multiplexing

Multiplexers

Transmission facilities are expensive and, often, two data terminal equipments communicating by coaxial cables, microwave links, or satellite, do not use the full capacity of the channel, wasting part of the available bandwidth. This problem is solved by means of equipment called multiplexers, which distribute the use of the transmission medium in several independent channels that allow simultaneous access to users, being totally transparent to the transmitted data.

At one extreme, multiplexers are equipment that receives multiple low-speed data streams and transforms them into a single high-speed data stream, which is transmitted to a remote location. At that location, another multiplexer performs the reverse operation, retrieving the original low-speed data streams. This function is called demultiplexing.

When several different transmissions will be made on the same line, the efficiency rate of the channel is notably improved.

There are two fundamental techniques for carrying out multiplexing:

• Frequency Division (FDM)
• Division in Time (MTC)

Frequency Division Multiplexing (FDM)

Frequency division multiplexing is a technique that consists of dividing the frequency spectrum of the transmission channel by means of filters and moving the signal to be transmitted within the margin of the corresponding spectrum by means of modulations, in such a way that each user has exclusive possession of his band. of frequencies (called subchannels).

At the end of the line, the multiplexer in charge of receiving the data demodulates the signal, obtaining each of the subchannels separately. This operation is performed transparently to line users. This type of multiplexing is used for telephone, radio, TV users who require continuous use of the channel.

This process is possible when the bandwidth of the transmission medium exceeds the bandwidth of the signals to be transmitted. Multiple signals can be transmitted simultaneously if each is modulated with a different frequency carrier, and the carrier frequencies are far enough apart that interference does not occur. Each subchannel is separated by guard bands to prevent possible overlapping interference.

The signal that is transmitted through the medium is analog, although the input signals can be analog or digital. In the first case, AM, FM and PM modulations are used to produce an analog signal centered on the desired frequency. In the case of digital signals, ASK, FSK, PSK and DPSK are used.

At the receiving end, the composite signal is passed through filters, each focused on one of the different carriers. In this way the signal is divided again and each component is demodulated to recover the signal.

The MDF technique presents a certain degree of standardization. A widely used standard corresponds to 12 voice channels, each one at 4.000 Hz (3.100 for the user and the rest for the guard band) multiplexed in the 60-108 Khz band. This unit is called a group. Many carrier service providers offer their customers a 48 to 56 Kbps leased line, based on a group.

Five groups (60 voice channels) can be multiplexed to form a super group. The next unit is the master group, which is made up of five supergroups (according to ITU standards) or ten groups (according to the Bell System).

Time Division Multiplexing (TCM)

Time division multiplexing is a technique for sharing a transmission channel among multiple users. It consists of assigning to each user, during certain "time slots", the totality of the available bandwidth. This is achieved by organizing the output message into units of information called frames, and assigning fixed time slots within the frame to each input channel. In this way, the first channel of the frame corresponds to the first communication, the second to the second, and so on, until the nth plus one corresponds to the first again.

The use of this technique is possible when the data rate of the transmission medium exceeds the rate of the digital signals to be transmitted. The time division multiplexer cyclically samples, or scans, the input signals (input data) from different users, and transmits the frames over a single high-speed communication line. MDTs are discrete signal devices and cannot accept analog data directly, but demodulated using a modem.

MDTs work at the bit level or at the character level. In a bitwise MDT, each frame contains one bit from each scanned device. The character MDT sends one character in each channel of the frame. The second is generally more efficient, since it requires fewer control bits than a bit MDT. The sampling operation must be fast enough so that each buffer is emptied before new data arrives.

MIC systems, a digital coding system, use the MDT technique to cover the capacity of the transmission media. The law of formation of the successive multiplexing orders responds to international standards, with a view to facilitating connections between various countries and compatibility between equipment from different manufacturers.

The ITU/ITU recommends, as the first rung of the time division multiplexing hierarchy, 24 or 32 (30 + 2) telephone channels, systems used in the United States and Japan the first and in Europe the second. According to ITU recommendation G-732, the European primary PCM system multiplexes at sample level 30 voice channels, in addition to an alignment channel and a signaling channel, forming a 256-bit frame (32 channels, one sample per channel and 8 bits per sample) at a frequency of 8 Khz (double the bandwidth of the telephone channel), resulting in a speed of 2.048 kbps.

In secondary, tertiary, etc. PCM multiplex equipment, time multiplexing (TDM) is carried out by pulse interleaving (bit by bit), unlike in primary PCM equipment.

The ITU has recommended four multiplexing hierarchies for PCM equipment. The digital multiplex equipment that combines the output signals of four primary PCM multiplex equipment is called second-order digital multiplex equipment. Third-order digital multiplexers would combine the output signals of four second-order multiplexers, etc.

Thus, the second level of multiplexing accepts four 2.048 kbps digital signals to form one 8.448 kbps signal. The third level groups four 8.448 kbps signals into one 34.368 kbps signal. The fourth level bundles four level three signals into one 13.9264 kbps signal. Finally, in the same proportion, the fifth level produces a 565 Mbps signal.

Statistical multiplexing

In real situations, no communication channel remains continuously transmitting, so that if a portion of the transmission time is automatically reserved for each channel, there will be times when, due to lack of data from the corresponding channel, nothing is transmitted and, instead, other channels wait unnecessarily. The idea of ​​this multiplexing consists of transmitting the data of those channels that, at each moment, have information to transmit.

Statistical TDM Multiplexers (STDMs) dynamically allocate time slots between active terminals and therefore line capacity is not wasted during terminal idle times.

The operation of these multiplexers allows the sum of the speeds of the input channels to exceed the speed of the output channel. If at one time all input channels have information, the global traffic cannot be transmitted and the multiplexer will need to store some of this information.

Statistical multiplexers have evolved in a short period of time into very powerful and flexible machines. They have virtually cornered the MDT market and are currently a serious competitor to MDF. These provide error control and data flow control techniques. Some provide the modulation circuitry to interface with analog networks.

Otherwise, it would be necessary to use separate modems. Flow control is used to prevent devices from sending data at an excessive rate to the buffers of multiplexers.