RAD Data comm Vmux-2100 operation manual Calculating Approximate Bandwidth Utilization

Chapter 1 Introduction Vmux-2100Installation and Operation Manual

Bandwidth Utilization

By using TDMoIP multiplexing and the voice activity detection, Vmux-2100 supports a higher number of voice channels over TDM than it is possible by utilizing conventional compression methods alone. TDMoIP multiplexing and grouping the timeslots of compressed voice together into bundles with a common IP address reduces the actual bandwidth used per channel to as low as 4 kbps (16:1), when all channels are active. Better compression, up to 20:1, is achieved when some of the voice channels are idle.

The actual bandwidth utilization is determined by the following factors:

•Header sizes:

Ethernet – 18 bytes

IP – 20 bytes

UDP – 8 bytes

TDMoIP – 4 bytes

VLAN (if exists) – 4 bytes.

•Size of the voice packet:

G.723 – 24 bytes + AAL2 header (3 bytes) + voice payload (4 bytes)

G.729 – 10 bytes + AAL2 header (3 bytes) + voice payload (4 bytes).

•Packet interval:

G.723.1 – 30 msec

G.729 – 10 msec.

•Number of timeslots in a bundle

•Silence percentage. Studies show that an average person speaks only 40% of the time during a telephone conversation. 50% of the time is spent listening to the other party, while the remaining 10% is spent quietly contemplating.

•Connectivity packets – 64 bytes per minute.

Calculating Approximate Bandwidth Utilization

Let us calculate an approximate bandwidth for 30 timeslots in one bundle with G.723.1 (6.4 kbps) compression, 60 % of silence:

[(50 ⋅ 1000/40) + (30 ⋅ 31 ⋅ 1000/30 ⋅ 0.4)] ⋅ 8	+ 0.00853 = 109.208 kbps
1000
where:

•50 – size of Ethernet, IP, UDP, TDMoIP headers

•1000/40 – packets per second (pps) transmission rate, calculated according to the packetizing interval chosen (40 msec in this example)

•30 – number of timeslots

•31 – size of the compressed G.732 packet plus AAL2 header and voice payload

•0.4 – 60% of silence

•8 – conversion from bytes to bits

•1000 in the denominator – conversion from bits to kilobits

•0.00853 – connectivity packets rate (64 bytes per minute) converted to kbps.

1-10 Functional Description