GSM Network and System Architecture:

Base Transceiver Station (BTS):

• Houses the radio transceivers that define a cell
• Handles radio-link protocols with the Mobile Station
• Speech and data transmissions from the MS are recoded
• Requirements for BTS:
ruggedness reliability portability minimum costs

BSC (Base Station Controller):

The BSC is a high capacity switch that manages all the radio-
related functions of a GSM network. It also provides physical links
between the MSC and BTS.
• Monitors and controls several BTSs
• Manages channel allocation on the radio interface during a call process
• Alarm Handling from the external interfaces and BTSs
• Performs inter-cell Call Handover
• Interface to OMC (Operation & Maintenance Centre) for BSS (Base
Station system) Management
• It contains the Cell configuration data (like Cell Identity, Frequencies etc)
• The BTS collects data on signal strength and quality of the neighbouring cells. The BSC uses these data to allocate a channel
during call setup or call handover
• Several BSCs can be controlled by an MSC
Network and Switching Subsystem (NSS):
• Mobile Switch Centre (MSC)
• Home Location Register (HLR)
• Visitor Location Register (VLR)
• Authentication Centre (AuC)
• Equipment Identity Register (EIR)
• Interworking Function (IWF)

MSC:
• Primary service delivery node for GSM
• Sets up and releases end-to-end connections
• Handles voice calls, SMS and supplementary services (e.g. conference calls, call hold etc.)
• Handles mobility management
• Handles hand-over requirements during the call (BSC to BSC as well as this MSC to other MSC)
• Handles generation of information used for billing

HLR(Home Loaction Register):

HLR is a permanent database of the Subscriber Services
• International Mobile Subscriber Identity (IMSI)
• Subscription information and services (like STD, ISD, Call Forwarding, Call Waiting, SMMO, SMMT, Roaming Subscription
Information, etc.)
• Current VLR address and LAI (current location of the MS)
• Referred to for an Incoming Call for MSRN (Mobile Subscribers Roaming Number)
A subscriber's data are deleted from the HR database only when his subscription is ended

VLR:
• Serves a specific geographical area.
• Temporary database of the subscribers who have roamed into its served area.
• Each base station is served by exactly 1 VLR. In other words, a subscriber can not be present in more than 1 VLR at any point of
time.
• Stores SIM number (IMSI).
• Stores authentication data.
• Stores MSISDN (subscriber number).
• Stores GSM services that are allowed for the subscriber.
• Subscribed access point (APN) for GPRS use.
• HLR address of the subscriber.

AuC:
• The authentication centre (AUC) is a function to authenticate each SIM card that attempts to connect to the GSM core network.
• Typically used when the phone is powered on.
• Generates an encryption key that is subsequently used to encrypt all wireless communication (voice, SMS etc.)
• A key element in an operator's strategy to avoid SIM cloning.
• Security of the process depends upon a shared secret between the AUC and the SIM called Ki.

EIR(Equipment Identity Register):

• The EIR is another database that keeps information about the identity of mobile equipment such as the International Mobile
Equipment Identity (IMEI) that reveals the details about the manufacturer, country of production, and device type.
• This information is used to prevent calls from being misused, to prevent unauthorized or defective MSs, to report stolen mobile
phones or check if the mobile phone is operating according to the specification of its type.
EIR has three databases:
• White list- For all known, good IMEIS
 • Black list- For all bad or stolen handsets
• Grey list- For handsets/IMEl's that are faulty or non-approved mobile equipment
• Database of mobile phones which are to be banned from network or monitored.
• Used to allow tracking of stolen mobile phones.
• Often integrated with the HLR.
• Some EIRs have the capacity to log handset attempts and store it in a log file.
• In theory, all data about all stolen mobile phones should be distributed to all ElRs in the world through a central EIR (C-EIR).

Identifiers Used in GSM System:

