2G GSM Network Architecture
Comprehensive analysis of 2G GSM network infrastructure, components, protocols, and security mechanisms for telecommunications security professionals.
Executive Summary
The 2G GSM (Global System for Mobile Communications) architecture represents the foundation of modern mobile telecommunications. Understanding its components, protocols, and security mechanisms is crucial for identifying vulnerabilities and implementing effective security measures.
BSS (Base Station Subsystem) handles radio communication between mobile devices and the network core.
NSS (Network Switching Subsystem) manages call routing, authentication, and subscriber data.
OSS (Operations Support Subsystem) provides network management, monitoring, and maintenance.
GSM Network Architecture Overview
The GSM architecture is divided into three main subsystems, each with specific functions and security considerations.
BTS (Base Transceiver Station)
- • Radio transceivers for air interface communication
- • Handles frequency hopping and power control
- • Vulnerable to IMSI catching and jamming attacks
- • Supports A5/1, A5/2, and A5/3 encryption
BSC (Base Station Controller)
- • Manages multiple BTS units
- • Handles handover decisions and frequency allocation
- • Controls radio resource management
- • Interfaces with MSC via A-interface
Security Vulnerabilities
- • Weak A5/1 and A5/2 encryption algorithms
- • IMSI transmission in plain text during location updates
- • Susceptible to fake BTS attacks
- • Limited authentication mechanisms
Security Mechanisms & Vulnerabilities
Understanding the security mechanisms in 2G networks and their inherent vulnerabilities is crucial for security assessment and penetration testing.
A3/A8 Algorithms
Used for subscriber authentication and key generation. A3 generates SRES (Signed Response) and A8 generates Kc (Ciphering Key).
A5 Encryption
- • A5/1: Stronger algorithm, export restricted
- • A5/2: Weaker algorithm, export version
- • A5/3: KASUMI-based, stronger encryption
- • A5/0: No encryption (plain text)
Vulnerabilities
- • A5/1 can be broken in real-time
- • A5/2 is cryptographically weak
- • 64-bit key length is insufficient
- • No forward secrecy
SS7 Protocol
Signaling System 7 is used for network control and subscriber management. Critical for call routing and service delivery.
Key SS7 Messages
- • MAP (Mobile Application Part) for subscriber data
- • TCAP (Transaction Capabilities) for database queries
- • SCCP (Signaling Connection Control Part) for routing
- • MTP (Message Transfer Part) for transport
SS7 Vulnerabilities
- • No authentication between networks
- • Trust-based security model
- • Susceptible to spoofing attacks
- • Limited access controls
Network Interfaces & Protocols
Understanding the interfaces and protocols used in 2G networks is essential for security analysis and penetration testing.
Radio interface between mobile station and BTS
Interface between BTS and BSC
Interface between BSC and MSC
Interface between MSC and VLR
Interface between MSC and HLR
Interface between HLR and VLR
Call Flow & Security Analysis
Understanding the call flow process and identifying security vulnerabilities at each step.
Channel Request
MS requests access to radio channel
Authentication
Network authenticates MS using A3/A8
Ciphering
A5 encryption activated for call
Call Setup
Call routing via MSC to destination
Step 1: Channel Request
- • IMSI can be captured in plain text
- • No authentication required
- • Susceptible to jamming
Step 2: Authentication
- • A3/A8 algorithms are weak
- • 64-bit challenge-response
- • No mutual authentication
Step 3: Ciphering
- • A5/1 and A5/2 are breakable
- • 64-bit key length insufficient
- • No forward secrecy
Step 4: Call Setup
- • SS7 signaling vulnerabilities
- • No message authentication
- • Trust-based security model
Security Recommendations
Best practices for securing 2G networks and mitigating known vulnerabilities.
- • Disable A5/2 encryption globally
- • Implement A5/3 where supported
- • Enable IMSI encryption (if available)
- • Implement SS7 firewall protection
- • Monitor for suspicious signaling traffic
- • Regular security audits and penetration testing
- • Migrate to 3G/4G/5G networks
- • Implement Diameter protocol security
- • Deploy IP-based signaling (SIGTRAN)
- • Implement strong authentication (AKA)
- • Use modern encryption standards
- • Implement network slicing security
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