uling from the QoS point of view: Delay in the backhaul. Different QCI in 4G and 5QI in 5G mobile networks have delay requirements that should be met. For example, for defined QCI in LTE-based mobile networks (up to 3GPP release 14) there is required backhaul delay of up to 20 ms for 98% of packets for high priority CoS or in uncongested conditions [20]. However, for 5QI defined for 5G mobile networks, there are even stricter delay requirements for delay critical services, such as below 5 ms for remote control services (Table 5.9), or below 10 ms for ITS, discrete automation or ITSs. Capacity of the backhaul. Small cells backhauling should carry traffic during both busy time intervals and quiet time peak traffic. So, the backhaul networking for N cells should satisfy the following: Backhaul capacity = Max {peak bitrate, N X busy time mean bitrate} Availability of the backhaul. This refers to the proportion of time that a backhaul con- nection is available (i.e. fully functional), while the outage time is the time when it is 5.9 Very Small Cell Deployments and Impact on QoS not available. Recovery from outage is referred to as resilience time. There is no strict value recommended for availability; however, the range of 99.9-99.99% availability is expected to cover most implementations [20], which is normally below the availabil- ity requested by the transport network (that is 99.999%, i.e. five nines) [2] because the transport networks connect different backhaul and core networks. Frequency and time synchronization. The backhaul needs to provide synchronization for small cells with accuracy of +0.1 ppm (parts per million) as well as phase accuracy for the TDD (time division duplex) systems of 1.5 us. Typically, backhaul network for small cells is Carrier Ethernet, which has built-in QoS support. For the purpose of synchronization of Carrier Ethernet (required also in macro mobile network backhaul, not only for small cells backhaul), the ITU-T SyncE standard, IEEE 1588-2008 standard (the Precision Time Protocol), o