Tutorial on T1/E1 Alarming, Dropping, and Inserting

While interest in IP networks continues to grow, designers must continue to push forward on using T1 and E1 links to deliver voice and data services. This tutorial highlights the alarm, drop, and insert mechanisms necessary to allow T1/E1 links to deliver both services.

E1 and T1 lines are used to carry data and voice between carrier links. Historically, the T1 circuit has been used in North America and the E1 circuit in Europe. T1 and E1 lines can support both voice and data on a single link. This gives substantial cost savings to the carriers by using existing data networks and routing the voice to other destinations.

When a link of T1 or E1 call setup/teardown data arrives at the data center, the data channel is extracted and voice is sent to a media backend for the voice call to be processed. This function is called drop and insert. Each T1 and E1 line has a protocol to notify the other end of any errors that occur. The method of notification is called alarms.

Alarms are used in T1 or E1 to notify a node of a problem on a link. Alarms classify the nature of the problem. If a problem arises in drop and insert mode on a link, both links must also be notified.

In this tutorial, we'll describe the E1 and T1 protocols, alarms, and framing formats. We'll also show how alarms are used in conjunction with drop and insert capabilities to promote the delivery of voice and data services over T1 links. Let's start the discussion by looking at E1.

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.

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 2.5 seconds. It is cleared after the LFA has been clear for at least one second.

E1 Double Frame
There are two E1 frame formats, the double frame and the multi-frame. The synchronization methods are different in the two frame formats. Synchronization can be achieved after receipt of three E1 frames in double frame format. The synchronization information is carried in timeslot 0. This is called the frame alignment signal (FAS).

The FAS is a pattern "0011011" that specifies the alignment of a frame. The FAS is in timeslot 0 in alternate frames (figure 2). Bits 2 through 8 are the FAS. The other frame's (N+1) bit 2 is set to 1. Frame alignment is reached if there is:

1. A correct FAS word in frame N.
2. Bit 2 = 1 in frame N+1
3. A correct FAS word in frame N+2.


What happens if synchronization is not achieved or has been achieved and lost? This condition is called LFA. If three in four alignment words are in error, an LFA is declared. This is if bit 2 in Frame N+1 is set to 0. The near end must respond to the far end that there is an alignment problem. This is done with the RAI alarm. The A bit (bit 3) in all N+1 frames is used for sending the RAI alarm to the far-end equipment.

E1 Multi-frame
In multi-frame format, the synchronization for multi-frame requires 16 consecutive good frames. The multi-frame structure also has two extra features. It provides channel associated signaling (CAS) and a cyclic redundancy check (CRC).

CAS is sent in timeslot 16 of each frame. It is CAS information that can denote on-hook and off-hook conditions of telephone calls. Figure 4 shows how CAS information is sent.


Figure 4: diagram illustrating how CAS information is sent.

In frame 1, the information for channels 1 and 16 is sent. In frame 2, the information for frames 2 and 17 is sent. Only 4 bits are used to denote on-hook and off-hook conditions. Of the four bits, not all are always used. Refer to figure 9 for the definitions of the ABCD bits for on hook/off hook conditions. Notice that timeslot 16 of frame 0 does not send this information.


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