AT&T DS1/DMi/ISDN-PRI manual 2INTRODUCTION

1-2INTRODUCTION

The bit stream of the DS1 protocol (1s and 0s) is transported over a DS1 line in a special way. The 1s are represented as alternating positive and negative pulses (called an alternate mark inversion (AMI) or bipolar signal); the 0s are represented as the absence of pulses. Two formats known as a DS1 line-coding formats can be used for encoding 1s into the bipolar bit stream. The DS1 channels, signaling, framing, and line-coding formats are all described in this section.

Two applications of DS1 service, known as DMI with bit-oriented signaling (DMI-BOS) and DMI with message-oriented signaling (DMI-MOS), are actually two different types of DMI interfaces. The term DMI-BOSis used when a DS1/DMI is optioned to provide BOS and when the interface is used to transport:

a.Data modes 0, 1, and 2 of 64K-bps digital data between the switch and a BOS-compatible computer (also mode 3 if calls are circuit switched)

b . Both 64K-bps data and voice between two customer-premises switches

c . Both 64K-bps data and voice between customer-premises switches and the public network

The term DMI-MOSis used when a DS1/DMI is optioned to provide message-oriented signaling and when the interface is used to transport:

a.64K-bps digital data (modes 0 through 3) between the switch and a MOS-compatible computer over private network facilities

b.64K-bps digital data between two customer-premises switches

Both DMI-BOS and DMI-MOS have the same channel structure, framing formats, and line-coding considerations, as well as metallic-cable considerations. Two significant differences between DMI- BOS and DMI-MOS are:

a.The way signaling information is encoded into the 24th channel

b.DMI-MOS bearer channels can transmit link-access procedure on the D-channel (LAPD) data (mode 3)

NOTE: The DMI-BOS and DMI-MOS are two separate, incompatible DS1 interfaces. Communication between the two is permitted by the switch interworking functions, which are described later.

A DMI uses 24 channels in a 23B + 1D arrangement. This means that a DMI uses 23 channels to carry either voice or data (called the bearer or "B" channels) and one channel to carry either data or signaling (called the data or “D” channel). The DMI is also the forerunner of the ISDN-PRI. The term ISDN-PRI,when used alone, refers exclusively to ISDN-PRI features or capabilities.

Over the past few years, ISDN has emerged as a powerful driving force in the evolution of business communication products and services. The increased demand for products that contain internationally sanctioned (CCITT) standard interfaces exists because of:

● Widespread confusion in the market place about multiple vendor/multiple proprietary interfaces

● Growing customer dissatisfaction with proprietary equipment interfaces