GSM

The protocol architecture of GSM, as depicted in Figure 4.7, encompasses various layers and entities, primarily focusing on the Um interface, which handles communication between mobile stations (MS) and base transceiver stations (BTS). Here's a breakdown of the key elements and functionalities described in the passage:

1. Physical Layer (Layer 1):

   • Responsible for radio-specific functions such as modulation/demodulation, synchronization, burst creation, and channel quality measurement.
   • Utilizes Gaussian Minimum Shift Keying (GMSK) for digital modulation.
   • Performs encryption/decryption of data between MS and BTS over the air interface.
   • Adjusts access timing based on round trip times (RTT) to minimize guard space and optimize channel usage.

2. Data Link Control Layer (Layer 2):

   • LAPDm protocol, derived from ISDN's LAPD, facilitates reliable data transfer, re-sequencing of data frames, and flow control between MS and BTS.
   • Offers services for segmentation/reassembly of data and acknowledged/unacknowledged data transfer.

3. Network Layer (Layer 3):

   • Radio Resource Management (RR):
     • Handles setup, maintenance, and release of radio channels.
     • Implemented partially in the BTS and partially in the Base Station Controller (BSC).
   • Mobility Management (MM):
     • Manages registration, authentication, identification, and location updating of mobile subscribers.
     • Provides temporary mobile subscriber identity (TMSI) for privacy over the air interface.
   • Call Management (CM):
     • Includes entities for call control (CC), short message service (SMS), and supplementary service (SS).
     • CC establishes point-to-point connections, manages call parameters, and facilitates DTMF signaling.
     • SMS enables message transfer over control channels, while SS offers various supplementary services.

4. Abis and A Interfaces:

   • Abis interface handles internal communication within the GSM system.
   • A interface connects BTS to BSC and utilizes PCM systems for data transmission.
   • LAPD is used for layer two at Abis, and BTSM manages BTS.

5. Signaling System No. 7 (SS7):

   • Facilitates signaling between Mobile Switching Centers (MSC), BSCs, Home Location Register (HLR), Visitor Location Register (VLR), Authentication Center (AuC), Equipment Identity Register (EIR), and Operation and Maintenance Center (OMC).
   • MSC can control BSS via BSS Application Part (BSSAP).

Overall, the GSM protocol architecture comprises layers and protocols designed to ensure efficient and reliable communication between mobile stations and the GSM network infrastructure.

One of the fundamental features of the GSM system is its capability for automatic worldwide localization of users, ensuring seamless connectivity regardless of the user's location. Here's how GSM achieves this and the key components involved:

1. Periodic Location Updates: GSM performs periodic location updates even when a mobile station (MS) is not in active use, ensuring that the Home Location Register (HLR) always contains information about the user's current location area.

2. Global Phone Number: The same phone number (Mobile Station International ISDN Number or MSISDN) is valid worldwide for a GSM user. This number is associated with the SIM card rather than the device itself.

3. Unique Identification: GSM uses the International Mobile Subscriber Identity (IMSI) for internal unique identification of subscribers. IMSI comprises a mobile country code (MCC), mobile network code (MNC), and mobile subscriber identification number (MSIN).

4. Temporary Identification: To protect user privacy, GSM uses the Temporary Mobile Subscriber Identity (TMSI), a temporary local subscriber identification selected by the Visited Location Register (VLR).

5. Roaming Support: GSM supports roaming, allowing users to maintain uninterrupted service while moving between different networks or countries. The Mobile Station Roaming Number (MSRN) is used to find a subscriber for incoming calls during roaming.

To illustrate how a call is routed to a GSM subscriber:

• Mobile Terminated Call (MTC):

  1. User dials the GSM subscriber's phone number.
  2. Fixed network forwards the call to the Gateway Mobile Switching Center (GMSC).
  3. GMSC identifies the subscriber's HLR and requests call setup.
  4. HLR validates the number, retrieves MSRN from the VLR, and forwards MSC information to GMSC.
  5. GMSC routes the call to the appropriate MSC.
  6. MSC initiates paging and establishes connection upon MS response.

