Telecommunication protocols application interaction

Let's look at the basics of transferring information between two application processes in the OSI model.

Say an application (in the picture above) needs to access a file service on another computer. It sends a request to the application layer. Based on it, the software generates a standard format message consisting of a data field and a service header. The latter contains a description of actions for the application layer of the other machine to perform, as well as instructions for the adjacent lower layer. The presentation layer, in turn, formats the data according to the instructions received and adds its own header (some protocol implementations also include service data at the end of the message), after which the information is sent to the session layer. This procedure, called encapsulation, is repeated further as data and service information move to the physical layer.

When arriving at the physical layer, i.e., the network, data passes through many other devices (network adapters, hubs, routers, bridges, etc.) and, upon reaching the destination (for example, a SIP phone), undergoes a reverse encapsulation process (right side of the picture).

Data networks can be of two types: local (local area network, LAN) and global (wide area network, WAN). The latter connects any two networks within the globe with dedicated or switched communication channels.
Local networks differ in transmission medium, topology, and access control. Depending on the transmission medium, they are divided into wired (using coaxial, twisted pair, or fiber-optic cable) and wireless (WLAN), and according to the type of topology, into networks with 'bus', 'ring', or 'star' topologies.

Each computer on the local network is connected to the transmission medium through a special adapter that "sees" what any other adapter on the same network is transmitting; such systems are called broadcasts.

The MAC protocol regulates the sequence of nodes' access to the transmission medium. It is implemented using two types of schemes: polling or contention-based MAC schemes.

MAC polling networks can be centralized or distributed. Centralized ones have a star topology with a hub as the control node. In contrast, distributed ones have a ring or bus topology with token passing.
Contention-based MACs are often implemented as carrier sense multiple access with collision detection (CSMA/CD) or carrier sense multiple access with collision avoidance (CSMA/CA). The first is used in all types of Ethernet LANs, and the second is used in most wireless WLANs.

The unrivaled leader among local networks is the IEEE 802.3 Ethernet network, where coaxial, twisted pair (designated by the letter T for 'twisted') and fiber-optic cables are used as transmission media. The transition to twisted-pair cable made it possible to implement full-duplex transmission over two twisted pairs, leaving the concept of collision meaningless since transmission and reception became independent operations. Ethernet has transmission speeds of 10, 100 ('Fast Ethernet'), 1000 Mbit/s (1 Gbit/s, 'Gigabit Ethernet'), and up to 400 Gigabit Ethernet.

Wireless local networks (wireless LAN, WLAN) have also become widespread. Their parameters are stated in the IEEE 802.11 specifications, known as Wi-Fi.

Today, the most common Wi-Fi 5 or 802.11ac defines the operation of wireless local area networks in the 2.4/5 GHz ranges with a data transfer rate of 1.3 Gbit/s. Shortly, speeds will reach 9.6 Gbit/s over 6 GHz frequency bands.

The range of most WLAN systems is 120 m, depending on the number and type of obstacles encountered. Additional access points will help expand the coverage area. Using two or three carrier frequencies instead of one can increase the number of users and network capacity.

Obviously, with Wi-Fi, network security becomes an important issue. A priori, wireless networks must be equipped with additional means of ensuring reliability compared to wired ones. WLAN uses direct sequence spread spectrum (DSSS) technology, which is highly resistant to data corruption and interference, including intentional ones. The problem of user authentication can be solved by introducing system identifiers. The wired equivalent privacy (WEP) mode is provided to transmit necessary information. In this case, the signal is encrypted with an additional algorithm, and the data is controlled using an electronic key.

Like all IEEE 802.3 standards, 802.11 refers to the lower two layers of the OSI model: physical and link. Any network application, operating system, or protocol (such as TCP/IP) will function equally well on an 802.11 network or Ethernet one.

The original 802.11 standard defines the basic architecture - the features of 802.11 services. The 802.11x specification only addresses the physical layer, introducing higher access speeds.

In conclusion, we should note that thanks to the close ties between Ethernet and the IP protocol, the scope of Ethernet applications is expanding, particularly in broadband subscriber access (BSA) networks.