Performance Management and Fault Management. 1 Dept. of ECE, SRM University

Transcription

1 Performance Management and Fault Management 1 Dept. of ECE, SRM University

2 Performance Management Performance management requires monitoring of the performance parameters for all the connections supported in the network and taking any actions necessary to ensure that the desired performance goals are met. Performance management is closely related to fault management. Fault management involves detecting problems in the network alerting the management systems appropriately through alarms. Also includes restoring service in the event of failures 2 Dept. of ECE, SRM University

3 The Impact of Transparency A pure transparent network means - a lightpath will be capable of carrying analog and digital signals with arbitrary bit rates and protocol formats. However, it is difficult to engineer because the various physical layer impairments that must be taken into account in the network design are critically dependent on the type of signal (analog versus digital) and the bit rate. It is difficult to manage because the management system may have no prior knowledge of the protocols or bit rates being used in the network. It is not possible to access overhead bits in the transmitted data to obtain performance-related measures. This makes it difficult to monitor the bit error rate. Unless the management system is told what type of signal is being carried on a lightpath, it will not be able to determine whether the measured power levels and signal-to-noise ratios fall within acceptable limits. 3 Dept. of ECE, SRM University

4 Current scenario We could design a network that carries data at a fixed bit rate (say, 2.5 Gb/s or 10 Gb/s) and of a particular format (say, SONET/ SDH only). Such a network would be very cost-effective to build and manage. However, it does not offer service providers the flexibility they need to deliver a wide variety of services using a single network infrastructure and is not future-proof at all. Currently The network is designed to handle digital data at arbitrary bit rates up to a certain specified maximum (say, 10 Gb/s) and a variety of protocol formats such as SONET/SDH, IP, ATM, Gigabit Ethernet, and ESCON. 4 Dept. of ECE, SRM University

5 BER Measurements The bit error rate (BER) is the key performance attribute associated with a lightpath. The BER can be detected only when the signal is available in the electrical domain, typically at regenerator or transponder locations. Parity check bytes present in framing protocols used in SONET and SDH provides a direct measure of the BER. Similarly, the digital wrapper overhead developed specifically for the optical layer also allows the BER to be measured. As long as the client signal data is encapsulated using the SONET/SDH or digital wrapper overhead, we can measure the BER and guarantee the performance within the optical layer. 5 Dept. of ECE, SRM University

6 Optical Trace Lightpaths pass through multiple nodes and through multiple cards within the equipment deployed at each node. It is desirable to have a unique identifier associated with each lightpath. For example, this identifier may include the IP address of the originating network element along with the actual identity of the transponder card within that network element where the lightpath terminates. This identifier is called an optical path trace. The trace enables the management system to identify, verify, and manage the connectivity of a lightpath. In addition it provides the ability to perform fault isolation in the event that incorrect connections are made. 6 Dept. of ECE, SRM University

7 Optical Trace A trace can be used in different layers within the optical layer. a lightpath passes through multiple nodes and potentially gets regenerated along the way. The end-to-end connectivity of a lightpath can be verified using an optical channel-path trace. This trace is inserted at the beginning of the lightpath and monitored at various locations along the path of the lightpath. In order to localize and verify connectivity between regenerator locations, we make use of an additional identifier called the optical channel-section trace, which is associated between each adjacent pair of regeneration points of the lightpath. Within an all-optical subnet, we can use a optical channel-transparent section trace. The latter two traces are inserted and removed at regenerator locations in the network. 7 Dept. of ECE, SRM University

