The aim of this system is to avoid loss of synchronization due to the coincidental appearance of the sequence "0011011" in a time slot other than the TS0 of a frame with FAS. To implement the CRC code in the transmission of 2 Mbit/s frames, a CRC-4 multiframe is built, made up of 16 frames. These are then grouped in two blocks of eight frames called submultiframes, over which a CRC checksum or word of four bits (CRC-4) is put in the positions Ci (bits #1, frames with FAS) of the next submultiframe. At the receiving end, the CRC of each submultiframe is calculated locally and compared to the CRC value received in the next submultiframe. If these do not coincide, one or more bit errors is determined to have been found in the block, and an alarm is sent back to the transmitter, indicating that the block received at the far end contains errors. The receiving end has to know which is the first bit of the CRC-4 word (C1). For this reason, a CRC-4 multiframe alignment word is needed. Obviously, the receiver has to be told where the multiframe begins (synchronization). The CRC-4 multiframe alignment word is the set combination "0011011", which is introduced in the first bits of the frames that do not contain the FAS signal. The CRC-4 method is mainly used to protect the communication against a wrong frame alignment word, and also to provide a certain degree of monitoring of the bit error rate (BER), when this has low values (around 10−6). This method is not suitable for cases in which the BER is around 10−3 (where each block contains at least one errored bit). Another advantage in using the CRC is that all the bits transmitted are checked, unlike those systems that only check seven bits (those of the FAS, which are the only ones known in advance) out of every 51 bits (those between one FAS and the next). However, the CRC-4 code is not completely infallible, since there is a probability of around that an error may occur and not be detected; that is, that 6.25% of the blocks may contain errors that are not detected by the code. The aim of monitoring errors is to continuously check transmission quality without disturbing the information traffic and, when this quality is not of the required standard, taking the necessary steps to improve it. Telephone traffic is two way, which means that information is transmitted in both directions between the ends of the communication.