The "Highway Exit" Problem

One of the most significant and costly drawbacks of the Plesiochronous Digital Hierarchy (PDH) was the difficulty in accessing a single, low-speed channel from within a high-speed trunk stream. This is often called the "branching problem" or the "add/drop problem."

Imagine a major highway (a 140 Mbit/s E4 stream) connecting two large cities. Now, suppose a small town located between these cities needs to access just one telephone line (a single 64 kbit/s channel) from this highway. In an ideal world, you would create a simple exit ramp for that one line. In the world of PDH, this was impossible.

Why Full Demultiplexing is Required

The core of the problem lies in the very nature of PDH multiplexing, which combines plesiochronous streams using bit interleaving and justification (bit stuffing).

Because the exact position of the bits belonging to a specific E1 stream within an E4 stream is not fixed (it depends on the cumulative justification performed at each multiplexing stage: E1-to-E2, E2-to-E3, and E3-to-E4), there is no way to "peek inside" the E4 stream and pull out the desired channel.

To access that single 64 kbit/s channel, the entire high-speed stream had to be completely disassembled and then reassembled. The process involved a large and expensive stack of equipment called a "back-to-back" or "drop-and-insert" multiplexer:

1. Demultiplex 140 Mbit/s to 34 Mbit/s: The E4 stream is fed into a DEMUX, which splits it back into four E3 streams.
2. Demultiplex 34 Mbit/s to 8 Mbit/s: The relevant E3 stream is then fed into another DEMUX to get four E2 streams.
3. Demultiplex 8 Mbit/s to 2 Mbit/s: The relevant E2 stream is fed into a third DEMUX to finally get four E1 streams.
4. Extract the Channel: The target E1 stream is identified, and from its 32 timeslots, the single 64 kbit/s voice channel can be extracted.
5. Re-Multiplex Everything: All the other channels and streams that were not dropped must now be passed through the entire multiplexing chain again in reverse (2->8, 8->34, 34-> 140) to recreate the E4 stream for onward transmission.

Consequences of the Branching Problem

This inability to easily add or drop individual channels had severe consequences for network operators:

• High Cost: The sheer amount of back-to-back multiplexer and demultiplexer equipment required at every intermediate node where a small amount of traffic was needed was enormous, making network expansion expensive.
• Physical Space and Power: The equipment was bulky, consumed a lot of power, and generated significant heat, requiring large, air-conditioned facilities.
• Lack of Flexibility: Network configurations were rigid and difficult to change. Adding or re-routing services was a major and costly undertaking.
• Reduced Reliability: Each piece of equipment in the multiplexing chain was another potential point of failure.

The branching problem was one of the most critical limitations of PDH. The need to solve it directly led to the development of Synchronous Digital Hierarchy (SDH) and SONET. In these synchronous systems, the position of every single low-speed channel within the high-speed frame is precisely known and fixed, allowing for simple and inexpensive devices called Add-Drop Multiplexers (ADMs) to easily extract or insert individual channels without tearing the entire structure apart.