Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
  • H04L9/12—Transmitting and receiving encryption devices synchronised or initially set up in a particular manner
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L9/00—Cryptographic mechanisms or cryptographic arrangements for secret or secure communications; Network security protocols
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04K—SECRET COMMUNICATION; JAMMING OF COMMUNICATION
  • H04K1/00—Secret communication
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04W—WIRELESS COMMUNICATION NETWORKS
  • H04W12/00—Security arrangements; Authentication; Protecting privacy or anonymity
  • H04W12/03—Protecting confidentiality, e.g. by encryption
  • H04W12/037—Protecting confidentiality, e.g. by encryption of the control plane, e.g. signalling traffic
• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
  • H04L2209/00—Additional information or applications relating to cryptographic mechanisms or cryptographic arrangements for secret or secure communication H04L9/00
  • H04L2209/80—Wireless

Definitions

• the present invention relates to a synchronizing method in a mobile radio system which lacks a global time reference, i.e. a system which lacks a time reference that is common to all mobiles and base stations in the system. More specifically, although not exclusively, thepresent invention relates to amethod of synchro ⁇ nizing a random bit flow which is superimposed cryptically on the normal traffic flow during call transmission or data transmission between the base stations and the mobiles.
• the proposed method can be applied solely to a so-called TDMA-system, i.e. a mobile radio system in which calls are transferred in frames and time slots by means of time multiplex.
• Calls between a base station and a mobile are cipheredby proces ⁇ sing the speech message in a scrambler, which functions to convert the speech signals into a random sequence in accordance with a given key.
• the speech signals can be superimposed with a random sequence of bits of relatively long duration (several minutes) .
• the cipher key consists in the knowledge of the total bit pattern of the sequence and also knowledge of the time at which the sequence was commenced. An authorized subscriber can plug into a circuit module in the apparatus which stores this sequence and said starting time- point, and is thus able to decode incoming calls.
• Some TDMA mobile radio systems lack a global time reference, i.e. means in the mobile telephone exchange which contains a clock common to the overall system and operative to produce the same time reference for all base stations and mobiles in the system.
• the purpose of such a common time reference is to enable the mobiles and the base stations to be synchronized with one another in certain frames and time slots when synchronizing from signal ⁇ ling or data/speech signals falls away for some reason or other.
• synchronization between the mobile and its base station may be lost, since a brief interruption will occur during the actual transmission or swithching of the call. If the call is also ciphered, a further problem can occur as a result of fall-off of the synchronism of the actual ciphering key concerned, thereby rendering deciphering impossible.
• the present invention is based on the time sharing in different frames of the traffic flow (TDMA-principle) and also on the access to a given ciphering sequence of relatively long duration (about 3 minutes) in relation to the time during which coupling or "hand- off” shall take place.
• the invention is also based on ciphering by superimposing a bit sequence on the normal traffic flow (data or speech and signalling) .
• non-ciphered signalling Prior to handing-off a call or during a given time interval during hand-off where synchronization of the ciphering sequence has ceased, non-ciphered signalling is effec ⁇ ted and thereafter the traffic flow is ciphered with a periodic bit sequency whose period is equal to a frame time interval.
• the object of the present invention is thus to achieve synchroni ⁇ zation of a cryptosequence when setting up or "handing-off" calls in a mobile telephone system which lacks a common time reference.
• Figure 1 is a schematic illustration of two base stations and a mobile unit
• Figure 2 is a time diagram for transmitting and receiving in accordance with the TDMA-principle
• Figure 3 is a time diagram for ciphering in accordance with the proposed method; and Figure 4 illustrates, in more detail, signalling during a given time interval in accordance with Figure 3.K1
• Figure 1 illustrates schematically two base stations BSl and BS2 and a mobile MS which is assumed to move from the base station BSl towards the base station BS2.
• the base station BSl serves traffic within the cell Cl and the base station BS2 serves traffic within the cell C2.
• the cells Cl and C2 have a common border G.
• the speech quality for a coupled call served by the base station BSl over a given radio channel Kl will fall-off.
• Switching of a new radio channel K2 to MS from BSl is effected, by measuring the field strength of the radio signal and making calculations in accordance with known principles. This new channel K2 is served by BS2.
• FIG. 2 illustrates a transmitting frame and a receiving frame (RX and TX respectively) for the mobile MS.
• This key consists of a bit sequence or a section of a longer, random bit sequence E (e.g. about 3 minutes long) , which is superimposed on the traffic flow through modulo 2-addition bit for bit.
• the key E is known to the mobile MS when this is authorized to transmit and receive calls, and the base station BSl transmit data concer- ning the starting time point, i.e. the place in the sequence E at which the bit flow shall commence, i.e. E2 is known to the mobile MS.
• This ciphering technique is known in the art.
• Figure 3 is a time diagram which illustrates application of the method during "hand-off". It is assumed that the base station BSl communicates with the mobile MS and that the communication (the speech) is ciphered by means of a crypto key E2 up to the time point t_. Hand-off takes place at the time t..
• the base station BS2 transmits a non-ciphered signal which denotes when ciphering in accordance with the key E2 shall recommence, i.e. BS2 announces the time point t .
• the traffic flow (speech/data) is ciphered in subsequent frames with a key El which consists of a periodic, random bit sequence having a period equal to the frame interval. This enables the mobile MS to count the number of frame intervals (announced by the non- ciphered signal) until ciphering in accordance with the key E2 is commenced by BS2.
• the ciphering key El can consist of zeros only, i.e.
• the traffic flow in subsequent frames is trans ⁇ mitted unciphered (interval t 2 -t_) .
• the mobile MS can anyway count the number of frame intervals in spite of absence of the periodic bit sequence in the key El, due to the presence of frame synchroni- zation in the control channel SY from the time point t .
• Figure 4 illustrates the sequence during the time interval t ? -t, more clearly.
• the mobile MS is synchronized to the new base station BS2 and the normal traffic flow (data, speech, synchronization) shouldhave commenced if no cipheringhas been employed. Synchronization with respect to the bit flow in the ciphering key E2 up to the time t_, however, has been lost.
• the base station therefore sends to MS a signal SI which indicates how many frames shall pass until ciphering according to key E2 is to be commenced.
• This frame number is assumed to be 13 in the Figure 4 illustration.
• This message can be transmitted in a non-ciphered form on the so-called FACCH (Fast Associated Control Channel) formed by a reserved time slot within a frame, see Figure 2.
• FACCH Fast Associated Control Channel
• This time slot is thus located in frame Rl.
• the ciphered call information is transmitted and ciphered with the key El.
• this key is a periodic bit sequence whose period equals one frame interval and which can be superimposed on the call information by modulo-2 addition. Consequently, the mobile MS is aware of the time at which counting shall commence and also of the number of frame intervals to be counted.
• a confirmation signal Al is transmitted back to the base station BS2 and received in frame R6.
• the base station BS2 thus waits a given length of time (in Figure 4 a time equalling 5 frame intervals) for receipt of the confirma ⁇ tion signal Al. If, for some reason or other, this signal is not received by the base station BS2 within a given period of time, a signal S2 is again transmitted in frame R7 and a new confirmation signal A2 is awaited.
• the signal Al can fall away because, e.g., of fading or difficult surrounding conditions at precisely the interval in which the signal Al is transmitted.
• the signal delay between base station and mobile has been assumed to be about 2-3 frame intervals ( «15 ms) in the Figure 4 illustra- tion. This delay should also be taken into account.
• the ciphering time point t_ will preferably be chosen so that t_-t 3 > 4 times the maximum propagation time. This calculation offers no difficulties, since the mobile is located on the border between two cells Cl, C2 according to Figure 1, i.e. at a maximum distance from a base station.

