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This assignment will explore and discusses several survivability techniques That are using in optical networks and will focus in Protection and Restoration. By means of the term survivability, stand for the ability of a network to continue to provide a service even after a failure occurs in the network. The most common failures in a optical network are:
- Broken/cut fiber
- Node failure
- Wavelength channel outage
- Management software bugs
- Incorrect configuration
Therefore designers of optical networks must create some protection/restoration mechanisms that needed to ensure network survivability. We can classify protection as an operation performed in network before any errors happens, to ensure that after a fault network will not fold down. Restoration defines as the procedure provided after any failure occurs in the network. To understand protection and restoration procedures in a optical network there are some characteristics that must be observed.
In terms of scalability the complexity of protocols/algorithms of the network should be kept slow and can be apply to whole network without any problem. New optical networks provide a lot of services not only voice as used in the past. Each service has to be determined by a service level agreement (SLA) which defines parameters such as the Quality of Service and the bandwidth that we have to use. SLA's contains technical service level specifications (SLS) are used in order to make an optimal use of the network, without waste available resources that can be used to obtain higher or better performance of the network i.e. Routing stability, service availability.
Another issue that we have to consider is the capacity efficiency of the network and this is relative to the efforts to make use every single part of the network without wasting the resources of the network. An important topic is the restoration time that the network needs to recovery from an error; the shorter is restoration time the faster the network will be restored to its original state and re starts working without problem.1.1 Gerenal
1.1 Techniques for Protection/Restoration in Optical Networks
There are two main protection schemes for the optical communication regarding to protection and restoration. First is refered as predesgined protection and the second as dynamic restoration.
1.2 Predisigned protection
In Predisigned protection network's designers have pre-configure how the network will recover from an error when were design it. The advantage of this technique is that it is fast and guaranteed because there is already a plan for recovering from an error situation. On the other hand there is a waste of resources becouse some resources of the network are using only for recovering and therefore the capacity is not efiicient.
For non-WDM networks the most efficient techniques are Automatic protection Switching(APS) and Self Healing Ring(SHR). APN mechanisms is used for optical point-to-point or linear topologies and has three main technologies.
One another techniques that can handle not only link failures but and node failures is SONET Self Healing Ring(SHR). This technique used in ring or mesh network topologies. Sonet is designed to support existing telephone network traffic and ISDN.
1.2 Dynamic restoration
In dynamic restoration there is none protection technique before an error occurs. After an error happens,it finds a protection path from the working elements/components of the network. The main advantage of this technique is that there is no waste of resources because network is not binding resources before an error occurs, and therefore the network is more eficient than the pre-planned preotection. A main drawbacks is the lack of ability to provide 100% recovery in particular in a big and demand network and therefore none can guarranted us that will be available resource of the network to use it for restoration. As a result this leads to have lower backup resource utilization, fast failure signaling rate. One other solution is proactive proactive restoration,this technique during the connection setup reserved backup capacity and can guarantee recovery under single link failure.The main disadvantage is that has low spare capacity utilization when no errors happens. There are some new techniques that uses hybrid survivability approaches and combine the positive effects of restoration and protection. The main idea is to increase the probability to recover from an error while reducing the overall spare capacity.
Wavelength Division Multiplexing (WDM) is a new type of network and the main purpose is to improve the use of the optical fiber Bandwidth. This can be achieve by separate different wavelengths and send them in a single optical fiber. By using this method more than one channel can be established at the same time; and as a result we can send more data through a single fiber. All the users in WDM communicate via links(Paths between nodes) in the network and every link has its own wavelength(λ1…λN) to communicate. When the connection between two nodes finishes the wavelength that was used is available for another connection. The advantage with this technique is that every link in the network uses the full available bandwidth in the particular wavelength, typical tr ansmission capable rates of 40 Gbps. WDM provide trunking capacity to higher-layer networks,supports efficient outing and fast failure recovery of demanding services as concering the bandwidth. Therefore WDM networks must efficienthly control the network. The major operation in this network is to establish connections between source and destination pairs on demand and when the connection finishes, to realease them.
As the WDM networks are development over the years , the protection and restoration issues are paid more and more consideration. Switching and routing of the lighpaths now is performed in a new domain called optical Domain without using electrical parts to convert it from electrical to optical and opposite Optical Layer leading to from point to point to wavelength routed protocols (WRN), it is wasier to groom the capacity for every service or network we use(Ethernet,fiber). In the past protection and restoration schemes are used for SONET/SDH but nowadays that networks become IP based. It is widely accepted that traffic over the WDM network using IP are more cost efficient especially to handling out failures but unfortunately Optical layert cannot handle higher layer equipment failures.
In WDM protection uses the optical layer to handle protection/restoration when a failure of a lightpath happen, when the primary link fail directly backup lightpaths are re-design to a new physical links but without notify the upper leyers for the failure that occurs.The main advantage of this protection is that the time to react and change from primary to backup link is very small.
In IP protection we use MPLS IP networks. Protection in the IP Layer can be established using a primary IP LSP and a secondary IP LSP as backup.Multiprotocol Label Switching(MPLS) is a method that in every hop attach a label in, every decision is based on that Label Switched Path(LSP). LSP's are configured using protocols like OSPF and RSVP.
LSRs performs label switching by establishing a connection from an input port/label to an output port/label.All IP LSPs are routed on a virtual topology in the WDM layer. To ensure “link-disjoin” of the primary and the secondary LSP there are using Shared-Risk Link Groups(SRLG). When using IP LSP protection we have to consider tha for every affected primary lightpath its input node must have the primary notification and after that a notification must be send to PSL. After the notifications LSR is responsible to switch the traffic to the backup IP LSP using the correct protection label.