The Challenge of "Almost" Synchronous Clocks
At the heart of PDH's biggest challenges is the concept of "plesiochronous" operation. Unlike a truly synchronous system where a single master clock governs every device, a PDH network consists of equipment with highly accurate but independent clocks. Even with the best technology, these clocks will inevitably have tiny frequency differences.
When a receiver with its own clock tries to read data from a transmitter running on a slightly different clock, a timing discrepancy occurs. To handle this, devices use . However, if one clock is consistently faster than the other, the buffer will either completely fill up (overflow) or completely empty (underflow).
Bit Slips: The Consequence of Overflow and Underflow
When a buffer overflows or underflows, the network is forced to either discard or repeat a block of data to reset the buffer. This event is known as a slip. A slip is the loss or repetition of an entire frame of data (e.g., 256 bits in an E1 stream) and is a major source of errors in PDH networks.
Impact of Slips on Different Services
- Voice (Uncompressed PCM):Slips are least noticeable here, typically perceived by the human ear as a minor "click" or "pop" in the audio. They generally do not affect the intelligibility of a conversation.
- Fax and Modem Data: These services are highly sensitive to timing. A single slip can corrupt a large portion of a transmission, almost always forcing a retransmission of a page (for faxes) or a block of data, leading to longer connection times.
- Digital Data (e.g., File Transfers): Slips are critical here, causing the loss of entire data blocks. This triggers higher-level protocols (like TCP) to retransmit the missing information, significantly reducing effective throughput.
- Compressed Video: A slip can be catastrophic for compressed video. Due to the inter-dependencies between video frames, a single lost block can cause severe, visible distortion (e.g., blocky artifacts, frozen picture) that can persist for several seconds until a new keyframe is received.
Managing Slips: The Hypothetical Reference Connection (HRX)
Since eliminating slips entirely is impossible in a plesiochronous network, international standards bodies like the ITU-T defined acceptable performance levels. They did this using a model called the Hypothetical Reference Connection (HRX).
The HRX is a standardized, theoretical end-to-end international connection with a total length of 27,500 km. Standards like ITU-T G.822 specify the maximum acceptable number of slips per day for a connection of this length. This model provides a benchmark against which operators can measure the performance of their real-world networks. For example, a common requirement might be no more than one slip in 70 days for the entire connection, with stricter limits on individual nodes within the chain.
The Path to a Solution: Synchronization
The accumulation of timing errors and the resulting slips across long chains of PDH equipment was a major limitation. It highlighted the need for a different approach. Instead of compensating for clock differences at every step, the next generation of technology, SDH/SONET, was designed to be fully synchronous.
In a synchronous network, all devices are timed by a single, highly accurate master clock. This network-wide timing eliminates the root cause of plesiochronous slips, enabling much higher levels of performance and reliability.