Topic: BusinessDecision Making

Last updated: June 8, 2019

University of Ottawa

ELG 5383
Prof. Hussein MouftahIn conformity for requirement of Project for course Survivable Optical Networks conducted at
University of Ottawa
April 2018
To increase the spectral efficiency of fibre with high data rate, a variable sized frequency range within any optical spectrum is developed which is called as Flexible grid (Flexi-grid). Basically, Flex grid networks enables different frequency slots on a single fibre. In this way it is possible to offer a mixture of several bit rates on a single fibre. With the current fixed grid, it is only possible to use a width of 12.5 GHz, 25 GHz, 50GHz, and 100 GHz. For future bit rates of more than 400 Gbits/s it would be advantageous to permit slot width between 50 GHz and 100 GHz. Media channel is an association that represents both the topology and resources that it occupies. In simple, Media channel is an effective frequency slot which are supported by a combination of different media elements. This report discusses about advantages of media channel in flex grid and the difficulties in achieving the network to be survivable.

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I am indebt thankful to Professor. Hussein Mouftah, for consistently motivating me towards the project and guided me to accomplish my goal from the course. His insightfulness is precious and motivation countless. The course is most interactive one and helped me built my intellectual skills. He showed me the way to be righteous and successful person in my future expeditions.
Secondly, I won’t forget to thank my friends who consistently supported my work and stepped in when I felt low about anything.

DWDM Dense Wavelength Division Multiplexing

FS Frequency Slot
FSC Fiber-Switch Capable
GMPLS Generalized Multi Protocol Label Switching
LSR Label Switching Router
MPLS Multi Protocol Label Switching
NCF Nominal Central Frequency
OCC Optical Channel Carrier
OCh Optical Channel
OCh-P Optical Channel Payload
OTN Optical Transport Network
OTSi Optical Tributary Signal
OTSiG: OTSi Group is a set of OTSiOSPF Open Shortest Path First
PCE: Path Computation Element
ROADM Reconfigurable Optical Add/Drop Multiplexer
RWA Routing and Wavelength Assignment
SSON Spectrum-Switched Optical Network
SWG Slot Width Granularity
1.Introduction 6
1.1 Evolution of IP Networks 6
1.2 Design Objectives of optical networks 7
2.Flex grid Networks 9
2.1 Difference between Flex grid and Fixed grid 10
2.2 Terminology 11
3.Media Channel in Optical Networks 12
4.Multi Protocol Label Switching (MPLS) 14
4.1 Routing and Spectrum Assignment 15
4.2 Constraints on Wavelength and Routing 16
4.3 Adaptive Routing 18
5.Generalized Multi Protocol Label Switching (GMPLS) 19
5.1 GMPLS control of Media Channel 20
5.2 GMPLS control of Flex Grid Network 21
5.3 GMPLS requirements of Flex Grid Networks 23
6.Open Issues in Flex Grid Network 24
7.Conclusion 25
Chapter 1 Introduction
The internet has revolutionized the computer and the communications world and has completely changed the life of the human being. As the importance of the internet grows, the strategies of constructing the infrastructure has become more critical. The internet is a core network, or a wide area network(WAN), which cuts across continents and countries for the purpose of interconnecting hundreds, or even thousands, of small sized networks such as Metropolitan area networks (MAN) and Local area networks (LAN). These small sized networks are also called as access networks which upload or download through a traffic glooming mechanism.

The idea of constructing networks with the fiber optics appeared with the break through in optoelectronics in the early 1980. However most of them were focused on access networks such as FDDI and SONET ring networks. With advances in optical technologies, DWDM has evolved where the amount of raw bandwidth available on fibre optic links has increased by several orders of magnitude. As a result, survivability issues emerged as the prevalence of using optical fibres with the DWDM technology in the internet core. In terms of interconnecting architectures, the next generation optical internet is expected to be a DWDM based mesh network with multiple fibers connecting each pair of nodes. Since DWDM technology has vastly increased the bandwidth along a single fibre, we need a new control and management strategies to make the most use of this revolutionary improvement in the enabling hardware.

