pitter patter on the keyboard

I recently started getting complaints from a client running VoIP services about lag and jitter. I wasn’t able to correlate the issue with any regular indicators and I have a number of people using VoIP services as a client on the same network who haven’t reported any issues. Looking closer at the router I noticed certain daemons lagging despite a low load average and I could see dmesg was flooded with this message:

ip_ct_ras: decoding error: out of bound

There isn’t exactly a heap of documentation on this situation online but I was able to infer it had something to do with H.323. Doing an lsmod I found:

ip_nat_h323
ip_conntrack_h323

I know some VoIP setups have to do a little magic to get around conventional NAT but since this network is completely public I assumed it was safe to rmmod these two modules and things seemed to clear up instantly. I’m still not sure how H.323 connection tracking got involved with good-ol-fashioned DMZ public IP routing and would appreciate any insights you VoIP geniuses could provide – especially since the client who complained is using SIP and not H.323.

I wasn’t going to write about the Meridian I adopted way back again because I planned on cleaning it up and selling the system. It turns out I have had better things to do. One night recently I decided to reward myself for being productive by dicking around with it for a bit and managed to get it working on a VoIP line by way of an ATA. I was so impressed with the quality of the audio I decided to keep the system for personal use. There is a subtle irony to having a three metre analogue bridge between two perfectly digital systems and all this fire-retardant 1990s-beige/grey plastic is getting to my head.

The POTS interface is generally terminated with a 25-pair BIX (Building Industry Cross-connect) block. These require a punch-down tool. My only punch-down tool at the moment has automatic snippers on one end which is useless as the position of the terminal teeth alternates from top to bottom.

I jammed the pairs in with the attached blade tool which is strongly discouraged as you invariably weaken the teeth and risk cracking the (especially ancient) plastic.

I, however, am a rebel.

The KSU had been unplugged some time and defaulted. Interestingly, the dialing mode defaults to pulse which, intuitively enough, doesn’t work with my ATA. It turns out you have to go in and set each line to tone dialing individually.

m7324: I rode this phone into Germany during WWII and rescued some POWs.

It took me forever to find out how to do this so pay close attention, I’m only going to remember this once:

• Punch FEATURE **CONFIG
  • **CONFIG is **266344
• The password should be CONFIG,  if lost will have to be reset.
• Press the top-rightmost indicated meta key, the display will read:
  • 1. Trk/Line Data
• Press the top-rightmost indicated meta key again. The display will read:
  • Show line: _
• This prompt expects three digits. To configure line one, press 001. The display will read:
  • Trunk data
• Press the top-rightmost indicated meta key once then the bottom-rightmost twice. The display will read:
  • Dial Mode: Pulse
• The rightmost display key will read
  • CHANGE
• Press the CHANGE display key and it will toggle between Pulse and Tone.
• Press Rls to exit the menu

Above you can see the data and software cartridges for the M8x24; one fits into the other which fits into the cabinet. If your KSU loses all of its settings when the power goes out you need to replace the backup capacitors mounted on the data board. They are 1 farad and 5.5 volts each.

These appear to be a very common capacitor configuration for data backup and shouldn’t be hard to find at a reasonable price.

Something neat I learned in my travels is that when these 24V phones are subjected to the ring voltage on a POTS line (90V in NA?) they tend to blow. What they may lack in ruggedness they more than make up for in ease of installation however, as their all-digital signalling makes their ports polarity agnostic.