Instruction manual

FEATURES AND SERVICES
Signaling Types
Signaling types refers to signaling used by DS1 on the T1 facility. Two signaling types can be
used in System 25, Robbed-bit and Common Channel.
Robbed-bit signaling (RBS) robs or replaces the least significant bit (of each DS0 channel’s
8-bit word in every 6th frame) with that channel’s signaling information. For D4 frame format,
that means the 6th and 12th frame carry RBS. For ESF frame format, this means the 6th,
12th, 18th, and 24th frames carry RBS. Robbed-bit signaling can not be used when the DS1
carries 64 kbps data and therefore limits the channel’s use to voice and analog voiceband
data applications.
Common Channel signaling places the signaling bits for DS0 channels 1 through 23 into the
8-bit word of the 24th channel. This restricts the DS1 from using the 24th channel for voice
or data transmission.
This signaling type is known as Digital Multiplexed Interface Bit
Oriented Signaling (DMI-BOS). Common channel signaling is acceptable when DS1 is used
in a data application.
A Remote Multiframe alarm occurs when Common Channel signaling is used and the far end
is unable to synchronize to the multiframe pattern in its incoming signal.
Line Coding Formats
The DS1 signal consists of a continuous stream of 1’s and 0’s. The bit stream is encoded
into bipolar pulses for transmission purposes. Actually, only the 1’s create bipolar pulses
while 0’s are represented as the absence of a bipolar pulse. The line coding formats serve
to guarantee that the bit stream maintains a minimum number of 1’s. In the T1 carrier
system, this is known as the ones-density requirement. A Blue alarm occurs when an all
ones pattern including the framing bits is received.
There are two line coding formats, Zero
Code Suppression (ZCS) and Bipolar 8 Zero Suppression (B8ZS).
The Zero Code Suppression line coding format monitors the transmit bit stream and forcibly
changes one of the zeros to a 1 when a string of 8 or more zeros are transmitted. When
RBS signaling is used, the overwritten bit has no effect on voice and voiceband data.
However, when common channel signaling is used, the ZCS format destroys digital data.
The Bipolar 8 Zero Suppression line coding format allows strings of 8 zeros in the DS1
signal, but encodes them into a unique binary sequence (known as a bipolar violation). These
special sequences are then detected at the receiving end and converted back to the correct
sequence.
Many of the network interface and transmission equipment devices currently
installed will not pass bipolar violations (while some will correct the violation). B8ZS offers no
advantage for voice or voice grade data, and must be used on unrestricted digital data
applications.
Digital Network Synchronization
The term synchronization refers to an arrangement where digital facilities operate from a
common clock. Whenever digital signals are transmitted over a communication link, the
receiving end must be kept in step or synchronized with the transmitting end in order to
receive the digital signals.
This is referred to as link synchronization.
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