A hairpin call occurs when a standard inbound telephony call is simply routed back out another telephony interface on the same voice gateway. A VoIP component is not present for this sort of call. This type of call is also commonly referred to as a POTS-to-POTS call, TDM switching, or a TDM hairpin call.
A scenario involving a hairpin call is illustrated in Figure 7-4. In this figure, a Cisco voice gateway is connected to the PSTN by a digital T1 or E1 circuit. Voice calls are routed via VoIP to Unified CM, whereas fax calls are "hairpinned" from the PSTN interface to another T1/E1 digital interface on the gateway, which connects directly to a fax server.
Figure 7-4 Hairpin Call
Regular VoIP (Non-Hairpin) Call
Regular VoIP (Non-Hairpin) Call
The most important aspect of any hairpin call on a Cisco voice gateway is whether the DSP can be dropped from the call after it is established. If the DSP can be dropped out of the call, a TDM connection through the voice gateway occurs, and this is the ideal scenario. All the bits transmitted between these ports are unaltered by the voice gateway. In Figure 7-4, a hairpin call with the DSP dropped is equivalent to the fax server being connected directly to the PSTN without the gateway present.
In some cases, because of hardware restrictions or user configuration, DSPs must remain involved for the call duration. With DSP involvement, the bits will always be altered to some extent as the DSP processes the call.
With fax calls, this DSP involvement can be plainly seen by running the command debug fax relay t30 all-level-1. You will see that by default T.38 fax relay occurs between the DSPs handling the hairpin call. This does not necessarily result in problems, but bypassing the DSP is the better option when it is possible. Naturally, when the DSP is bypassed, these debugs will not be present because the DSP has been removed from the call path.
A pure TDM hairpin call in its simplest form occurs within a single module slot of a voice gateway. This intraslot TDM hairpin can occur on digital or analog voice ports and is typically dependent on the module installed in the slot. For example, on ISR voice gateways such as the 2800 and 3800 series, a two-port FXS card (VIC2-2FXS) inserted into an HWIC slot on the motherboard module slot 0 will automatically perform TDM hairpins between the two FXS ports.
The other type of "DSP-less" TDM hairpin calls occur between module slots on a Cisco voice gateway. An interslot hairpin call requires that the voice gateway contain a TDM backplane to link the module slots and that the modules themselves participate in the timing that is occurring across this backplane.
Although numerous voice card and voice module combinations are possible when it comes to TDM hairpin calls, a few basic rules apply when planning for TDM hairpin calls on Cisco voice gateways:
• Both analog and digital voice ports support TDM hairpin calls. In addition, the two ports involved in a hairpin call do not have to match from an analog and digital perspective. You can have one port be an analog port and the other be a digital port during a TDM hairpin call.
• The command local-bypass is enabled by default, and it controls the TDM hairpin call feature for a particular module slot on a Cisco voice gateway. The negation of this command, no local-bypass, forces the DSP to be involved for all hairpinned calls involving this module slot. This command is configured under the voice-card submenu.
• Performing TDM hairpin calls across module slots (interslot) requires that the gateway have a TDM backplane, such as the 2800 and 3800 series of Cisco voice gateways. Other gateways without a TDM backplane are only capable of intraslot TDM hairpin calls. These gateways can pass calls between slots, but they will not be in a true TDM fashion, and the DSP will be involved.
• When performing interslot TDM hairpin calls, the DSP types must be the same. You cannot have C549 DSPs attached to one voice port and C5510 DSPs being used by the other voice port. For example, the NM-HDV module uses C549 DSPs and is capable of interslot TDM hairpin calls only with another NM-HDV module. Hairpin calls between an NM-HDV and an NM-HDV2 or other C5510-based module require that DSPs be involved.
• Both module slots in an interslot TDM hairpin call must be part of the gateway's TDM backplane clocking scheme. This is accomplished using the network-clock-participate command. If a module slot is not tied to the clocking used on the TDM backplane, the DSP must stay involved with the transmission, and it cannot drop out.
• Notable modules that do not support intraslot or interslot TDM hairpin calls are the older NM-1V, NM-2V, and NM-HDA.
You should always strive for TDM hairpin calls where the DSP is dropped from the call to ensure the best call success rate. However, for situations where a TDM hairpin call is not possible, a hairpin call with DSP involvement and T.38 fax relay between the DSPs should suffice.
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