TICTOC L. He Internet-Draft F. Su Intended status: Informational ZTE Corporation Expires: April 30, 2009 October 27, 2008 Time synchronization method in packet-switched transport network for mobile backhaul draft-su-tictoc-time-sync-mode-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on April 30, 2009. Abstract This document introduces a time/phase transfer application mode employing popular packet-based method IEEE Std 1588-2008 i.e. PTP with support of common physical layer method Synchronous Ethernet in a packet-switched transport network for mobile backhaul and a preliminary thought of time transfer error detection. He & Su Expires April 30, 2009 [Page 1] Internet-Draft Time sync mode in mobile backhaul October 2008 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions used in this document . . . . . . . . . . . . . . 3 3. Time distribution in packet-switched transport network . . . . 3 3.1. Equipment type . . . . . . . . . . . . . . . . . . . . . . 4 3.2. Synchronous Ethernet support . . . . . . . . . . . . . . . 4 3.3. Building of time synchronization path . . . . . . . . . . 5 3.4. Frequency and time distribution path switch . . . . . . . 8 4. Fault detection . . . . . . . . . . . . . . . . . . . . . . . 10 4.1. Problem of time synchronization OAM . . . . . . . . . . . 10 4.2. Detection of time transfer error . . . . . . . . . . . . . 11 5. Security Considerations . . . . . . . . . . . . . . . . . . . 13 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13 7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13 8. Informative References . . . . . . . . . . . . . . . . . . . . 13 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14 Intellectual Property and Copyright Statements . . . . . . . . . . 15 He & Su Expires April 30, 2009 [Page 2] Internet-Draft Time sync mode in mobile backhaul October 2008 1. Introduction As the transport network for mobile backhaul has being migrated from circuit-switched technology to packet-switched technology such as Ethernet, IP/MPLS, packet-switched transport network is expected to provide timing and synchronization distribution function as SDH/SONET network does. In 2G mobile backhaul network (e.g. GSM), only frequency synchronization is needed largely by locking base station clock onto a PRC traceable frequency source (i.e. E1/T1 links). By introduction of 3G cellular system (e.g. UMTS TDD, CDMA2000, TD- SCDMA, WiMAX), base stations need to be aligned each other in phase/ time in order to guarantee smooth handover. 3G system with TDD mode stringently requires micro-second level time/phase synchronization. So currently the prevalent way to satisfy this requirement is that base station is synchronized to UTC time by installing GPS receiver on each of them. Due to the high cost and security issues, cellular operators are seeking solutions to replace or minimize the use of GPS and thus to distributes time/phase synchronization through transport network to base stations. This I-D describes how to transport time/phase employing popular packet-based method IEEE Std 1588-2008 i.e. PTP with support of common physical layer method Synchronous Ethernet in a packet- switched transport network for mobile backhaul and a preliminary thought of time transfer error detection. 2. Conventions used in this document The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC-2119. 3. Time distribution in packet-switched transport network The transport network in mobile backhaul between RNC and base station in general comprises aggregation part and access part. The RNC is connected to aggregation equipment (e.g. router or switch) which could be selected as time server or Grand Master (GM) in terms of PTP protocol. In this case, the equipment receives the UTC time by dedicated time interface (e.g. 1PPS + ToD) from GPS or BITS. The time server distributes time information by PTP interface to RNC and downstream equipment respectively. At the far end, access equipment outputs time signal to base station via dedicated time interface (e.g. 1PPS + ToD) or synchronizes the base station via PTP interface. He & Su Expires April 30, 2009 [Page 3] Internet-Draft Time sync mode in mobile backhaul October 2008 The later case requires that the base station support PTP protocol. The time distribution path from the GM to end equipment is logically a tree like topology. 3.1. Equipment type There are basically two types of equipment in the time synchronization network, named PTP equipment and non-PTP equipment. PTP equipment includes standalone or integrated devices that implement Ordinary clock, Boundary clock, End-to-End transparent clock, Peer-to-Peer transparent clock or Management nodes. In this I-D, we focus on application of PTP equipment with Ordinary clock and Boundary clock. Other types of devices would be discussed in future. PTP equipment could enable BMC algorithm and each PTP port could be configured to use Delay Request-response measurement or Peer Delay measurement. Non-PTP equipment refers to switches or routers that deal with PTP packet as general data unit. Normally, the number of nodes in aggregation network is much smaller than that in access network. So the employment of non-PTP equipment in aggregation network, in some respects, avoids the accumulated phase error induced by Boundary clock at cost of increasing the processing load of GM as well as introducing more PDV. In real network engineering, the PTP packet from GM may traverse some non-PTP equipments and is terminated by equipments with Boundary clock at the demarcation point of aggregation network and access network. 