GB2306861A

GB2306861A - Handling erroneous data frames on a multi-hop communication link

Info

Publication number: GB2306861A
Application number: GB9523093A
Authority: GB
Inventors: John Hughes; David John Chater-Lea; Mark Edward Adey; Kwan Yee Lee
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1995-11-03
Filing date: 1995-11-03
Publication date: 1997-05-07
Anticipated expiration: 2015-11-03
Also published as: GB2306861B; GB9523093D0

Abstract

At an intermediate hop site (300) Fig.3, (not shown), in a multi-hop communication link between a transmitter and receiver, a received transmission is processed to handle erroneous data frames. A data stream is recovered from the transmission which includes one or more data frames, each having corresponding error detection information (410). When a particular data frame is detected as being erroneous, based at least in part on an evaluation of the corresponding error detection information (420), the data stream is modified to include an indicator representing that the particular frame does not contain valid data (430), and the data stream is retransmitted (440). Thus bad frame indication is performed at the intermediate station(s) and bad frame substitution (mitigation) is left for the receiver, (after checking if the receiver is capable of receiving such indication).

Description

METhOD AND APPARATUS FOR HANDLING ERRONEOUS DATA FRAMES ON A MULTI-HOP COMIMUNICATION LINK Technical Field This invention relates in general to communications systems, and particularly, communication systems supporting communication links over multiple hops.

Background Many communications systems support data transfer between two points through a communication link comprised of smaller links (hops). The data may be segmented into frames for transmission. The frames are protected by error detection or error correction information transmitted with each frame. An error occurring during transmission of data may result in a retransmission of the affected frame. However, in some cases, retransmission of data may be impractical, and error mitigation for the affected frames may be preferable.

For real time applications, such as voice or video transmissions, a bad frame substitution (BFS) technique is often employed to estimate or correct for errors in the received frame. An effective BFS technique typically requires some knowledge of the expected data. For example, with voice data, knowledge of the speech coding in use is advantageous to accomplish effective frame substitution.

FIG. 1 shows a prior art voice communication system 10 in which a transmitter 12 establishes a communication with a receiver 18 over a communication link extending across multiple hop sites 14, 16. FIG. 2 is a block diagram of the communication system of FIG. 1 showing selected functional aspects. In this example, the transmitter 12 and receiver 18 are subscriber units capable of wireless digital voice communications over radio frequency (RF) channels, and the hop sites 14, 16 are base stations wired together to form an infrastructure. Communication systems so organized are common in the art.

Generally, the transmitting subscriber unit 12 encodes speech into data frames, and supplements the data frames with encoded error detection information for channel transmission. The transmitting subscriber unit 12 then modulates the data frames on a carrier signal that is communicated to a first base station 14. The first base station 14 receives and demodulates the channel transmission. The first base station 14 then performs channel decoding while checking for transmission errors using the encoded error detection information. When a transmission error is detected that indicates a particular data frame is bad, a corresponding BFS requirement is identified. As a typical prior art hop site, the first base station 14 performs a BFS or some other action to mitigate the error.By the compensating for the error in the particular frame, subsequent transmission of the particular data frame does not create an error condition in the next stage of the communication link, unless additional errors occur.

Performing BFS at intermediate hop sites of the communication link can have significant drawbacks. A particular communication may have BFS performed at different intermediate hop sites along the communication link.

Moreover, substantial processing may be needed at each hop site to effectuate BFS. For example, in real time voice applications, BFS may be best carried out by decoding and using other speech parameters transmitted proximate to, or in conjunction with, the particular problem data frame. If speech decoding and encoding is required for BFS, complexity is increased and tandeming occurs, wherein portions of the speech may be degraded. For speech which is highly compressed, tandeming can be very detrimental to voice quality and is thus undesirable. Tandeming also significantly increases processing requirements and the delay over the link.

An additional problem occurs in a system wherein the transmitted data is encrypted. Generally, a decryption key is necessary to perform effective BFS. If the decryption key resides only at the receiver, important parameters which could be derived from other data frames transmitted in association with the bad data frame is not available at the intermediate sites to assist in BFS, thereby necessarily limiting BFS options.

As in other prior art communication systems employing multi-hop communication links, BFS can be performed more than once over a particular link between a transmitter and a receiver. This approach to error mitigation has negative consequences including the possibility for complex processing at each intermediate site, and performance degradation from tandeming. Constraints imposed by the capabilities of existing systems, and standards for interoperability within a given system, impact the availability of solutions for the above described problems. Whereas it is desirable to provide an improved method and apparatus for handling erroneous data frames on a multi-hop communication link, it is also desirable to provide a solution with some interoperability with some non-conforming systems and standards.

