Basically, Router selects the path/paths (including PSTN and Internet) for intra/inter network traffic.
To start with an analogy – A car driver selects the route using GPS and then follows that preassigned path as selected by GPS. In another case, for a VVIP, the associated driver follows the specially defined route as guided by the VVIP team understanding security and other related pros and cons on the VVIP route. The first case is like Packet -switched network where logically addressed small packets traverse across preassigned different digital networks through various nodes and follow networking protocols from origin to final destination. In the second case, is like Circuit-switched dedicated path being selected before actual traffic commences.
Circuit switching: Users establish a dedicated circuit or channel between the source and the destination before they can connect. It is mostly preferred for voice and video calling systems.
Optical routing: Provides a circuit-switched dedicated connection based on out-of-band signalling control, paradigm – GMPLS. Control information and data transfer follow two separate paths. GMPLS allows the participation of Layer 3 (the network layer) devices within the signalling circuits. These devices stand to benefit from path protection as well as the reestablishment of MPLS within the circuit.
Optical Packet Routing: Here O/E converter in the router converts incoming data packets from optical to electrical form for processing. Similarly, outgoing data passes through E/O converters before traversing the next optical link in the network. An optical packet switch is similar to an electronic router but does not require O/E and E/O converters. Optical routing does not solve the energy problem, and neither conventional nor slow-light delay lines do away with the storage problem. In particular, planar integrated optical buffers (including slow-light buffers) occupy a larger chip area than their electronic equivalents, dissipate more power, and are limited in capacity to a few IP packets. Optical-fibre-based buffers have low power dissipation but are bulky. Instead, the data remains in optical form throughout the network, including the switches. Optics is well suited for transmission of data, and electronics is well suited to buffering and manipulation of data. Its operation is based on dedicated wavelength, being assigned for each path from source to the destination and obeys the principle of wavelength routing.
Packet routing: Here logically addressed packets are forwarded in the network or over multiple networks on establishing a defined path from the source to the ultimate destination using intermediate nodes. Data plane forwards IP packets as in the case internet over same established path according to topology information and using the forwarding table – Source address, Destination address and Total number of packets. So, data with routing information follow the same assigned path hop by hop from the source to its final destination and it’s called Datagram-based architecture followed by IP and MPLS (Multiple Pre-configured Label Switching). In another approach – Virtual circuit switch, the assembled and numbered packets travel across a predefined route, sequentially used by X.25 or in ATM.
Optical Packet Switching: The worldwide multi-service transport architecture of choice had conceptualized after being inspired by the emerging convergence of packet (IP/MPLS), multi-layer network control-plane, induction of reconfigurable DWDM. It improves overall network operational performance. Here, I would like to mention generally, Router has inter-network i.e., IP layer3 functionality while Switch functions intra-network i.e., uses MAC or physical address. In OPS, variable length of packets can be switched in the optical domain. But OPS is not feasible currently due to lack of optical RAM technology.
Optical Burst Switching (OBS): This technology is a compromise between Optical Packet Switching (OPS) and Optical Circuit Switching (OCS). OBS-control information is sent separately in a reserved optical channel well in advance of the data payload. Data being processed electronically, allows time to setup an optical light path to transport the soon-to-arrive payload, called as delayed reservation. OBS technique provisions dynamically sub-wavelength details by optimally combining electronics and optics. OBS considers sets of packets with similar properties called bursts and provides more bandwidth flexibility than wavelength routing. Data packets are aggregated into data bursts at the edge of the network to form the data payload. It offers More efficient bandwidth utilization.