8 Alarm Management In a network, a single failure event may cause multiple alarms to be generated all over the network. Alarm suppression is accomplished by using a set of special signals, called the forward defect indicator (FDI) and the backward defect indicator (BDI). When a link fails, the node downstream of the failed link detects it and generates a defect condition. For instance, a defect condition could be generated because of a high bit error rate on the incoming signal or an outright loss of light on the incoming signal. If the defect persists for a certain time period (typically a few seconds), the node generates an alarm. Upon detecting a defect, the node inserts an FDI signal downstream to the next node. The FDI signal propagates rapidly and nodes further downstream receive the FDI and suppress their alarms.the FDI signal is also sometimes referred to as the alarm indication signal (AIS). A node detecting a defect also sends a BDI signal upstream to the previous node, to notify that node of the failure. If this previous node didn't send out an FDI, it then knows that the link to the next node downstream has failed. 8 Dept. of ECE, SRM University

9 Indicator Signals Suppose there is a link cut between OLT A and amplifier B. Amplifier B detects the cut. It immediately inserts an OMS-FDI signal downstream indicating that all channels in the multiplexed group have failed and also an OTS-BDI signal upstream to OLT A. The OMS-FDI is transmitted as part of the overhead associated with the OMS layer, and the OTS-BDI is transmitted as part of the overhead associated with the OTS layer. 9 Dept. of ECE, SRM University

10 Indicator signal propagation Amplifier C downstream receives the OMS-FDI and passes it on. OADM D, which is the next node downstream, receives the OMS-FDI and determines that all the lightpaths on the incoming link have failed. Some of these lightpaths are dropped locally and others are passed through. For each lightpath passed through, the OADM generates OCh-TS-FDIs and sends them downstream. The OCh-TS-FDIs are transmitted as part of the OCh-TS overhead. At the end of the all-optical subnet, at OLT E, the wavelengths are demultiplexed and terminated in transponders/regenerators. Therefore the OCh-TS layer is terminated here. OLT E receives the OCh-TS-FDIs. It then generates OCh-P-FDI indicators for each failed lightpath and sends that downstream to the ultimate destination of each lightpath as part of the OCh-P overhead. Finally, the only node that issues an alarm is node B. 10 Dept. of ECE, SRM University

11 Reasons for using indicator signals Defects are used to trigger protection switching. For example, nodes adjacent to a failure detect the failure and may trigger a protection-switching event to reroute traffic around the failure. At the same time, nodes further downstream and upstream of the failure may think that other links have failed and decide to reroute traffic as well. A node receiving an FDI knows whether it should or shouldn't initiate protection switching. For example, if the protection-switching method requires the nodes immediately adjacent to the failure to reroute traffic, other nodes receiving the FDI signal will not invoke protection switching. On the other hand, if protection switching is done by the nodes at the end of a lightpath, then a node receiving an FDI initiates protection switching if it is the end point of the associated lightpath. 11 Dept. of ECE, SRM University

12 Data Communication Network (DCN) and Signaling The element management system (EMS) communicates with the different network elements through the DCN. The DCN can be transported Through a separate out-of-band network outside the optical layer. Carriers can make use of their existing TCP/IP or OSI networks for this purpose. If such a network is not available, dedicated leased lines could be used for this purpose. Through the OSC on a separate wavelength. This option is available for WDM line equipment that processes the optical transmission section and multiplex section layers, where the optical supervisory channel is made available. Through the rate-preserving or digital wrapper inband optical channel layer overhead techniques

13 Signaling In addition to the DCN, in many cases, a fast signaling network is needed between network elements. This allows the network elements to exchange critical information between them in real time. For instance, the FDI and BDI signals need to be propagated quickly to the nodes along a lightpath. Other such signals include information needed to implement fast protection switching in the network, Just as with the DCN, the signaling network can be implemented using dedicated out-of-band connections, the optical supervisory channel, or through one of the overhead techniques. 13 Dept. of ECE, SRM University

14 Policing One function of the management system is to monitor the wavelength and power levels of signals being input to the network the acceptable power levels will depend on the signal types and bit rates. The types and bit rates are specified by the user, and the network can then set thresholds for the parameters, at which alarms must be set off. The thresholds depend on the data rate, wavelength, and specific location along the path of the lightpath, and degradations may be measured relative to their original values. more important function is to monitor the actual service being utilized by the user. 14 Dept. of ECE, SRM University

