Cisco Cisco 1800 2800 3800 Series AP-AG 802.11a b g High-Speed WIC
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“Bandwidth remaining ratio” provides a method of balancing the minimum bit rates that traffic classes will
receive when there is congestion. The algorithm works as follows in 3-level policies:
“Bandwidth remaining” equals the 1
st
level shaped rate minus the 2
nd
level strict priority rate. For
example, if the top level shaped rate is 1.5mbps and the strict priority class rate is 100kbps,
“bandwidth remaining” is 1.4mbps.
“Bandwidth remaining ratio” defines the poriton of “bandwidth remaining” for a traffic class, and
is defined in the CS3/AF3x/CS4/AF4x traffic class AND the class-default; Note that if the
“bandwidth remaining ratio” is not configured on a class in the 2
nd
level policy (e.g. class-default)
the default value is 1, which may not be yield the desired result.
Continuing the example above, if bandwidth remaining is 1.4mbps, and a class “CS3-4” is
defined with “bandwidth remaining ratio 4” and “class-default” is defined with “bandwidth
remaining ratio 10”, class “CS3-4” will be provided a minimum of 400kbps while class “class-
default” will be provided a minimum of 1mbps. In this example, assuming the service rate was
500kbps and a voice class (strict priority) was assigned 100kbps, this is sufficient, as the “CS3-4”
class would be shaped regardless at 400kbps (so the total of the CS5/EF + CS4/AF3x/CS4/AF4x
classes do not exceed the service rate, to avoid the network dropping the excess). The sample
configuration for ISR43xx/44xx uses “bandwidth remaining ratio”.
-
Voice Considerations: To support voice, the bandwidth needed to support the maximum number of
simultaneous calls must be calculated. This bandwidth need is the value for the strict priority bit rate
defined in the 2
nd
level policy’s voice traffic class. As voice packets are small in size, packet overhead
has a greater effect on the bandwidth required for a voice call. To determine the total bandwidth needed
in the strict priority queue, the amount of bandwidth per call is multipled by the maximum number of
simultaneous calls.
The bandwidth per call is determined by taking the codec type and IP overhead into account. Two
popular codecs are G.711 and G.729. The bandwidth required for various scenarios is shows below.
Codec
LAN
LTE, MPN +
DMNR
LTE, MPN +
DMNR + GETVPN
LTE, MPN +
DMVPN
LTE, MPN +
IPsec
G.711
87 kbps
96 kbps
120 kbps
131 kbps
125 kbps
G.729
31 kbps
41 kbps
65 kbps
76 kbps
70 kbps
These estimated values are based on test results,60 byte overhead for GETVPN, the IPsec overhead
calculator at
https://cway.cisco.com/tools/ipsec-overhead-calc/ipsec-overhead-calc.html
and values
from
http://www.cisco.com/c/en/us/support/docs/voice/voice-quality/7934-bwidth-consume.html
As an example, if 3 simultaneous G.729 calls over MPN with DMNR are desired, 3 x 41kbps or 123 kbps
would be set in the 2
nd
level policy map for the strict priority class (class “CS5-EF” in the examples that
follow). Call admission control would define this site as a “location” and limit the voice traffic to 125kbps,
the phones at this site wold be in a device pool assigned to a region that supported only G.729.
More information on Unified Communications Call Admission Control can be found here:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab11/collab11/cac.html