GSM in quick general revision

Mobile station (MS)

• The Mobile Station is the interface between the user and the network.

The MS consists of two independent parts:

 Subscriber Identity Module (SIM) card

 Mobile equipment (ME).

Base transceiver station (BTS)

• A BTS acts as the interface between MS’s and the network, by providing radio coverage functions from their antennae to provide the coverage area for one cell.

• Converts the GSM radio signals into a format that can be recognized by the BSC

• Ciphering using the ciphering key

• Channel coding and interleaving

• Modulation and demodulation.

• Records and passes to the BSC the Signal strength measurements

Base station controller (BSC)

• The BSC is the central node within a BSS and co-ordinates the actions of Base Stations. The BSC controls a major part of the radio network

• Hand over calculations.

• Power control calculations.

• Supervises the transmission network and the operation of each BTS

Mobile switching center (MSC)

• The primary node in a GSM network is the MSC. It is the node, which controls calls both to MS’s and from MS’s.

The primary functions of an MSC include the following:

 Switching and call routing to or from MS.

 Charging.

 Service provisioning.

 Control of connected BSC’s.

 Direct access to Internet services.

Gateway mobile switching center (GMSC)

• Gateway functionality enables an MSC to interrogate a HLR in order to route a mobile terminating call. It is not used in calls from MS’s to any terminal other than another MS.

• For example, if a person connected to the PSTN wants to make a call to a GSM mobile subscriber, then the PSTN exchange will access the GSM network by first connecting the call to a GMSC

Home location register (HLR)

• The HLR is a centralized network database that stores and manages all mobile subscriptions belonging to a specific operator.

• It acts as a permanent store for a person’s subscription information until that subscription is cancelled.

• The primary functions of the HLR include:

 Stores for each mobile subscriber:

• Basic subscriber categories.

• Supplementary services.

• Current location.

• Allowed/barred services.

• Authentication data.

 Subscription database management

Visitor location register (VLR)

• The role of a VLR in a GSM network is to act as a temporary storage location for subscription information for MSs, which are within a particular MSC service area.

• Thus, there is one VLR for each MSC service area. This means that the MSC does not have to contact the HLR (which may be located in another country) every time the subscriber uses a service or changes its status.

• The VLR is always integrated with the MSC

• For the duration when the MS is within one MSC service area, then the VLR contains a complete copy of the necessary subscription details, including the following information:

 Identity numbers for the subscriber

 Supplementary service information (e.g. Does the subscriber has call waiting activated or not)

 Activity of MS (e.g. idle or busy)

 Current Location Area of MS

Authentication center (AUC).

• To protect GSM systems, the following security functions have been defined:

 Subscriber authentication: by performing authentication, the network ensures that no unauthorized users can access the network, including those that are attempting to impersonate others.

 Radio information ciphering: the information sent between the network and a MS is ciphered. A MS can only decipher information intended for it.

Equipment identification register (EIR)

• Because the subscriber and equipment are separate in GSM, it is necessary to have a separate authentication process for the MS equipment.

• The equipment identification procedure uses the identity of the equipment itself (IMEI) to ensure that the MS terminal equipment is valid.

Operation and maintenance center (OMC)

• The operation and Maintenance center (OMC) is connected to all equipment in the SS (the GMSC, MSCs, HLR, VLR, AUC, EIR and the BSCs).

• It can be viewed as a computerized monitoring center were staff can monitor and control the network remotely

Traffic Channel

 Carries either encoded speech or user data up and down link between a single mobile and a single BTS.

Types of traffic channel:

 Full rate (TCH)

 Transmits full rate speech (13 Kbits/s). A full rate TCH occupies one physical channel.

 Half rate (TCH/2)

 Transmits half rate speech (6.5 Kbits/s).

 Two half rate TCH’s can share one physical channel, thus doubling the capacity of a cell.

Control Channels

 These are used to carry signaling or synchronization data. They are divided into three types:

 Broadcast Channels (BCH)

 Common Control Channels (CCCH)

 Dedicated Control Channels (DCCH’s)

1. Broadcast Channels (DL)

 From Single BTS to all the mobiles in the area

 Frequency Correction Control Channel (FCCH)

 Pure signal is transmitted to help the MS to lock on the frequency on the BTS

 Synchronization Channel (SCH)

 Carries 2 important pieces of information

 TDMA frame number (max = 2715684 )

 Base station identity Code (BSIC)

 Broadcast Control Channel (BCCH)

 Broadcasts some general cell information such as:

 Location Area Identity (LAI),

 maximum output power allowed in the cell

 The identity of BCCH carriers for neighboring cells.

 Cell parameters (Power , idle mode parameters)

2. Common Control Channels To or from a certain BTS to a single mobile

 Paging Channel (PCH)

 BTS Transmits a paging message to indicate an incoming call or short message. The paging message contains the identity number of the mobile subscriber that the network wishes to contact. (DL)

 Random Access Channel (RACH)

 MS Answers paging message on the RACH by requesting a signaling channel of SDCCH. (UL)

 Access Grant Channel (AGCH)

 Assigns a signaling channel (SDCCH) to the MS. (DL)

3. Dedicated Control Channels

 Stand alone Dedicated Control Channel (SDCCH)

 Used for signaling purposed: call setup, location update, IMSI attach – detach.

 Used to send / receive SMS in idle mode.

 Slow Associated Control Channel (SACCH)

 Always allocated with traffic channel to transmit measurement reports.

 DL: Commands from the network to the MS to adjust its power, Timing advance (TA).

 UL: information about the MS own power, received signal strength, quality and SS for neighbor cells.

 Used to send SMS in active mode.

 Fast Associated Control Channel (FACCH)

 Transmits handover information.

Antenna Tilting: Direction of the antenna vertical beam. Types of tilting:

 No Tilt: The vertical beam of the antenna is directed towards the horizon.

 Down Tilt: The vertical beam of the antenna is directed lower the horizon.

 Up Tilt: The vertical beam of the antenna is directed above the horizon.

Types of Tilting:

 Mechanical Tilting : tilting the physical body of the antenna

 Electrical Tilting: change the phase of the current fed the internal dipoles which will result in tilting the main beam.

What is the better, Mechanical tilting or Electrical tilting …????

 Mechanical tilting :

-With mechanical down tilting, the main beam will be down tilted but the back lobe will be up tilted that make interference. - With mechanical tilting you can’t change the tilt of one band alone.

 Electrical tilting :

- With electrical down tilting, the main beam will be down tilted but the back lobe will not be up tilted so back lobe interference decreased. - With electrical tilting you can change the tilt of any band alone. So Electrical Tilting is better than Mechanical Tilting