Summary of the Invention These needs and others are substantially met through the provision of a method and apparatus for handling erroneous data frames in a transmission to a receiver over a multi-hop communication link. Pursuant to the present invention, at an intermediate hop site along the communication link to the receiver, a data stream is recovered that comprises one or more data frames. Each data frame is protected by corresponding error detection information. When a particular data frame is determined to be erroneous, such as by evaluation of the error detection information, the data stream is retransmitted with an indicator representing that the particular frame does not contain valid data.

In a first embodiment, the data stream is modified by substituting, within the particular data frame, data containing a pattern which is recognizable by the receiver as signifying that the particular data frame contains invalid data. The data stream is then retransmitted with error detection information that no longer signifies that the particular data frame contains invalid data. Preferably, in order to co-exist in a heterogeneous infrastructure system, the hop site determines whether the receiver is capable of recognising and handling the indicator. When the receiver is not capable of handling the indicator, the data stream is retransmitted with an error detection code for the particular frame that reflects an error in the particular frame.

In a second embodiment, the hop site alters a portion of the data stream to characterize the particular data frame as a stolen data frame, according to a particular communication protocol, so as to indicate that the particular data frame is not a valid data frame.

Brief Description of the Draws FIG. 1 is a prior art communication system having a communication link extending across multiple hop sites.

FIG. 2 is a block diagram of the communication system of FIG. 1 showing selected functional aspects.

FIG. 3 is a block diagram of a base station hop site, operable to incorporate a frame substitution requirement indicator into a data stream, in accordance with the present invention.

FIG. 4 is a flow chart of procedures used by the base station hop site of FIG. 3 during a communication link, in accordance with the present invention.

FIG. 5 is a flow chart of procedures according to a particular embodiment of the base station of FIG. 3, in accordance with the present invention.

FIG. 6 is a flow chart of procedures used by a receiver in response to communication routed through the base station hop site of FIG. 3, in accordance with the present invention.

Detailed Description ofthe Preferred Embodiment While the specification concludes with claims defining the features of the invention that are regarded as novel, it is believed that the invention will be better understood from a consideration of the following description in conjunction with the drawing figures, in which like reference numerals are carried forward.

Referring to FIG. 3, a block diagram of a hop site 300 is shown, in accordance with the present invention. In the preferred embodiment, the hop site 300 is a base station that interfaces with an infrastructure for a radio frequency (RF) communication system . The base station selectively acts as an intermediate hop site in a multi-hop communication link. A multi-hop communication link may include wireless channels, such as RF channel, or wired channels, such as over a local area network, or a wide area network.

The base station 300 may receive a wireless radio frequency (RF) signal from a subscriber unit to be transmitted to the next hop site in a communication link, or the base station 300 may receive communication from another hop site from the communication infrastructure on a wired or wireless link.

When the base station 300 receives a wireless RF communication, this communication comprises an information signal modulated on a carrier signal, and the information signal is encoded with error protection information for transmission over the RF communication channel. When the base station receives communication over a wired link, such information may be encoded for channel transmission but may not be modulated on a carrier signal.

The base station 300, includes a controller 350 that provides operational control including the processing of incoming and outgoing signals. The controller 350 is coupled to a receiver block 310, a transmitter block 320, an infrastructure interface 330, an error detector 360, and a data modifier 370.

The receiver block 310 comprises a demodulator 312 and a channel decoder 314, for processing incoming transmission signals. The transmitter block 320 includes a modulator 322, and a channel encoder 324, to process outgoing signals. The infrastructure interface 330, provides access to the infrastructure for the communication system. The error detector 360 processes incoming information signals for errors using encoded error detection information. The data modifier 370 tags outgoing data with a bad frame indicator, in accordance with the present invention. Preferably, the controller also has access to information on target receiver capabilities from a source 380, such as from a database, or from information decoded from received information signals.

The receiver block 310 operates to recover a data stream from a transmission received by the base station 300. Generally, the data stream is segmented into a plurality of data frames. Preferably, each data frame is protected by error detection information associated therewith, such as in the form of an error detection code. In the preferred embodiment, the error detection code comprises cyclic redundancy code (CRC) information, that is generated for, and associated with, each data frame, when the data is encoded for transmission on a communication channel. When the base station 310 receives a transmission signal from a wireless subscriber unit, that transmission signal is demodulated to recover the information signal. If the information signal was transmitted across a wired link, such as from an infrastructure source, no demodulation might be necessary.However received, the base station 300 recovers from the transmitted information signal, a data stream which comprises one or more data frames that have been encoded with error protection information.

Similarly, a base station may transmit information over a wired or wireless link, to a target receiver, such as a subscriber unit, or another hop site in a communication link. Generally, information transmitted over the communication channel, whether using a wired or wireless link, is encoded with error protection bits for transmission. For wireless communications, a signal is generated from the information to be transmitted, and this signal is modulated on a radio frequency carrier signal for transmission on the RF channel.

