TV a partir de uma rede

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should make sure there is a reliable network connection between the transmitter and receiver, otherwise the broadcast won‘t be reliable. RTP on the other hand is not a fundamental protocol like TCP or UDP. On the contrary, RTP is a protocol layer based on UDP. This means that as a basis UDP is always used for RTP. RTP offers a number of functions that greatly improve live audio and video transmissions: • Data identification • Time stamps for audio/ video synchronization • Maintaining the order of MPEG frames • Special handling of MPEG key frames This just goes to show that RTP should be used whenever all the devices involved support this protocol. With Multicasting an audio/video stream is simply transmitted without any particular receiver in mind. This transmission is routed to all users by the network and it‘s up to the users to decide if they want to receive it or not. Obviously this is extremely practical if more than one user wants to receive the same stream. As many users as desired can tune in without the need for additional network bandwidth between the transmitter and the switch. Since the switch has to process numerous data packets, it could very quickly become overloaded when dealing with a cheap SOHO product (Small Office, Home Office) while at the same 154 TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine — 09-10/2013 — www.TELE-audiovision.com 1. Wireshark capturing the network traffic. Beware that lots of data is captured during a IPTV broadcast. You should only capture data for some seconds, otherwise it will take ages to process all captured packets. 2. After capturing the network traffic for a short period, there are many analysis tools available within Wireshark. This picture shows where the traffic originated. It can be clearly seen that the devices with IP-address 192.168.1.149 and 192.168.1.210 generated a huge amount of traffic. Notice the destination IP: they are multicasting IPs 3. All captured packets can be analyzed individually and the contents can be viewed, while the header is shown in a more detailed way. time additional data is exchanged between users. If, for example, User 2 doesn‘t want to take part in Multicasting and would rather send a large file to User 3, it could happen that the flow of this data transmission is reduced to a snail‘s pace or it 1 might not function at all. To utilize Multicasting, with regard to the IP destination address it would be good enough to configure using any IP address from 224.0.0.0 to 239.255.255.255. This address range would be used by all of the Multicast de- 2 3
4. This graph shows the bandwidth during the captured transmission. Interestingly, the bandwidth is not constant, when displayed in millisecond interval. 5. This graph shows the bandwidth of the same transmission, but as Bits/ second. With just two transponders fed into the network, a 100MBPS switch is working at its limit. In fact, we tried to use a 100MBPS switch and it froze vices. Note that Multicast is not only used for IPTV; it is used whenever the same data needs to be sent to multiple computers, such as, when multiple computers need to be installed all at the same time via a network. That‘s how IT professionals, for example, in universities or large company‘s install an operating system on hundreds of computers all at once. With Unicasting however, the stream - unlike Multicasting - is sent to one specific receiver. This has the advantage of not loading down the network for all the remaining users allowing data traffic to function normally. The data is routed in the switch between the inputs of the servers and the users while the remaining switch ports remain free. The disadvantage is the fact that for every additional user the stream would have to be individually retransmitted. In no time at all the network bandwidth after a few seconds. A regular operation was only possible using a 1000MBPS switch. 6. Using DekTec’s StreamXpert software, which already has been tested in TELE-satellite 11-12/2012, together with the professional DekTec DTE-3137 receiver, one has much more feedback about the IPTV broadcast. Notice the lower left corner: a graph shows 5 6 156 TELE-audiovision International — The World‘s Largest Digital TV Trade Magazine — 09-10/2013 — www.TELE-audiovision.com continuously the occupied network bandwidth. 7. StreamXpert can show all possible information about the streamed transponder, including the programme clock reference (PCR), required for correct synchronization of audio and video. This allows editing the timestamps in order to compensate for any detected errors. 4 7
Page 1 and 2: FEATURE Faça-você-mesmo IPTV TV a
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TV a partir de uma rede

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