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DOCSIS Timing Protocol (DTP) Generating Precision Timing Services from a DOCSIS System

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DOCSIS Timing Protocol (DTP) Generating Precision Timing Services from a DOCSIS System The DOCSIS Timing Protocol (DTP) Generating Precision Timing Services from a DOCSIS System John T. Chapman, CTO ATTG & Fellow, [email protected] Rakesh Chopra, Principal Engineer, [email protected] Laurent Montini, Technical Leader, [email protected] Abstract generation of precision timing protocols such as NTP, PTPv2 (IEEE Std1588-2008) and New market opportunities for DOCSIS Synchronous Ethernet that can serve new include applications such as cellular and evolving CPE devices with traceable backhaul of femtocell, picocell, microcell, time and frequency synchronization and macrocells. These applications may requirements. require network timing in terms of time and frequency. With the deployment of IP and Ethernet based networks, PTP (IEEE 1588) TABLE OF CONTENTS and Synchronous Ethernet have become popular approaches for distributing carrier- Table of Contents 1 class network timing over a network. Introduction 2 DOCSIS and the HFC plant present Introducing DTP 2 many challenges why it is difficult to Wireless Basics 2 propagate network timing information from Mobile Backhaul Synchronization 3 the headend, through a DOCSIS network and Radio Frequency Synchronization 4 into a CPE device with any degree of Phase/ToD Synchronization 5 What about GPS? 6 accuracy. These challenges include: Alternative to GNSS solutions 6 • HFC plant asymmetry, Synchronous Ethernet 7 • DOCSIS asymmetry due to the IEEE 1588 8 upstream scheduler variability, Evolution History 9 Network Node Types 9 • unknown asymmetrical plant delay PTP Protocol Overview 11 between CMTS and CM, Achieving Frequency Synchronization 13 Achieving Time Synchronization 14 • unknown delay of CMTS and CM Time Synchronization Error Sources 15 PHYs, Improving Packet-Based Timing Accuracy 15 • uncalibrated ranging. DTP Operation 16 System Description 16 This paper proposes a solution called CM Frequency Synchronization 16 DOCSIS Timing Protocol (DTP) and CM Time Synchronization 18 discusses how DTP can address these Time Offset Technique 20 challenges in a DOCSIS system and what DOCSIS Path Latencies 20 specification and product changes are DOCSIS Ranging 20 needed to the DOCSIS CM, CMTS, and DTI Measuring Round Trip Delay 21 Server. The resulting design can support the An Example 23 1 Caveats 23 Precision timing in the context of DTP First Pass Approximation 24 refers to protocols such as Network Time Measuring DOCSIS Asymmetry 24 Protocol (NTP), Precision Time Protocol Offset Math 25 (PTP), or hardware interfaces such as a Pulse What about DPV? 26 Per Second (1PPS) output and Synchronous DTI Server Recap 26 Ethernet. Additional Sources of Error 27 Reference CM Precise Timing Output 27 Any specific reference to supporting a DTI Server Propagation Delays 27 PTP output on the CM in this white paper Differences in CM Hardware & Software 27 inherently could include other protocols such Differences in CMTS Hardware & Software 27 as NTP and/or 1PPS-style interfaces and Ranging Accuracy 27 their associated protocols. A standard Upstream Interleaver 28 telecom 1PPS output is not defined at this Methodology 28 time but is under study by the ITU-T. Deployment Scenarios 29 All references to PTP and IEEE 1588 Scenario 1 30 imply the latest version of PTP that is PTPv2 Scenario 2 31 as defined by IEEE Std 1588-2008. Scenario 3 32 Summary of Design Changes 33 Wireless Basics CM 33 CMTS 33 For clarification, it is useful to review DTI Server 34 the basics of wireless terminology for this DOCSIS Protocol Changes 34 paper. DTI Protocol Changes 34 Conclusion 34 Wireless includes cellular technology such as LTE, GSM/UMTS and others as well Acknowledgements 35 as non-cellular technologies such as Wi-Fi References 35 and DECT. This white paper targets cellular wireless technologies since they generally require highly precise time and frequency synchronization. INTRODUCTION Each wireless technology generally has a base station that acts as a coupling point Introducing DTP between the wireless and wired network. The The DOCSIS Timing Protocol (DTP) is classification of base station is related to its the proposed name for a series of hardware, coverage and usage. Common classifications software, and protocol modifications to the used today along with typical coverage and DOCSIS system to support the highly typical usage areas are listed on the next accurate and traceable generation of page. Each technology and cell site produces precision timing from the CM. slightly different results. The DOCSIS system in the context of Standard GSM macrocell range is DTP includes the CM, the CMTS, and the limited to 35 km but an extended range cell DTI Server. may go up to 60-100 km in certain areas. Range depends on various parameters including technology, power, area or 2 coexistence with other cells of same or Such trends lead to optimization of the distinct radio technology. The example mobile backhaul infrastructure. ranges below are for WCDMA in open air (reduced coverage inside of