DOCSIS 3.1 MAC SUBLAYER DTP – DOCSIS Time Protocol John Chapman Fellow, CTO Cable BU Cisco Corporation © 2014 CableLabs®. All rights reserved. DTP Introduction What is DTP trying to achieve? • Goal – To provide precise frequency and time to an external system that is connected to the network port of a DOCSIS CM. – Running timing protocols over the top of DOCSIS adds timing error due to upstream scheduling jitter and asymmetrical delay. – DTP allows conversion to and from other timing protocols. • Application – Wireless/cellular backhaul of pico, femto & macrocells over DOCSIS. – Any service requiring precision timing. © 2014 CableLabs®. All rights reserved. Time Protocols A Brief History © 2014 CableLabs®. All rights reserved. Timing Protocols A brief history Name Standard Typical Accuracy Signaling Server - Client Comments Time Protocol RFC 868 1983 seconds Req time Send time Used for ToD in DOCSIS Extremely basic. NTP v4 RFC 5905 1985-2010 1 – 100 ms Delay Req Delay Rsp Depends upon network topology. OTT system. 32/64/128 bit timestamp PTP v2 IEEE 1588 2002-2008 1 – 100 us Sync Delay Req Delay Rsp Modifies nodes in networks to achieve precision. 64 bit timestamp. SyncE G.8261 0 ppm delta ESMC Locked to source clock. Initially HW only. SONET-like. 1 PPS Evolving Not a true protocol (yet) HW Pulse marks a describable event. © 2014 CableLabs®. All rights reserved. Synchronous Ethernet A hardware centric design Frequency Synchronization Ethernet Network OC-X STM-X SONET/SDH Network Hybrid Hybrid EEC T1 Engineered Timing Path • Initially for frequency only. – Follows a Stratum clock hierarchy. • Extended with ESMC (Ethernet Synchronization Message Channel) – SyncE/ESMC are intended to replace SONET/SSM © 2014 CableLabs®. All rights reserved. IEEE-1588 Precision Timing Protocol (PTP) A software centric design with hardware support • PTP is a message based SYNC and two-way time transfer (TWTT) protocol for synchronizing distributed nodes in time and frequency. • PTP defines a hardware and software method for passing clocking through a network node. – Timestamps are placed in an Ethernet packet and replaced or corrected as they pass through a node. – If a network node can be made to look like a constant delay, and since the network link is usually constant delay, then the overall network can have predictable and precise timing coupled through it. © 2014 CableLabs®. All rights reserved. PTP Clock Types Definitions for all network scenarios • PTP defines various clock types GM: Grandmaster Clock GM is the ultimate source of time in a PTP domain Clocking is terminated (Slave) and regenerated (Master) at a network node. BC: Boundary Clock TC: Transparent Clock Clocking passes through a network node and receives a correction factor. OC: Ordinary Clock End node clock © 2014 CableLabs®. All rights reserved. Frequency Synchronization Time Synchronization c & sp n y s y_re a del sync & delay_resp S SyncE SyncE OC GPS GM S M de M la y_ re q Non-Participant Nodes M Non-Participant Nodes sync + corrA TC TC TC GM : Grand Master BC : Boundary Clock OC : Ordinary Clock TC : Transparent Clock M : Master port S : Slave port a y_ syn c _req delay +A rrB + co delay_req del BC res p sync + corrA+B TC TC TC delay_req + corrB S OC delay_req © 2014in CableLabs®. All rights reserved. Note: TC is often used end-to-end a network. DTP Network Protocol Support DOCSIS 3.1 can interface with 1588 and SyncE IEEE-1588 (PTP) • CMTS/CM can be a Grandmaster Clock (GC) – Frequency and time derived from CMTS. PTP originated in CM. • CMTS/CM can be a Boundary Clock (BC) – CMTS syncs to PTP. CM syncs to CMTS. BC is regenerated • CMTS/CM can be a Transparent Clock (TC) – CMTS and