PDH Branching and Add-Drop Access
One limitation of the Plesiochronous Digital Hierarchy (PDH) was access to a single low-rate channel inside a high-rate trunk stream. This is called the "branching problem" or the "add/drop problem."
For example, an E4 stream at 139.264 Mbit/s can connect two network nodes while an intermediate node needs one 64 kbit/s telephone channel. PDH does not provide that channel directly from the E4 stream. Access requires processing the intervening hierarchy levels. In the four-level construction of ITU-T G.751 Method 1, an E4 stream contains four E3 streams, each E3 contains four E2 streams, and each E2 contains four E1 streams. This path therefore requires three demultiplexing stages before the target channel is available. G.751 also defines Method 2, which multiplexes sixteen E2 streams directly into E4, so the number of stages depends on the E4 construction. In a CAS E1 structure, 32 timeslots include 30 user channels and the overhead slots TS0 and TS16.
Demultiplexing Path to a 64 kbit/s Channel
PDH multiplexing combines streams using and justification (bit stuffing).
The position of a specific E1 stream inside E4 changes with the cumulative justification at each multiplexing stage: E1-to-E2, E2-to-E3, and E3-to-E4. Direct extraction from E4 therefore requires recovery of the intervening hierarchy levels.
For the four-E3 path above, access to the 64 kbit/s channel required demultiplexing the high-rate stream and then multiplexing the remaining traffic again. The equipment was commonly arranged as a "back-to-back" or "drop-and-insert" multiplexer:
- Demultiplex 139.264 Mbit/s to 34.368 Mbit/s: The E4 stream is fed into a DEMUX, which splits it back into four E3 streams.
- Demultiplex 34.368 Mbit/s to 8.448 Mbit/s: The relevant E3 stream is then fed into another DEMUX to get four E2 streams.
- Demultiplex 8.448 Mbit/s to 2.048 Mbit/s: The relevant E2 stream is fed into a third DEMUX to finally get four E1 streams.
- Extract the Channel: The target E1 stream is identified, and from its 32 timeslots, the single 64 kbit/s voice channel can be extracted.
- Re-multiplex the remaining traffic: The remaining channels and streams pass through the multiplexing chain in reverse (2.048->8.448, 8.448->34.368, 34.368->139.264) to recreate the E4 stream for onward transmission.
Every stage must recover its frame alignment and process justification information. An intermediate node therefore needs equipment for the complete E4-to-E1 path even when it drops one channel. The return direction requires the corresponding multiplexing chain. This requirement also applies when the node forwards the remaining channels unchanged.
Consequences of PDH Branching
The need to process the full hierarchy to add or drop an individual channel affected network operation in several ways:
- High cost: Each intermediate node requiring a small amount of traffic needed additional multiplexer and demultiplexer equipment, increasing expansion costs.
- Physical space and power: The equipment occupied large spaces, consumed power, and generated heat, which increased facility and cooling requirements.
- Limited flexibility: Network configurations were difficult to change, so adding or rerouting services required work at several hierarchy levels.
- Additional failure points: Each device in the multiplexing chain introduced another possible point of failure.
The branching problem contributed to the development of Synchronous Digital Hierarchy (SDH) and SONET. Their standardized containers and multiplexing structures let Add-Drop Multiplexers (ADMs) access lower-rate tributaries without demultiplexing the complete higher-rate hierarchy.