Max Connections

Similar to physical DS0 limitation, Max-Connections uses the concept of limiting the number of simultaneous calls to help protect the quality of voice conversations. Unlike physical DS0 limitation, the max-conn command is a physical gateway configuration applied on a per-dial peer basis.

The first advantage that Max-Connections offers over DS0 limitation is the capability to provision for the oversubscription of TDM interfaces on an originating gateway without compromising the quality of the voice conversations being carried over the WAN. Figure 8-8 illustrates a T1 PRI connection from the PSTN and a T1 PRI connection from a PBX. Because a T1 PRI connection has the capability of supporting 23 channels, it is theoretically possible to have 46

simultaneous voice conversations on the host site router that are attempting to traverse the WAN to reach endpoints in the remote site.

Figure 8-8 Max-Connections

Figure 8-8 Max-Connections

In this example, nine concurrent calls can be supported. Assuming that the data requirement for the IP WAN circuit is 256 kbps, and the codec in use is G.729 at 50 pps without the use of VAD or header compression, the maximum number of calls that can be successfully supported and protected across the 512-kbps WAN link is 9, as shown in the following:

((Total circuit bandwidth) - (Total data requirements)) XT , . . , ,,

—-——-—r—--—---— = Number of protected calls

(Bandwidth per call)

NOTE

This calculation does not take into account the bandwidth required for routing updates. Instead, this calculation shows the theoretical maximum number of calls that can traverse this link assuming no packets other than the listed data requirements are present on the link.

If the theoretical maximum of 46 calls over the 2 PRIs is attempted, voice quality for each conversation will suffer greatly, because the current bandwidth of the WAN can provide for a total of 9 simultaneous calls without voice-quality degradation. Example 8-1 shows how you can configure the max-conn command on the host site router to limit the number of simultaneous calls on the VoIP dial peer to 9. For this example, assume that all endpoints in the remote site have a directory number consisting of 4 digits, and each directory number begins with the number 12 followed by 2 additional digits.

Example 8-1 Max-Connections Host Site Router

dial-peer voice 100 voip

!Sets the maximum number of

connections (active admission control).

max-conn 9

destination-pattern 12..

ip precedence 5

session target ipv4:10.1.1

2

Assume that all endpoints in the host site have a directory number consisting of 4 digits and each directory number begins with the number 5 followed by 3 additional digits. Example 8-2 shows how you can configure the max-conn command on the remote site router to limit the number of simultaneous calls on the VoIP dial peer to 9.

Example 8-2 Max-Connections Remote Site dial-peer voice 100 voip

!Sets the maximum number of connections (active admission control), max-conn 9

destination-pattern 5...

ip precedence 5

session target ipv4:10.1.1.1

The second advantage that Max-Connections offers over DS0 limitation is the capability to limit the number of calls that will be allowed to traverse an IP WAN between multiple sites. Because the max-conn command is applied on a per-dial peer basis, a clear understanding of the call volume desired and the available bandwidth between sites must be achieved. To limit the total number of aggregate voice conversations allowed to traverse the WAN link, the max-conn command must exist on each VoIP dial peer. The aggregate of these configurations must not exceed the bandwidth provisioned for the call volume.

Suppose, for example, that the two additional remote sites are added to the preceding example, each with the same data requirements of 256 kbps. In this scenario, 9 simultaneous calls can be protected from each remote site, as shown in the following:

((Total circuit bandwidth) - (Total data requirements)) XT , . . , ,,

—-——-—r—--—---— = Number of protected calls

(Bandwidth per call)

The total circuit bandwidth for the host site is increased to a full T1 to handle the two additional sites, as shown in the following:

((Total circuit bandwidth) - (Total data requirements)) / (Bandwidth per call) = Number of protected calls

(1536 kbps - (3 * 256 kbps)) / 26.4 kbps = 29 calls

NOTE These calculations do not take into account the bandwidth required for routing updates. Instead, this calculation shows the theoretical maximum number of calls that can traverse these links assuming no packets other than the listed data requirements are present on the link.

Figure 8-9 illustrates this multiple-site converged network.

Suppose that endpoints in all remote sites have a directory number consisting of 4 digits, and each directory number at Remote Site 1 begins with the number 12 followed by 2 additional digits. Directory numbers at Remote Site 2 begin with the number 13 followed by 2 additional digits. Directory numbers at Remote Site 3 begin with the number 14 followed by 2 additional digits. Example 8-3 shows how you can configure the max-conn command on the host site router to limit the number of simultaneous calls per location on the VoIP dial peer to 9 with an aggregate maximum of 27 calls allowed across the IP WAN.

Example 8-3 Max-Connections Host Site Router per Site

dial-peer voice 1 voip

! 9 calls allowed on VoIP

Dial

Peer

to

Remote

Site

1

max-conn 9

destination-pattern 12..

ip precedence 5

session target ipv4:10.1

!

