Introduction to network transmission media

In addition to the communication equipment itself, the communication network also includes the transmission media that connects these devices, such as coaxial cables, twisted pairs, and optical fibers. Different transmission media have different characteristics, which directly affect many aspects of communication, such as line encoding, transmission speed, and transmission distance.

simple network

Two terminals, connected by a physical medium (also called a transmission medium) that can carry data transmission, form the simplest network.
The need for terminals to transmit information to each other and share resources is the main reason for the emergence of networks.
The terminal can generate, send and receive data. The network is the medium through which the terminal establishes communication. The terminal establishes connections through the network. The carrier used to transmit data is called a medium. Networks can use various media for data transmission, including physical cables, radio waves, etc.
A network is a large-scale, powerful system that interconnects terminals through media, allowing numerous terminals to easily transfer information to each other and share information resources.

Media - Coaxial Cable

Coaxial cable is an early transmission medium. There are two standards for coaxial cable, 10BASE2 and 10BASE5 . Both standards support a transmission rate of 10Mbps and a maximum transmission distance of 185 meters and 500 meters respectively. Generally, 10Base2 coaxial cable uses BNC connectors, and 10Base5 coaxial cable uses N- type connectors.
10BASE5 and 10BASE2 are two early Ethernet standards, both of which use coaxial cables as transmission media. The diameters of the coaxial cables used in 10BASE5 and 10BASE2 are 9.5mm and 5mm respectively , so the former is also called a thick cable, and the latter is also called a thin cable.
Now, the transmission rate of 10Mbps can no longer meet the current needs of enterprise networks, so coaxial cables are rarely used in current enterprise networks.

Medium - Twisted Pair

Compared with coaxial cable, twisted pair ( Twisted Pair ) has lower manufacturing and deployment costs, so it is widely used in enterprise networks. Twisted pair cables can be divided into shielded twisted pair cables (Shielded Twisted Pair , STP) and unshielded twisted pair cables (Unshielded Twisted Pair , UTP) . Shielded twisted pair cables have a metal shield between the twisted pair cables and the outer insulating envelope to shield against electromagnetic interference. There are many types of twisted pairs, and different types of twisted pairs generally support different transmission rates. For example, Category 3 twisted pair supports 10Mbps transmission rate; Category 5 twisted pair supports 100Mbps transmission rate; Category 5e twisted pair and higher-level twisted pair support Gigabit Ethernet transmission. Twisted pair cables use RJ-45 connectors to connect network devices. To ensure that the terminal can correctly send and receive data, the pins in the RJ-45 connector must be arranged in a certain line sequence.

Medium - Fiber Optic

Twisted pairs and coaxial cables use electrical signals to transmit data, while optical fibers use optical signals to transmit data. The transmission rates supported by optical fiber include 10Mbps , 100Mbps , 1Gbps , 10Gbps , and even higher. According to the different optical signal transmission modes of optical fibers, optical fibers can be divided into single-mode optical fibers and multi-mode optical fibers. Single-mode fiber can only transmit one mode of light and has no inter-modal dispersion, so it is suitable for long-distance and high-speed transmission. Multimode fiber allows different modes of light to be transmitted on one fiber . Due to the large inter-mode dispersion, the signal pulse broadening is serious. Therefore, multimode fiber is mainly used for short-distance transmission in local area networks. There are many types of optical fiber connectors, and commonly used connectors include ST , FC , SC , and LC connectors.

Media - Serial Cable

Various serial port cables are often used in network communications. The commonly used serial cable standard is RS-232 , which is also the recommended standard. However, the transmission rate of RS-232 is limited, and the transmission distance is only 6 meters. Other serial cable standards can support longer transmission distances, such as RS-422 and RS-485 with transmission distances up to 1200 meters. RS-422 and RS-485 serial cables usually use V.35 connectors, which were eliminated in the 1980s but are still used on traditional networks such as Frame Relay and ATM . V.24 is the European version of the RS-232 standard. RS-232 itself does not define a connector standard, and commonly used connector types are DB-9 and DB-25 . Now, RS-232 has been gradually replaced by new standards such as FireWire and USB , and new products and devices have generally used the USB standard.

conflict domain

Signal conflicts may occur in shared networks.
As shown in the picture is a 10BASE5 Ethernet. Each host uses the same coaxial cable to communicate with other hosts. Therefore, the coaxial cable here is also called a shared medium, and the corresponding network is called a shared medium network. Or simply called a shared network. In a shared network, when different hosts send data at the same time, signal conflicts will occur. The method to solve this problem is generally to use Carrier Sense Multiple Access / Collision Detection technology ( Carrier Sense Multiple Access/Collision Detection ).
The basic working process of CSMA/CD is as follows:
The terminal device constantly detects the status of the shared line. If the line is idle, data can be sent; if the line is not idle, detection will continue after a period of time (the delay time is determined by the backoff algorithm).
If another device sends data at the same time, the data sent by the two devices will conflict.
After the terminal device detects a conflict, it will immediately stop sending its own data and send special blocking information to strengthen the conflict signal so that other stations on the line can detect the conflict as early as possible.
After the terminal device detects a conflict, it waits for a period of time before sending data (the delay time is determined by the backoff algorithm).
The working principle of CSMA/CD can be simply summarized as follows: listen first and then transmit, listen while transmitting, stop transmitting due to conflict, and retransmit after random delay.

duplex mode

Both duplex modes support bidirectional data transmission.
Half-duplex: In half-duplex mode, both communicating parties can send and receive data, but not at the same time. When one device sends, the other can only receive, and vice versa. Walkie-talkies are a classic example of half-duplex.
Full-duplex: In full-duplex mode , both communicating parties can receive and send data at the same time. The telephone network is a classic example of full duplex.
The communication modes on Ethernet include half-duplex and full-duplex:
In half-duplex mode, both communicating parties sharing the physical medium must use the CSMA/CD mechanism to avoid conflicts. For example, the communication mode of 10BASE5 Ethernet must be half-duplex mode.
In full-duplex mode, both communicating parties can achieve two-way communication at the same time. This mode does not cause conflicts, so there is no need to use the CSMA/CD mechanism. For example, the communication mode of 10BASE-T Ethernet can be full-duplex mode.
The duplex mode of two devices connected on the same physical link must be consistent.

Chapter summary

Q: Which transmission medium is used when deploying Gigabit Ethernet in enterprise networks?

Answer: Gigabit Ethernet transmission must use twisted pairs of Category 5e standards and above, or Gigabit and higher optical fibers.

Q: What is a conflict domain?

Answer: A collision domain is a collection of all nodes transmitting in both directions over a shared physical medium. When hosts in the same collision domain send data at the same time, a collision may occur before the data reaches its destination.

Q: What is the role of CSMA/CD?

Answer: CSMA/CD is a mechanism for detecting and avoiding conflicts on shared networks.