1、 TECHNICAL REPORT ATIS-0900004 INTRA-OFFICE SYNCHRONIZATION ARCHITECTURE As a leading technology and solutions development organization, ATIS brings together the top global ICT companies to advance the industrys most-pressing business priorities. Through ATIS committees and forums, nearly 200 compan
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3、clude standards, specifications, requirements, business use cases, software toolkits, and interoperability testing. ATIS is accredited by the American National Standards Institute (ANSI). ATIS is the North American Organizational Partner for the 3rd Generation Partnership Project (3GPP), a founding
4、Partner of oneM2M, a member and major U.S. contributor to the International Telecommunication Union (ITU) Radio and Telecommunications sectors, and a member of the Inter-American Telecommunication Commission (CITEL). For more information, visit . Notice of Disclaimer in packet networks, this signal
5、is traceable to a Primary Reference Time Clock (PRTC); see ITU-T G.8272. ATIS-0900004 6 Figure 1 - Phase Synchronization (from G.8260) 3.1.16 Primary Reference Time Clock (PRTC): A reference time generator that provides a reference time signal traceable to an internationally recognized time standard
6、 (e.g., UTC). 3.1.17 Time clock: Network equipment that provides the elapsed time from a reference epoch. 3.1.18 Time synchronization: The process of distribution of a time reference from a PRTC to the real-time clocks of a telecommunications network. All the associated nodes have access to informat
7、ion about time (in other words, each period of the reference timing signal is marked and dated) and share a common timescale and related epoch (within the relevant time accuracy requirement). Examples of timescales are: UTC TAI UTC + offset (e.g., local time) GPS PTP Local arbitrary time Distributin
8、g time synchronization is one way of achieving phase synchronization. tttiming signal recovered by system Atiming signal recovered by system BSystem ASystem BBReference timing signalto system AReference timing signalto system BATIS-0900004 7 Figure 2 -Time Synchronization (from G.8260) 3.2 Acronyms
9、BC Boundary clock BITS Building Integrated Timing Source CC Composite Clock CES Circuit Emulation Service CO Central Office (building) DSL Digital Subscriber Network DTI DOCSIS Time Interface (ITU-T J.211) GNSS Global Navigation Satellite Systems GPS Global Positioning System MRTIE Maximum Relative
10、Time Interval Error MTIE Maximum Time Interval Error NE Network Element NGN Next Generation Network NTP Network Time Protocol PDV Phase Delay Variation PMC Packet Master Clock PON Passive Optical Network PP2S Packet per 2 seconds (0.5 Hz) PRS Primary Reference Source PRTC Primary Reference Time Cloc
11、k PTP Precision Time Protocol QoS Quality of Service TAI International Atomic Time System ASystem Btttiming signal recovered by system Atiming signal recovered by system B00:01:0200:01:0100:01:0000:01:0200:01:0100:01:00Ex.: UTC, UTC + n x hoursGPS Time, Local arbitrary TimeATIS-0900004 8 TDEV Time D
12、eviation TDM Time Division Multiplexing ToD Time of Day TSG Timing Signal Generator UTC Coordinated Universal Time 3.3 Abbreviations None. 4 Introduction The distribution of both frequency and time within a central office (CO) revolves around the timing signal generator (TSG). A CO is a building own
13、ed by a telecommunication operator with a centralized synchronization distribution mechanism covering different types of network elements. Traditionally, the timing architectures were divided into intra-office and inter-office, but this document will start to change this idea in certain cases. The T
14、SG can have various network interfaces that are detailed in this technical report. As shown in Figure 3, there can be frequency only references as shown to NE-1 and NE-2 or packet interfaces as shown to NE-5, NE-6, and NE-7. (Please note that the diagram in Figure 3 shows examples of the different t
15、ypes of NE in a central office and there may be many more of each type of NEs in a real CO.) NE-6 and NE-7 are assumed to be stand-alone and not co-located with another TSG. Figure 3 - Reference Model for Intra-office Synchronization Architecture The connection between the Timing Signal Generator (T
16、SG) and served network element (NE) can be of two basic types as depicted in Figure 3. The two fundamental modes are point-to-point and networked. Ethernet connections (see Ref. 5) can be used for either case. The point-to-point case can be achieved using conventional (existing) cabling means such a
17、s the single twisted pair cable used for DS1 and CC (composite-clock) timing signals, though for two-ATIS-0900004 9 way time transfer the transmission is done in a ping-pong fashion to provide signals in both directions. The preponderance of existing Central Office timing distribution is achieved us
