Network card working principle diagram

The network cards we use daily are all Ethernet network cards. At present, network cards can be divided into 10M network cards, 10/100M adaptive network cards and Gigabit (1000M) network cards according to their transmission speed. If it is only for general use, such as daily office work, it is more suitable to use 10M network cards and 10/100M adaptive network cards. If it is used in product fields such as servers, you must choose a Gigabit network card.
1. The main features of the network card

Network interface card (NIC), also known as a network adapter, is a device that connects a computer to a local area network. Whether it is an ordinary computer or a high-end server, as long as it is connected to a local area network, a network card needs to be installed. If necessary, a computer can also install two or more network cards at the same time.

When computers communicate with each other, data is transmitted in frames rather than streams. We can regard frames as data packets, which contain not only data information, but also the location of data transmission, location of data reception, and data verification information. A network card includes two layers of the OSI model - the physical layer and the data link layer. The physical layer defines the electrical and optical signals, line status, clock reference, data encoding and circuits required for data transmission and reception, and provides a standard interface to the data link layer equipment. The data link layer provides addressing mechanisms, data frame construction, data error checking, transmission control, and provides standard data interfaces to the network layer.

The network card has two main functions: one is to encapsulate the computer's data into frames and send the data to the network through the network cable (electromagnetic waves for wireless networks); the other is to receive frames transmitted from other devices on the network, reassemble the frames into data, and send them to the computer where they are located. The network card can receive all signals transmitted on the network, but under normal circumstances, it only accepts frames sent to the computer and broadcast frames, and discards the rest of the frames. Then, it is transmitted to the system CPU for further processing. When the computer sends data, the network card waits for the right time to insert the packet into the data stream. The receiving system notifies the computer whether the message has arrived intact. If there is a problem, it will ask the other party to resend it.

2. Graphical Network Card

Take the most common PCI interface network card as an example. A network card is mainly composed of PCB circuit board, main chip, data mercury, gold finger (bus slot interface), BOOTROM, EEPROM, crystal oscillator, RJ45 interface, indicator light, fixing plate, etc., as well as some diodes, resistors and capacitors, etc. Let's take a look at the main components.

Main chip

The main control chip of the network card is the core component of the network card. The performance and function of a network card mainly depend on the quality of this chip. Take the common RTL8139C and RTL8139D launched by Realtek as examples. First of all, the two are slightly different in packaging. The former is 128pin QFP/LQFP and the latter is 100pin. Secondly, in the matching EEPROM, 8139C supports 93c56 more than the latter, while 8139D supports 93C46. However, in terms of function, 8139D is stronger. It provides more support for PCI Multi-function and PCI-bridge I/F. PCI Multi-function allows the RTL8139D chip and other functional chips (such as hardware modem chips) to be designed on the same PCB board to work together to make different types of multi-function cards. Among them, the role of 8139 is to distinguish between LAN signals and PCI bus signals; 8139D also enhances the power management function.
If classified by the speed of the network card main chip, common 10/100M adaptive network card chips include Realtek 8139 series/810X series, VIA VT610* series, Intel 82550PM/82559 series, Broadcom 44xx series, 3COM 3C920 series, Davicom DM9102, Mxic MX98715, etc.

Common 10/100/1000M adaptive network card chips include Intel's 8254* series, Broadcom's BCM57** series, Marvell's 88E8001/88E8053/88E806* series, Realtek's RTL8169S-32/64, RTL8110S-32/64 (LOM), RTL8169SB, RTL8110SB (LOM), RTL8168 (PCI Express), RTL8111 (LOM, PCI Express) series, VIA's VT612* series, etc.

Figure 4 VIA's VT6120 Gigabit chip

It should be noted that network card chips also have "soft" and "hard" distinctions, especially for network card chips on-board the motherboard (LOM). What's going on? As we all know, the Ethernet interface can be divided into the protocol layer and the physical layer.

The protocol layer is implemented by a single module called a MAC (Media Access Layer) controller.

The physical layer consists of two parts, namely the PHY (Physical Layer) and the transmitter.

Common network card chips integrate MAC and PHY in one chip, but many motherboard south bridge chips currently include Ethernet MAC control functions, but do not provide physical layer interfaces. Therefore, an external PHY chip is required to provide Ethernet access channels. This type of PHY network chip is commonly known as a "soft network card chip". Common chips with PHY functions include RTL8201BL, VT6103, etc.

"Soft network card" generally transfers the computing part of the network control chip to the processor or south bridge chip to simplify the circuit design and reduce the cost, but it will occupy more system resources.

