Optical network Technology

1 Optical network background


The emergence of optical networks mainly stems from the inability of traditional networks to adapt to the high bandwidth requirements brought about by the rapid growth of IP services. With the rapid growth of IP services, the demand for network bandwidth has become increasingly high. Traditional networks mainly transmit data through copper cables, with slow transmission rates and high power losses, which cannot meet the bandwidth requirements of modern networks and new business expansion. In addition, with the significant increase in copper cable prices, continuing to use copper raw materials to increase bandwidth costs is too high. Against the backdrop of the urgent need for bandwidth acceleration, optical networks have emerged. They use optical fibers instead of cables for communication, greatly improving data transmission rates and meeting the demands of modern networks for large bandwidth.


Overview of 2 Optical Networks


Optical network is the abbreviation of fiber optic communication network, which generally refers to a wide area network, metropolitan area network, or newly built large-scale local area network using fiber optic as the main transmission medium. Optical networks provide high-capacity, long-distance, and highly reliable link transmission methods through optical fibers. At the same time, on the basis of optical fibers as the transmission medium, advanced optical switching technology is used to introduce control and management mechanisms, achieving interconnection between multiple nodes and flexible configuration functions based on resource and business requirements.


3 Principles of Optical Networks


  • Optical transmission technology


Optical transmission technology is a technology that uses optical fibers as the transmission medium to transmit optical signals between the sender and receiver in the form of optical signals. Optical transmission technology utilizes the principle of total reflection of light, which is achieved by converting the electrical signal carrying data into an optical signal through an optical transmitter and coupling it into an optical fiber. The optical signal generates total reflection at the interface between the fiber core and the cladding, and forms a light lock that propagates forward inside the fiber core; The optical signal is continuously reflected on a uniform and transparent glass fiber core, and finally transmitted from the optical transmitter to the optical receiver at the other end. The information carried by the optical carrier is restored by the optical receiver. Due to the small diameter of the fiber core, light travels along the glass fiber core, and the loss of optical signals is much lower than that of electrical signals transmitted in network cables.


  • Optical switching technology


Optical switching is a technology that directly exchanges input optical signals to any optical output without undergoing any optical/electrical conversion in the optical domain. It can eliminate the optical/electrical and electrical/optical switching processes in traditional switching technologies and reduce losses. Optical switching technology can be divided into optical path switching technology and packet switching technology.


○ Optical path switching


Optical path switching includes three types: space division (SD) optical switching, time division (TD) optical switching, and wavelength division/frequency division (WD/FD) optical switching. Among them, spatial optical switching is the exchange of optical signals in the spatial domain, achieved by changing the transmission path of optical signals in space; Time division optical switching is based on the principle of time division multiplexing, which exchanges optical signals in the time domain; Wavelength division optical switching is based on the principle of wavelength division multiplexing, using wavelength selection or exchange methods to achieve switching.


○ Optical packet switching


Optical packet switching is a technology that uses packet switching for optical communication. Its implementation process is similar to that of electrical packet switching, which uses hop by hop addressing and forwarding. Optical packet switching technology uses optical packets as the minimum switching unit to divide data into multiple optical packets, transmit multiple optical packets simultaneously on a physical line, and then remove the packet header and reassemble each data field in order to form a complete message after the optical packet reaches the receiving end, completing the optical switching process.


Due to the characteristics of dynamic sharing and statistical multiplexing of bandwidth resources, optical packet switching can improve the utilization of network bandwidth resources and make the network highly flexible. However, due to technical reasons, optical packet switching technology has not yet reached practicality.


4 optical network structure


4.1 Composition and architecture of optical networks


The optical network is mainly composed of optical communication equipment, fiber optic cables, and optical modules. Among them, fiber optic cables are used for data transmission in optical networks, and optical modules are applied to optical communication equipment, serving as a bridge connecting optical communication equipment and fiber optic cables.


Generally speaking, optical network architecture can be divided into three layers: core layer, aggregation layer, and access layer, each layer playing different roles and achieving different functions in the optical network. However, with the development of technology, the three-layer network architecture in optical networks has gradually been simplified.


