CAPITULO 2. II MARCO TEORICO
2.1 REDES DE COMUNICACIÓN
2.3.3 SEGURIDAD EN LAS REDES INALÁMBRICAS
Reconfigurable optical core is an emerging technology, enabling dynamic provisioning o f connection by setting up and tearing down optical paths on-demand, thus offering better bandwidth utilisation with respect to that in the static WRONs. This brings an advantage to the schemes of routing, protection and restoration as they can be coordinated at both the IP and the optical layers [Rafl], [Yel]. The protocol stack can, therefore, be simplified by avoiding SDH and other intermediate layers. Typical switching times in this architecture can vary from a few seconds to a few hours or days. An extensive research has been carried out towards OXC functional models, which can be found in [Rafl ] and references therein.
En d-to-end IP connectivity
Q
IP layer lightpath (R?-^Ra) lightpath 1 lightpath 2 (R^-^R^)x_
WDM layer ^3 Electronic IP router^ Dynamically re-configurable OXC
Figure 2.2. IP over optical networks with re-configurable WDM core. An example of two dynamically re-configurable lightpaths, (Rz-^I^) and (Rz—^Rj). Dashed line shows an
alternative in provisioning the lightpath (R ]^ !^ ) in response to changes in network operational conditions or traffic pattern. For simplicity, the case o f uni directional
connectivity is shown.
In an IP over re-configurable optical core architecture, router interfaces from the IP routers are connected to the ports of WDM cross-connects, as shown in Figure 2.2. In an automatically re-configurable OXC, any o f its input ports can be connected
CHALLENGES IN QOS PRO VISIONING IN INTERNET-O VER-OPTICAL NETWORK ARCHITECTURES
to any of its output ports. The cross-connects can be configured in such a way that it is possible to connect a given router interface to any other router interface at any other router.
The first step in QoS provisioning in re-configurable WDM networks is traffic classification, aiming at mapping the arriving packet into pre-defined optical service classes, and admission control, ensuring fair packet servicing [Kahl], carried out at the edge o f optical network. At the same time, the optical domain is responsible for the following main areas o f QoS provisioning:
A. Dynamic routing and wavelength assignment. B. Identification of optical transmission characteristics. C. Prioritised lightpath allocation.
The functionality of these areas, as well as the associated issues in QoS provisioning, are discussed below.
A. Dynamic routing and wavelength assignment (dynamic RWA)
The dynamic RWA algorithms aim at minimising the amount o f blocked connections, or maximising the number of connections, established for a given period o f time [Zanl]. Consequently, the performance of a given dynamic RWA scheme, in terms of request blocking, has a direct impact on minimisation o f PLR and packet end-to-end delays due to congestion at the ingress o f the network, and hence, on maximisation of network throughput. Thus, the implementation o f an efficient dynamic RWA is the key factor of QoS provisioning in the IP networks with re-configurable optical core.
As in the static RWA case, the problem is usually divided into two sub problems: dynamic routing and dynamic wavelength assignment. The dynamic routing includes three main strategies, specifically, fixed routing, fixed-alternate routing, and
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Table 2.4. Dynamic routing strategies.
Routing strategy and
references
Algorithm Perform ance
Fixed routing [Xul], [Rami]
The same fixed route for a given node pair is always chosen. An example is widely used fixed shortest path routing (see [Zanl] for discussion), where the shortest paths for each node-pair in the network are pre-calculated.
Although the route computation is very simple, it implies poor balancing of available resources, and hence, high blocking probabilities. Additionally, it cannot handle failure restoration. Fixed-alternate
routing [Xul], [Rami]
An ordered set of multiple link-disjoint routes is maintained between each source-destination pair.
Major advantage is the simplicity of
managing restorations after link
failures. Additionally, lower blocking probability with respect to that in fixed routing can be achieved.
Adaptive routing [Lil], [Mokl],
[Spai], [Chal]
A path is chosen amongst any routes that can be found between a given node-pair, according to the network state and a certain policy, determined by the link weights. These policies include adaptive unconstrained routing (AUR) [Mokl], least-congested path
routing (LCP) [Lil], and their
variations.
Adaptive routing results in the lowest blocking probability with respect to
fixed and fixed-alternate routing
strategies. However, it imposes extra
requirements on network control
protocols, responsible for the
distribution and update of network state information.
To carry out the dynamic wavelength assignment, a number of heuristic approaches can be employed, as reviewed in Table 2.5. To date, very limited comparative analysis o f the wavelength assignment approaches can be found in the literature. This is because the performance o f individual heuristics very much depends on such operational conditions as wavelength conversion capability, single-fibre or multi-fibre networks, regular or mesh physical topologies. Table 2.5, therefore, presents the generally expected heuristic performance in terms o f blocking probability
[Zanl].