CM update correction fields in PTP messages Synchronous Ethernet • CM derives Ethernet Frequency from OFDM Baud clock. © 2014 CableLabs®. All rights reserved. DTP DOCSIS Time Protocol IEEE-1588, SyncE, NTP and DOCSIS as an Integrated System © 2014 CableLabs®. All rights reserved. DTP Scope CMTS Clock CM Clock DTP IEEE-1588 IEEE-1588 SYNC-E SYNC-E DTI NTP NTP GPS • • CMTS synchronizes DOCSIS to a network source. DTP adds support to DOCSIS to support protocol conversion. • • CM generates precision timing using DOCSIS timing to support network protocols DOCSIS latency and asymmetry are measured and compensated for by DTP. © 2014 CableLabs®. All rights reserved. Timestamp - The Concept of EPOCH " PTP typically uses the same epoch as Unix time (Midnight, 1 January 1970). Whereas Unix time is based on Coordinated Universal Time (UTC) and is subject to leap seconds, PTP is based on International Atomic Time (TAI) and moves forward monotonically. The PTP grandmaster communicates the current offset between UTC and TAI so that UTC can be computed from the received PTP time.” - Wikipedia • • • The first thing that is needed is a more accurate timestamp. EPOCH anchors a timestamp to time of day. DOCSIS 3.1: – Uses a 64 bit timestamp – Uses EPOCH of Midnight, 1 Jan 1970 – Uses TAI monotonic counting method • This allows for easier conversion between D3.1 and IEEE-1588. © 2014 CableLabs®. All rights reserved. DOCSIS 3.1 Timestamp D3.0 timestamp is contained in the D3.1 timestamp 8 bytes 23 bits 32 bits Epoch 112 years D3.0 Timestamp 7 min 97.66 ns 10.24 MHz 5 bits 4 bits ÷ 20 ÷ 16 4.88 ns 204.8 MHz with roll-over 305 ps 204.8 MHz x 16 10.24 MHz x 320 Comparison Size Precision Max Time DOCSIS 3.0 32 bits 97.6562 ns 7 min DOCSIS 3.1 64 bits 305 ps 112 years IEEE-1588 64 bits 1 ns 584 years NTP 64 bits 232 ps 136 years © 2014 CableLabs®. All rights reserved. True Ranging Offset • • • • DTP defines a True Ranging Offset (TRO) The TRO is the measured ranging offset of the CM between two defined reference points. 1. This value will be the same for all CMs 2. This value equals the round trip delay of the HFC plant. The TRO is measured at the CM between: A. the time the first bit of a packet is transmitted in the upstream from the CM, and B. the time the first bit of a packet is expected to arrive at the CMTS (MAP entry) Since the measurement is done when an upstream packet is transmitted, (after buffering), all upstream scheduling jitter is eliminated. © 2014 CableLabs®. All rights reserved. PTP and DTP System View CMTS CMTS Clock CM CM Clock DTP PTP NSI/DTI PHY Buffering Switching Buffering DOCSIS PHY HFC Plant DOCSIS PHY Buffering Switching Buffering CMCI PHY PTP Legend: green = fixed delay path blue = variable delay path • IEEE-1588 (PTP) runs external to CMTS/CM • DOCSIS DTP runs internal to CMTS/CM • Timing is hardware coupled to the device edges Slide 15 © 2014 CableLabs®. All rights reserved. IEEE-1588 Syncing The tricks of the trade • All timing protocols use a variant of TWTT – Two-Way Time Transfer • A timestamp is sent from A to B • Messaging is used to determine the network delay and asymmetry. • Timestamp B is corrected to match timestamp A. • DTP has this information built into DOCSIS Slide 16 © 2014 CableLabs®. All rights reserved. DTP True Ranging Offset TWTT is achieved by measuring the result of DOCSIS Ranging • The TRO is the measured ranging offset of the CM between two defined reference points. – TRO is a measured (or derived) value that is different than the actual implemented ranging offset a CM might use in its communication with the CMTS. • The value of TRO is the equivalent to the round trip delay of the combined downstream and