AudioCodes Mediant 4000 SBC, Руководство пользователя

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CHAPTER 24 Configuring Call Admission Control
Mediant 4000 SBC | User's Manual
24 Configuring Call Admission Control
You can implement Call Admission Control (CAC) to regulate the volume of voice traffic handled by
the device.
CAC configuration is done using two tables with parent-child type relationship:
■ Call Admission Control Profile table: This is the parent table, which defines a name for the
CAC profile.
■ Call Admission Control Rule table: This is the child table, which defines the actual CAC rules
for the profile.
You can configure up to 625 CAC profiles and up to 625 CAC rules. In addition, a CAC profile can
be configured with up to 8 CAC rules.
Once you have configured a CAC profile with CAC rules, you need to assign it to any of the fol-
lowing SIP configuration entities (using the 'CAC Profile' parameter):
■ IP Group (see Configuring IP Groups on page 346)
■ SIP Interface (see Configuring SIP Interfaces on page 334)
■ SRD (see Configuring SRDs on page 324)
CAC rules define the maximum number of allowed concurrent calls (SIP dialog-initiating requests)
for the assigned SIP entity (listed above) and per registered user belonging to the SIP entity. This
can also include the maximum number of allowed concurrent SIP dialogs per second (
rate ). The
CAC rule can be defined for a specific SIP message type (e.g., only INVITEs) as well as for a
specific call direction (e.g., only outbound calls).
The CAC feature supports SIP-dialog rate control using the token-bucket mechanism. Token
bucket is a control mechanism that determines the rate of SIP dialog processing based on the
presence of tokens in the bucket. Tokens in the bucket are removed ("cashed in") for the ability to
process each dialog. If there are no tokens, the device rejects the dialog request with a SIP 480
(Temporarily Unavailable). Configuration of the token-bucket mechanism involves the following:
■ Configuring the number of tokens that are added to the bucket per second. This is referred to as
rate . To process (allow) a SIP dialog, the device needs a token from the bucket.
■ Configuring the maximum number of tokens that the bucket can hold and thus, the maximum
number of tokens that can be used for processing SIP dialogs that are received at one time.
This is referred to as
burst .
For example, assume that the rate is configured to 1 and the burst to 4:
■ One token is added to the bucket every second.
■ The maximum number of tokens that the bucket can hold is four.
■ If SIP dialogs have never been received by the device, the bucket is filled to its maximum,
which is four tokens (i.e., burst), regardless of the number of seconds that have passed.
■ If four SIP dialogs are received at the same time (i.e., burst), the device uses the four tokens to
process the dialogs. The bucket is now left with no tokens at that given moment, but after a
second, a new token is added to the bucket (due to the rate). If there are no calls for the next
three seconds, the bucket fills up again to four tokens (and no more).
■ If the bucket contains four tokens (i.e., full) and five SIP dialogs are received at the same time,
the device uses the four tokens to process four of the dialogs and rejects one.
■ If the bucket has one token and SIP dialogs are then received every second, the device uses
the token to process the first dialog, adds a token to the bucket after a second and processes
the second dialog, and so on.
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