Understanding MIC Techniques and ISDN Functional Groups

Posted on Nov 25, 2024 in Electronics

MIC Techniques: Permit digital-analog conversion to optimize signal transmission. Digital operations are based on three processes:

  • Sampling: Transforming an analog signal into a series of pulses (PAM or Pulse Amplitude Modulation). The sampling frequency must be greater than twice the maximum frequency of the original signal (Nyquist criterion, Fmues ≥ 2.fmax). Commonly used frequencies are 8 KHz (8,000 samples per second).
  • Quantization: Replacing infinite values with a discrete number of values, dividing the operating range into quantization intervals.
  • Coding: Encoding each sample into a binary word (e.g., 8-bit MIC). The first bit represents the sign, the next three the segment, and the last four the quantification range within the segment.

After encoding, the bits are inverted. The voice channel becomes an 8000 samples/sec x 8 bits/sample = 64 Kbit/sec channel (MIC channel). To recover the analog signal, the MIC channel is decoded, using the average value of the quantified interval, and then filtered. The recovered signal may have quantization distortion.

Multiplex MIC: Combines MIC techniques with Time Division Multiplexing (TDM). TDM allows sending multiple signals simultaneously. European standards recommend a 30-channel MIC multiplex. A MIC frame is a binary word of 32 intervals x 8 bits/interval = 256 bits. Interval 0 is used for frame alignment, interval 16 for signaling, intervals 1-15 and 17-31 for voice channels. The frame period (Tframe) is 1/fm = 1sec / 8000 = 125 µs, the interval duration (Tinterval) is 125 µs / 32 = 3.9 µs, and the bit duration (Tbit) is 3.9 µs / 8 bits = 488 ns. The digital flow in a 30-channel multiplex MIC has a transmission speed of 8000 frames/sec x 32 intervals/frame x 8 bits/interval = 2048 Kbit/sec = 2 Mbit/sec.

ISDN Functional Groups and Reference Points:

Network Terminations (TR):

  • TR1: Marks the boundary between the public and private network, isolating the subscriber loop. It has an RJ45 connector for a passive bus.
  • TR2: Performs switching, routing, or concentration functions. TR1 and TR2 are relevant for ISDN network users.

Adapter Terminal (AT): Provides compatibility for non-ISDN equipment.

Reference Points:

  • R: Interface between non-ISDN equipment and AT or analog terminals.
  • S: Defines communication between a computer and TR2 (ISDN). It’s a 4-wire interface (2 for reception and 2 for transmission).
  • T: Similar to S, representing the separation between user facilities and online processing equipment.
  • U: Adapts signals to the local loop.
  • V: Represents the separation between transmission and switching functions within the ISDN local exchange.

Bus Configurations:

  • Short Passive Bus: Cable up to 200m with up to 10 rosettes (8 terminals connected).
  • Extended Passive Bus: Reaches 500m to 1000m, allowing 4 terminals in the last 50m.

ISDN Functional Groups

  • ET1 (Terminal Equipment Type 1): Designed to connect directly to ISDN (e.g., ISDN telephone).
  • ET2 (Terminal Equipment Type 2): Not originally designed for ISDN, requires an adapter (e.g., a modem).
  • AT (Terminal Adapter): Connects ET2 equipment to ISDN (e.g., V.35 interface adapters).
  • TR2 (Network Termination Type 2): Performs switching, concentration, and control functions within the customer premises.
  • TR1 (Network Termination Type 1): Interconnects the indoor 4-wire user network with the external 2-wire network.
  • TL (Terminal Line): Located in the local exchange, functionally similar to TR1.
  • CT (Central Termination): Located in the local exchange, connects information channels with switching stages.

The set of reference points and functional groups provides a generic reference configuration for describing ISDN user access.