Wi-Fi. Формат кадра | Курс "Компьютерные сети"

Name: Wi-Fi. Формат кадра | Курс "Компьютерные сети"
Uploaded: 2016-11-20T17:39:02.000Z
Duration: 28 min 22 s

Understanding Wi-Fi Frame Formats

Overview of Wi-Fi Frame Formats

The lecture introduces the topic of Wi-Fi frame formats, explaining that it operates on both physical and data link layers, similar to the internet.

It highlights that Wi-Fi uses two different frame formats at the LLC (Logical Link Control) and MAC (Media Access Control) sublayers, with the LLC format being identical to Ethernet.

Differences Between Ethernet and Wi-Fi Frames

A key distinction is noted: Wi-Fi frames utilize four addresses instead of two as in Ethernet. This design choice supports infrastructure mode where wireless networks connect to wired networks for internet access.

The three primary devices involved in data transmission are identified: a computer transmitting data wirelessly, an access point, and a wired device connecting to the internet.

Addressing in Wi-Fi Frames

Addresses used in Wi-Fi frames include:

Receiver Address: Identifies the device receiving data from the wireless medium.

Transmitter Address: Indicates the sender's address; often matches with sender's MAC address when sending directly.

Destination Address: Refers to the MAC address of the wired device forwarding data to the internet.

Data Transmission Process

The process of transmitting a frame from a computer through a wired router to the internet is explained.

In this scenario:

The first field contains the MAC address of the access point (receiver).

The second field has the computer’s MAC address (transmitter/sender).

The third field includes destination information for further routing.

Reverse Data Transmission

When data travels back from a wired device through an access point to a computer:

The first field shows the computer's MAC address as receiver.

The second field lists the access point’s MAC address as transmitter.

The third field indicates sender information from the router.

Advanced Addressing Scenarios

Peer-to-Peer Communication

In peer-to-peer communication within one network segment:

Receiver and sender addresses match their respective devices' addresses.

Additional identifiers for peer-to-peer networks are generated automatically.

Special Cases in Addressing

An unusual case involves two wired computers communicating via a wireless network. Here:

Sender's and receiver's addresses are specified across multiple fields indicating their roles through various points like access points.

Frame Structure Details

Frame Size Limitations

Notably, Wi-Fi allows for larger maximum payload sizes compared to Ethernet—up to 2304 bytes versus Ethernet’s limit of 1500 bytes.

Error Handling Mechanisms

If checksum verification fails during transmission, erroneous frames are discarded. This ensures integrity in communication over potentially unreliable wireless connections.

Control Fields in Wi-Fi Frames

Types of Frames Used in Wi-Fi

Three types of frames utilized by Wi-Fi include:

Data Frames: Similar functionally to those found in Ethernet.

Management Frames: Essential for maintaining network operations such as acknowledgments or control signals like RTS/CTS.

Control Frames: Facilitate various services including connection establishment and authentication processes.

Directional Flags

Fragmentation in Wi-Fi Data Transmission

Understanding Data Fragmentation

The transmission of data frames larger than 1000 bytes can lead to errors; thus, fragmentation is necessary. Each fragment must be smaller than 1000 bytes to ensure successful transmission.

Fragmentation technology divides a single frame into smaller parts for separate transmission, utilizing two header fields: the fragment flag and sequence control.

Sequence Control in Fragmentation

The sequence control field consists of two subfields: the sequence number (indicating the original frame's ID) and the fragment number (indicating each part's order).

For example, a 1500-byte frame may be split into three 500-byte fragments. All fragments share the same sequence number while having unique fragment numbers.

Reassembly Process

The receiving side recognizes that it has received three fragments of the same frame based on their shared sequence number. The fragment flag indicates whether more fragments are forthcoming.

The reassembly process requires collecting fragments in order, starting with fragment one, followed by two and then three.

Retransmission Mechanisms

Handling Acknowledgments

If an acknowledgment for a transmitted frame is not received by the sender, it will retransmit that frame with an RT flag set.

This situation may arise if the acknowledgment was sent but did not reach the sender; hence, retransmission ensures reliability.

Power Management in Wi-Fi

Energy Efficiency Techniques

Power management is crucial for mobile devices using Wi-Fi to extend battery life. Devices operate in active or sleep modes to conserve energy.

In sleep mode, devices notify access points before entering this state. Upon waking up, they check for buffered frames from access points.

Frame Management During Sleep Mode

When a device wakes up and requests frames from an access point, it can return to sleep mode after receiving them.

Two flags manage power: one indicates power management usage while another signals additional data availability at the access point.

Wi-Fi Frame Structure

Frame Types and Addressing

Wi-Fi frames consist of three types: data transfer frames similar to Ethernet frames, control frames, and management frames used for service implementation.

Unlike Ethernet frames which use two MAC addresses (sender and receiver), Wi-Fi employs four MAC addresses including those transmitting and receiving data wirelessly.

Conclusion on Frame Formats