The optical module application characteristics and test parameter values

1 Introduction Today, Ethernet technology has become an indispensable for LAN, also can not be replaced by technology. With the popularization of network, the network scale expanding, Ethernet access technology rapid development, the network transmission rate increase, and the network interconnection and next generation network technology applications, Ethernet transmission method, transmission capacity, service quality more and more attention, in which the transmission distance, transmission rate is the Ethernet transmission ability important reflect, is from the traditional LAN Ethernet technology to the metropolitan area network technology and wide area network technology key. However, from a technical point of view, the transmission rate is higher, limited transmission distance is short; from the application demand, the more high speed, more may be used for transmission, the transmission distance is long. Also because of this pair of contradiction, and high-speed Ethernet to the larger campus backbone and metropolitan area application of rapid expansion, the Ethernet standard transmission distance constraints often encounter challenges: why is the standard distance constraints? In order to meet the actual needs can break through the distance? In this paper, based on the optical media of Gigabit Ethernet standards as reference, emphatically from the media access control method, transmission loss, chromatic dispersion of the transmission of Ethernet transmission distance limitation and breakthrough approach. 2 Gigabit Ethernet standard distance limit Since the 1998 June IEEE 802.3z Gigabit Ethernet standard ( about 1000 Base-SX, 1000 Base-LX and 1000 Base-CX interface ) formally adopted since, has passed the IEEE 802.3ab ( about 1000 Base-T interface ) Gigabit Ethernet standard and IEEE 802.3ae ( about 10 GBase-SR, 10 GBase-LR, 10 GBase-ER, 10 GBase-SW, 10 GBase-LW, 10 GBase-EW and 10 GBase-LX4 10 G Ethernet interface ). But on the long distance transmission of Gigabit Ethernet, the main concern is with the fiber medium related Gigabit Ethernet standard -- IEEE 802.3z. According to the IEEE 802.3z standard, the different optical fiber bandwidth corresponding to the wavelength, the maximum transmission distance as shown in table 1. Wherein, wavelength of 850 nm corresponding to 1000 Base-SX, 1310 nm 1000 Base-LX wavelength corresponding. Table 1 Gigabit Ethernet optical fiber types, corresponding to different wavelength of maximum transmission distance Optical fiber type wavelength ( nm ) mode bandwidth ( MHz• km ) maximum transmission distance ( m ) 62.5 μm multimode 850160220 62.5 μm multimode 850200275 50 μm multimode 850400500 50 μm multimode 850500550 62.5 μm multimode 1310500550 50 μm multimode 1 310400/500550 10 μ m single mode 1 310N/A5 000 In Table 1, and the transmission distance limit is closely related to an important parameter is the modal bandwidth, is a section of the fiber can be adopted by the maximum modulation frequency pulse modulation frequency and the length of the fiber product, it is the embodiment of optical fiber transmission of information capacity, mainly embodies the dispersion on optical fiber system transmission rate, transmission distance effects. Table 1 the relevant parameters in the given condition analysis:①using LD light source rather than low rate system that uses LED light source;②considered the different types, different levels of fiber, especially the traditional optical fiber, the optical fiber emerging not listed but it will provide a better performance;③single-mode fiber mode dispersion can be ignored no, the corresponding mode bandwidth value high enough; the maximum transmission distance without relay amplifying means, dispersion compensation of time distance. 3 key factors influence the transmission distance Effects of Ethernet transmission distance of many factors, such as noise, crosstalk, which is the key factors are the main media access control method, signal transmission attenuation and signal transmission dispersion. The following are their principle, effect analysis, improvement measures. 3.1 media access control method for transmission distance Ethernet media access control method for CSMA/CD is to restrict the transmission distance is the most basic factor, it appears with the Ethernet technology, and with the transmission rate limiting distance shorter. The basic idea of CSMA/CD is to say after hearing, when interference when looking for a chance to say, namely, for each node on the same network, sharing the same transmission medium, monitoring the same network segment of the state, and may try to send data, but at the same time there can be only one node can send all data, other nodes can receive the transmitted data, if the node transmits the data signals are confounded by other signals, that have been in conflict, using backoff algorithm for collision avoidance. Typical backoff is truncated binary exponential backoff algorithm ( Truncated Binary Exponential Backoff ): ( 1 ) when sending a first data when there is a conflict, and m=2; ( 2) give time for r time slices, each slice of time equal to network end-to-end round-trip propagation delay, R 0~2m random number; ( 3) n times when there is a conflict, m (=min n, 10); ( 4) n the maximum value is set to 16, exceeded discard data instead of retransmission. In 10 Mbps, 100 Mbps Ethernet, the minimum frame length of 64 bytes, time slice to send 512 bits required for the 1 Gbps Ethernet, delay, minimum frame length is 64 bytes, time slice is to send 4096 bits required delay. The minimum frame length constant is to ensure that the Gigabit Ethernet Compatibility, but based on CSMA/CD, to ensure that conflicts can instant, effective detection, the round-trip distance must be small enough so that the short frame, first in the last one before sending back the entire network, according to the signal transmission speed of 200000 km/s calculation, for 10 Mbps, 100 Mbps and Gigabit Ethernet round-trip distance were 10240 m, 1024 m and 102.4 M. Considering the delay caused by the connectors and other equipment, especially rate to send the short frame shorter time, thereby greatly reducing the collision domain diameter, segment length becomes unacceptable tens of meters. A solution to this problem is in