Physical DS0 Limitation

Physical DS0 limitation is a design methodology that limits the number of physical DS0 connections into the gateway. This limitation, in conjunction with other queuing methods, ensures that the gateway can successfully provide IP bandwidth across the WAN for each voice conversation originating from the individual DS0 trunks.

If you have determined that there is 158.4 kbps of WAN bandwidth available to handle 6 simultaneous G.729 calls, for example, DS0 limitations can be implemented by allowing only 6 DS0 connections between the PBX and the originating gateway. These 6 DS0 connections can be in the form of time slots on a digital T1/E1 trunk or individual analog connections, such as FXS, FXO, and E&M trunks.

Figure 8-6 shows a network using physical DS0 limitation to provide CAC.

This CAC design method works well in a situation where there is a TDM-to-IP gateway; however, it not effective in an IP telephony environment where a TDM-to-IP conversion does not exist on the WAN router. Calls originated from IP Phones are presented to the WAN router as an IP stream, without a physical TDM interface to provide a DS0 limitation. Another CAC mechanism must be used to solve this issue. Figure 8-7 demonstrates this concept.

Figure 8-6 VoIP Physical DS0 Limitation

Figure 8-6 VoIP Physical DS0 Limitation

Figure 8-7 IP Telephony PhysicalDS0 Limitation

Catalyst Switch

There are no physical TDM interfaces available in an IP telephony environment to provide DS0 Limitation CAC.

CallManager Publisher

Catalyst Switch

1 —1—-J 1

fjTJ 1

.J —

v IH

rl

CallManager Subscriber

CallManager Subscriber

There are no physical TDM interfaces available in an IP telephony environment to provide DS0 Limitation CAC.

Remote Site

Voice Gateway

Voice Gateway

Catalyst

Switch

Phone A

Remote

Phone B

Remote

Phone C

Restricting physical DS0s on the gateway offers the following advantages:

• Adds no extra CPU usage on the gateway or bandwidth overhead to the network

• Predominant CAC mechanism deployed in toll-bypass networks today

• Protects the quality of voice conversations on the WAN link by limiting the number of voice conversations that are allowed

• Offers a known maximum bandwidth consumption rate based on the total number of possible simultaneous calls

The physical DS0 CAC method has the following limitations:

• Not effective for IP telephony applications

• Limited to relatively simple topologies

• Does not react to link failures or changing network conditions

Table 8-5 evaluates the physical DS0 limitation mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-5 DS0 Limitation CAC Evaluation Criteria

Table 8-5 evaluates the physical DS0 limitation mechanism against the CAC evaluation criteria described earlier in this chapter.

Table 8-5 DS0 Limitation CAC Evaluation Criteria

Evaluation Criteria

Value

VoX supported

Independent of the VoX technology used

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 DSO/trunk (per call)

Topology awareness

None

Guarantees QoS for duration of call

None

Postdial delay

None

Messaging network overhead

None

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