Method and apparatus for maintaining continuous synchronous encryption and ...

WHEN THE LINK ESTABLISHMENT PROCEEDS TO A POINT AT

WHICH USER COMMUNICATION CAN BEGIN, THE MICROPROCESSORS IN THE FCU AND IN THE PCU LOAD THE

PIN INTO THE ESCs AND ENABLE THE INCREMENTATION CLOCKS FOR THE ESCs CONTAINED THEREIN RESPECTIVELY AT THE TIME OF THE NEXT SYNC MARKER FOR THE FCU TRANSMIT TIME SLOT BEING USED FOR THE LINK

I

FOR THE DURATION OF THE ESTABLISHED LINK
THE INCREMENTATION CLOCKS IN THE PCU AND
IN THE FCU ARE KEPT SYNCHRONIZED TO EACH
OTHER BY THE RECURRENT SYNC MARKER

609

FIG. 6

f A PCU LINKED WITH A FIRST FCU DETERMINES A NEED FOR A HAND-OFF\ TO A SECOND FCU, SELECTS A TIME CONCURRENT WITH A FUTURE SYNC MARKER AS THE TIME FOR HAND-OFF COMPLETION, AND COMPUTES AS A CONTINUATION VALUE THE EXPECTED CONTENTS OF y THE PCU ESC AT THE TIME SELECTED FOR HAND-OFF COMPLETION

1 2

encrypted mode after sufficient time has elapsed for METHOD AND APPARATUS FOR MAINTAINING encryption synchronization to be reestablished. This of CONTINUOUS SYNCHRONOUS ENCRYPTION course implies that each hand-off is accompanied by a AND DECRYPTION IN A WIRELESS brief period in which the security of the transmitted

COMMUNICATION SYSTEM THROUGHOUT A 5 information is compromised.

HAND-OFF Consequently, what is needed is an encryption tech

nique that overcomes the aforementioned problems of FIELD OF THE INVENTION conventional encryption techniques. That is to say, an

This invention relates in general to radio communica- encryption technique that can continue operating in the tion systems, and more specifically to two-way radio 10 encrypted mode throughout a hand-off with no loss of communication systems comprising a method and appa- information is needed. An encryption technique that ratus for performing synchronous encryption and de- does not degrade the voice quality is needed. Furthercryption. more, an encryption technique that can be built with a

„. _„.„_ r^T„ T lower cost and a smaller size than allowed by conven

BACKGROUND OF THE INVENTION ,5 tional encryption techniques is needed.

iS'Sl^^^^^S&^Z SUMMARY OF THE INVENTION

are well known in the art. Such systems normally em- One aspect of the present invention is a method in a

ploy separate encryption and decryption devices in wireless communication system comprising a plurality

each communication unit for encrypting and decrypting 20 of fixed communication units (FCUs), the FCU com

a transmit and receive path, respectively. The use of prising an FCU encryption synchronization counter

separate encryption and decryption devices in each (ESC), and at least one portable communication unit

communication unit adds to the cost and size of the (PCU), the PCU comprising a PCU ESC, the PCU and

communication unit, as well as complicating the syn- pCU for generating and receiving transmissions of in

chronization of the two devices. 25 formation having a digital format and comprjsing a

In order to maintain synchronization between two periodically recurring synchronization marker for syn

linked communication units, conventional systems must chromzing the transmissions, the method for maintain

transmit an encryption synchronization signal (E-sync) ■ a synchronized encryption and decryption of the

along with the encrypted information. Transmitting the information without interruption throughout a hand

E-sync is not a problem when the niformation is stored 30 Qff ^ ... ^ ^ Qf lo ...

data, which can be interrupted without concern. On the establishment of a second wireless link ^tween a PCU

