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Automatic Registration Via DTMF for Roaming Cellular Subscribers.

Automatic Registration Via DTMF for Roaming Cellular Subscribers

Among the plans proposed for accommodating the automatic registration of mobile cellular units when roaming away from the subscriber's home system is the use of CCITT's Recommendation 7 for computer-to-computer query/response.

The CCITT (International Consultative Committee for Telephone and Telegraph) 7 concept may eventually become the acceptable method, but it ignores current realities and near-term projections. For example, the current Direct Distance Dialing (DDD) network structure has not yet provided universal CCITT 7 connectivity down through Class 4 offices. Since cellular systems will be providing network connectivity either through a Class 5 office or as a Class 5 office, the availability is simply not there. Also, some cellular systems may not have CCITT 7 capability, especially in smaller markets. And the initial connect expense for providing CCITT 7 will be large per subscriber served by small systems. This may create expense-driven rate increases, thus discouraging traffic and general public acceptance of cellular service.

An economical method should be considered for the initial period of cellular service. To do otherwise causes financial hardship on those operating companies which would bring cellular service to the largest portion of the North American continent. Assuming that CCITT 7 type connectivity is available through all Class 4 offices (or LATA tandems), extensions of larger-based metropolitan systems along major traffic routes (such as the interstate highway system) would occur, precluding smaller systems for operating autonomously in such areas.

A number of parties preparing applications for the 91-plus markets, including non-Cellular Geographic Service Areas (CGSAs), have expressed apprehension over the spectre of immediate or near-term CCITT 7 on other packet-switched registration requirements. ITT believes that an entire segment of the emerging cellular industry may be disadvantaged by adoption of "too much too soon.' This segment consists of those parties who are currently preparing to enter the industry within the 91-plus markets. Some of these parties may not have previously participated as a telecommunications common carrier.

Therefore, to allow the industry time to establish itself, and during the period of restructuring of the wireline network, a complementary arrangement should be considered. This should be reasonable and generally available for the present and near-term to allow positive verification of roamer traffic.

A roamer is a cellular subscriber to one system who appears in a different system. This visited system has no data on this roamer and consequently must determine how and what service to provide. This may necessitate obtaining the data from either the roamer's home system or an equivalent data base (clearing house) using a packet-switched network or the North American telephone network as the transport facility.

Signaling Overview

In the North American telephone network, there are different methods of inter-office signaling--Dial Pulse (DP), Multi-frequency (MF), Modem, Common-Channel Inter-office Signaling (CCIS) [currently limited to Class 4 and higher offices], eventually CCITT 7 (currently not available in North America) and finally Dual-Tone Multi-Frequency (DTMF). We'll Look at the features of each of these in the following section.

Dial Pulse (DP). Operates a bit at a time and requires simple terminal equipment. Offers combined transport (voice and signaling), and is widely available. DP is too slow. In addition, it may experience difficulty being transmitted through the network.

Multi-Frequency (MF). Operates a digit at a time, with relatively simple terminal equipment. Also has wide availability and combined transport.

Modem. Operates a character at a time, with more sophisticated equipment than MF. Has wide availability and combined transport. There are several drawbacks to this approach--the modems are not an integral part of the office switch. Since the modems must be added on, the architecture will not easily support them, and the use of modems would probably preclude the subscriber from directly communication from the subscriber instrument.

Common Channel Inter-office Signaling (CCIS). Operates a character at a time, with sophisticated terminal equipment. Has limited availability and provides separate transport. CCIS is similar to CCITT 7, and has not acheived widespread use in the DDD network. CCIS is being replaced by CCITT 7.

CCITT 7. Operates a character at a time with sophisticated terminal equipment. Has limited availability and provides separate transport. GCITT 7 is an evolving system and currently does not have wide availability. Projections cannot be made as to its speed of evolution or its availability. This mechanism will require complex terminals, a dedicated transport network and packet switches. This will require a considerable investment.

The fixed cost of the terminal equipment and access charges to the packet switching network must be pro-rated over the number of roamers served by a specific office. For those offices serving few roamers, the pro-rated cost per roamer is relatively high. This suggests that CCITT 7 is inappropriate for these offices.

