I help maintain a network with over 40 locations, each one with a leased T1 line connected via a Cisco 1841 router with an integrated T1 DSU/CSU card. The routers are very reliable but occasionally the T1 card will fail, whereas we then have to overnight a new T1 card to the location. I understand the T1 card is the first point of contact with the outside world (lightning, power outage from our service provider could damage the card).

My question is this: should these T1 cards be failing so easily when the rest of the router is completely fine each time?

  • I always preferred external CSU / DSU. What does Cisco say?
    – dbasnett
    Oct 8 '10 at 12:36
  • Who determines that it fails? Define 'occasionally'. What model WIC is this?
    – Aaron
    Aug 1 '11 at 15:16
  • How can you replace an integrated card?
    – joeqwerty
    Aug 1 '11 at 23:17

In short, no, your T1/DS1 WIC should not be failing "so easily." While just about all (I am not a Ph.D EE so I cannot confidently say "all") semiconductors experience degradation, decay, and aging -- mostly in the dielectric, top-end ASIC and device manufacturers (read: Cisco's enterprise lines) have very high safety margins and lifetimes.

If you are going through T1/DS1 WIC's "so easily" I would look to other causes. See below.

Background -- Skip Ahead if you like

It is rather rare that your DS1 comes off the street and goes straight into your WIC. Generally, the incumbent local exchange carrier (ILEC) (read: last mile copper owner) will terminate your DS1 to a "smart jack" in a shelf (metal box with cards in it). This "smart jack" serves as the physical point of demarcation for the DS1. It can be used to isolate facilities/provider problems from customer premises equipment (CPE) problems.

Most modern metropolitan T1/DS1 circuits are delivered from the central office (CO) to the smart jack as HDSL2/4 -- riding on a single pair. At the smart jack the HDSL turns into traditional B8ZS, ESF two pair DS1 (there are variations). HDSL line voltage can (but shouldn't) approach 200VDC. Traditional T1/DS1 is generally under 3.3VDC. Quite a difference.

If you skipped ahead -- start here

Make sure that the shelf in which the smart jacks live is properly grounded with minimum 6AWG wire. If the shelf has been there a while -- in a high traffic communal telco room there exists the possibility of the grounding wire being bumped or removed.

The smart jacks can build up a lot of extra charge and if there is not a proper ground on the smart jack/shelf it can either damage the smart jack itself or the T1/DS1 WIC behind it.

I have been witness to a handful of smart jacks and T1/DS1 Cisco WIC's failing several times on account of improper grounding of the smart jack/shelf.

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