# Future Scenarios for Software-Defined Metro and Access Networks and Software-Defined Photonics
## 3 Solutions for Software Defined Metro and Access Networks
### Physical Layer Re-Configurability
#### 3.2.4 Optical Switching at the Physical Layer  
Wavelength routing described in the previous paragraph is just one of the possibilities to implement optical switching at the physical layer, namely optical channel switching (OCS). OCS and other more sophisticated optical switching technologies are summarized in Figure 8, i.e., optical burst switching (OBS), optical label switching (OLS), and optical packet switching (OPS). These techniques allow the network adaptability in a most granular way to traffic variations, since they facilitate statistical multiplexing to efficiently share wavelength channels among multiple users [10]. Optical switching techniques are receiving greater attention in recent years as a possible alternative to conventional IP-based electrical packet switching (EPS), which has been an efficient solution for relatively low-speed, bursty traffic (less than 1 Gbit/s). Emerging services, such as video streaming or large file transfer, require increased data rates, low packet latency, energy efficiency, and low switching overhead. For these applications, OCS could be the optimal solution since it enables end-to-end optical communication over dedicated lightpaths. Nevertheless, the heterogeneity and burst of the traffic, according to the scenario described in the Section 2.1, requires major flexibility [37]. The ideal approach to conjugate both needs would be to replicate the packet-switching operated by conventional EPS entirely in the optical domain, which is the aim of OPS. OPS would allow the application of the IP over WDM (IPoWDM) concept, considered the 'holy grail' of optical communications. IPoWDM would reduce OpEx by simplifying the protocol stack, since the IP packets would be switched and routed over the all-optical WDM network without excessive electronic processing in the data plane. OPS systems can be categorized in a number of ways: synchronous versus asynchronous packet switching, fixed-length versus variable-length packet switching, and store-and-forward versus cut-through packet switching [60]. The header control can be in-band, where both header and payload are carried via the same wavelength, or out-of-band, where control headers are carried via a dedicated control wavelength. There are three basic in-band header control techniques: subcarrier multiplexed, orthogonal modulation, and time-domain multiplexing [48]. Figure 8: Optical switching technologies sorted by complexity/performance and hypothetical phases of implementation. Since actual technologies do not allow the complete actuation of OPS, a more practical approach is represented by OLS, where only the header (or label) is approached in the electrical domain. An even simpler solution is OBS, where large aggregated packets with the same destination and the same QoS, i.e., bursts, are routed by processing out-of-band control signals in the electrical domain. OBS networks initially emerged as very fast reconfigurable OCS networks and are based on a complete separation of control plane and data plane. In the control plane, burst header packets (BHPs) are read using electronic processing and control the switching fabric; in the data plane, burst data packets (BDPs) are switched all-optically, as shown in Figure 9. Burst assembly and disassembly operations are performed in the edge routers, ingress, and egress respectively. OBS uses two typologies of signaling protocols for burst management by optical routers. In two-way reservation protocols, also called tell-and-wait (TAW) protocols, an acknowledgment is required from the receiving node before the burst transmission, e.g., wavelength routing (WR) protocols. In one-way protocol signaling, also called tell-and-go (TAG) protocols, no acknowledgment is required and therefore latency is reduced, e.g., just-in-time (JIT) and just-enough-time (JET) protocols. In the JET protocol, a BHP is sent on the control channel shortly before the burst transmission begins, specifying the destination of the burst and the channel on which the burst is being transmitted [60].