Call Throttling and the Code Yellow State
This chapter provides information about Call throttling which
Cisco Unified Communications Manager to automatically throttle (deny) new call
attempts when it determines that various factors, such as heavy call activity,
low CPU availability to
Cisco Unified Communications Manager, routing loops, disk I/O limitations, disk
fragmentation or other such events, could result in a potential delay to dial
tone (the interval users experience from going off hook until they receive dial
Call Throttling Feature
Call throttling occurs automatically when
Cisco Unified Communications Manager determines such conditions to be
present, and the system exits throttling automatically when such conditions are
alleviated. You can configure the parameters that are associated with entering
and exiting call throttling through several service parameters in
Cisco Unified Communications Manager Administration ( ) although Cisco does not advise modification of these parameters
unless recommended by Cisco customer support. See topics related to service parameter configuration
Cisco Unified Communications Manager Administration Guide for information
on accessing and configuring service parameters.
Cisco Unified Communications Manager uses the values
that are specified in the call-throttling-related parameters to evaluate the
possibility of a delay to dial tone and also to determine when conditions no
longer necessitate call throttling. When throttling is necessary to prevent
excessive delay to dial tone,
Cisco Unified Communications Manager enters a Code Yellow state, and new
call attempts are throttled (denied). You can disable call throttling via the
System Throttle Sample Size service parameter, but Cisco does not recommend
disabling call throttling. The following list defines several of the call
throttling-related service parameters:
Code Yellow Entry Latency defines the maximum allowable delay, in
milliseconds, to handle SDL messages that are sent to
Cisco Unified Communications Manager by the various devices in the system
as well as the wealth of internal messages that are received and sent by
Cisco Unified Communications Manager for various activities such as
KeepAlives, change notification, and many more types of internal messaging. If
the calculated average expected delay is more than the value that is specified
in this service parameter,
Cisco Unified Communications Manager enters a Code Yellow state to initiate
call throttling, and stops accepting new calls.
Code Yellow Exit Latency Calculation determines the acceptable
percentage of Code Yellow Entry Latency to specify exit criteria for leaving
the Code Yellow state (Code Yellow exit latency) when
Cisco Unified Communications Manager has initiated call throttling. The
basis for the value that you specify in this parameter comprises a formula that
uses the value in the Code Yellow Entry Latency parameter, which specifies the
delay in milliseconds. To arrive at a percentage, use the following formula:
Code Yellow Entry Latency value multiplied by the Code Yellow Exit Latency
value. For example:
Code Yellow Entry Latency service parameter value: 20 msec
Code Yellow Exit Latency service parameter value: 40%
Code Yellow Exit Latency value = 20 X 0.4 = 8 msec, which means
Cisco Unified Communications Manager exits Code Yellow state if the
calculated message latency drops to 8 msec or lower.
To get out of the Code Yellow state,
Cisco Unified Communications Manager ensures that the average expected
delay is less than the value of the Code Yellow exit latency.
Code Yellow Duration specifies the number of minutes that a
Cisco Unified Communications Manager system can remain in a Code Yellow
state (call throttling). If this duration is met and the system is still in
Code Yellow state,
Cisco Unified Communications Manager enters a Code Red state, which
Cisco Unified Communications Manager has remained in a Code Yellow state
for an extended period and cannot recover. When
Cisco Unified Communications Manager enters a Code Red state, the Cisco
CallManager service restarts, which also produces a memory dump that may be
helpful for analyzing the failure.
System Throttle Sample Size indicates the size of the sample, in
seconds, that is used to calculate the average expected delay for
Cisco Unified Communications Manager to handle an SDL message. For example,
a sample size of 10 means that
Cisco Unified Communications Manager must calculate a non-zero latency
value for 10 consecutive seconds before it will calculate the average expected
delay and compare it to the value in the CodeYellow Entry Latency parameter.
You can disable call throttling via this parameter.
When delay to dial tone is calculated to be over the
threshold that is configured in the call-throttling-related service parameters,
Cisco Unified Communications Manager begins rejecting new calls. When call
throttling is engaged, a user who attempts a new call will receive reorder tone
and, depending on the phone model, may also receive a prompt on the phone
display. Call throttling effectively avoids the problem in which a user tries
to place a new call, but the length of delay between going off-hook and
receiving dial tone is excessive enough to cause a reaction in the user (such
as complaining to the system administrator or questioning whether the system is
down or the phone is broken, for example).
Cisco Unified Communications Manager uses a complex algorithm to constantly
monitor the system to anticipate when such latency could occur.
When the delay to dial tone is within the guidelines of the
call-throttling-related service parameters,
Cisco Unified Communications Manager ceases throttling calls by exiting the
Code Yellow state and new calls events are again allowed.
Troubleshooting Call Throttling
CCM/SDI and SDL trace files record call throttling events and can provide useful information. Also, you generally will require performance monitoring data for debugging. The Cisco CallManager System Performance object (viewable in the Real Time Monitoring Tool) includes a counter called ThrottlingSampleActivity, which indicates whether Cisco Unified Communications Manager has calculated a non-zero value for latency and helps you understand how busy the system is. Frequent non-zero values in this counter could indicate a potential overload condition on the system. To try to circumvent the possibility of a Code Yellow event, consider the possible causes of a system overload, such as heavy call activity, low CPU availability to Cisco Unified Communications Manager, routing loops, disk I/O limitations, disk fragmentation or other such events, and begin to investigate those possibilities.
Generally, repeated call throttling events require assistance from the Cisco Technical Assistance Center (TAC). TAC will likely request these trace files for closer examination.