18、ing dedicated cabling. Some of the important considerations of the networked connection mode are: a. There are intervening devices between the TSG (“master/server”) and timed network element (“slave/client”). The presence of such devices, packet-switches, or routers, implies that packet delay variat
19、ion (PDV) could be inserted in the timing flow path. b. It is possible to serve multiple NEs from a single TSG port. Care must be taken in provisioning the port for this purpose. c. It is possible for the NE to accept timing information from the TSG on a port that also carries other traffic. d. Ther
20、e could be asymmetry in the transmission delay characteristics of the paths in the two directions (TSG to/from NE). e. The service provided by this method of connectivity is principally protocol layer synchronization (i.e., packet-based methods). f. The physical layer for connectivity is typically E
21、thernet (see Ref. 5). Some of the important considerations of the point-to-point connection mode are: a. There are no intervening devices between the TSG (“master/server”) and timed network element (“slave/client”) and consequently there is minimal packet delay variation (PDV) inserted in the timing
22、 flow path. The type of PDV introduced in this situation arises typically from time-stamping errors b. It is not possible to serve multiple NEs from a single TSG port. There is a one-to-one correspondence between TSG port and NE port. c. It is not possible for the NE to accept timing information fro
23、m the TSG on a port that also carries other traffic. A dedicated NE port is required for timing. d. There is minimal asymmetry in the transmission delay characteristics of the paths in the two directions (TSG to/from NE). Asymmetry, if any, arises primarily because of circuit element variability and
24、 is usually very small. Some asymmetry can be introduced if separate pairs are used for the two directions of transmission. e. The service provided by this method of connectivity can be protocol layer synchronization (i.e., packet-based methods) or physical layer synchronization or a hybrid case. f.
25、 The physical layer for the connectivity can be Ethernet (see Ref. 5) or traditional twisted pair cabling. 4.1 Timing Signal Generator The TSG serves as the focal point for delivering timing to the various network elements (NE) within a Central Office. In a TDM scenario, the timing feed is one-way b
26、etween the TSG11and the NEs and serves as a frequency reference. That is, the NE clock aligns itself with the TSG in terms of frequency; phase alignment is possible at a coarse level. In Next Generation Network (NGN) scenarios where the alignment must be in terms of frequency as well as time/phase,
27、the timing feed involves two-way transmission, but the flow of timing information is always from the TSG to the NE. That is, the TSG is the “master” and the NE is the “slave” device. Whereas legacy feeds are based on physical layer timing transfer, NGN feeds can have a protocol layer aspect as well.
28、 The principal functions of the TSG are: 11Note that a Building Integrated Timing Supply (BITS) is sometimes incorrectly used interchangeably with BITS. This document will use TSG and explains the difference between TSG and BITS in 4.1.1. ATIS-0900004 10 a. To enable the synchronization of all the C
29、O-based network elements to a common reference. In TDM (e.g., SONET) terms, this synchronization is in terms of frequency alignment. In COs that require timing for DS0 support, composite clock methods provide phase alignment as well (the phase alignment is coarse by NGN standards). In NGN terms, the
30、 alignment is in frequency as well as, possibly, phase/time. b. To provide holdover. In the event that the CO loses its timing reference for any reason, the BITS enters holdover. The holdover performance of a clock is based on the quality of its oscillator. It is economically advantageous to focus t
31、he holdover capability in the TSG, rather than in each NE. c. To provide reference filtering. The TSG is usually a slave clock, deriving its timing reference from a Primary Reference Source (PRS) either directly or indirectly. The direct feed from PRS to TSG is possible when the two equipments are c
32、ollocated in the CO. It is common to obtain a reference that is traceable to a distant PRS over the trunk transmission network. These reference signals could have accumulated jitter and wander that needs to be filtered out and this clean-up is performed by the TSG. That is, the TSG can be part of th
33、e synchronization network. d. In an NGN context the TSG can obtain its time/frequency reference from a suitable source such as GPS and thereby serve as the host to facilitate the functionality of an NTP NTP-stratum-1 server or a PTP Grandmaster. NOTE: The term “NTP-stratum-1” is used to indicate tha
34、t the server derives its time/timing reference from a source other than NTP. The term “stratum” as used in NTP differs from the usage of the same word in a telecommunications. It is generally clear from the context which meaning is implied. e. For reliability reasons, each NE is provided two timing