1.1 Evolution of IP networks:
Since the late 1980s, router-based cores have been largely adopted by the service provides, where the traffic engineering was realized by simply manipulating cost functions and link states metrics of Interior gateway protocol (IGP), such as Open Shortest Path First (OSPF). With increase of internet traffic, several limitations to bandwidth provisioning and TE requirements have emerged. While the IP over ATM core networks initially solved the problems by using the IP routing cores for ISPs, the expenses of the increased complexity in overlaying the IP and ATM control mechanisms stimulated the development of various multilayer switching technologies. The evolution of control and management for the IP networks began a new era in 1998, when Multi-Protocol Label Switching (MPLS) was standardized by the internet engineering and designed to unify the multilayer switching architectures.

1.2 Design Objectives of Optical Networks:
The four important design principles and objectives upon which the efficiency of any optical networks is determined. The issues of interest include
Class of service
Capacity of efficiency
A network is considered to be a survivable if it can maintain services continuity to the end users during the occurrences of any failure on the transmission media, switching devices, and the protocols, by a suite of real time mechanisms of protection. Network survivability has become a critical issue as the prevalence of DWDM technology in the optical core, by which a single fiber cut may influence a huge amount of bandwidth in the transmission and cause service interruptions to innumerable end users. With under utilised capacity in the networks, the most widely used strategy is to find the protection resources which are physically disjointed from the working path, over which affected data flow can be switched to the protection paths during any failure along the working path.

Network fault scan be divided into four categories:
Path failure (PF)
Path Degraded (PD)
Link failure (LF)
Link Degraded (LD)
PD and LD are the results of the Loss of signals (LoS), in which t he quality of the optical flow is unacceptable to light path terminating nodes. For the protection of LoS Failure, preparation of the spare network capacity is the most commonly adopted strategy. In case of PF and LF failure, the continuity of a link or a path is damaged. This kind of failure can be detected by a Loss of Light detection performed at each optical network element so that fault localization can be easily performed.

In the networks with static traffic, where all the traffic demand is defined prior to the network operation, the issue of scalability refers to the size of the network. For a network with dynamic traffic where connection/ disconnection requests, or network events, arrive at the network one by one without prior knowledge of the future arrivals, the scalability issues are not limited to the size of the networks, but are also subject to the characteristics of the traffic pattern. The important factors include
Traffic dynamicity
Traffic granularity
Provisioning speed
Class of Service:
In the optical network layer, evaluating the quality of service for a connection request is no longer limited to delay, jitter or packet-discard policies which play major roles in differentiating services in traditional packet switching networks. In general, there have been two criteria defined in services of bandwidth provisioning in the optical domain, namely
Provisioning priority
Restoration time
Capacity- Efficiency:
One of the most important ways by which ISPs can increase revenue from their internet services is by accommodating more connection requests into their networks at a moment. Capacity efficiency is defined as a measurement of effective bandwidth that a network can provision at a moment. The capacity efficiency of an optical network can be evaluated alternatively with a couple of performance indexes. The choice of performance index strongly depends on the network environment and design objective. The performance indexes could be throughput, which is defied as the amount of bandwidth provisioned within a given period of time. For a network with dynamic traffic, blocking probability usually serves as a performance index for evaluating the performance. Blocking probability is defined as the probability for a connection request to be rejected under a certain arrival and departure rates of a network events.

Chapter 2 Flex grid Networks:
In recent days, advances such as using multi rate system on a same WDM networks by using different types of transponders. The main challenge in these networks are the trade off between the cost and the efficiency of the total network. As a solution, the flexible grid network proves to be a promising architecture which overcomes the limitations of fixed grid by allowing the wide channel selection as per the requirement. These networks are built using the variable switches that are used to create an efficient optical spectrum slot.

Figure 1: Fixed grids and FlexgridIn the above diagram,
Figure 1(a) shows a mixture of signals which are up to 100Gbit/s that are operating over a 50GHz fixed grid.
As speeds increase in figure 1(b), high speed 400Gbit/s and 1Tbit/s signals straddle multiple slots and the signals are notched as it passes through a fixed filter.

