scope:

This standard provides transmission performance objectives for point-to-point voiceband data (VBD), voice-grade special access (VGSA) services covering the bi-directional path between an exchange carrier's (EC) end-user network interface (NI) and an interexchange carrier's (IC) point of termination (POT) (see figure 1). Throughout the document, the term, VGSA, implies point-to-point VGSA. Moreover, throughout the document, for consistency with previous standards, the designation, exchange carrier (EC), is used, but is intended to include all providers of special access services.

Transmission performance objectives for multipoint services require further study and therefore are not covered. Within this document, the term, IC, refers to interexchange carriers and any other connecting entities. Both originating (access) and terminating (egress) special access services are covered, and are referred to in this standard simply as special exchange access. VGSA is an element of voice-grade special services similar to that provided in what has historically been referred to as private line, leased line, or private network services.

The transmission performance objectives specified in this standard are based on end-user needs and applications, characteristics of voiceband data modems, and capabilities of evolving telecommunications networks. A discussion of end-user application needs, and the relationship of these needs to voiceband channel transmission performance, can be found in annex A. The objectives are considered to be reasonable criteria for circuit acceptance and restoral¹⁾. Actual in-service performance will depend on the initial values and variance of parameters, not the objectives alone. Therefore, the objectives may not be met on all circuits or on any particular circuit at all times. In such cases, the carriers and end-users should work cooperatively to achieve a level of performance within transmission maintenance limits specified by the EC. This standard does not preclude the possibility of separate negotiations between carriers to implement other levels of service.

Purpose

This standard is intended to be used by the telecommunications industry to cooperatively provide service quality appropriate for end-user application needs. The use of this standard helps to ensure the ICs about the quality of the VGSA services and enable ECs to implement and maintain VBD-VGSA network arrangements. This standard is a product of a number of considerations. Among them are customer service needs, network architectures, the technical capabilities of transmission and equipment systems, as well as operational and economic concerns. See annex B, Bibliography, for examples.

Application

Architecture

This document specifies transmission performance objectives that are based on end-user application needs. This standard assumes that the emerging ITU-T V.34 modem recommendation operating at signaling data rate of 19.2 kbit/s is the most demanding application need²⁾. Many network architectures and facility and equipment combinations can be used to achieve these objectives. Generally, circuits will be routed through one or more serving offices between the POT and the NI.

Facilities

The actual facilities and equipment used to provide VBD-VGSA will vary, depending on the service provider, the geographical location, the embedded plant, economic considerations, and many other factors. Circuits may utilize digital facilities, analog facilities, or combinations of the two. Although the vast majority of the facilities in modern telecommunication networks are digital, cable extensions are commonly used from the NI to the first carrier system channel bank and between tandemed carrier systems. It is recommended that if cases arise that have not been addressed in this standard, the EC and the IC cooperate to negotiate needed performance.

Low-bit-rate-encoding techniques such as described in American National Standard for Telecommunications - Digital processing of voice-band signals - Algorithms for 24-, 32-, and 40-kbit/s adaptive differential pulse-code modulation (ADPCM), ANSI T1.303-1989, have a severe impact on the transmission of high data rate VBD. A separate standard, American National Standard for Telecommunications - Network performance tandem encoding limits for 32 kbit/s adaptive differential pulse-code modulation (ADPCM), ANSI T1.501-1988, provides tandem encoding limits and guidelines for functional usage of 32 kbit/s ADPCM. Therefore, the performance effects of ADPCM are not covered in this document. In order to ensure satisfactory VBD transmission performance, the recommendations of ANSI T1.501 should be followed in addition to meeting the objectives specified in this standard.

NOTE - Existing standard measurement methods in American National Standard for Standard methods and equipment for measuring the transmission characteristics of analog voice frequency circuits, ANSI/IEEE 743-1984 (R1993), do not reflect the effects of ADPCM on VBD performance. As part of a project to revise that standard, new measurement methods are currently being developed to identify low bitrate encoding and to provide better measures of signal-to-total distortion and intermodulation.

Specification groups

In order to accommodate a wide range of end-user needs and applications, three groups of specifications, referred to as Group 1, Group 2, and Group 3 are provided. Group 1 is intended to meet the needs of modern high data rate modems and the most critical applications (e.g. V.34 at 19.2 kbit/s),²⁾ and has stringent objectives for signal-to-noise ratio and intermodulation distortion. Group 2 is intended for lower data rate modems and less critical applications, and is specified in two tiers. Tier 1 has more stringent objectives for attenuation distortion and envelope delay distortion than Tier 2 and is intended for modems having compromise linear equalization. Tier 2, on the other hand, is intended for modems having adaptive equalizers. Group 3 objectives are the least stringent but are still adequate for many less demanding end-user applications. The Group 3 specifications are intended to be the minimal objectives for point-to-point VBD-VGSA.

Measurements

This document provides objectives for a set of analog parameters that relate to VBD-VGSA transmission performance between the POT and the NI. The numerical values contained in this standard include the contributions of digital-to-analog (D/A) conversion at a digital POT as found in commonly used channel banks. The numerical values, however, do not include the contributions of impairments caused when digital loss is placed in the measurement path to test terminations of digital equipment. For further information on the effect of digital loss pads, see annex B of American National Standard for Telecommunications - Network performance transmission specifications for switched exchange access network, ANSI T1.506-1990. As discussed below, measurements can be made using analog or digital methods. Furthermore, under specific conditions, measurements can be made at other interfaces.

Measurement method - Analog

At an analog test access point, the parameters can be measured directly using methods defined in ANSI/IEEE 743. At a digital POT, using analog measurement methods, objectives in this standard apply to measurements that include encoding or decoding with a 4-wire D4 channel bank, or equivalent, except that its contributions to the loss measurement error shall be less than 0.1 dB (see Digital Channel Bank Requirements and Objectives, annex B, Bibliography.)

Measurement method - Digital

Measurements of analog parameters that are performed directly on a digital bit stream at the DS0 rate (64 kbit/s) may use a technology that does not contribute significant degradations that are associated with the D/A conversion process. It follows that the results of such measurements should generally be better than or at least equal to those that would be obtained after such a conversion. Therefore, the objectives in this document can be used as bounds or guidelines for digital measurements. This standard does not provide guidelines for discounting the effects of D/A conversions on the analog objectives. Note that the digital measurement method is not covered in ANSI/IEEE 743, but is commonly used.

Measurement interfaces

The objective values contained in tables 1, 2, and 3 apply between the NI and the POT. However, for practical reasons, measurements are sometimes made at other interfaces. Such measurements must be corrected to compensate for the contribution between the actual measurement interface and the POT or NI.

¹⁾ Parameters measured for circuit acceptance and restoral will depend on the network provider's service offerings and may not include all the parameters specified in this standard.

²⁾ Note that V.34 might be specified to have a maximum data rate as high as 28.8 kbit/s.