Synchronization is critical for telecommunication system performance. Frequency and time (time-of-day or wall-clock) synchronization is crucial for mobile wireless networks because the radios used in these networks operate in very strict bands that need separation to avoid channel interference which reduces the call quality and network capacity. Poor synchronization has also negative impact for the handover between base stations.
Mobile handsets generally derive the accurate frequency that they transmit and receive from the base stations. If the transmission frequencies are not very closely matched between adjacent cell sites, then “clicks” can occur when the call is being handed over (that is, switches) between base stations. In the worst case, the call would drop because the mobile handset would not be able to immediately lock onto and acquire the new signal.
Failure to meet the timing requirements of the relevant standards would cause performance degradation for the radio access channels. In particular, this failure could compromise cell handover (especially for travelling mobile stations) and producing excess of dropped calls.
With increasing interest in packet networks as a common mode of communication, packet-based synchronization solutions are in high demand as alternative to PDH (plesiochronous digital hierarchy)/SDH (synchronous digital hierarchy) and GPS based solutions. Equipment vendors and telecom service providers are looking for new packet synchronization solutions with very high accuracies beyond those attainable using the traditional packet methods like Network Time Protocol (NTP) (see Mills, D., “Network Time Protocol (Version 3) Specification, Implementation and Analysis”, IETF RFC 1305, March 1992).
It is desirable that such new solution are also be designed for applications (e.g., base stations) that cannot bear the cost of a GPS receiver at each node, or for which GPS signals are inaccessible (for example due to location).
The IEEE Standard 1588 Precision Time Protocol (PTP) (IEEE Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control Systems, IEEE 1588-2008), is the latest addition in packet timing technology. Originally designed to provide precise timing for critical industrial automation applications it is now providing the highest level of accurate frequency and time to wireless backhaul networks. The backhaul portion of the network comprises the intermediate links between the core network, or backbone, of the network and the base stations. Currently standardized in 2008 as Version 2 (IEEE-1588v2, superseding IEEE 1588-2002), PTP is designed to overcome the Network Time Protocol (NTP) latency and jitter issues, providing accuracy in the nanosecond range.
Base stations have traditionally met synchronization requirements by locking their internal oscillators to a recovered clock from the T1/E1 TDM (time division multiplexing) backhaul connection. While Ethernet has proven to be a ubiquitous and inexpensive medium for connectivity, it has not been well-suited for applications requiring precise synchronization. When the backhaul transitions from TDM to Ethernet, the base station becomes isolated from its traditional network sync feed. New base station designs are incorporating IEEE 1588 PTP slave clocks to meet the 50 ppb (parts per billion) accuracy requirement. These PTP slave clocks in the base stations rely on access to a PTP grandmaster clock deployed in a mobile switching center (MSC). Sync and Follow_Up packets flow from the grandmaster clock to the slave clocks in the base stations.
IEEE 1588v2 PTP is fully compatible with all Ethernet and IP networks. Additionally, the protocol is designed to enable a properly designed network to deliver frequency and phase or time with precision rivalling a GPS receiver. An IEEE 1588v2 PTP Technique for Frequency Synchronization implementation can supply FDD (frequency division duplexing) and TDD (time division duplexing) radio systems and CES-based (circuit emulation services) transport systems with the synchronization signals they require as illustrated in FIG. 1. This greatly reduces the costs of clocking all wireless base station equipment using other means.
A primary reference is a source of time and or frequency that is traceable to international standards. A recognized standard time source is a source external to PTP that provides time and/or frequency as appropriate that is traceable to the international standards laboratories maintaining clocks that form the basis for the International Atomic Time (TAI) and Universal Coordinated Time (UTC) timescales. Examples of these are Global Positioning System (GPS), NTP, and National Institute of Standards and Technology (NIST) timeservers.
Although IEEE 1588v2 PTP systems add a small amount of additional traffic to the network load, they have several advantages. First, they work in the data path, and also benefit from the redundancy and resiliency mechanisms of the network, resulting in “always on” operation. Next, multiple transmission paths reduce redundant clock system costs. They also use a single synchronization session for all base station traffic. IEEE 1588v2 PTP supports any generic packet-based transport (such as IP, MPLS). The protocol also features configurable synchronization packet rates for network conditions to maintain accuracy.
The transmission of the clock information over a packet network eliminates the need for alternative mechanisms, such as GPS or prohibitively expensive oscillators placed at the receiving nodes. This provides significant cost savings in network equipment as well as in ongoing installation and maintenance. This synchronization solution transmits dedicated timing packets, which flow along the same paths with the data packets, reducing the cost of synchronization and simplifying implementation.
To ensure that packet technologies (Ethernet, IP, MPLS) have the necessary attributes to be truly carrier grade, operators and vendors are introducing several key technologies for the transport of timing and synchronization over packet networks. Of these IEEE 1588v2 PTP is perhaps the most important because it provides both frequency and time distribution.
The grandmaster is the root timing reference in a domain and transmits synchronization information to the clocks residing in its domain. In IEEE 1588v2 PTP messages are categorized into event and general messages. All IEEE 1588 PTP messages have a common header as shown in FIG. 2.
