Multiplexing of MPEG-2 streams

MPEG-2 is an international standard for video coding and transmission. It is the standard used in the current European digital television standard, DVB-T (and also in other TV standards). This digital television standard uses the MPEG-2 standard for the encoding of the video and also uses the MPEG-2 standard for the transmission and signaling of the video. Previous post detailed the DVB-T television standard, the video encoding standard, and the signaling standard.

To transmit the compressed video, and also other extra media content and information, all the streams must be combined. The process of combining these multiple streams is known as multiplex. A multimedia program consist of compressed video, compressed audio, control data and user data. In the multiplex process, these multiple streams are combined into one signal over a shared channel. Currently there are several shared channels where the multiplexed stream can be transmitted such as radio frequency (UHF/VHF), satellite, cable TV, standard terrestrial communication, digital networks (ADSL), etc. 

Multiplexing techniques

The multiplexed stream is transmitted over a communication channel, also know as shared channel or transmission link. Today they coexist several communication channels (UHF, cabe TV, terrestrial communication…) and the physical transmission mediums are very different.  To provide communication services to multiple users, multiplexing techniques are required. There are currently several multiplexing techniques, also known as access methods, and the use of each one depends on the characteristics of the medium such as bandwidth or if its wired or wireless.

Several multiplexing techniques are used depending on the transmission system, bandwidth and characteristics of the medium. There are the following basic multiplexing methods for the transmission of TV signals to multiple users:

  • Frequency Division Multiple Access (FDMA): The TV programs are carried by different independent carriers (in different frequencies), sharing the transponder bandwidth. 
  • Time Division Multiple Access (TDMA): The digitalized TV programs are time multiplexed onto a signal carrier. This technique is used when the transmission bitrate is higher than the bitrate needed.
  • Code Division Multiple Access (CDMA): It employs spread-spectrum technology and a special coding scheme to allow multiple users to be multiplexed over the same physical channel. The modulated coded signal has a much higher data bandwidth than the data being communicated. 


The multiplexing process

Once the A/V signals have been digitalized and compressed the different bitstreams are combined and multiplexed to be transmitted. The streams are combined onto the following streams:

Packet Elementary Stream (PES)

The elementary stream (ES) is packetized by encapsulating sequential data bytes inside PES packets. PES packets consist on a header and the Playload (0-64kB). Once PES packets are created from the elementary stream data, these PES packets are encapsulated inside Transport Stream (TS) or Program Stream (PS).

Program Stream (PS)

Program Streams are created by combining one or more PES packets (audio, video or data), which have a common time base, into a single stream. Audio and video PES packets includes the Time Stamp for a correct synchronization. PS are designed for reasonably reliable media such as DVDs, CDs, HDDs…

Transport Stream (TS)

Transport Stream specifies a container format encapsulating packetized elementary streams, with error correction and stream synchronization features for maintaining transmission integrity when the signal is degraded. TS are designed for broadcast applications.

Null Packet Optimization

Standards like ATSC or DVB impose strict constant bitrate requirements on the transport stream. In order to maintain a constant bitrate, the multiplexer insert some additional null packets (not contain any data). The receiver ignore its contents.

Program Clock Reference (PCR)

The PCR is a 42 bits number transmitted in the Transport Stream packets to enable a decoder to present synchronized content.


Conditional Access

Conditional Access is the protection of content by limiting the access or requiring certain criteria. In the DVB standard is achieved by a combination of scrambling and encryption.


The data stream is scrambled with an 48-bit secret key, the control word (CW). The CW have been encrypted with the service key and the service key have been encrypted with the user key. The control word is encrypted as an Entitlement Control Message (ECM). The CA subsystem in the receiver will decrypt the control word only when is authorized by and Entitlement Management Message (EMM). The contents of ECMs and EMMs are not standardized and depends on each CA system provider.


CAT tables are used to indicates to the receiver the position of EMMs and important information and conditional access descriptors.

Technologies of conditional access systems

  • Symulcript: The CW is transmitted through different ECMs at once. This allows the use of several conditional access systems at the same time. It uses multiple set-top boxes, each using a different CA system, to authorize the programs for display.
  • Multicrypt: The CW is sent sequentially through different conditional access modules and its based on a DVB standardization. It allows two or more different encryption systems to co-exist in the same receiver.



