Research on TD-LTE Backhaul Network and Time Synchronization Solutions (Part 3)

4. PTN interoperability study

　　The implementation of single SGW equipment in the TD-LTE era has led to the need for core layer PTN intercommunication networking in the PTN L3 solution. In addition, in order to ensure service and cost and enhance network openness, the PTN networks in various cities may use products from two or more equipment vendors to form a network. In order to achieve end-to-end management of bearer services, PTN also has a need for networking intercommunication. Intercommunication application scenarios mainly include layered intercommunication and domain-based intercommunication. Intercommunication between PTN devices mainly includes intercommunication at five levels: service, protection, OAM, QoS and synchronization.

　　The PTN interworking model mainly includes the UNI (user network interface) interworking model and the NNI (network-network interface) interworking model. The UNI interworking model refers to the interconnection of PTN devices at both ends through the service adaptation module, that is, the service interface is interconnected (see Figure 4). The device at one end performs PWE3 decapsulation on the MPLS-TP tunnel to restore the service message and transmit it to the device at the other end. The device at the other end then performs PWE3 encapsulation on the service and enters the MPLS-TP tunnel. The NNI interworking model refers to the interconnection of PTN devices at both ends through the line adaptation module, that is, the network/line interface is interconnected (see Figure 5). The devices at both ends directly realize MPLS-TP tunnel docking.

Figure 5: NNI interoperability model

　　For each end device of the intercommunication, the UNI intercommunication model can be regarded as a client-side device like a base station and various customers, and the services come from the intercommunication interface. Therefore, in the PTN intercommunication research, the NNI intercommunication model is more critical and more difficult to implement.

　　Among the interoperability requirements at the five levels of service, protection, OAM, QoS and synchronization, service interoperability is the most basic requirement and prerequisite. First of all, it is necessary to ensure that services can be interoperable based on the UNI interface and the NNI interface. On this basis, the focus is on achieving protection interoperability and OAM interoperability, which will be analyzed separately later. In addition, PTN equipment must support end-to-end QoS in DiffServ mode and use a consistent QoS priority mapping table to achieve mutual mapping of DSCP, VLAN PRI, and TC. Finally, frequency and time synchronization interoperability is achieved. Frequency synchronization is mainly based on standard synchronous Ethernet, and time synchronization is achieved using IEEE 1588v2 in-band Ethernet or 1PPS+TOD out-of-band interface.

　　4.1 PTN protection mutual

　　PTN can provide 50ms end-to-end protection for base stations and various customer services to ensure network reliability. PTN protection is divided into UNI interface protection and network protection. When interconnecting, the UNI interface needs to support 1+1/1:1 MSP protection interconnection (for SDH interface) and LAG protection interconnection (for Ethernet interface). The PTN network needs to support the 1:1 LSP linear protection mode interconnection defined by MPLS-TP (based on the NNI interconnection model).

　　Since the ITU-T linear protection standard for MPLS-TP has not yet been completed, the current equipment implementation mainly refers to the ITU-T G.8031 Ethernet linear protection standard. When PTN adopts 1:1 linear protection mode, under normal circumstances, the service runs on the working path, and the service is switched to the protection path during the fault period; when the service is restored, the two end node PTN devices do not immediately switch the service back to the working path, but set WTR timers respectively, and wait for the WTR timer to end before switching back. The main function of WTR is to prevent frequent protection switching operations caused by intermittent faults.

　　During the test of the Shenzhen TD-LTE large-scale trial network, China Mobile found that the ITU-T standard had defects in the WTR (wait for recovery) state processing mechanism for 1:1 linear protection, which led to differences in the implementation of PTN equipment manufacturers and the protection switching back time exceeded the standard during intercommunication. Since the WTR settings of the PTN equipment at both ends may be inconsistent or the settings may be consistent but there is an instant difference in the WTR end time, the end that ends the WTR first immediately returns to the working path (i.e., the short timeout solution), while the WTR of the other end device has not yet ended, so the service may still reside on the protection path (i.e., the long timeout solution). Two manufacturers that use the short timeout and long timeout solutions respectively may cause service interruption when performing linear protection intercommunication.

　　Therefore, the linear protection interworking solution is improved, and it is recommended to adopt a long timeout solution, and not perform switching actions for the newly defined "intermediate state" in the ITU-T standard, so that the end that ends the WTR first can wait for the other end's WTR to end before switching back together. This solution has been adopted by the CCSA (China Communications Standards Association) domestic standard and the ITU-T international standard, and the mainstream manufacturers' equipment has also supported and passed the test. 4.2 OAM mechanism interworking based on ITU-T G.8113.1 PTN OAM is mainly to cooperate with the network management to realize fault detection and rapid location, delay, packet loss rate and other performance monitoring functions on the equipment, which needs to be implemented based on specific OAM messages. PTN equipment mainly uses Ethernet interfaces to carry services. The UNI interface and NNI interface need to support multiple OAM functions. For example, the Ethernet service OAM function based on the IEEE 802.3ah standard and the ITU-T Y.1731 protocol needs to be implemented on the UNI side. On the NNI side, the latest ITU-T G.8113.1 standard should be followed to realize the OAM interworking at the PTN network level (see Table 1).

Table 1: Field description of PTN OAM message encoding format using G.8113.1

　　In order to realize G.8113.1-based OAM interoperability within the PTN network, China Mobile proposed unified requirements for the message encapsulation fields of MPLS-TP OAM, with major improvements including Ether Type, Channel Type, TC, TTL, MEL, Version, etc. The relevant content has been adopted by the CCSA standard.