Cryptography – Communication system using cryptography – Wireless communication
Reexamination Certificate
1999-02-12
2004-11-02
Meislahn, Douglas J (Department: 2137)
Cryptography
Communication system using cryptography
Wireless communication
C380S273000
Reexamination Certificate
active
06813355
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method and arrangement for ciphering information transfer. The invention can be advantageously applied in a time division multiple access (TDMA) cellular system offering broadband circuit switched services.
The prior art will be now described, discussing first the use of time slots in the GSM (Global System for Mobile communications) system and the coding of information in a burst transferred in a time slot. Then it will be described a known method for ciphering information transfer in said system as well as the disadvantages related to it.
Current mobile communication networks generally use the time division multiple access (TDMA) method. For example, in the GSM system each traffic channel uses TDMA frames comprising eight time slots. In mobile communication systems a call is conventionally established in such a manner that one time slot is reserved for the call and the transmission channel provided by that time slot is then used for the whole duration of the call. If, however, the mobile station moves from the area of a base station to that of another, a handover is carried out and a channel using a new time slot is established between the new active base station and the mobile station.
FIG. 1
shows a GSM TDMA frame comprising eight time slots
0
to
7
. Separately shown are transmission frame TX and reception frame RX. Here, transmission frame means a frame sent by the mobile station, i.e. an uplink TDMA frame, and reception frame means a frame received by the mobile station, i.e. a downlink TDMA frame. A cross in
FIG. 1
marks the time slot
1
which in the call depicted by the example is used in both uplink and downlink transmission. It should be noted that in the downlink and uplink directions there is a delay between the frames, which is why time slots represented by corresponding numbers are not simultaneous in the different transfer directions.
Broadband high speed circuit switched data (HSCSD) services, in which a call uses more than one time slot in order to speed up the communications rate, have been introduced especially for data transmission services. The number of uplink time slots may be equal to that of downlink time slots, in which case the configuration is symmetrical, or it may be unequal, in which case the configuration is asymmetrical. Time slots used are specified during call establishment and the system indicates the time slots used as well as the related parameters to the mobile station. Said parameters include, for example, the ciphering key used in ciphering/deciphering. The number of time slots used can also be changed during a call.
FIG. 2
shows a TDMA frame in conjunction with a HSCSD call using two time slots
1
and
2
in the uplink direction TX and three time slots
0
to
2
in the downlink direction RX.
FIG. 3
illustrates the use of a time slot in the GSM system. A burst transferred in a time slot contains training sequence symbols TSS
33
, two sequences IS
1
and IS
2
consisting of information symbols,
31
and
32
, and tail symbols TS
1
and TS
2
,
30
and
34
, respectively. In addition, time slots are separated by guard periods GP,
35
. A conventional GSM system uses GMSK modulation to modulate the data into the burst.
Furthermore, there are new solutions to increase the transfer capacity by changing the method of modulation of the burst transmitted in a time slot. One such solution is the so-called EDGE (Enhanced Data rates for GSM Evolution) system which is now being developed and is based on the GSM system. In that solution, GMSK modulation may be replaced by binary order quadrature amplitude modulation (B-O-QAM), quadrature order quadrature amplitude modulation (Q-O-QAM) or by code pulse modulation (CPM), for example. Possible characteristics of the EDGE system are described e.g. in [1]. To illustrate the invention we will examine in this patent application some of the arrangements to implement the EDGE system discussed in said document. Those arrangements will be below called the “EDGE system” although the characteristics of the eventual implemented EDGE system might be different from those described here.
When using fast modulation, the symbol rate can be generated from a 13-MHz clock frequency by dividing by 36, for example, while in the conventional GSM system the divisor is 48. Thus the symbol rate becomes 361.111 ksps (kilosymbols per second). When using Q-O-QAM modulation, a symbol comprises 2 bits, so the modulation bit rate is 722.222 kbps (kilobits per second). When using B-O-QAM modulation, a symbol comprises one bit, so the modulation bit rate is 361.111 kbps.
Table 1 below lists the most important modulation characteristics of the GSM system and the system using QAM modulation.
TABLE 1
Modulation
GSM
B-O-QAM
Q-O-QAM
Time slot length
576.92 &mgr;s
576.92 &mgr;s
576.92 &mgr;s
Clock frequency divisor
48
36
36
Symbol rate
270.833 ksps
361.111 ksps
361.111 ksps
Symbol sequence length
3.692 &mgr;s
2.769 &mgr;s
2.769 &mgr;s
Modulation bit rate
270.833
361.111 kbps
722.222 kbps
kbps
Symbols in burst
156.25
208.333
208.333
Symbols in TDMA frame
1250
1666.666
1666.666
So, using QAM modulation, a burst in one time slot can transfer 208.333 symbols, whereas the GSM system can only transfer 156.25 symbols.
Table 2 below shows the time slot sequence lengths in the GSM system and in the system based on QAM modulation. The portion of the stealing flag is shown separately in the numbers of information symbols and bits.
TABLE 2
Modulation
GSM
B-O-QAM
Q-O-QAM
Tail symbols /TS
3
2
2
Information symbols /IS
57 + 1
81 + 1
81 + 1
Information symbols
114 + 2
162 + 2
326 + 2
/burst
Symbols in training
26
28
28
sequence /TSS
Guard period GP
8.25
12.333
12.333
(30.462 &mgr;s)
(34.153 &mgr;s)
(34.153 &mgr;s)
In the GSM system the ciphering of information transferred is based on the use of the so-called A5 ciphering algorithm. The ciphering algorithm is used to produce a 114-bit pseudo-random ciphering sequence which is used to encrypt the 114 information bits transferred in one burst. A ciphered 114-bit sequence is produced by performing an exclusive-or (xor) operation between the unciphered information and the ciphering sequence. Similarly, the ciphered information is deciphered at the receiving end by producing the same ciphering sequence and carrying out an xor operation between the ciphering sequence and the received bit sequence.
The A5 algorithm is not public but as regards its structure it is a conventional ciphering algorithm using two input parameters. The first input parameter, so-called COUNT value, is derived from the TDMA frame number and transferred on the synchronization channel SCH. The COUNT value is used for producing ciphering blocks for bursts in sequential TDMA frames. The second input parameter is a call specific ciphering key Kc which is transferred on a data transmission channel prior to call establishment.
Different connections and time slots within a TDMA frame are distinguished using separate ciphering keys. If a connection uses more than one time slot, ciphering key Kc is used in time slot
0
if that is in use. In addition, ciphering key Kc is used to produce the ciphering keys Kcn (n=0 to 7) for the other time slots.
The method above is used for creating for all bursts different ciphering bit blocks within a TDMA frame and between TDMA frames. The use of multiple input parameters in the A5 algorithm makes it possible to avoid long text sequences ciphered with one and the same ciphering block. This way, the encryption function of the conventional GSM system can be made comparatively reliable.
Ciphering methods for the GSM system are described in more detail in [2], chapter 4.
Prior-art arrangements, however, have limitations. The reliability of encryption largely depends on how much information is transferred using the same ciphering algorithm and key. The greater the amount of information transferred using the same algorithm/ke
Meislahn Douglas J
Nokia Mobile Phones Limited
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