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# How to calculate the service air-interface bandwidth

Issue Description
If the Microwave RTN 900 product transmits Ethernet services, the service air-interface bandwidth is not equal to the port (FE/GE) bandwidth.

Alarm Information
Null

Handling Process
To correctly plan the air-interface channel spacing and modulation mode, learn the following from the wireless department:
(1) The number of base stations mounted to RTN equipment
(2) The port bandwidth of each base station, normally, the peak bandwidth
(3) Length of service packets, and the port bandwidth for each type of packet length

The following calculation formula can be used:
Service air-interface bandwidth = [(L + 8 + En. Len)/(L+20)] x Service port bandwidth
L is service packet length; 8 is frame encapsulation at air interface; 20 is Ethernet frame interval and preamble; En. Len is service encapsulation on RTN equipment. If the service is transmitted over a QinQ link, the encapsulation overhead is 4. The number of base stations, L and service port bandwidth are provided by the wireless department.

For example, four base stations are mounted to the RTN 950. The service port bandwidths of each base station are 6 Mbit/s (128 bytes) and 16 Mbit/s (256 bytes). The upstream air-interface bandwidth of the RTN 950 is
[(128 + 8 + 0)/(128 + 20)] x 6 x 4 + [(256 + 8 + 0)/(256 + 20)] x 16 x 4 = 83.27 Mbit/s

Root Cause
The port bandwidth transmits services containing frame intervals and preambles, and the service air-interface bandwidth transmits services containing frame encapsulation, service encapsulation, and air-interface compression. Therefore, for Ethernet services, the air-interface bandwidth is not simply equal to the port bandwidth.

Suggestions
Before the planning and caculating, we have to get enough information from wireless department or customer.

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