It is also important to note that, when planning the backhaul for small cells, operators should take into account their broader RAN evolution strategies for LTE and LTE-Advanced. These transport networks are going to be the foundation for Heterogeneous Network (HetNet) topologies in which layers of small cells and macro sites are coordinated. Discussed below are challenges that network planners are encountering in the backhaul design for small cells.

Backhaul Moves to Layer 3

While mobile backhaul networks have undergone a shift from dedicated TDM-based circuits to Layer 2 Carrier Ethernet-based transport in recent years to keep up with demands for more flexible capacity provisioning, small cells introduce a new paradigm and need for Layer 3 protocols into backhaul transport. The small cells will use Layer 3 IP-based backhaul almost without exception. For LTE networks, regardless of base station size, Layer 3 backhaul is required.

To monitor performance on the backhaul links in the macro network, operators rely on Layer 2 OAM standards, such as the ITU-T’s Y.1731 or the IEEE’s 802.1ag. For small cell backhaul, it will be critical to support Layer 3 test methods both as a passive reflector and generator in order to get a full view of network performance from small cells to macro sites and mobile switching centers.  As Figure 2 shows, transport networks cross multiple Layer 2 network domains, the number of which depends on factors such as the scale of the deployment, the type of backhaul used and the number of third-party providers involved. In a small cell backhaul scenario, a single Layer 3 domain sits above the Layer 2 domains. End-to-end network performance monitoring can be achieved in the single-domain Layer 3 (IP) because it is not limited by the boundaries of operator domains as is the case at Layer 2.

Standards for Layer 3 test and performance monitoring methods include two-way active measurement protocol (TWAMP, RFC 5357), one-way active measurement protocol (OWAMP, RFC 4656), or the ITU-T’s Y.1564 standard.

[Figure 2. View of Layer 2 and Layer 3 Networking]

Left image shows that Layer 2 is not appropriate to be used for Performance Monitoring (PM) because it cannot traverse outside its Layer 2 domain. Right image shows Layer 3 Performance Monitoring (PM) that spans across several Layer 2 domains.

Enter the evolved packet core

LTE networks require a new core network architecture which is a flatter, all-IP Evolved Packet Core. With the EPC comes the new X2 interface that creates a direct link between base stations (that is, eNodeB’s) for the first time in the history of cellular network design. The X2 interface opens new paths for traffic to flow between base stations in order to optimize the use of finite radio resources, increase operating efficiency in the RAN, reduce processing load in the core network and improve cell-edge performance. This is particularly relevant to LTE small cells deployed to offload voice and data traffic from capacity constrained macro sites in high usage areas. Since handovers are enabled between small cells, traffic can remain on the small cell layer, transiting from one small cell to another without having to traverse the whole of the transport network and back again, which is an efficient way to hand off traffic to adjacent small cells within the same macro network span of control.