Table 2.5. Comparison of heuristics for the dynamic wavelength assignment. Heuristics are
presented in the order of decreasing request blocking probability [Zanl]. L - number of links;
- number of wavelengths per link; N - number of nodes.
Heuristic and
references Algorithm ComplexityComput. Performance
Least-used (LU/SPREAD) [Mokl]
LU maintains information on overall wavelength usage in the network and for a given request it selects the least- used wavelength.
0(LW O LU/SPREAD tends to assign short
lightpaths first (those, traversing 1 or
2 hops), resulting in an unfair
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Table 2.5 (continued).
Heuristic and
references Algorithm Comput.
Complexity Performance Random wavelength assignment (RA) [Harl], [Mokl], [Birl]
RA first creates a list of all fi-ee wavelengths. Then, a wavelength to be assigned is
selected amongst them
randomly.
O(wo One of the simplest heuristics in
terms of computational complexity.
Min-product (MP) [Jeol]
MP is proposed for multi fibre networks, aiming at minimising the number of fibres by packing
wavelengths into them.
CKNWÙ Computational complexity is high,
due to intensive calculations, involved in minimising the number of fibres. First-Fit (FF) [Birl], [Harl], [Chl2], [Banl], [Rami], [Mokl], [Zhul]
First, the wavelengths are ordered. Then, they are searched in the ascending order such that the first wavelength, available end-to- end, is selected.
0(fVi) Computational complexity is one of the lowest (comparable to that in RA), since no global knowledge is maintained. FF proves efficient in terms of both blocking and fairness.
Most-used (MU/PACK) [Harl], [Mokl], [Zhul] In contrast to LU, MU selects the most-used wavelength in the network.
O(LW0 MU/PACK attempts at saving less- used wavelengths. The blocking probability is up to one order of magnitude lower than that in RA. However, it is achieved at the expense of additional computational overhead with respect to that in FF. Least-loaded
(LL) [Karl]
LL is proposed for multi fibre networks. It searches for a wavelength that has the highest remaining capacity on the most-loaded link along a given route.
O(NW0 In multi-fibre networks, LL maintains the blocking probability, up to one order of magnitude lower than that in MU and FF.
Max-sum
(ML)
[Bar?]
M l is also designed for multi-fibre networks, although it can be used in single-fibre networks as well.
W V o MZ considers all possible paths in the network and attempts to maximise the remaining path capacities after lightpath establishment. However, the computational complexity is one of the highest amongst the proposed heuristics.
Relative capacity loss (RCL) [Zhal], [Zanl]
RCL computes the relative capacity loss for each path on each available wavelength, and then selects a
wavelength such that the sum of the relative capacity loss on all the paths is minimised.
RCl’s computational complexity is also one of the highest. The blocking probability is similar to that in MZ, but further comparative research is required.
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Two main factors that determine the performance o f these heuristics are the request blocking probability and the computational complexity. It can be seen from Table 2.5 that the First-Fit (FF) offers the lowest computational overhead, i.e. 0(Wi).
Furthermore, contrarily to the Random Assignment (RA), FF was shown to be a fair policy, equally servicing lightpaths independent o f the number o f hops traversed [Zhul]. This appears to be an important feature, since employing the wavelength reservation and protecting threshold techniques [Birl] to support longer lightpaths in the unfair heuristics, can lead to extra computational overhead and increased blocking probability for all requests, regardless of the lightpath length. At the same time, the difference, in terms of the blocking probability, between FF and such heuristics as the Least-Used (LU/SPREAD), Min-Product (MP), or Most-Used (MU), was shown to be lower than one order of magnitude [Zhul], [Zanl]. On the other hand, an improvement in the blocking probability in the Maximum-Sum (ME) or Relative Capacity Loss (RCL), is traded off against the increased complexity o f calculations that iterate through the entire wavelength space (see Table 2.5). Hence, FF can be considered as one of the heuristics, most suitable for practical implementation, especially in single- fibre networks.
Two above-mentioned RWA sub-problems (i.e. routing and wavelength assignment) can also be solved jointly, e.g. by employing an exhaustive wavelength search for the shortest available path in combination with the adaptive shortest-path routing (AUR/EXHAUSTIVE) [Mokl]. This strategy is expected to outperform those based on separated routing and wavelength assignment steps (see [Mig4] and references therein). However, this type o f dynamic RWA algorithms would also combine the computational complexity o f the dynamic routing with that of the wavelength assignment heuristics.
It should also be noted that the use o f wavelength converters can be considered as a means o f decreasing blocking probabilities [Karl], [Lowl]. However, it may lead to the opposite effect, i.e. to the increase in blocking with respect to that in the no conversion case, especially when the number o f supported wavelengths per fibre is decreased to approximately 50% o f the non-blocking wavelength requirement [Anal]. On the other hand, as shown in [Zhul], the same gain as that, obtained using
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wavelength conversion in poor-performance heuristics (e.g., RA), can be achieved without the conversion capability by employing more efficient heuristics (e.g., FF).
B. Identification of optical transmission characteristics
The quality of traffic transport in the optical domain depends on such characteristics as signal-to-noise ratio (SNR), dispersion, cross-talk, and a number of other physical impairments in the fibre, as they uniquely identify transmission properties of a given lightpath, including bit-error rate (HER). Hence, these parameters should be considered when allocating a wavelength to the requests with QoS guarantees. The dynamic RWA algorithms, selecting the best route according to the value o f HER amongst all possible routes that can serve a given connection, can be found in [Ram3]. Additionally, resource allocation strategies for QoS-based routing in the optical domain were proposed in [Juki] and [Juk2]. In these works, QoS performance required by an end- user was mapped into the quality o f network service performance, whilst taking into account the quality of optical signal within the physical devices along the optical paths. These allocation strategies can be based on the RWA with adaptive weight functions, identifying the properties of different wavelengths [Juk2], and implemented as a decentralised path selection framework [Juk3].
C. Prioritised lightpath allocation
The wavelength pool can be partitioned so as to ensure different blocking probabilities for different traffic classes. The simplest way, ensuring prioritised lightpath allocation, is to pre-allocate a set o f wavelengths for each traffic class such that the number of wavelengths, given to a higher-priority class, is higher than that, given to the lower- priority classes. However, when the requests from each service class arrive in a non- uniform fashion, this strategy can easily starve requests that have high arrival rate, whilst wasting free bandwidth, allocated to the other service classes [Chel].
Alternatively, requests from different service classes can “borrow” lightpaths from each other according to certain criteria [Kahl]. For instance, lower-priority traffic can be allowed to borrow lightpaths from higher-priority traffic. However, this strategy raises an issue o f servicing the higher-priority traffic, requesting a lightpath, already
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borrowed by the lower-priority traffic. A pre-emption strategy can be implemented as proposed in [Penl], where an already established lower-priority connection can be tom down to re-allocate the corresponding lightpath to a higher-priority connection. However, this may not be efficient, since QoS requirements o f the higher-priority traffic might not be satisfied with transmission characteristics o f lightpaths, given to the lower-priority traffic [Kahl]. Additionally, such pre-emption would result in the increased blocking rate o f low priority traffic.
Another method, providing the prioritised lightpath allocation, is the proportional differentiation technique, considered in [Yanl], [Kahl]. According to this method, the QoS metrics can be quantitatively set to be proportional to certain differentiation factors defined by the network operator. By assuming C to be the number of service classes, to be the value of a QoS metric for a service class f, and 5, to be the differentiation factor for the class /, it is possible to formulate the proportional differentiation by the following condition:
where U j= 1, 2 ,..., C.
The above three areas to the provision o f QoS in the re-configurable WDM networks can be incorporated into a general architecture, supporting the QoS-capable optical transport domain. Examples of such architecture, referred to as differentiated optical services (DOS), were proposed in [Goll] and [Kahl]. For each lightpath, the DOS architecture maintains a set o f optical parameters, including HER, jitter etc., as well as a set o f behaviours in terms o f monitoring, protection, and security. The combination of optical parameters and behaviours uniquely identifies the quality of optical service available over a given path. This way, the IP service classes can be mapped into the optical services.
The shift towards QoS-capable re-configurable WDM networks from purely- static WRONs highlights the first significant potential improvement in the optical bandwidth utilisation. However, one o f the major challenges in the implementation of this architecture is the development o f network control/management processes.
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allowing for the interaction between the IP and the WDM network domains, thus ensuring the dynamic lightpath provisioning on-demand. These processes can potentially be based on Generalised Multi-Protocol Label Switching (GMPLS), as well as on the Automatically-Switched Optical Network (ASON) architecture (see [Soil] and references therein). These approaches are discussed in appendix C. Another approach to automating the lightpath setup will be addressed in chapter 6 of this thesis, which describes the original contribution to the design and development of an integrated IP/WDM network management system, allowing for QoS-capable automatic end-to-end service provisioning over the re-configurable optical core [Ser2].