: [General] HYBRID 2 and HYBRID 1. When a POTS splitter fails, the modem will see the home run as an in-home bridged tap and report Hybrid #2 if it is even capable of syncing. " Another thought also is if the end user is or only has one main line inside the premise and goes right to a filter. A failing filter will look like a bridge tap in that configuration as well. Though a short distance bridge tap it will report one as such.

    2013-05-30 22:05:10 (David See Profile)



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by d_l See Profile
last modified: 2009-04-06 14:20:04