GSM Wireless Communications (3)
GSM
§ Interleaving
With an interleaver between the two
encoders serving to make
the two encoded data sequences approximately statistically
independent of each other.
§ Ciphering and Encryption
The purpose of ciphering is to encode the
burst so that it cannot be interpreted by any other
device than the intended receiver.
The ciphering algorithm in GSM is called the A5
algorithm.
It does not add bits to the burst, meaning that the input
and output to the ciphering process is the same as the input: 456
bits per 20 ms.
§ Burst Formatting
Every transmission from an MS/BTS must
include some extra information
such as the training sequence.
The process of burst formatting
is to add these bits (along with some others such as tail bits) to the basic
speech/data being sent.
In GSM, the input to burst
formatting is the 456 bits received from ciphering. Burst formatting adds a
total of 136 bits per block of 20 ms, bringing the overall total to 592.
§ Ciphering and Encryption
The purpose of ciphering is to
encode the burst so that it cannot be interpreted by any other device than the
intended receiver.
The ciphering algorithm in GSM is
called the A5 algorithm.
It does not add bits to the
burst, meaning that the input and output to the ciphering process is the same
as the input: 456 bits per 20 ms.
GSM service security:
GSM was designed with a moderate level of service security. The system
was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications
between the subscriber and the base station can be encrypted. The development
of UMTS introduces an
optional Universal Subscriber Identity Module (USIM), that uses a longer authentication key to give greater
security, as well as mutually authenticating the network and the user – whereas
GSM only authenticates the user to the network (and not vice versa). The
security model therefore offers confidentiality and authentication, but limited
authorization capabilities, and no non-repudiation.
GSM uses several cryptographic algorithms for security. The A5/1, A5/2 and A5/3 stream ciphers are used for ensuring over-the-air voice privacy. A5/1 was
developed first and is a stronger algorithm used within Europe and the United
States; A5/2 is weaker and used in other countries. Serious weaknesses have
been found in both algorithms: it is possible to break A5/2 in real-time with
a cipher text-only attack, and in January
2007, The Hacker's Choice started the A5/1 cracking
project with plans to use FPGAs that allow A5/1 to be
broken with a rainbow table attack. The system supports
multiple algorithms so operators may replace that cipher with a stronger one.
GSM Services:
§
Fax calls
§
Data calls
§
Short messages
service (SMS)
These GSM non-Speech services mean that GSM is a telecommunication
network rather than a telephony network.
GSM Evolution:
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