# Congestion Analysis of Transport Layer in a Multicell 5G DL Communication System
## 2 Models and Methods
The slow-start threshold is another variable utilized in TCP. This threshold is an estimate of the congestion window's most recent value when the congestion process has failed to create it. The threshold must be changed during each congestion event after it is established at the sending window. When the window size value matches the slow-start threshold value, the slow-start process comes to an end. The next stage of congestion control then begins. If the user equipment (UE) does not initiate any congestion events, the slow-start procedure of the congestion control algorithm causes the congestion window to grow exponentially until the slow-start threshold is reached [34]. In wireless networks, packet loss is caused by transmission defects or other factors unrelated to congestion. TCP guarantees dependable transfers and can detect lost segments. There are two types of TCP congestion control scheme: mild congestion and severe congestion. The authors of [35] emphasized that TCP considers the network to be lightly congested when it encounters mild congestion. Three duplicate acknowledgements are received in this case, resulting in quick retransmit. When only one segment is lost, the fast retransmit is considered a success. TCP performs a multiplicative decrease in this case, and the congestion window is divided by 2. This new congestion window value becomes the next slow-start threshold. On the other hand, when the network's retransmission timer expires, it becomes severely congested. The first segment is retransmitted by TCP, and the slow-start threshold is set to 50% of the congestion window. The TCP begins a slow-start process after the congestion window takes its initial value. FTP refers to one set of rules that control how computers transfer files from one system to another via the Internet. The network protocol for transferring files between entities across TCP/IP connections was here assumed to be FTP. FTP is categorized as an application layer protocol as it uses the TCP/IP suite. According to 3 GPP TR 36.814 V9.2.0 [36], traffic models for system performance evaluations can be classified as full-buffer models, used for downlink and uplink continuous traffic, and non-full-buffer FTP models, used for downlink and uplink burst traffic. System throughput studies must be evaluated using a full-buffer traffic model that captures constant and non-varying traffic interference. Continuous traffic in the downlink and uplink uses the complete buffer model. On the other hand, the downlink and uplink burst traffic uses the non-full-buffer FTP model. The crucial factor in burst traffic is the duration of time that bursts are transferred for. This transfer time is influenced by the scheduling delay, the throughput, and the percentage of transmission time intervals in which the burst is scheduled. Additionally, burst traffic models must be used for evaluations with time-varying interference. There are two different FTP traffic models under the heading of non-full-buffer models: FTP model one and FTP model two. The file size for FTP traffic model one is currently set to 2 Mbytes; however a size of 0.5 Mbytes can be used to accelerate simulations. Using a Poisson distribution and an arrival rate of \(\lambda\), the number of users, denoted K, can be computed. According to the 3 GPP standard mentioned above, the possible ranges for the arrival rate are as follows: [0.5, 1, 1.5, 2, 2.5] for a file size of 0.5 Mbytes and [0.12, 0.25, 0.37, 0.5, 0.625] for a file size of 2 Mbytes. The user arrival time with FTP traffic model two, referred to as a general traffic model, which corresponds to most circumstances, is shown in Figure 2.  
The number of users is fixed at two in FTP model two, and the file size is set to 0.5 Mbytes. The reading time (_d_) is a random variable that has an exponential distribution with a mean value of five seconds (rate \(\lambda\) = 0.2). The probability density function of \(d\) can be estimated as follows:  
\[f_{d}=\lambda e^{-\lambda d},d\geq 0,\,\lambda=0.2 \tag{1}\]