The present invention generally relates to data packet retransmission and Forward Error Correction (FEC) for protection of data packet transmissions against packet loss or packet corruption due to noise on wire-bound or wireless links, like for instance a Digital Subscriber Line (DSL) or a Wireless Local Area Network (WLAN) link. Data packet in the context of the current patent application means any fixed length or variable length packet conveying information of whatever nature or service (voice, video, TV, Internet, gaming, multimedia, data files, . . . ) over links of a communication network.
Due to noise, wire-bound and wireless physical layers are prone to bit errors that ultimately may translate in data packet loss. At the link layer, solutions exist for protection against bit errors on the physical layer. In general, two main techniques exist for error protection in packet based networks: retransmission and Forward Error Correction (FEC). These techniques must ensure that the services (e.g. voice, video, data transfer) can run at the desired quality of experience (QoE). In order to achieve for instance an acceptable video quality, the viewer of High Definition Television (HDTV) should not be faced with more than one Visible Distortion in Twelve hours (1 VDT) caused by loss of video packets. When the packets that contain the video information are sent over an indoor wireless link, the packet loss rate can amount to several percents. Typical packet loss rates on indoor wireless links are 2% to 5%. Hence, protection of the video packets through retransmission or FEC is indispensable. Also in a wired scenario where the video packets are for instance sent over an interleaved DSL line, the objective of 1 VDT cannot be guaranteed without proper protection of the video packets. Worst case packet loss rates on DSL lines are in the range of 10−4 to 10−5, leading to approximately 1 visible distortion each 30 seconds for HDTV at 8 Mb/s
Retransmission consists in transmitting a copy of an earlier transmitted data packet that got lost or corrupted, either on request of the receiver or automatically when a certain time period has lapsed and no receipt acknowledgement has been received. Retransmission techniques are efficient in terms of overhead—only data packets that are effectively lost or corrupted, are retransmitted—but the delay or latency associated with retransmission can be very large. This is in particular the case when the retransmissions are requested from a remote buffer, in case of a slow link, or in case the number of requested retransmissions is high. For Broadcast TV services for instance, the maximum acceptable zapping delay puts an upper bound of about 150 milliseconds on the allowed latency. In case where retransmission is used to recover video packets that were lost or corrupted during transmission over a DSL line, the round-trip delay—this is the time required to request retransmission from the Set-Top Box (STB) to the DSLAM plus the time required to retransmit the packet from the DSLAM to the STB over the DSL line—can amount up to 40 milliseconds. In the wireless scenario where the wireless router is equipped with a retransmission buffer from which retransmission of a video packet can be requested in case of an error due to transmission over the indoor wireless link, the round-trip delay usually stays below 5 milliseconds. In the Wireless LAN scenario where a video packet gets lost in a channel that is the concatenation of a DSL line and an indoor wireless channel, and where retransmission must be requested from the DSLAM, the round-trip delay consequently may be expected to be around 45 milliseconds. Concluding, although retransmission techniques are economical in sending overhead information on the link, the major bottleneck related to retransmission is the introduced latency which restricts the maximum amount of retransmissions. Depending on the service and the round-trip delay of the physical layer, the acceptable number of retransmissions for a data packet might be as low as 2 or 3 (e.g. in case of video service over a DSL link).
Forward Error Correction (FEC) techniques add parity packets or FEC packets to the content stream in order to enable the receiver to reconstruct lost or corrupted packets without having to request retransmission. A drawback of FEC techniques is that all packets are protected through FEC packets, also the packets that are received free of errors. FEC techniques in other words introduce a permanent, additional overhead which can be too large in some cases. Further, FEC techniques introduce delays as well, because collecting the packets upon which the FEC decoding has to be calculated takes time since these packets do not arrive instantly but arrive at the rate of the link. In a wired scenario where for instance video packets are sent over a DSL loop operating at 20 Mbps, an overhead of more than 6% cannot be tolerated. This restriction of low overhead and low latency (the zapping delay must stay below 150 ms) impedes the use of for instance powerful binary FEC codes to protect video packets sent over DSL lines. On indoor wireless links, more powerful FEC codes can be used since the allowed overhead is substantially in excess of that on a DSL link. A wireless link can operate at 54 Mbps while HDTV requires a video bit rate of about 8 Mbps. Assuming that about 30 to 35 Mbps is effectively at the disposal for transmission of video packets over the wireless indoor link, it is clear that the allowed overhead can be much higher than the 5 a 6% acceptable on DSL links. Studies have shown that powerful binary codes on wireless links require a very high overhead, in excess of 60%, in order to comply with the viewers demand of less than 1 VDT. Reed-Solomon codes are an alternative to their binary counterparts, requiring only 20 a 30% overhead on wireless links, but Reed-Solomon codes are less appealing because of their higher decoding complexity.
