# Congestion Analysis of Transport Layer in a Multicell 5G DL Communication System
## 1 Introduction
For single-user downlink and uplink Internet protocol (IP) packet transfers, these performance requirements are expected;
* Ultra-reliable and low-latency communication (URLLC): The 5G technology must meet the stringent requirements for capabilities such as throughput, latency, and availability for applications such as wireless control of industrial production and manufacturing processes, remote medical surgery, distribution automation in smart grids, and transportation safety [15]. Notably, 5G release 15 established a basis for URLLC by incorporating new features, such as configurable sub-carrier spacing, a subslot-based transmission scheme, a new channel quality indicator, new modulation and coding scheme tables, and configured-grant transmission with automated repetitions [16]. Incoming URLLC users cannot be delayed to the next processing time slot due to severe delay limits. Hence, entering URLLC users pre-empt eMBB resources and begin transmission at the border of the next mini-slot after arriving [17]. Thus, URLLC can be used for public safety, intelligent transportation, remote healthcare, and industrial automation. These applications require ultra-low latencies, dependable communication, and high availability;
* Massive machine-type communication (mMTC): mMTC is a communication link that enables a high number of devices to speak with one another or with the Internet, with or without the involvement of humans, providing support for a very large number of connected devices, each of which normally sends a very low stream of data that is not time-sensitive [18]. Applications consist of sensor networks, smart homes, and smart cities. Machine-to-machine (M2M) communication is machine-centric communication between devices and sensors without human contact. Sensor networks under the IoT paradigm are driving machine-type communication with low-cost, low-power, and long-lasting sensors and devices. These devices and sensors normally transmit modest amounts of data that are not susceptible to delays. As the authors of [19] demonstrate, the primary problem is establishing massive communication lines for low-cost devices spread across vast regions while consuming ultra-low power. Thus, 5G networks create intelligent networked communication environments by connecting people, data, applications, and transportation systems. It is going to require different protocols to accommodate their varied characteristics [20; 21]. When thinking about mobile terrestrial access, one goal is to reduce delay as much as possible to less than 1 ms. As a result, there is ongoing research on the IP stack on the network layer. The TCP and user datagram protocol (UDP) are the two protocols that are used at the transport layer of networks [23]. TCP is built to disassemble a message into a series of data packets and to ensure that each packet properly reaches its destination as quickly as feasible, offering high data rates [24]. Hence, TCP is defined as a connection-oriented communication protocol that permits devices or applications to exchange data over a network and check their successfully delivery. UDP is a protocol that can be used instead of TCP; however, it does not offer error correction and its reliability is worse. Thus, the user datagram protocol (UDP) is a message-oriented communication protocol that allows devices and applications to traffic information via a network without verifying its delivery (incorrect sequencing cannot be detected or corrected), but it is the most suitable protocol for real-time communication (live data transmission), where the loss of a few packets is insignificant. Each of these variants was developed with the intention of using a congestion control method. Additionally, routing protocols can be added in order to increase the quality of service and the network's performance, such as the destination sequence distance vector, Babel, dynamic source routing, Dymo, and the ad hoc on-demand distance vector [25].