# mIoT Slice for 5G Systems: Design and Performance Evaluation
## 3 Gap Analysis for 4G System
### EPC Consideration  
An assessment of the EPS connectivity model applied to the mIoT use case highlights severe drawbacks in the CN. If, on the one hand, the sometimes strict mIoT QoS requirements may favor a EPS connection-oriented model, on the other hand, the sporadic and limited data transmission of mIoT devices and the consequent CP vs UP traffic ratio makes the Always-ON concept questionable.  
First, default bearer establishment requires time, computation and storage resources for CP (eNB (evolved NodeB) and MME (Mobility Management Entity)) and UP (eNB-SGW, S1, and SGW-PGW, S5) entities. EPC network elements can simultaneously handle only a target maximum number of bearers (\(\sim\)10\({}^{6}\)-10\({}^{7}\)) and their complexity and cost increase accordingly. Supporting the massive CP load and handling bearers on UP even for initial mIoT scenarios on carrier grade would require either enhanced (and more expensive) EPC network elements, or network re-engineering, re-planning and re-deployment. Even with an approximate quantitative evaluation, we may estimate a thousand LTE cells to lead to up to one billion mIoT devices EPS attached (idle most of the time) and assuming best practice 4G network topologies, this goes clearly beyond EPC network element capability.  
In addition, even assuming enhanced CP and UP capable of handling the required number of bearers at core network side, the approach would lead to an extremely inefficient resource utilization at EPC. The Always-ON concept presumes the UE, while attached, will require frequent data exchange. This justifies the resources allocated to support the default bearer. Such assumption clearly does not apply to mIoT, where small and infrequent data transmission appears to be the dominant model. Consequently, a novel solution is needed to reduce the signaling. A possible approach has been proposed in [13], which introduces the concept of connection-less transmissions where devices transmit packets without any bearer establishment. The paper claims the gain relating to a connectionless access can be estimated evaluating the overhead reduction. In a first approximation, according to [13], connectionless access may lead to an overhead reduction of up to an order of magnitude, on both UL and downlink (DL). Although having a clear benefit from an access signaling reduction point of view, the lack of bearers involves the impossibility of handling QoS (thus limiting the applicability from a use case perspective). Moreover, it should be stressed that, again, the solution included in [13] focuses on RAN only, and does not consider at all the issue relating to massive connectivity, yet QoS constrained, at Core Network.  
This short 4G EPC gap analysis suggests guidelines for the definition of the Core Network part of the 5G mIoT E2E Slice architecture and procedures: the need to provision QoS favors the adoption of a connection oriented connectivity model, while the expected traffic patterns clearly highlight the \"Always-ON\" principle should be superseded to allow efficient CP signaling and UP resources utilization.  
The next Section will focus on the design of a 5G mIoT slice able to support above mentioned goals through an enhanced bearer-oriented approach.