So you’ve decided you want to be a Cisco voice engineer, but you cannot figure out why people keep talking to you about analog. As a voice engineer, analog is like the monster in every horror film – it will always find you and it will never die. Since it’s not going away anytime soon, let’s examine some often overlooked aspects when planning for analog lines.
An analog inbound PSTN connection at a business is, for all intents and purposes, like what you have at your home, should any of you still maintain that dinosaur of a landline. When connecting this type of circuit at a business you will typically need an FXO card inserted into a Cisco voice gateway for the connection, but before you get all gung ho and start trying to plug this analog circuit in, a little planning needs to go into the process. We’ll discuss two specifics that are often glossed over in this analog planning but can be serious show stoppers when doing a PSTN line implementation.
The first detail most engineers fail to take into account is confirming that the gateway can actually support the FXO card itself. This confirmation is easy enough, but it does require a little work on the part of the engineer, so I assume that’s why it’s often overlooked.
First, a “Show inventory” on the router should be done. The output will look something like this:
SAMPLEGATEWAYI#show inventory NAME: "CISCO2901/K9 chassis", DESCR: "CISCO2901/K9 chassis" PID: CISCO2901/K9 , VID: V01 , SN: NAME: "3rd generation four port FXS DID voice interface daughtercard on Slot 0 SubSlot 0", DESCR: "3rd generation four port FXS DID voice interface daughtercard" PID: VIC3-4FXS/DID , VID: V02 , SN: NAME: "VWIC2-2MFT-T1/E1 - 2-Port RJ-48 Multiflex Trunk - T1/E1 on Slot 0 SubSlot 1", DESCR: "VWIC2-2MFT-T1/E1 - 2-Port RJ-48 Multiflex Trunk - T1/E1" PID: VWIC2-2MFT-T1/E1 , VID: V01 , SN: NAME: "VWIC2-1MFT-T1/E1 - 1-Port RJ-48 Multiflex Trunk - T1/E1 on Slot 0 SubSlot 2", DESCR: "VWIC2-1MFT-T1/E1 - 1-Port RJ-48 Multiflex Trunk - T1/E1" PID: VWIC2-1MFT-T1/E1 , VID: V01 , SN: NAME: "PVDM3 DSP DIMM with 192 Channels on Slot 0 SubSlot 4", DESCR: "PVDM3 DSP DIMM with 192 Channels" PID: PVDM3-192 , VID: V01 , SN: NAME: "C1941/C2901 AC Power Supply", DESCR: "C1941/C2901 AC Power Supply" PID: PWR-1941-2901-AC , VID: , SN:
Of all this lovely command line output, the part you should be concerned with is the line about PVDMs. PVDMs, and the DSPs they contain, are integral to making VoIP magic happen. When a router boots, it sees the FXO card, as well as any other modules inserted, and tries to allocate DSPs. If you don’t have any or enough DSPs, your module(s) will call you dirty names and refuse to function. OK, that first result may not be entirely accurate, but the last one certainly is.
If you do happen to install an FXO card into a router that does not allocate enough DSPs, all the lights on the FXO card will stay amber – a sign that the router is crying on the inside – and you won’t see the voice ports show up in the running configuration. What I mean is that typically when you do a “show run” on a gateway with a working FXO module, you will see something like this:
SAMPLEGATEWAY#show run | sec voice-port voice-port 0/0/0 voice-port 0/0/1 voice-port 0/0/2 voice-port 0/0/3
However, when you install a FXO module into a router without the required DSPs, you won’t see these voice-port statements. They just aren’t there – the router is punishing you for your lack of preparedness and it holds quite the grudge. You will find you will be unable to configure any of the voice ports for the module and that your particular voice project is going nowhere fast. Do note, however, that even if you are in this state and were to issue a “show inventory” as seen previously, the FXO module would show up as a module installed in the router, just not a very usable installed module.
Once you have your PVDM count, the next step you need to do is peruse your running configuration for what is commonly referred to as media resource configurations. You are basically looking to see how much of your PVDMs you’ve already allocated to other voice stuff. The command I like best for this and a sampling of the type of output it produces is seen below:
SAMPLEGATEWAY#show run | sec dsp dsp services dspfarm dspfarm profile 1 transcode codec g729r8 codec g722-64 codec g711ulaw maximum sessions 30 associate application SCCP dspfarm profile 2 conference codec g729r8 codec g711ulaw maximum conference-participants 32 maximum sessions 1 associate application SCCP
From the output, what you are really interested in is the max-sessions for each media resource that has been defined. Armed with this information and the PVDM count, it’s finally time to head over to the Cisco DSP calculator to crunch some numbers. While a tutorial on how to use the DSP calculator is out of scope here, just be sure to use “estimate on the high end” as a guiding rule of thumb when entering details of your configuration.
Once you confirm you have the necessary DSPs to run your FXO module, you can check your voice gateway’s hardware compatibility matrix to be sure you order an FXO card compatible with your router. Please don’t skip this step because, as was previously alluded to, routers are picky and I promise they will not look kindly upon an incompatible FXO module.
The second and also often overlooked aspect of FXO planning is inbound call capacity planning. FXO lines, like other analog devices, can only handle one call at a time. This call can be setup either inbound or outbound, but only one call is active on the circuit at any given time. If your business needs to be able to have four simultaneous calls at once, you better order four lines from the carrier and a FXO card with at least four ports to plug the connections into.
Also, if you decide you do need four simultaneous calls, it’s not always enough to just order four analog lines from a carrier, because it’s voice and it’s never *that* easy. The way companies typically like to do business is to publish a single number that the outside world calls, not four different numbers. However, when putting in a new circuit(s), calls will not automagically roll to the next FXO line ordered without some intervention from the carrier.
In these cases, the carrier sets up what is usually called a hunt. This hunt is what allows the second and third and so on calls to the primary number to roll over to the next FXO lines. This is not something carriers do unless you ask, so be sure to specify a hunt setup when ordering the PSTN lines. As a word to the wise, be absolutely certain to confirm this behavior after the circuit is in. Just because you ordered it one way doesn’t mean it will get deployed per specifications. You are dealing with carriers after all.
Now that we’ve covered the two most commonly overlooked details in FXO planning, you can be certain once you get your FXO module installed in your router, that you will be ready to configure voice ports and receive inbound calls just as the business intended.