Plesiochronous clocks and frequency differences
PDH's main synchronization issue comes from plesiochronous operation. Devices use clocks with the same nominal rate, but the clocks are independent and are not phase-locked. Small frequency offsets accumulate over time.
Incoming data is written to an with the recovered clock and read with the local clock. If the write and read rates differ persistently, the buffer approaches a full state (overflow) or an empty state (underflow), which can cause a slip.
Slips at elastic-buffer limits
When a buffer reaches an overflow or underflow condition, equipment may repeat or delete a block of data to restore its operating range. ITU-T G.810 defines a slip as the repetition or deletion of a block of bits caused by a discrepancy between read and write rates. In the ETSI PDH path function, positive justification cancels one 64 kbit/s octet once, while negative justification reads that octet a second time. For a 64 kbit/s service, standards commonly evaluate controlled octet slips. Equipment can use a larger internal block, and an E1 frame has 256 bits, but the G.822 objective is expressed as octet slips. A slip is a timing event rather than a random single-bit error in the transmission medium.
Impact of slips on different services
- Voice (uncompressed PCM).A slip is often heard as a brief "click" or "pop" in the audio. A single event usually does not make a conversation unintelligible.
- Fax and modem data. These services are sensitive to timing. A slip can corrupt part of a transmission and may force retransmission of a fax page or data block.
- Digital data. A slip can make a framed data block unusable. A higher-layer protocol such as TCP may retransmit the missing information, reducing effective throughput.
- Compressed video. A damaged payload can affect decoding of dependent frames, producing artifacts or a frozen picture until the next intra frame. The duration depends on the codec and intra-frame interval.
Controlling slips with the Hypothetical Reference Connection (HRX)
ITU-T defines slip-rate objectives for plesiochronous operation with the Hypothetical Reference Connection (HRX) model.
The HRX is a theoretical international connection with a total length of 27,500 km. Under its stated clock-accuracy assumptions, ITU-T G.822 gives an objective of no more than one controlled octet slip in 70 days for each plesiochronous interexchange link. This is a planning value under specified conditions, not a universal guarantee. In one G.822 example, 12 plesiochronous link intervals across 13 nodes give one slip in 5.8 days. Another example with four international links and synchronized national ends gives one slip in 17.5 days. Temperature wander, timing interruptions, and network configuration can change the observed rate.
Why synchronization was introduced
Accumulated timing differences and slips limited long chains of PDH equipment. SDH/SONET uses synchronous transport and a network timing hierarchy, so tributaries can share a controlled timing domain.
In a synchronous network, equipment follows a common timing reference ultimately derived from a highly accurate master clock. This removes the steady frequency offset that drives plesiochronous slips, while timing faults, jitter, and wander still require control. ITU-T G.823 and G.824 specify jitter and wander limits for the 2048 and 1544 kbit/s hierarchies.