Flex grid that allows a mixture of different sized demands as shown in figure 1(c).

The term Flexible grid is defined by international Telecommunication standardization section which gives an updated set of central frequency, channel spacing, optical spectrum management and allocation that are defined in order to allow efficient and flexible allocation of spectral bandwidth that have higher bit rate systems. The main target of flex grid network is to allow efficient and effective allocation of optical spectral bandwidth for high bit rate. It enables flexible, application-based testing of bandwidth use. The key element of the Flex grid DWDM networks is use of the variable switches which can be dynamically modified according to the needs. Flexible transponders used along with the flex grid are called as bandwidth variable transponders (BVT) which allows to choose the modulation format and the parameters that give efficient performance.

Source: wikipedia The above shown diagram is general overview of OTN layer which is a independent layer network with client/ server relationships between them. In OTN layer, media layer is the server layer. The optical layer is guided to its destination node by media layer with the help of network media channel. Signal layer is the client layer.

2.1 Difference between Flex grid and Fixed grid:
The main difference is that fixed grid uses a fixed size of optical frequency range or a frequency slot with typical channel separation of 50 GHz, whereas the flex grid can select its media channel with a more flexible choice of spectral slot width. In any networks flex grid networks is a layered network architecture in which media layer is a sever layer and signal layer is client layer. For future traffic growth, 50 Ghz grid will be no longer used for wide channel spectrum. As a solution, fixed grid with various sized frequency slots can be used depending on the demand.

Figure 3: Flexible grid Vs Fixed grid
Source: wikipedia2.2 Terminology:
A new WDM frequency grid is defined with aim of allowing flexible optical spectrum management in which frequency slot width of various frequency range allocated to different channels are flexible. The most important terminologies used in flex grid networks are
Frequency slot: the frequency range allocated to a channel and it is unavailable to all other channels within a flexible grid. A frequency slot is defined by its nominal central frequency and its slot width.

Central frequency: Each of the maximum allowed frequency as per the definition of the flex-grid networks.
Central frequency is calculated by using formula
f = 193.1 THz + n x 0.00625 THz
where 193.1 – anchor frequency
n – positive or negative integer including 0
Slot width: Slot width is defined in terms of Full width in Hz of a frequency slot which is a multiple of 12.5 GHz.

Figure 4: calculation of central frequency and slot width
Source: wikipediaChapter 3 Media Channel in Optical Networks:
A Media Channel is an association that represents both the topology (path through the media) and the resource (frequency slot) that it occupies ie, an effective frequency slot supported by a concatenation of different media elements such as fibers, amplifiers, switching matrices. There are 2 types of media channels as follows:
Media channel
Network Media Channel
Media Channel:
Each element is defined as a container and includes a set of attributes. The module also includes the data types for the type of modulation, the optical technology such as Forward Error Correction (FEC). – Media Channel (two types). A frequency slot is effective and is supported by a combination of media elements (fibers, amplifiers, filters, switching matrices.)
Network Media Channel:
A Network media channel is also a Media Channel, that transports a single optical tributary signal.

Optical Tributary signal:
The optical signal is placed within any network media channel and is transported across its end to end optical networks. Optical tributary signal comprises of a single modulated carrier or a group of different optical carriers. Each optical tributary signal is carried by one network media channel.

Composite media channel:
As the name suggests, a composite media channel is a media channel which consist of group of network media channel. A composite media channel carries a group of Optical tributary signal which is also called as optical tributary signal group (OTSiG). Each optical signal is carried by one individual network media channel. This OTSi signals are grouped as one OTSiG and is carried over a single fiber. In some case, the effective frequency slots may be contiguous or non-contiguous i.e., there is no spectrum between the frequency slots and they can be used even for other network media channels.

Media Channel Frequency Slot
| |
| Frequency Slot Frequency Slot |
| +———–X———–+ +———-X———–+ |
| | OTSi | | OTSi | |
| | o | | o | |
| | | | | | | |
-4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12
;- Network Media Channel -; ;- Network Media Channel -;
;———————— Media Channel ———————–>
X – Frequency Slot Central Frequency
o – Signal Central Frequency
Figure 5: Example of a Media channel with its network media channel
Source: paper titled “Framework and Requirements for GMPLS-Based Control
of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks”
Flexi-grid Layered Networks:
An OTN network is an layered architecture where network media channel consists of more media channel matrices. Network media channel is in built between two channel port which together called as OTSi signal. Extensions include a new DWDM grid which defines a set of nominal central frequencies, channel spacings, and the concept of frequency slot. Data plane connections are switched based on allocated, variable-sized frequency ranges within the optical spectrum.

| OTSi |
O – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – O
| |
| Channel Port Network Media Channel Channel Port |
O – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – O
| |
+——–+ +———–+ +——–+
| (1) | | (1) | | (1) / |
| —-|—————–|———–|——————-|—–/ |
+——–+ Link Channel +———–+ Link Channel +——–+
Media Channel Media Channel Media Channel
Matrix Matrix MatrixFigure 6: Simplified Layered Network Model
Source: paper titled “Framework and Requirements for GMPLS-Based Control
of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks”
Chapter 4 Multi-Protocol Label Switching (MPLS):
MPLS finalizes the evolution of multi layer switching of internet technology and is built upon the efforts of various proprietary multi layer switching solutions. MPLS is famous for its strong functionality in traffic engineering and class of service and can provide a robust and scalable platform for facilitating control and management on the internet. The most important benefits of using MPLS control plane are summarized as the provision of a shim layer between the IP and the underlying data transportation layers and the label swapping algorithm that makes the setup of LSPs scalable. An LSP is functionally equivalent to a virtual circuit with specific bandwidth and is defined as an ingress to egress path through a network. The first and last LSR along an LSP is called ingress and egress LSR of the LSP respectively. An LSP is created by concatenating one or more label switch routers which is router equipped with the existing IP protocols with TE extensions as well as the MPLS based forwarding plane.

The control box is equipped with the MPLS control plane, which contains the following three functional units. First, the constraint based shortest path first (CSPF) path selection module can perform path selection upon different constraints such as maximum reservable bandwidth, wavelength availability and diversity requirement of a link according to database.

Figure 7: MPLS/GMPLS internetworking model
Source: wikipediaMPLS has been concluded as an effective solution to the stringent requirements for traffic engineering and class of service in the packet switching core networks. The control component uses a standard routing protocols such as Open shortest path first (OSPF), IS-IS (intermediate system- intermediate system) and BGP-4 (-Border gateway protocol) to exchange information with other routers to build and maintain a forwarding table.

4.1 Routing and Spectrum Assignment:
Routing and wavelength assignment constitutes one of the most fundamental elements in the control and management of an optical networks. The constraints imposed on a wavelength routed optical network includes topics of physical constraint and diversity constraint. RWA also called as path selection in the optical domain is a key process of provisioning light paths in response to the connection requests. In the event that all optical wavelength conversion is allowed in some or other network nodes, the wavelength continuity constraint does not always hold where a light path may take network resources on different wavelength planes.

The two types of RWA process are
Static RWA process
Dynamic RWA process
Static RWA:
As the name says the traffic distribution in the network will not be changed while the network is in the operation. In the static case, a traffic matrix is given, which specifies the bandwidth demand between any node pair of the network. Then the light paths are allocated to a network according to the traffic matrix. The design objective can be either to achieve a maximum throughput given the total network capacity or to satisfy all the traffic demand with a least amount of fibers along each link or wavelengths contained in each fiber.
The static RWA problem can be performed through an optimization process such as Integer Linear Programming (ILP), which is notorious for an NP-complete computation complexity. Therefore, the optimization process is only feasible for small sized networks. To reduce the computation complexity, the optimization process can be divided into two sub processes, namely physical path selection and wavelength assignment for deriving an approximate optimal deployment of light paths.

