In MSTP+ networks. There are two kinds of services, traditional TDM services and Packet services. According to ITU standard, the switching time of protection must be less than 50 ms for TDM. The customer want to ensure that the switching time is less than 50 ms for both TDM and packet services. Because of the packet service is asynchronous and use buffers, we cannot calculate the accurate time of switching. this case describe a method how to ensure the switching time is less than 50 ms and show a example using JDSU SmartClass Ethernet.
1) Configure JDSU SmartClass (specify the frame length and the load bit rate)
2) calculate or locate the tolerable packet loss:
* its recommended to use values from table (image2)
* else, use this formula:
(0.05*bit rate (Mbps)*1000000)/ (Frame length(byte)*8)
3) cute the active fiber, and check the packet loss with JDSU.
4) Check if the packet loss is less than the calculated threshold
JDSU SmartClass Ethernet can calculate the number of packet loss. the idea is configure the analyzer to send a fixed length frames (specified by user) with a fixed bit rate (specified by user). After forcing the switching, the analyzer will detect errors and it will calculate the number of packet loss. in this case, the switching time will less than:
Tmax= (Frame length (byte)*Number of packet loss*8)/load bit rate (bps)
You can refer to the photo_1 to understand the source of formulas.
In fact, we need just to be sure that the switching time is less than 50 ms. so it's enough to know the maximum tolerable packet loss. So the engineer will have a table (image_2) which contains the tolerable packet loss according to frame length and bit rate. And he will just check if the packet loss calculate by JDSU is less than this value (value in the table).
In order enhance the accuracy. It’s better to use a short frame with a high bit rate. So it's recommended to set the frame length to 64 byte, and load bit rate to 1000 Mbps.