# Congestion Analysis of Transport Layer in a Multicell 5G DL Communication System
## 2 Models and Methods  
With regard to transport layers, as mentioned above, two main protocols have been designed: UDP and TCP. The TCP/IP model uses the network access layer to transfer data between two directly connected devices. The network access layer manages data transfer across the interface for 5G networks. TCP is used to decompose a message into data packets and to ensure that the message reaches its intended recipient as soon as possible [26]. TCP's replacement, UDP, is less dependable and does not offer error correction. TCP, which has several different operating versions, is the subject of this research. These versions are all built with a congestion control mechanism. Congestion control (CC) is an algorithm that limits the number of packets at network nodes. It works by restricting the available bandwidth. In this way, we can avoid overloading any node with an excessive quantity of packets [27]. A network is said to be congested when the rate of the packet flow towards any node is greater than its capacity to handle those packets. Congestion control can be implemented in a 5G network at either the network or transport layer, depending on the use case. It is reasonable to add the congestion control algorithm at the transport layer since this layer already facilitates the movement of data. One of the algorithms used is the slow-start algorithm, which is employed to prevent the need to wait for too many round trips for a large enough congestion window. When a node cannot handle the rate at which packets are flowing toward it, congestion occurs. In a 5G network, congestion control can be performed at either the network or transport layers. Since it provides for high-quality data delivery, congestion control should be included in the transport layer. The main features of the CC algorithms are [28; 29]:  
* Preventing congestion collapse: The receiver can control the sender's data flow using window-based flow control in the original TCP. The receiver's data buffer space is protected by this flow control;
* High bandwidth utilization: CC algorithms optimize performance regarding throughput, delay, and loss. As the Web grows, Internet consumers desire high-bandwidth and low-delay communication rather than a slow background file transfer;
* Fairness: CC algorithms guarantee an acceptable, equal share of the available bandwidth among all competing flows sharing the same channel. TCP connections can share a congested link to distribute bandwidth fairly across similar flows;
* Fast convergence to fairness: CC algorithms are capable of rapid reactions to ensure the increase of the new flow and the reduction of the old flow until fairness is achieved. All flows need compatible congestion control techniques to share bandwidth equally;
* TCP-friendly: For deployment purposes, CC algorithms intended to be used in an uncontrolled network (e.g., the Internet) should coexist with other CC algorithms by maintaining fairness. The TCP's congestion control scheme holds the congestion window (CWND), which is in charge of limiting the amount of data transferred by the TCP in a single transmission interval [30; 31]. It is crucial to remember that the transmission control block stores the current value of the CWND for each TCP connection. The best way to avoid congestion is to initiate the CWND since the sender is unaware of any network congestion that would prevent the data from reaching the receiver. The TCP connection must wait a long time between round trips before using the available bandwidth. The TCP congestion management technique, which involves round-trip incrementation of the CWND by a number of bytes equal to the maximum segment size (MSS), induces this situation. The slow-start algorithm is used to avoid too much time being spent waiting for a large enough congestion window. The main goal of the TCP slow-start phase is to quickly arrive at a reasonable value for the congestion window [32; 33]. The congestion window is doubled with every round trip during a slow-start process.