Implementing T1 CAS for VoIP

CAS is the transmission of signaling information within the voice channel. In addition to receiving and placing calls, CAS also processes the receipt of DNIS and ANI information, which is used to support authentication and other functions.

Various types of CAS are available in the T1 world. The most common forms are loop-start, ground-start, Equal Access North American (EANA), and E&M.

The biggest disadvantage of CAS is its use of user bandwidth to perform signaling functions. CAS is often referred to as robbed-bit-signaling because user bandwidth is "robbed" by the network for other purposes.

Service-provider application for T1 CAS includes connectivity to the public network using T1 CAS from the Cisco router to the end-office switch. In this configuration, the router captures dialed-number or called-party-number information and passes it to the upper-level applications for IVR script selection, modem pooling, and other applications. Service providers also require access to ANI for user identification, billing account number, and, in the future, more complicated call routing.

Service providers who implement VoIP include traditional voice carriers, new voice and data carriers, and existing internet service providers. Some of these service providers might use subscriber-side lines for VoIP connectivity to the PSTN; others use tandem-type service-provider connections.

New CAS functionality for VoIP includes all CAS and E1/R2 signaling already supported for supported Cisco platforms in data applications, with the addition of dialed-number and calling-party-number capture whenever available.

This feature supports the following T1 CAS systems for VoIP applications:

• E&M--E&M robbed-bit signaling is typically used for trunks. It is generally the only way that a CO switch can provide two-way dialing with direct inward dialing. In all E&M protocols, off-hook is indicated by A=B=1 and on-hook is indicated by A=B=0. For dial-pulse dialing, the A and B bits are pulsed to indicate the addressing digits. There are several further important subclasses of E&M robbed-bit signaling:
  • EandM Wink Start--Feature Group B

In the original Wink Start protocol, the terminating side responds to an off-hook from the originating side with a short wink (transition from on-hook to off-hook and back again). This wink indicates that the terminating side is ready to receive addressing digits. After receiving digits, the terminating side goes off-hook for the duration of the call. The originating side maintains off-hook for the duration of the call.

• • E&M Wink Start--Feature Group D

In Feature Group D Wink Start with Wink Acknowledge Protocol, the terminating side responds to an off-hook from the originating side with a short wink just as in the original Wink Start. After receiving digits, the terminating side provides another wink (called an acknowledgment wink) to indicate that the terminating side has received the digits. The terminating side goes off-hook to indicate connection when the ultimate called endpoint has answered. The originating side maintains off-hook for the duration of the call.

• • E&M Immediate Start

In the Immediate Start Protocol, the originating side does not wait for a wink before sending addressing digits. After receiving digits, the terminating side goes off-hook for the duration of the call. The originating side maintains off-hook for the duration of the call.

• Ground Start/FXS--Ground Start Signaling was developed to help resolve glare when two sides of the connection tried to go off-hook at the same time. This is a problem with loop start because the only way to indicate an incoming call from the network to the customer premises equipment (CPE) using loop start was to ring the phone. The six-second ring cycle left a lot of time for glare to occur. Ground Start Signaling eliminates this problem by providing an immediate-seizure indication from the network to the CPE. This indication tells the CPE that a particular channel has an incoming call on it. Ground Start Signaling differs from E&M because the A and B bits do not track each other (that is, A is not necessarily equal to B). When the CO delivers a call, it seizes a channel (goes off-hook) by setting A to 0. The CO equipment also simulates ringing by toggling the B bit. The terminating equipment goes off-hook when it is ready to answer the call. Digits are usually not delivered for incoming calls.

To configure T1 CAS for use with VoIP, perform the following steps.

Note	The following shows how to configure the voice ports as ds0-group for channelized T1 lines.

1.    enable

2.    configure terminal

3.    controller {t1| e1} slot / port

4.    framing type

5.    linecode type

6.    ds0-group group-number timeslots range type type {dtmf| mf} {ani| dnis| ani-dnis}

7.    Repeat steps 4 to 6 for each additional controller (there are 12). Be sure to increment the controller number and ds0-group number.

8.    dial-peer voice tag type

9.    dial-peer voice tag type

10.    Repeat steps 8 and 9 for each dial peer.

11.    exit

--

1.    debug cas

2.    show controllers t1 | e1 dial-shelf / slot / port

3.    show isdn status

4.    show running-config

5.    show voice port

Router# show debug 
Router# debug cas slot 1 port 0 
CAS debugging is on 
Router# 
debug-cas is on at slot(1) dsx1(0) 
Router# show debug 
CAS debugging is on

