Nortel NT1R20, Description

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Page 40 of 290 Description
553-3001-105 Standard 5.00 June 1999
Digital line call operation
Digital line call operation is controlled entirely by use of messages between
the digital telephone set and the Meridian 1 system. These messages are
carried across the TCM loop interface. There is no call connection sequence
similar to the one used for analog telephone line operation.
Line-side T1 call operation
The line-side T1 card’s call operation is performed differently depending on
whether the T1 link is configured to process calls in loop start mode or
ground start mode. Configuration is performed through dip switch settings on
the line-side T1 card.
The line-side T1 card performs calls processing separately on each of its 24
channels. Signaling is performed using the “A/B robbed bit” signaling
standard for T1 communication. A/B robbed bit signaling simulates standard
analog signaling by sending a meaningful combination of ones and zeros
across the line that correlates to the electrical impulses that standard analog
signaling sends. For example, to represent that an analog line interface unit is
idle, the analog line card provides a ground on the tip lead and -48Vdc on the
ring lead. The line-side T1 card accomplishes the same result by sending its
A bit as 0 (translated as ground on the tip lead) and its B bit as 1 (translated
as -48Vdc on the ring lead). However, measuring the voltage of the ring lead
on the T1 line would not return -48Vdc, since actual electrical impulses are
not being sent.
Call operation will be described by categorizing the operation into the
following main states:
— Idle (on-hook)
— Incoming calls
— Outgoing calls
— Calls disconnected by the Central Office (CO)
— Calls disconnected by the telephone