upstream propagation delays of the HFC plant, the CMTS and CM PHY paths. • Since the measurement is done between the downstream clock path and when an upstream packet is transmitted (after buffering), all jitter and delay from internal packet queues are eliminated from the measurement. Slide 17 © 2014 CableLabs®. All rights reserved. DTP Timing Model Generic Model for CMTS, CM, and HFC Plant DTP Calibration System CMTS HFC Plant CM CMTS Clock ts-cmts-in t-cmtsds-i CM Clock t-cmtsds-o t-cmtsds-p t-hfcds-o t-hfcds-p t-cmds-o t-cmds-p t-cmtsus-o t-cmtsus-p t-hfcus-o t-hfcus-p t-cmus-o t-cmus-p t-cmds-i t-cmadj ts-cm-out t-tro • The accuracy of the system depends upon the accuracy of the parameters in the model. Slide 18 © 2014 CableLabs®. All rights reserved. PTP Timestamp PTP Timestamp Sync & Conversion (t-cmts-ds-i = 0) Sync & Conversion (t-cm-adj = 1800) (t-cm-ds-i = 0) CMTS Clock CMTS PHY CM PHY CM Clock DOCSIS RANGING, DTP Signaling 2000 t-cmtst-cmtsds-o ds-p 500 25 SYNC t-hfcds-p 750 t-hfcds-o 0 t-cmds-o 25 500 3800 4000 500 750 0 25 5800 6100 RNG REQ 20 t-cmtsus-o 8800 2000 2200 25 MAP (Tx=7000) 7000 200 t-cmds-p 30 t-cm -us-p 750 t-hfcus-p 50 t-hfcus-o 30 t-cmus-p 500 20 4000 4300 t-cmus-o 5200 7000 True Ranging Offset (TRO) Example TRO Round Trip Delay Measured Between Reference Points 500 ✔ known + 25 ✔ characterized + 750 ? to be measured + 25 ✔ characterized + 500 ✔ known + 50 ✔ characterized 7000 + 800 ? to be measured measured transmit - 4300 + 50 ✔ characterized time ====== ==== True 2700 = Ranging Offset 2700 MAP entry Offset needed = 2000 – 200 = 1800 © 2014 CableLabs®. All rights reserved. CMTS Timetamp Reference CMTS PHY CM PHY CM Timestamp Reference Calculating Latency DOCSIS RANGING 2000 500 25 Ti/2 TRO = known delays + unknown delays 200 SYNC 3800 TDS-HFC 25 TDS-CM 4000 500 CMTS CM 750 TDS-CMTS 25 500 HFC Plant 2000 Ti/2 MAP (Tx=7000) 2200 2700 – (500+25+25+500+50+50) = 1550 4000 DS HFC Plant Latency = 1550 / 2 * (known plant asymmetry) = 750 750 25 5800 6100 RNG REQ 500 4300 50 800 50 7000 TUS-HFC TDS-CM 5200 TUS-CMTS 8800 Where: - All values are in arbitrary time units for sake of example. 7000 True Ranging Offset Total DS Latency = 500+25+750+25+500 = 1800 Corrected CM Timestamp = 7000 + 1800 = 8800 © 2014 CableLabs®. All rights reserved. DTP Signaling CMTS is DTP Master • Used for generic CM with CMTS centric design. Larger scale. • Note the extra INFO message. Unique. Slide 21 © 2014 CableLabs®. All rights reserved. DTP Signaling CM is DTP Master • Used for higher end CM centric designs • CMTS can still override specific parameters Slide 22 © 2014 CableLabs®. All rights reserved. Multi-System Designing across systems CMTS 1 T-cmts-error Timing Protocol Source HFC 1 T-hfc-error CM 1 Timing Protocol T-cm-error T-source-skew T-cm-cm-skew T-cmts-error CMTS 2 T-hfc-error HFC 2 T-cm-error CM 2 Timing Protocol • 1588 timing needs consistency across systems. • DTP defines five levels of performance between two systems. Slide 23 © 2014 CableLabs®. All rights reserved. DTP Recap • CMTS synchronizes DOCSIS to a network source. – DTI, IEEE-1588v2, etc • DTP manages DOCSIS latency and asymmetry. • CM generates precision timing using DOCSIS timing • Accuracy is dependent upon accurate modeling of CMTS, CM, and HFC plant. – Target accuracy of better than a few μseconds © 2014 CableLabs®. All rights reserved. DTP Summary It’s about time. • The DOCSIS system is already based upon highly precise timing. – DTP leverages this asset. • Rather than run NTP or PTP over-thetop, the DOCSIS system can be modified to generate or correct these timing protocols with a very high degree of precision. © 2014 CableLabs®. All rights reserved.