[VoIP] Channel Banks

[Russ Price]

Jayson Smith wrote:
> When getting a channel bank, do you have to
> get one with either all FXS or all FXO, or can you mix and match within a
> single channel bank, E.G. sixteen FXS and eight FXO? What types of channel
> banks will work, and are there any that won't work? There's a bunch of
> untested channel banks that don't give much info about them on eBay, some
> for $50 or less. Are these most likely unpopulated channel banks, with no
> FXS or FXO ports at all? Sorry for all my newbie questions, I'm new to all
> this! Unlike most of you, I haven't spent my life working at a phone
> company, or collecting old telephone bits to build an SXS switch, etc.

I haven't worked for a phone company, or built an SXS switch, either. 
*grin*  If I had been born 25 years earlier, I probably would have gone 
to work for Ma Bell, though...

A T1 circuit has 24 channels.  Depending on the channel bank, these 
could be FXS, FXO, a mixture of them, or various other types of 
interfaces such as E&M or ISDN.

As an example, the Adtran 750 has six slots that can be used for ports, 
usually with quad FXS or quad FXO cards, which can be mixed and matched. 
  Other cards are also available, like an Nx56/64 (think old-school 
Dataphone), an E&M trunk interface, or a U-BR1TE (ISDN BRI interface).

Some Adtran 750-specific recommendations:
1. Get a unit that has been tested, and CHECK THE SELLER'S FEEDBACK.  I 
got burned with one unit - as received, the BCU (Bank Control Unit, 
basically the brains of the channel bank) was fried - in the words of a 
tech support guy at Adtran, "smoked like a cheap cigar."  The seller 
offered to exchange it, but never sent anything in return after I sent 
the bad BCU back.  So now I have a spare chassis and six spare FXS 
cards, along with a battery box/120 VAC power supply combo (predictably, 
the batteries were as dead as disco and went to a recycler).
2. FXO ports are scarcer than hen's teeth, and channel banks that have 
FXOs installed tend to cost a lot.  If you want FXO ports, be prepared 
to go to a reseller like NetLink <http://www.netlinkweb.com/> and get a 
factory refurbished unit.
3. Some units have a battery box with an integrated AC power supply, 
others have just a "cube" (actually rectangular) that bolts onto the 
side of the unit.  If it has a battery box, it's *very* heavy, due to 
four 12V sealed lead-acid batteries inside.  Chances are, the batteries 
will be junk. The "cube" power supply can be hooked to a 48 VDC battery 
string and act as a charger.  Unless you really need battery backup, get 
the cube - the battery box has an alarm relay that will continuously 
tick if the batteries are bad or missing.
4. The top of the unit has a retaining bar with a Phillips-head captive 
screw that prevents the cards from being yanked out.  The BCU (second 
card from the left) has two female DB-9 ports.  The one on the left is 
an RS-232 admin port, which can be used with a terminal emulator and a 
9-pin straight-through cable to access the BCU's control and line 
provisioning functions.  The one on the right is a port to connect a DS0 
test set.  Do not confuse the two!
5. The T1 card for the computer will cost far more than the second-hand 
channel bank.

	Russ
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