# Simultaneous Transmission of Photonic Services over One Fiber with an ITU 100 GHz Grid
## 1 Introduction  
At present, the literature suggests that data transmission will keep following the current trend, i.e., the continuous increase in the amount of data being transferred. Based on the report by Cisco [1], it is clear that in 2017, \(\approx\)122 EB of data were transmitted in a month, whereas in 2018, this figure reached \(\approx\)156 EB. By 2022, this amount of data is expected to increase up to 396 EB. One of the reasons for the increasing data transfer could be a wider deployment of high-speed interfaces on backbone networks and access networks. Customers of the access network at the edge of the millennium shared \(\approx\)155 Mbps. Fortunately, today's standards allow for sharing a transmission capacity of 10 to 100 Gbps [2; 3; 4; 5; 6; 7]. To support higher transfer rates, wavelength division multiplex (WDM) technology is used. WDM technology has been known for over 40 years [8; 9; 10]. Its primary purpose was to replace space division multiplex (SDM) technology. SDM used multiple fibers to separate services on a single channel. However, the spatial separation of individual services was very uneconomical. Due to developments in the field of radiation sources, more advanced techniques could be used to connectmultiple services into a single fiber by using coarse wavelength division multiplex (CWDM) or dense wavelength division multiplex (DWDM) technologies [11; 12]. Both of these methods are widely used today to transfer/separate multiple services in a single optical fiber. For the purpose of multiple services being present in one fiber, multiple so-called channels are used. Each of the channels uses a different wavelength. For CWDM, the wavelength spacing is 20 nm, with the first carrier at 1270 nm and the last at 1610 nm. Overall, it is possible to create 8-16 channels with a different data formats. To use all 18 channels, it is best to use the International Telecommunication Union (ITU) G.652D fiber, as this fiber are not susceptible to the wavelength range of 1360-1450 nm [13]. DWDM technology is considerably more complicated to implement because it requires use of precise laser sources of radiation that must maintain stable wavelengths to avoid the next channel being disturbed. The channel spacing is only 0.8 nm or less. The transmission rate of the channel may be 10 Gbps, and the total number of available channels ranges from 40 to 80. A great advantage is the reach of approximately 100 km without the need to amplify the transmitted signals. Currently, data are considered to be the base unit being transmitted. The increasing amount of transferred data is also a result of services such as streaming, Internet protocol television (IPTV), and video on demand (VoD) in high definition (HD) or ultrahigh definition (UHD) that are becoming increasingly popular. Unlike commercial telco operators, the National Research and Education Network (NREN) intends to enable transmission of advanced photonic and special services. These services can include accurate time transmission [14; 15; 16], stable frequency [17; 18; 19], or real-time network service. Real-time network service is not specifically related to performance or speed but rather to strict time constraints. As an example, one can think of the transfer of surgery over the network to doctors on the other side of the globe, etc. Generally, photonic services can be defined as an end-to-end connection between two or more endpoints. These services are defined by their photonic path and allocated bandwidth. The photonic path is the path that defines where the light travels from the source to the target(s). The recovery of a photonic path is a focus of many publications today, e.g., [20; 21; 22]. The allocated bandwidth represents a portion of the dedicated spectrum for the photonic service over the entire photonic path. The optical path requirements used for the transmission of time and/or frequency depend primarily on the choice of the utilized method.