Bit Rate Calculation

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 configured first in RAW UDP and then in RTP.

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)

Audio Format	Audio Data Rate (Kbit/sec)	Packet/sec	Overhead (bit)	Total Bit Rate (Kbit/sec)
PCM 16 bit 8 KHz, mon	128	13.551	524	135.1
PCM 16 bit 24 KHz, mono	384	42.213	524	406.12
uLaw 8 KHz, mono	64	6.7	524	67.5
ALaw 8 KHz, mono	64	6.7	524	67.5
ALaw 24KHz, mono	192	20.28	524	202.62
uLaw 24 KHz, mono	192	20.28	524	202.62
MPEG1 48KHz quality 0, stereo	88	16.46	524	96.62
MPEG1 48KHz quality 4, stereo	144	22.56	524	155.82
MPEG1 48KHz quality 7	192	27.81	524	206.57
MPEG1 48KHz quality 4, mono	96	16.08	524	104.42
MPEG1 44.1KHz, quality 0, mono	65	12.27	524	71.43
MPEG1 44.1KHz, quality 4, mono	90	15.76	524	98.25
MPEG1 44.1KHz, quality 7, mono	140	25	524	153.1
MPEG2 16KHz quality 0, mono	28	5.8	524	31.03
MPEG2 16KHz quality 4, mono	44	7.58	524	47.97
MPEG2 16KHz quality 7	64	11.5	524	70

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 = 620 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)

Audio Format	Audio Data Rate (Kbit/sec)	Total Bit Rate (Kbit/sec)
ALaw 8 KHz, mono	64	95
MPEG1 48KHz, quality 0, stereo	88	113.8
MPEG1 48KHz, quality 4, stereo	144	169.85
MPEG1 48KHz, quality 7, stereo	192	217.85
MPEG1 48KHz, quality 4, mono	96	121.85
MPEG1 44.1KHz, quality 0, mono	65	88.7
MPEG1 44.1KHz, quality 4, mono	90	113.7
MPEG1 44.1KHz, quality 7, mono	140	163.7
MPEG2 16KHz, quality 0, mono	28	45.2
MPEG2 16KHz, quality 4, mono	44	61.2
MPEG2 16KHz, quality 7, mono	64	81.2