●BOOTROM

BOOTROM socket is also known as diskless boot ROM interface, which is used to construct diskless workstation through remote boot service. Remote boot service (Remoteboot, also commonly called RPL) makes it possible to boot a workstation on the network by using the software on the server hard disk instead of the workstation hard disk. The network card must be equipped with a RPL (Remote Program Load) ROM chip to achieve diskless boot. Each RPL ROM chip is made for a specific type of network interface card and they are not interchangeable. The network interface card with RPL sends broadcasts of boot record requests, and the server automatically establishes a connection to respond to it and loads the MS-DOS boot file into the workstation's memory.

In addition, there is usually a 93C46, 93LC46 or 93c56 EEPROM chip in the center of the BOOTROM slot (93C56 is a 128*16bit EEPROM, while 93C46 is a 64*16bit EEPROM). It is equivalent to the BIOS of the network card, which records the vendor ID of the network card chip, the subsystem vendor ID, the MAC address of the network card, and some configurations of the network card, such as the address of the PHY on the bus, the capacity of the BOOTROM, whether to enable the BOOTROM boot system, etc. The EEPROM information of the motherboard network card is generally integrated in the motherboard BIOS.

LED indicator

Generally speaking, each network card has more than one LED (Light Emitting Diode) indicator to indicate the different working states of the network card, so that we can check whether the network card is working properly. Typical LED indicators include Link/Act, Full, Power, etc. Link/Act indicates the connection activity status, Full indicates whether it is full duplex, and Power is the power indicator (mainly used on USB or PCMCIA network cards).

●LAN wake-up interface

Currently, mainstream independent network cards or motherboard onboard network cards comply with PCI2.2 and above specifications, so this interface is no longer needed. To enable the network wake-up function, just enable the "Wake on PCI Card" function in the motherboard BIOS.

●Data Mercury

Figure 10 Data Mercury
Data Mercury is a device that all consumer-grade PCI network cards have. Data Mercury is also called a network transformer or a network isolation transformer. It has two main functions on a network card. One is to transmit data. It uses a differential mode coupling coil to couple and filter the differential signal sent by PHY to enhance the signal, and couples it to the other end of the network cable with different levels through electromagnetic field conversion; the other is to isolate the different levels between different network devices connected by the network cable to prevent different voltages from being transmitted through the network cable and damaging the equipment. In addition, data mercury can also provide certain lightning protection for the equipment.

●Crystal Oscillator

Fig.11

Crystal is the abbreviation of quartz oscillator. Its English name is Crystal. It is the most important component in the clock circuit. Its function is to provide reference frequency to each part of accessories such as graphics card, network card, motherboard, etc. It is like a ruler. Unstable working frequency will cause unstable working frequency of related equipment, which is naturally prone to problems. Due to the continuous improvement of manufacturing technology, the important technical indicators of crystal oscillator such as frequency deviation, temperature stability, aging rate, and sealing are very good, and it is not easy to fail, but you can still pay attention to the quality of crystal oscillator when selecting.

For example, the clock circuit of a certain network card uses a high-precision SKO25MHz crystal oscillator, which reliably guarantees the precise synchronization of data transmission and greatly reduces the possibility of packet loss. In addition, the line design is as close to the main chip as possible, which greatly shortens the length of the signal routing and further increases reliability. If a low-quality crystal oscillator is used, although this can reduce the cost of the network card a little, it is very easy to cause data packet loss during transmission due to the accuracy of the frequency.

●Network cable interface

Common network card interfaces in desktop consumer-level network cards include BNC interface and RJ-45 interface (similar to the interface of a telephone), and there are also dual-port network cards with both interfaces. The choice of interface is related to the network wiring form. In a small shared LAN, the BNC port network card is directly connected to other computers and servers through coaxial cables; the RJ-45 port network card is connected to a hub or switch through twisted pair cables, and then connected to other computers and servers through hubs or switches.

At present, network cards with BNC interfaces are rare, mainly because there are relatively few networks that use thin coaxial cables as transmission media and there are many problems with networking methods. RJ-45 is an 8-core wire, while the interface of the telephone line is a 4-core wire, usually only 2-core wires are connected (ISDN telephone lines are connected to 4-core wires); but you can take a closer look, in fact, the RJ-45 socket of the 10M network card also only uses 1, 2, 3, and 6 pins, while the 100M or 1000M network card has all eight pins, which is also a way to distinguish 10M and 100M network cards (see Figure 8 above).

● Transmission medium type

Speaking of network cards, let's talk about the twisted pair connected to the network card.