3.2. Synchronous Ethernet support PTP is based on the idea that master clock periodically adjust slave clock by calculating time difference between, compensated by transmission delay and latency. So the frequency accuracy and stability of slave clock has considerable impact on the performance of PTP system. If the slave, for example, has local clock with frequency accuracy of 1ppm, the generated time offset rate will be 1us per second. This could be unacceptable for design and engineering of the network. In fact, the frequency accuracy of local clock has worse freerun or holdover performance than 1ppm in most of equipments particularly in access network. On the other hand, the better frequency accuracy makes the slave clock come into steady state as soon as possible, i.e. minimize the convergence time of the PTP. The less convergence time is also addressed by operators in case time source and/or time synchronization path protection switch occurs. A possible way to enhance the performance under condition of low quality local clock is to increase the sync packet rate of master clock in PTP, e.g. 100 or 1000 packet per second, but this would not He & Su Expires April 30, 2009 [Page 4] Internet-Draft Time sync mode in mobile backhaul October 2008 be always effective if the congestion is introduced by intermediate non-PTP equipments. Obviously, the high rate of sync packet brings out more bandwidth consumption and spending of CPU resources. Alternatively, the Synchronous Ethernet is a physical layer method that locks local clock to external stable frequency source or line timing signal from frequency synchronization network traceable to PRC. This provides an easy and reliable scheme for the time synchronization path to build the PTP operation on well-synchronized frequency distribution path. Each Boundary clock implements Synchronous Ethernet function including SSM detection and processing. 3.3. Building of time synchronization path The overview of time synchronization path construction is given by the flow chart in Figure 1 Initially, a frequency synchronization path is formed on a link by link basis. The primary frequency source of the backhaul network could be the same source of time, e.g. given by GPS. In this case, all PTP equipments (with Ordinary clock or Boundary clock) hierarchically follow the frequency output of GM and a tree like frequency synchronization path is established from the GM to end equipments in the access network. The frequency source cloud be otherwise given from existing synchronization network and/or other external frequency source (e.g. BITS/SSU). The PTP equipment should perform clock source selection among multi frequency sources (e.g. recovered line timing from GE or 10GE Synchronous Ethernet links and/or external frequency source) by means of SSM detection or pre- configuration. It is noted that, the timing loop in frequency synchronization path should be prevented by network engineering or some enhanced automatic SSM mechanisms in order to avoid clock quality degradation. After all nodes clock have entered into locked mode, each NE decides the PTP port state according to frequency source state. O The slave port of PTP could be uniquely the physical port from which the timing signal is received and selected as reference frequency source. When multi PTP time source exist, the BMC chooses the best master clock from set of priorities and thus logically selects the only PTP slave port. The rest of valid clock source ports bound with PTP should be set as PTP master port. He & Su Expires April 30, 2009 [Page 5] Internet-Draft Time sync mode in mobile backhaul October 2008 O Particularly, if the frequency reference source of the node is internal clock (e.g. fed by BITS/SSU), no PTP slave port is selected and all other ports are configured as PTP master port. O The invalid or non-configured frequency source ports shall be treated as PTP passive port. O If the node enters into holdover mode or before it locks/relocks to certain frequency reference source, time holdover steps in and PTP will neither change the port state nor receive and send sync packets until frequency stable state is recovered. He & Su Expires April 30, 2009 [Page 6] Internet-Draft Time sync mode in mobile backhaul October 2008 +-------+ | Start | +-------+ | v +-----------------+ NO | | YES +------------| Freq sync port |-----------+ | | | | | +-----------------+ | v v +--------------+ +------------------+ |Frequency sync| |Non freq sync port| |of NEs | |recovers clock | +--------------+ |from PTP port | | +------------------+ | | v v +-------------+ +------------------+ |Enable PTP | |Syntonize local | |for time sync| |clock & sync local| +-------------+ |time | | +------------------+ | | v | +---------------+ | |Time holdover | | |when NEs switch| | |freq source | | +---------------+ | | | v | +------------------+ | |Time sync restores| | |when NEs re lock | | |to new freq source| | +------------------+ | | | v | +-------------+ | | End |<----------------------------------+ +-------------+ Figure 1 He & Su Expires April 30, 2009 [Page 7] Internet-Draft Time sync mode in mobile backhaul October 2008 3.4. Frequency and time distribution path switch To show the relationship between time distribution path and frequency distribution