Metropolitan network is important part of NGN. Such a network serves for concentration of traffic from several local networks and switches it into core network. Looking on contemporary networks, metropolitan networks are bottleneck. Traditionally, these networks are based on simple topologies, like circle or star. They deploy optical fiber in conjunction with WDM technology to provide sufficient transport speed.
STARNET is one of the well-known examples of star topology. This network is based on broadcast of data from central point. Such data are sent and receiver selects only data addressed to it. Ring based networks are even more interesting, particularly RPR (Resilient Packet Ring), which has biggest potential of application, are certainly most important.
RPR has been proposed to respond to the problems of both SONET and Ethernet in MAN environment. Resilient Packet Ring (RPR) is the technology proposed for packet oriented MAN network established over ring architecture using optical fiber. RPR has been standardized by IEEE as 802.17 standard [48], [49].
RPR architecture (Figure below) is composed of two rings oriented in different directions. Advantage is that each node can communicate with both neighbors directly without necessity for whole ring way. Each packet controls traffic passing through the node. In the case, when the packet is aimed at it, the node drops it from the ring. Otherwise, it passes transparently through this node. In the case, when the node wants to add a new packet to the ring, it can do that only in the case, if there is enough long gap on the ring. The gap can be created also by dropped packet.
RPR can be characterized by following properties: