Cryptography – Communication system using cryptography – Symmetric key cryptography
Reexamination Certificate
1999-08-13
2004-08-17
Barrón, Gilberto (Department: 2132)
Cryptography
Communication system using cryptography
Symmetric key cryptography
C380S261000, C380S270000, C380S265000, C380S274000
Reexamination Certificate
active
06778670
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to communications systems, and, more particularly, to a method and apparatus for encrypting and decrypting data in a communications system.
2. Description of the Related Art
Modern day electronic products, such as computers and telecommunications devices, are generally faster and more efficient than their predecessors. Two important factors that have contributed to the improved performance of today's electronic products is the efficiency and faster rate at which these products transmit and receive signals. However, given the customer demand for faster and more responsive products, designers are constantly seeking ways to achieve a higher bandwidth while controlling the costs.
The overall performance of applications, particularly real-time telecommunication applications, can be improved by increasing the bandwidth available for transmitting signals. One example of a real-time telecommunication application where an increased bandwidth is desirable is a Wireless Local Loop (WLL) network.
Wireless Local Loop is quickly emerging as the technology that promises to deliver telephone service in geographical areas where using conventional copper telephone lines is cost prohibitive, or in a case where a telephone line exists, radio access technologies such as WLL provide other companies an opportunity to provide competitive services. Installing the last quarter of a mile of the telephone wire to a subscriber station is usually one of the most costly portions of the traditional telephone network, primarily because of the expenses associated with labor and maintenance. The telephone companies, which are usually responsible for maintaining the telephone wire, are frequently plagued with the daunting task of repairing the damage to the telephone lines caused by inclement weather, falling trees, digging, and by the recurring problem of copper wire theft. Accordingly, to circumvent the problems that are typically associated with the “traditional” telephone network, system designers turned to WLL technology, which is proving to be a promising and viable alternative.
FIG. 1
illustrates a block diagram of a WLL network
100
. The WLL network
100
includes a Wireless Subscriber Unit (WSU)
110
at a subscriber station
115
that communicates with a remote Basestation Transceiver System (BTS)
120
. The data flow from the WSU
110
to the BTS
120
is referred to as an uplink connection, and the data flow from the BTS
120
to the WSU
110
is referred to as a downlink connection. The BTS
120
links the WSU
110
to a central office
130
, thus allowing a user at the subscriber station
115
to communicate with other subscriber stations (not shown) through the central office
130
. A connection
132
between the BTS
120
and the central office
130
may be made via a wire-line, fiber or microwave link, depending on the bandwidth, distance, and the terrain.
The heart of the WLL network
100
is the “wireless” interface between the BTS
120
and the WSU
110
, where the need for the copper loop is eliminated. The WSU
10
, located in the home or office, provides a radio frequency (RF) interface to an existing phone
135
or modem
140
, usually through an RJ-11 type plug (not shown). The connection between the subscriber station
115
and the central office
130
is typically as reliable and clear as the copper wire version.
The counterpart to the WSU
110
is the BTS
120
, which is generally located in the field. As is common in most wireless systems, the BTS
120
serves as a control station for the WSU
110
by providing, over a designated pilot channel, synchronization and control information necessary to initiate and maintain two-way communication. In communication systems employing time division multiple access (TDMA), a process well known in the art, the WSU
110
selects the channel frequency and the specific time slot based upon the availability and quality of the channels in the coverage area.
Most wireless communication systems, including the WLL network
100
, operate in accordance with industry defined standards. For example, two popular standards for the WLL network
100
are Personal Handyphone System (PHS) and Digital Enhanced Cordless Telecommunications (DECT). The PHS and DECT standards, as well as other WLL standards, define the format for transmitting and receiving data, error checking algorithm, retransmission scheme, and other such parameters that are relevant to wireless communications systems.
The DECT standard for the WLL network
100
, for example, defines a 10-millisecond TDMA frame that comprises twenty-four time slots, where generally twelve slots are reserved for transmitting and twelve for receiving. DECT supports voice, analog data, and packet data communications. Voice and analog data communications are full duplex, whereas packet data communications are simplex in nature. Packet data communications can take advantage of directing all slots in one direction, uplink or downlink, with the exception of one slot for the reverse acknowledgement channel. Depending upon the bandwidth allocated by the system, an uplink or downlink channel can utilize between one and twenty-three slots for transferring packet data. When no data is waiting to be sent, the bandwidth is de-allocated and assigned to other users. Thus, in an uplink connection, the BTS
120
of the WLL network
100
can transmit data on up to twenty-three slots that are allocated for transmitting data per each frame to the WSU
110
. Assuming all of the data is successfully transmitted over the twenty-three time slots, then new data may be transmitted on the twenty-three transmit time slots of the next frame. On the other hand, if not all of the data is successfully transmitted to the BTS
120
because of transmission errors, then that data is retransmitted over the twenty-three transmit time slots in accordance with the DECT retransmission scheme.
The retransmission scheme of a DECT WLL network
100
requires an acknowledgement to be generated by the peer station on a slot-by-slot basis for duplex bearers. Double simplex bearers' acknowledgements are on a logical bearer number (LBN) basis contained in a MAC-MOD
2
-ACK message in a reverse bearer. The BTS
120
or the WSU
110
may be the peer station, depending on whether the connection is an uplink or downlink connection. That is, in an uplink connection, the BTS
120
is the peer station, and in a downlink connection, the WSU
110
is the peer station. A “good” acknowledgement, a request to advance, from the peer device indicates a good transmission and new data should be transmitted in the next frame. On the other hand, a “bad” acknowledgement, a request to retransmit, indicates a bad transmission, and thus requires retransmission of data that was not received correctly by the peer station.
In communications systems, particularly real-time in communications systems, it is desirable to have the ability to quickly and efficiently encrypt and decrypt data. Efficient means of encryption and decryption can generally enhance the overall performance of the communications systems. One communications system that employs encryption and decryption is the DECT WLL network
100
.
In the WLL network
100
, the exchange of data between the WSU
110
and the BTS
120
occurs at very high speeds, and generally involves encryption on the transmitting end and decryption on the receiving end. Data is typically encrypted using a unique cipher key, which is also then required for decryption to recover the original data. A variety of encryption algorithms may be employed in communications systems. For example, in the DECT WLL network
100
, encryption of a serial data stream involves generating, based on the cipher key, a key stream sequence of a length same as that of the data stream and then performing an exclusive “OR” of the data stream with the key stream. And, for decryption of a data stream encrypted in such a manner, the same key stream needs to be generated and exclusive OR
Bharath Jagannathan
Larson David N.
Sharma Sandhya
Barrón Gilberto
Gurshman Grigory
Legerity Inc.
Williams Morgan & Amerson P.C.
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