When you are servicing customers at the last 10% of the reach for any bandwidth service there are bandwidth stealers in the plant that need to be addressed. I spend a lot of time with these issues when the capacity exceeds 80%.
 
The choices the DSL modem make can also be either conservative, where the modem chooses to allocate fewer bits per bin than it possibly could, a choice which makes for a slower connection; or less conservative in which more bits per bin are chosen in which case there is a greater risk case of error should future signal-to-noise ratios deteriorate to the point where the bits-per-bin allocations chosen are too high to cope with the greater noise present.

This conservatism involving a choice to using fewer bits per bin as a safeguard against future noise increases is reported as the signal-to-noise ratio margin or SNR margin. The telephone exchange can indicate a suggested SNR margin to the customer's DSL modem when it initially connects, and the modem may make its bits-per-bin allocation plan accordingly.

A high SNR margin will mean a reduced maximum throughput but greater reliability and stability of the connection. A low SNR margin will mean high speeds provided the noise level does not increase too much, otherwise the modem will drop.

A customer sent me an email explaining that their company has decided to use figure 8 non shielded (109 messenger) in applications where the span is too long for a traditional drop and anchoring and guying is a challenge for cable and strand. This would typically be a service drop scenario.

He stated that their company has worked hard over the years to eliminate all non shield plant (with the exception of aerial drops). He is of the opinion that if the span is too long for a traditional span clamp to hold, than the span is too long for a non shielded facility. Their DSL penetration rate is around 66%. It seems to him that this new practice (while it may make it easy to build) creates potential problems.

I have tested a multitude of circuits where unshielded drops and cables decrease the bandwidth from 1 to 2 megabits in spans from 100 to 300 feet. The longer the unshielded drop or cable the less the maximum achievable rate is obtained and SNR drops drastically. I also have seen a decrease in bandwidth when binding post washers are corroded or 2 wires are placed between 2 washers.

These issues are solved by using shielded drops and cables plus good housekeeping when placing wires on binding posts. A UDC connection in aerial terminals or splicing the drop to the cable pair is more stable than binding posts on that last 10% of the reach.

Please email or call me with your results: 831.818.3930 or dmccarty@mccartyinc.com.

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