GSM distinguishes between user & equipment and subscriber identity & the telephone number. To manage all the complex
function, GSM deals with many addresses and identifiers, which are:
• International Mobile Station Equipment Identity (IMEI):- Every mobile equipment has a unique identifier. The IMEI is allocated
by the equipment manufacturer and registered by the network operator in the Equipment Identity Register (EIR). For know IMEI
no. type *#06#, this identifier called IMEL.
- serial number which distinctively identifies a mobile station internationally.
- allocated by the equipment manufacturer and registered by the network operator, who stores it in the Equipment Identity
Register (EIR).
- Using IMEI, one recognizes obsolete, stolen, or non-functional equipment.
• International Mobile Subscriber Identity (IMSI):- The IMSI is stored in the SIM card and secured by the operator. A mobile
station can only be operated when it has a valid IMSI. The IMSI consists of several parts.
- 3 decimal digits of Mobile Country Code. For India its 404.
- 2 decimal digits of Mobile Network Code. This uniquely identifies a mobile operator within country.
- maximum 10 decimal digits of mobile subscriber identification.
• IMSI(International Mobile Subscriber Identity): A unique identifier for a GSM subscriber,stored on the SIM card.
• MSISDN (Mobile Station International Subscriber Directory Number): The authentic telephone number of a mobile station is
the Mobile Subscriber ISDN Number (MSISDN). Based on the SIM, a mobile station can have many MSISDNs, as each subscriber is
assigned with a separate MSISDN to their SIM respectively.
• LAI(Location Area Identity): Identifies a specific geographic area within a network.
• IMSEI (International Mobile Equipment Identity): Unique identifier for a specific mobile device.
• MSRN (Mobile Subscriber Roaming Number): Temporary number assigned to a mobile device when it roams.
- Mobile Station Roaming Number (MSRN) is an interim location-dependent ISDN number, assigned to a mobile station by a
regionally responsible Visitor Location Register (VLA).
- Using MSRN, the incoming calls are channeled to the MS.
• TMSI(Temporary Mobile Subscriber Identity): Temporary identifier used to protect the subscriber's privacy
- Temporary Mobile Subscriber Identity (TMSI) can be assigned by the VLR, which is responsible for a subscriber’s current
location.
- The TMSI needs to have only local significance in the area handled by the VLR.
- This is stored on the network side only in the VLR and is not passed to the Home Location Register (HLR).
• Local Mobile Subscriber Identity (LMSI)
• Each mobile station can be assigned with a Local Mobile Subscriber Identity (LMSI), which is an original key, by the VLR.
• This key can be used as the auxiliary searching key for each mobile station within its region.
• It can also help accelerate the database access. An LMSI is assigned if the mobile station is registered with the VLR and sent to
the HLR.
• Cell Identifier (CI)
• Using a Cell Identifier (CI) (maximum 2 × 8) bits, the individual cells that are within an LA can be recognized.
• When the Global Cell Identity (LAI + CI) calls are combined, then it is uniquely defined.

GSM Channels:
• The Global System for Mobile Communications, GSM sends a lot of data to and from mobile phones. Typically each piece
of data is short.
 • Although it would be possible to have a different radio
frequency channel for each type of data, this would be a very
inefficient way of segregating the data.
• Instead the data can all be carried over the same radio
frequency signal, but they can be sorted into different
messages that can be sent. These messages are tagged in a
way that the system or mobile knows what they are.

Channel Structure: The spectrum is divided into 200kHz

channels, with 124 Channels available for each direction.
TDMA/FDMA: GSM employs Time Division Multiple Access
(TDMA) and Frequency Division Multiple Access (FDMA) to efficiently allocate spectrum.
Time Slots: Each 200 kHz channel is further divided into 8 time slots, allowing 8 users to share the same frequency.
Cell Structure:
• A GSM network is divided into cells, each served by a base station.
• As a mobile device moves, it switches between cells to maintain connectivity.
• This process is called cell handover or handoff.

• Physical channels: The physical channels are determined by the fact that they have a particular position within the time-
slot. In this way, they are physical, adopting a particular position within the signal. It can be further summarised by saying that
several recurring time-slots on a carrier constitute a physical channel.
• Logical channels: The logical channels are contained within the physical channels. The channels can also be defined by the
purpose they perform. They can be grouped according to their function.
• Broadcast channels: The broadcast channels are channels which provide data to all mobiles to enable them to provide tasks
such as registering.

Control channels: Control channels are those which give commands that are needed for the operation of the link, etc.
• Broadcast Control Channel (BCCH): Transmits system information, such as cell ID, operator identity, and available services.
• Frequency Correction Channel (FCCH): Provides frequency reference signals for synchronization.
• Synchronization Channel (SCH): Transmits timing information for frame synchronization.
• Paging Channel (PCH): Used to page mobile stations for incoming calls.
• Random Access Channel (RACH): Mobile stations use this channel to request access to the network.
• Access Grant Channel (AGCH): The network uses this channel to grant access to a mobile station.
• Stand-Alone Dedicated Control Channel (SDCCH): Used for initial call setup and other control signaling.
• Slow Associated Control Channel (SACCH): Carries slow-rate control information, such as power control commands and
measurement reports.
• Fast Associated Control Channel (FACCH): Used for urgent control information, such-as handoff requests.

Traffic channels: Traffic channels are those which carry the payload data
• Full-rate Traffic Channel (TCH/F): Supports high-speed data rates, such as voice and high-speed data.
• Half-rate Traffic Channel (TCH/H): Supports lower data rates, suitable for basic voice and low-speed data.