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• Engineering & Computer Science (AREA)
• Computer Security & Cryptography (AREA)
• Computer Networks & Wireless Communication (AREA)
• Signal Processing (AREA)
• Mobile Radio Communication Systems (AREA)
• Time-Division Multiplex Systems (AREA)

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Applications Claiming Priority (2)

Publications (1)

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Citations (3)

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• 1989
  • 1989-09-12 SE SE8902994A patent/SE464553B/sv not_active IP Right Cessation
• 1990
  • 1990-07-19 KR KR1019910700472A patent/KR970002747B1/ko not_active IP Right Cessation
  • 1990-07-19 WO PCT/SE1990/000497 patent/WO1991004620A1/en active Application Filing
  • 1990-07-19 AU AU63535/90A patent/AU625469B2/en not_active Ceased
  • 1990-07-19 JP JP2512660A patent/JP2978243B2/ja not_active Expired - Lifetime
  • 1990-07-19 CA CA002039699A patent/CA2039699C/en not_active Expired - Fee Related
  • 1990-07-31 NZ NZ234720A patent/NZ234720A/xx unknown
  • 1990-08-09 GB GB9017473A patent/GB2236458B/en not_active Expired - Fee Related
  • 1990-08-09 MY MYPI90001333A patent/MY106831A/en unknown
  • 1990-08-23 DE DE4026698A patent/DE4026698C2/de not_active Expired - Fee Related
  • 1990-09-05 FR FR909011029A patent/FR2652468B1/fr not_active Expired - Lifetime
  • 1990-09-12 CN CN90107657A patent/CN1023283C/zh not_active Expired - Fee Related
• 1995
  • 1995-04-20 HK HK58695A patent/HK58695A/xx not_active IP Right Cessation

Patent Citations (3)

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