Character Saves Life

E1 Protocol Basics
The E1 frame is composed of 32 timeslots (Figure 1). Timeslots are also called DS0s. Each timeslot is 8 bits. Therefore, the E1 frame will be (32 timeslots * 8 bits) = 256 bits. Each timeslot has a data rate of 64,000 bits/second. There will be 64,000 bits/second/8 bits = 8000 frames a second. The E1 frame will arrive every 1 second/8000 frames/sec = 125 microseconds. The line rate will be (32 channels * 8 bits/channel)/ frame * 8000 frames/second = 2048000 bits/second.


Figure 1: Diagram of the E1 frame.
Timeslot 0 is used for frame synchronization and alarms. Timeslot 16 is used for signaling, alarms, or data. Timeslot 1 to 15 and 17 to 31 are used for carrying data.
An alarm is a response to an error on the E1 line or framing. Three of the conditions that cause alarms are loss of frame alignment (LFA), loss of multi-frame alignment (LFMA), and loss of signal (LOS).
The LFA condition, also called an out-of-frame (OOF) condition, and LFMA condition occur when there are errors in the incoming framing pattern. The number of bit errors that provokes the condition depends on the framing format. The LOS condition occurs when no pulses have been detected on the line for between 100 to 250 bit times. This is the highest state alarm where nothing is detected on the line. The LOS may occur when a cable is not plugged in or the far end equipment, which is the source of the signal, is out of service.
The alarm indication signal (AIS) and remote alarm indication (RAI) alarms are responses to the LOS, LFA, and LFMA conditions. The RAI alarm is transmitted on LFA, LFMA, or LOS. RAI will be transmitted back to the far end that is transmitting frames in error. The AIS condition is a response to error conditions also. The AIS response is an unframed all 1's pattern on the line to the remote host. It is used to tell the far end it is still alive.
AIS is the blue alarm, RAI is the yellow alarm. A red alarm that can occur after a LFA has existed for...