Precision Time Protocol for indoor mobility of client devices

What is Precision Time Protocol?

"The Precision Time Protocol (PTP) is a protocol used to synchronize clocks throughout a computer network." - Wikipedia

A Celona private mobile network uses LTE-TDD, the important part here being 'TDD' which stands for 'Time Division Duplex', meaning available upload and download bandwidth is allocated by time slot.

And, a PTP Grandmaster is a network-connected device that uses multiple GPS signals to create an accurate timing lock.

Why is it Required?

Indoor networks with multiple Celona access points (AP) are designed to take advantage of overlapping coverage.

If the APs are not accurately time synchronized they could transmit at the wrong time, interfering with signals from other APs in the coverage area, resulting in problems for handover and throughput.

How is PTP deployed in a Celona private mobile network?

PTP Grandmaster

Celona has tested and verified the Trimble Thunderbolt PTP Grandmaster Clock. The PTP Grandmaster device is deployed in a suitable physical location that allows connection to your network, and the placement of an outdoor antenna to receive GPS signals.

The Trimble Thunderbolt must be installed and synchronized prior to integration with your Celona network. If you have already completed this step you can move on to Configuring Celona for PTP.

Configuring the Trimble Thunderbolt GM200

Before starting the configuration of the GM200, connect the GPS antenna, which must have a clear, uninterrupted, view of the open sky.

By default, the GM200 will ship with the Management port (Eth2) set to IPv4 address 192.168.2.250.

• Set a static IP on your laptop (e.g. 192.168.2.10) connect to port Eth2 on the GM200.

• Open a web browser session to 192.168.2.250, Login is trimblesuper, password is Tbolt_<serial number>.

• Configure Port IP addresses. Note that the Management Port and the PTP Port need to be on different subnets, else the GM200 will trigger an alarm.

• Click the gear icon (top-right menu) to enter edit mode.

• Select the preferred IP mode, Static or DHCP.

• Enter static IP info if required.

• Click the wrench icon to save these settings.

• Update your laptop IP address settings and reconnect to the GM200 web interface.

With the management interface settings complete you can move on to set Ethernet Port 1, which is the port used for PTP network traffic. Once configured and saved, connect Ethernet Port 1 to your LAN.

Next, select Synchronization Management from the left-hand menu.

• Click the gear icon to enable editing.

• Select Ethernet Port 1

• Set the following;

  • PTP Port Status: Enabled

  • PTP Profile: G8275.2

  • Sync Mode: One-Step

  • Transport Protocol: IPV4

  • IP Mode: Unicast

  • Delay Mechanism: E2E

  • Domain Number: 44

  • Clock Class: blank

  • Announce Interval: 0

  • Announce Timeout: 3

  • Sync Interval -7

  • Delay Request Interval: -7

  • Priority 1: 128

  • Priority 2: 128

  • P2P Delay Request Interval: 0

  • DiffServ Code Point 0

• Click the download icon (top-right menu) to save the config.

Once all your settings are saved you can check the status of GPS Synchronization by selecting System Status > GNSS from the left-hand menu. The image below shows a successfully synchronization.

Your Trimble Thunderbolt is now ready for use with the Celona network.

Configuring Celona for PTP

The first step is to add the PTP Grandmaster information to you site. Log in to your Celona Orchestrator account, and select Sites from the left-hand menu.

Click the '+' icon to add your external time sync server.

Give your PTP server a name, enter the IP address and select the appropriate Packet Rate. Recommended configuration is as follows:

• Up to 16 APs @ 128 packets per second

• Up to 32 APs @ 64 packets per second

• Up to 64 APs @ 32 packets per second

Click save then select the Access Points menu to begin configuring your APs to use the PTP Grandmaster.

IMPORTANT: Any changes to Timing Sync settings will cause all APs using that timing sync source to reboot and begin the synchronization process again. This can take up to 30 minutes, during which time APs will be offline. Therefore we suggest that any updates to timing sync settings are conducted out of normal operating hours.

Multiple PTP Master can be added to the network for redundancy, follow the same process to add each PTP device. Once your PTP devices are added to the system, go ahead and configure each AP with the appropriate PTP servers. In future releases, you will be able to bulk edit APs or configure via the API.

Click Time Sync Configuration for each AP, selecting the primary and secondary PTP sources as shown in the screenshot below.

You can also access this configuration from the Acceess Points page by entering the AP details page.

From this page click the gear icon for Time Sync Configuration.

Once time sync servers has been selected the AP will reboot and begin the time synchronization process, which can take up to 30 minutes.

Monitor status of PTP sync directly from the Devices page as shown above, status message definitions are listed below.

• Initializing: Waiting for Time of Day hard-sync to complete.

• Acquiring: Time synchronization in progress.

• Freq_Lock: Frequency is synchronized.

• Synced: Both Frequency and Phase are synchronized.

• Failed:

  • High uncertainty, sync may be disqualified.

  • No time sync reports received Or No active server found

  • No time sync reports received

• Unknown: Invalid state.

You can also confirm PTP client status via the Trimble GM200 web interface by selecting System Status > Timing from the left-hand menu.

The PTP client IP addresses should reflect those of your Celona APs.

Enabling device group and application specific QoS policies and seamless integration with an existing enterprise network with custom traffic forwarding rules are next. To learn more, check out our next article in the series.