Two Worlds of Digital Transmission

The lack of a single global standard for PDH led to the development of two major, fundamentally incompatible systems for the first level of the digital hierarchy. The European standard, known as E1, differs significantly in structure, capacity, and signaling method from its North American counterpart, T1. Understanding these differences is key to appreciating the complexities of legacy international networks and the motivation behind the unified SDH/SONET standards that followed.

E1 vs. T1: Head-to-Head Comparison

The following table provides a detailed comparison of the key parameters for the PCM-30/32 (E1) and PCM-24 (T1) systems.

Detailed Breakdown of Differences

• Frame Structure and Bit Rate: The most apparent difference is the size. The E1 system bundles more channels, resulting in a larger frame (256 bits vs. T1's 193 bits) and a higher overall bit rate (2.048 Mbit/s vs. T1's 1.544 Mbit/s), all within the same 125 µs timeframe.
• Signaling Method: This is a critical functional difference.
  • E1 uses Channel Associated Signaling (CAS), dedicating a full timeslot (TS16) exclusively for signaling. This keeps user data pristine but sacrifices one channel's worth of bandwidth.
  • T1 uses Robbed-Bit Signaling. In this technique, the system periodically "steals" the least significant bit from a user's voice channel to transmit signaling. This provides 24 full channels for payload but slightly impacts the quality of voice transmission.
• Companding Law: Both systems use non-linear quantization to optimize voice quality, but they use different, incompatible algorithms.
  • North America and Japan use µ-law.
  • Europe and most of the rest of the world use A-law.
  While both achieve a similar result, a digital stream encoded with µ-law cannot be directly decoded by a system expecting A-law, requiring conversion at gateways.