Hardware Time-stamping in PTP 1588

The Hardware that supports PTP event-message time-stamping makes the synchronization much more accurate. As you can see in the figure below, the same timestamps can be gotten from various layers:

  1. Software
  2. Kernel
  3. Driver
  4. MAC
  5. PHY

The lower (nearer) we go to the physical-layer, the more accurate and exact time we can use to find when a particular PTP packet was received from or transmitted to the line/wire.

content-security

...that's not all folks!

To continue reading the article, please Login/Register

(c) AVChrono 2021, All Rights Reserved

PTP Transparent clocks 1-step 2-step

Why Transparent Clock

Transparent clocks are used in PTP topologies in order to improve the accuracy of the end-station synchronization. The reason is that time-unaware devices introduce varying amount of path-delay between the clock-source (master) and end-station (slave). Unless the master and slave are directly connected, or all the devices in between then are time-aware the inaccuracy increases due to varying amounts of queue-delays and path-delays of different nodes and links.

How Transparent Clocks work

Transparent clocks are much simpler that full-featured Boundary clocks because the aim of a transparent clock is to make itself invisible (of sorts) from their own effect on transport of time across the PTP topology. They do this by timestamping PTP packets as they ingress (with a timestamp T1) and at egress (with a timestamp T2) and putting this delta-difference (T2 - T1) as a correction-field (CF) in the PTP packets. Such a device (called an End-to-End or E2E TC) removes its internal delay (and thus delay-variation) by informing the end-station about its own CF.

Working of a PTP Transparent Clock

Types of Transparent clocks

There are 2 types of Transparent clock:

  • The ones that measure their own delay (E2E)
  • The ones that also measure the link-delay (P2P)

The additional logic in the latter TC is that, it uses PDelay_Req and PDelay_Response packets with its peer-device to calculate the link-delay and adds that to the residence time (T2-T1) calculation ie.

Correction Field = Link Delay + Residence time

There is another way to classify Transparent clocks:

  • The ones that use 1-step clock
  • The ones that use 2-step clock

The difference in the latter is that they use a Follow-up message to convey the Correction-Field as they cannot update the delta (residence time) on the fly.

content-security

...that's not all folks!

To continue reading the article, please Login/Register

(c) AVChrono 2021, All Rights Reserved

IEEE 1588 PTP Clock Modes and Transports

IEEE-1588 PTPv2 has various clock-modes that can work with different Transport-layer technologies like Ethernet, IPv4, IPv6 as well as MPLS, DeviceNet, ControlNet, etc. Here we look at the various scenarios and cases on how PTP can serve different network use-cases with its different configuration combinations.

Ethernet Mode

S. No. Transport Clock Mode Delay Mechanism Clock Steps
1 Ethernet Boundary End-to-End 1
2 Ethernet Boundary E2E 2
3 Ethernet Boundary Peer-to-Peer 1
4 Ethernet Boundary P2P 2
5 Ethernet Transparent E2E 1
6 Ethernet Transparent E2E 2 (2-step)
7 Ethernet Transparent P2P 1
8 Ethernet Transparent P2P 2
Table 1: IEEE-1588 Clock Modes with Ethernet Transport

The PTP communication in Ethernet mode using the specific Multicast MAC address(es). These come in two varieties, the general Multicast address (01-1B-19-00-00-00) and the reserved Multicast address (01-80-C2-00-00-0E). While the former is used to deliver General (Announce etc) and event (Sync, Delay_Req etc) messages, the latter is specifically used for peer-delay (Pdelay_Req etc) messages.

1588 Sync and Delay Req

Let us look at the PTP messages and then map them to each of the modes and transports tabulated above.

S. No. Message Type Destination MAC Timestamps
1 Sync Event 01-1B-19-00-00-00 T1, T2
2 Follow_Up General 01-1B-19-00-00-00 T2'
3 Delay_Req Event 01-1B-19-00-00-00 * T3
4 Delay_Resp General 01-1B-19-00-00-00 * T4
5 PDelay_Req Event 01-80-C2-00-00-0E T1, T2
6 PDelay_Resp Event 01-80-C2-00-00-0E T3, T4
7 #P_Follow_Up General 01-80-C2-00-00-0E T3'
8 Announce General 01-1B-19-00-00-00 NA
9 Management General 01-1B-19-00-00-00 NA
10 Signaling General 01-1B-19-00-00-00 NA
Table 2: PTP Message Types and Destination

content-security

...that's not all folks!

To continue reading the article, please Login/Register

(c) AVChrono 2021, All Rights Reserved

You cannot copy content of this page
%d bloggers like this: