tldr - Cellular concept, GSM network architecture, GSM network layer and GSM radio link
Cellular Phone
Frequency channels used for radio wave transmission
Duplex device (has 2 channels to transmit and receive simultaneously)
Comparison
LAN: Switch used to interconnect PCs
Cell phone network: Cell tower via a switching office used
Pop Quiz
Question - If a telco has 700 channels, how many users can make use of the cell phone service at the same time?
Answer - Each user requires 2 channels ∴ max users = 700 / 2 = 350
Problem - 1 cell tower for whole city
Not enough channels to support more users
Cell phones require powerful antennas to reach the cell tower
Solution - Cell Concept
Divide city into small cells (hexagons)
Cells can be sized at about 2 x 2 square miles
Each cell has base station (antenna tower and building containing radio equipment)
Base station uses low-power transmitter
Benefits of Low-power Transmitters
Allows cellular phones to have longer battery lives
Allows frequency re-use
Transmissions from base station do not travel very far outside its cell
Hence, non-adjacent cells can use same set of frequencies without interference
By using the same set of frequencies extensively across a big area, it reduces the number of frequencies required to support all users
Frequency Re-use
Base Station
Manages, sends and receives traffic from all mobiles in its geographical area to a distant cellular telephone switch called Mobile Telecommunications Switching Office (MTSO)
Mobile Telecommunications Switching Office (MTSO)
Authenticates wireless customers before establishing connection
Places calls from land-based telephones to wireless customers
Switches calls between cells as mobiles travel across boundaries
Cellular Phone Technologies
Code Division Multiple Access (CDMA)
Time Division Multiple Access (TDMA)
Analog and inefficient. Also obsolete
Global System for Mobile Communications (GSM)
Adopted by Singapore
Global System for Mobile Communications (GSM)
Types of GSM Networks
GSM Network Architecture
Consists of 3 main subsystems
3 main subsystems
Mobile Station (MS)
Mobile Equipment (ME)
Identified internal by IMEI (International Mobile Equipment Identity)
IMEI is allocated by equipment manufacturer
IMEI is registered into EIR (Equipment Identity Register) by the telco
Example: Mobile Phones
Subscriber Identity Module (SIM)
Contains IMSI (International Mobile Subscriber Identity)
IMSI identifies a user on a network
IMSI = Mobile Country Code (MCC) + Mobile Network Code (MNC) + Mobile Subscriber Identity Number (MSIN)
Base Station Subsystem (BSS)
Base Transceiver Station (BTS)
a.k.a. Base Station
Defines a cell’s boundary
Responsible for radio link protocols with MS
Transmits and receives signals from mobile users using different frequencies
Found on elevated locations (e.g. Towers, poles, water tanks, rooftops)
Has a multi-channel frequency band
Base Station Controllers (BSC)
Controls multiple BTSs
Manages radio channel setups and handovers
Connects MS to MSC
Network Switching Subsystem (NSS)
Mobile Switching Centre (MSC)
a.k.a. Mobile Telephone Switching Office (MTSO)
Handles multiple BSCs
Manages locations of mobiles
Manages setup of calls to / from MSs
Manages security
Collects call billing data and sends to billing system
Collects traffic stats for performance monitoring
Home Location Register (HLR)
Stores info about each subscriber belonging to the MSC
Visitor Location Register (VLR)
Maintains info about subscribers currently physically in the region
Authentication Center (AuC)
Databases used for handling authentication and encryption keys
Equipment Identity Register (EIR)
A list of all valid MEs in the network by keeping all valid IMEIs in use
GSM Signalling Protocols
GSM components talk to each other using signal-based protocols
3 layers
Layer 1: Physical Layer
Layer 2: Data Link Layer
Layer 3: Network Layer
Layer 3 is divided into Connection, Mobility and Radio Resource
Radio Resource Management (RRM)
Manages the switching of an ongoing call to a different cell / channel
Components involved
MS
BSS
MSC
2 types of handover - internal and external
Internal Handover (Handled by BSC)
Intra-cell Handover
Transferring call connection to another channel on the same BTS
Switch channel
Same BTS
Intra-cell Handover
Intern Inter-cell Handover
Transferring call connection to another BTS on the same BSC
Switch BTS
Same BSC
Intern Inter-cell Handover
External Handover (Handled by MSC)
MSC Intern Handover
Transferring call connection between BTSs of different BSCs on the same MSC
Switch BTS
Different BSC
Same MSC
MSC Intern Handover
MSC Extern Handover
Transferring call connection to another BTS under the control of a different MSC
Switch BTS
Different BSC
Different MSC
MSC Extern Handover
Mobility Management (MM)
Facilitate location updating
Network needs to be aware of location of MS is order to transfer calls to the proper cells
Handles
Location management & other mobility issues
Authentication & other security issues
Components involved
HLR
VLR
MSC
How it works
Cells are grouped into location areas (LA)
When a MS crosses from one LA to another, it sends location updating messages to MSC and VLR of new LA
New VLR records the location of MS
Location info is sent to HLR from new VLR
HLR sends subset of subscriber’s info to new VLR required for call control
HLR informs previous MSC / VLR to cancel registration of the MS
Radio Link / Air Interface
2 frequency bands used
Uplink (MS to BS)
Downlink (BS to MS)
In GSM-900, both links use 25 MHz frequency bands
Uplink: 890 - 915 MHz
Downlink: 935 - 960 MHz
GSM uses a combination of FDMA & TDMA
Each 25 MHz band divided into 124 frequency sub-bands using FDMA
Each channel allocated 200 kHz carrier spacing
Total channels of both frequency bands = 248 channels
Combined with an 8 time-slot TDMA scheme, total logical channels = 1984
If there is a cluster of 7 cells sharing 1984 logical channels, each cell would use 283 channels