• Mobile Originated Call (MOC):

  1. MS transmits a call setup request to the Base Station Subsystem (BSS).
  2. BSS forwards the request to the Mobile Switching Center (MSC).
  3. MSC verifies user authorization and resource availability.
  4. If resources are available, MSC establishes a connection with the fixed network.

Throughout call setup and communication, various messages are exchanged between the MS, BTS, and MSC to ensure security, authentication, channel assignment, and connection establishment. These steps ensure seamless and secure communication for GSM subscribers worldwide.

GSM provides robust security services leveraging confidential information stored in the Authentication Center (AuC) and the individual Subscriber Identity Module (SIM) cards. These services include access control and authentication, confidentiality, and anonymity. Let's delve deeper into each aspect:

1. Access Control and Authentication:

   • Users must authenticate themselves to access the SIM card, typically using a PIN.
   • Subscriber authentication is based on a challenge-response mechanism. The Authentication Center (AuC) generates a random challenge (RAND), which is sent to the SIM. The SIM computes a signed response (SRES) using the authentication key (Ki) and returns it to the network for verification.
   • Algorithm A3 is employed for authentication.

2. Confidentiality:

   • Once authenticated, GSM ensures the confidentiality of user-related data. Encryption is applied to voice, data, and signaling between the Mobile Station (MS) and Base Transceiver Station (BTS).
   • This encryption protects communication between the MS and BTS but does not extend end-to-end throughout the entire fixed GSM/telephone network.
   • Algorithm A5 is used for encryption.

3. Anonymity:

   • GSM preserves user anonymity by encrypting all transmitted data and avoiding the use of user identifiers over the air interface.
   • Instead of transmitting user identifiers directly, GSM utilizes temporary identifiers (TMSI), newly assigned by the Visitor Location Register (VLR) after each location update. The VLR can change the TMSI at any time, enhancing user privacy.

4. Security Algorithms:

   • Three algorithms are specified for security services in GSM: A3 for authentication, A5 for encryption, and A8 for generating a cipher key (Kc).
   • Initially, only algorithm A5 was publicly available. However, A3 and A8 were later published on the internet in 1998, demonstrating the ineffectiveness of security by obscurity.
   • These algorithms are implemented in the SIM and AuC and can be proprietary. Only A5 must be identical for all providers.

5. Encryption Process (A5 and A8):

   • Encryption ensures privacy by encrypting all user-related messages over the air interface.
   • The cipher key (Kc) is generated using the individual key (Ki) and a random value (RAND) by applying algorithm A8. Both the SIM and the network calculate the same Kc based on the random value RAND.
   • The key Kc is not transmitted over the air interface, enhancing security.

Overall, GSM's security services provide robust protection against unauthorized access, eavesdropping, and identity exposure, ensuring the integrity and confidentiality of user communications within the GSM network.

High-Speed Circuit-Switched Data (HSCSD) is an enhancement to GSM's data transmission capabilities, allowing for higher data rates by bundling multiple Traffic Channel (TCH) slots within a TDMA frame. In HSCSD, data is transmitted using circuit-switched technology, which establishes a dedicated connection between the user's mobile device and the network for the duration of the data transfer. This is different from packet-switched technology, where data is divided into packets and sent over the network as needed.

How HSCSD Works:

1. Allocation of TCHs:
   • Mobile Stations (MS) request one or more TCHs from the GSM network, allocating several TDMA slots within a frame.
   • Allocation can be asymmetrical, with more slots on the downlink than the uplink, reflecting typical user behavior of downloading more data than uploading.
2. Software Upgrades:
• HSCSD typically only requires software upgrades in both the MS and the Mobile Switching Center (MSC) to split a traffic stream into multiple streams using separate TCHs and combine them again.

Advantages of HSCSD:

1. Higher Data Rates:
   • By bundling multiple TCHs, HSCSD can achieve higher data rates, theoretically up to 115.2 kbit/s.
2. Symmetric/Asymmetric Allocation:
• Allows for flexible allocation of TCHs, catering to varying uplink and downlink requirements.

