Ever since Voice over Internet Protocol (VoIP) was introduced, all calls going to the Public Switched Telephone Network (PSTN) have been plagued with line or electrical echo. I often get asked where these electrical echoes come from and why it is inevitable when using VoIP. We’ve given many seminars on the subject, especially in the Asterisk space where people are suddenly confronted with this problem when building equipment. I’ve put together a basic introduction explaining why echo cancellation is needed when using VoIP over PSTN.
Electrical or line echo is inherent to the PSTN, it has always been present and cannot be avoided because of the nature of the lines connected to homes and offices. These local or subscriber loop circuits use two wires to carry the voice signals while the voice channels use four wires for bi-directional communications beyond the first switch that is the local exchange. The conversion between the four wire and two wire electrical circuits is done by transformer called a telephone hybrid whose goal is to separate the signals’ directions and adapt the impedance of both circuits. Like all analog circuits, the hybrids can’t perfectly match the impedances and that causes part of the signal …
As video becomes one of the most demanding services for network traffic and Internet video becomes an important part of modern life, the quality of experience needs to meet the user’s expectations, regardless of the device or network being used. More and more video is being consumed on smartphones and tablets. The smaller screens allow for lower bit-rates, but the video playback need to start quickly and remain smooth throughout.
Internet Video delivery is challenging because of factors such as high bitrates and sensitivity to delay or packet loss.
Video Streaming
In the past, video streaming was typically associated to RTSP, with RTP used for transmission. This protocol uses “VCR-like” commands such as PLAY and PAUSE. In this scheme, the server has to keep track of the client’s state. The server starts playing a stream when giving the PLAY command, and has to maintain the state of each session in order to know what packet to send next. The video stream is based on a single “track”, a file with a fixed encoding profile that cannot change. The quality would quickly suffer when there was a shortage of bandwidth such as congestion leading to packet loss.
Today, HTTP based adaptive …
A lot of noise and fuss has come out of the HTML5 video debate. There are in fact (at least) two debates going on. The first is Flash vs. HTML5. Here, it’s an old-world vs. new-world debate. The PC-era was good to Flash, but as mobile devices take on more clout, things like battery life, security, and touch-interfaces are used as arguments to kill flash and move to something new. The second debate is from within the HTML5 camp itself, where the choice of video codec has become a prickly subject. That subject should be left for another post.
The big news last week was that Microsoft announced how their forthcoming Internet Explorer 10 (IE10) will NOT support plug-ins, including Flash. Actually, this isn’t entirely true. The “Metro-style” version of IE10 will not support these plug-ins. There will also be a desktop version, but it’s not where Microsoft is headed. Desktop applications are seen as legacy support. This blog describes the differences quite well. This was obviously a bold decision, but anchored in the same reasoning as what I mentioned above. IE10 will be part of Windows 8 which is promoting touch interfaces, and better integration with Windows Phone 7.…
As a follow up to my article on Wireless Expansion in Developing Areas I wrote not long ago for Wireless Design and Development magazine, I asked myself if I could go one step further and explain how we made it possible to provide backhaul. More specifically, how our client can easily leverage the features we added to the latest DSP to provide cellular and backhaul concurrently using a single multi-core device. All of this while keeping everything at low cost, low power and not affecting the ease of installation.
In my previous article, we were using a System on Chip (SoC) DSP that would be the core processor of a compact BTS setup. The very same SoC can be used to provide the backhaul simultaneously to the cellular portion. With its multiple independent RF interfaces, our clients can connect both cellular and backhaul antenna directly on one device. The backhaul is also flexible, offering the option to use OFDM or cellular UE standard with high performance radio. Since we wanted that our clients be able to customize and adapt the backhaul modem to their specific needs (LOS, NLOS), we designed our DSP to be C programmable. That’s a lot …
Summer time is the perfect chance for people to hit the road with their families and see the sights. Roadtrips have been a North-American tradition for decades. They always sound like a good idea when you head out, but by the third day of “Are we there yet?” the parents usually want to strangle someone. There was a clear trend this year however: instead of seeing 100$ DVD players strapped in front of children’s faces, I saw lots of iPads in their place. I was also guilty of this. Playing video on a tablet is just so much easier than shuffling through DVD discs, the custom battery charger, carrying an extra gadget. And most importantly, the screen is a lot bigger. But this isn’t an ad for Apple. The same would have been true for any tablet. In fact, less expensive tablets will probably start showing up more frequently, attached to headrests in cars everywhere in lieu of cheap DVD players.
Fast-forward a few years, and what do you think we’ll see? Those same tablets will have pervasive 3G/4G connections. And those same kids will grow up a little. Within 1-2 years, I’m convinced we’ll see the proliferation of “backseat …
