PDH Regional Standards (E1/T1/J1)

Three regional PDH families

Plesiochronous Digital Hierarchy (PDH) did not emerge as one globally interoperable line format. ITU-T G.702 specifies hierarchy bit rates, while regional bodies defined framing, signaling, and physical interfaces. The classic families are the European E-carrier, the North American T-carrier, and the Japanese 1544-based hierarchy. Their different rates and framing rules complicated direct interconnection between continents.

• The European international structure, known as the E-carrier system.
• The North American structure, known as the T-carrier system.
• The Japanese structure, standardized nationally through TTC documents.

The European Hierarchy (E-carrier)

Early European deployment was coordinated by CEPT. ITU-T G.702 lists the 2048-based rates, and ETSI EN 300 166 profiles European 2048 kbit/s interfaces through E4 with references to G.703 and G.704. The E1 level multiplexes 30 64 kbit/s bearer channels.

Hierarchy levels

• E1 (Level 1). This base stream combines 30 64 kbit/s bearer channels. In PCM-30/32, TS16 can carry channel-associated signaling and TS0 carries frame alignment information.
  Total bit rate. $32 \times 64 \text{ kbit/s} = 2.048 \text{ Mbit/s}$
• E2 (Level 2). Created by multiplexing four E1 streams.
  Total bit rate. $4 \times 2.048 + \text{overhead} = 8.448 \text{ Mbit/s}$
• E3 (Level 3). Created by multiplexing four E2 streams.
  Total bit rate. $4 \times 8.448 + \text{overhead} = 34.368 \text{ Mbit/s}$
• E4 (Level 4). Created by multiplexing four E3 streams.
  Total bit rate. $4 \times 34.368 + \text{overhead} = 139.264 \text{ Mbit/s}$ (often rounded to 140 Mbit/s).
• Beyond E4. G.702 and ETSI EN 300 166 end the classic E-carrier sequence at E4. Some later carrier tables use 565.148 Mbit/s as an E5 label, outside that sequence.

Each E2 through E4 stage combines four tributaries. Framing and positive-justification overhead make the output rate higher than a simple multiplication of the input rates.

The North American Hierarchy (T-carrier)

AT&T introduced the North American T-carrier system. ANSI and ATIS documents define its DS interfaces and formats. The base stream multiplexes 24 DS0 channels and uses different rates and multiplication factors from the European hierarchy.

Hierarchy levels

• T1 / DS1 (Level 1). The base stream combines 24 64 kbit/s DS0 channels with one framing bit used for alignment.
  Total bit rate. $(24 \times 8 + 1) \times 8000 = 1.544 \text{ Mbit/s}$
• T2 / DS2 (Level 2). Created by multiplexing four T1 streams.
  Total bit rate. $4 \times 1.544 + \text{overhead} = 6.312 \text{ Mbit/s}$
• T3 / DS3 (Level 3). Created by multiplexing seven T2 streams.
  Total bit rate. $7 \times 6.312 + \text{overhead} = 44.736 \text{ Mbit/s}$
• T4 / DS4 (Level 4). Extended T-carrier tables use six T3 streams for this level.
  Total bit rate. $6 \times 44.736 + \text{overhead} = 274.176 \text{ Mbit/s}$

The successive multiplexing factors are x4, x7, and x6. The DS designation is the North American naming convention.

Interworking between regional hierarchies

Japan developed a third hierarchy around the 1.544 Mbit/s base rate. TTC JT-G702 conforms to the ITU-T hierarchy but removes the 2048 and 44.736 Mbit/s branches used in Europe and adds a Japanese fifth level. Its sequence is J1 at 1.544 Mbit/s, J2 at 6.312 Mbit/s, J3 at 32.064 Mbit/s, J4 at 97.728 Mbit/s, and J5 at 397.200 Mbit/s. J1 therefore shares the T-carrier base rate, while the higher levels diverge.

Different rates, framing, and signaling required interworking equipment. A gateway connecting a European E4 stream (139.264 Mbit/s, often rounded to 140 Mbit/s) to a North American network terminated the incoming format and rebuilt the outgoing one. It recovered the tributaries and mapped the required 64 kbit/s channels or lower-order streams into the T-carrier hierarchy. Demultiplexing all the way to individual voice channels was one possible implementation, not a requirement at every interface. This process required specialized gateway equipment at the interconnection points that required conversion.

These interworking costs helped motivate SDH and SONET. ITU-T G.707 and ANSI T1.105 defined synchronous transport structures that simplified tributary multiplexing and network management across regions.