35、feeds (redundancy) from diverse ports on the TSG. It is recommended that the two feeds (active and standby) be located on different TSG circuit packs so as to allow for uninterrupted service during maintenance/repair action. f. For operational reasons, there is a deterministic mapping available to i
36、dentify the connection between TSG port and NE. 4.1.1 BITS versus TSG A timing signal generator (TSG) is a synchronization network element that traditionally has three main functions: input, filtering/holdover, and output. Telcordia defined the TSG in Telcordia GR-378-CORE, a TSG equipment specifica
37、tion. The interfaces of a TSG are defined in ATIS-0900101. The input has been DS1 for frequency and the output has been DS1 or CC. There are some manufacturers who add a primary reference source (PRS) to the TSG functionality. A PRS can be GPS or atomic clock (cesium) based. The GPS has the added ad
38、vantage that it can provide time as well as frequency. Historically, the BITS (Building Integrated Timing Supply) concept is important in the large wireline service providers in North America. The BITS concept states that all timing from outside of the office will go through one piece of equipment f
39、or filtering and distribution. This replaced the previous system that had separate synchronization systems for different types of equipment. The BITS concept also helps with maintenance of the synchronization system since a problem is either localized between the TSG and the NE or is widespread arou
40、nd the office (due to an input problem on the TSG). Note that some service providers do not accept the BITS concept due to the size of their offices or the widespread use of line-timed SONET/SDH network elements in long chains. The BITS concept is implemented with TSGs, but a TSG can be used without
41、 BITS. 4.1.2 Multiple BITS Instances In very large Central Offices, the number of NEs deployed can be substantial, exceeding the physical port count of a single BITS device. Most manufacturers provide the notion of expansion shelves to increase the port count, but the limit can still be exceeded. Co
42、nsequently, multiple instances of BITS may be required, possibly generating different timing domains. If all the distinct BITS equipments are provided a common reference, then they can be considered as part of the same domain. ATIS-0900004 11 A common frequency reference in a telecommunications cont
43、ext can be construed as the case when two devices are aligned relative to each other to an accuracy of better than 1x10(-11). A common time/phase reference is more challenging, since it implies that the long term frequency alignment between two devices is “perfect” and the time/phase alignment is be
44、tter than “X” ns. A suitable value for “X” is still the subject of ongoing development. If “X” is small enough, it is possible that two equipments, each with its own GPS receiver, may be considered as time-mis-aligned because the time error between the two exceeds “X” even though experience has show
45、n that it is straightforward to ensure that the same two devices are frequency aligned in normal operation. 4.2 Frequency Distribution Historically, frequency distribution from the TSG to the SONET Network Elements has used DS1 signals. Over time, the set of network elements requiring frequency sync
46、hronization has expanded to include xPON and xDSL access modules, microwave transmitters, and Synchronous Ethernet nodes. Both SONET and Synchronous Ethernet network elements also have derived DS1 outputs to provide optional references into the TSG from interoffice transmission lines. SONETNESynchro
47、nous EthernetSwitchTSG(BITS)Derived DS1Various TransportNetwork ElementsOC-NxPONxDSLMicrowaveSyncEPTP GMGNSS AntennaGNSS ReceiverDerivedDS1Figure 4 Intra-office Synchronization Distribution Example for Frequency Distribution ATIS-0900004 12 In addition, the distribution technique for frequency now i
48、ncludes the use of PTP packet transmissions from a IEEE-1588 Grandmaster Clock internal to the TSG or located in a separate network element. An example of frequency distribution within an office is shown in Figure 4. The frequency is transported from the TSG to the network elements either by DS1 (in
49、 the solid lines) or by packet (primarily PTP) in the dashed lines. For this example, a local GNSS (Global Navigation Satellite Systems) receiver is used to provide a frequency reference to the TSG and a phase reference to the PTP GM. It is expected that both of these receivers are GPS-based, but that may change in the future. If the office does not have a GNSS receiver, the PTP GM would be a BC (boundary clock) which provides the same output, but has a PTP input and gets its phase reference from another network element outside of the office. In addition to the me