Summarizing, the latency introduced by conventional retransmission techniques is often too large to attain an acceptable quality of experience (e.g. a good zapping performance). This is so because in order to reach a packet loss ratio that is low enough (e.g. at most 1 VDT for video services), certain packets need to be retransmitted more than once. In particular on wireless links where the packet loss ratio can amount to several percents or on wire-bound links where the round-trip delay equals several tens of milliseconds, conventional retransmission techniques may not perform satisfactory. FEC techniques on the other hand introduce overhead on top of the payload packets, and the overhead might be too large. This is so because in order to reach a packet loss ratio that is low enough, powerful FEC codes may be required, introducing unacceptably high permanent overhead.
It is an object of the present invention to disclose a packet retransmission technique that achieves optimal performance with minimum latency, minimum overhead and minimum complexity, both in wire-bound and wireless scenario's.
The above object is achieved by the data packet retransmission arrangement defined in claim 1, having:
Indeed, the basic idea underlying the current invention is a new retransmission strategy. The number of retransmissions is for each packet restricted to a certain value K. If a packet is lost K times in succession, this packet will be grouped with L−1 recently transmitted packets, and this set of L packets is protected by N FEC packets, transmitted immediately after the Kth retransmission of the lost data packet. The N FEC packets will enable to reconstruct the data packet in case of a subsequent loss (during the Kth retransmission), and eventually will enable to recover one or more of the L−1 recently transmitted packets that are used for the FEC packet calculation. If the integer K is chosen adequately, the latency can be kept under the desired bound and if the FEC parameters are chosen adequately, the overhead and complexity can be kept under control while at the same time attaining a rate of distortions (packet losses or packet corruptions) that stays below the maximum acceptable distortion rate for a certain quality of experience. No permanent FEC overhead will be transmitted for packets that are transmitted free of errors or packets that can be recovered within K−1 retransmissions. The latency will not extend beyond the delay introduced by K retransmissions, because the Kth retransmission must enable recovery of the packet, either through the retransmission or through FEC decoding. The FEC encoding/decoding complexity can be kept simple by choosing for instance one or more copies of the L packets as FEC packets.
It is noted that the above objects of the current invention are further achieved through the method for retransmitting data packets defined by claim 10.
An additional feature of the packet retransmission arrangement according to the current invention is, as defined by claim 2, that K is preconfigured such that K retransmissions of the packet still arrive within a predefined, acceptable delay bound. This way, K−1 FEC-less retransmissions and one FEC-enhanced retransmission of the data packet can be used for packet recovery without affecting the quality of experience. The delay bound is predefined and dependent upon location of the retransmission buffer, application, nature of the physical medium, bitrate, DSL mode, availability of other error resilience modes, etc.
For instance in case of video services offered over a DSL loop whereby the retransmission buffer is integrated in the DSLAM, the delay bound could be chosen equal to 150 milliseconds, as in claim 3. This way, K−1 FEC-less retransmissions and one FEC-enhanced retransmission of the data packet will not exceed the maximum zapping delay of 150 milliseconds, acceptable for viewers that use a video or TV service.
An optional feature of the packet retransmission arrangement according to the current invention is that N might be chosen equal to 0, as defined by claim 4. Thus, even if the functionality is available to perform a last retransmission enhanced with FEC, the parameters of a packet retransmission arrangement according to the current invention may be configurable such that the Kth and last retransmission will be a simple retransmission of one copy of the requested packet.
An optional feature of the packet retransmission arrangement according to the current invention is that L might be chosen equal to 1, as defined by claim 5. In this case, no recently transmitted packets other than the requested packet will be used in the FEC calculation. In a particular implementation, the N FEC packets may be N ordinary copies of the packet requested to be retransmitted, thereby minimizing the complexity while still performing better than prior art retransmission or FEC systems because K+N copies of the packet are now available for recovery within the delay bound.
As is indicated by claims 6 to 9, a packet retransmission arrangement according to the current invention might be integrated in different kinds of network equipment, i.e. access nodes like a Digital Subscriber Line Multiplexer (DSLAM), a Digital Loop Carrier (DLC) a Cable Modem Termination System (CMTS), an optical fibre aggregator, etc.; end-user equipment like a home gateway, a Set-Top Box (STB), a DSL modem, a wireless router, a PC, a video codec, etc.; switching/routing gear like an edge IP router, a core IP router, a switch/router, etc.
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Although the present invention has been illustrated by reference to a specific embodiment and specific drawings, it will be apparent to those skilled in the art that various changes and modifications may be made within the spirit and scope of the invention. It is therefore contemplated to cover any and all modifications, variations or equivalents that fall within the spirit and scope of the basic underlying principles disclosed and claimed in this patent application. For example, the retransmission and FEC arrangement might be used in a completely different environment, where it is integrated in a node that is not a DSLAM or access aggregating network node in order to control retransmission and forward error correction for packets of various applications. The retransmission and FEC arrangement may for instance form part of a home gateway where it keeps track of the number of retransmissions of packets sent over wireless inhouse links. Based on that number, it is decided whether there is sufficient time left for a regular retransmission over the wireless link or whether Forward Error Correction has to be applied for a final retransmission. From an architectural point of view, it will be understood by a person skilled in the art of designing network equipment, that the different functional blocks shown for instance in
Number | Date | Country | Kind |
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05292530.2 | Nov 2005 | EP | regional |