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How do I configure ISDN call-back ?
Perle Router ISDN Callback Setup Procedures
The ISDN Callback feature of the Perle router is a powerful feature to control security and ISDN call charge billing. In order to use it for your application properly, you will require an understanding of both the feature itself and the ISDN line configuration.
This article will present an overview of this feature, and will provide three (3) real-world application examples that you may use as templates for your application.
Callback, or as it is sometimes called, dial-back, has been used in dial-up MODEM applications for over fifteen years. The primary purpose of this feature in those applications was to control access to a secure resource. It was an effective control mechanism and it was difficult to bypass. As a result, most secure dial-in applications required some form of this dial-back feature.
The basic premise of callback in a security application was that if you called me, and you are in my calling database, I will try to call you back at the pre-configured number I have in my database. If you can be reached at this number, you must be okay. Today, it is not accepted solely as a security mechanism, as there are now more secure methods available. However, it has found renewed application to control the call charge billing of an application, by allowing or forcing a particular location to pick up the call charges. It is this aspect of callback that will be the focus of our three applications.
Before you can use the ISDN callback feature in your application, a feature called Caller ID must be enabled on the ISDN circuit provided to you by the Telephone company. The Caller ID feature presents the "caller" to the Perle router in the form of a Telephone number. It is this Telephone number that will be used to decide who is calling and where the callback should take place.
Simple Callback Scenario
In a simple callback scenario, router (A) would like to call and connect to a Perle router (B). Router (A) would like the Perle router (B) to pay for the call charges. The steps that would be executed are as follows:
The router (A) would place an ISDN call to the Perle router (B) (B) will not actually answer this incoming ISDN call, but instead it will interrogate the ISDN Call Signaling Setup packet to determine who the caller is. (B) verifies that this is a number located in its callback database. When (B) decides that it is a callback number, it will reject the Call Setup (forcing a disconnect) and it will then start the callback procedure. (B) will call the number that was pre-configured in its database to reach (A). (A) answers the call and the callback procedure is complete.
Given this simple example, router (B) would now be the caller who would be billed for the ISDN call. Because router (B) did not accept the original incoming call, there are no charges incurred by (A).
The callback functionality can be used in many different applications and has the versatility to work well in all. There are a range a features that work in tandem with Callback to fine tune the application of the feature. In the examples provided below, Central Site Router refers to the router that is only to receive calls, while the Remote Site Router is the router that must call the central site to incur the billing charges.These references are made for the purpose of clarifying the applications presented below and to overview the different responsibilities of each router. It is quite feasible to reverse the roles so that the Central Site Router would be the Callback device. Of course, you must select which router should perform a given role to meet your specific application and to modify the configuration accordingly. The initial call from the Central Site Router to alert the Remote Site Router to call back is commonly referred to as a tickle. It is this tickle that starts the callback procedure.
Wait just a second!
One issue that must be dealt with in this very first scenario is that the Remote Site Router can actually call back the Central Site Router before the original tickle call has been completely disconnected by the Telephone company. If this happens, the circuit will be busy and the Remote Site Router will not be able to complete the callback call. Although the Remote Site Router will retry this connection, it only serves to delay the required result.
To solve this problem, the Perle router provides a hold-off timer called Callback Delay. This timer can be adjusted to control the exact period that the Remote Site Router will wait from the time it receives the tickle to the time it will initiate the call back.
This time period is very important and should be considered closely. The period selected must be a period where the circuit can fully disconnect the call, but before the Central Site Router initiates another tickle call. A little experimentation will provide the exact setting needed. Normally, the default setting will be quite adequate.
There are two menus located in the Remote Site setup of the Perle router that control the application of Callback. Presented below are the two menus.
Device: DEV802915 V 05P184.108.40.206 REMOTE SITE 2 (CSR) CONNECTION SET-UP ISDN CALL SET-UP MENU Option Value Description 1. Advanced settings menu - Advanced ISDN call settings 2. ISDN number [none] - Set ISDN number 3. Alt. ISDN number [none] - Set alternate ISDN number 4. Group  - Specify dial group 5. Wildcard [none] - Set wildcard ISDN number 6. Call you [none] - Set call you prefix 7. Call me [none] - Set call me prefix 8. Callback [disabled] - Enable/disable callback Device: DEV802915 V 05P220.127.116.11 CONNECTION SET-UP ISDN CALL SET-UP ADVANCED SETTINGS MENU Option Value Description 1. Callback delay [2 s] - Time to wait until callback 2. Redial timer [10 s] - Time to wait until redial 3. Redial count  - Number of redials to try
Callback to the same number that called you This is the simplest of the callback scenarios. In this example, when the Central Site Router calls the Remote Site Router, the calling number presented in the Caller ID field is the number you need to call back.
The parameters that need to be adjusted on the Remote Site Router are:
ISDN number - This must be set to the ISDN number that will be sending the tickle (in our example, Central Site Router) Alt. ISDN number - If a 128Kbps access rate is required this would be the number to establish the second link to the Central Site Router.
Call you - If the number you are calling is long distance this would be where you can include the long distance prefix such as the long distance access code, the country code or the area code. If you are calling long distance, it is suggested that you use this feature and not include the area code in the ISDN number parameter.
Callback - Enabled This it the parameter that enables Callback on for this remote site.
Callback delay - This may need to be adjusted to a larger value than the default 2 seconds. The symptoms of this parameter being set to a value smaller than required would be that when the Remote Site Router calls the Central Site Router back, it will receive a busy signal and be disconnected.
Callback to a different number This scenario presents the situation where the tickle is actually originated on a different, perhaps reserved, telephone circuit. Although this calling number will be consistent, the Remote Site Router must call a different number to reach the intended location.
In order to deal with this example, we are now going to establish a Wildcard that will be used to match the incoming call. The Wildcard entry is the number that calls and it will be used to select the correct Remote Site profile in the Perle router to establish the return call.
The parameters that need to be adjusted are:
ISDN number - This must be set to the ISDN number of the Central Site Router. Alt. ISDN number - As above Wildcard - This must be set to the ISDN number that would be seen in the Caller ID field of the call setup message during the tickle.
Call you - As above. Callback - Enabled - This turns Callback on for this remote site. Callback delay - As above.