For offices where CCITT 7 is inappropriate, a secondary mechanism must be explored. Such a mechanism would be to use DTMF. Consequently, this proposal accepts CCITT 7 and proposes DTMF as the two transport mechanisms to be used by the cellular mobile systems in handling roamers.

Dual-Tone Multi-Frequency (DTMF). Operates a digit at a time, with relatively simple terminal equipment. Has wide availability and provides combined transport. The use of DTMF is recommended because the equipment required is simple terminal equipment comprising a pool of DTMF receivers that are normally part of the telephone switch. A DTMF generator is required and generally exists, or can easily be implemented using hardware or software techniques. Such types of receiving and generating equipment must be part of the cellular office to allow it to interconnect to the land-based telephone network. This allows mobile-to-land and land-to-mobile calls.

The following paragraphs summarize DTMF features.

Simple terminal for in-band signaling. This type of signaling can be sent over the existing voice path network; thus it does not require any special facilities to be developed. The land-based telephone switches are transparent to this signaling since they merely provide a speech path for these signals. Historically, these signals have been sent over the dial-up network, with great success, over a wide variation of configurations and conditions.

Compatibility with cellular subscriber end instrument. An advantage of the DTMF signaling scheme is that it provides a mechanism by which the roaming cellular subscriber may communicate directly with the home office (as it will be shown in the call cases described later) without requiring operator intervention. By placing calls to specified numbers, the subscriber can be connected to a DTMF termination. The subscriber can then administer features using the DTMF keypad. This common mechanism has far-reaching implications for feature administration in addition to roamer treatment and is usable by home subscribers as well as roamers.

DTMF Precedence

The remainder of this article describes the treatment of those offices where CCITT 7 is not economically justified.

Precedence exists in using DTMF in the manner proposed. DTMF is currently employed in bank-by-phone between the subscriber's end instrument and the bank's computer. The bank-by-phone experience has established that data base security may be maintained; customer parameters may be administered with commonly available DTMF instruments without abuse.

The Federal Aviation Administration is currently testing a system that allows pilots to obtain actual weather information for pre-flight planning. This system uses a dialed-up connection to access a computer. By using any DTMF telephone to control this computer, weather information voice messages are generated.

Since the cellular subscriber data base can be accessed by the cellular subscriber's instrument via DTMF, and since land-based DTMF instruments also send DTMF, it would be possible for a subscriber to concurrently update approved portrions of that subscriber's data base from either a mobile-based or a land-based DTMF phone.

When a cellular subscriber unit is detected by a cellular system and the system has no data on this subscriber (roamer), the system must determine how and what service to provide. The necessary data from either the roamer's home system or an equivalent data base (clearing house) is obtained automatically without operator intervention via a dial-up call to the home data base or clearing house using an exchange of DTMF tones. Note that this exchange is initiated by the visited office.

As an additional benefit of having installed the mechanism to support the abover roamer requirement, the subscriber may directly activate the following:

Extended Call Forwarding. Calls can be forwarded anywhere in the telephone network.

Local Call Forwarding. Allows for calls to be forwarded within the local dialing area of the subscriber.

Mobile Call Forwarding to Land Base. For those land-based offices which are unable to support call forwarding, the ability now exists to route all calls to the mobile office. The subscriber can, by administrative features, cause call forwarding to the designated land-based office. Consequently, the subscriber has the benefit of call forwarding even though the land-based office does not support this feature.

The following sections will offer some typical call cases, beginning with automatic roamer registration (see Figure 1).

Roamer registration is used when the subscriber unit detects that it has entered a foreign service area. When the roamer attempts to register, it will, in addition to sending currently required information, also send a category bit. This category bit, set to 0, may be interpreted by the visited system as a request not to contact the home system (thus, automatic call forwarding should not be invoked). With the category bit set to 1, it may be interpreted as permission to contact the home system (thus being billable) to invoke whatever special vertical features may be appropriate in the visited system. Note that existing units will still function in a cellular system but will always have the benefits of this enhancement.

Since the category bit is not currently part of the EIA CIS-3 specification, a request from the FCC for inclusion of this category bit should be submitted to EIA (Electronic Industries Association in Washington, D.C.).

