How Many Links Can Be Established over One Fiber Strand?

Introduction

Optical fibers, the transmission media of optical signals, can replace copper cables in the infrastructure network of a hospital. Featuring wide frequency band, low loss, and strong resistance to interference, optical fibers can provide more flexible and reliable data transmission for medical service systems. However, most of hospitals do not reserve sufficient trunk fiber resources (fiber strands) for a P2P all-optical network architecture. As a result, fiber resources cannot be added to the network in pace with the increase of transmission links triggered by business growth. The wavelength division multiplexing (WDM) technology can improve the utilization of each fiber strand.

Overview of optical fibers and WDM

Definitions:

An optical fiber is a fiber made of glass or plastic and used as an optical transmission medium.

WDM is a technology that allows two or more optical signals of different wavelengths to be transmitted over different optical channels in an optical fiber.

A link is a physical P2P connection between two passive devices.

1.1 How many strands can a fiber optic cable have?

A fiber optic cable generally contains 1-288 strands. Generally, the strand count is an even number. Follow the instructions below to determine the number of strands in a fiber optic cable:

(1) Determine the purpose of the cable, such as data transmission or video/voice/image transmission, and the number of fiber strands required. To establish an optical link on a traditional network, two fiber strands (Tx and Rx) are required for full-duplex transmission.

(2) After determining the purpose, determine the number of strands based on the 1:1 backup principle. That is, use four fiber strands for each optical link. If a fiber optic cable is used for multiple purposes on a trunk link of the campus network, the dual redundancy deployment can be used. For example, the imaging department of a hospital needs to connect to the two core switches through two uplinks, which means an 8-strand fiber optic cable is required. To ensure network security, two fiber optic cables can be used.

Note: If BiDi transceivers (optical modules) are used to transmit and receive data over the same fiber strand, one fiber strand is enough for establishing a complete full-duplex link.

1.2 How WDM works

The WDM technology combines multiple information carrier signals of different wavelengths into one beam and transmits it along one fiber. The multiplexer at the transmit end aggregates optical signals and couples them to the same fiber for transmission. The demultiplexer at the receive end divides the beam into optical signals of different wavelengths, which are then processed by the optical receiver to restore the original signals. This technology allows multiple signals to be transmitted over the same fiber. Each signal is transmitted through light of a specific wavelength, which is called a wavelength channel.

The working mechanism of WDM can be explained through an analogy with a superhighway. Vehicles driving to different destinations run on the same superhighway. After passing a toll gate, the vehicles drive in different directions toward their destinations.

WDM improves the transmission capacity and utilization of optical fibers.

1.3 WDM classification

WDM is generally implemented in two models: coarse WDM (CWDM) and dense WDM (DWDM).

(1) CWDM

To build an ordered information superhighway, WDM needs to control wavelengths of optical signals (like the interval between running vehicles). If the wavelength spacing is too small, optical signals may "crash". If the wavelength spacing is too large, the fiber utilization is low.

In the early stage of WDM, wavelengths are controlled at several tens of nm due to immaturity of the technology. This model is called coarse WDM, or CWDM. The International Telecommunication Union (ITU) sets the center wavelength range for CWDM to 1271–1611 nm, with a spacing of 20 nm.

(2) DWDM

With the development of this technology, the wavelength spacing decreases gradually to several nm. Then, dense WDM, or DWDM, is implemented.

In the DWDM model, the wavelength spacing can be 1.6 nm, 0.8 nm, 0.4 nm, or 0.2 nm, and 40, 80, or 160 wavelengths can be accommodated. The wavelength range is 1525–1565 nm (C band) and 1570–1610 nm (L band). DWDM usually uses the C band, with a wavelength spacing of 0.4 nm.

