Shulou(Shulou.com)06/02 Report--

The design of disaster recovery communication link is an important step to ensure that users can successfully realize the construction of disaster recovery system under reasonable communication cost. Different communication links have different attributes, such as distance support, bandwidth capacity, etc., while different disaster recovery technologies and applications have different requirements for communication links.

Selection of disaster recovery Communication Link

For disaster recovery scheme, no matter which kind of disaster recovery communication link is used, it is necessary to determine the type of risk from the actual needs of information system disaster preparedness, analyze the different disaster recovery requirements of each business system, and make clear the goals of RTO and RPO of disaster recovery system. Users also need to choose the appropriate data transmission mode according to the characteristics of the application data and the affordable cost. The following questions need to be answered in the selection of disaster recovery communication links:

The distance between the disaster recovery communication link (that is, the distance from the production center to the disaster recovery center) needs to be determined according to the type of risk resisted. For example, remote disaster recovery is required for regional disasters, local disaster preparedness can be selected for site disasters, and disaster preparedness for system or equipment failures can be selected in the same computer room.

For the bandwidth of disaster recovery communication links, it is necessary to identify the requirements of RTO and RPO according to the analysis of business applications, so as to determine which kind of bandwidth links and how many links are needed.

After selecting the disaster recovery communication link, it is also necessary to verify whether the designed link meets the expected goal according to the data change of the application system and the reliability of data transmission. At present, the main ways, advantages and disadvantages and suitable transmission distance of remote data transmission are shown in the table.

Disaster recovery link connection mode

At present, the communication links of disaster recovery schemes in the industry basically adopt "bare optical fiber direct connection switch mode, bare optical fiber connection mode through DWDM equipment, IP network mode" and so on, each method has its own advantages and disadvantages. Disaster recovery system is built based on application disaster recovery technology, mainly using standard IP network connection, communication links can be ATM, E1/E3, IP, etc., if the technology based on storage or virtual storage is used to build disaster recovery scheme You can use bare fiber, DWDM, SONET, SDH and other communication links, and you can also use ATM, E1/E3, IP and other communication links through FCIP equipment.

At present, the main transmission medium is optical fiber, which can be divided into single-mode optical fiber and multimode optical fiber according to the transmission mode of data in optical fiber. Due to the existence of mode dispersion, multimode fiber will narrow its bandwidth and reduce its transmission capacity in long-distance transmission, and its effective transmission distance is 2~4km. Therefore, single-mode optical fiber is generally used in long-distance (greater than or equal to 100KM) transmission.

The quasi-synchronous digital series PDH (Plesiochronous Digital Hierarchy) uses electrical signals for transmission before using optical signals. The transmission rate is less than 150m.

Synchronous optical fiber network SONET (Synchronous Optical Network) and synchronous digital series SDH (Synchronous Digital Hierarchy) are early optical transmission technologies. One optical fiber can only transmit one optical signal. At present, the network is still widely used, but new technologies have not been developed on it. Is being gradually replaced. Transmission rates include 155Mbps, 622Mbps, 2.5Gbps, and 10Gbps.

Wavelength division multiplexing technology WDM (Wavelength Division Multiplex), which transmits multiple optical signals on one optical fiber at the same time, has two kinds: DWDM (Dense WDM) and CWDM (Corase WDM). At present, the mainstream is DWDM.

The transmission rate of a single optical signal includes 2.5Gbps, 10Gbps, 40Gbps, 100Gbps and 200Gbps.

The number of wavelengths that can be transmitted by a single optical fiber: 32, 40, 100, 80, 96, 106, 160. At present, the mainstream applications are 40-wave and 80-wave systems.

Selection of link multiplexing mode

Single-mode fiber has the characteristics of large transmission bandwidth and high transmission cost. In order to make effective use of the transmission bandwidth and reduce the transmission cost, the multiplexing technology is usually adopted, that is, a physical channel is divided into multiple logical channels, so that the multiple transmission signals are transmitted in the same physical channel at the same time. In order to effectively use the bandwidth of the transmission medium and improve the transmission efficiency of the channel.