In the absence of such subscriber-transmitted information, which is the current situation, the visited system must determine how it wishes to deal with roamer registration. This determination should be left up to the individual operating company. This could include the checking of hot lists, positive verification to the home system (clearing house), or doing nothing. Following discussions at previous FCC working groups, it was decided that if the visited system should choose to contact the home office for information regarding the subscriber, it would not be permissible to bill the subscriber for the cost incurred.

A Mobile Scenario

The following scenario applies to those systems which do not support CCITT 7. The visited system, having received the Mobile Identification Number (MIN), automatically performs the required translation to determine the home system and its associated directory number. This directory number is either for the home system or for the clearing house to which it subscribes. The visited system places a call over the land-based telephone network to the home system or its clearing house using the directory number. The home system answers this circuit, connects a DTMF sender-receiver, and sends a pulse of tone to the visited system to signify answer. Using DTMF tones, the visited system transits the subscriber's MIN, serial number and System Identification Number (SID) of the visited system, plus a code indicating that this is a roamer category type of message. The home system then responds to the visited system via DTMF with the subscriber information, such as automatic call forwarding allowed, financial responsibility and special features.

If the visited system receives automatic call forwarding permission from the home system, it then sends a temporary B number to the home system and disconnects.

When a subscriber returns to the home system, the visited system need not be informed because the home system will not route calls to the previously visited system. The visiting system will maintain the temporary B number for a period of time following the last registration number (the time period is solely at the discretion of the visiting system). If the roamer reappears in the visiting system after the timeout has occurred, the visiting system will simply re-notify the home system and reassign a temporary B number.

With manual roamer registration (Figure 2), when a roamer manually contacts the visited system and requests roamer status, the visited system may assign a temporary number for use while in the visited system. The subscriber may distribute this temporary number. Calls placed to this temporary number will be routed by the telephone network to this visited system. No notification or communication is required between the visiting system and the home system about the mobile unit's location. Credit validation may be required.

Looking at call origination (Figure 3), a roaming cellular subscriber may or may not have registered in the visited system. Even if the roamer registered, the visited system may have chosen not to contact the home system for status at that time. In this case, when such a roamer attempts to originate a call, the visited system may choose not to contact the home system before completing the call. If a verification is desired before completion of the call, the mobile will be connected via a voice channel to a recorded message informing the subscriber that status is being checked and that the call will be processed as soon as possible.

The visited system will concurrently place a call over the land-based telephone network to the home system. When answering this cricuit, the home system connects a DTMF sender-receiver and sends a pulse of tone to the visited system signifying answer. The visited system transmits via DTMF tones the subscriber's MIN, Serial Number, the SID of the visited system and a code indicating that this is a roamer category type of message. The home system then responds to the visited system, via DTMF, with information about the subscriber. If the visited system received automatic call forwarding permission from the home system, it sends a temporary B number and then disconnects. The visited system then completes the call in the normal manner.

With automatic call termination call forwarding), shown in Figure 4, a roaming cellular subscriber has registered in a visited system, and the visited system has advised the home system as described previously.

The home system, upon receiving a call for this subscriber, establishes a telephone network path to the visited system by dialing the temporary B number previously received by the home system.

The home system then makes the connection between the originating and the visited systems. Having established this speech path, billing treatment can now be initiated. The visited system upon receiving this incoming call for the temporary B number will associate it with the MIN of the roamer and follow normal terminating call procedures, such as paging.

Note that a number of billing records may be generated across the network by this call, including: originating party's toll charges to reach the home system; home system billing to extend the call to a visited system; land-based tandem switched billing to the home system; visited system billing for the call.

These billing assignments may be worked out through revenue-splitting arrangements, which are outside the scope of this article. Note that this call was established without the need for DTMF signaling; thus, no additional cost or delays were incurred.

Call Termination

Looking at the call termination (via a temporary number made known to the subscriber) chart in Figure 5, a call can be directly terminated to a roamer if the A-party knows the temporary B number assigned to the roamer by the visited system. The A-party dials the temporary B number and the call is routed by the network to the visiting system. Upon receiving this B number, the visited system will associate it with the MIN of the roamer and follow normal call terminating procedures, such as paging.

Note that automatic call forwarding and manual roame registration are not mutually exclusive mechanisms; thus, a subscriber can request and be accessed by both automatic call forwarding from the home system and directly dialed calls to the temporary B number.

