Framing - E1

E1 is the European standard and is the framing format that is widely used in almost all countries outside the USA, Canada and Japan. The E1 frame is composed of 32 timeslots or channels. 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 (8000 samples/sec * 8 bits/sample). As each slot has to be repeated every 1/8000 sec or 125 microseconds, the entire frame frequency is also 125 microseconds. Therefore the E1 line rate will be (32 channels * 8 bits/channel)/ frame * 8000 frames/second = 2048000 bits/second or 2.048Mbs.

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. In double frame format , synchronization can be achieved after the receipt of three E1 frames. 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 (Frame N) . Bits 2 through 8 are the FAS. The other frame's (i.e Frame 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 or loss of frame alignment. If three in four alignment words are in error, an LFA is declared. 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 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 1 shows how CAS information is sent.

Figure 1

 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 2 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. 

Figure 2

The extra feature to multi-frame is the addition of a CRC. This resides in timeslot 0 (Figure 3). The Cx bits are for the four-bit CRC which resides in bit 1 of frames 0, 2, 4, 6, 8, 10, 12, and 14. The E and S bits are for international use.

Figure 3

The FAS pattern for multi-frame is also "001011". This is in bit 1 of frames 1, 3, 5, 7, 9, and 11. Notice now that the FAS is 1 bit in each of the frames vertically. This pattern is called a multi-frame alignment, when all 16 frames are correct. Also note that a Double frame alignment is achieved before multi-frame alignment.

If synchronization is not achieved or has been achieved and then lost, a LMFA condition will be declared. This denotes that the FAS was not received correctly in 16 frames. If double frame alignment has been lost, LFA will also be declared. The LMFA and LFA conditions are handled differently. When the LMFA condition exist at the near end of a link, the near end will send a RAI alarm the to far end of the link. The RAI is transmitted by setting the Y bit to 1 in timeslot 16 (Figure 4). The LFA alarm will be handled as it is in double frame by setting the A bit in bit 3 of every N+1 frame.

Figure 4

The AIS is sent as all 1's in the frame. All timeslots will be filled with 1's. This is sent in double frame and multi-frame when the LFA occurs. When LMFA condition occurs, AIS will be sent only in timeslot 16.