Dynamic RWA process:
In addition to the static one, the other type of RWA problem is for a network with dynamic traffic. The type of RWA process is also called a dynamic RWA, where the computation complexity directly influences light path provisioning speed. Static RWA process cannot be applicable to dealing with traffic distribution that is changing from time to time. The dynamic RWA is aimed at satisfying light path setup requests one at a time with a goal of maximizing the probability of successful allocation for the subsequent demands. The most commonly used performance index in the dynamic network is the blocking probability under specific potential traffic load for each S-D pair. Any change of the traffic distribution has to update the link state database. Based on the database, the next connection request can be allocated.

4.2 Constraints on Routing and Wavelength Assignment:
The routing constraints can be divided into three major categories:
Constraints imposed by physical transmission medium
Constraints imposed by diversity requirements
Wavelength continuity constraint
Physical Constraints:
These constraints are imposed upon the transparency of the optical networks which may impair the availability of light path. The task of defining a domain of transparency much depends on the physical apparatus and communication equipment adopted and may be a group of network nodes and link. An optimal or near optimal deployment of transponders along with a properly designed routing and wavelength assignment strategy is essential for maximizing and economizing the network resource utilization. The physical impairments may distort optical signals in a linear or non-linear manner as the optical flow traverses along a fiber or through a switch which leads to a loss of optical signal quality. The loss of optical signal quality can be attributed to major factors:
Signal power loss
Diversity Constraints:
One of the major reasons for imposing the diversity constraint in path selection is for the purpose of achieving survivability. In the optical networking layer, two or more light paths are said to be diverse if they will never be subject to a single failure at the same time. To determine whether two or more light paths are subject to a single failure we have to define a hierarchy of shared risk link group(SRLGs).
Wavelength continuity constraint:
The wavelength continuity constraint is unique to optical networks with WDM as the core technology and is a consequence of multiplexing several wavelength channels into a single fiber. Due to the wavelength continuity constraint, the WDM networks are intrinsically different from the other connection-oriented networks in terms of bandwidth allocation. The purpose of modelling a WDM network into a graph is to facilitate the use of any adaptive routing scheme such as Dijkstra’s shortest path algorithm. A network can be full wavelength convertible when all wavelength channels on different wavelength planes are exchangeable during the routing process. So, the wavelength conversion capability of a node as the maximum number of light path that can be converted in this node at the same time.

Dynamic path selection:
The modeling and approaches for optimal routing which is used for dynamic path selection can be either fully adaptive or partially adaptive. Dynamic path selection can be performed in a centralized manner, in which all the computation tasks are taken by the Network Management System (NMS).
4.3 Adaptive Routing:
Due to the wavelength continuity constraint, most of the existing studies use either Fixed or alternate routing scheme to select a physical route for each connection call. To achieve better performance and adaptability to traffic variation, a fully adaptive approach is desired. Fully adaptive routing can be achieved good performance in finding the shortest paths in networks based on the dynamic link state and a custom designed cost function. The most commonly seen adaptive routing scheme uses Dijkstra’s shortest path first algorithm, which consumes a reasonable computation complexity O(N2), where N is the number of network nodes. The other type of adaptive routing is called partially-adaptive routing which includes Fixed Routing (FR) and Fixed Alternate Routing (FAR). Since a single path is present for each S-D pair with FR, the RWA problem is degraded to a task of wavelength assignment along the fixed path, which leads to an ultra-fast routing decision making process at the expenses of performance. The dynamic path selection can be performed in either a fully adaptive way in which the network wide dynamic link states are necessary for making a routing decision, or a partially adaptive way the routing decisions are made according to a limited amount of dynamic per wavelength link states.
Dynamic Wavelength Assignment:
The dynamic RWA problems in the multi fiber networks have also been extensively used due to its extremely high computation complexity, the task of dynamic RWA is separated into two independent sub tasks-routing and wavelength assignment. For achieving better performance, dynamic link state can be taken into consideration while selecting a physical path so that the wavelength assignment is conducted together with the physical path selection. The proposals of wavelength assignment that cooperate with FAR are Random Fit, First Fit, Most Used, least loaded routing
Chapter 5 Generalized MPLS Architecture:
The emergence of Generalized multi Protocol Label Switching (GMPLS) has opened a new era for the automatic control and management for the optical internet with a core technology of DWDM. GMPLS can be described in brief as an optical extension to the MPLS based control plane, which is in its on-going progress of standardization for supporting optical traffic with multi granularity and provides generalized labels to traffic flows with different switching types. In GMPLS architecture, labels in the forwarding plane of label switch routers (LSR) are not only limited to the packet headers and cell boundaries but also time slots, wavelengths or any physical ports. A connection can be only established between any interfaces of the same type, which is termed a G-LSP in the context of GMPLS.

