Telecommunications Blog

Call Setup

Different procedures are necessary depending on the initiating and terminating party:

• Mobile Originating Call MOC: Call setup, which are initiated by an MS

• Mobile Terminating Call MTC: Call setup, where an MS is the called party

• Mobile Mobile Call MMC: Call setup between two mobile subscribers; MMC thus consists of the execution of a MOC and a MTC one after the other.

• Mobile Internal Call MIC: a special case of MMC; both MSs are in the same MSC area, possibly even in the same cell.

 Mobile Originating Call MOC

1. Channel Request: The MS requests for the allocation of a dedicated signaling channel to perform the call setup.

2. After allocation of a signaling channel the request for MOC call setup, included the TMSI (IMSI) and the last LAI, is forwarded to the VLR

3. The VLR requests the AC via HLR for Triples (if necessary).

4. The VLR initiates Authentication, Cipher start, IMEI check (optional) and TMSI Re-allocation (optional).

5. If all this procedures have been successful, MS sends the Setup information (number of requested subscriber and detailed service description) to the MSC.

6. The MSC requests the VLR to check from the subscriber data whether the requested service an number can be handled (or if there are restrictions which do not allow further proceeding of the call setup)

7. If the VLR indicates that the call should be proceeded, the MSC commands the BSC to assign a Traffic Channel (i.e. resources for speech data transmission) to the MS

8. The BSC assigns a Traffic Channel TCH to the MS

9. The MSC sets up the connection to requested number (called party).

Remark: This MOC as well as the MTC described in the following describes only the principles of an MOC / MTC, not the detailed signaling flow.

  It was agreed that any new security architecture must be based on an evolution of GSM and must adopt four basic principles:

•   It will take into account the additional features needed for actual or predicted change in the operating environment

•   It will maintain compatibility with GSM wherever possible

•   It will retain those features of GSM that have proved to be robust and useful to the user and network operator

•   It will add or enhance features to overcome actual or perceived weaknesses in 2G system

1-  Additional features :

  One of the main reasons for the development of the 3G system to make higher value services available to as many users as possible world wide, using a universal design of the handset. However, this increases the number of relationships, as the number of Users, Service Providers, and Network Operators in the market expands.

  This increased level of service interaction increases the number of potential attackers and the opportunities open to them. This was not too much of a problem in the initial roll out of GSM, as there were a relatively small number of operators and the risk of compromise was low.

  For 3G, the networks are getting smaller and more numerous, so opportunities for hackers and other abusers of networks will increase. Even if deliberate abuse is not considered likely, unintentional mishaps may occur as a result of the complexity and the rate of new service introduction.

    If an adversary knows certain information (IMSI, IMEI, MSISDN) about the subscriber or a mobile phone, he can then identify individual phone calls, although this does require extensive technical effort.

    With the aid of IMEI it is possible to selectively filter calls out from the datastream on the mobile network radio relay links. Calls can also be identified in the public telephone landline network, for which knowledge of the subscriber call number is necessary. With the appropriate interception equipment, the IMSI and IMEI can be ascertained directly on the radio link between mobile phone and base station.

     The SIM card can be protected against unauthorised access with a four- to eight-digit Personal Identification Number ( PIN). After switching on the mobile phone, the subscriber identifies himself to the card by entering this PIN. If an unauthorised person gains possession of a SIM card, he cannot use it without also knowing the PIN. To prevent improper use of the SIM card, the PIN should therefore be kept in safe place.

      The subscriber identifies himself to the network provider on registering by means of the SIM card and the cryptographic algorithms held on it. Authentication is effected with the aid of an authentication key which is known only to the network provider in the AUC and the subscriber on the SIM card.

     Normally the data is only transmitted encrypted on the radio link between the mobile phone and the base station. Encryption is not used on any of the other transmission paths, either in the

      An MS communicates with a BTS by transmitting or receiving radio waves, which consist of electromagnetic energy. The frequency of a radio wave is the number of times that the wave
oscillates per second. Frequency is measured in Hertz (Hz), where 1 Hz indicates one oscillation per second. Radio frequencies are used for many applications in the world today.
Some common uses include :
                           • Television  --------------> 300 MHz approx.
                           •  FM Radio  --------------> 100 MHz approx.
                           •  Police radios  ----------> Country dependent.
                           •  Mobile networks  ------> 300 - 2000 MHz approx

      The frequencies used by mobile networks varies according to the standard being used2. An operator applies for the available frequencies or, as in the United States, the operator bids for frequency bands at an auction.

Advantages of 2G GSM

• GSM is mature; this maturity means a more stable network with robust features.
• Less signal deterioration inside buildings.
• Ability to use repeaters.
• Talktime is generally higher in GSM phones due to the pulse nature of transmission.
• The availability of Subscriber Identity Modules allows users to switch networks and handsets at will, aside from a subsidy lock.
• GSM covers virtually all parts of the world so international roaming is not a problem.
• The much bigger number of subscribers globally creates a better network effect for GSM handset makers, carriers and end users.

Disadvantages of 2G GSM

• Pulse nature of TDMA transmission used in 2G interferes with some electronics, especially certain audio amplifiers. 3G uses W-CDMA now.
• Intellectual property is concentrated among a few industry participants, creating barriers to entry for new entrants and limiting competition among phone manufacturers.[citation needed]
• GSM has a fixed maximum cell site range of 35 km, which is imposed by technical limitations.[2]

   The GSM network is divided into two systems. Each of these systems are comprised of a number of functional units which are individual components of the mobile network. The two systems are:
              • Switching System (SS)
              • Base Station System (BSS)
    In addition, as with all telecommunications networks, GSM networks are operated, maintained and managed from computerized centers.

AUC     Authentication Center