Minggu, 28 Maret 2010

PCM 30

PCM30 describes an application of pulse-code modulation (PCM) in which 30 telephony analog signals are binary coded into a digital signal stream.

The term is used today mostly as a synonym for the encoding of 30 channels each with a signalling rate of 64-kbit/s. This rate is also used in the first stage of European PDH technique, so PCM30 is also known as E1.

Originally it described a device in communications technology which converted the 30 analog telephone signals into a digital bit stream of 2048 kbit/s .

PCM30 system

The PCM30-base system, the analog switching technology in Europe, and served on the digital transmission of telephone traffic. It has 30 coders, each with a phone channel in an 8-bit digital word (byte) can implement. The 30 bytes are, together with a frame ID word, and a channel identifier, bytewise successively sent #Multiplexer#. Thus, a large 32-byte frame. In receiving direction, the 30 channels are, using a de-multiplexer, converted back to 30 analogue signals. The time slot 0 is used as a frame ID word, and as word detection is used. These are sent alternately. Time slot 16 is a channel identifier for the speech channels. Each voice channel is assigned 4 bits, one after the other in 16 frame are sent, there is an over-under with a frequency of 500 Hz and a length of 16 ⋅ 32 ⋅ 8 # 4096 bits.

Operation summery

The analog input signals are initially passed through a low-pass filter. Then, the signals from the multiplexer are cyclically sampled at a frequency of 8 kHz. There is a PAM signal all injected signals. This is after the A-curve hörgerecht quantized by an analogue to digital (A/D) converter as 8-bit binary word coded. Since 8000 samples each second, each with 8 bits to be made is that each bit stream at 64 kbit/s. The sum of all 32 channels #30 channels + 2 telephone management channels# the bit rate of 2048 kbit/s.

For the transmission is still not suitable binary signal. Conducted DC transmission requires freedom and constant synchronous information. The binary bit stream is therefore on the intermediate stage of the AMI code in the HDB3 code umcodiert. The entire bit stream of 32 Fernsprechkanälen is HDB3 encoded with 2.048 Mbit/s on the distant opposite body. Distances> 3.5 km with regenerators, the so-called Line equipment bridges.

Since 2005 in Germany but no PCM30 lines on HDB3 base provided. Instead, the 2B1Q - encoding used by the baud rate #Vs# at a constant transmission rate #VUE# in half and allows the signal to a lower frequency band to relocate. This in turn has a positive effect on the attenuation of the signal and its range.

Specification of PCM30 system

Number of channels #time slots#: 32
Fernsprechkanäle Number: 30
Frame duration: 125 microseconds
Channel length: 3.9 microseconds
Kanalbitzahl: 8 bit
Bit duration: 0.488 ms
Bitrate: 2048 kbit / s #2.048 Mbit / s#
Bit rate per channel: 64 kbit/s
Clock frequency: 8 kHzrame bits)