Shulou(Shulou.com)06/01 Report--

Figure 1: communication model between two hosts

Agreement

When the voltage is applied at the A terminal, the voltage signal can be received instantly at the B terminal. The voltage signal is divided into high voltage and low voltage, that is, 0 and 1. If An and B have an agreement in advance, A sends 100 voltage signals per second to BMagazine B to receive 100 voltage signals per second. This requirement to send and receive semaphores per unit of time is called a protocol.

Network speed

The tenth power of 2 is 1024, which is about 10 to the third power. The 20th power of 2 is 1024 multiplied by 1024, which is about 10 to the 6th power. In daily life, we often hear that the bandwidth is 4Mbps, that is, 4 x 1024 x 1024 byte (bytes) messages are sent per second, which translates into bit (bits) is 1byte = 8 bites, and the network speed we use is 512Kb/s.

Network card

The network speed also has something to do with the network card, which represents the amount of signals that the host can modulate per second, such as 100-megabit network card and gigabit network card, but it is also limited by the transmission medium.

Figure 2: multi-host communication model

Broadcast

In the model of multi-host communication, if A wants to communicate with host B, it first sends a message to all hosts asking who is host B. this kind of communication is called broadcast.

MAC mechanism

Each host can receive the information of A, and when it is found that it is not sent to itself, it will lose the information. This identification mechanism is called MAC media access control.

CMACD

Because multiple hosts use one medium to communicate, this kind of cable can only allow one kind of voltage signal communication at a time, so there must be some problems. Before each host uses the cable to communicate, send out a detection signal to ensure that other hosts are not using the cable. If other hosts are in use, it will pause a certain random time and detect again. This mechanism is called underlying channel requisition, CMACD. The daily so-called Ethernet is the network characterized by this mechanism.

Hub (HUB)

As shown in the figure, if you want to add another mainframe, you need to remove another interface for the cable, which is generally called a T-port. For convenience, we can think of the whole cable as a device with multiple interfaces, which we call a hub.

Repeater

With the increase of transmission distance, the signal will attenuate to a certain extent, so we need to add signal amplification equipment to a certain distance, which we call repeaters.

Figure 3: two sets of host models

Collision domain: an area that can occupy channels with each other.

Network bridge

When the number of hosts on the same cable increases, that is, the number of hosts in the same conflict domain increases and the communication efficiency decreases, then we need to isolate the network, which is called a bridge.

Functions of the bridge:

Each subnet is connected to the interface of a bridge, and there is a chip inside the bridge that records which hosts are on which interface. When a host in a subnet wants to communicate, it will send a broadcast to the network. The interface on the bridge will receive its data message, and then judge whether the target host is in the subnet according to the information on the chip. If it is, it will be forwarded. In this way, the goal of isolation can be achieved.

Figure 4: switch network model

Exchanger

If you limit the subnet to which the bridge is connected, that is, there is only one host per interface, then the device is the switch. There is an intersection array inside the switch, and the communication between the third host and the fourth host will not be affected when every two hosts communicate.

Record table

Dynamic learning method: it is suitable for the time when the host on the switch changes frequently. The switch chip will send messages to all interfaces at regular intervals to determine whether the host on the interface is still there. If a new host is connected, the information in the log table is updated.

Static learning method: suitable for when the host on the switch is relatively fixed. At this point, you can manually change the information in the record table.

Dynamic learning requires a high load on the switch.

Broadcast storm

Although the cross-point array within the switch does not affect the communication after the connection is established, data packets are still broadcast to all hosts on the switch before the host establishes the connection. When the number of hosts on a switch increases, a broadcast storm is formed.

Figure 5: multi-switched network model

When the number of hosts on switch m increases, considering the broadcast storm, it is necessary to divide the large network into several subnets and try to add another switch. when host A starts to communicate, it sends a broadcast to the network. Switch n will also receive the broadcast message of A, and the host on m can also receive it, and the purpose of isolating the broadcast has not been achieved.

Logical address

It is possible to add a device between the two switches so that it can intelligently determine that if the destination address is a device on the switch, the message will not be forwarded. At this time, the concept of logical address should be introduced.

First of all, it is important to understand that hosts on the same switch must be communicating through physical addresses. The MAC address, which is unique to each Nic in the world.

Communication process:

Host A wants to communicate with host E, the message style is: 1.1Murmuri 2.1, then encapsulate its own source MAC address externally and start broadcasting to the network. The message arrives at the upper port of the router, and the router interface also has a MAC address. The MAC address is taken apart and the logical address of the target host is 2.1. it is determined that the host is not in the 1.0 network, and this interface begins to forward data packets to the lower port of the router. If it is 1.3, then the number of packets will not be forwarded, and the purpose of isolating broadcasts can be achieved.

The lower port encapsulates its own MAC address on the outside of the packet, and then begins to broadcast to all hosts in the 2.0 network. When the packet arrives at the 2.1host, it disassembles the external message and finds that the logical address of the target is the same as its own. It begins to send its own MAC address to the lower port, and the lower port will forward the MAC address to host A, so that host An and host B establish communication.

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