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

Wireless local area network is actually a kind of shared network. Each wireless connection is half-duplex.

To achieve full-duplex mode, it must be transmitted at one frequency and received at another frequency, but the IEEE 802.11 standard does not allow full-duplex mode.

With the exception of broadcast and multicast frames, all outgoing frames must be responded to. This is a basic feedback mechanism to confirm that the frame has been received correctly.

IEEE 802.11 uses CSMA/CA to avoid conflicts. IEEE 802.3 wired networks detect conflicts, while IEEE 802.11 networks avoid conflicts as much as possible.

The IEEE 802.11 standard allows multiple wireless clients to communicate directly with each other without the need for other network connections, which is called peer-to-peer wireless network (Ad Hoc) or independent basic service set.

The IEEE 802.11 basic Service set (BSS) contains an access point (AP) that acts as a hub for that service set.

The higher the frequency, the shorter the transmission distance. 2.4G travels farther than 5G. The higher the frequency, the more it is absorbed by the object.

AP requires the client to meet the following conditions to allow access:

1. Matching SSID

2. Compatible wireless rate

3. Authentication credentials

Applying for membership from AP is called association, and the client must send an association request message, and AP approves or rejects the request by sending the association reply message. After the association, the data going to and from the client must pass through the AP.

A BSS contains only one AP and is not connected to regular Ethernet, in which the AP and its associated clients form a separate network.

AP is responsible for mapping VLAN to SSID. Therefore, the AP must be connected to the switch through a trunk link (Trunk) that transmits data for multiple VLAN.

That is, when AP uses multiple SSID, it connects the VLAN to the end user through wireless media, the end user must use the appropriate SSID, and the SSID is mapped to the corresponding VLAN.

The coverage of the signal depends on the antenna radiation pattern of the AP. The radiation pattern is three-dimensional. In a multi-storey building, the upper and lower floors will also be covered.

In order to determine the location and coverage of the AP, the best way is to conduct a site survey. Test the coverage of the AP in a real environment, where obstacles may interfere with client connections.

By adding AP, cellular coverage is used to cover all areas to which the client may move. There should be a certain degree of overlap between the honeycombs.

If two adjacent AP use the same frequency, they will interfere with each other. Throughout the coverage area, the frequency of the AP should change alternately.

AP cellular is actually a half-duplex shared medium, and all clients will share its bandwidth and transmissible time.

By reducing the transmission power to reduce the size of the cell, reduce the number of clients in the coverage area, and improve the communication quality.

In radio frequency (RF) communication, a device transmits a vibration signal, which is received by one or more devices. The vibration signal is based on a constant and is called frequency.

The sender uses a fixed frequency, and the receiver can adjust to the same frequency in order to receive the signal.

The carrier signal is only a stable signal with a fixed frequency, and the carrier signal itself does not contain any audio, video or data, because it is only used to carry other signals.

To send other information, the transmitter must modulate the carrier signal and insert the information (that is, coding) in a unique way, and the receiving station must do the opposite processing to demodulate the signal to restore the original information.

Modulation technology: amplitude modulation (AM), frequency modulation (FM)

The idea of WLAN modulation is to encapsulate as much data as possible in wireless signals and minimize the amount of data lost due to rain interference or noise. Because the data must be retransmitted after loss, it will take up more wireless resources.

Characteristics of RF: reflection, refraction, absorption, scattering, diffraction, Fresnel zone

When the power is reduced by half, the DB value will decrease by about 3%; with each doubling of the power, the DB value will increase by about 3%.

Omni-directional antenna: suitable for large rooms or entire floors

Semi-directional antenna: suitable for covering many rooms in the corridor, there is almost no signal behind this kind of antenna

Highly oriented antenna: suitable for establishing a building-to-building or site-to-site link where two AP or antennas are far apart.

The IEEE 802.11 standard defines how wireless Lans operate at layer 1 and layer 2, including wireless frequencies and channels, as well as throughput, security, and mobility.

Manage frames: used to manage WLAN advertisements and client membership

Control frame: used to control client association with AP

BSSID is the MAC address of the AP, which uniquely identifies a set of devices for the AP service.

802.11b:2.4G

If two wireless workstations use adjacent channels, there will be such a problem: their signals penetrate into each other's channels, resulting in the destruction of both channels. The solution is to use as many channels as possible that do not overlap each other, which makes only channels 1, 6, 11 available.

The wireless client and the AP must agree on the supported rate before the client can associate to the AP.

Higher rates can be successfully achieved only when close to the AP.

802.11g:2.4G

802.11a:5G

802.11n:2.4G or 5G

802.11ac:5G

802.11e:QoS

802.11i: security

There are two ways to supply power:

1 、 Cisco CILP

2 、 IEEE 802.3af

When the switch port is in the DOWN state, the switch stops power on that port, and then the switch must constantly detect whether the port is connected to a device that needs power, and if so, the switch must start supplying power so that the device can initialize and operate properly before the Ethernet link is established.

If you are using Cisco CILP, the switch can attempt to exchange Cisco device Discovery Protocol (CDP) messages with the device. If CDP information is returned, the switch can know the type of device (such as IP phone) and its actual power requirements, and then the switch can reduce the power to the value required by the device. This process can be obtained using debug ilpower controller and debug cdp packets.

Under the interface, you can configure the power options provided by POE.

Use the following command to view the POE status of the switch port:

In IEEE 802.3af, the switch first provides a lower voltage on the transmission and reception pins connected by the copper twisted pair, and then detects whether the current comes from the device by measuring the resistance between the pins. If the resistance is measured to be 25 ohms, it indicates that a device that needs power is connected.

The Catalyst switch waits for 4s after powering up on the port to see if any device is activated, and if not, it will be powered off on that port.

If the switch port was originally connected to a wireless access point or IP phone, be careful when removing it and plugging it into a regular Ethernet device. Within 4 seconds after the device is removed, there will still be power supply. if a device that does not support the built-in power supply is plugged in during this period, it may be damaged. After removing the device, wait 10 seconds before connecting other devices to the same port.

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