Instruction manual

DS1 Facility Interface
DS1 Facility Interface
Description
The DS1 facility interface feature provides connectivity between System 25 and a T1 facility.
The DS1 facility is a transmission system to transport digital signals of the DS1 format. The
System 25 forms voice signals using the DS1 format. By using the DS1 format the following
advantages are provided; calls to other digital PBXs or central offices remain digital and
signals don’t need to be converted to analog for acceptance by the connecting tie or tandem
trunk. A T1 trunk can replace up to 24 analog loop-start, ground-start, direct-inward dialing,
tie, and off-premises station trunks and associated System 25 circuit packs. One T1 trunk
can transport a 1.544-Mbps signal consisting of twenty-four 64 kbps channels.
DS1 Format
The term DS1 format stands for Digital Signal 1.
Twenty-four Digital Signal 0 signals, each
operating at 64 kbps plus framing bits, are multiplexed forming a DS1 signal of 1.544 Mbps.
Each DS0 channel within the DS1 signal corresponds to a port or a trunk.
Framing Formats
To identify each DS0 channel within the DS1 signal, the DS0 channels are segmented into
blocks of 193 bits (known as a frame). A frame consists of twenty-four 8-bit plus one
framing bit which is inserted at the beginning of each frame. A framing bit appears every
193rd bit of the DS1 signal. Frames repeat at a rate of 8000 per second. Each frame repeats
DS0 channels 1 through 24 of the DS1 signal sequentially.
Two framing formats exist, D4 and Extended Super Frame (ESF). System 25 accepts either,
and the selection must match the framing format at the far end.
A D4 frame consists of 24 eight-bit time slots and one framing bit that alternates between a
one and a zero every other frame.
The receiver uses the framing information for
synchronizing to the start of each frame and to identify which frames contain signaling
information (see Robbed-bit signaling). The framing information repeats once every 12
frames, defining the D4 superframe.
The advantage of this framing format is that it is
universally used by all DS1 equipment.
The ESF framing format extends the 12 frame superframe of D4 to a 24 frame extended
superframe, making significantly different use of the framing information. The ESF framing
format consists of a 24-bit pattern, with 6 of the bits used to synchronize to the start of each
frame and signaling frame. Six of the remaining 18 bits consist of an error detection code
known as a Cyclic Redundancy Check (sum) over the superframe. The remaining 12 bits are
used as a facility data link signal providing maintenance and facility supervision. The
advantage of ESF framing format is that it is able to detect more errors than D4 framing
format. However ESF is not universally used by DS1 equipment.
A Red alarm occurs when frame synchronization on the DS1 interface is lost.
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