| Interleaving |
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| Interleaving is the processes of rearranging the bits.
Interleaving allows the error correction algorithms to correct more of
the errors that could have occured during transmission. By interleaving
the code, there is less possibility that a whole chuck of code can be
lost. |
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| Consider this example to see how interleaving works.
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We need to transmit 20 bits. Furthermore, 10 bits can
be transmitted in one transmission burst, and the error correcting mechanism
can correct 3 errors per 10 bits. Take a look at the following two scenarios:
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| With interleaving the receiver is able to get all 20
bits correctly but without interleaving we lose 1 complete burst. |
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| In GSM the interleaving is much more complicated than
the simple example above. The 456 bits outputed by the convolutional encoder
are divided into 57 bit blocks by selecting the 0th, 8th, 16th through
448th bits in the first block, the 1st, 9th 17th through 448th bits in
the 2nd block and so on to have 8 blocks [2].
Then the bits in the first 4 blocks are placed in the even bit positions
for the total block of 456 bits, and the bits in the second set of 4 blocks
are placed in the odd positions [2]. |
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| Multiple
Access |
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| GSM allows many users to use their cellphones at the
same time. GSM uses a combination of Time-Division Multiple Access (TDMA)
and Frequency-Division Multiple Acess (FDMA) to share the limited bandwith
that is provided by regulators to the service providers. |
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| FDMA divides the spectrum into small slices, and then
each frequency slice is seperated in time into many blocks by TDMA. An
individual using GSM receives a block every several blocks. |
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| The transmission of the voice signal is no longer continious
because of the division of the the frequency slice in time, but the data
is transmitted in bursts. The burst assembly operation takes the final
encoded data and groups it into bursts. |
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| Please go to the Multiple
Access Section to learn the specific GSM FDMA and TDMA implementations,how
a frequency is chosen for a particular user, and how the data is divided
into bursts. |
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| Ciphering |
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| Ciphering is used to encrypt the data so that no one
can overhear the conversation of another user. |
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| In GSM the two parties involved in encrypting and decrypting
the data are the Authentication Center (AuC) and the SIM card in the mobile
phone. Each SIM card holds a unique secret key, which is known by the
AuC. The SIM card and AuC then, follow a couple algorithms to first authenticate
the user, and then encryt and decrypt the data. |
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| For authentication, the AuC sends a 128-bit random number
to the mobile phone [1]. The SIM card
uses it's secret key and the A3 algorithm to perform a function on the
random number and sends back the 32-bit result [1].
Since the AuC knows the SIM card's secret key, it performs the same function,
and checks that the result obtained from the mobile phone matches the
result it obtained. If it does, the mobile user is authenticated. |
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| Once authentication has been performed, the random number
and the secret key are used in the A8 algorithm to obtain a 64-bit ciphering
key [1]. This ciphering key is used
with the TDMA frame number in the A5 algorithm to generate a 114 bit sequence
[1]. Note: the ciphering key is constant
throughout a conversation, but the 114 bit sequence is different for every
TDMA frame. |
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| The 114 bit sequence is XORed with the two 57 bit blocks
in a TDMA burst [1]. The only user
that can decrypt the data is the mobile phone or the AuC since they are
the only ones that have access to the secret key, which is needed to generate
the ciphering key, and the 114 bit sequence. |
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| Note that the A3, A5, and A8 algorithms are not known
to the public domain, however some information about A5 has been leaked.
It is known that A5 has a 40-bit key length, which allows for the encryption
to be broken in a matter of days, but since cellular calls have a short
lifetime, the weakness of the algorithm is not an issue [3]. |
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