path, and also the switch process of each, Figure 2 and Figure 3 are given below. In Figure 2, PTP equipment A, B, C, D, E and F are interconnected by Synchronous Ethernet links in a ring topology. The time and frequency reference source of the network are co-located. The port I of each node is set to priority 1, the port II of each node is set to priority 2 in accordance with ITU-T G.781. Each node selects frequency reference source based on SSM code in QL- enabled mode as defined in ITU-T G.8264 and G.781. The frequency distribution path is indicated by flow arrows when all nodes are in locked mode. Port III of node A is configured as PTP slave port and port I and II are configured as PTP master port. Other nodes assign the PTP port state by above given rules. In this example, no PTP port is assigned as passive state. Finally, the time distribution path is formed congruent with frequency distribution path. Figure 3 shows the case when the link between node E and node F is broken. Time distribution path stays unchanged but node E enters into time holdover and stop transmitting PTP message at master port. At the same time, node E sends SSM code DNU to inform node D to switch the frequency source. When node D finishes clock source switch, node E will follow line timing from port II of node D. At last, node D and node E re-entered into locked mode, thus a new frequency distribution path is built indicated by flow arrows. The PTP port state of node D and node E are consequently switched. Time transfer restores with the new time distribution path along rebuilt frequency distribution path. He & Su Expires April 30, 2009 [Page 8] Internet-Draft Time sync mode in mobile backhaul October 2008 +---------------+ | Freq and time | | source | +---------------+ | III M +---------+ M +-----------I-| A |-II-------------+ | +---------+ | S | II +------ -------+ I | S +---------+ | | +---------+ | F | | | | B | +---------+ | | +---------+ M | I | | II | M | | | | S | II | | I | S +---------+ | | +---------+ | E | | v | C | +---------+ | +---------+ M | I | II | M | +-------------------> | | S +---------+ M | +---------II--| D |--I-------------+ +---------+ Figure 2 He & Su Expires April 30, 2009 [Page 9] Internet-Draft Time sync mode in mobile backhaul October 2008 +---------------+ |Freq and time | |source | +---------------+ | | M +---------+ M +-------------| A |----------------+ | +---------+ | S-| +------ -------+ | S +---------+ | | +---------+ | F | v | | B | +---------+ | +---------+ M | | | M | | | M | | | S +---------+ ^ | +---------+ | E | | | | C | +---------+ | | +---------+ S | | | | M | +-----------------------+ | | M +---------+ S | +-------------| D |----------------+ +---------+ Figure 3 4. Fault detection 4.1. Problem of time synchronization OAM Although PTP is considered an effective way to deliver accurate time in telecom transport network, the measurement and diagnosis of time transfer error is not yet accomplished. Telecom network requires omplete OAM functionality to improve the robustness and availability of network. So the time synchronization OAM will be a key to the full deployment of PTP in telecom transport network. So far, the approach to know the time transfer defects is to test the time offset between particular PTP node or end equipment and time reference represented by metrics such as MTIE or TDEV. But in a complex transport network with thousands of intermediate nodes and/or end equipments, it is not practical to make such test from one piece of equipment to another. Moreover, this kind of test is normally carried out using GPS to provide time reference source. This would really increase the OPEX of operators. He & Su Expires April 30, 2009 [Page 10] Internet-Draft Time sync mode in mobile backhaul October 2008 There are many unknown reasons giving rise to time inaccuracy. In a PTP time transfer network connected by Boundary clock, the accumulation of phase error needs to be considered. Another major factor affecting PTP performance is the transmission delay asymmetry. The strict control of transmission media length (e.g. fibers) in network engineering is not always a simple thing. PTP does not specify the correction scheme of asymmetry but allows compensation options. Currently, the delay asymmetry could be manually compensated by network management though it is subject to mis- configuration. In a large scale network, the mis-configuration would unpredictably change the values of asymmetry compensation at some PTP equipments without interfering with time offset measurement of other nodes on the time distribution path. Therefore, a detection operation is needed to indicate the location of defected nodes or links. 