the Ethernet frame is increased after a data phase difference extended field, whose length is equal to the time slice can send bits minus the minimum number of bits of the frame. This also resulted in Gigabit Ethernet transmission efficiency reduction, solution is introduced into the frame of the emergency mechanism, i.e. once successfully sent a frame, the station can not need to compete and continue to send other frames, the maximum burst size is limited to 65536 bits. 3.2 signal attenuation on the transmission distance Signal on the transmission medium, the energy will gradually loss, thus determines the signal in the absence of the relay when the maximum transmission distance. When the signal propagating in the fiber, is transmitted to theL average light power and incident when the average optical power exponentially decreasing, namely P ( L ) =P ( 0) 10 -α L/10, attenuation coefficientαunit is the dB/km. For Gigabit Ethernet, in Table 1, the maximum transmission distance is determined according to the corresponding transmission system loss characteristics, including the fiber loss, insert loss, optical path penalty, transmission power, sensitivity, rich degree. In IEEE802.3z, based on the worst conditions considered, given in the maximum transmission distance when the link power budget, see table 2. The implementation of the project, if the relevant parameters are consistent with the data in Table 2, the standard distance within the application is sure to succeed. In practical engineering, there are often beyond the standard distance application requirements, Table 2 the worst conditions of link power budget Parameters of 850 nm, 62.5 μ m850 nm, 50 μM1, 300 nm, 62.5 μM1, 300 nm, 50 μM1, 300 nm, 10 μ m, SMF Link power budget ( dB ) 7.57.57.57.57.57.57.58.0 Link distance ( m ) 2202755005505505505505 000 Channel insertion loss ( dB ) 2.382.603.373.562.352.352.354.75 Link power price ( dB ) 4.274.294.073.573.485.083.963.27 Minimum average transmit power ( dBm ) -9.5-11.5-11.5-11.5-11.0 Receiver sensitivity ( dBm ) -17-19 Extinction ratio ( dB ) 9 Affluence ( dB ) 0.840.600.050.371.670.071.190.16 Of or based on the cost and other factors to consider, need to adopt the standard distance shorter but low cost plan, therefore, it is necessary to analyze table parameters: (1 ) standard numerical determination of the premise: considering a variety of possible situations, such as different manufacturers, different types of connectors, different manufacturers, different types of optical transmission characteristics vary widely, such as the early optical fiber. ( 2) the standard on the transmission distance of extended leave " backdoor ": if consistent with other standard conditions, beyond the range is acceptable. ( 3) the table parameters: ①link power budget and transceiving equipment: link power budget values for the minimum average transmission power and receiver sensitivity difference, therefore, if the actual sending and receiving devices with different parameter values in the table, as long as the difference with the link power budget requirements, should also be able to meet the actual needs. ②channel insertion loss: including the link section of optical fiber connection loss, line loss. ③link power price with the exception of the link loss related, and link dispersion, noise and the sending end of the extinction ratio and so on, its reason is the waveform distortion resulting receiver sensitivity decline. ④link power budget and rich degree: a segment to be successfully implemented, is to meet the requirement of bit error rate signal transmission, the link power budget should be large enough, or the channel insertion loss and link power price should be small enough, in order to make the system power is rich and non insufficient, should meet: Margin = link power budget - channel insertion loss - link power price > 0 ( 4) : data in the table for further analysis: In Table 1 channel insertion loss value should contain at least two of the connector insertion loss and optical fiber line loss. With MT-RJ joint each on the maximum insertion loss value of 0.75 for dB, two for maximum insertion loss value of 1.5 dB, 62.5μ m if the use of multimode optical fiber in the 850 nm band transmission signal, according to the maximum optical attenuation 3.75 dB/km calculation, the maximum distance is 220 m 0.825 dB loss, so a total of 2.325 dB channel insertion loss, still meet the 2.33 dB channel insertion loss budget. For the commonly used SC, ST and FC models of the connector, the insertion loss is much smaller, with a company of Wuhan product as an example, the insertion loss of single mode::≤ 0.3 dB, multimode:≤ 0.1 dB, even MT-RJ connector, its actual insertion loss: single-mode:≤ 0.7 dB, multimode:≤ 0.45 dB; on the other hand, a new multimode fiber attenuation has also been reduced in 850, NM < 3 dB/km, 1300 nm < 1 dB/km. Thus, a single channel insertion loss value, the actual power budget has quite rich. ②table link power budget for 7.5 dB or 8 dB, the actual equipment to provide greater link power budget, it means that the system can provide farther transmission distance. The 3.3 dispersion on Ethernet transmission distance The dispersion of fiber for optical signals of different frequency components and different patterns of components caused by the different transmission speed, it makes the optical fiber bandwidth is narrowed, thus limiting the optical fiber transmission capacity, but also limits the optical signal without electric relay transmission distance. The dispersion of fiber are the main mode of chromatic dispersion, dispersion and polarization mode dispersion, which in the fiber type, the transmission rate of the system, light source, modulation mode of different systems have different effects on. At a high rate, long distance optical fiber transmission system, dispersion of the system has significant effect on. As for the 10 G Ethernet, if the G.652 single-mode optical fiber ( NDSF ), and the EA modulator, only 1550 nm wavelength, light source for chirped single longitudinal mode laser source, at this time, dispersion limited distance is mainly composed of frequency chirp power price and the chromatic dispersion power, its dispersion limited distance of about 34 km; the same environment for the 2.5 Gb/s system, its corresponding dispersion limited distance of about 600 km