DDN synchronization and Network Management

I. Network Synchronization

There are three methods for network synchronization: quasi-synchronization, master-slave synchronization, and mutual synchronization. Quasi-synchronous according to ITU-T G.811 recommendations, often recommended for international use. Master-slave synchronization synchronizes data by locking the slave clock phase to the reference timing of the master clock. This synchronization can be divided into master-slave synchronization with a number structure and master-slave synchronization with an external reference mode, the former uses the PCM High-Order Group as the master and the lower-order group as the slave. The latter has a master clock responsible for the frequency distribution of all switches. At this time, the clock pulse and information bit stream are transmitted through different channels. Mutual synchronization is a synchronization method without a unique reference clock. At this time, each switch clock is locked on the average value of all incoming clock.

Next we will discuss DDN synchronization in four points.

1. DDN network synchronization mode

DDN is a synchronization network, that is, it must ensure the timing signal consistency of all nodes across the network, in order to provide high-quality dedicated circuits. DDN network synchronization mode is: the quasi-synchronous mode is adopted internationally, in the digital circuit of DDN international interconnection, the timing requirements should comply with the timing requirements of ITU-T G.811 for international digital link quasi-synchronous operation benchmark clock output; DDN nodes in China comply with China's requirements on the hierarchical master-slave synchronization mode for digital networks. For details, see section 1. section 3. The DDN node level should be the same as that of the long-distance bureau or local telephone Bureau. When DDN users access the network, they should first choose the timing provided by the network to maintain the same speed as the network.

2. DDN node clock and timing

Generally, the DDN node uses a crystal oscillator as the clock source. for medium and large nodes, the third-level clock source is required.+4.6 × 10-6; for small nodes, refer to the requirements of four-level clock source, its long-term frequency tolerance is+25 ~ 50*10-6.

DDN nodes should be able to select the master and slave timing modes. The active/standby timing method uses the clock source of the current node as the Scheduled mode. The scheduled operation is based on a reference frequency. The scheduled operation is performed when the clock source of the current node is locked.

DDN nodes should allow the following reference frequency sources:

① The standard frequency signals provided by the Bureau are provided in a unified manner. The DDN node should give priority to the unified Bureau clock to maintain synchronization with the digital transmission network;
② Timing signal extracted from the set channel interface;
③ Directly use the scheduled signal on the data interface. The DDN node should be able to select the scheduled signal on the V.24, V.35, and X.21 data interfaces to meet the special connection needs.

The DDN node should be able to set multiple ports to obtain the reference signal in priority order, and automatically detect port faults, and automatically switch to obtain the Reference reference signal port in priority order.

3. DDN network node synchronization
(1) DDN network nodes should be synchronized in the same way as China's Digital Network

According to the synchronization level of China's digital synchronization network, China's DDN synchronization network is divided into four levels, as shown in the following table. With the implementation of the national digital synchronization network, the DDN node should adopt a unified clock with the local bureau as the reference signal.

　

Changtu Network	Level 1	Reference clock
	Level 2	Class	1. Level 1 long-distance center and international clock
		Class B	Layer 3 and Level 4 long-distance Center clock
Internet	Level 3	Board and end clock
	Level 4	Remote module, digital PBX, digital terminal device clock

Note:

Class A Clock: Synchronize directly with the benchmark clock through the synchronization link;
Class B clock: the synchronization link is controlled by class A clock and indirectly synchronizes with the benchmark clock.

(2) master-slave hierarchical Synchronization

If you cannot use the same clock as the local clock of the digital synchronization network, nodes on the DDN Network adopt the master-slave hierarchical synchronization mode. Each DDN node should prioritize the extraction of reference signals from the connected to high-level digital channels based on its location.

To ensure the reliability of DDN network synchronization, each node on the DDN primary and secondary trunk networks should be set based on the priority to obtain reference signals from multiple digital circuits.

　

4. User inbound Synchronization

Users should be arranged to use the timer provided by the network whenever possible. When users cannot use the timer network, DDN nodes should insert buffer memory at the user interface to reduce sliding caused by timed deviation between the two sides. After the buffer memory is inserted at the data circuit transfer, the sliding interval is related to the buffer memory length, interface speed, timing deviation between the two parties, and other factors.

Ii. Network Management and Control
1. Network Management Control Center settings
(1) National and Provincial Network Management Control Centers

The DDN network is configured with two levels of Network Management Control Centers (NMC) at the national and provincial levels. The NMC is responsible for the management and control of the level-1 backbone network, and the provincial NMC is responsible for the management and control of networks in the province, municipality, or autonomous region.

On a local network with a large number of nodes and complex network structures, you can also set up a local network management control center to manage and control the Internet.

(2) network management control terminal (NMT)

According to the needs of network management and control, as well as the needs of business organization and management, you can set up several network management control terminals (NMT) on the primary and secondary trunk networks respectively ). NMT should be able to exchange network information and business information with NMC, and manage and control it within the permitted scope of NMC. NMT can be assigned to the responsible users of the virtual private network (VPN.

(3) node management and maintenance terminal

Each DDN node should be able to configure the management and maintenance terminal of the node, be responsible for the configuration of the node, control the running status, and monitor and direct the business situation, the user line of the node should be maintained and measured.

(4) The higher-level network management system can observe the operation status of the lower-level networks step by step. alarms and fault information should be promptly reflected in the higher-level network management center, so as to achieve unified network management.
2. Network Management Control Information Communication path
(1) communication between nodes and the Network Management Control Center

When network management control information is exchanged between the network management control center and its nodes, use the appropriate capacity path specified by the DDN network, you can also use other channels provided by public branch networks or telephone networks.

(2) Communication Between Network Management Control Centers

Communication between NMC and provincial NMC and between NMC and NMT is required to exchange network management control information.

To achieve this communication path, you can use a dedicated circuit configured on the DDN network, or a connection circuit through a public branch network or a telephone network.