4.2. Detection of time transfer error Basically, the telecom network operator needs some alarm indication mechanisms to know whether the time transfer is in normal condition. Then some fault location progress might be invoked. This I-D describes a preliminary method that automatically detects and reports the status of time distribution path applicable to a ring topology. It could be a complementary scheme to the node-by-node test. The PTP equipment on the path is Boundary Clock and interconnected with Synchronous Ethernet links as shown in Figure 4. The following discussion is under the assumption that the time distribution path in the ring has been established and no frequency and/or time path switch occurs during the detection period. O The detection process starts from locating the test point. The test point could be selected arbitrarily from PTP nodes on the ring. In this example, node 1 is chosen for convenience. The frequency and time distribution path originated from node 1 are indicated by dash flow arrows. O An OAM message is generated from the test node on both directions of the ring. The OAM message on each direction is time-stamped leaving the PTP port. The test node records the original timestamp value (e.g. t0) and the original node ID. A direction flag field is also set in the OAM message. O Taking direction I for example, the timestamp value of the OAM message sent to node 2 is updated at the ingress PTP port by recoding the arriving time plus the transmission delay. The transmission delay is estimated by ingress PTP port using Peer Delay measurement. Node 2 will forward this OAM message to next He & Su Expires April 30, 2009 [Page 11] Internet-Draft Time sync mode in mobile backhaul October 2008 hop on time distribution path (i.e. node 3). At the egress PTP port, the timestamp value is re-written to the time when the message is leaving. The new timestamp value includes the residence time of the OAM message at the node. The forwarding of the OAM message continues by all other nodes on the ring with the same procedure that the timestamp value is updated. O When the OAM message travels through all nodes on the ring back to the test node (node 1), node 1 will terminate the OAM message by checking the original node ID and record the arriving time of the OAM message at the ingress PTP port (e.g. ta). Node 1 also extracts the timestamp value from the OAM message which contains the record of instant when the message leaves the last node on the ring (e.g. tr). The test node (node 1) might compensate this value by adding the transmission delay of incoming link. O Based on these time values, the test node could calculate the accumulated time error (delta) of all PTP nodes on the ring on direction I as follows: delta(I) = ta(I) - t0(I) - tr(I) Similarly, the test node could get the value of delta on the direction II using the same mechanism and calculation: delta(II) = ta(II) - t0(II) - tr(II) The test node judges the time transfer status of the ring comparing the difference between delta(I) and delta(II) with pre- determined threshold value. If the difference is out of the threshold value, it is indicated that the time transfer is problematic somewhere and the test node thus reports alarm. The interpretation of this method is that the effect of time error resulted from path asymmetry could be revealed by differentiating accumulated time offset between test node and all other nodes on two opposite directions. If the asymmetry of a single link exists, it would be clearly observed. And if the asymmetry takes place on several links where they have negligible composite effect, the difference would be too small to detect, but this situation might be of very much low possibility. He & Su Expires April 30, 2009 [Page 12] Internet-Draft Time sync mode in mobile backhaul October 2008 II ******************* +---------+ <********************* * +-------------| 1 |----------------+ * * | +---------+ | * * | +---- -------+ | * * +---------+ | ****> ***** | +---------+ * * | 6 | | * I * | | 2 | * * +---------+ | * * | +---------+ * * | | * * | | * * | | * * | | * * | | * * | | * * +---------+ | * * | +---------+ * * | 5 | | * * v | 3 | * * +---------+ | ***************** +---------+ * * | | | * * | +--------> | * * | +---------+ | * * +-------------| 4 |----------------+ * * +---------+ * * * ********************************************************** ------> time distribution path ******> OAM message path Figure 4 5. Security Considerations The time distribution given in this document may have security concerns as described in informative reference [TICTOC]. 6. IANA Considerations There have been no IANA considerations so far in this document. 7. Acknowledgments It is appreciated that Mr. Su Hui, Mr. Yu Zhiyong and Mr. Shen Ruiwu have paid substantial work and contributions to this document. 8. Informative References [1588] IEEE, "Standard for A Precision Clock Synchronization He & Su Expires April 30, 2009 [Page 13] Internet-Draft Time sync mode in mobile backhaul October 2008 Protocol for Networked Measurement and Control Systems", IEEE Std 1588-2008 . [G.781] ITU-T, "Synchronization Layer Functions", G.781, September 2008. [G.8261] ITU-T, "Timing and Synchronization Aspects in Packet Networks", G.8261, April 2008. [G.8264] ITU-T, "Distribution of Timing through Packet Networks", Draft G.8264, February 2008. [TICTOC] Frost, T., Dowd, G., and K. O' Donoghue, "Architecture for the Transmission of Timing over Packet Networks", draft-stein-tictoc-modules-02.txt . Authors' Addresses Li He ZTE Corporation R.D. Building 3, ZTE Industrial Park, LiuXian Road Shenzhen 518055 P.R.China Email: he.li4@zte.com.cn Fei Su ZTE Corporation R.D. Building 3, ZTE Industrial Park, LiuXian Road Shenzhen 518055 P.R.China Email: su.fei@zte.com.cn He & Su Expires April 30, 2009 [Page 14] Internet-Draft Time sync mode in mobile backhaul October 2008 Full Copyright Statement Copyright (C) The IETF Trust (2008). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Intellectual Property The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79. Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org. He & Su Expires April 30, 2009 [Page 15]