Time Waits for No Man - and Surely Not in Networks

By Dan Joe Barry, Napatech

Precise timing has become essential in modern packet networks.  Time-sensitive services, such as VOIP, IPTV, financial trading, real-time data transfers, and mobile infrastructure, require time-stamping of packets. Lack of precise synchronization can lead to low service quality and a bad reputation for a service provider. It also causes problems when handing off wireless calls between networks. And, of course, timing is a necessity in charging for services or capacity at every level.

The required level of timing precision has increased greatly in recent years. The latest high-speed networks, operating at 10 Gbps or higher, must measure time accurately to tens of nanoseconds. For example, a 10 Gbps network can handle up to 15 million 64-byte packets per second – that’s one packet every 67ns!

Measuring  latency accurately requires a network adapter capable of performing precise time-stamping  with an accuracy of 10 ns. That is, if two packets are received on two different ports within 10ns of each other, they will be stamped with the same exact time. All the network adapter needs is a reading for the Coordinated Universal Time (UTC) and a Pulse per Second (PPS) on which to trigger the internal clock.

The network adapter can thus provide a time-stamp accuracy of 10ns from packet to packet, but what about from one adapter to another?  Ensuring that all system network adapters are operating on the same clock requires an overall  time synchronization solution. Such a solution provides a PPS output to all the adapters and a UTC time. Each adapter will then use this reading to adjust its internal clock for skew.  The approach provides accuracies of at least 100ns from the PPS source and across adapters. The PPS signal can be fed to each adapter individually or adapters can be “daisy-chained”, whereby each one passes the signal on to its successor.

Several time synchronization sources are available, including GPS, CDMA, Network Time Protocol (NTP), and IEEE1588. GPS is accessible everywhere, but suffers from the fact that its antennae must be placed where they can receive the satellite signals. This is typically on a rooftop. CDMA networks, on the other hand, distribute GPS time from each basestation, providing a strong signal that can be received inside buildings.  However, the CDMA clock precision is only 10 µs, sufficient for many situations, but not as accurate as GPS, which can provide 15ns precision.  

IEEE1588 provides a means of distributing GPS or CDMA clock signals effectively in Ethernet networks. A hardware based solution providing 100ns accuracy, it is an alternative to NTP, which is a software-only solution limited to millisecond accuracy.

With these capabilities in place, we can ensure that each packet will be time-stamped with high precision. The network will then be able to support even the most demanding real-time applications. Users will not miss the action in a crucial soccer match or lose a stray $100 billion due to a financial trade failing to occur at the correct time!

Dan Joe Barry is VP Marketing at Napatech. You can reach him at djb@napatech.com.