25-04-2012, 11:41 AM
Security In Wireless Cellular Networks
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Introduction
Cellular Communication has become an important part of our daily life. Besides using cell phones for voice communication, we are now able to access the Internet, conduct monetary transactions, send text messages etc. using our cell phones, and new services continue to be added. However, the wireless medium has certain limitations over the wired medium such as open access, limited bandwidth and systems complexity. These limitations make it difficult although possible to provide security features such as authentication, integrity and confidentiality. The current generation of 3G networks have a packet switched core which is connected to external networks such as the Internet making it vulnerable to new types of attacks such as denial of service, viruses, worms etc. that have been used against the Internet.
Generations Of Cellular Networks
Cellular Networks have been around since the 1980s and each year their subscribers increase at a very fast rate. First generation (1G) networks were the first cellular networks introduced in the 1980s. They were only capable of transmitting voice at speeds of about 9.6 kbps max. In the US the system was known Advanced Mobile Phone System (AMPS) and in Europe the Nordic Mobile Telephony (NMT). Both these technologies used analog modulation to transmit data as a continuously varying waveform.
1G systems had some limitations such as no support for encryption, poor sound quality and inefficient use of the spectrum due to their analog nature. Second generation (2G) cellular networks also known as personal communication services (PCS) introduced the concept of digital modulation meaning that voice was converted into digital code, and then into analog (radio) signals. Being digital, they overcame certain limitations of 1G systems. Various 2G technologies have been deployed around the world. Code Division Multiple Access (CDMA), North American Time Division Multiple Access (NA-TDMA) and digital AMPS (D-AMPS) have been deployed in the US whereas Global System for mobile communication (GSM) has been deployed in Europe and USA and Personal Digital Cellular (PDC) has been deployed in Japan.
2G And 2.5G
GSM is the most widely adopted 2G technology in the world. Although it was initially employed in Europe, it has become a global technology with subscribers in about 197 countries. Its specifications were completed in 1990 and service began in 1992. This paper will not delve into the techniques of 2G/2.5G because it will soon be replaced by 3G. Interested readers are encouraged to look at [Imai05] for more details. However, some of the data services which are part of the 2.5G extension are
• Short Messaging Service (SMS): Transfer of messages between cell phones. Large messages are truncated and sent as multiple messages.
• High-Speed Circuit-Switched Data (HSCSD): This was the first attempt at providing data at high speeds data over GSM, with speeds of up to 115 kbps. This technique cannot support large bursts of data. HSCSD was not widely implemented and GPRS became a more popular technique.
• General Packet Radio Service (GPRS): This technique can support large bursty data transfers. In order to support this two new elements have to be added to existing networks. Service GPRS support node (SGSN) for security mobility and access control and Gateway GPRS support node (GGSN) in order to connect to external packet switched networks.
• Enhanced Data Rates for GSM Evolution (EDGE): The standard GSM uses GMSK modulation. Edge uses 8-PSK modulation. GPRS and EDGE combined provide data rates of up to 384 kbps.
• Cellular Digital Packet Data (CDPD): CDPD is a packet based data service. CDPD is able to detect idle voice channels and uses them to transfer data traffic without affecting voice communications.
Security Issues In Cellular Networks
The infrastructure for Cellular Networks is massive, complex with multiple entities coordinating together, such as the IP Internet coordinating with the core network. And therefore it presents a challenge for the network to provide security at every possible communication path.
Limitations Of Cellular Networks
Compared to Wired Networks, Wireless Cellular Networks have a lot of limitations.
1. Open Wireless Access Medium: Since the communication is on the wireless channel, there is no physical barrier that can separate an attacker from the network.
2. Limited Bandwidth: Although wireless bandwidth is increasing continuously, because of channel contention everyone has to share the medium.
3. System Complexity: Wireless systems are more complex due to the need to support mobility and making use of the channel effectively. By adding more complexity to systems, potentially new security vulnerabilities can be introduced.