Automatic protection switching. OmniBER playing a more active role

Transcription

1 Automatic protection switching OmniBER playing a more active role

2 In the world today our networks run under the ground and roads are often dug up, buildings are constructed, earthquakes occur. Whenever the ground is disturbed, cables can be damaged. Faulty network equipment and severe signal degradation can also cause failures. Service providers, whose income is based on fault-free service, are now using service reliability as a differentiator between themselves and the competition. This is transferred into pressure on the network equipment manufacturer to produce ever more reliable components. Rigorous testing of equipment to ensure fault free protection switching in a real network environment is therefore essential. Automatic protection switching (APS) Most modern networks have working or service channels and protection channels. Networks can be configured in either linear or ring topologies [Figure 1]. Traffic signals usually travel on the service channels. However, sometimes faults can occur. This is usually due to either a signal fail (SF), where there has been a catastrophic failure of the signal path, or a signal degrade (SD), where the signal has degraded beyond prescribed limits. When faults occur, the traffic signal moves over to the protection channel [Figure 2]. Figure 1 Automatic protection switch (APS) 2

3 Networks can either be unidirectional or bi-directional. During fault free operation in either uni-directional or bidirectional linear networks, the traffic signal will travel along the service channels. Figure 2 Linear network with service and protection channels If there is any kind of signal failure or signal degradation, unidirectional or bi-directional networks behave slightly differently: Uni-directional networks The traffic signal from the faulty service channel will move onto its associated protection channel, while the traffic signal on the associated backward reporting channel will continue to travel on those fault free service channels. Bi-directional networks Both the forward and backward reporting channels are moved over to the protection channel. Initiating protection switching K1/K2 bytes Automatic protection switching is initiated by network elements when there is either a signal failure or the error rate exceeds specified limits (determined by the operator), known as signal degrade. The network elements communicate using the K1/K2 bytes in the overhead to divert the traffic signal [Figure 3]. Figure 3 How protection switching is implemented in SDH 3

4 The layout and operation of the K1/K2 bytes vary for linear and ring networks. And the values of the K1/K2 bytes are determined by Telcordia GR-253 and ITU-T recommendations G.783 for linear networks and G.841 for ring networks [Tables 1 and 2]. Protection switching protocol - Linear networks K1 byte Condition Bits Lockout of protection 1110 Forced switch 1101 Signal fail high priority 1100 Signal fail low priority 1011 Signal degrade high priority 1010 Signal degrade low priority 1001 Unused 1000 Manual switch 0111 Unused 0110 Wait-to-restore 0101 Unused 0100 Exercise 0011 Unused 0010 Reverse request 0001 Do not revert 0000 No request Bits 5-8 Selects channel used by APS messages K2 byte Bits 1-4 Bits 5 Condition Selects bridge channel used Determines automatic protection switch architecture Bits Reserved for future use 001 Reserved for future use 010 Reserved for future use 011 Reserved for future use 100 Reserved for future use 101 Reserved for future use 110 MS-RDI 111 MS-AIS Tables 1 and 2 Protection switching protocol - Ring networks K1 byte Condition Bits Lockout of protection (span) or signal 1110 Forced switch (span) 1101 Forced switch (ring) 1100 Signal fail (span) 1011 Signal fail (ring) 1010 Signal degrade (protection) 1001 Signal degrade (span) 1000 Signal degrade (ring) 0111 Manual switch (span) 0110 Manual switch (ring) 0101 Wait-to-restore 0100 Exercise (span) 0011 Exerise (ring) 0010 Reverse request (span) 0001 Reverse request (ring) 0000 No request Bits 5-8 Desination node ID K2 byte Bits 1-4 Bits 5 Condition Source node ID Path code: 0 = short path 1 = long path Bits Idle 001 Bridged 010 Bridged and switched 011 Reserved for future use 100 Reserved for future use 101 Reserved for future use 110 MS-RDI 111 MS-AIS 4

5 Using the Agilent OmniBER to test automatic protection switching The Agilent OmniBER communications performance analyzer can be used to test automation protection switching (APS). It has two modes, passive and active. Passive APS allows you to test the responses of a network element to various K1/K2 APS messages sent by the OmniBER, or faults generated in the service channels. The OmniBER can be used to register and display any response from the network element. This allows you to check whether the network element will respond appropriately to fault conditions in a real-life situation by performing automatic protection switching correctly. The OmniBER now also provides Active APS testing. An example set-up uses two OmniBERs to emulate the behaviour of a network element [Figure 4]. The OmniBER now looks intelligently at the request it receives from the network element and generates an appropriate response. This lets the network element know whether it can use a particular channel or not and hence avoids the network element going into oscillatory mode. It also independently verifies the network element algorithm. Again uni-directional and bidirectional networks behave slightly differently. Uni-directional networks: The response (step 3 of Figure 4) is simply a confirmation that the switch has been completed. Bi-directional networks: The response (step 3 of Figure 4) is both a confirmation and a request to switch the backward reporting channel. The network element is then in no doubt about which channel can be used avoiding going into the undesirable oscillatory mode. And it independently verifies the algorithm. Figure 4 Testing automatic protection switching 5

6 Summary To meet the ever increasing demands of testing today s network element s ability to survive, the OmniBER now provides Active APS and can still detect signal degrade, measure service disruption time, and generate and display APS messages. Active APS allows the OmniBER to fully simulate the role of a network element during automatic protection switching. It does this by generating the K1/K2 switching protocols and responding intelligently to the signals received from the network element. By testing in this way, you ll have greater confidence that your equipment can withstand the rigors of real network operation. You can also be confident that it complies with Telcordia and ITU-T recommendations. 6

7 Related Literature The complete global tester to 2.5 Gb/s (OmniBER 718) E OmniBER analyzer family complete 2.5 Gb/s test solutions EN OmniBER 718 technical specification EN OmniBER 718 configuration guide EN OmniBER product notes: OmniBER communications performance analyzer Offers a modular, upgradeable onebox solution for installation, commissioning, and field maintenance. This rugged, portable tester allows comprehensive functional testing of SONET/SDH, PDH, ATM and POS equipment including jitter generation and test. A 3.5-inch disk drive eases results retrieval, test firmware upgrades, and analysis. Protection-switching time measurements: understanding the OmniBER analyzer s measurement technique and the alternative test methods EN Measuring service disruption times in high-speed ATM networks E Physical layer jitter testing in an ATM environment E Verifying the policing functions in the ATM network E ATM perfomance testing E 7

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