Twisted pair is a data transmission line composed of many pairs of wires in an insulating jacket. Its characteristic is that it is cheap. Most of the current network cards use twisted pair as transmission cable. Twisted pair is generally used for wiring connection of star network. RJ-45 heads (crystal heads) are installed at both ends to connect the network card and the hub. The maximum length of the network cable is about 100 meters.

There are two types of twisted pair cables: STP (shielded twisted pair) and UTP (unshielded twisted pair). STP twisted pair cables have a metal isolation film inside, which can reduce electromagnetic interference during data transmission, so it has higher stability. UTP does not have this metal film, so its stability is poor, but its advantage is that it is cheap. Among them, STP (shielded twisted pair) is mainly divided into two types of cables, 3 and 5, and UTP (unshielded twisted pair) is mainly divided into 3/4/5/5e/6. Generally, the network mainly uses 5-category twisted pair cables, which have a thick outer protective rubber layer and are marked with "CAT5". 5e twisted pair cables are unshielded twisted pair cables. Compared with ordinary 5-category twisted pair cables, 5e twisted pair cables have less attenuation when transmitting signals and stronger anti-interference ability. In a 100M network, the interference level of user equipment is only 1/4 of that of ordinary 5-category cables, and it is also the mainstream application at present.

●Bus interface

The network card must be connected to the computer to be used normally. There are endless various interfaces on the computer, which also leads to the various types of bus interfaces used by the network card. In addition, when it comes to bus interfaces, it should be noted that people generally refer to this type of interface as "gold fingers". Why are they called gold fingers? This is because the wire pins of this type of card are plated with titanium (or other metals), which ensures reliable contact during repeated plugging and unplugging, increasing its own anti-interference ability and reducing interference with other devices.

For your convenience, we will illustrate the various common interface types of network cards below.

①ISA interface network card

Fig.13

ISA is a bus interface used by early network cards. ISA network cards use program request I/O to communicate with the CPU. This method has a low network transmission rate and occupies a lot of CPU resources. Most of them are 10M network cards. Currently, there are basically no ISA bus type network cards on the market. The author found several ISA network cards from the old parts pile, including D-LINK products. They can still be used after cleaning the gold finger with an eraser.

② PCI interface network card

Fig.14

The PCI (peripheral component interconnect) bus slot is still the most basic interface on the motherboard. It is based on a 32-bit data bus and can be expanded to 64 bits. Its operating frequency is 33MHz/66MHz. The data transfer rate is 132MB per second (32*33MHz/8). At present, PCI interface network cards are still the absolute mainstream in the home consumer market. ③ PCI-X interface network card

PCI-X is an extended architecture of the PCI bus. It is different from the PCI bus in that the PCI bus must frequently exchange data between the target device and the bus, while PCI-X allows the target device to exchange data only with a single PCI-X device. At the same time, if the PCI-X device does not have any data to transmit, the bus will automatically remove the PCI-X device to reduce the waiting cycle between PCI devices. Therefore, at the same frequency, PCI-X will be able to provide 14-35% higher performance than PCI. Currently, server network cards often use network cards with this type of interface.

④PCI-E interface network card

The PCI Express 1X interface has become a must-have interface for current mainstream motherboards. Unlike parallel transmission, the PCI Express interface uses a point-to-point serial connection method. The PCI Express interface varies according to the different bit width requirements of the bus interface, and is divided into PCI Express 1X (standard 250MB/s, bidirectional 500MB/s), 2X (standard 500MB/s), 4X (1GB/s), 8X (2GB/s), 16X (4GB/s), and 32X (8GB/s). Most network cards using the PCI-E interface are Gigabit network cards.

⑤USB interface network card

It is hard to find a computer without a USB interface (Universal Serial Bus). The USB bus is divided into USB2.0 and USB1.1 standards. The theoretical value of the transmission rate of USB1.1 standard is 12Mbps, while the transmission rate of USB2.0 standard can be as high as 480Mbps. Most of the current USB wired network cards are USB2.0 standard.

⑥PCMCIA interface network card

The PCMCIA interface is a special interface for notebook computers. The PCMCIA bus is divided into two categories, one is 16-bit PCMCIA and the other is 32-bit CardBus. The maximum throughput of the CardBus network card is close to 90Mbps, which is the mainstream of the notebook network card currently on the market.

The MiniPCI interface is an interface standard suitable for laptops that was expanded based on the desktop PCI interface. Its speed is equivalent to the PCI standard. Many such products are wireless network cards.