1-2
INTRODUCTION
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-BOS is 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-MOS is 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 āDā 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:
āWidespread confusion in the market place about multiple vendor/multiple proprietary interfaces
āGrowing customer dissatisfaction with proprietary equipment interfaces