other hand, transmitting the E-sync m a voice commu- , JIV,,T. , , «. rt. Tw-tt

. ,. \ , 6 \. ,. , and a second PCU for receiving a hand-off of the PCU

nication system becomes somewhat more difficult, be- - _ . . . ... ... °r t, ,

3 ■ ■ c ... .. , ,, from a first wireless link with a first FCU to the second

cause the voice information is continuous and cannot be

interrupted periodically for an E-sync transmission 35

without noticeable gaps or noise bursts in the received _ „ T , ... ,

voice audio signal FCU' the continuatlon value comprising a value ex

Some conventional voice encryption systems "steal" Pfcted ^hf\PCU ESC concurrent w*th hand-°fff c°m" bits periodically from the voice information and use the Pletl0n- ^ loadinS stef comprises the steps of selectstolen bits for the transmission of the E-sync, the theory 40 mS m the pcu a tlme of a sPeclflc future occurrence of being that if the bits are stolen only infrequently, their the periodically recurring synchronization marker subabsence will not seriously degrade the voice audio. Still, sequent to the establishment of the second wireless link missed bits do degrade the voice audio quality some- as the time selected for hand-off completion, and calcuwhat, causing encryption systems based on the use of latinS in the pcu the continuation value from the constolen voice bits not to rate as well in subjective tests of 45 tents of the PCU ESC at an occurrence of the periodiaudio quality while in an encrypting mode as they rate cally recurring synchronization marker prior to the in an unencrypted, i.e., "clear" mode. time selected for hand-off completion in response to the

Yet another problem with conventional encryption selecting step, the continuation value being calculated techniques used in wireless communication systems that m a manner that predicts the contents of the PCU ESC can hand-off a portable communication unit (PCU) 50 a* the time selected for hand-off completion. The loadfrom one fixed communication unit (FCU) to another is ing step further comprises the step of transmitting the associated with the hand-off procedure. The problem completion value and the time selected for hand-off occurs because when two communication units are completion from the PCU to the second FCU in relinked, the encryption device for the transmit path of sponse to the calculating step. The method further corneach of the two communication units supplies the E- 55 prises the step of incrementing synchronously the secsync signal for the decryption device in the correspond- ond FCU ESC and the PCU ESC, starting from the ing receive path of the other communication unit. Thus, continuation value loaded in the loading step and beginafter a hand-off to a new FCU, the encryption synchro- ning at the time selected for hand-off completion, nization is lost for a period of time required to resyn- Another aspect of the present invention is a method chronize the decryption device in the PCU with the 60 in a wireless communication system comprising a plunew E-sync from the new FCU, and the decryption rality of fixed communication units (FCUs), the FCU device in the new FCU with the E-sync from the PCU. comprising an FCU encryption synchronization As the loss of encryption synchronization would counter (ESC), and at least one portable communicacause the loss of all communicated information during tion unit (PCU), the PCU. comprising a PCU ESC, the the ^synchronization period following the hand-off, 65 PCU and FCU for generating and receiving transmisconventional encryption systems for sending continu- sions of information having a digital format comprising ous information such as voice must revert to the clear time division multiplexing to provide at least one transmode prior to each hand-off, followed by a return to the mit channel and at least one receive channel, the at least

wireless link with the second FCU, a continuation value for the contents of a second FCU ESC in the second 3 4

one transmit channel and at least one receive channel FCUs are coupled to the Public Switched Telephone

being carried on a single radio frequency (RF) carrier, Network (PSTN) 114 by a plurality of telephone lines

the digital format further comprising a periodically 116. Those skilled in the art will recognize that the

recurring synchronization marker for synchronizing the wireless communication system according to the pres

transmissions, the method for effecting a synchronized 5 ent invention can also be used with telephone systems

encryption and decryption of the information by gener- other than the PSTN, e.g., a private branch exchange

ating a single encryption sequence for controlling the (PBX). The system further comprises at least one porta

synchronized encryption and decryption in the commu- ble communication unit (PCU) 120 having hand-off

nication units. The method comprises the step of load- capability and transmitting and receiving in a time divi

ing, after establishment of a wireless link between a 10 sjon multiple access and time division duplex

PCU and an FCU, identical start-up contents into the (TDMA/TDD) digital format. The system further

FCU ESC in the FCU and the PCU ESC in the PCU. comprises at least one wired telephone set 124 for send

The loading step comprises the steps of transmitting the i„g md receiving calls to and from a PCU 120. A PCU

start-up contents in a pre-determined encrypted format 120 may ^ communicate with another PCU 120

from the PCU to the FCU over the wireless link, the 15 through one or more of the FCUs 102. pre-determined encrypted format being for protecting Further enabling the wireless communication system

the security of the start-up contents, and decrypting the according to the present invention is the use of a stan

start-up contents in the FCU according to a Pfe-deter- dafd protocol for communication between the plurality

mln*d decrypt'0n, JPr°fCSS f°r l0admg lnt° thC FCU of FCUs and he at least one PCU. The standard protoESC. The metfiodfurther comprisess mcrementmg syn- 20 j defmes messa md procedures for requesting and

chronously the FCU ESC and the PCU ESC, starting ^hlishi wireless communication links, for transmit

