Application of PTN time synchronization scheme in mobile network

2020-05-26 13:13:26

The development of mobile technology from 2G to 3G and LTE is driving the evolution of mobile transmission networks from traditional TDM technology to higher bandwidth, lower cost, and more flexible packet technology. PTN (Packettransportnetwork) is a connection-oriented transport technology. Based on the packet architecture, it draws on SDH's complete protection switching, rich OAM, and good synchronization performance. It also combines MPLS / Ethernet technology packet switching, QoS management, and statistical multiplexing Such ideas provide a good solution for operators to build a unified and integrated transport network that can be managed and operated.
The strategic significance of PTN to meet the requirements of mobile network synchronization The mobile network has strict requirements for high-precision synchronization. The switching and roaming of base station operations require precise time control to avoid users dropping out during the handover between base stations and affecting other users. The phenomenon, the current requirements of various wireless technologies for synchronization are shown in the table.

Currently deployed TD-SCDMA (hereinafter referred to as TD) networks all use GPS solutions to solve the problem of mobile network synchronization. However, the GPS antenna installation needs to meet the 120 Â° clearance requirement, which is difficult for engineering installation, and the GPS cost is high and maintenance is difficult. China Mobile has been actively seeking new technologies to replace GPS. On the one hand, it transmits precise time synchronization signals through a wired transmission network; on the other hand, it uses the Beidou satellite independently launched by China as a time signal source, uses dual-mode timing of the Beidou satellite and the GPS satellite, and communicates with each other. Main standby. In the end, the two aspects of time signal source and transmission are combined to completely get rid of the dependence on GPS.
At present, the industry has set their sights on PTN to provide high-precision synchronization solutions on PTN packet networks, so as to achieve GPS replacement.
PTN network related technology to achieve time synchronization In PTN-based packet networks, synchronization technology can be achieved by synchronizing Ethernet G.8261 and precise time synchronization protocol IEEE1588V2. G.8261 recovers the sender's clock from the serial data stream through the Ethernet physical layer PHY chip. This method is the same as the SDH clock recovery method, and it can obtain SDH-like clock accuracy and achieve network clock synchronization. The clock synchronization quality is close to SDH, and will not be affected by data network congestion, packet loss, and delay. But at present, synchronous Ethernet can only support the transmission of frequency signals, but not the transmission of time signals, so the simple synchronous Ethernet solution is only suitable for scenarios that do not require time synchronization. The core idea of â€‹â€‹IEEE1588 is to use the master-slave clock method to encode time information, and use the network symmetry and delay measurement technology to achieve master-slave time synchronization. The key lies in delay measurement, as shown in Figure 1. Although IEEE1588V2 can achieve frequency synchronization independently of synchronous Ethernet when performing frequency synchronization, compared with synchronous Ethernet, PTP has a longer convergence time, and the frequency accuracy depends on the granularity of the time stamp.

PTN Time Synchronization Solution Several key issues of network application Time synchronization is a new requirement for 3G mobile standard prompt. At present, China Mobile does not have an accurate time synchronization network, and the deployment of time synchronization solutions should adopt a gradual construction method.
The time source is first deployed on the RNC side of the core computer room of the local network, and the RNC is timed. The RNC transmits the PPS (frequency) and TOD (time) information to the PTN equipment through out-of-band methods. Each base station side. Most of the base stations deployed on the existing network do not support the 1588V2 protocol. If the existing network is to be transformed, the cost and workload are huge. At this time, the PTN equipment at the access end uses out-of-band methods to transmit synchronization information to the base station. If the base station already supports the 1588V2 protocol, the PTN device at the access end simultaneously transmits the service and time information to the base station through the in-band FE / GE interface (as shown in Figure 2).

The PTN network runs the protocol mode of SSM and BMC, which realizes the automatic protection switching of the time link and ensures the reliable transmission of time.
PTN time synchronization scheme is applied in the network, there are several key issues worth discussing.
Accuracy problem
The TD-SCDMA air interface synchronization requirements are very accurate, with a frequency accuracy of 0.05 ppm and a time synchronization accuracy of 1.5 us. This is the end-to-end specification of the air interface, and the allocation to the bearer network is generally about 900 ns. If the network is large and there are many nodes that pass through, does the accuracy meet the requirements? Generally speaking, G.8261 based on physical layer synchronization can provide a higher-quality reference clock, but it cannot achieve time synchronization; IEEE1588V2 synchronization can achieve both frequency synchronization and time synchronization, but 1588 messages are complicated In the data network, jitter and asymmetry are uncontrollable, which makes it difficult to guarantee the accuracy of clock and time recovery from 1588 packets. Therefore, in practical applications, a combination of two synchronization methods can be considered. Accurate frequency synchronization is accomplished through G.8261, 1588V2 time synchronization is achieved on the basis of G.8261, and hardware implements accurate timestamp insertion and extraction in 1588 protocol. Reduce the frequency of 1588 message sending, speed up the convergence speed, and effectively improve the time synchronization accuracy.
Compensation Problems The compensation mentioned here involves two aspects: 1PPS + TOD cable delay compensation and fiber optic line asymmetry compensation.
As mentioned earlier, the key to 1588 is delay measurement. Synchronization time source generally transmits 1PPS and TOD information to PTN equipment through out-of-band cables. In addition, most base stations in the current network do not have 1588 capability. PTN equipment also needs to pass 1PPS and TOD to the base station through out-of-band . If the time source and base station do not have the delay compensation capability, the PTN equipment needs to complete the delay compensation introduced by this part of the cable, and add the compensation result to the TOD information.
Reliability issues To ensure the reliable transmission of time information by the PTN network, a reliable protection design must be implemented for the input time source and transmission link.
Firstly, the PTN equipment is timed with a high-precision time source through the 1PPS + TOD interface to ensure that the system operates reliably and stably within a certain period of time. When conditions permit, two high-precision time sources are used to increase the original time redundancy .
The physical link of the PTN transmission network generally adopts a ring network. The convergence layer and the access layer use an intersecting ring or a tangent ring to achieve interconnection. Each external time source accesses the main link and the backup through different two points in the ring link link. The BMC algorithm is enabled in the PTN network. The BMC algorithm can select the optimal time source based on the clock quality and the shortest node path, which effectively reduces the accumulated error of the clock quality and increases the redundant backup capability of the clock source.

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