Mobile network operators (MNOs) need to assess the demands small cells are placing on the evolved packet core (EPC). As MNOs expand their 4G/LTE capacity and coverage with small cells, the EPC may be called upon to deliver a significant increase in scale, capacity, and performance beyond that which was required initially to support the macro-cellular network. To meet new small cell requirements, the existing EPC must be re-assessed to determine if it can support potentially hundreds of thousands of new cells within a metro area that may also provide Wi-Fi® network access in addition to 3G/4G cellular.
In re-examining the evolved packet core in support of small cells there are 3 factors to consider:
- EPC network architecture for small cells
- User plane and control plane scalability
- Data offload
1) Evolved packet core network architecture
The 3GPP mobile standards define 3 different architecture variants for the 4G/LTE small cell network to connect to the EPC. Two of the options involve the addition of a small cell gateway to aggregate control and/or user traffic from a group of small cells back to the EPC, while a 3rd provides direct connectivity from each small cell to it.
The MNO must consider the tradeoffs:
- Adding a small cell gateway reduces the scaling/capacity requirements of the EPC but increases the network and operations complexity
- Connecting the EPC directly to each small cell significantly increases its scalability and performance requirements yet keeps the network flat
Another factor to consider is whether the small cell network must support 3G in addition to 4G and/or Wi-Fi. Small cell gateways are required for 3G small cell networks but not for 4G, which has implications for the evolved packet core and the small cells network architecture.
2) Scalability and performance
The EPC must support the scaling and performance of the additional small cell network load. If it’s directly connected to the small cell network, the biggest impact is on the control plane and the mobility management entity (MME) -- with all of the additional signaling that’s required. The MME must also support small cell closed subscriber groups and associated paging and signaling optimizations to limit the amount of additional signaling inherent to small cells.
The evolved packet core should also support -- in an integrated and operationally simple model -- the mobile gateway functions of evolved packet data gateway (ePDG) and trusted wireless access gateway (TWAG) in support of small cells with integrated Wi-Fi access. This enables seamless continuity of services and handovers between cellular and Wi-Fi networks while reducing the number of nodes in the network.
3) Data offload
A number of local breakout options can be implemented in small cell networks to offload data traffic that provides little to no added-value to the MNO, and therefore need not be forwarded to the evolved packet core handling mobile services. In this case, the EPC must support the requirements necessary to redirect traffic to the appropriate gateway and packet data network.
As small cell networks increase in both size and number, the EPC may not provide sufficient scale on existing hardware platforms and require an evolution to network functions virtualization (NFV). This evolution will allow massive distribution of the EPC functions with data-offload capability, paving the way for 5G introduction.
A virtualized EPC will have the horizontal and vertical scalability needed today to support the expanding macro and small cells network. But it must also retain the reliability and high availability necessary to meet end-user quality of service.
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