Referring to FIG. 4, a flow chart of procedures 400 used by the base station in a first embodiment is shown, in accordance with the present invention. The base station receives data by recovering a data stream from an incoming transmission, step 410. When the transmission is received via a wireless communication channel, the base station performs demodulation of the information signal. Generally, the data is encoded for transmission on the communication channel, and thus the base station performs a channel decoding function. Decoding of the information signal provides a plurality of data frames, each having a corresponding error detection code. The base station then checks for a frame that does not contain valid data, step 420.

Using the error detector module, the base station processes the data of each frame for comparison against corresponding error detection information to uncover frame errors, such as caused by transmission errors or otherwise, as is well known in the art. According to the present invention, when a frame error is detected, the base station modifies a data stream to include an indicator or indicator flag, preferably separate from the error detection code, indicating that the particular frame does not contain valid data, step 430.

The indicator flag directly represents, without any further reference to or processing of the data frame or stream, that the particular frame requires error mitigation. The base station, does not attempt to do error mitigation, such as bad frame substitution, at this point in the communication link.

Rather, the base station retransmits a data stream that includes an indicator flag to alert compatible receivers later on in the communication link that an error was detected with respect to a particular frame, step 440.

FIG. 5 is a flowchart of procedures 500 of a preferred embodiment, in which the indicator flag is sent by substituting data containing a pattern within the particular data frame, which pattern is recognizable by a target receiver as signifying that a particular data frame was detected as not containing valid data, i.e., as a bad frame. Preferably, the base station determines whether the ultimate target receiver of the data is compatible with the indicator flag, such that it is capable of recognising the particular frame as not containing valid data and to take the appropriate action, step 510. The information on receiver capability can be provided through access to a data base containing receiver type information, or can be communicated within the data transmitted on the communication link.If the target receiver is indicator compatible, the data stream is modified with the indicator reflecting that a particular frame does not contain valid data, steps 520, 530. Additionally, the base station preferably adjusts the error detection code associated with the particular frame upon retransmission, so as not to reflect an error in the particular frame. Consequently, subsequent hop sites along the communication link to the receiver will not be affected by the error detected by the base station.

The substitution pattern that serves as an indicator flag is preferably reserved for bad frame identification. Thus, steps will be taken within the communication system to ensure that valid data is not represented by the particular data pattern. In a heterogeneous system having several types of communication equipment and subscriber units, it is possible that such data pattern could in normal course be represented in data transmitted across the communication network. For example, if data is encrypted between transmitter and receiver, it may be unlikely that the data pattern selected as the indicator flag would not occur when there is no error. Such incompatibility may be addressed, according to the present invention, by determining whether the target receiver or hop sites within the communication link to the target receiver, are incompatible with the pattern substitution. In such cases, the base station may elect to retransmit the data stream while deliberately inducing an error for the particular frame, by coding the corresponding error detection information to reflect an error, steps 520, 540.

In a second embodiment, it is assumed that the data communication protocol within the communication system includes the concept of a stolen frame. The use of stolen frames can be found in prior art communication systems. In a common implementation, a data frame can be transformed into a stolen data frame by special signaling designated for such purposes, which signaling is contained in the data stream. Generally, stolen data frames are used for signaling between one participant and another of a communication link. The second embodiment contemplates use of the stolen data frame to function as an indicator flag that the particular data frame does not contain valid data. For systems incorporating stolen data frames, the use of a stolen data frame as an indicator flag for representing that a particular frame does not contain valid data may be preferable for compatibility purposes.Generally, communication units supporting stolen data frames perform error mitigation, such as bad frame substitution, to account for the lack of valid data in the stolen frame. When an error is detected, the base station, according to the present invention, modifies the data stream such that the particular erroneous data frame is treated as a stolen data frame and the associated data previously received in the erroneous data frame is either discarded or modified to facilitate error mitigation at the target receiver.

In a further extension, according to the present invention, a base station, upon receipt of a data frame containing an indicator flag for a particular data frame, and with knowledge that the target receiver is incompatible with the indicator flag being sent, deliberately induces an error for the particular data frame. Consequently, the receiver will, upon decoding the particular frame, conclude that the data frame was received in error.

The receiver will then perform its error mitigation routine, such as a bad frame substitution. A base station including this extension increases the level of compatibility in a heterogeneous communication system.

FIG. 6 is a flow chart of procedures 600 used at a receiver that is compatible with an indicator flag, in accordance with the present invention.

The receiver receives a transmitted data stream and demodulates and decodes, where necessary, to recover a data stream from the transmitted signal, steps 610, 620. The data stream will contain a plurality of data frames. When one such data frame has an associated indicator flag signifying that the data frame does not contain all valid data, the receiver performs error mitigation, such as bad frame substitution, steps 630, 640.

