Occasionally, you must route calls between POTS dial peers. CAC might cause this, such as when a call from a PBX is denied admission to the WAN and sent back to the PBX. Or a call from an analog phone might be routed over the PSTN. The ability to switch calls between POTS dial peers is enabled by default on Cisco gateways.
Many of the CAC mechanisms that are described in this chapter use hairpinning, or tromboning, in which a call both enters and exits on the same interface. Suppose that a router doing toll bypass receives an analog call from its PBX. This call is matched to an incoming POTS dial peer. Typically the analog call is terminated, regenerated as a VoIP call, and then routed out a VoIP dial peer. However, the CAC configuration of the gateway might cause the call to be rejected for sending over the IP network. When a call is rejected, you can redirect (or hairpin) it back to the PBX. Both legs of the call thus traverse the same router interface. The PBX then looks for an alternate route, such as out to the PSTN.
A hairpinned POTS call is terminated on the gateway and then routed out another POTS dial peer as a new call leg. When this happens, DSPs are assigned to the incoming leg of the POTS call and also to the outgoing (hairpinned) leg. These DSP resources stay assigned for the duration of the call; thus, each hairpinned call doubles the DSP usage.
Routing between ISDN POTS dial peers can use TDM switching, but this must be done across a bus that supports it. With TDM switching, DSP resources are assigned when the call is received, but media DSPs are dropped after the call is switched to another POTS port. No DSP is needed for the media because an internal TDM connection is made between the incoming and outgoing ports on the TDM bus. This is an advantage with nonvoice calls, such as modem, fax, and video.
Older routers, such as the 1700, 2600, 3600, and 3700 series, support TDM switching only between ports on certain network modules. Both legs of the call must stay on the same module. Cisco Integrated Services Routers (ISR) support TDM switching across the backplane; thus, POTS-to-POTS calls can be routed between different network modules. Other Cisco gateways, such as the Access Server (AS) and Integrated Access Device (IAD) lines, also support TDM switching across their backplane, and thus, can route POTS calls between interfaces. These are typically used in a service provider environment.
Modules that support intramodule TDM switching include these:
ISRs allow intermodule TDM call routing between the following modules:
You must synchronize clocking when you are switching between these ports and modules. Chapter 7, "Connecting to PBXs," describes how to do this.
Part I: Voice Gateways and Gatekeepers
Gateways and Gatekeepers
Part II: Gateways
Media Gateway Control Protocol
Session Initiation Protocol
Connecting to the PSTN
Connecting to PBXs
Connecting to an IP WAN
Influencing Path Selection
Configuring Class of Restrictions
SRST and MGCP Gateway Fallback
Using Tcl Scripts and VoiceXML
Part III: Gatekeepers
Part IV: IP-to-IP Gateways
Cisco Multiservice IP-to-IP Gateway
Appendix A. Answers to Chapter-Ending Review Questions