1.4 Advantages and applications of CWDM

(1) Low equipment cost, helping to reduce the cost of network construction and operation

CWDM devices are passive, small sized, and available in multiple forms, including ABS case, LGX plug-in module, and 1U rack-mounted chassis. They are easy to maintain and environmentally friendly. As CWDM devices support only 2–18 channels, they provide only 18 x 10 Gbps bandwidth at most. Generally, 16-channel CWDM devices do not have special requirements for optical fibers and can use G.652, G.653, or G.655 fibers. They can transmit signals of multiple services over one or two fiber strands in existing fiber optic cables. A CWDM system can significantly improve the transmission capacity and utilization of optical fibers. Shortage of fiber resources and high cost of leased fibers are common problems for all Metropolis Area Network (MAN) projects. Currently, a typical CWDM system provides 16 optical channels. According to ITU-T G.694.2, the number of optical channels can reach up to 18.

(2) Small size and no power consumption

CWDM devices are passive. In a CWDM system, lasers of CWDM optical modules do not need a thermoelectric cooler or the temperature control feature, so that their transceiver modules are small. The simplified structure also reduces the size of a CWDM device, saving space in the equipment room. Each laser in a CWDM system consumes only 0.5 W of power, whereas each laser in a DWDM consumes about 4 W of power. Compared with the traditional time-division multiplexing (TDM) model, CWDM is more suitable for high-speed data transmission on MANs, because it achieves data rate and protocol transparency. On a MAN carrying traffic of different protocols and rates, CWDM provides multiple channels on one fiber. These channels have different rates and are transparent to protocols, such as Ethernet, ATM, POS, and SDH. In addition, the transparency and add-drop multiplexing feature of CWDM allow users to add or remove a wavelength for signals without converting the format of original signals. That is, the optical layer provides a transmission structure independent of the service layer.

(3) High flexibility and scalability

It is important for a network to provide high-speed transmission and expand with business growth. CWDM supports quick service provisioning within one day or even several hours. In addition, the network capacity can be expanded by simply inserting new optical transport units (OTUs) to support increasing service traffic.

(Commonly used WDM technology)

How many links can be established over one fiber strand?

In summary, WDM can increase the transmission rate tens of folds without adding optical fibers. How is this achieved?

It is known from basic characteristics of light that a white light beam can be split into beams of seven colors when passing through a prism. These beams are combined into a white light beam again after passing through another prism. This optical phenomenon tells us that even light of the same color can be sub-divided.

As we all know, light is a type of electromagnetic wave and has a certain wavelength and frequency. Generally, the wavelength and frequency are used to identify a light wave. Light in an optical fiber is composed of multiple wavelengths. It is found that each wavelength can carry different information independently, and therefore more information can be transmitted over multiple channels than a channel with only one wavelength. If a WDM system contains 16 wavelengths, it can transmit 16 times as much information as a single wavelength. To make it easier to understand, we can consider a fiber-optical link as an urban tunnel:

During specific night hours, only maintenance vehicles can drive into the tunnel to clean it. This is like the traditional transmission model where one fiber supports only one service link.

When WDM is used, this tunnel provides three lanes, and each lane is like a service link on an optical fiber. The WDM technology turns each lane into 16 lanes to transmit 16 signals of different wavelengths (16 service links or eight full-duplex links in each direction for bidirectional transmission). In this way, the capacity of fiber channels is expanded.

The traffic volume on each lane is the total load of all vehicles on this lane. If a service upgrade is required, for example, each CT image needs to contain more information, the vehicles carrying CT images can be replaced with larger ones. Then, the total load of the lane is increased greatly without affecting the total number of lanes and traffic on the other lanes. The replacement of smaller vehicles with larger ones in this example is equivalent to the replacement of optical modules at both ends of a link. For example, if 1G optical modules are replaced with 10G optical modules, the speed of this link will be increased 10 folds. The other seven full-duplex links on the same fiber remain unchanged. The WDM technology improves the transmission speed of each fiber and allows more links to be established more flexibly to satisfy customers' need for high bandwidth.

Summary

Low-cost, small-sized, and passive CWDM products can help customers establish multiple links over one fiber strand. This deployment greatly reduces the fibers used, allows for more flexible service upgrades, and improves both the network bandwidth and transmission speed.