PCM30
PCM30 refers to an application of the method described with pulse code modulation (PCM), in which an analog signal is binary coded. It is used, for example, when making calls.
Today the term is mostly used as a synonym for the 30 • 64 kbit / s signal. This bit rate is also used in the first stage of European PDH technology, which is why PCM30 is also referred to as E1 .
It originally referred to a device in communications engineering that can convert 30 analog telephone signals into a digital bit stream of 2048 kbit / s.
The PCM30 system
The PCM30 basic system was / is used in the digital switching technology of Europe and served for the digital transmission of telephone traffic. It has 30 encoders, each of which can convert a telephone channel into an 8-bit digital word (byte). The 30 bytes are sent one byte at a time, together with a frame code word and a code channel ( multiplexer ). This creates a 32-byte frame . In the receiving direction, the 30 channels are again distributed to the 30 digital-to-analog converters by means of a demultiplexer ; analog signals are generated again. The time slot 0 is used as a frame code word and as a message word. These are sent alternately. Time slot 16 is used as the flag channel for the speech channels. Each speech channel is assigned 4 bits, which are sent one after the other in 16 frames, creating a superframe with a frequency of 500 Hz and a length of 16 ⋅ 32 ⋅ 8 = 4096 bits.
Structure of the PCM30 frame
- Explanation of the management channels 0 and 16
- Like every PCM30 channel, these channels are 8 bits wide.
- Odd frame, channel 0
- Channel 0 transmits the frame password. Since the password word is only 7 bits long, the eighth bit was used as an additional checksum (CRC4 bit) in later applications. The top bit was not used in the PCM30 system. The frame code word is transmitted in every second frame, so it alternates with the message word and is used for synchronization.
- Even frame, channel 0
- This contains the message word. This is used to alarm system malfunctions. Here, too, the message word is only 7 bits long, the eighth bit is used to form superframe synchronous words.
- Channel 16
- Channel 16 transmits the identification information for two subscriber connections, 4 bits each. They are used to transmit and control the remote switching center for the individual speech channels.
- Organization of the license plate channels
- Channel 0 with the multiframe password, channel 16 with the superframe password. 16 125 µs basic frames are combined into a 2 ms unit and synchronized independently.
How it works (short version)
The analog input signals are initially limited by a low pass. The signals are then cyclically sampled by the multiplexer at a frequency of 8 kHz. A PAM signal is created for all the signals supplied. This is quantized aurally according to the A characteristic and is binary coded as an 8-bit word by an 'A / D converter' . Since 8000 samples with 8 bits each are made every second, a bit stream with 64 kbit / s is generated. In the sum of all 32 channels (30 telephone channels + 2 management channels) the bit rate of 2048 kbit / s.
The previously binary signal is not suitable for transmission. Conducted transmission requires freedom from direct current and constant synchronous information . The binary bit stream is therefore recoded into the HDB3 code via the intermediate stage of the AMI code . The entire bit stream of 32 telephone channels is HDB3-coded transmitted at 2.048 Mbit / s to the remote terminal. Distances greater than about 3.5 km are bridged with regenerators , the so-called line equipment.
Since 2005, however, no PCM30 routes based on HDB3 have been provided in Germany. Instead, the 2B1Q code is used, which halves the baud rate (Vs) while maintaining the same transmission rate (Vü) and makes it possible to move the signal transmission to a lower frequency band. This in turn has a positive effect on the attenuation of the signal and its range.
Data of the PCM30 system
- Number of channels (time slots): 32
- Number of telephone channels: 30
- Frame duration: 125 µs
- Channel duration: 3.9 µs
- Channel bit number: 8 bit
- Bit duration: 0.488 µs
- Bit rate: 2048 kbit / s (2.048 Mbit / s)
- Bit rate per channel: 64 kbit / s
- Clock frequency: 8 kHz