There are currently two main technical routes:


The PON (Passive Optical Network) network technology roadmap, such as the POL (Passive Optical LAN) all optical network solution, adopts a two-layer networking architecture, using passive splitters for fiber distribution, fiber 1: N indoor, multi-point shared broadband, and deploying ONU (Optical Network Unit) indoors, simplifying the optical network structure.


  • Ethernet all optical network technology roadmap, such as the minimalist Ethernet all optical network solution launched by Ruijie Network, which replaces active aggregation with passive transparent aggregation, achieving complete passivity of aggregation nodes. While simplifying the optical network structure, it strengthens the scalability of the network, and also involves fiber in the room. However, compared to the POL solution, the minimalist Ethernet all optical network solution adopts direct point-to-point connection from the core to the access, Realize exclusive broadband access for each access, providing users with a better network experience.


4.2 Optical communication equipment


Optical communication devices refer to communication devices that use light waves to transmit information in optical networks. According to the location and function of the network architecture, optical communication equipment can be divided into access switches, aggregation switches, and core switches. Among them, the core switch is located at the top core layer, the aggregation switch is located between the core layer and the access layer, and the access switch is located at the bottom access layer, directly connecting to the client.


4.2.1 Access switch


Access switches usually refer to switches that are designed for users to connect or access the network, directly connected to clients. Their main role in optical networks is to provide network access services, allocate services and bandwidth, and address the mutual access needs of users within local network segments. Due to its targeting of direct users, access switches have the characteristics of low cost and high port density.


Application scenario:


Access switches are widely used and have many application scenarios, especially in offices, small computer rooms, multimedia centers, and other scenarios. At present, there are many access switch products on the market, and product development is increasingly focusing on the convenience and flexibility of product use. For example, the RG-IF2920 series Ethernet all optical network indoor switch of Ruijie Network adopts a small size design (240mm long and 86mm wide), paired with an INC controller. After the device is connected, it can automatically complete registration and configuration, achieve plug and play, and installation is convenient and flexible, supporting office workstation and desk bottom installation There are various installation methods such as wall hanging, and in addition, it can be bound to the user identity of the information database through the INC controller, achieving no change in IP address, and the security policy varies with the identity, effectively ensuring user permissions and high network security.

4.2.2 Convergence switch


A convergence switch is a convergence point for multiple access layer switches. Its function in an optical network is to uniformly export access nodes, and also perform forwarding and routing, achieving functions such as resource access control and flow control. It can handle all traffic from access layer devices and provide uplink to the core layer, so aggregation switches need to have higher forwarding performance compared to access switches. A convergence switch is usually a three-layer switch.


Application scenario:


In optical networks, there are no fixed requirements for aggregation switches, and not every network must be equipped with aggregation switches, depending on the size of the network environment and the forwarding ability of the devices. The convergence switches on the market are represented by the RG-S5750-DP series switches of Ruijie Network. This series of switches adopts an integrated power supply/fiber optic transmission solution, simplifies network deployment, and supports VSU (Virtual Switching Unit) virtualization technology to achieve millisecond level fault recovery. It adopts hardware multiple protection to ensure the continuous operation of devices without dropping wires, and has a good network experience.


The aggregation switch needs to have power access, and corresponding weak current rooms should be deployed and configured with power supply in network deployment, which increases the complexity of network deployment and occupies a large space. To improve this issue, there have been transparent aggregation devices on the market that replace aggregation switches. They do not require power connections, so there is no need to deploy weak current rooms or configure power supplies in optical network deployment. This can effectively reduce line deployment, reduce external device failure rates, and achieve management and maintenance free weak current rooms. For example, the RG-DEMUX/MUX series Ethernet all optical network combiner/transparent convergence products of Ruijie Network, combined with unique color light technology, can achieve core to access bandwidth transparent transmission, with 1:1 exclusive bandwidth at the access end, and 10 Gigabit/Gigabit access, which can meet high bandwidth demand for device access. The device is plug and play, making it easy to go online and quickly expand capacity.

