This is the original method of formatting T1 datastreams. In AMI a zero is always sent by doing nothing, at the time when a pulse might otherwise be sent, a pulse is not sent to represent a zero.
A one is sent on an AMI T1 by sending a pulse, as opposed to not sending a pulse.
The alternating mark rule means that if the last pulse sent was of a positive going polarity, the next pulse sent must be negative going.
If an AMI T1 device receives two pulses in a row and they are of the same polarity a bipolar violation (BPV) has occurred.
Thus AMI has a rudimentary error checking capability with a 50% probability of detecting altered, inserted or lost bits end to end.
Since a T1 uses a single pair of wires in each direction and the only signals on those wires are the pulses which represent data; the only way to recover clock and retain synchronization on a T1 is by detecting the rate at which pulses are being received. All of the equipment in a T1 circuit must operate at the same rate because all the equipment must sense the T1 at the correct time in order to determine if a pulse (1) or no pulse (0) has been received at each bit time.
Since only ones are sent as pulses and zeroes are represented by doing nothing, if too many zeroes are sent at a time there will be no pulses on the T1 at all and the clock circuitry in all of the hardware will rapidly fall out of synchronization. Thus the design of AMI requires that a certain ONES DENSITY be maintained, and that a certain minimum of the bits over a certain period of time be guaranteed to be a ONE (pulse). This is why AMI circuits require DENSITY enforcement.
Briefly stated; on average one bit in eight must be a one and no more than (varies according to specific standard) so many zeroes may be sent in a row.
In order to be able to satisfy the ones density requirement on an AMI T1 one bit out of every eight is taken away from the user, not available for voice or data traffic, and that 1 bit in 8 is always sent as a one. Once this has been done the requirement for ones density is satisfied and the user is free to send any data pattern in the remaining bandwidth.
The rate of a T1 is 1.544 megabits per second. 8K is used for framing leaving 1.536MBPS. The 1.536 is usually divided into 24 timeslots (DS0s) or "channels" each being inherently 64KBPS. By taking the 1 bit in 8 that is reserved to satisfy ones density the user is left with 56K per timeslot.
When configuring a CSU for an AMI T1 the selection for density must be other than NONE.
When configuring a DSU (such as a DIU2130) for an AMI T1 the data rate per timeslot must be 56K as opposed to 64K.
An improved method of guaranteeing ones density was devised for the more modern format technique B8ZS such that each timeslot may be operated at 64K. For data applications in particular,B8ZS offers advantages over AMI.