ATIS T1.TR.12 1991 Edition, September 1, 1991
Application Guidelines for use of the DS 1 Extended Superframe Format Data Link
Background of Extended Superframe Format for DS1
Requirements for the Extended Superframe (ESF) Format were developed as part of the Carrier-to-Customer Installation – DS1 Metallic Interface standard, ANSI T1.403-1989. The ESF description is referenced in a variety of Digital Hierarchy standards.
The DS1 signal is transmitted at 1544 kbit/s. When formatted, it is blocked into 193-bit frames repeated every 125 microseconds. The first bit of the frame is designated the F bit, and corresponds to an eight kbit/s channel. In the ESF, three functions are interleaved into the F-bit channel. Two kbit/s are used for a repetitive pattern used for terminal equipment framing purposes. Another two kbit/s are used for a block error check using a CRC-6 code. The remaining four kbit/s form a communications channel, or facility data link.
With typical network architecture the date link may be discontinuous relative to the DS1 payload, and end-to-end continuity of the data link cannot be guaranteed. In a switched or reconfigurable DS1, reframing of the DS1 ESF may cause momentary interruptions during circuit connection and disconnection. The effect of these interruptions may be performance data in a number of PRMs that is not reflective of the facility performance outside the periods of interruption.
In designing the ESF, it was acknowledged that the standardization effort involving the data link would be focused on maintenance of the transmission quality and availability of the DS1 facility. It was further recognized that the same ESF format would be used in a variety of applications, some crossing Customer Installation (CI)-to-network interfaces, some totally within one or more carrier networks, and some totally within customer installations. The signals and functions included in the standard for ESF include only the minimum essential subset of functionality needed for all applications, not the totality of features that might be desirable for one or another of the applications. In writing the standard in that way, the working group recognized that additional roles for the data link would likely be identified in the future. This report summarizes the views of the working group as to the best way to accommodate other uses of the data link without compromising the standard functionality.
This Technical Report may be used with the ANSI T1.403-1989 standard for Carrier to Customer Installation – DS1 Metallic Interface, as well as with the ISDN Primary Rate – Customer Installation Metallic Interfaces Layer 1 Specification standard, ANSI T1.408-1990. The report also complements those sections of ANSI T1.102-1987, Digital Hierarchy – Electrical Interfaces, and ANSI T1.107-1988, Digital Hierarchy – Formats Specifications concerning DS1 digital signals with the Extended Superframe Format.