scope:

This standard addresses the steady-state operation of both Plain Old Telephone Service (POTS) and Digital Subscriber Line (DSL) services. Various tests and requirements are described to minimize mutual interactions in terms of continuous noise or signal degradation. Transient events such as impulsive noise and large current spikes introduced by POTS signaling or DSL state changes have not been considered and are left for further study. The use of In-Line filters complying with this standard does not ensure the absence of data errors introduced by POTS signaling.

Various digital data over voice systems require the signal isolation of the voice grade premises equipment from the telephone inside wiring in the premises. Existing premises wiring is being used for access to broadband networks, connection of high-speed data within the premises, and access to services such as the Internet.

Standardization of the In-Line filter benefits the users and providers of data services and voice services by aiding the interoperation and performance of these services. By standardizing on this filter, the industry benefits by simplifying the interoperation of CPE by defining only one filter. Maintenance is simplified and interoperability is enhanced for all vendors.

Splitterless deployment of DSL requires the shared use of in-premises wiring by POTS and broadband xDSL devices. In many situations, there may be interactions that can degrade POTS and broadband performance. Examples of such interactions include:

- Reduction of the desired DSL signal amplitude due to low offhook and/or onhook POTS device impedance within the DSL band.

- Nonlinear impedances of POTS devices, causing translation of the DSL signal energy to undesired frequency bands through intermodulation distortion (IMD) products.

- Ingress of DSL signals into POTS devices, causing audible POTS interference through nonlinear interactions inside the POTS device.

- DSL band impedance differences when POTS devices are onhook or offhook.

- Un-terminated wire stubs can cause deep nulls in the in-premises Phoneline network frequency band. These nulls can impair the operation of an in-premises Phoneline network.

Not all phones or other voice grade equipment will require the use of the filter. It will be up to the installation process to identify and suggest which devices require filter protection, as well as where to place the filters for optimum performance. It is important to keep the number of filters to a minimum, as each added filter impacts POTS performance in general.

The differences between an In-Line filter and a POTS splitter are defined by the placement of the filter more than the function. POTS splitters are designed to be used at the premises demarcation point and provide separation of the DSL signals from the voice band at a single location, while the In-Line filters are distributed at each voice grade terminal. From the POTS splitter into the premises, the DSL signals are contained on separate wiring. The In-Line filter on the other hand is designed to be placed "in line" with each voice grade terminal between the wall and the equipment. The DSL signals are delivered over the entire premises wiring, and voice grade equipment is protected by separate In-Line filters.

Alarm Systems that use the premises wiring are substantially different in nature from POTS phones (voice-grade equipment) and are beyond the scope of this document.

The purpose of covering up to 12 MHz is to characterize the filter to show stability in the design up to those frequencies. It is expected that there may be newly defined services in this range.

Analysis in the frequency range of 12 to 20 MHz indicates extreme difficulty in standardizing this range due to wide variations in inside house wiring, lengths, loads, and frequency, and this standard will therefore restrain its scope to 0 - 12 MHz.

The performance of POTS in the presence of series stacked filters (see 3.16.2) is beyond the scope of this standard.

Filters designed to this standard are intended to work with phones that meet CFR 47 Part 68 of FCC regulations. Phones that have characteristics significantly outside the norm may not function satisfactorily with a filter specified in this standard.

Performance Objectives

- Isolate POTS device load impedance from premises phone wiring to prevent attenuation and harmonic distortion of DSL signals.

- Prevent ingress of DSL signals into POTS devices, potentially causing audible interference through nonlinear conversion inside the POTS device.

- Minimize VB transmission effects for up to 5 installed filters and maintain VB signal levels such that the operation of VB devices currently deployed (e.g., V.90 modems) are not significantly impaired.

Extended Performance Objectives

Annex A describes an optional Figure of Merit (FOM) that extends performance to address broader applications, longer reach, higher data rates, and improved POTS service quality. The purpose of the FOMs is to enable the In-Line filter standard to better address the needs of technologies that cover the entire non-loaded resistance design service area. The Figure of Merit addresses increased DSL stopband attenuation.