from the start-up contents loaded in the loading step m d and ... rf ]m d for transmitti and order to generate identical, synchronized encryption ^ ^mi£unications> and for defini the

sequences tor encrypting and decrypting the intorma- XDMA^DD format used therefor. An

example of

tion on the at least transmit channel and at least one 25 , * A j .. i • A T-»- i n v^j , . „. ,. nTM, , such a standard protocol is the Digital European Cordreceive channel present in the PCU and in the FCU, and , _ . F . .. /T>_° . * , _,

c £ rrlT , • 4. Ti/^tt ^ u less Telecommunications (DECT) standard. The

performing m the FCU and in the PCU the synchro- ~—. J J • *■ J • T- T- ,

. , B .. , , .. ?■ c .. DECT standard is defined in the European Telecommzed encryption and decryption of the information in . __„ *1_

f,,;^ A- ;„ mumcation Standard document prETS 300 175 entitled

accordance with the encryption sequences generated in . ,_ _ _ ,r _

„„„ rl> tn t. ;„„,„m„„t;„„ io "Digital European Cordless Telecommunications Cornresponse to the incrementing step. 30 °, - ,. , , . .„„.

mon Interface, dated August 1991, and produced by

BRIEF DESCRIPTION OF THE DRAWINGS the European Telecommunications Standards Institute,

FIG. 1 is a block diagram of a wireless communica- applicable parts two, three, five and seven of which

tion system in accordance with a preferred embodiment document are hereby incorporated by reference, of the present invention. 35 Wlth reference to FIG. 2, a TDMA/TDD frame 201

FIG. 2 is a diagram showing the contents of a time constructed according to the DECT standard as used in

division multiple access and time division duplex accordance with the preferred embodiment of the pres

(TDMA/TDD) frame constructed according to the ent invention comprises twelve time slots 202 for FCU

Digital European Cordless Telecommunications transmission and twelve time slots 203 for PCU trans

(DECT) standard as used in accordance with the pre- 40 mission. The time slots 202, 203 are paired on a posi

ferred embodiment of the present invention. t,onal basls for transmitting and receiving information.

FIG. 3 is a block diagram of a fixed communication F°r example, an FCU 102 (FIG. 1) transmitting in the

unit (FCU) in accordance with the preferred embodi- tlme slot 202 labeled "0" would receive m the time slot

ment of the present invention. 2^ a's0 labeled "0." Each time slot 202, 203 comprises

FIG. 4 is a block diagram of a portable communica- 45 a synchronization part 204 comprising a synchronization unit (PCU) in accordance with the preferred em- tion marker for synchronizing a linked PCU 120 (FIG. bodiment of the present invention. 1) and FCU 102, and a data part 205. The data part 205

FIG. 5 is a block diagram of an encryption synchroni- comprises a control part 206 for passing control infor

zation counter (ESC) in accordance with the preferred mation, e.g., frame and slot identification and other

embodiment of the present invention. 50 control messages between the linked PCU 120 and FCU

FIG. 6 is a flow chart of a method of effecting a 102, and a user data part 208 for carrying user data, e.g.,

synchronized encryption and decryption of information speech.

by generating a single encryption sequence in accor- The synchronization part 204 and the control part

dance with the preferred embodiment of the present 206 are used to synchronize the encryption and decryp

invention. 55 tion of the information carried in the user data part 208

FIG. 7 is a flow chart of a method of maintaining a as well as any user signaling that is carried in the control

synchronized encryption and decryption of information part 06 in accordance with the present invention. By

without interruption throughout a hand-off in accor- synchronizing the encryption and decryption processes

dance with the preferred embodiment of the present through the use of the existing parts 204, 206 of the

invention. 60 DECT time slot 202, 203, it is not necessary to steal bits

... „c . ... from tne user data part 208. Thus, the encryption and

DESCRIPTION OTA PREFERRED decryption technique according to the present inven

EMBODIMENT tjon does not degrade voice quality, as do conventional

With reference to FIG. 1, a preferred embodiment of encryption and decryption techniques that employ bit

a wireless communication system according to the pres- 65 stealing for synchronization. Also, by synchronizing the

ent invention comprises a plurality of fixed communica- encryption and decryption circuitry by means of syn

tion units (FCUs) 102 that provide radio coverage in a chronization signals already available in the FCU 102

plurality of radio coverage areas 108, 110, 112. The and the PCU 120 (FIG. 1), fewer parts are needed for