Research on MAC Sub-layer Access Technology in WLAN (1)

I. Introduction:

Mobile computing network solutions can be divided into two types: Wide Area solutions and local area solutions. The wide-area scheme mainly relies on the wireless cellular data communication network and satellite communication network as the physical network of mobile computing. The WLAN scheme has higher transmission rate and lower communication cost, it can be used as an extension and replacement of a wired LAN, and is particularly eye-catching.

WLAN uses multiplexing channels as the basis for communication. In this way, compared with a point-to-point connection network, there is a key technical problem: when the use of channels leads to competition, how to assign the right to use a channel. In WLAN, the media access control MAC (medium access control) Sub-layer of the data link layer DLC is enabled. It can be said that the network performance (throughput, latency, etc.) of WLAN depends entirely on the Access Protocol of the MAC Sub-layer. Therefore, appropriate MAC Sub-layer specifications should be formulated to effectively configure channel resources based on network business characteristics, improve the efficiency of wireless resource usage, and improve the system capacity and transmission quality, it is an important topic for future WLAN research.

Ii. Classification of MAC access mechanisms

The central topic of the MAC layer is how to allocate channel resources between competing users. The method by which multiple terminals share the same channel resources is called channel access mode or multi-address mode. In a wireless LAN, MAC Sub-layer commonly used multi-access mechanisms can be divided into the following three types:

1. Random competition, such as the Aloha Series. A random competition Protocol generally uses a public channel. terminals connected to this channel can send broadcast information to the channel. If the terminal needs to send messages, it competes for the channel's right to use in some way. Once the right to use is obtained, all terminals can receive information from any terminal, if it is detected that it is sent to itself, accept it; otherwise, discard it.

2. Pay-as-you-go allocation (or reservation or no competition), such as token ring. The principle of this method is that the network asks each terminal whether data is sent in a certain cyclic order. If yes, it sends data immediately. Otherwise, the network immediately redirects to the next terminal. Round Robin is characterized by the fact that each substation can obtain the Channel Access Control fairly, which is suitable for situations where the communication traffic changes over time and such changes are unpredictable. This method is easy to implement and has been widely used in real-time distributed measurement and control systems.

3. fixed allocation classes, such as FDMA, TDMA, and CDMA. The principle is to divide a shared channel into several independent subchannels, each sub-channel is assigned to one or more users.

The above three multi-access technologies are applicable to different communication services. For voice-oriented networks with stable traffic, the fixed allocation class can provide reliable services while maintaining high channel utilization. The pay-as-you-go assignment class does not involve information collision. However, a dedicated channel is usually required. All users apply for a call through the channel in a fixed or random way, it is applicable to situations where the communication traffic changes randomly and is unpredictable. Fixed allocation and On-Demand allocation are suitable for businesses with high real-time requirements, while random competition is more suitable for businesses with non-latency sensitivity for users with intermittent work.

Iii. Analysis of advantages and disadvantages of various multi-access modes

Fixed allocation methods, such as time division multiple access (TDMA) and frequency division multiple access (FDMA), are fixed to divide the channel into different sub-channels and assign them to each user. This distribution method is channel-oriented and suitable for continuous stream services, such as voice services. When a user does not send information, the channel allocated to it will be wasted.

CDMA is a combination of a fixed allocation method and a random allocation method. It has some notable advantages, such as zero-channel access latency, high bandwidth utilization, and good statistical multiplexing, in addition, the use of CDMA technology can greatly reduce the impact of hidden terminal problems. However, the disadvantage is that the transmission rate is limited and the complexity of the base station is high.

The random access method is a competitive method of multi-access technology. It is suitable for scenarios with high traffic spikes. ALOHA is a simple random access method that is prone to business flow collisions when the business volume is heavy. The CSMA technology allows the whole channel bandwidth to be shared by all users. Users can send information only when the channel is idle. This method reduces the probability of collision. CSMA/CA closely associates the division of time domains with the frame format to ensure that only one terminal sends messages at a certain time point, thus achieving centralized control of the network system. However, it must be seen that latency, hidden terminals, and exposed terminals are inherent problems of WLAN. Currently, the main solution is To use the short message handshake mechanism (RTS/CTS (Request To Send/Clear To Send. Multi-address Access conflict prevention protocol MACA (Multiple Access Collision Avoidance), MACAW (improved MACA), and FAMA (Floor Acquisition Multiple Access) and other mainstream MAC Sub-layer protocols all use the RTS/CTS short group.

The pay-as-you-go allocation method retains the bandwidth for the user, and the user provides clear information to the network considering their bandwidth needs. The Network allocates bandwidth Based on the Data Length of the user's business. When the user is idle, the bandwidth allocated to the user will be allocated to other users. By allocating bandwidth on demand, the waste of network bandwidth resources is minimized, and there is no wasted bandwidth or latency due to resource competition, so as to achieve high information throughput. A simple on-demand distribution method is that the central controller takes turns to ask users. When users need to send information, the central controller allocates bandwidth, which requires high stability of the central controller.

It must be noted that the multi-access mode is also closely related to the network communication mode. The pre-distribution mode or competition mode is applicable to point-to-point (with only one destination terminal) communication mode, but the pure competition mode is not applicable to the communication mode of broadcast (with multiple destination terminals, because of One contention channel, it is difficult to ensure that multiple destination terminals can receive it smoothly. However, it is feasible to use a combination of contention and reservation.

All in all, a good MAC Sub-layer protocol should compromise in the following aspects:

1. share bandwidth resources fairly and effectively;

2. obtain the highest possible throughput;

3. Minimize latency.