A hierarchy can be built if an interface can multiplex several G-LSPs with the same technology, which is also called a “nesting of traffic flows”. In the optical domain, there is no difference between the processing of various traffic since all of them must be demultiplexed into wavelength level flows to be further processed. Thus, we must have at least three classes of granularities: lambda-(or wavelength), waveband and fiber switching in which several lambdas switching flows can be nested into a waveband- switching flow, and in turn several waveband switching flows can be nested into a fiber switching traffic flow. The enhanced capabilities of GMPLS include:
neighbor discovery
distribution of link information
provisioning of topology management
path management
path protection and recovery.

In next generation network GMPLS is most promising architecture as most of the communication entities could be connected on fiber and increase coarse granularity. Within a flexible DWDM grid, optical spectrum can be allocated in multiples of a width granularity, depending on client signal rate and modulation format. A control plane can be used for efficient and dynamic provisioning and recovery of flex grid connections. Flex grid technology in terms of increasing the no of optical channels established over optical links, however may not be sustainable because of the associated increase in optical amplified power. New GMPLS protocol are proposed on which to integrate the optical power control process in the control plane. As a result, controlling optical power from flex grid technology is high and reduces optical connection blocking.

5.1 GMPLS control of Media Channel:
Media Channel deals with the establishment of media channels, which are switched in a media channel matrix. The association of the three components i,e a filter, a fibre and a filter is a media channel in its most basic forms. The control plane is used to support media channels which are characterized by a single frequency slot. This representation of a media channel in GMPLS control plane is called Flexi grid LSP.

Figure 8: Basic Media channel
We can concatenate several media channels with its intermediate nodes to create a new single media channel. From a control-plane perspective, the main difference between the network media channels is that the LSP that represents a network media channel still includes the endpoints (transceivers connected at the end), including the cross-connects at the ingress or incoming node and egress or outgoing nodes. The ports towards the client side can be still represented as the interfaces from the control-plane.

Figure 7: Extended Media Channel
5.2 GMPLS Control of Flex Grid Networks:
The flex grid LSP of any network media channel is represented by a control plane perspective. In case of GMPLS controlled system, the switched element is represented as the label whereas in the flex grid networks, the switched element is represented as a frequency slot, so these labels represent the frequency slot of network media channels. These labels of network media channel is used to describe the important details which is used to obtain the general characteristics of a frequency spectrum.
In general, any optical network domain comprises of various optical switches and various fiber links. These optical switches usually function as Reconfigurable Optical Add Drop Multiplexers (ROADMs) which deploys a flex-grid technology, and as a result it supports many optical connections of one or more number of contiguous frequency slot of 12.5 GHz. MPLS routers of each optical switch are connected to the ROADM via a tunable transceiver. In other hand, these tunable transponders can be connected to IP/MPLS routers ports, so that each generated signal can directly enter into the optical network domain. Both these methods are almost equivalent, in every term of its cost and functionality. The traffic entering into the optical switch is routed all over the optical network in all of its optical connections.

Figure 8: Flexi-Grid LSP Representing a Media Channel That Starts at the Filter of the Outgoing Interface of the Ingress LSR and Ends at the Filter of the Incoming Interface of the Egress LSR
This IP/MPLS router can be:
The destination of some of the packets in the optical network domain, will be forwarded to its destination through other domains
An intermediate hop, in which the related packets will re-enter the optical network which are to be forwarded to its destination. The light paths are always bidirectional and the transponders act both as transmitters and receivers.
5.3 GMPLS Requirements of Flex Grid Networks:
The general requirements of a flex grid networks can be best described in terms of its signaling aspect and routing aspects.

