Network Performance – Switched Exchange Access Network Transmission Specifications
|Publication Date:||1 January 1997|
This standard provides performance specifications for the analog transmission parameters of a switched exchange access network covering the bi-directional transmission path between an exchange carrier's (EC) end office (EO) and an interexchange carrier's (IC) point of termination (POT) (see figure 1). Within this document the term, IC, refers to interexchange carriers and any other connecting entities. The transmission parameters included are specified in clauses 4 and 5. (Absolute round trip delay guidelines are offered for the transmission path between the NI and the POT.) At this time, transient impairments affecting voiceband data are not included. Both originating (access) and terminating (egress) switched services are covered, and are referred to in this standard simply as exchange access services. For the purposes of this standard, originating (access) refers to the bi-directional transmission channel between the calling party's serving end office and an interexchange carrier POT; terminating (egress) refers to the bi-directional transmission channel between an interexchange carrier POT and the called party's serving end office.
The performance specifications are given in terms of acceptance limits, restoral limits, and immediate action limits. In-service parameter performance will be distributed statistically. The actual parameter performance is characterized by the parameter distributions and not by the limits alone. On this basis, although the parameter distributions are not requirements of this standard, network providers and equipment vendors should take into account the parameter distributions as well as the individual trunk limits in their plans and designs. The statistics in annex A reflect an estimate of the characteristics of the statistical distributions that are expected for a large number of channels.
This standard is intended to be used by the telecommunications industry to provide high-quality service to end users. The use of this standard helps to assure the ICs about the quality of exchange access services and to provide exchange carriers with the ability to implement and maintain the exchange access network. This standard is a product of a number of considerations. Among them are customer service perception, network architectures (see SR-TSV-002275 and GR-334-CORE. See annex C), the technical capabilities of transmission and switching systems (see TR-NPL-000037 and Analog voice and voiceband data transmission performance characterization of exchange access plant) and terminal equipment (as described in ANSI/EIA 470-A-1987), as well as operational and economic concerns. Some of these considerations are discussed in annex B.
This standard applies to switched exchange access provided by an EC to an IC between an EC end office and a POT. Switched exchange access service is an element of voicegrade service similar to that provided in what has historically been referred to as the "public switched network."
This network allows for direct and tandem switched connections between an end office and a POT. The standard provides one set of requirements between the end office and the POT that apply independent of routing of the call through the exchange carrier's network. Guidelines are provided between the POT and the EC access tandem (AT).
When the standard was developed, the arrangement was modeled after the capabilities of a service offering provided by many ECs and identified as Feature Group D. However, the standard is intended to be completely generic and may apply to future EC service offerings. For arrangements that differ from the original Feature Group model used to develop this standard, values for absolute delay and other performance parameters, such as post-dialing delay, need to be considered. Moreover, arrangements involving more than one EC between a POT and an end office were not evaluated in developing this standard. Such arrangements require further study.
This standard applies generally to digital facilities and limited-mileage cable facilities. (In all cases, guidelines are not part of ANSI T1.506.) Specifically, it covers channels provided by the following facilities and combinations:
a) 64-kbit/s pulse code modulation (PCM) systems using 8-bit encoding, with or without Robbed-Bit signaling, and a μ-law characteristic with μ=255. Examples are channels using newer-design digital systems (D2, D3, D4, D5 channel banks, or their equivalent) on wire, radio, or fiber media;
b) Digital facilities of any length with cable extensions that are 1 mile or less in length;
c) Cable that is 1 mile or less in length;
d) Any combination of the above facility types, with or without intermediate switching (total cable not to exceed 1 mile in length).
The limits reflect the transmission performance capabilities of
newer-design digital facilities (D2, D3, D4, D5 channel banks or
their equivalent). Excluded are the early-design 7-bit PCM systems
(such as D1 or the equivalent) and low-bit-rate-encodin
The development of this standard did not include the effects of systems such as digital cross-connect systems (DCS). Deployment of such systems may increase impairments such that exchange access service may not meet requirements.
This document specifies performance limits and allows for numerous architecture configurations and facility and switch combinations applicable to exchange access services. The actual facilities and switches used to provide service are a function of the existing telephone plant and may be influenced by the location of the POT. The existing telephone plant may vary by geographical area and between any pair of points within a geographical area (i.e., both analog and digital facilities may coexist between the same two points). Therefore, not all the specific facilities and combinations described in this standard might be offered or available between all point pairs within an EC area. It is recommended that if cases arise that have not been addressed in this standard, the EC and the IC cooperate to negotiate needed requirements.
This standard does not include the effects of low-bit-rate encoding techniques such as those employed in 32-kbit/s ADPCM as specified in ANSI T1.302-1989, and ITU-T Recommendation G.726. A separate standard, ANSI T1.501, deals with the performance impact and deployment guidelines for ADPCM. This exchange access standard is independent of ADPCM deployment. In addition to the requirements of this standard, the deployment guidelines contained in ANSI T1.501 are needed to control service performance.
This document provides specifications for a set of analog parameters that relate to voice and voiceband data network performance between an end office and a POT. 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. These numerical values, however, do not include the contribution of impairments caused when digital loss is placed in the measurement path to the test terminations of a digital AT or digital IC switch. For further information on the effect of digital loss pads, see annex B. As discussed in 126.96.36.199 and 188.8.131.52, measurements can be made using analog or digital methods. Further, under specific conditions, measurements can be made switch-to-switch.
Measurement method - analog
At an analog test access point, the parameters may be measured directly, using methodologies defined in ANSI/IEEE 743. At a digital POT, using analog measurement methods, specifications in this standard apply to measurements that include encoding or decoding with a 4-wire D4 channel bank, or equivalent, except that its contribution to the loss measurement error shall be less than 0.1 dB. (See Digital Channel Bank Requirements and Objectives. See annex C.)
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 specifications in this document can be used as bounds or guidelines for digital measurements. This standard does not provide guidelines for discounting the effects of digital-to-analog conversion on the analog specifications. Note that the digital measurement method is not covered in ANSI/IEEE 743, but is commonly used in remote measuring systems.
The specific recommended numeric values contained in the requirements clauses 4 and 5 in the standard apply between the end office and the POT. However, testing is usually most conveniently done switch-toswitch. If the IC switch is co-located with the POT, the standard may be used as the basis for switch-toswitch testing after accounting for the effect of the IC switch and office wiring, if any. If the switch and POT are not co-located, the switch-to-switch analog test results may need to be corrected by compensating for the contribution of the IC transmission facilities between the POT and the test position at the IC switch.