buildings) Packet-based networks allow the operators a cost-effective way to fulfill the • Macrocell necessary improvements in bandwidth, coverage and access. Today, IP over Ethernet • WCDMA: 43 dBm/30m = 1 km is the most utilized transmission option for • Rural areas or along highways the aggregation networks. • Microcell The choices for the last mile access • WCDMA: 33 dBm/20m = 400 m transport technologies include Ethernet • Malls, hotels (fixed and microwave), Passive Optical • Picocell Network (PON), Digital Subscriber Line (DSL) or legacy TDM line such as T1/E1 or • WCDMA: 24 dBm/10m = 200 m SONET/SDH (either fixed or microwave). • Transportation hub, airplane Cable HFC (hybrid fiber coax) networks are • Femtocell being considered also as the bandwidth of DOCSIS (Data over Cable Service Interface • WCDMA: 24 dBm/1m = 71m Specification) based systems continue to • Actual coverage area is usually increase. less due to being inside of a building. The first generation of this mobile • Residential home backhaul network evolution called for the support of legacy TDM circuits (e.g. for 2G GSM base stations). Replacing these circuits Mobile Backhaul Synchronization created a market for circuit emulation service over packet networks. Because T1/E1 In the last few years, synchronization in requires accurate and stable clocking, circuit access networks has become an important emulation services (or TDM pseudo-wires topic because of the evolution from TDM- services) over packet networks inherited the based to packet-based networks. In need for frequency synchronization. particular, the mobile wireless operators are struggling to increase their backhaul capacity DOCSIS 1.1 introduced circuit that is required by the newest radio emulation support and was able to leverage technologies in order to provide greater inherent DOCSIS frequency transfer such as bandwidth and improved services. NCR (Network Clock Recovery) via Symbol Clock Lock. Other access technologies such Although the introduction of smaller as SHDSL or GPON also have bit timing capacity base stations (namely microcell, (physical layer) capability. But this requires picocell and femtocell) permits mobile and equipment at both ends to be able to either broadcast operators to improve the wireless receive or to retransmit clock signal. Timing service by providing better coverage, it also distribution must then be accordingly demands increasing the number of network planned. connections. Classic Ethernet has no such synchronous clocking capability. Adaptive 3 Clock Recovery (ACR) – that is, recovering synchronization between base stations allows frequency from a packet flow – from Circuit user handsets to seamlessly handover Emulation Services (CES) traffic was the between base stations, reduces interference first method developed to support TDM between cells and optimizes radio bandwidth pseudowires [G.8261]. If such a solution was capacity. sufficient in some cases for CES application, it appeared to be sub-optimal to support base To improve timing services available station radio interface requirement. from networks, particularly Ethernet based, ITU-T Question 13 in Study Group 15 took Radio Frequency Synchronization the leadership on investigating solutions and defining the appropriate specifications. Indeed, one critical aspect of mobile Focus was first given to frequency base stations (from GSM to LTE or distribution because of the CES application WiMAX) and broadcast transmitters is their and 2G/3G base stations. need for synchronization of their radio interface. Accurate frequency This focus led to adopting Synchronous Radio Frequency Phase or Time Synchronization Cell (Base Station) Technology or Type Read: better than… Read: less than… Service Macro ±50 ppb GSM N/A Pico ±100 ppb ±50 ppb WideArea ±16 ppb (OBSAI) WCDMA (and Medium/LocalArea N/A LTE) FDD ±100 ppb (micro/pico-cell) Home BS (femtocell) ±250 ppb WideArea ±50 ppb WCDMA TDD ±2.5 µs between base stations LocalArea ±100 ppb WideArea ±50 ppb TD-SCDMA ±3 µs between base stations LocalArea ±100 ppb WideArea ±50 ppb ±3 µs between base stations (may LTE TDD LocalArea ±100 ppb range from ±0.5 µs to ±50 µs) Macro ±50 ppb ToD (UTC) sync should be less than CDMA2K Pico and Femto ±100 ppb 3 μs and shall be less than 10 μs Up to ±1 ppb (with an Usual values between ±0.5µs and ±5 WiMAX Mobile average target of ±15 µs ppb) MB SFN Service ±50 ppb ±1 µs LTE-Advanced Services (CoMP, ±0.5 µs [±1 µs] Up to ±5 ppb (CoMP) relaying function, carrier aggregation…) may be < ±0.2 µs (TBC) DVB SFN Up to ±1 ppb General agreement: ±1 µs Table 1 – Cellular Accuracy Requirements 4 Ethernet, a physical layer method that As for frequency synchronization, those demands hardware changes in Ethernet wireless phase/ToD requirements apply to equipment as well as to define the radio interfaces, particularly if Time recommendations to support packet-based Division Duplexing (TDD) is being used frequency transfer with no hardware changes (e.g., WCDMA or LTE TDD). TDD is a in packet network elements. method allowing radio interface to transmit and receive in different time slots on the As mentioned earlier, frequency same media or frequency band.
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