For digital voice applications, the receiver ordinarily includes a speech decoder, that is used to decode and assemble speech parameters. With this increased level of information, the receiver is usually better able to provide effective bad frame substitution, than intermediate hop sites along a communication link. For data frames received without an associated indicator flag, but with incorrect error detection code information, the receiver similarly provides for error mitigation through bad frame substitution, steps 630, 640. Preferably, in both cases, error mitigation is performed by the receiver regardless of the source of the frame error.

The present invention provides significant advantages over the prior art. In radio communication systems using multi-hop communication links, error mitigation for received frame errors is performed at the most appropriate point in the communication link, not necessarily where error detection occurred. Generally, the most suitable error mitigation site is at the target receiver, but this may be at other sites along the communication link. This deferral of error mitigation to the most appropriate site can yield significant enhancement in system performance. Yet, this solution can be integrated into an existing communication system, or a system conforming to existing standards, which, for example, support the performance of error mitigation at the point where the error is detected. Thus, the present invention is particularly useful in heterogeneous communication systems.

While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the appended claims.

What is claimed is:

Claims

Claims 1. A method of handling erroneous data frames in a transmission between a transmitter and a receiver over a multi-hop communication link, comprising the steps of: at an intermediate hop site between the transmitter and the receiver: recovering a data stream from the transmission, the data stream comprising a plurality of data frames each having a corresponding error detection code; detecting when a particular frame of the plurality of data frames is erroneous based at least in part on the corresponding error detection code; modifying the data stream to include an indicator representing that the particular frame does not contain valid data; and retransmitting the data stream, including the indicator.
2. The method of claim 1, further characterized by the step of processing, at the receiver, the data stream including performing bad frame mitigation for the particular frame represented by the indicator.
3. The method of claim 1 or 2, characterized in that the step of modifying the data stream comprises the steps of: substituting data containing a pattern within the particular data frame, which pattern is recognizable by the receiver as signifying that the particular data frame was detected as a bad frame; and adjusting the corresponding error detection code to no longer signify the particular data frame as a bad frame.
4. The method of claim 2, wherein the step of substituting data is further characterized by the step of substituting a pattern reserved for bad frame identification.
5. The method of claim 1 or 2, characterized in that the step of modifying the data stream comprises the step of: providing the indicator by altering a portion of the data stream to characterize the particular data frame as a stolen data frame.
6. The method of claim 5, wherein the step of altering is further characterized by the step of adjusting the corresponding error detection code to no longer signify the particular data frame as a bad frame.
7. The method of claim 2, 3, or 4, wherein the step of performing bad frame mitigation is further characterized by the step of providing substitute data for information represented in the particular frame.
8. A method of handling bad frame substitution in a transmission between a transmitter and a receiver over a multi-hop communication link, comprising the steps of: at an intermediate hop site between the transmitter and the receiver: recovering a data stream from the transmission, the data stream comprising a plurality of data frames each having a corresponding error detection code, the plurality of data frames including a particular frame identifiable as erroneous; determining that the receiver is indicator capable when the receiver is capable of interpreting a particular indicator, separate from the error detection code, as directly indicating that the particular frame does not contain valid data; when the receiver is indicator capable:: retransmitting the data stream with the particular indicator, and with an error detection code for the particular frame that does not reflect an error in the particular frame; when the receiver is not indicator capable: retransmitting the data stream with an error detection code for the particular frame that reflects an error in the particular frame.
9. The method of claim 8, characterized in that the step of determining that the receiver is indicator capable comprises the step of determining whether the receiver is capable of recognizing a data pattern as signifying that the particular data frame as signifying that the particular data frame does not contain valid data.
10. The method of claim 9, characterized in that the step of retransmitting the data stream with the particular indicator comprises the step of substituting data containing the data pattern within the particular data frame.
11. The method of claim 8, 9, or 10, characterized in that the step of recovering a data stream comprises the step of recovering a data stream comprised primarily of speech data.
12. The method of claim 11, further characterized by the step of processing, at the receiver, the data stream, including performing bad frame substitution for the particular frame represented by the particular indicator.
13. An apparatus for retransmitting a received signal containing erroneous data frames in a multi-hop communication link, comprising: a demodulator responsive to the received signal to provide a demodulated signal; a decoder coupled to the demodulator, and responsive to the demodulated signal to recover a data stream, the data stream comprising a plurality of data frames each having corresponding error detection information; an error detector coupled to the decoder, and responsive to the data stream to provide an error signal, when evaluation of the particular frame according to the corresponding error code indicates an error; and a transmitter, coupled to the data modifier, for transmitting a modified data stream; characterized by: : a data modifier coupled to the decoder and error detector, the data modifier being responsive to the error signal to provide the modified data stream that includes an indicator, separate from the error detection information, which indicator directly represents that the particular frame contains invalid data.