In cases where a special connection is not established between the devices and there are common connections in communication, more than one device communicates at the same time over more than one communication, the same line, the same cable, or the same channel. Multiple Access Protocols is an OSI Layer 2 definition that determines which data can be transferred to which network environment in which time interval. The Data Link Layer is responsible for operations such as Link Controls and Multiple Access Protocols in the communication established between the sender and the receiver.
Multiple Access Protocols are used to reduce the collision that may occur in the channel during transmission, to enable more than one user to access the same channel at the same time, and to reduce the congestion that may occur in traffic. Multiple Access Protocols are examined under three different headings: Random Access, Channel Access, and Control Access. This article will talk about Random Access Protocols.
Channel access methods are used in TCP communications where are provided over the physical layer. There are different methods are used to communicate using the signal transmission. These are;
- Communication with using the same frequency but dividing the time one by one users
- Communication with using unique codes
- Communication with using different frequencies for different users
- Communication with using the same frequency but dividing the areas where the signal is used by different users
- Communication with using different power values for different users
When the receiver and transmitter want to communicate, they communicate over certain channels with the help of a multiplexer. Multiplexer works with a kind of switchboard logic. It allows different users to communicate with each other by being on different channels. Thus, options such as conflicts and data loss are minimized.
TDMA – Time Division Multiple Access
Time Division Multiple Access is one of the channel access techniques and that was first used by Western Union for the Westar3 communication satellite in 1979. TDMA is a channel access method that allows a certain number of users to communicate at different times via using the same frequency. These users can have full bandwidth for their allotted time in communication. Users can use the same frequency for communication in the same network. Also, kind of a router device be located in the center and that can decide the communication time for the receivers and transmitters for using the same frequency. For example, GSM used by mobile phones uses TDMA technology for signal transmission. Devices are using GSM technology cause regular generation of electromagnetic waves.
Thus, data belonging to two or three different communications are loaded on one frequency and which is transmitted to the target. Milliseconds are divided according to different communications and data sends to the destinations one by one respectively. In simultaneous communications, delays can reach high levels. If the number of time slots to be used is full, a new user has to wait for one of the existing communications to finish before starting a communication. TDMA technology can be compared to a conventional switchboard application.
FDMA – Frequency Division Multiple Access
Frequency Division Multiple Access is one of the channel access techniques. High-performance filters are used in FDMA technology. FDMA helps prevent problems that occur in close-range communications that may reduce the quality of communication. Communication is provided to users by allocating a frequency or multiple different frequency bands. A frequency cannot be used in the communication of several users at the same time, the frequency allocated to each user is on a different channel or in a different frequency band. Thus, it allows limited access to communication. All users can transmit and receive different frequencies. FDMA can work with both digital and analog signals.
It is possible, for more than one user to communicate using different frequencies in the same environment at the same time. As a disadvantage for FDMA, crosstalk can be given as an example, which can cause interference between different frequencies in the environment and interrupt the transmission. Having too many frequencies in the same medium increases the likelihood of signals overlapping. FDMA does not require synchronization or time control.
CDMA – Code Division Multiple Access
Code Division Multiple Access is one of the channel access techniques. It is the technology that makes different unique codings in the same bandwidth, at the same time, and on the same frequency. So, that users can communicate with each other. It allows multiple users to communicate using a single frequency without dividing the time. The data sent by the sender is transmitted by converting to certain codes. The codes received by the receiver are converted back to the original signal, and the data is brought into an understandable format. Each communication uses different and unique encodings. Also, before communication begins, the sender transmits a packet to the receiver containing its unique encoding format. Thus, the receiver knows how the incoming packets are encoded and easily converts the received packets to their intelligible form before transmission. Communication between sender and receiver can thus be easily distinguished. This reduces waiting times and provides a certain level of security with all possible conflicts. It enables wireless communication in data transfers where high speeds are required. It increases the available bandwidth and plays an effective role in the most efficient use of the available bandwidth.
The frequency spectrum suitable for CDMA is between 800 MHz and 1900 MHz. Theoretically, unlimited users of the same communication channel can be used at the same time. The reason for this is the implementation of the Walsh Coding method. However, as the number of users increases, the noise ratio starts to increase and the quality gradually decreases. It is a cost-effective technology that can operate at low power levels.
Devices are using GSM technology cause regular generation of electromagnetic waves. However, devices are using CDMA technology do not generate electromagnetic waves. Devices working with GSM technology emit an average of 28 times more radiation compared to devices using CDMA technology. In terms of data transfer rate, both technologies have similar speeds.
SDMA – Space Division Multiple Access
Space Division Multiple Access is a channel access technique. Data transmission is provided separately for users physically located in different areas. Thanks to the advanced antenna technology, the spread area of the antenna are separated into more than one special area. This process is accomplished by pointwise transmission of the same signal in each of the coverage areas, called beams, which provide the propagation of an antenna. Within wireless data transmissions, it is similar to cellular radio systems, Lidar, Lo-Fi or Wi-Fi technology. Base stations used in GSM technology have a similar working structure. It is very advantageous to use for wireless devices working on the go.
Since traditional base stations do not have information about the location of mobile devices, they spread the generated signal in all directions within the reach. Thus, some of the power is wasted and can cause interference between stations close to each other. It also makes it difficult to distinguish weak signals at the extremes from interference. Advanced antennas can be tuned to optimize both data transmission and data reception for each device. The base station quickly adjusts the phase of signals from several antennas. Thus, as well as providing region-based communication, different region-based configurations, different permissions or rules can be introduced.
Using the same frequency range as SDMA, communication between different wireless cells is ensured and the cells must be sufficiently separated from each other to avoid interference. This limits the number of cells and the availability of frequency, but the limit values can be increased with the use of advanced antenna technology. Using the same frequency to communicate with different points is about routing the transmitted signal only to the specific device without a widespread. Instead of propagating within the signal reach area, it is transmitted pointwise to provide access to the device. This technology is also known as intelligent signal processing. Advanced antenna technology allows the use of SDMA.
PDMA – Power Division Multiple Access
Power Division Multiple Access is a channel access technique. By using different power values over the same channel in communication, it enables users to be distinguished from each other with different power values. Users with the same power values communicate with each other. It is generally used in satellite communication systems. Accesses are provided by being distributed over a satellite to multiple devices in the Hub&Spoke structure. It is used to provide high performance while transmitting different information in a single channel to the points at the same time.
