## 2 Network Slicing Concepts
### Hard Network Slicing  
Despite the apparent advantages that an overlay MPLS tunnels could provide, there are some disadvantages. Overlay tunnels built by data encapsulation have neither visibility nor control of the underlying physical network. With more and more tunnels deployed on shared physical infrastructure, network congestion necessarily becomes an attention point. Thus, VRFs, VSIs or Optical Data units (ODus) cannot be managed by their corresponding  
\begin{table}
\begin{tabular}{l l l} \hline \hline
**Applications** & **Definition** & **Example** \\ \hline Ultra Reliable Low & Requires support for 1 ms & Autonomous driving or \\ Latency Communications & latencies, 0.001\% packet loss, & Industrial automation \\ (uRLLC) & user mobility up to 100 km/h & \\ \hline Massive Machine & Requires support for 1 million & \\ Type Communications & devices per square kilometer, & \\ (mMTC) & tens of bps bandwidth, & \\  & and latency minimization & \\  & for battery life optimization & \\ \hline Enhanced Mobile & Requires Gbps bandwidth, & Immersive UIs based \\ Broadband (eMBB) & real time or not & Augmented Reality \\  & & Virtual Reality. Streaming \\  & & of High Quality Video \\ \hline \hline \end{tabular}
\end{table}
Table 1: Possible Applications derived from the 5G implementations.  
tenants directly because they are part of the same administrative domain represented by the physical device, and they need to be operated by the network administrator. Due to the massive number of tunnels and services in a complex network, the QoS and Traffic Engineering policies management becomes a crucial task to guarantee the right SLAs to its customers [13; 16].  
The solution to this limitation and the way to move the networking industry to more automated scenarios is Hard slicing. Hard slicing refers to the provision of dedicated resources to a specific Network Slice Instance. For example, Data-plane resources are provided by allocating time-domain multiplexed resources such as a Flex Ethernet channel or as a service such as an MPLS hard-pipe, route diversity (disjoint paths), wavelength selection, among others.  
Disaggregated routers is an example of hard-slicing. In the disaggregated case, the data plane runs in the physical device, and the control plane runs outside the device in a remote cloud or server. In that case, the 1:1 relationship between physical device and routing logic is disassociated, enabling the support of multiple virtual routers over a single physical network device. Each of these virtual routers is a router in its full sense, being able to host multiple VRFs and VSIs, or to be managed independently of the other virtual routers running in the same device. Virtual routers are thus administratively separated from each other, which means separate virtual routers can be assigned to different tenants, and each tenant can manage the virtual routers directly, without the need from the operator that owns the physical network to intervene intervene (e.g., following an approach similar to the one described in [17]).  
Network slicing at the IP transport level can then be accomplished by grouping multiple virtual routers running over a shared physical network infrastructure into a common virtual infrastructure under its separated administrative domain. Virtual routers under the same administrative domain are then known as hard network slice, so different hard slices can be assigned to different tenants.