The main technologies of multiplexing are frequency division multiplexing (Frequency Division Multiplexing,FDM), time division multiplexing (Time Division Multiplexing,TDM) and code division multiplexing (Code Division Multiplexing,CDM). The main multiplexing methods used in disaster-tolerant optical fiber links are frequency division multiplexing and time division multiplexing.

Synchronous digital series

Synchronous Digital Family (Synchronous DigitalHierarchy SDH) is an international standard based on SONET. It is a set of standardized digital signal hierarchy for synchronous digital transmission, multiplexing and cross-connection. It is also a time division multiplexing technology. SDH multiplexes signals at the circuit layer. The basic principle is to package PDH, ATM, IP and other signals into packets, put them in STM-N frames, and then be carried by STM-N signals and transmitted over the SDH network. The multiplexing methods adopted are synchronous multiplexing and flexible mapping structure (which can be understood as time division multiplexing).

Before the advent of wavelength division multiplexing (Wavelength Division Multiplexing,WDM) technology, each pair of optical fibers in a typical ultra-long-haul optical transmission link transmitted only one SDH or SONET signal. Due to the loss of optical fiber, 3R optical-electro-optical (OEO) regeneration is needed for every 80km transmission. The regeneration distance is determined by the attenuation coefficient of optical fiber and the sensitivity of the receiver. The network consists of a series of OEO regenerators.

The cost of OEO regenerator is high, so the large-capacity data transmission network based on this technology is very expensive.

Wavelength division multiplexing

Wavelength division multiplexing technology is a special frequency division multiplexing technology based on optical layer, which combines two or more optical carrier signals of different wavelengths (carrying all kinds of information) at the transmitter through a multiplexer (also known as combiner, Multiplexer) and coupled to the same optical fiber of the optical line for transmission. At the receiving end, the demultiplexer (also known as demultiplexer or demultiplexer, Demultiplexer)

The optical carriers of various wavelengths are separated and then further processed by the optical receiver to recover the original signal.

According to the channel spacing, WDM can be subdivided into CWDM (sparse wavelength Division Multiplexing) and DWDM (dense wavelength Division Multiplexing). The channel interval for CWDM is 20nm, while the channel interval for DWDM is from 0.2nm to 1.2nm. There is also optical fiber loss in the transmission network of WDM. The solution is to use an erbium-doped fiber amplifier (EDFA) to amplify all DWDM optical signals after each 80km span. EDFA can amplify the optical signals of 40 to 80 wavelength channels at the same time, so the extra cost is shared by all channels. With the increase of the number of channels, the cost of transmitting per bit will be further reduced.

In the first generation DWDM system, the maximum transmission distance allowed by the technology is about 640km, and the typical distance supported by the relay is 400km, which is generally used in the metro core network. The emerging third-generation LHDWDM network adopts ULH technology, and the signal transmission distance can reach 3000km or even longer without OEO electrical regeneration, so it can be used in long-distance backbone network. Before arriving at the destination, the signal is not only in the transport layer, but also in the optical layer, which can further remove the redundant OEO regenerator and further reduce the transmission cost and transmission delay.

In a word, different multiplexing methods of disaster recovery communication links should be selected according to distance and cost. Based on the existing optical fiber network, metro WDM transmission network is recommended when disaster recovery distance is about 100km, and SDH transmission network, long-distance backbone WDM transmission network or SDH+WDM hybrid transmission network can be used when disaster recovery distance is 1000km or above. The components of DWDM devices are as follows:

Wavelength conversion: converts the service to be transmitted into an optical signal of the corresponding wavelength; includes the optical transmitter and receiver described earlier

MUX: combining multiple wavelength signals into a single optical fiber

DEMUX: separate multiple wavelength signals on an optical fiber, using the same device as MUX

DCM: dispersion compensation, optional

Optical amplifiers: amplifying optical power

Optical fiber: transmission media

In short, the wavelengths of all kinds of light used in the DWDM system are different, that is, specific standard wavelengths. In order to distinguish from the common broad-spectrum wavelength of SDH and other systems, the wavelength of WDM system is sometimes called color light (Colored), and the optical interface of ordinary optical system is called black and white light (Grey). WDM wavelength division multiplexing is the transmission of "light" of different colors (wavelengths) in the same optical fiber.

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