15 Optical Layer Overhead 15 Dept. of ECE, SRM University

16 Pilot Tone or Subcarrier Modulated Overhead This overhead is realized by modulating the optical carrier (wavelength) of a lightpath with an additional subcarrier signal. This signal is also sometimes called a pilot tone. As long as the modulation depth of this signal is kept small compared to the data, typically between 5-10%, and the subcarrier frequency is chosen carefully, the data is relatively unaffected as a result. The pilot tone itself may be amplitude or frequency modulated at a low rate, say, a few kilobits per second. At intermediate locations, a small fraction of the optical power can be tapped off and the pilot tones extracted without receiving and retransmitting the entire signal. Note that the pilot tones on each wavelength can be extracted from the composite WDM signal carrying all the wavelengths without requiring each wavelength to be demultiplexed. The advantages of the pilot tone approach are that it is relatively inexpensive and that it allows monitoring of the overhead in transparent networks without requiring knowledge of the actual protocol or bit rate of the signal. The disadvantages are that it cannot be used to monitor the BER, and the pilot tone can be modified only at the transmitter or at a regenerator and not at the intermediate nodes.

17 Optical Supervisory Channel OSC is used to convey information associated with monitoring the state of the amplifiers along the link, particularly if these amplifiers are in remote locations where other direct access is not possible. The OSC is also used to control the line amplifiers, for example, turning them on or turning them off for test purposes. It can also be used to carry the DCN, as well as some of the overhead information. The OSC is carried on a wavelength different from the wavelengths used for carrying traffic. It is separated from the other wavelengths at each amplifier stage and received, processed, and retransmitted The OSC wavelength could be located within the same band as the traffic-bearing channels, or in a separate band located away from the traffic-bearing channels. 17 Dept. of ECE, SRM University

18 OSC Choices Usage of wavelengths in the network. Traffic is carried on the O (original), S (short), C (conventional), or L (long) wavelength bands. Raman pumps, if used, are located about nm below the signal. 18 Dept. of ECE, SRM University

19 Rate-Preserving Overhead This overhead includes several bytes that are currently unused. Some of these bytes can be used by the optical layer. These bytes can also be used to add forward error correction (FEC), which improves the optical layer link budget. This technique can be used only at locations where the signal is available in electrical form, that is, at regenerator locations or at the edges The advantages it can be used with the existing equipment in the network. it retains the existing hierarchy of bit rates in the SONET/SDH The disadvantages the number of unused bytes available is limited and may not offer sufficient bandwidth to carry all the optical layer overhead and FEC while the SONET/SDH standards specify the set of unused bytes, several vendors have already made use of some of these bytes for their own proprietary reasons, which makes it difficult to determine which set of bytes are truly unused! it does not work with signals that don't use SONET/SDH framing, such as Fibre Channel or Gigabit Ethernet

20 Digital Wrapper Overhead a new set of overhead bytes is added to the signal as it enters the optical layer and removed when the signal is handed back to the client layer. digital wrapper defines a new set of overheads associated with the optical layer and can be used instead of the SONET/SDH overhead. It is being standardized in the ITU. The advantages First, sufficient overhead bytes can be added so as to provide adequate FEC and support the DCN as well as to allow for future needs. Second, a new standard based on this technique would allow better interoperability among multiple vendors through regenerators. Third, the technique is not limited to SONET/SDH signals. The wrapper can be used to encapsulate a variety of different signals, such as Fibre Channel and Gigabit Ethernet. The main disadvantages it is not suitable for use with legacy equipment, and that it requires the development of a new set of components to support the new hierarchy of bit rates. However, new components have already been developed to support the wrapper, and it is now available on many WDM products. The digital wrapper is ideally suited to carrying OCh-section and path layer traces and defect indicators, as well as providing other overheads for management, such as those used by an automatic protection-switching (APS) protocol for signaling between network elements during failures.

21 Applications of different optical layer overhead techniques 21 Dept. of ECE, SRM University