5.4 Aggregation and Backbone Tiers

As mentioned in the beginning of the chapter, transport systems used in MBH upper tiers, i.e. in aggregation and backbone networks, are similar to those used in fixed networks. Characteristics of MBH aggregation network is that it is not only transferring bits, but also processes and combines traffic streams from high number of sources.

In legacy networks aggregation site may be the start-point of several micro-wave radios having a large common PDH/SDH multiplexer. In 2G or low-capacity 3G mobile networks even the first hops in the aggregation level are realized by wireless links. In upgraded networks they have been replaced partially (or wholly) by new NG-SDH/MSPP nodes which include packet switching capabilities. In legacy aggregation networks bit rates are from STM-1 to STM-16 and in backbone networks typically STM-16, in a few cases also STM-64; in high capacity networks these signals may be optically multiplexed, i.e. DWDM is used to increase fiber capacity. Topologically these networks are usually rings, so that alternative path is always available and fast protection switching can be applied.

In newer packet-based aggregation and backbone networks the main elements are routers (IP or IP/MPLS), Ethernet switches and layer 1 transport links connecting them together. In the simplest case the connecting links are just dark fibers between the optical interfaces of routers and/or Ethernet switches – in both cases most often high speed Ethernet interfaces (1G, 10G or higher bit rate). Typical routers and switches have tens of Gbit/s interface cards, can handle several protocols and are highly redundant. They form Points of Presence (POPs) for transmission network connections to IP services and also work as edge router in the edge of network clouds.

In other cases there may be extensive optical networks, an optical layer, connecting the sites where the routers and switches are located; in these cases the connection may be based on a wavelength in DWDM systems, routed through that optical network to connect the wanted router/switch interfaces. Capacities in these networks can be significantly higher than in the above mentioned legacy networks; in aggregation networks already up to 10Gbit/s and in backbone networks n*10G or n*40G or even higher. Also in these networks alternative routes usually exist between all important nodes; however, due to packet switching, topologies are not limited to rings but are of partial mesh type.