Frame Structure for GSM:

• Frequency Band of Operation The most common spectrum allocated to cellular mobile
communication applications is around 900 MHz.
• Number of Logical Channels or Number of Time Slots in TDMA Frame In order to justify the
additional costs of multiplexing, let the minimum number of time slots per TDMA frame be 8 so
as to serve eight simultaneous users.
• Channel Bandwidth The current channel bandwidth being used for analog FM cellular systems
in Europe is 25 kHz. To serve 8 mobile subscribers using TDMA technique, the channel bandwidth
should not exceed 200 kHz.
• Maximum Cell Radius (R) To provide radio service to high traffic in rural areas, let the maximum
cell radius be 35 km.
• Maximum Vehicle Speed ( Vm ) To accommodate mobile
subscriber units traveling on expressways or high- speed trains,
the maximum vehicle speed be 250 km/h.
• Maximum Delay Spread (Δm ) Delay spread is the difference in
propagation delay among different multipath signals arriving at
the same Rx antenna. Typical delay spread in mountainous
regions is about 10 seconds.
• Maximum Coding Delay To avoid unnecessary delays within
the fixed wireless network, which may involve satellite links,
maximum coding delay be Approximately 20 milliseconds.

GSM Call Procedures:

Registration:
• The mobile device registers with the network to obtain
a temporary identifier(TMSI).
• This process involves authentication and authorization.
• The network stores information about the device's
location in the Home Location Register (HLR) and Visitor
Location Register (VLR).

Mobile-Originated Call:
1. Initiation: The mobile device initiates a call by sending
a request on the RACH.
2. Channel Assignment: The network assigns a dedicated control channel
(SDCCH) to the mobile device.
3. Authentication and Encryption: The network authenticates the device and establishes a secure connection.
4. Call Setup: The network determines the routing information and initiates the call setup process.
5. Traffic Channel Assignment: The network assigns a traffic channel (TCH) to the mobile device for voice or data transmission.
6. Call Connection: The call is established, and voice or data communication begins.

GSM Hand-off Procedures:

Handoff Procedures:
Handoff is a crucial process in GSM networks to ensure
seamless connectivity as a mobile device moves from one
cell to another.

Handoff Process:
1. Signal Strength Measurement: The mobile device
continuously monitors the signal strength of the current
cell and neighboring cells.
2. Handoff Decision: The network makes a decision to initiate a handoff based on signal strength, interference levels, and other
factors.
3. New Channel Allocation: The network allocates a new channel in the target cell for the mobile device.
4. Handoff Execution: The mobile device is switched to the new channel and the call is seamlessly transferred.
5. Confirmation: The network confirms the successful handoff and releases resources in the old cell.

There are different types of handoffs:

1. Intracell Handoff:
• Occurs within the same cell to improve signal quality or reduce interference.
• The network reassigns the mobile device to a different time slot or frequency channel.
2.Intercell Handoff:
• Occurs when a mobile device moves from one cell to another.
• The network selects a new base station with a stronger signal and reassigns the call to the new cell.
There are three types of intercell handoff:
• Intercell-Intra-BSC Handoff: Handoff between cells within the same BSC.
• Inter-BSC Intra-MSC Handoff: Handoff between cells in different BSCs but within the same MSC.
• Inter-MSC Handoff: Handoff between cells in different MSCs, often involving roaming.

GSM Services and Features:

GSM Services:
Voice Services:
• Voice calls, including domestic and international calls.
• Voicemail and call forwarding.
• Caller ID and call waiting.
Data Services:
• SMS (Short Message Service): Text messaging.
• MMS (Multimedia Messaging Service): Multimedia messaging, including images, audio, and video.
• GPRS (General Packet Radio Service): Data transfer over cellular networks.
• EDGE (Enhanced Data Rates for GSM Evolution): Faster data speeds than GPRS.
Supplementary Services:
• Call barring: Restricting incoming or outgoing calls.
• Call hold and call waiting.
• Call transfer and conference calls.
• Divert calls: Diverting incoming calls to another number.
• CLIP (Calling Line Identification Presentation): Displaying the caller's number.
• CLIR (Calling Line Identification Restriction): Withholding the caller's number.

GSM Service Quality:

• Call Setup Time: The time it takes to establish a call.
• Call Drop Rate: The rate at which calls are dropped due to network issues.
• Handoff Success Rate: The success rate of handoffs between cells.
• Bit Error Rate: The rate of errors in data transmission.
• Delay: The time it takes for data to be transmitted and received.

GSM Power Management:

• Power Classes: GSM devices are categorized into power classes based on their power output capabilities.
• Power Control: The network dynamically adjusts the transmit power of mobile devices to optimize coverage and battery life.
• Sleep Mode: Mobile devices can enter a low-power sleep mode to conserve battery power when idle.

GSM Security:
• Authentication: Ensures that only authorized users can access the network.
• Encryption: Protects user data from interception and eavesdropping.
• IMSI Privacy: The International Mobile Subscriber Identity (IMSI) is replaced with a temporary identifier (TMSI) to protect user
privacy.

GSM Frequency Hopping:

• Interference Reduction: Frequency hopping helps to reduce interference from other users and external sources.
• Improved Security: It makes it more difficult for unauthorized users to intercept communications.