AT&T DS1/DMi/ISDN-PRI manual Voice Voice-grade data Digital data, 8INTRODUCTION

1-8INTRODUCTION

Voice

Voice-grade data

Digital data

Analog voice date is encoded into 64K-bps pulse-code modulation (PCM) samples using an encoding technique known as the Mu-255 law. Details of this encoding technique are not given here. The important point is that each DS1 channel can transport PCM-encoded 64K-bps voice signals.

Voice grade data is also called PCM Data and voiceband analog data. Modems receive digital data, convert the data to an analog voiceband signal, and transmit it over analog phone lines. Whenever the modem connects to a digital switch, the modem analog output signal undergoes the same PCM encoding process as voice. Therefore, the modem output is termed voice-grade data.

This two-step process of first converting digital data to analog data and then to 64K-bps PCM data is necessary for transmitting data on DS1/DMI facilities that are either administered for robbed-bit signaling (RBS) or routed over a combination of digital and analog sections.

An attribute of voice-grade data is that signaling information can be inserted into the least-significant bit (LSB) of the PCM words without destroying the data. This capability cannot be done for those DS1 facilities that transmit digital data (described below).

Voice-grade data calls placed over DS1/DMI facilities, which use RBS, require the use of a modem to permit this two-step conversion. Actually, the modem pool (modem-to-switch) interface does this conversion.

NOTE: Voice-grade data is limited to speeds provided by the modem (typically 19.2K-bps or less). However, DS1 channels accommodate data at rates up through 56K-bps.

Digital data operates at 64K-bps and 56K-bps rates. Computers and data terminals generate digital data. The computer ports and data terminals interface to data modules. Data modules transmit the digital data (in digital form) to the switch. When this digital data is switched into a DS1/DMI channel without any intervening processes (such as modem pool conversion or embedded signaling information), the channel is said to provide 64K-bps data capability (also known as mode-1 data). The important point here is that when a DS1/DMI signal consists of digital data, every bit that goes in at one end must come out the other end unaltered; otherwise, the data would be destroyed.

Data modules support 56K-bps digital data over robbed-bit facilities.

NOTE: Although the digital data channels transmit synchronous 64K-bps data, computer ports and data terminals do not typically generate digital data at this rate. Data modules provide data rate adaptation (modes 0, 1, and 2) and generate nulls or fill characters (as required) for maintaining the 64K- bps data rate.