Event messages are timed messages in that an accurate timestamp is generated at both transmission and receipt of each message. Event messages have to be accurately timestamped since the accuracy in transmission and receipt timestamps directly affects clock distribution accuracy. A timestamp event is generated at the time of transmission and reception of any event message. General messages are not required to be timestamped. The set of event messages consists of Sync, Delay_Req (both of which have the format shown in FIG. 4), Pdelay_Req, and Pdelay_Resp. The set of general messages consists of Announce (which has the format shown in FIG. 3), Follow_Up (which has the format shown in FIG. 5), Delay_Resp (which has the format shown in FIG. 6), Pdelay_Resp_Follow_Up, Management, and Signalling.
The Sync, Delay_Req, Follow_Up, and Delay_Resp messages are used to generate and communicate the timing information needed to synchronize ordinary and boundary clocks using the delay request-response mechanism. A Sync message is transmitted by a master to its slaves and either contains the exact time of its transmission or is followed by a Follow_Up message containing this time. In a two-step ordinary or boundary clock, the Follow_Up message communicates the value of the departure timestamp for a particular Sync message.
A Delay_Req message is a request for the receiving node to return the time at which the Delay_Req message was received, using a Delay_Resp message.
The format of the PTP message timestamp fields is shown in FIG. 7. A timestamp is the current time of an event that is recorded by a device. IEEE 1588 PTP allows for two different types of timestamping methods, either one step or two-step. One-step clocks update time information within event messages (Sync and Delay-Req) on-the-fly, while two-step clocks convey the precise timestamps of packets in general messages (Follow_Up and Delay-Resp).
The basic pattern of synchronization message exchange is illustrated in FIG. 8. The message exchange pattern is as follows. The master 101 sends a Sync message (M101) to the slave 102 and notes the time T1 at which it was sent. The slave 102 receives the Sync message and notes the time of reception T2. The master 101 conveys to the slave 102 the timestamp T1 by one of two ways: 1) Embedding the timestamp T1 in the Sync message. This requires some sort of hardware processing (i.e., hardware timestamping) for highest accuracy and precision. 2) Embedding the timestamp T1 in a Follow_Up message (M102). Next, the slave 102 sends a Delay_Req message (M103) to the master 101 and notes the time T3 at which it was sent. The master 101 receives the Delay_Req message (M103) and notes the time of reception T4. The master 101 conveys to the slave 102 the timestamp T4 by embedding it in a Delay_Resp message (M104).
At the end of this PTP message exchange, the slave possesses all four timestamps {T1, T2, T3, T4}. These timestamps may be used to compute the offset of the slave's clock with respect to the master and the mean propagation time of messages between the two clocks. The computation of offset and propagation time assumes that the master-to-slave and slave-to-master propagation times are equal.
The IEEE 1588 PTP based frequency recovery technique described in Section 12.1 of IEEE 1588-2008 Standard requires estimation of the mean path delay between server (master) and client (slave) which may include accounting for path asymmetry. In order to accurately synchronize to their master, slave clocks must individually determine the network transit time of the PTP messages. The network transit time is determined indirectly by measuring round-trip time from each slave to its master.
Like all message-based time transfer protocols, PTP time accuracy is degraded by asymmetry in the paths taken by event messages. Any asymmetry in the forward and reverse path propagation times and introduces an error into the computed value of the link delay. Asymmetry can be introduced in the physical layer, e.g., via transmission media asymmetry, by bridges and routers, and in large systems by the forward and reverse paths traversed by event messages taking different routes through the network. Systems should be configured and components selected to minimize these effects guided by the required timing accuracy. In single subnet systems with distances of a few meters, asymmetry is not usually a concern for time accuracies above a few 10 s of ns. Asymmetry is not detectable by PTP; however, if known, PTP corrects for asymmetry. If two-step clocks are used, then the network has to be designed such that the general message takes the same path as the event message through a transparent clock. Failure to do this will result in a condition where the transparent clock does not calculate path delay properly. This condition is undetectable and may introduce additional jitter and wander, but it will not break the protocol.
Other IEEE 1588 based techniques (such as those discussed in T. Neagoe and M. Hamdi, “A Hardware IEEE-1588 Implementation with Processor Frequency Control,” Arrow Electronics White Paper; T. Neagoe M. Hamdi and V. Cristea, “Frequency Compensated, Hardware IEEE-1588 Implementation,” IEEE International Symposium on Industrial Electronics, 9-13 Jul. 2006, pp. 240-245 and S. Balasubramanian, K. R. Harris, A. Moldovansky, “A Frequency Compensate Clock for Precision Synchronization Using IEEE1588 and its Applications to Ethernet” IEEE-1588 Workshop, September 2003 assume that the PTP GrandMaster Clock (GMC) sends Sync messages at fixed intervals, an assumption which might not necessary hold true in practice. The above receiver clock recovery mechanisms are designed based on this assumption.
An object of the present invention is to provide a method and system for frequency synchronization that allows one or more receivers (slaves) to frequency synchronize to a transmitter (master). Other applications of frequency synchronization are in process and manufacturing industries like paper mills, printing presses, automation and robotic systems, test and measurement instruments and systems, etc.
Another object of the present invention is to provide a synchronization technique based on, for example, the IEEE 1588 Precision Time Protocol (PTP) that allows frequency to be distributed over a packet network from a PTP server (master) to synchronization clients (slaves).