Officially an Engineer

Today is a very special day, I graduated in Electrical and Computer Engineering (aka Telecommunications Engineering) from the Technical University of Catalonia (UPC). After five years of study and a year abroad I finally completed my studies.

I started in 2007 with a bachelor’s degree in Electrical and Computer Engineering, specializing in Audiovisual Systems. I graduated after three years and my final thesis was titled NortheasternDevelopment of a Video on Demand Service for the iPhone. In 2010 I extended my studies with a 2.5 year program withi UPC. After finishing all courses in Spain, I had the opportunity to complete my studies with a year abroad. I was working as a research scholar in the Mathworks SMART Laboratory, in Northeastern University, Boston (USA). I developed KiFall, a Privacy Preserving Fall Detection Using Smartphones and Wireless Camera Sensors. The year in Boston was, personally and professionally, one of the best experiences of my life.

Today when I went to pay for the title, I realized this was the last time I will go to the Telecommunication Engineering School at UPC as a student. I have spent so much time in these buildings, I have attended hundreds of lectures, I have spent thousands of hours studying in the library (and a few hours in the bar), I have learnt so much and I have met a lot of nice people. I suffered and enjoyed the studies at the same time. But overall, my balance is very positive.


What’s next? This is the big question I keep asking myself over and over again. I have just graduated and now I have many possibilities: looking for a job, creating my own company, continuing my studies, starting a career as a researcher, etc. And, what is also very important, where? After a year abroad and seeing how is the situation in Spain, we’ll see where I end up… The future is full of opportunities!

RPL protocol

Low power and Lossy Networks (LLNs) are a class of network in which both the routers and their interconnect are constrained. RPL routing protocol, which provides a mechanism whereby multipoint-to-point traffic from devices inside the LLN towards a central control point, as well as point-to-multipoint traffic from the central control point to the devices inside the LLN, is supported. Support for point-to-point traffic is also available.

RPL protocol is easy to adopt to network changes and allows to have multiple logical topologies over a single physical topology. Moreover, it provides a loop avoidance and detection mechanism, and it takes into account both link and node properties when choosing paths. However, it assumes other mechanism can be used to provide security.

Nowadays, RPL routing protocol is used in 6LoWPAN and generic LLN networks. There were some successful implementations in TinyOS, and with the deployment of IPv6 it is expected to have more implementations and successful results.

My Top10 iPhone apps

Over 500.000 apps are on the iPhone App Store: for business, navigation, social networks, playing, texting…even making calls! Browsing the App Store is the best way to realize there’s no limit to what an iPhone can do. However, as an iPhone user, I will show you my 10 choices among hundreds of thousands more:

  • iStudiez: if you are a student this is the best way to organize and manage your courses.
  • Evernote: a note-taker to stay organized, keeping track of ideas and writing things down.
  • Reeder: a feed reader which offers plenty of options for sharing, syncing and reading your news.
  • Dropbox: sync all your computers and devices to a cloud in order to access to your documents anywhere you go.
  • Whatsapp: a messaging app to exchange messages (text, images, audio and video media messages) with your friends.
  • Tweetbot: the most powerfull twitter app.
  • Camera+: camera app that offers many features to take photos, edit, add special effects and more.
  • TuneIn Radio: app that lets you listen to radio stations and podcasts from all around the world.
  • AVPlayer: play all video formats (without mp4 conversion). Options of subtitles or streaming from the computer are also supported.
  • Garmin: GPS app which includes off-line maps, 3D landmarks, lane assisting feature and speed limit alerts.

Which are your favourite ones? Comment and share yours!

Following the way of technology

Hello world!

I’m Bruna and I’m a student of Telecomunications Engineering on the Universitat Politècnica de Catalunya (UPC).

This blog has been created to share the contents of the subject New Generation of Internet, wich is simoultaniously coursed at the UPC in Barcelona and UPV in Valencia. The conferences are given by an expert professor from Barcelona, Valencia or Madrid through a videoconference system. In this blog I will share topics such as HTML5, blogs, wikis… as well as other things I find interesting.

Do you want to follow the way of technology?