Disadvantages of HSCSD:

1. Connection-Oriented Mechanisms:
   • HSCSD still uses the connection-oriented mechanisms of GSM, which may not be efficient for bursty and asymmetric computer data traffic.
2. Idle Channels:
   • Channels may remain idle most of the time during typical web browsing, leading to inefficient use of allocated resources and increased service costs.
3. Signaling Overhead:
   • Each channel requires signaling during handover, connection setup, and release, leading to increased signaling overhead and higher probability of blocking or service degradation.
4. Cost Implications:
• Users are typically charged for each channel allocated, making HSCSD less economical for bursty internet traffic where channels may not be fully utilized.

The General Packet Radio Service (GPRS) represents a significant advancement in mobile data transmission, offering fully packet-oriented transfer and a range of benefits over traditional GSM data services. Here's a detailed breakdown of GPRS and its key concepts:

Overview of GPRS:

1. Packet-Oriented Transfer:
   • GPRS provides packet mode transfer suited for applications with varying traffic patterns, such as frequent transmission of small volumes or infrequent transmissions of small to medium volumes.
2. Efficiency and Quality of Service (QoS):
   • Designed to use network resources more efficiently, GPRS offers a selection of QoS parameters for service requesters, allowing for broadcast, multicast, and unicast services.
3. Cost Structure:
   • Network providers typically charge based on volume rather than connection time, aligning with the usage patterns of packet-oriented internet applications.
4. 'Always On' Connectivity:
   • GPRS offers an 'always on' characteristic, eliminating the need to set up a connection prior to data transfer, which is beneficial for users accessing internet services.
5. Evolution Towards UMTS:
   • GPRS serves as a precursor to Universal Mobile Telecommunications System (UMTS), utilizing similar infrastructure and paving the way for more advanced mobile data technologies.

Key Concepts and Characteristics:

1. Dynamic Allocation of Resources:
   • GPRS allows for dynamic allocation of time slots within TDMA frames, based on current load and operator preferences, enabling efficient resource utilization.
2. Flexible Data Rates:
   • Depending on coding schemes and channel allocation, GPRS can achieve transfer rates of up to 170 kbit/s, with operators typically reserving at least one time slot per cell to ensure a minimum data rate.
3. Packet Transfer Services:
   • GPRS supports point-to-point (PTP) packet transfer services, including both connection-oriented and connectionless network services, catering to various application requirements.
4. Quality of Service Profiles:
   • Users can specify QoS profiles, determining service precedence, reliability class, delay class, and user data throughput, with adaptive resource allocation to meet these specifications.
5. Mobility Management:
   • GPRS includes mobility management functions for authentication, location management, and ciphering, ensuring seamless connectivity and security for mobile users.
6. Protocol Architecture:
   • GPRS utilizes protocols such as GPRS Tunneling Protocol (GTP), Subnetwork Dependent Convergence Protocol (SNDCP), and Base Station Subsystem GPRS Protocol (BSSGP) for efficient packet transfer and routing.

Security Features:

1. Authentication and Access Control:
   • GPRS provides authentication and access control mechanisms to verify user identities and ensure secure access to the network.
2. User Information Confidentiality:
   • Confidentiality measures protect user data from unauthorized access or interception during transmission.
3. Anonymous Service:
   • GPRS supports anonymous services, allowing users to access certain services without revealing their identity.

Network Elements:

1. Gateway GPRS Support Node (GGSN):
   • Acts as an interworking unit between the GPRS network and external packet data networks, performing routing, address conversion, and encapsulation.
2. Serving GPRS Support Node (SGSN):
   • Supports mobile stations via the Gb interface, handling user authentication, location management, billing, and security functions.
3. GPRS Register (GR):
   • Stores GPRS-relevant data and coordinates with SGSNs for user management and mobility tracking.

Data Transmission and Protocol Stack:

1. End-to-End Transparency:
   • Applications interact with GPRS networks via standard protocols such as TCP/IP, with data routed seamlessly from mobile stations to external networks.
2. Address Translation:
• GGSNs perform Network Address Translation (NAT) to assign private IP addresses to mobile stations, enhancing security and privacy.