Multiplex communications – Communication over free space – Having a plurality of contiguous regions served by...
Reexamination Certificate
1998-02-24
2001-05-22
Ton, Dang (Department: 2661)
Multiplex communications
Communication over free space
Having a plurality of contiguous regions served by...
C370S329000, C370S336000, C370S230000, C370S233000, C370S235000
Reexamination Certificate
active
06236647
ABSTRACT:
BACKGROUND OF THE INVENTION
The widespread availability of personal computers at low cost has lead to a situation where the general public increasingly demands access to the Internet and other computer networks. A similar demand exists for wireless communications in that the public increasingly demands that cellular telephones be available at low cost with ubiquitous coverage.
As a result of their familiarity with these two technologies, the general population now increasingly wishes to not only have access to computer networks, but also wishes to access such networks in wireless fashion as well. This is of particular concern for the users of portable computers, laptop computers, hand-held personal digital assistants (PDAs), and the like, who would prefer and indeed now expect to be able to access such networks with the same convenience they have grown accustom to when using their cellular telephones.
Unfortunately, there is still no widely available satisfactory solution for providing low cost, high speed access to the Internet and other networks using the existing wireless infrastructure which has been built at some expense to support cellular telephony. Indeed, at the present time, the users of wireless modems that operate with the existing cellular telephone network often experience a difficult time when trying to, for example, use the Internet to view web pages. The same frustration level is felt in any situation when attempting perform other tasks that require the transfer of relatively large amounts of data between computers.
This is at least in part due to the architecture of cellular telephone networks, which were originally designed to support voice communications, as compared to the communication protocols in use for the Internet, which were originally optimized for wireline communication. In particular, the protocols used for connecting computers over wireline networks do not lend themselves well to efficient transmission over standard wireless connections.
For example, cellular networks were originally designed to deliver voice grade services, having an information bandwidth of approximately three kilohertz (kHz). While techniques exist for communicating data over such radio channels at rate of 9600 kilo bits per second (kbps), such low frequency channels do not lend themselves directly to transmitting data at rates of 28.8 kbps or even the 56.6 kbps that is now commonly available using inexpensive wireline modems. These rate are presently thought to be the minimum acceptable data rates for Internet access.
This situation is true for advanced digital wireless communication protocols as well, such as Code Division Multiple Access (CDMA). Even though such systems convert input voice information to digital signals, they were also designed to provide communication channels at voice grade bandwidth. As a result, they have been designed to use communication channels that may exhibit a bit error rate (BER) of as high as approximately one in one thousand bits in multipath fading environments. While such a bit error rate is perfectly acceptable for the transmission of voice signals, it becomes cumbersome for most data transmission environments.
Such a high bit error rate is certainly unacceptable for Internet type data transmissions. For example, the Transmission Control Protocol/Internet Protocol (TCP/IP) standard in use for Internet air transmission uses a frame size of 1480 bits. Thus, if a bit error is received in every frame, such as detected by a frame check sequence, it would appear as though every single frame might have to be re-transmitted in certain applications.
SUMMARY OF THE INVENTION
The present invention is implemented via a protocol converter disposed between a physical communication layer, such as may be associated with implementing a wireless communication protocol, and a network layer, such as may be associated with implementing a network protocol.
The protocol converter first splits messages in the form of network layer frames into multiple subframes prior to formatting them for transmission. The subframes are each assigned a position number such that they may be reassembled into the proper order to reconstruct the network layer frame at the receiver end.
The protocol preferably makes use of multiple physical layer connections such as radio links as needed to transmit the subframes at an overall desired data transmission rate. When this is the case, a link sequence identifier is added to identify the order in which the subframes are sent over a given sub-channel in a link.
On the receiver side, the subframes are then reassembled into the network layer frames using the subframe position numbers, and then passed the reassembled frame up to the network layer. Thus, the receiver side includes a protocol converter that performs the inverse function.
The protocol converters at both the sender and receiver also take steps to automatically and dynamically adjust the size of the subframes based upon an observed rejected subframe rate in order to optimize overall throughput. An average rate at which frames are rejected can be determinined by counting good subframes and bad subframes. For example, at the receive end, a subframe with a bad cyclic redundancy check code (CRC) is discarded and counted as a bad subframe. By keeping track of the sequence numbers of the good subframe received, the receiver can determine that a particular subframe sequence number, namely the frame with the sequence number between the last good frame and the next good frame is missing. The receiver then explicitly requests retransmission of the bad frame by sequence number. This so called selective reject feature of the transmission permits both the receiver and the sender to know the number of frames received in error from the tally of selective reject orders.
From the count of the number of frames sent and the number of selective reject order received, the sender then dynamically adjusts the size of later transmitted subframes. Preferably, the subframe size is adjusted based upon a formula which depends upon the ratio of the actual data transferred to the number of bits actually used to carry the transmission, including the frame overhead and re-transmissions. For example, the number of data bytes, X, in a given subframe can be adjusted according to the formula:
X
=
-
H
+
(
X
current
+
H
current
)
*
H
/
R
)
where H is the new frame overhead, in bytes, including any shared frame synchronization flag (7E) between frames, X
current
and H
current
are, respectively, the immediately prior values of X and H, and R is a ratio of the observed number of frames transmitted successfully to the number of frames that are not transmitted successfully.
Particularly noisy channels may be subjected to down speed procedures or error coding techniques in order to improve the bit error rate observed in a particular channel.
In order to optimize throughput on overall basis, the subframe size calculation is preferably carried out on each channel separately. Otherwise, any good channels, that is, those channels which do not experience particularly noisy environments, might suffer down speed procedures needed to accommodate the weakest channels.
In one specific embodiment of the invention, the physical layer radio links may be implemented as 9.6 kbps channels such as can be reliably provided using CDMA cellular protocols and subchannel coding techniques.
The invention is particularly advantageous in environments such as requiring the communication of TCP/IP protocols since the number of channels needed to carry a single data stream at burst rates of 56.6 or 128 kbps can be quite large. For example, carrying such TCP/IP frames at these data rates may require up to and including 20 channels operating at 9.6 kbps. Because the probability of at least one relatively weak channel may be significant, by optimizing the throughput of each channel separately, the invention obtains the best overall system throughput in such environments. Simulations of the implementation of the invention indicate that it may be used t
Hamilton Brook Smith & Reynolds P.C.
TANTIVY Communications, Inc.
Ton Anthony
Ton Dang
LandOfFree
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