Special notes on the infamous "All 1's condition"
When an upstream data source is not functioning, the DACs t-1 switch
that recieves the "empty" signal will generate all 1's to the
customer. The customer gear will show an alarm indication and AIS alarm will show up on the router. The t-1 will appear to be down to the end customer; no data can be transferred across the line.
Execute a show controller t1 or show service-module command depending on the type of t1 card in the cisco router. In the case where you have multiple t1s coming into a router, make sure to read the controller or interface number so you can match up the correct t1. Cisco routers number the first controller or interface at ZERO not ONE always.
Output will look similar to the below; in this case 4 t1 interfaces exist, and the 4th - service module serial 0/3/0 is having the AIS:
Interface Serial0/3/0 Module type is T1/fractional Hardware revision is 1.2, Software revision is 20080312, Image checksum is 0x4144A7, Protocol revision is 0.1 Transmitter is sending AIS. Receiver has no alarms.
The Transmitter is sending AIS is the key. This tells us that the t1 local loop between the telco switch and the customer prem is working. The problem is NOT on the local loop, but is instead in the network - somewhere upstream.
Why AIS is important
Often telco techs will "test" only the local loop part of the t-1, from the customer prem to the first telco DACs. This is because the most common part of a t1 circuit to have a problem is the local loop. These local loop tests will pass of course, because in AIS condition the local loop itself is fine. Once the local loop has been tested, an inexperienced tech will push the problem back on the "customer equipment" to avoid further work/expense on the part of the telco. Pushing the support team to escalate the issue is often easier when you mention AIS as it forces the techs to start to look at their switching paths end to end instead of focusing on you (the customer) having a "dead router".
How does AIS get generated
The upstream DACS, Frame-Relay or MPLS Switch, SMDS Hub, or ATM switch port is down, misconfigured, or disconnected from the network (partitioned)
and not sending signal from the upstream network to the local
Central Office or customer. As soon as the port is swapped, or
re-enabled, all 1's will clear, and layer 2 will come up. Some CSUs
can read the bits coming in off the line. If you have such a CSU, be
on the lookout for all 1s, as this means the repair call should be
directed farther up the food chain than the local central office.
Always ook for the AIS alarm in the service module or show controller output. This is
fairly common, so be sure you can identify AIS quickly.
From the Cisco Web Site:
AIS: alarm indication signal. In a T1 transmission, an all-ones signal
transmitted in lieu of the normal signal to maintain transmission
continuity and to indicate to the receiving terminal that there is a
transmission fault that is located either at, or upstream from, the
If you are receiving an AIS alarm on the router csu this means that the
last mile is o.k. (demarc, smart jack, router csu/dsu are all ok), but the
problem is somewhere upstream (Local Bell Central Office, MCI, etc.)
AIS Alarms are sometimes caused by telco grooming
Telco DACS troubleshooting
Situation Presents as:
1. layer 1 is clean
2. Telco can loop smart jack, and they can loop the router WIC or controller built-in T1 CSU
3. but they cannot do a payload loopback (individual channels loopback)
Problem: Telco has a DACs assignment issue.
Sometimes a t1 loop must pass through multiple DACS at multiple
Central Offices at multiple companies (Verizon, ATT, Qwest etc). If one of
the DACS is setup to map the wrong channels, then these symptoms will
result. Telco tech must be encouraged to check *all* DACS.
Notes below are specific to MCI/Worldcom now Verizon frame/t1 networks
Normally T1 and fractional T1 services run on an aggregate channelized ds3 when the
service is 384K and greater. If the service is less than 256K, it runs on
standard t1 service. 256K service can be configured either way. Techs
will sometimes have to move a circuit termination point from one switch to
another if spare port capacity on the DACS is not available when an upgrade
is being performed.
Grooming teams will come behind install and upgrade jobs to clean pvcs,
and relocate circuits.