Signaling Aspects: These give the information about how the signals are routed through the destination. The frequency slot width used for signaling plays an important role in defining the flex grid networks. The main aspects considered are
Signal compatibility Information
Identifying the slot width requirements
Identifying the central frequency assigned to a LSP
Routing Aspects: They give the information regarding the connection and signal processing. They also convey the necessary information about the frequency range to transfer the link information.

Related information such as connectivity matrix, signal compatibility and processing.

Available frequency range of each link (link information)
Drawbacks of Flex Grid Networks:
There are some potential problems related to internetworking between the fixed grid and flex grid networks. Additionally, when two flex grid networks are combined due to different grid properties they lead to a link property conflict and thus results in limited internetworking. Devices or the application that make use of flex grid networks might not be able to support all possible frequency slot width.

Chapter 6 Open Issue in Flex Grid Networks:
There are some open issues which need to be solved before utilizing flex grid technology to its maximum potential. The three main open issues are
Spectrum Fragmentation
Traffic behaviour
Spectrum Fragmentation:
In general, spectral continuity constraint is used to avoid the optical to electrical conversion at nodes which are costlier method. In flex grid networks, this problem still exists with a higher value due to the different sized demands. This can be controlled by two methods. The first method involves by defragmenting the frequency spectrum and the second method is by applying the fragmentation in the RSA algorithms.
The second issue is the controllability which is due to different network elements. A well-defined control plane is required which can effectively communicate between the network elements about the frequency spectral requirements.

Traffic Behaviour:
The third issue arises in case of dynamic traffic demands. If different traffic demands need the same spectral bandwidth at the same time, then the flex grid network can not allow the sharing of the additional spectrum. The other main issue between the flex grid and fixed grid networks are related to the property of contiguous i,e whether all the flex grid networks are Contiguous or Non-Contiguous and whether these networks can support all non-contiguous frequency and can flex grid LSP can use some of the non-contagious frequency on the link. Some proposed requirements do not fit in the current framework:
Finer frequency slot granularity
Media Channel resizing
Media Channel with multiple frequency slots
Chapter 7 Conclusion:
Main Target of the Flex grid is to allow the efficient allocation of optical spectrum Bandwidth by enabling different frequency slots on a single fibre. It can support network media channel with frequency slots width ranging from minimum of 6.25 GHz to a maximum of entire C band with granularity if 6.25 GHz. It allows resizing of frequency slots. In some scenarios the resizing of frequency slots may cause temporary data plane disruptions. Currently, there is no clear view on hitless resizing at the data plane.

1. Ramon Casellas, Josep Ma.Fabrega, “GMPLS control of flexi-grid DWDM optical networks using OFDM transmmsion”- Journal of optical communication and networking, nov 2012
2.Turus, I., Fagertun, A. M., ; Dittmann, L, “GMPLS control plane extensions in support of flex-grid enabled elastic optical networks”- In Proceedings of OPNETWORK 2013 OPNET
3. O. Gonzalez de Dios, D. Ceccarelli, X. Fu, F. Zhang, I. Hussain, R. Casellas, “Framework and Requirements for GMPLS-Based Control of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks”
4. Gangxiang Shen1, Hong Guo1, Sanjay K. Bose2, “Survivable elastic optical networks: survey and perspective”- Springer Science+ and Business Media New York 2015
5. P. Papanikolaou, K. Christodoulopoulos, and E. Varvarigos, “Multilayer flex-grid network planning” – 2015 International Conference on Optical Network Design and Modeling (ONDM)
6. Raul Muñoz1, Víctor López2, Ramon Casellas1, Óscar González De Dios2, “IDEALIST Control and Service Management Solutions for Dynamic and Adaptive Flexi-grid DWDM Networks” – IIMC International Information Management Corporation, 2013 ISBN: 978-1-905824-37-3
7. P. Papanikolaou, P. Soumplis, K. Manousakis, G. Papadimitriou, ” Minimizing energy and cost in Fixed grid and Flex- grid networks” – Vol. 7, No. 4/April 2015/J. Opt. Commun. Netw.


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