Evolution of Cellular Systems 11
became of major concern. 2G cellular systems, in general, comprises of GSM, digital
AMPS (D‐AMPS), CDMA, and personal digital communication (PDC).
GSM is the most successful and widely‐used standard in cellular communications
throughout the world, as part of 2G cellular networks. It includes GSM900, GSM‐
railway (GSM‐R), GSM1800, GSM1900, and GSM400. 2G phones using GSM were
first introduced around 1990, first deployed in Finland in July 1991. IS‐95, or
CDMAONE, another technology under the 2G umbrella, based on CDMA, unlike
GSM, which is Time Division Multiple Access (TDMA)‐based. However, the use of
GSM is much wider in scale than IS‐95. The successor of GSM is wideband CDMA
(W‐CDMA), while the successor of IS‐95 is CDMA 2000. In order to understand the
security measures in 2G cellular networks, it is convenient to first focus on the security in the GSM. Supplemental Code Channel (SCH) was introduced in the version of
IS‐95B. It is also known as packet mode transmission for increased efficiency.
1.4.5 Security in GSM
GSM tries to focus on four aspects of security that include authentication of a user,
ciphering of data and signaling, confidentiality of user identity, and the use of subscriber
identity module (SIM) as a security module. SIM is another distinguishing feature of 2G
cellular networks. The SIM is basically a detachable smart card containing subscriber
information, and used for proving its identity with the operator along with the information regarding the kinds of services it is allowed to access. It plays a vital role in the
security process. Authentication requires any particular user to prove that they are a
valid customer requesting the service from a particular operator. Ciphering takes care
of the interception of all the data and signaling. In order to handle confidentiality, GSM
uses international mobile subscriber identity (IMSIs) and, more particularly, uses
Temporary Mobile Subscriber Identity(TMSI) to provide confidentiality for the user, by
making sure that the information of any particular user being in any particular area
is not disclosed to anyone to avoid any intrusion of confidentiality. The SIM card
uses algorithms to develop a secure connection with the operator to carry out safe communication. In case the SIM card is taken by an unauthorized person, there is still a PIN
code security measure in place.
GSM uses A3 and A8 algorithms between a mobile station and the GSM operator.
These are the symmetric algorithms where the same key is used for the encryption and
decryption. These algorithms have a one‐way function, meaning that output can be
found if the inputs are known, but the opposite is not possible. These algorithms are
implemented in the SIM card. The technical details of these algorithms are further
explained in [12].
1.4.5.1 IMSI
International Mobile Subscriber Identity (IMSI) represents the unique number for
every subscriber in the world, and carries the information regarding the home network
of the subscriber and country it belongs to. This particular information can be read
from the SIM if local access to the SIM exists. It basically comprises of up to 15 decimal
became of major concern. 2G cellular systems, in general, comprises of GSM, digital
AMPS (D‐AMPS), CDMA, and personal digital communication (PDC).
GSM is the most successful and widely‐used standard in cellular communications
throughout the world, as part of 2G cellular networks. It includes GSM900, GSM‐
railway (GSM‐R), GSM1800, GSM1900, and GSM400. 2G phones using GSM were
first introduced around 1990, first deployed in Finland in July 1991. IS‐95, or
CDMAONE, another technology under the 2G umbrella, based on CDMA, unlike
GSM, which is Time Division Multiple Access (TDMA)‐based. However, the use of
GSM is much wider in scale than IS‐95. The successor of GSM is wideband CDMA
(W‐CDMA), while the successor of IS‐95 is CDMA 2000. In order to understand the
security measures in 2G cellular networks, it is convenient to first focus on the security in the GSM. Supplemental Code Channel (SCH) was introduced in the version of
IS‐95B. It is also known as packet mode transmission for increased efficiency.
1.4.5 Security in GSM
GSM tries to focus on four aspects of security that include authentication of a user,
ciphering of data and signaling, confidentiality of user identity, and the use of subscriber
identity module (SIM) as a security module. SIM is another distinguishing feature of 2G
cellular networks. The SIM is basically a detachable smart card containing subscriber
information, and used for proving its identity with the operator along with the information regarding the kinds of services it is allowed to access. It plays a vital role in the
security process. Authentication requires any particular user to prove that they are a
valid customer requesting the service from a particular operator. Ciphering takes care
of the interception of all the data and signaling. In order to handle confidentiality, GSM
uses international mobile subscriber identity (IMSIs) and, more particularly, uses
Temporary Mobile Subscriber Identity(TMSI) to provide confidentiality for the user, by
making sure that the information of any particular user being in any particular area
is not disclosed to anyone to avoid any intrusion of confidentiality. The SIM card
uses algorithms to develop a secure connection with the operator to carry out safe communication. In case the SIM card is taken by an unauthorized person, there is still a PIN
code security measure in place.
GSM uses A3 and A8 algorithms between a mobile station and the GSM operator.
These are the symmetric algorithms where the same key is used for the encryption and
decryption. These algorithms have a one‐way function, meaning that output can be
found if the inputs are known, but the opposite is not possible. These algorithms are
implemented in the SIM card. The technical details of these algorithms are further
explained in [12].
1.4.5.1 IMSI
International Mobile Subscriber Identity (IMSI) represents the unique number for
every subscriber in the world, and carries the information regarding the home network
of the subscriber and country it belongs to. This particular information can be read
from the SIM if local access to the SIM exists. It basically comprises of up to 15 decimal