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The difference between 1+1 and 1:1 of SDH Network

SDH (Synchronous Digital Hierachy) has become the basis for building high-speed broadband digital transmission networks. SDH is a set of structural levels of standardized digital signals for simultaneous information transmission, multiplexing, add-drop and cross-connect, while SDH networks are composed of some basic network elements (NE) on the transmission medium (optical fiber, microwave etc). A transport network for simultaneous signal transmission, multiplexing, add-drop, and cross-connect, with a unified network node interface (NNI) worldwide. The NNI mentioned here refers to the interface of the network node interconnection, which includes two basic devices of the transmission network, namely the transmission device and the network node device. Transmission equipment includes systems such as fiber-optic communication, microwave communication, and satellite communication, and network nodes include many types, such as 64 kbps circuit nodes, broadband switching nodes etc. In modern transmission networks, in order to unify the specifications of the above technologies and devices, it is necessary to have a uniform interface rate and a corresponding frame structure, and the SDH network has this feature.

SDH network 1+1 and 1:1 comparison

The 1+1 finger in the SDH network sends the same information (concurrent) on both the primary and spare channels. The receiving end selects the service on the primary channel under normal conditions, because the services on the primary and spare channels are same (all are used as main service), so when the primary channel is damaged, the primary service is restored by switching the spare channel. This type of switching is also called single-ended switching (switching only at the receiving end), and the switching speed is fast.

The 1:1 mode in the SDH network means that the originating end sends the primary service on the primary channel at normal time, and the additional service (low-level service) is sent on the alternate channel, and the receiving end receives the primary service from the primary channel and receives the additional service from the alternate channel. When the primary channel is damaged, in order to ensure the transmission of the primary service, the originating end sends the primary service to the alternate channel, and the receiving end switches to the spare channel to select the primary service. At this time, the extra service is terminated, and the primary service is transmitted and get restored. This switching mode is called double-ended switching (switching both ends of the receiving/transmitting), and the switching rate is slow. Since the transmission of the extra service is terminated when the primary channel is damaged, the extra service is also called an unprotected service.

1:n means that an alternate channel protects n primary channels. At this time, channel utilization is higher, but one spare channel can only protect one primary channel at the same time, so system reliability is reduced.

1+1 is also called hot backup, which means that two systems transmit the same service at the same time, and the selection is preferred.

1:1 is also called cold backup. When it is normal, only the primary system transmits traffic, and the backup system does not transmit traffic or other secondary services. Once the primary system fails, the backup system is used to transmit traffic and discard secondary traffic. It can also be divided into 1:1A and 1:1B, that is return type and non-return type. The difference the change and the switchover, the main system returns to normal and the service can returns to the main system or not

1+1 is that the protection channel works simultaneously with the working channel under normal conditions, so that no switching is required when the switching occurs, and the speed block is switched, but the 1+1 channel utilization is low.

1:1 is the protection channel does not work under normal conditions (or bears additional services), after the working channel is broken, it switches to the working state, which is slower than 1+1, but the 1:1 channel utilization is high.

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