An example of this is a New York cellular subscriber roaming to Los Angeles. The roamer can have automatic roamer registration, in which case the visited system assigns a temporary B number (as previously described). In addition, the roamer can request to be informed of the temporary B number. The roamer can then advise local subscribers (LA) of this temporary B number. These local subscribers can then originate calls to the roamer on a local basis rather than via the roamer's home system (NY), thus saving the cost of two transcontinental toll charges. Calls to the home system can still be forwarded.

With call termination via an operator (Figure 6), a roamer can receive terminating calls via a local operator. This can occur when an originating subscriber knows that a roamer is in a service area (CGSA), but does not know the temporary B number even if one has been assigned. This call can be completed for the originating subscriber by accessing an operator in the visited system. This operator will obtain the home B number from the originating subscriber and then request a global page using the MIN. If the page is answered, the visted system will then complete the call.

Note that at no time did the originating subscriber obtain the temporary B number, if one had been assigned, thereby ensuring roamer confidentiality.

Multiple visited systems represents an insolvable problem to the cellular operators if there is no change to the CIS-3 specification.

The cause of the problem is that the mobile unit stores and remembers the last four visited cellular systems. The effect of this may be demonstrated by the following example: A mobile leaves its home system and enters visiting system A. Visited system A, by mechanisms previously described, contacts the home system and invokes automatic call ?? Thus land-based calls to the mobile unit arriving at the home system are extended to visited system A.

The mobile then roams to visiting system B from visited system A. Visiting system B, by mechanisms previously described, contacts the home system and invokes automatic call forwarding. Thus land-based calls are extended from the home system to visited system B.

The mobile subscriber then goes back to visited system A. At this point, since the subscriber unit remembers already registering in visited system A, the unit will not attempt to re-register until the register timing increment has been satisfied (this could be a long period of time). The problem is now clear: the home system believes the mobile unit is in visiting system B and the subscriber is actually in visiting system A, but has no way of notifying the home system. Thus, all of the land-based calls to this subscriber will be extended to visiting system B where the subscriber is not now located.

Bit in Overhead Train

It is proposed to modify the CIS-3 specification to include a bit in the overhead train. This bit, under control of the NCS, when set requires the mobile unit to bypass its internal check of SIDs associated with previously visited systems. The mobile unit, when re-entering visiting system A will then re-register, thereby ensuring that the home system correctly extends calls.

The penalty for using this bit will be an increase in the number of registrations. The existing units will still function in a cellular system, but will not have the benefits of this enhancement.

In addition, a modification should be made to CIS-3 to add a message on the reverse access channel as a new type of registration response. This will consist of a normal registration response plus an instruction to the mobile unit to turn off the roamer light, if so equipped.

In summary, this article proposes a secondary mechanism for handling automatic registration of roaming mobile units by using DTMF signaling.

In proposing the use of DTMF signaling, the justification is two-fold: (1) the DTMF approach reduces the cost per roamer, especially in low-volume roamer traffic systems, and (2) CCITT 7 signaling is not currently available and is difficult to project its speed of evolution and availability.

Photo: Figure 1. Automatic roamer registration is used when the subscriber unit directs that it has entered a foreign service area. See text for more detailed explanations.

Photo: Figure 2. Manual roamer registration.

Photo: Figure 3. Call origination.

Photo: Figure 4. Automatic call termination (call forwarding).

Photo: Figure 5. Call termination (via temporary number made known to the subscriber).

Photo: Figure 6. Call termination via operator.

Photo: Figure 7. Roamer multiple visited systems.

Photo: Figure 8. Another roamer multiple visited system example. See text.

Photo: Figure 9. Existing roamer multiple visited systems.

Photo: Figure 10. Proposed roamer multiple visited systems.

Photo: Figure 11. Summary diagram of proposals presented in article.
COPYRIGHT 1984 Nelson Publishing
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Copyright 1984 Gale, Cengage Learning. All rights reserved.

Article Details
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Author:Rollin, H.E.
Publication:Communications News
Date:Feb 1, 1984
Words:3441
Previous Article:Business Users the First to Benefit from the Promise of Cellular Radio.
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