Tellabs Blog

I'm encouraged by the progress we've made with synchronization in the past year.

I have a unique perspective of the synchronization issues that operators, suppliers and vendors around the world are facing. This is mostly thanks to my participation as a member of the International Telecom Sync Forum (ITSF) steering group and contributor to the ITU-T synchronization experts group (Q13). 

A two phase approach
Many operators are now looking at network synchronization as a migration to IEEE 1588 in two stages. This approach benefits operators because it enables them to address the immediate business needs of 2G, 3G, and LTE-FDD*. It also enables operators to address the longer term developments for LTE-TDD:** LTE-advanced.

In the first phase, which is already well underway, operators replace the TDM circuits they use for synchronization with packet clocks connected over Ethernet backhaul. During the second migration, operators are especially focused on LTE-TDD and LTE-Advanced. This second migration enables operators to support high accuracy phase synchronization across the transport network. This solution offers an alternative to requiring GPS on every cell site.

Packet clocks becoming mainstream
The general agreement from both the conference and standard perspective is that IEEE 1588 packet clocks are finally moving into mainstream deployment. In December, the ITU-T agreed on 3 new recommendations for frequency delivery via packet clocks. These recommendations are directly applicable to mobile backhaul.

1. G.8260: Definitions and terminology for synchronization in packet networks.
2. G.8261.1: Packet Delay Variation Network Limits Applicable to Packet-based Methods
   This standard specifies the deployment case where external equipment provides frequency synchronization to a mobile base station. It specifies the PDV input network limits (noise of packet network) and the output network limits (filtering capability of the clock).
3. G.8263: Timing Characteristics of Packet-based Equipment Clocks
   This standard provides a model of a packet clock for frequency delivery. It also indicates the need for a good quality local oscillator to meet the G.8261.1 output tolerance mask during worst case network conditions.

Together, these 3 standards represent a considerable advancement in both the industry's understanding of synchronization technology and operators' commitment to deploying packet clocks.

Standardization brings credibility
The ITU-T has now essentially completed standardization work on packet clocks for frequency, as the focus has clearly moved to phase and time synchronization.

The next phase of standardization work is the G.827x series of recommendations for time and phase distribution across Ethernet networks. The ITU-T approved the first standard of this series as G.8271. This standard provides the requirements and architectural model for using IEEE1588v2 boundary clocks in a transport network environment together with Synchronous Ethernet.

Packet clocks are no longer new, but despite successful trials and considerable live deployment, they have lacked the credibility of SDH and Synchronous Ethernet. The progress the ITU-T made in 2011 provides packet clocks with more credibility through models, limits, metrics and a common vocabulary.

Network operators will no longer get stuck discussing how bad the clock is or how good the network is. With the help of G.8261.1, operators can deploy packet clock technologies into Ethernet Backhaul networks more confidently.

*LTE-FDD: LTE-Frequency Division Duplex
** LTE-TDD: LTE-Time Division Duplex