4.2.3 Core Switch

A core switch refers to a switching device placed in the backbone (core layer) of a network, mainly responsible for reliably and quickly transmitting a large amount of data flow in an optical network. Generally, it is a three-layer switch, which adopts a chassis style (mostly used in large networks) or box style (mostly used in small and medium-sized networks) appearance, has a large capacity interface bandwidth, and supports link aggregation function to provide sufficient bandwidth for the traffic sent by the distributed layer switch to the core layer switch. In addition, to ensure the availability of the core layer, the core layer equipment has high requirements for redundancy, reliability, and transmission speed. Therefore, when conducting network planning and design, the core layer equipment usually accounts for the majority of the investment.

Application scenario:

There are no fixed requirements for core switches, which depend on the size of the network environment and the forwarding ability of the devices. Core switches are often deployed in scenarios such as campus optical networks or data centers, and they need to be selected according to the needs of different scenarios. For example, for a campus network, it is not only necessary to consider the reliability of switch data transmission and forwarding, but also the energy consumption of the switch.

The RG-S7808C-V2 Ethernet all optical network switch of Ruijie Network adopts advanced CLOS orthogonal architecture, which can achieve non blocking forwarding and high-speed transmission without packet loss. At the same time, it supports industry-leading VSU virtualization technology, which can virtualize multiple physical devices into a single logical device, unify operation and management, reduce network operation and maintenance management costs, and also achieve fast switching of 50-200ms link failures to ensure uninterrupted transmission of critical services. In addition, RG-S7808C-V2 supports dynamic power management, which can save power at low loads. The intelligent fan supports 256 level speed regulation, precise temperature control, energy-saving and noise reduction, and can work for a long time at high temperatures, greatly reducing energy consumption. In addition to being deployed in the campus network, RG-S7808C-V2 can also be deployed in scenarios such as data centers and metropolitan area networks as needed.

Fiber optic is a type of fiber made of glass or plastic that serves as a medium and tool for transmitting optical signals in optical networks. According to the transmission mode of light in optical fibers, optical fibers can be divided into single-mode fibers and multimode fibers.

1. Single mode optical fiber

Single mode fiber can only transmit one mode of light at its working wavelength, usually using a laser as the light source. Light propagates in a straight line in a single mode fiber without reflection, so the signal strength loss is relatively small in optical transmission and can be used for medium to long-distance transmission of over 5 kilometers. The core of single-mode optical fiber is relatively thin, with a diameter generally ranging from 8um to 10um. The cladding diameter is 125um, and a yellow protective sleeve is used.

2. Multimode fiber

Multimode optical fibers can carry multiple fiber optic signals and transmit multiple modes of light, usually using LED as the light source. The light source is relatively scattered and will reflect light. Therefore, there is a significant loss of signal strength in optical transmission, and they are mostly used for short distance fiber optic transmission within 2 kilometers. The diameter of multimode optical fibers generally ranges from 50um to 62.5um, with a cladding diameter of 125um and mostly orange or water green protective sleeves.

3. Optical cable

Fiber optic cable is a communication line that realizes optical signal transmission, generally composed of several parts such as cable core, reinforced steel wire, filler, and sheath. In addition, it may also include components such as waterproof layer, buffer layer, and insulated metal wire as needed. Among them, the cable core is composed of one or more optical fibers in a certain way.

According to the different types of optical fibers used in optical cables, they can be divided into single-mode optical cables and multimode optical cables. That is, optical cables using single-mode optical fibers are single-mode optical cables, and optical cables using multimode optical fibers are multimode optical cables. Therefore, single mode optical cables and multimode optical cables each have the corresponding optical transmission characteristics of the fiber.