Router# p 1.1.1.2
Type escape sequence to abort. 
Sending 5, 100-byte ICMP Echos to 1.1.1.2, timeout is 2 seconds:
*Mar 2 00:17:45: dsx1_alloc_cas_channel: channel 0 dsx1_timeslot 
1(0/0): TX SEIZURE (ABCD=0001)(0/0): RX SEIZURE_ACK (ABCD=1101)(0/1): 
RX_IDLE (ABCD=1001)(0/2): RX_IDLE (ABCD=1001)(0/3): RX_IDLE 
(ABCD=1001)(0/4): RX_IDLE (ABCD=1001)(0/5): RX_IDLE (ABCD=1001)(0/6): 
RX_IDLE (ABCD=1001)(0/7): RX_IDLE (ABCD=1001)(0/8): RX_IDLE 
(ABCD=1001)(0/9): RX_IDLE (ABCD=1001)(0/10): RX_IDLE (ABCD=1001)(0/11): 
RX_IDLE (ABCD=1001)(0/12): RX_IDLE (ABCD=1001)(0/13): RX_IDLE 
(ABCD=1001)(0/14): RX_IDLE (ABCD=1001)(0/16): RX_IDLE (ABCD=1001)(0/17): 
RX_IDLE (ABCD=1001)(0/18): RX_IDLE (ABCD=1001)(0/19): RX_IDLE 
(ABCD=1001)(0/20): RX_IDLE (ABCD=1001)(0/21): RX_IDLE 
(ABCD=1001).(0/22): RX_IDLE (ABCD=1001)(0/23): RX_IDLE 
(ABCD=1001)(0/24): RX_IDLE (ABCD=1001)(0/25): RX_IDLE (ABCD=1001)(0/26): 
RX_IDLE (ABCD=1001)(0/27): RX_IDLE (ABCD=1001)(0/28): RX_IDLE 
(ABCD=1001)(0/29): RX_IDLE (ABCD=1001)(0/30): RX_IDLE 
(ABCD=1001)...(0/0): RX ANSWERED (ABCD=0101). 
Success rate is 0 percent (0/5) 
Router# 
*Mar 2 00:18:13.333: %LINK-3-UPDOWN: Interface Async94, changed state to up 
*Mar 2 00:18:13.333: %DIALER-6-BIND: Interface As94 bound to profile Di1 
*Mar 2 00:18:14.577: %LINEPROTO-5-UPDOWN: Line protocol on Interface Async94, changed 
state to up 
Router# p 1.1.1.2 
Type escape sequence to abort. 
Sending 5, 100-byte ICMP Echos to 1.1.1.2, timeout is 2 seconds: 
!!!!! 
Success rate is 100 percent (5/5), round-trip min/avg/max = 160/180/236 ms

Router# clear int dialer 1 
Router#
(0/0): TX IDLE (ABCD=1001)(0/0): RX IDLE (ABCD=1001) 
*Mar 2 00:18:28.617: %LINK-5-CHANGED: Interface Async94, changed state to reset 
*Mar 2 00:18:28.617: %DIALER-6-UNBIND: Interface As94 unbound from profile Di1 
*Mar 2 00:18:29.617: %LINEPROTO-5-UPDOWN: Line protocol on Interface Async94, changed 
state to down 
et2-c3745-1# 
*Mar 2 00:18:33.617: %LINK-3-UPDOWN: Interface Async94, changed state to down

Router# p 1.1.1.2
 
Type escape sequence to abort. 
Sending 5, 100-byte ICMP Echos to 1.1.1.2, timeout is 2 seconds:
*Mar 2 00:18:40: dsx1_alloc_cas_channel: channel 5 dsx1_timeslot 
6(0/5): TX SEIZURE (ABCD=0001)(0/5): RX SEIZURE_ACK 
(ABCD=1101)....(0/5): RX ANSWERED (ABCD=0101). 
Success rate is 0 percent (0/5) 
Router# 
*Mar 2 00:19:08.841: %LINK-3-UPDOWN: Interface Async93, changed state to up 
*Mar 2 00:19:08.841: %DIALER-6-BIND: Interface As93 bound to profile Di1 
*Mar 2 00:19:10.033: %LINEPROTO-5-UPDOWN: Line protocol on Interface Async93, changed 
state to up 
Router# p 1.1.1.2 
Type escape sequence to abort. 
Sending 5, 100-byte ICMP Echos to 1.1.1.2, timeout is 2 seconds: 
!!!!! 
Success rate is 100 percent (5/5), round-trip min/avg/max = 160/167/176 
ms

Router# 
(0/5): TX IDLE (ABCD=1001)(0/5): RX IDLE (ABCD=1001) 
*Mar 2 00:19:26.249: %LINK-5-CHANGED: Interface Async93, changed state to reset 
*Mar 2 00:19:26.249: %DIALER-6-UNBIND: Interface As93 unbound from profile Di1 
*Mar 2 00:19:27.249: %LINEPROTO-5-UPDOWN: Line protocol on Interface Async93, changed 
state to down 
Router# 
*Mar 2 00:19:31.249: %LINK-3-UPDOWN: Interface Async93, changed state to down

Router# 
(0/0): RX SEIZURE (ABCD=0001) 
*Mar 2 00:22:40: dsx1_alloc_cas_channel: channel 0 dsx1_timeslot 
1(0/0): TX SEIZURE_ACK (ABCD=1101)(0/0): TX ANSWERED (ABCD=0101) 
Router# 
*Mar 2 00:23:06.249: %LINK-3-UPDOWN: Interface Async83, changed state to up 
*Mar 2 00:23:06.249: %DIALER-6-BIND: Interface As83 bound to profile Di1 
*Mar 2 00:23:07.653: %LINEPROTO-5-UPDOWN: Line protocol on Interface Async83, changed 
state to up

Router# show controllers t1 1/0/0
T1 1/0/0 is up.
  Applique type is Channelized T1
  Cablelength is long gain36 0db
  No alarms detected.
  alarm-trigger is not set
  Framing is ESF, Line Code is B8ZS, Clock Source is Line.
  Data in current interval (180 seconds elapsed):
     0 Line Code Violations, 0 Path Code Violations
     0 Slip Secs, 0 Fr Loss Secs, 0 Line Err Secs, 0 Degraded Mins
     0 Errored Secs, 0 Bursty Err Secs, 0 Severely Err Secs, 0 Unavail Secs