## 2 OBS Networks
### 2.3. QoS Support
#### 2.3.1 Differentiated QoS Provisioning  
In general, differentiated QoS provisioning can be achieved by introducing _differentiation_ at some point in the network. In OBS networks this differentiation can be included in the functional mechanisms described in the subsection above, and various proposed schemes providing QoS can be classified in the following four categories based on the stage at which service differentiation is performed [28]: assembly-time schemes, reservation schemes, scheduling schemes, and contention resolution schemes.  
The assembly-time category includes differentiation preformed during the assembly of packets into bursts. Examples of these schemes include a burst length differentiation technique which assigns different burst sizes for different classes of traffic [29]; prioritized assembly using composite burst assembly techniques along with burst segmentation [6, 22, 30, 31]; and intentional dropping, where lower priority bursts are intentionally dropped at assembly under certain predetermined conditions [28, 32], some of theme providing absolute QoS differentiation in OBS networks [8, 33, 34].  
Figure 5: QoS mechanisms in OBS (based on [27]).  
The reservation category comprises service differentiation schemes using different reservation policies for different classes. Proposals in this category include offset-time based mechanisms, providing service differentiation in terms of burst loss probability by setting different offset times for bursts with different priorities [35; 36; 37; 38; 39]; the forward resource reservation scheme, where the authors present a transmission mechanism embedded at ingress nodes to efficiently reduce the end-to-end delay [40]; preemptive reservation protocols, where a node may drop, or preempt, a low priority burst already scheduled to make room for a high priority one (similar to intentional dropping but more efficient, because it drops only those necessary to schedule the high priority burst) [41; 42; 43; 44], and a probabilistic variant to provide relative burst loss rate differentiation by setting different preemption probabilities for multiple classes of traffic [45; 46]; and wavelength limited schemes where lower priority bursts are restricted to use only a certain set of wavelengths, while those with higher priority can use a larger, or complete, set of wavelengths [34; 47].  
Proposals in the scheduling category provide differentiated QoS by allocating different amounts of bandwidth to different service classes [48], where the processing of a low priority control packet is delayed at the core node by a certain period of time, allowing higher priority CPs which arrive later to reserve bandwidth ahead of the low priority bursts; a slot-based prioritized scheduling scheme is also proposed in [49]; and an approach relying on a centralized request server that performs the scheduling for the entire network trying to minimize delay [50].  
Contention is the major cause of burst loss in OBS networks. Considering that most of the QoS mechanisms of OBS networks have burst loss or blocking probability as their main QoS metrics of interest, then, contention resolution is also considered in most of them. Examples of such mechanisms using priorities and burst segmentation can be found in [6; 30; 31]. Once in OBS networks data loss may occur when bursts contend for network resources, there have been several proposals to resolve contentions in order to minimize burst loss (some presented above). These localized contention resolution techniques react to contention, but do not address the more fundamental problem of congestion. Hence, there is a room for network level contention avoidance using TE techniques in order to minimize burst loss. This can be achieved by load-balanced routing techniques, as described bellow.