17
Frame size (L2 octets)
L1 capacity
(Mbit/s)
L2 capacity
(Mbit/s)
64
1000
761,9
1500
1000
986,8
8000
1000
997,5
Table 2. GbE interface L2 capacity also depends on the frame size.
If the average frame size were only 64 octets, there would be a problem fitting the 2+0 maximum capacity
at 56 MHz and 512QAM into a single GbE interface. This is because the total L2 speed is 2 x 394 = 788M,
which would need over 1 Gbit/s at interface L1 speed (2 x 517 = 1034M). In other words two modems
could send more packets than a single GbE interface can handle.
In practise the average packet size is always much larger than 64 octets, perhaps 500-1000 octets, and then
the GbE interface can handle all the packets delivered by two modems.
The compression can become an interpretation problem when measuring the link capacity with the
smallest frame size. If the capacity is defined using the smallest frames only, that capacity cannot be
achieved with real traffic and a larger average frame size. This may cause SLA problems between the
operator and the end customer. It is recommended that the capacity is defined and measured using the
largest possible frames which will remove the L1/L2 difference. Then the real capacity achievable is always
slightly larger than the measured one.
Table 3 shows iPasolink radio capacities with each available modulation and channel spacing. This value is
practically identical with the L1 and L2 capacity when the average frame size is 1500 octets or larger.
(1024QAM and 2048QAM are preliminary values).
Radio capacity (Mbit/s)
Modulation
Channel spacing
7MHz
14MHz
28MHz
56MHz
QPSK
10
22
45
91
16QAM
22
45
91
183
32QAM
27
56
113
228
64QAM
33
67
136
274
128QAM
39
79
159
320
256QAM
45
90
182
366
512QAM
-
-
205
412
(1024QAM)
-
-
(228)
(458)
(2048QAM)
-
-
(251)
(504)
Table 3. iPasolink radio capacity.