{"id":534,"date":"2013-09-28T19:14:04","date_gmt":"2013-09-28T16:14:04","guid":{"rendered":"https:\/\/telcocloudbridge.com\/?p=534"},"modified":"2013-09-28T19:14:04","modified_gmt":"2013-09-28T16:14:04","slug":"what-your-dwdm-vendor-did-not-tell-you-about-latency-for-data-center-interconnect","status":"publish","type":"post","link":"https:\/\/telecomlighthouse.com\/?p=534","title":{"rendered":"What your DWDM vendor did not tell you about latency for your Data Center Interconnect"},"content":{"rendered":"

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\"DWDM<\/a>Are you concerned about\u00a0 latency in your data center links running\u00a0 over optical network ?<\/p>\n

Or, are you planning for Data Center Interconnectivity in near future and want to know\u00a0 the impact of delays involved. The read on, for some useful tips to know your options.<\/p>\n

Chances are, you are already using DWDM to connect data centers<\/a> but there are few things that you should be\u00a0aware of as an operator regarding\u00a0the sources of delays in your optical links in order to either mitigate them or at least know how to properly dimension your network for delay. As they say , NOT all DWDM systems are equal when it comes to latency in them so it would help to know what are the choices available when comparing DWDM systems.<\/p>\n

In this era of Cloud networks, it would be naive\u00a0to ignore the impact of delay in optical networks that run clouds. Impact of delay for financial markets that do High Frequency Transactions ( HFT) can be considerable\u00a0and far reaching; fraction of millisecond can impact revenue and as per one estimate,these fractional delays can contribute to difference of revenues over the year as high as 100 Million USD.<\/p>\n

Lets see, what are these factors that cause delay and what measures can be taken to deal with them. Perhaps your DWDM vendor did not tell them to you, yet.<\/p>\n

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1. Fiber Delay<\/strong><\/h3>\n

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Delay Impact = 5 micro seconds \/ KM<\/p>\n

Speed of light in vacuum is 299,792,458 meters \/ seconds. This equates to 3.33 mirco seconds\/KM. Since optical fiber has different refractive index compared to vacuum therefore\u00a0 latency is more and equals 5 microseconds\/ KM.<\/p>\n

So if there is a 100 KM of fiber between two network elements the delay would be 500 micro seconds ( This does not include delay of the network elements themselves)<\/p>\n

Not surprisingly, fiber delay\u00a0 is the biggest culprit and contributes to the major delay\u00a0 in optical network\u00a0 and it would impact no matter which DWDM system you use. Long fiber link causes more delay than shorter fiber link.Little\u00a0 can be done to mitigate the latency owing to fiber except selecting shortest route if it is an option.If there are two diversity routes available between two stations, operator is advised\u00a0to use the shortest route as the primary path and use the longer one as secondary or backup path; if\u00a0 new locations for data centers are being planned, the\u00a0 locations can be strategically selected in order to\u00a0have minimum latency owing to fiber itself. While calculating latency coming from fiber, optical lengths should be considered and not geographical length. Operators should have good record of fiber and optical distances<\/a> to dimension the delay properly. A good OTDR trace is very handy to know the optical length of the route.<\/p>\n

Recommendation:<\/strong><\/em><\/h4>\n

wherever possible use shorter distance for connectivity. Avoid using longer routes.<\/p>\n

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2. Transponding\/Muxponding including FEC<\/strong><\/h3>\n

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Delay Impact = 10 micro seconds to few hundred micro seconds ( depending on model and make)<\/p>\n

Also called “color conversion”. There are several processes involved in transponding\/muxponding. Some of them are \u2022Data encapsulation \u2022Forward Error Correction ( FEC) \u2022Performance Monitoring<\/p>\n

Of these forward error correction is perhaps the biggest culprit. Delay due to it ranges from 15 to 100 microsecond.<\/p>\n

A word about FEC:<\/p>\n

Low OSNR\u00a0is directly related to poor BER performance<\/a> in DWDM networks. FEC or Forward error connection is a method used to achieve\u00a0coding gain for higher bit rates. It is a method of encoding optical signal with extra error detection and correction overhead bytes enabling optical receivers to detect errors and correct them. Thus FEC can reduce\u00a0BER and effectively increases distances reachable by high speed signals without regeneration.<\/p>\n

Forwards Error Correction is a must have feature when going on longer distances ( several hundred kilometers)\u00a0 but it does not bring considerable value at shorted distances ( metro links). There are some models of transponders\/muxponders that can switch off the FEC option . There are other models specially designed without FEC option and optimized for low latency. Some vendors are offering transponders with “No-FEC” option for shorter distances that can give ultra low latency of few nano seconds. They claim that they can carry\u00a0 signal in its native form without processing and\u00a0 recommend them for data center connectivity.<\/p>\n

Recommendation:<\/strong><\/em><\/h4>\n

Whenever possible use\u00a0 latency optimized transponder\/muxponder from your vendor. Check with the vendor if it provides any such model. For green field deployment,do make this a benchmark for evaluating, which vendors to go with by comparing all variants of transponders\/muxponders, the vendors have to offer. Not all vendors provide these kind of variants. If possible, go without FEC for shorter distances.<\/p>\n

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3. Native Optical<\/strong><\/h3>\n

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Always remember a simple rule of thumb : The lesser\u00a0 the processing of an optical signal, the better it is for latency. Said in another way, the lower we are in the OSI layers the better it is for latency.<\/p>\n

So when it comes to latency<\/p>\n