10.3. Load Calculation 
The following bit rate calculations only take one direction into account.
The route back in the opposite direction requires the same bit rate.
If a “shared medium“ is used for both directions, i.e. both directions
are routed via the same line, then both directions also have to be included
in the calculation. This is the case with 10Base5, 10Base2 and 10BaseT
or 100BaseT half-duplex.
The Ethernet Media Access Layer (MAC) is realized pursuant to the
IEEE 802.3 / DIX Ethernet II Standard with MAC-Type 0800 (IP) in the
case of the IPDA components. The alternative pursuant to IEEE 802.3 and
802.2 LLC/SNAP is not customary and would require greater bandwidth.
The calculations have been performed with active VLAN Tagging pursuant
to IEEE 802.1q. This yields the higher network load. Without VLAN Tagging,
the packet size is reduced by 4 octets (bytes).
IMPORTANT:
The bit rate calculation used in this manual differs
from that used in previous versions. By popular demand, the bit rate
is no longer specified on the Physical Layer (PHY), but instead on the
Media Access Layer (MAC). The 8 bytes for the preamble are no longer
included in the calculation.
Payload transport with RTP (Realtime Transmission Protocol)
Table 51. Load calculation for an RTP connection (VoIP)
|
Codec
type
|
Sample
size
[ms]
|
Payload
[Octets]
|
Ethernet
packet size
[Octets]
|
Ratio
Overhead / Payload
|
Ethernet load
No preamble
[kbps]
|
Ratio
RTP / ISDN
bit rate
|
|
G.711
|
20
|
160
|
222
|
39%
|
88.8
|
139%
|
|
G.711
|
30
|
240
|
302
|
26%
|
80.5
|
126%
|
|
G.711
|
60
|
480
|
542
|
13%
|
72.3
|
113%
|
|
G.729A
|
20
|
20
|
82
|
310%
|
32.8
|
51%
|
|
G.729A
|
40
|
40
|
102
|
155%
|
20.4
|
32%
|
|
G.729A
|
60
|
60
|
122
|
103%
|
16.3
|
25%
|
The G.711 codec generates a sample every 125 µs. The G.729A codec
generates a 10-octet frame every 10 ms.
The overhead calculation is based on the following:
Table 52. Overhead with RTP connections
|
Protocol
|
Overhead
|
|
RTP
|
12
|
|
UDP
|
8
|
|
IP
|
20
|
|
IEEE 802.1Q VLAN Tagging
|
4
|
|
Ethernet MAC (DA,SA,TYPE,FCS - no preamble)
|
18
|
|
Total
|
62
|
The transmission bit rate depends not only on the codec type used
and the sample size, but also on the stack layer on which the bit rate
is calculated. Details on configuring the RTP payload can be found in
Section 3.3,
“Calculation Basis - Configuring Payload Packets”.
In this document, the “worst case“ on the physical layer is always
considered, i.e. the Ethernet MAC frame including FCS with activated
VLAN tagging. The bit rates on higher stack layers are also important
when calculating the load following implementation on other media. The
table below provides an overview. The values are specified in Kbps and
apply to an RTP connection in one direction:
Table 53. Load for an RTP connection [Kbps]- by stack layer
|
Stack layer
(Protocol)
|
Codec type / Sample size
|
|||||
|
G.711
|
G.729
|
|||||
|
20 ms
|
30 ms
|
60 ms
|
20 ms
|
30 ms
|
60 ms
|
|
|
RTP
|
68.8
|
67.2
|
65.6
|
12.8
|
10.4
|
9.6
|
|
UDP
|
72.0
|
69.3
|
66.7
|
16.0
|
12.0
|
10.7
|
|
IP
|
80.0
|
74.7
|
69.3
|
24.0
|
16.0
|
13.3
|
|
Ethernet
No preamble
No
VLAN TAG
|
84.0
|
79.5
|
71.7
|
31.2
|
19.6
|
15.7
|
|
Ethernet
No preamble
With
VLAN TAG
|
85.6
|
80.5
|
72.3
|
32.8
|
20.4
|
16.3
|
|
Ethernet
With preamble
No
VLAN TAG
|
87.2
|
81.6
|
72.8
|
34.4
|
21.2
|
16.8
|
|
Ethernet
With preamble
With
VLAN TAG
|
88.8
|
82.7
|
73.3
|
36.0
|
22.0
|
17.3
|
RTP load during inactivity of VAD or DMC master connections
If “idle“ is detected when Voice Activity Detection has been enabled,
audio signals will not be transmitted temporarily. Nonetheless, the load
on the RTP connection does not drop completely to zero because data continues
to be transmitted on the RTP connection to check the connection and (depending
on the codec) to transmit the background noise. This data is referred
to as SID frames (Silence Insertion Descriptor).
The RTCP connection is not affected by VAD.
Table 54. Load on an RTP connection [kbps] with VAD during inactivity
|
Stack Layer
(Protocol)
|
Codec Type
|
|
|
G.711
|
G.729
|
|
|
RTP
|
1.8
|
1.1
|
|
UDP
|
2.5
|
1.8
|
|
IP
|
4.1
|
3.4
|
|
Ethernet, No preamble, No VLAN TAG
|
4.9
|
4.2
|
|
Ethernet, No preamble, With VLAN TAG
|
5.2
|
4.5
|
|
Ethernet, With preamble, No VLAN TAG
|
5.5
|
4.8
|
|
Ethernet, With preamble, With VLAN TAG
|
5.8
|
5.1
|
The payload connection is controlled via the parallel RTCP connection (Realtime Transmission Control Protocol)
A Sender Report message is sent by each relevant connection
partner every five seconds for every RTP connection. No Receiver Reports
are sent. This produces a load of 0.2 Kbps in each direction. The packet
size was once again calculated with VLAN tag and preamble.
Table 55. Load calculation for an RTCP connection
|
Report type
|
Report interval
[s]
|
Average Ethernet packet size
[Octets]
|
Ethernet load excl. preamble
[Kbps]
|
|
Sender report
|
5
|
130
|
0.2
|