ST 2110 Launch Pad – PTP

Presenters : Andrew Scott - Telestream

Year : 2022

dlp_document_download : PTP: The Key to ST 2110 Timing Andrew Scott, Telestream Designed to synchronize clocks via a computer network, with much higher accuracy than NTP Widely used in industries such as telecom and finance, now being adopted for video applications to replace analog sync Version 1 published as IEEE 1588-2002, not used for video Version 2 published as IEEE 1588-2008 Updated with IEEE 1588-2019 ("Version 2.1") Augmented with standards such as SMPTE ST 2059-2 (since 2015) Some terminology has changed: Master/Slave  Leader/Follower Precision Time Protocol – History User Datagram Protocol (UDP) used for transport Port 319 for Event messages Port 320 for General messages Multicast group address for most messages: 224.0.1.129 Exchange of PTP messages may be multicast, unicast, or a mix of both PTP over IPv4 Event message class Sync Delay_Req General message class Announce – used to establish the synchronization hierarchy Delay_Resp Follow_Up Management – used to query and update "data sets" used by PTP instances Differentiated Services used by switches to enforce high priority for event messages Common PTP Message Types Sync messages sent periodically (e.g. 8 per second) from leader to followers Follow_Up message needed only if Sync timestamp is not included in the Sync message Followers send Delay_Req , typically at same rate as Sync After complete exchange, followers have all 4 timestamps Clock Synchronization 𝑡 𝑚𝑠 =  𝑡 2  −  𝑡 1 𝑡 𝑠𝑚 =  𝑡 4  −  𝑡 3 𝑚𝑒𝑎𝑛𝐷𝑒𝑙𝑎𝑦 =  𝑡 𝑚𝑠  +  𝑡 𝑠𝑚 2 𝑜𝑓𝑓𝑠𝑒𝑡 =  𝑡 𝑚𝑠  −  𝑡 𝑠𝑚 2 Follower clock can be adjusted using these values There is also a provision for asymmetric network delays   Clock Synchronization Ordinary Clock PTP nodes with a single port on the network Can be either a follower or a leader Typically in video networks, we have grandmaster (GM) clocks that are leader-only and devices (cameras, etc.) that are follower-only Boundary Clock Multi-port device (i.e. switch) with one follower (synced to the GM) and many leaders on all other ports Provides scalability in large PTP networks Transparent Clock Switch that updates timestamps in PTP messages but does not assume a role PTP Clock (Node) Types Redundant network for resiliency Leaf/Spine switches (boundary clocks) for scalability Hybrid facility with PTP  analog to support legacy devices Media Network Architecture Spine Leaf Leaf Leaf Leaf Leaf Leaf Leaf Leaf Spine Amber (primary) network Blue (backup) network Camera Display PTP Only Analog Sync Signals Legacy Devices Sync Gen Grandmaster GNSS Media Node Announce messages are sent periodically (typ. 1 per second) and include (in priority order): Priority 1 – user-configured Clock Class – GNSS locked, holdover, free-running, etc. Clock Accuracy – estimate based on time source, e.g. "within 25 ns" Clock Variance – computed stability of local clock Priority 2 – user-configured Clock Identity – (tie-breaker) based on MAC address Potential leaders use the Best Master Clock Algorithm (BMCA) to decide which node is the sync source Lead Clock Determination Leader clocks listen for other Announce messages Start sending Announce when better than received Announce Or when no Announce received (timeout is usually 3s) Stop sending Announce (go to Passive) when a better leader is seen Best Master Clock Algorithm Power On, Reset Listen Passive Grandmaster Announce from better leader No announce from better leader No announce from better leader Announce from better leader Domain is simply a numeric identifier (0-255) for a logical PTP network Multiple domains can co-exist on a physical network For example, separate audio (AES67) and ST 2110 domains Every PTP message includes the domain number so nodes can accept or discard the message as appropriate PTP Domains A profile is a set of PTP options and attributes for a given application Message rate defaults and permitted limits (Announce, Sync, etc.) Communication models (e.g. multicast, unicast, mixed) IEEE 1588-2019 specifies the "Delay Request-Response Default PTP Profile" SMPTE 2059-2:2021 specifies the "SMPTE profile for synchronization in a professional broadcast environment" AES67-2018 specifies the "PTP profile for media applications" Other profiles for telecom and other applications PTP Profiles Specifies the PTP profile used by ST 2110 Permits three communication models: Multicast for all messages Unicast for all messages. Followers are configured with a list of potential leaders and use a "grant" mechanism to request that the leader send unicast Announce, Sync, and Delay_Resp messages Mixed – Announce, Sync (& Follow_Up ) and Management messages are sent by multicast. Delay_Req / Delay_Resp are sent by unicast (without grant) Adds a synchronization metadata TLV in Management messages to signal time code information to media nodes (e.g. next jam time) SMPTE 2059-2 Create a solid network plan Redundant amber/blue networks Switches that support PTP, ideally Boundary Clock mode Scalable switch architecture – Spine/Leaf Use PTP-only link between amber/blue networks so that GMs can see each other’s Announce messages All devices see the same GM on both networks Configure switch ports "role master" to prevent rogue leaders Use SMPTE 2059-2 profile and default message rates Use Mixed Multicast/Unicast communication model Best Practices PTP monitoring and measurements Wireshark packet captures for offline analysis Troubleshooting Tools Any Questions?