{"id":13492,"date":"2022-10-04T16:17:26","date_gmt":"2022-10-04T14:17:26","guid":{"rendered":"https:\/\/www.linphone.org\/?post_type=actualites&p=13492"},"modified":"2025-10-03T17:04:31","modified_gmt":"2025-10-03T15:04:31","slug":"error-correction-the-key-to-good-transmission","status":"publish","type":"actualites","link":"https:\/\/linphone.gtec.pro\/en\/news\/error-correction-the-key-to-good-transmission\/","title":{"rendered":"Error correction, the key to good transmission"},"content":{"rendered":"

Our team is currently doing research and developing prototypes about the use of\u00a0Forward Error Correction<\/strong><\/span>\u00a0for video transmissions.<\/p>\n

Forward Error Correction (FEC)<\/strong>\u00a0[1] is a well-known technique for controlling errors in data transmission. In IP networks, errors actually translate into the loss of an entire UDP or TCP datagram, which is why\u00a0FEC<\/strong>\u00a0is also referred to as Forward Erasure Correction. The goal is then to be able to-recreate a lost packet, based on other transmitted packets and redundant information included in specific\u00a0FEC<\/strong>\u00a0packets.<\/p>\n

Why are there packet losses?<\/span><\/h2>\n

Packet losses [2] are common in telecommunications due to the nature of transmissions: for example, a wireless network can be subject to radio frequency interference. Other causes of packet drops are temporary congestion occurring in routers.<\/p>\n

In the real world, it is common and usually acceptable to observe around a 1\u20122 % loss rate for many kinds of internet connectivity (xDSL, LTE, 5G, Wifi …).<\/p>\n

Why are packet losses a problem?<\/span><\/h2>\n

In most situations of internet usage, packet losses are handled by the TCP protocol: the receiver acknowledges the received packets, and stops until the sender retransmits the lost packets.<\/p>\n

However, this technique is not applicable in the field of real-time communications, because it is not acceptable to stop the streaming until the retransmitted packets arrive.<\/p>\n

This is the reason why real-time audio and video streams are preferably transmitted over UDP protocol. Though application-level retransmissions can be implemented efficiently over UDP, they are often not suitable because of round-trip delays between the sender and receiver.<\/p>\n

Packet losses are well handled for audio streams thanks to Packet Loss Concealment (PLC), which makes them almost un-noticeable to the end users as long as the loss rate remains below 5%.<\/p>\n

However, they are far more complicated to handle for video data. Indeed, the nature of video coding means that a single loss within a video frame makes the whole frame un-decodable, and the next frames too, until a special frame called a “key-frame” is received. This is a consequence of the differential coding that is used for video compression.<\/p>\n

How are packet losses handled in real-time video transmissions?<\/span><\/h2>\n

When a video receiver is observing a packet loss, it may notify the remote sender of the loss of information thanks to special RTCP packets (PLI – packet loss indication [3]), so that the sender can immediately generate a key-frame to let the receiver continue the rendering of the video sequence.<\/p>\n

This technique, which has been implemented in linphone for many years, has the drawback of freezing the video display until the said key-frame is received.<\/p>\n

For a medium quality video stream and with a 1% loss rate, the freeze events will not happen frequently.<\/p>\n

However, for a high quality (HD) video stream, the amount of data is so high that even a 0.5% loss rate will result in video freezing almost every second, causing a non-smooth, jerky effect for the end-user.<\/p>\n

Why can FEC help?<\/span><\/h2>\n

FEC offers a solution to compensate the loss rates typically observed over the Internet. At the expense of a relatively low overhead and small latency, the redundancy information transmitted by the sender lets the receiver re-create the missing packets, as long as the loss rate remains compatible with the amount of redundancy sent by the encoder.<\/p>\n

As a result, video can be played smoothly, without any observable effect for the end-user.<\/p>\n

How will this be handled in Linphone?<\/span><\/h2>\n

There are currently various FEC standard techniques and approaches documented at IETF.<\/p>\n

Our team has selected the Flexible Forward Error Correction scheme (flexfec) documented in RFC8627 [4], because it combines low-complexity redundancy coding (XOR based) with a high-level of flexibility for the FEC parameters, which is key to adjusting the redundancy overhead according to the observed loss rate and network bandwidth.<\/p>\n

The results look very\u00a0promising<\/strong>!<\/p>\n

We will be soon very pleased to announce a future release of linphone and linphone-sdk, incorporating an FEC algorithm for video transmission!<\/p>\n

Annex :<\/strong><\/u><\/span><\/p>\n

[1]\u00a0https:\/\/en.wikipedia.org\/wiki\/Error_correction_code#Forward_error_correc…<\/a><\/span><\/p>\n

[2]\u00a0https:\/\/en.wikipedia.org\/wiki\/Packet_loss<\/a><\/span><\/p>\n

[3]\u00a0https:\/\/en.wikipedia.org\/wiki\/Packet_loss_concealment<\/a><\/span><\/p>\n

[4]\u00a0https:\/\/datatracker.ietf.org\/doc\/html\/rfc8627<\/a><\/span><\/p>\n","protected":false},"author":10,"featured_media":13493,"parent":0,"template":"","meta":{"_acf_changed":false,"inline_featured_image":false},"actualites-category":[42],"class_list":["post-13492","actualites","type-actualites","status-publish","has-post-thumbnail","hentry","actualites-category-tech"],"acf":[],"yoast_head":"\nError correction, the key to good transmission - Linphone<\/title>\n<meta name=\"robots\" content=\"noindex, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Error correction, the key to good transmission - Linphone\" \/>\n<meta property=\"og:description\" content=\"Our team is currently doing research and developing prototypes about the use of\u00a0Forward Error Correction\u00a0for video...\" \/>\n<meta property=\"og:url\" content=\"https:\/\/linphone.gtec.pro\/en\/news\/error-correction-the-key-to-good-transmission\/\" \/>\n<meta property=\"og:site_name\" content=\"Linphone\" \/>\n<meta property=\"article:modified_time\" content=\"2025-10-03T15:04:31+00:00\" \/>\n<meta property=\"og:image\" content=\"https:\/\/linphone.gtec.pro\/wp-content\/uploads\/2024\/10\/correction_derreur_1128_x_500_px-4.png\" \/>\n\t<meta property=\"og:image:width\" content=\"1128\" \/>\n\t<meta property=\"og:image:height\" content=\"500\" \/>\n\t<meta property=\"og:image:type\" content=\"image\/png\" \/>\n<meta name=\"twitter:card\" content=\"summary_large_image\" \/>\n<meta name=\"twitter:label1\" content=\"Est. reading time\" \/>\n\t<meta name=\"twitter:data1\" content=\"4 minutes\" \/>\n<script type=\"application\/ld+json\" class=\"yoast-schema-graph\">{\"@context\":\"https:\/\/schema.org\",\"@graph\":[{\"@type\":\"WebPage\",\"@id\":\"https:\/\/linphone.gtec.pro\/en\/news\/error-correction-the-key-to-good-transmission\/\",\"url\":\"https:\/\/linphone.gtec.pro\/en\/news\/error-correction-the-key-to-good-transmission\/\",\"name\":\"Error correction, the key to good transmission - 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