What Is a Hybrid Cable?
A hybrid cable incorporates optical fibers and copper wires within the same jacket, and can supply power to devices while transmitting data. On campus networks, hybrid cables are typically used to connect access switches and WLAN APs, so that the access switches can supply PoE power to the APs while exchanging data with them. As future-proof WLAN technologies, such as Wi-Fi 6 and Wi-Fi 7, gain momentum, the conventional twisted pairs, however, cannot keep pace with the bandwidth needs of these technologies. Not only this, optical fibers cannot be used for PoE power supply. This is where the hybrid cable comes in.
Why Do We Need Hybrid Cables?
Typically, communication cables fall into optical cables and copper cables based on their transmission media. To elaborate, optical fibers are used most often as a means to transmit light based on total internal reflection, and have advantages such as large bandwidth, low loss, and long transmission distance. However, they are made of glass fibers that cannot conduct electricity. Copper cables consist of copper wires that transmit data via electromagnetic waves. As a good conductor of electricity and heat, copper cables can transmit both data and electrical signals, but will heat up rapidly in this process. This may lead to a serious propagation loss, and therefore copper cables are not applicable to long-distance transmission.
Some network devices, such as 5G base stations and video surveillance cameras, are installed in complex environments where we can hardly find proper receptacles to supply power to them. Facing this, the entire industry is finding ways to design a cable that can maximize the advantages of the optical and copper cables while minimizing their disadvantages. This is where the hybrid cable comes in. It transmits data signals via optical fibers and transmits electrical signals via copper wires, enabling long-distance power supply without deteriorating the data transmission speed.
The hybrid cable was first developed by Sumitomo Electric in 1978, and was mainly used for submarine transmission of optical and electrical signals. After years of evolution, the hybrid cable is now widely used in numerous scenarios, such as signal coverage for 5G base stations and fiber to the apartment (FTTA) cabling. In addition, as Wi-Fi technologies keep on developing, they pose higher requirements for the campus network bandwidth and PoE power supply quality. To meet these requirements, many vendors in the industry begin to explore ways to deploy hybrid cables on their campus networks.
The following figure shows the cross section of a hybrid cable. Drawing on a special fiber-copper incorporation structure and a custom protective layer, the hybrid cable ensures that optical signals and electrical signals do not interfere with each other during transmission. Not only this, it can be deployed in various network systems, significantly reducing network construction costs.
Cross section of a hybrid cable
Why Do Campus Networks Need Hybrid Cables?
Conventionally, twisted pairs are routed between access switches and APs to transmit data between them while supplying PoE power to APs. However, as Wi-Fi technologies continue to develop, they pose increasingly higher requirements on cable performance. For example, as Wi-Fi 6 is commercially used on a large scale, it requires the cable to deliver up to 10 Gbit/s bandwidth. The future-oriented Wi-Fi 7 standard further requires the cable to deliver a maximum of 40 Gbit/s bandwidth while supplying PoE power to APs over long distances. In most cases, APs are installed in complex environments and require PoE power supply over a distance of more than 100 m. In some special scenarios, the power supply distance will be much longer. For example, APs in some stadiums require 300 m or even longer power supply distances. The conventional twisted pairs, however, can only supply PoE power over a distance of up to 100 m.
Applications of hybrid cables on campus networks
As shown in the following figure, if twisted pairs are used, the network bandwidth can only be 10 Gbit/s at most without sacrificing the PoE power supply distance. If the network bandwidth reaches up to 25 Gbit/s, the power supply distance will be as short as only 30 m, which cannot cater to most PoE power supply scenarios. If optical fibers are used, the data transmission speed will not be limited, but we need to add a power supply system to supply power to the APs. However, it is hard to route power cables and manage this power supply system as APs are most often installed in complex environments. Facing this, we need to design a cable that can supply PoE power to APs without limiting the bandwidth evolution. This is where the hybrid cable comes in.
Transmission speed and distance bottlenecks of twisted pairs
How Does a Hybrid Cable Work?
Hybrid cable is a key component in Huawei's All-Optical Campus Network Solution. The following figure shows its appearance. The question is why hybrid cables can support long-term bandwidth evolution and long-distance PoE power supply, while twisted pairs or optical fibers cannot.
Appearance of the hybrid cable
The answer falls into two parts:
First, the hybrid cable maximizes the advantages of optical fibers. It transmits data signals via optical fibers that are used most often as an optimal means to transmit data over long distances and at high bandwidth. In contrast, twisted pairs transmit data signals via copper wires, and the transmission quality is easily affected by the wire resistance and capacitance, inevitably leading to attenuation and distortion of data signals. Not only this, the extent to which data signals are attenuated is also related to the cable length. A longer cable usually means a higher attenuation value. To prevent this, the regulation on integrated network cabling states that the cabling distance of twisted pairs cannot exceed 90 m and the total link length cannot exceed 100 m. Data transmission over optical fibers, however, is not affected by resistance or capacitance. This is because they transmit data based on total internal reflection. In this process, we will not see energy loss due to the thermal effect of current, or crosstalk caused by electromagnetic induction. What we do see are longer transmission distance and higher bandwidth.
Second, the hybrid cable minimizes the disadvantages of copper wires. Copper wires in a hybrid cable transmit only DC electrical signals. This means that the power supply distance can be quite long. According to Huawei's tests, hybrid cables can supply 60 W PoE power even at a distance of 300 m. That said, copper wires still have resistance, and thermal effect still occurs during transmission, meaning there is still some energy attenuation. Therefore, the transmission distance over copper wires is limited even for DC signals. Essentially, the transmission distance of a hybrid cable depends on the transmission distance of DC signals over copper wires. But, as technologies and practices improve in the near future, we will see transmission distances reaching beyond 500 m, which will meet the requirements for long-distance PoE power supply in most scenarios.
- Author： Huang Mingxiang
- Updated on： 2021-09-02
- Views： 134
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