Call Forwarding -- Early or Late

What is difference between Early and Late Call Forwarding? Here is a possible explanation ....
Cellular telephones have become increasingly prevalent in today's society. The cellular telephone makes use of a cellular network and traditional telephone networks to route calls using radio communication signals. Two types of cellular networks have evolved—Global Systems for Mobile communications (GSM) and American National Standards Institute-41 (ANSI-41) type networks. ANSI-41 is also sometimes referred to as IS 41 (TIA/EIA [Telecommunications Industry Association/Electronic Industries Association] Interim Standard 41).
The two types of cellular networks use message formats that are not completely compatible with one another as they are. Thus, if a mobile subscriber (MS) using cellular communication, roams from one type of network to another, the messages sent by the MS may not be understood by the network into which it roams unless some sort of conversion is performed. In order to allow roaming between “foreign” type networks, a signaling message converter, referred to herein as an IWU (InterWorking/interoperability Unit) operates to interconnect calling and called Mobile services Switching Centers (MSCs). The IWU passes signaling messages back and forth between network entities while converting the format of these messages such that the messages can be properly interpreted by the receiving MSC.
However, even with known signal message converters, GSM standards are inherently different from the ANSI-41 standards mentioned above. One of these differences is in the operation of “late call forwarding” or the action of the system when a call to a mobile subscriber (MS) cannot be terminated, i.e. connection completed, for any of several specific reasons like call collision, no page response of a page request sent to the called MS, and the like. A GSM system expects the terminating Mobile service Switching Center (MSC) to handle the call forwarding while an ANSI-41 system expects the originating or home MSC of the MS to handle the call forwarding.
As is known by those skilled in the art, call forwarding may occur when a call, to a given MS, cannot be terminated. The call forwarding can be “early” or “late”. Early call forwarding is defined as occurring when a contact with the MS has failed due to predetermined circumstances occurring while attempting to extend the call to another network. Late call forwarding is defined as occurring when call forwarding is initiated after the call has been extended to the last known “foreign” public land mobile network location and contact with the MS has failed due to predetermined circumstances.
In view of the above, it would be advantageous to have a converter mechanism that provides for late call forwarding from the originating MSC when a MS roams from an ANSI-41 network to a GSM network.

In summary, If call is forwarded by HPLMN then it is ECF if by VPLMN then it is LCF.

Source: http://www.freepatentsonline.com/6615037.html

Call Forwarding in SIP

This article arises from a problem we face today as how does SIP stack take care of Call Forwarding.

Suppose A is calling B and B wants to enable Call Forwarding to Caller C.

Our main issue is: Whether SIP stack at A would come to know that the call has been forwarded from B to C or would it be transparent for stack running on A.

Some basics:

Call Forwarding lets you forward your incoming wireless calls to another phone number. Airtime charges, plus any applicable long distance and/or roaming charges, are incurred for calls forwarded via the wireless network.

With Call Forwarding activated:

• You can continue to make outgoing calls from your telephone. Incoming calls will be forwarded.
• When the telephone number to which your calls are being forwarded is busy, callers to your number will receive a busy signal.
• To change the number to which your calls are forwarded, deactivate the feature. Then, follow the activation instructions to enter the new number.

Our problem got resolved from the tutorial on http://www.tech-invite.com/Ti-sip-service-7.html
It clearly indicates that

1. A signaling message would be sent to client, i.e. 181, Call is being forwarded.
   
2. No response message is sent from the client side to the proxy for this 181 message.
3. Rest of the signaling messages received by the client are same, i.e. 180 Ringing, 200 Okay.
   

Hence, Call forwarding at the destination does not really matter to the originating client from protocol signaling aspects.