Fig.1 Main Screen

The main operation screen is divided into 4 main parts:

- receivers window
- tape recorder window
- target list/base stations list
- protocol window

Fig.2 Receivers window

1.Receiver number
2.Channel/Timeslot number
3.Receiver status
4.Traffic channel
5.Signal level indicator of forward and reverse channels 6.Identity of intercepted call or name of the network    operator

Fig.3 Receivers setup screen

Fig.4 Target List

Fig.5 Target List Edit window

Fig.6 Base Stations List

Fig.7 Tape Recorder Window

1. Receiver number
2. Voice Record Level Indicator
3. TMSI or IMSI number
4. Duration
5. Reset Call button
6. Pause button
7. Speaker ON/OFF button

Fig.8 Protocol window

Fig.9 Toolbar

The system has 3 main operational modes:

1. Random mode

2. Classmark mode

3. IMSI/TMSI mode

Only one of these main modes can be chosen and used at any given time.

In addition the system has 4 more operational modes:

4. Distance mode

5. Phone number mode

6. Reverse channel mode

7. IMEI mode

The additional operational modes are used together with main modes. The additional modes can be used together in any combination.


2.1 Main Operational modes


2.1.1. Random mode

This mode is usually used to intercept all calls to or from a given area if the particular phone number is unknown.

2.1.2. Classmark mode

All mobile phone are distinguished by their classmarks. The classmark is one of the characteristics of mobile phones which is never changed. As soon as some phone conversation is intercepted the classmark of the active phone is displayed in the protocol window and can be put on the target list. Subsequently this parameter can be used for monitoring calls made from or to the specific mobile phone.



2.1.3. IMSI/TMSI Mode

The real mobile phone number is never transmitted over the air in GSM networks. Instead of phone numbers GSM networks use special identities (IMSI or TMSI) for mobile phone authentication.

Using a special technique the system discovers the identity that corresponds to the particular phone number. These identities are stored in the target list and used for monitoring by specific phone number (see Fig.5).

One of these identities (TMSI) is changed from time to time. The system automatically follows all TMSI modifications and automatically updates them in the Target List without intervention by the system operator.



2.2 Additional Operational Modes


2.2.1. Distance mode

This mode allows interception of those conversations, and only those conversations, being made to or from mobile phone located at a given distance from Base Stations. The distance can be specified in the Receiver Setup Screen (see Fig.3).

This mode may be used effectively when there is some "Place of Interest" and there are no particular phone numbers or other information about mobile phones for that place.



2.2.2. Reverse Mode

When reverse mode is active only conversations with active reverse channel (i.e. from mobile phones located near the interceptor) will be intercepted.

This mode can be very useful for the interception of calls being made to or from a specific area when the interceptor is in the area.

Another very effective implementation is a combination of (main) Random Mode and (additional) Distance and Reverse Modes together with a uni-directional antenna. In this case a "Place of Interest" can be defined very closely in terms of distance from one side and by azimuth from he other side.



2.2.3. Phone Number Mode

When this mode is active the only calls intercepted will be calls made from or to a phone number defined in the Target list (see Fig.5 Target List Edit window).


2.2.4. IMEI Mode

Some GSM networks use IMEI identity. This parameter can be very useful as it is a characteristic
of mobile phones which is never changed.

In addition this identity contains the model of the mobile phone. If IMEI is used by the GSM network, Interceptor will display models of mobile phones in the Protocol Window.


3. Effective Radius


The Inteceptorґs effective radius may depend on the direction of transmission. In mobile communication there are two directions:

* The direction outward from a base station to a mobile phone is considerd the forward channel.

Normally the system can intercept traffic in the forward channel at a distance of 3 to 10 km
and even more because a signal in the forward channel is strong.


* The opposite direction from a mobile phone to base staion is considered the reverse channel.

Normally the system can intercept traffic in the reverse channel at a distance of only 100 to 600 meters because a signal in the reverse channel is significantly weaker. The exact effective radius for the reverse channel depends on many factors including walls and their thickness, relative positioning of the system and mobile phone, terrain and more.

As long as base station transmitters and mobile phone transmitters differ in power the effective radius of the Interceptor will differ according to channel, but in order to hear both sides of the conversation, the Interceptor needs to be close enough for the weaker channel - normally 600 meters or less.

However, the Interceptor can also be used as a stationary device with uni-directional antennae. In this way the effective radius for the reverse channel may be increased to as much as 1,000 meters.



4. How to choose the right configuration

Although the obvious answer and often the best is "the more channels the better", still various factors such as budget may make this guideline less useful in practice. For choosing a price/performance level there are three main performance considerations:

1. Ability not to miss calls

2. Ability not to miss a more important call than the one currently being intercepted

3. Ability to intercept simultaneously numerous calls


Letґs analyze each of those.


4.1 Ability not to miss calls

At the beginning of interception all existing receivers dedicated to forward channels must be tuned to the nearest-to-target-phone-base-stations. Depending on its environment a mobile phone can communicate with a GSM network via one of several preferred base stations located nearby.

In Fig.1 the mobile phone may choose among 4 base stations so the interceptor needs at least 4 forward receiver channels. If an Interceptor with forward receiver channels is used in this situation the chances of missing the call will be around 50%.



4.2 Ability not to miss a more important call than the one currently intercepted

This is another important feature of the Interceptor. Suppose some call is intercepted. One of the forward receiver channels is then receiving voice traffic of one of the base stations and, if there are no additional free receivers, cannot monitor other calls of the base station. Therefore one of the preferred base stations is not under surveillance. If at this stage another call occurs that is more important, it cannot be missed. For that reason it is very important to have a surplus of forward receiver channels over the number of preferred base stations.


4. Number of simultaneously intercepted calls

For this model of the Interceptor the number of reverse channels is usually equal to the number of forward channels. One of our standard models has 4 forward and 4 reverse channels. In practice this means that such an Interceptor can monitor and record four conversations simultaneously (including both sides of the conversation).


The Interceptors can be produced and supplied in various configurations depending on customer needs and budget.


The following system configurations are standard:

2+2, 3+3, 4+....8+8.

(2+2) means that the system has four receiver channels: two forward channels and the other two reverse.

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