1.2

dial-peer voice 2 voip

! 9 calls allowed on VoIP

Dial

Peer

to

Remote

Site

2

max-conn 9

destination-pattern 13..

ip precedence 5

session target ipv4:10.1

!

2.2

dial-peer voice 3 voip

! 9 calls allowed on VoIP

Dial

Peer

to

Remote

Site

1

max-conn 9

destination-pattern 14..

ip precedence 5

session target ipv4:10.1

3.2

Figure 8-9 Max-Connections Multi-Site

Figure 8-9 Max-Connections Multi-Site

Assume that all endpoints in the host site have a directory number consisting of 4 digits, and each directory number begins with the number 5 followed by 3 additional digits. Example 8-4 shows how you can configure the max-conn command each remote site router to limit the number of simultaneous calls on the VoIP dial peer to 9, with an aggregate maximum of 27 calls allowed across the IP WAN.

Example 8-4 Max-Connections Remote Site 1

dial-peer voice 1 voip

!VoIP Dial Peer from Remote Site 1 to Host Site max-conn 9

destination-pattern 5...

ip precedence 5

session target ipv4:10.1.1.1

Example 8-5 shows the configuration of the max-conn command at Remote Site 2.

Example 8-5 Max-Connections Remote Site 2

dial-peer voice 1 voip

!VoIP Dial Peer from Remote Site 2 to Host Site max-conn 9

destination-pattern 5...

ip precedence 5

session target ipv4:10.1.2.1

Example 8-6 shows the configuration of the max-conn command at Remote Site 3.

Example 8-6 Max-Connections Remote Site 3

dial-peer voice 1 voip

!VoIP Dial Peer from Remote Site 3 to Host Site max-conn 9

destination-pattern 5...

ip precedence 5

session target ipv4:10.1.3.1

After the maximum number of calls specified by the max-conn command has been reached, another mechanism must be used to connect the call via an alternate route. This is achieved by configuring a second dial peer with the same destination pattern, but with a higher preference. Remember that the dial peer with the lowest preference, that can route the call, will be matched.

Example 8-7 shows the configuration of an alternate path using the max-conn command. In this example, dial-peer voice 1 voip is defined with a preference of 1 and dial-peer voice 100 pots is defined with a preference of 2. This indicates that dial peer 1 is preferred over dial peer 100. The two dial peers share the same destination-pattern, meaning that they will both match any dialed digits beginning with 12; however, they will attempt to connect the call using different paths. Dial peer 1 will attempt to connect the call over the IP network sending the dialed digits, whereas dial peer 100 will prefix the digits 91404555 to the dialed digits and attempt to connect the call using the PSTN. Dial peer 1 will connect calls until the number of active calls reaches the configured max-conn of 9. When this maximum has been reached, dial peer 1 can no longer connect calls. At this point, dial peer 100 will begin to connect calls using the alternate path to the PSTN.

Example 8-7 Max-Connections Alternate Path

dial-peer voice 1 voip

IDefines first priority for

call routing.

preference 1

!Sets the maximum number of

connections (active admission control).

max-conn 9

destination-pattern 12..

ip precedence 5

session target ipv4:10.1.1

!

2

dial-peer voice 100 pots

!Defines second priority for call routing, preference 2 destination-pattern 12.. direct-inward-dial port 0:D

!Adds prefix 91404555 in front of the called number before sending the digits to the PSTN prefix 91404555

!Defines second priority for call routing, preference 2 destination-pattern 12.. direct-inward-dial port 0:D

!Adds prefix 91404555 in front of the called number before sending the digits to the PSTN prefix 91404555

Max-Connections also offer the capability to limit the number of calls allowed on a POTS dial peer by making the value of the max-conn command for that POTS dial peer lower than the physical number of time slots that are available on a T1/E1 connection between the PSTN or a PBX and an originating gateway.

Although the Max-Connections feature is useful in many scenarios, it has the following drawbacks:

• Although it provides some protection for the voice gateway egress WAN link, it provides little or no protection for links in the network backbone.

• It does not work for IP telephony applications that do not use dial peers.

• It is limited to simple topologies.

• It does not react to link failures or changing network conditions.

Table 8-6 evaluates the Max-Connections mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-6 Max-Connections CAC Evaluation Criteria

Evaluation Criteria

Value

VoX supported

All VoX that use dial peers

Toll bypass or IP telephony

Toll bypass only

Platforms and releases

All voice gateways and all Cisco IOS releases

PBX trunk types supported

All

End to end, local, or IP cloud

Local

Per call, interface, or endpoint

Per dial peer

Topology awareness

None

Guarantees QoS for duration of call

None

Postdial delay

None

Messaging network overhead

None

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