Difference between revisions of "Bit Rate Calculation"
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Revision as of 08:08, 24 March 2009
BIT RATE CALCULATION WITH BARIX DEVICES
With this document we want to explain how to calculate the total bit rate flowing on an Ethernet cable connected to a Barix device.
The total bit rate is equal at the audio bit rate plus the overhead due to the encapsulation of audio packet inside the layers of OSI stack [1].
We have used for the test an Instreamer 100 (with standard firmware) configured first in RAW UDP and then in RTP, and an Exstreamer 1000 (with STL firmware).
RAW UDP STREAMING MODE
For the structure of the Ethernet packet see here: [2]
The Total Ethernet Overhead per packet is 300 bit.
For the structure of the IP packet see here: [3]
The Total IP Overhead per packet is 160 bit.
For the structure of the UDP packet see here: [4]
The Total UDP Overhead per packet is 64 bit.
Total Bit of Overhead per Packet = Ethernet Overhead + IP Overhead + UDP Overhead = 524 bit
The Audio Bit rate is expressed in bit/sec but the Overhead is expressed in bit/packet.
So we have to translate the Overhead (bit/packet) in Overhead rate (bit/sec).
For this we need to know the amount of packet/sec, that depends by Audio Format and is measurable with Network Protocol Analyzer software like Wireshark.
The values of Audio Data for audio format, sampling frequency and quality (MP3 bitrate average), come from Instreamer Manual (firmware V3.10 – page 18 - [5])
Overhead/sec = Overhead/Packet x Packet/sec
Total Bit Rate (Kbit/sec) = Audio Data Rate (Kbit/sec) + Overhead/sec (Kbit/sec)
(Kbit = 1000 bit)
RTP STREAMING MODE
For the structure of the RTP packet see here: [6]
The Total RTP Overhead per packet is 128 bit (counting also the MP3 MPA wrapper inside the RTP payload, which has an extra 32 bit header).
Total Bit of Overhead per Packet =
Ethernet Overhead + IP Overhead + UDP Overhead + RTP Overhead = 652 bit
The values of Audio Data for audio format, are the same of above, since the transmission protocol doesn't affect audio bit rate.
Overhead/sec = Overhead/Packet x Packet/sec
Total Bit Rate (Kbit/sec) = Audio Data Rate (Kbit/sec) + Overhead/sec (Kbit/sec)
(Kbit = 1000 bit)
Without Ethernet Overhead
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