MAC protocol in Wireless Networks (1)

In the previous article, we analyzed the MAC protocol of the traditional wired network. Next we will give a detailed introduction to the MAC protocol of the wireless network. How does MAC work in wireless networks? The MAC standard in a wireless LAN (WLAN) is IEEE 802.11. IEEE 802.11 MAC combines two work modes: Distributed Control (DCF) and central control (PCF) there are two ways to work:

1. distributed Control Mode (DCF), similar to CDMA/CD, uses the carrier listening mechanism to transmit burst and random group data in distributed networks, supports non-competitive real-time businesses and competitive non-real-time businesses ｡

2. The central control mode (PCF) is built on the DCF operating mode and only supports competing non-real-time services. It is suitable for networks with central controllers ｡

Distributed Control Mode (DCF)

The DCF mechanism is the most basic media access control mechanism in the MAC protocol layer. The DCF mechanism is based on CSMA/CA (CSMA/Collision Avoidance carrier listener multi-access/Collision Avoidance ), the media access mode is supported by the RTS/CTS message switch ｡

CSMA/CA Protocol

CSMA is a random competitive MAC protocol with simple algorithms and rich performance. Therefore, CSMA has been widely used in practical LAN applications, due to the inherent characteristics of wireless transmission media and the impact of mobility, MAC of a wireless LAN is different from that of a wired LAN in terms of Error Control and terminal hiding. Therefore, CSMA of a WLAN is different from that of a wired LAN. CSMA/CA (CSMA) is used for WLAN. /Collision Avoidance) protocol, the biggest difference with CSMA/CD is that it adopts a way to avoid conflict ｡

Unlike CSMA/CD, the CSMA/CA (CSMA/Collision Avoidance) protocol is used on the MAC layer of WLAN media access control, which is difficult to detect conflicts in RF transmission networks, therefore, the protocol uses conflict detection instead of conflict detection used by the 802.3 protocol, and uses the conflict avoidance mechanism to minimize the probability of conflict collision, the protocol uses the channel idle evaluation (CCA) algorithm to determine whether the channel is idle, this is accomplished by testing the antenna energy and determining the received signal strength. In addition, conflict is reduced using the RTS, CTS, and ACK frames. Data Encryption is the same as the common LAN equivalent encryption (WEP) algorithm, use a 64-bit key and RC4 encryption algorithm ｡

CSMA/CA process: 

When the transmitter wants To Send data, it first checks whether the media is idle. If the media is idle, it sends a Request To Send (Request To Send ), the RTS signal includes the transmitting end address, the receiving end address, and the next data transmission time. After receiving the RTS signal, the receiving end returns the Short Signal CTS (Clear To Send ), the duration of continuous transmission recorded in the CTS signal. When the transmitting end receives the CTS packet, it starts to send the packet. After the receiving end receives the packet, the CRC (Cyclic Redundancy Check, Cyclic Redundancy Check) value in the packet is used to Check whether the packet data is correct. If the test result is correct, the receiving end will respond to the ACK packet, inform the transmitting end that the data has been successfully received. When the transmitting end does not receive the ACK packet from the receiving end, it will think that the packet is lost during transmission, and it will be re-sent ｡

Difference between CSMA/CA and CSMA/CD

1. carrier Detection Method: CSMA/CD and CSMA/CA are detected in different ways due to different transmission media. CSMA/CD is detected by changing the voltage in the cable. when data is collided, the voltage in the cable changes. CSMA/CA uses three idle channels: Energy Detection (ED), carrier detection (CS), and energy carrier Hybrid Detection ｡

2. comparison of channel utilization: the channel utilization of CSMA/CS protocol is lower than that of CSMA/CD Protocol. However, due to the characteristics of wireless transmission, CSMA/CD Protocol cannot be used in a wireless LAN. The channel utilization is affected by the transmission distance and the degree of openness. When the distance is long or there are obstacles, there will be hidden terminal problems, reduces channel utilization ｡