One element that is key to all networks rarely gets discussed. Network timing (or network clocks) involves hardware or processes to make sure that all parts of a network are in synch.
Timing and synchronization are critical for network services that depend on precise, synchronized timing on network devices. Accurate and reliable synchronization of any network device helps manage the security, availability, and efficiency of the network devices. Timing is essential for the function of telephone, cellular, and broadband networks.
There are multiple kinds of timing in use.
Frequency Synchronization. This makes sure that all electronics inside a network operate using the same clock rate or frequency. Many kinds of network gear come with built-in clocks, and having different parts of a network using different clocks will result in data loss, corruption, or misinterpretation of bits. Frequency synchronization forces all of the clocks inside the network to operate in unison by matching the frequency of each clock to a source clock. There are different sources for frequency synchronization:
- Synchronous Ethernet (SyncE) chooses one clock and forces the other clocks to match.
- Networks can be synchronized to external clocks such as BITS or the GPS satellites. BITS can choose any reliable external clock.
- Many networks use Precision Time Protocol (PTP), which eliminates the danger of losing the connection to an external clock.
- A network can use a free-running internal oscillator chip that holds an accurate clock.
Many networks have used GPS for frequency synchronization. A GPS satellite carries a highly stable atomic clock that provides precise time signals, which can be converted into frequency references by a GPS receiver. While the atomic clock provides highly precise time and frequency information, GPS is not as reliable when there isn’t a clear view of the sky during weather events.
Phase Synchronization makes sure that the phase of network signal is consistent throughout the network. Phase refers to a specific point in time on a waveform cycle. Phase synchronization ensures that electronics agree on the timing of the start and end of each bit in a data stream. This is critical in applications where data from multiple sources have to be combined or compared, such as in a cellular network.
Time Synchronization, also called Time of Day (ToD) ensures that all electronics agree on the current time, which is critical in applications where timing is crucial. Networks differ in the need for precise time. Network Time Protocol (NTP) can be used to provide millisecond accuracy, while PTP can provide nanosecond accuracy along with phase synchronization.
Thanks for the interesting discussion! I think a lot of signals are self clocking, so we don’t need precise frequency or phase control at each end of the wire. In streaming data (audio, video, or whatever), we need the source and destination to agree on the data rate to avoid buffer overflow or underflow at the receive end. In systems I’ve designed, we used audio sample rate converters between the incoming AES3 audio and the DSP so we did not have to sync the DSP to the incoming audio (and potential phase differences between multiple AES3 pairs carrying more audio channels). But sample rate converters corrupt the binary signal. While the corruption is not audible, it messes up devices downstream that are using the precise binary code. Another approach is to have the receiver adjust its “playback speed” based on how much data is in its receive buffer. Ideally the variation in speed would be small, but we need to avoid under and over flow.
Thanks for the discussion!