This is the 25th day of my participation in the August Challenge

The book goes back to modems, hubs, and Bridges:

Modem Modem

  • Modem is the tool of signal conversion, which is divided into modulation and demodulation two functions;

  • Modulation is to convert the original signal into the signal type suitable for transmission, and demodulation is the reverse process, the combination of the two to achieve two-way communication;

  • A modem is a physical layer device that changes the physical way signals travel.

  • The types of modems we commonly use in communication:

    • An antenna system that converts digital signals into wireless electromagnetic waves;
    • An optical modem that converts digital signals into optical signals;
    • ADSL modem that converts digital signal to analog signal;

  • For example, if the operator wants to use existing telephone lines to connect users to the Internet, it needs to install a modem at the user’s home or in the operator’s computer room to convert digital signals into analog signals that can be transmitted over the phone line.

  • For example, optical fiber into the building, all need to install optical cat, the optical fiber signal into digital signal, for the room equipment to access the Internet, because equipment generally can only use RJ45 network cable;

  • Therefore, the connection sequence of the general home network to the Internet is host–> router –>Modem–> Internet. The combination of router and Modem is often called gateway.

Hub Hub

  • Hub is the most primitive communication equipment, its structure and function are very simple, is to solve the LAN interconnection;

  • The hub does not parse the packet, but only amplifies and forwards the signal. It works at layer 1, the physical layer.

  • The communication principle of the hub is that the packets received by any port are broadcast on other N-1 ports. A hub is a broadcast domain, and the hub can be regarded as a big speaker. The host that receives the broadcast responds if the data is sent to itself, and discards the data if the data is not sent to itself.

  • When broadcasting, other ports can only listen to this packet, and other packets must wait in a queue. Therefore, a hub is also a conflict domain. It can be understood that there is only one common channel inside the hub. All ports are connected to this common channel, and this channel can transmit only one message at a time.

  • The working mode is half duplex, that is, cannot receive and send at the same time. In order to avoid conflicts, HUB adopts CSMA/CD conflict avoidance mechanism.

  • Hubs are suitable for small networks, but because the transmission efficiency is not high, now are basically replaced by switches. Let’s look at the upgrade equipment of hub — network bridge;

Bridge Bridge,

How do you connect multiple Lans and control conflict domains after you form a rudimentary LAN through a hub? So the bridge came into being

  • The bridge can identify the MAC address carried in the data frame. It is a device running at layer 2, data link layer.

  • The bridge has the ability to learn. It has a MAC address table, which records the mapping between MAC addresses and ports in the connected subnets.

  • Working mechanism of network bridge:

    • When the source host broadcasts data frames to the bridge port, the bridge registers the MAC address of the source host in the bridge table, so that the mapping between MAC and port is realized.
    • When the host sends a data frame and arrives at the bridge port, the bridge will parse the target MAC address in the data frame and check whether it is in its bridge table.
    • If the MAC address belongs to the same port as the source MAC address, the fw does not forward the MAC address. If the MAC address belongs to different ports, the FW broadcasts the MAC address to the destination port.
    • If it is not in the bridge table, it broadcasts on another port;

  • The two ends of the bridge are connected to two Lans respectively, but the network segment of the two Lans must be the same, otherwise they must communicate through routing. Therefore, the existence of the bridge actually expands the physical scope of the LAN.

  • In contrast to hubs, Bridges do not shrink the broadcast domain. For broadcast messages, they are also broadcast on all ports because Bridges are still connected to the same LAN/subnet.

  • However, the network bridge does divide the conflict domain, through MAC identification and filtering, each port corresponds to a conflict domain, so as to optimize the performance of the LAN, can carry more hosts;

  • The bridge also has caching function. First of all, the CSMA/CD anti-collision mechanism is adopted, and when the bandwidth at both ends of the bridge connection is not the same, cache buffer is also needed.

At this point, you should already be able to vaguely see the prototype of a switch. A bridge is an upgrade of a hub, and a switch is an upgrade of a bridge, with more ports, more bandwidth, and less collision domains, as we’ll explain in the next section.

Thank you for reading, if there are inaccurate and wrong place please comment, I will immediately correct, thank you!





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