3. Optoelectronic hybrid cable

Optoelectronic hybrid cable is a composite cable that combines fiber optic transmission and power transmission in a network system. It is a new type of access method that integrates optical fiber and transmission copper wire, which can simultaneously solve the problems of equipment electricity and signal transmission. It has the advantages of small outer diameter, light weight, and small space occupation, as well as good scalability, superior bending performance, and good lateral pressure resistance.

In network business, to ensure its normal operation, it is generally necessary to solve the two problems of equipment power supply and data transmission through cables. However, in some network deployment scenarios, the installation environment is more complex, and it is difficult to route optical cables and cables separately. In this case, choosing an optoelectronic hybrid cable can solve power and data transmission problems simultaneously through only one cable, reducing the difficulty of routing.

4.Optical module

Optical modules are optoelectronic devices that perform optical/electrical and electrical/optical conversion, consisting of optoelectronic devices (including emission and reception parts), functional circuits, and optical interfaces. Optical modules are mainly used in optical communication equipment, connecting the main board of the optical communication equipment to the fiber optic cable network, and serving as the carrier for transmitting signals between optical communication equipment and other devices.

According to the packaging form classification, optical modules can be divided into SFF, GBIC, SFP, SFP+, XFP, XENPAK, X2, SFP28, CFP, QSFP, QSFP+, QSFP28 types, among which SFP, SFP+, XFP, QSFP+are currently the most commonly used. SFP is a gigabit optical module and is currently the most widely used type of optical module. SPF+is an enhanced version of SPF, which is a 10 Gigabit optical module like XFP. However, compared to SPF+, the XFP optical module encapsulates signal modulation, serial/deserializer, MAC, clock and data recovery (CDR), as well as electronic dispersion compensation (EDC) functions. Therefore, the size is also larger compared to SPF+. When the above functions are not encapsulated in the device board, the XFP optical module can be selected. QSFP+is a 40G optical module, which has a larger size compared to SPF and SFP+optical modules. CFP and QSFP28 are both 100G optical modules. Although the transmission rate is relatively high, considering application requirements and cost issues, gigabit and gigabit optical modules are still the mainstream at present.

Advantages of Optical Network Technology

Comparison between fiber optic and copper cables

○ Transmission distance and rate. The transmission distance of copper cables generally does not exceed 100 meters, and the current maximum transmission rate can reach 40Gbps; The maximum transmission distance of single mode fiber can reach 100km, and the maximum transmission distance can reach 100Gbps or higher.

○ Service life. Copper cables have a short lifespan and are prone to aging, requiring redeployment after aging; And the optical fiber has a long lifespan and is corrosion-resistant.

○ Transmission performance. Copper cables have significant power loss, are susceptible to electromagnetic interference, and are prone to theft, resulting in low security; And the power loss of optical cables is small, not affected by electromagnetic interference, and theft is more difficult, with higher security.

Comparison between optical networks and traditional networks

Taking Ethernet all optical network as an example, compare traditional network and optical network.

○ Good transmission performance. Optical networks use optical fiber as the medium, which has significant advantages in data transmission distance, transmission rate, transmission capacity, and security compared to traditional networks using copper cables as the medium.

The network structure is simple. Traditional Ethernet generally uses a three-layer physical architecture, namely core, aggregation, and access. However, optical networks have evolved from the initial three-layer physical architecture to the current two-layer architecture based solution.

○ Strong scalability. Traditional network information points are complex and require re planning and deployment of routing when bandwidth changes, which cannot meet the needs of rapid iteration of enterprise business. However, optical networks are more convenient in network expansion. For example, the minimalist Ethernet all optical network adopts a fiber optic in room solution. When the bandwidth changes, the installation and deployment of desktop switches in the room are more convenient, and business expansion is more convenient.


With the development of artificial intelligence, big data and other technologies, the amount of data transmission and processing is showing a rapid growth trend, so the demand for bandwidth is also constantly increasing. Due to its fast and efficient fiber optic transmission technology and network structure advantages, optical networks are gradually replacing traditional copper cable communication networks and becoming an important component of many large-scale network infrastructure. With the maturity of trends such as 5G, future optical networks will also play an important role in our increasingly digital world.

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