# A Framework for the Joint Placement of Edge Service Infrastructure and User Plane Functions for 5G
## 2 Related Work and Motivation
### UPF Placement  
The 5G UPFs are analogous to Service Gateways (SGWs) and Packet Gateways (PGWs) in Long-Term Evolution (LTE) networks. However, unlike SGWs and PGWs, UPFs only perform user-plane related functions. The placement of SGWs and PGWs has been addressed in a wide variety of research works [5; 16; 17; 18; 19] comprising different points of view.  
The authors in [5] put forward the necessity of avoiding mobile gateway relocations through the optimal placement of SGWs over a distributed network of data centers. They formulate the optimization problem as an ILP and propose a heuristic solution based on a greedy algorithm. Their main target is to minimize the overall cost of SGW relocations subject to SGW capacity restrictions. In [16], an algorithm for creating and placing virtual instances of PGWs is proposed. Its main goal is to reduce costs while ensuring QoE to users. In this regard, the load assigned to PGWs as well as their imbalance are optimized by considering the application/service type and geographical location when selecting a PGW to attend a user's request. The PGWs placement is modeled as a nonlinear optimization problem and three heuristics are proposed. [PERSON] et al. [17] introduce a set of solutions to tackle the placement of virtual gateways. Their main objective is to determine Virtual Network Functions (VNFs) best locations, so that, the path between users and PGWs is minimized while, at the same time, SGW relocations are reduced. Three placement strategies are formulated by considering delay and relocations requirements. Nonetheless, the VNF maximum capacity restriction is overlooked.  
Other papers such as [18; 19] address the SGWs and PGWs placement in 5G networks upon the concept of Control and User Plane Separation (CUPS). [PERSON] et al. in [18] separate the SGW functionalities into SGW-U and SGW-C and define an algorithm for the SGW-C placement. This algorithm is aimed at reducing SGW relocations and at balancing their traffic load. Nevertheless, they do not take into account latency requirements for the gateway placement. In [19], the possibility of using a 5G hybrid architecture where the PGW and SGW functions could be deployed by using either VNFs or SDN elements is presented. Taking as reference this architecture, three optimization models are proposed. Their main aim is to determine the optimal sizing and planning for the data center, PGW and SGW elements. Although data and control plane latency requirements are considered, the effects of users mobility on network functions (NFs) placement are neglected.  
Despite the plethora of papers tracking the placement of mobile gateways (i.e., SGWs and PGWs), the existence of a model capable of integrating aspects such as mobility, latency, reliability, costs reduction and gateway available capacity is missing. Moreover, all the works mentioned above take as reference the mobile network architectures based on LTE systems. In a previous work [20], we propose a framework to place 5G NFs and to optimize VNF infrastructure (VNFI) resources following the 3 GPP 5G architecture [21]. Our main focus in this precedent research was the placement of control plane NFs, where despite the assignment of UPFs to the Session Management Functions (SMFs) was addressed, it started from the criterion that UPFs were already optimally located.