Broadband DSL and Triple Play Overview

Broadband DSL and Triple Play Overview

More than 60% of Triple Play implementations worldwide are based on DSL Broadband, a dominant communications technology of today. So it makes sense to learn a technology that is driving some of the most happening things and social behavior on the Internet. “Triple” drives from the fact that it is a single bundle of three major types of services, namely; Internet, Video and Telephony.

Although Internet, Video and Telephony are three different types of service, they are delivered on a communication network that is built on one protocol – the Internet Protocol. Internet Protocol is what runs the Internet.

The majority of subscribers worldwide access the Internet using DSL Broadband technology. DSL Broadband technology allows the use of existing telephone lines to the subscribers homes and offices for this purpose. Subscribers can use their telephone connection for both Triple Play services and their conventional ‘plain old’ telephone service at the same time.

Plain Old Telephone service (POTS) should not be confused with Telephony which is offered as part of Triple Play. Telephony offered by Triple Play is a kind of digital telephony that is superior in voice clarity and other features compared to conventional telephony. One example of Triple Play Telephony is Voice over IP or VOIP.

Two popular examples of Triple Play Video service are TV over IP or IPTV and Video on Demand or VoD. IPTV is common in Europe and parts of North America and China as of early 2012, according to the Broadband Forum.

DSL Broadband network that supports Triple Play yet is backward compatible with a vast legacy of ATM based implementations.

Network elements are devices that makeup a digital network. The arrangement and inter-linking of various network elements that make up a network is Topology. The Digital Subscriber Line Modem(DSL Modem), Digital Subscriber Line Access Multiplexer(DSLAM) and Broadband Remote Access Server(B-RAS) cover the last mile or last several miles of the network which connects the subscriber’s home or office. But there’s more in the network that extends beyond the DSLAM and B-RAS and towards the worldwide internet. In order to better understand Triple Play on DSL, we will take a detailed look at topology of a typical backward compatible DSL Broadband Network that connects a DSL customer’s home or office through the CO and the Communication Service Provider’s network all the way to the internet.

So why are we interested in a backward compatible network? DSL Network elements have advanced over many generations; however, worldwide investment has been made in older generations of devices while newer ones evolved. Thus,in current DSL networks, elements with newer technology must remain compatible with older ones. This is backward compatibility.

In earlier implementations of DSL, digital data streams between the DSLAM and the DSL Modem were transmitted via Asynchronous Transmission Mode or ATM links. These links were built on top of Broadband DSL. Although DSLAMs and DSL Modems have moved away from ATM towards Internet Protocol or IP, many DSLAMs still retain ATM capability for backward compatibility with older local loops and modems.

The DSLAM aggregates digital data on these multiple local loops and places the data on one or more upstream links. Upstream links are high capacity links usually built on Gigabit Ethernet or, in older implementations, Asynchronous Transmission Mode (ATM) technologies. Through these links, the DSLAM connects to an element called Broadband Remote Access Server(B-RAS) that sits at the edge of the Communication Service Provider’s network.

One key function of the B-RAS is to authenticate each subscriber to prevent fraudulent or unauthorized use of DSL services. In the earliest days, the DSLAM used to connect to the B-RAS on a single ATM link. Later, it was found that a single ATM link met only the performance needs of internet access, whereas the Video and Telephony parts of Triple Play required more stringent performance that was not met by this single ATM link. Internet access could be best effort, which means that internet browsing and download related data streams could be slowed down or stopped for short intervals when the DSL network was not performing at its peak, before recovering the streams again.Yet, the subscriber would not mind such short intervals of performance degradation most of the time because as they were browsing or downloading from the internet in bursts,these degradations didn’t cause appreciable issues.

However, Video and Telephony over DSL are not forgiving of such performance degradations. So, later generation DSL implementations used separate ATM links for internet access, TV and Telephony data streams between the DSLAM and B-RAS. These links were possible due to ATM’s Switched Virtual Circuit (SVC) architecture. This reduced contention over network resources by multiple services and established priority for Video and Telephony data streams such that these streams would not slow down or stop when running.

Beyond the B-RAS and towards the Communications Service Provider network, the data that the DSLAM aggregated from all subscriber local loops were routed to Edge Routers over Gigabit Ethernet links. The Edge Routers carried subscribers data into and from the internet. In more recent implementations, the B-RAS has been merged with another entity called Broadband Network Gateway (BNG )because the BNG has more functionality over and above B-RAS.

So, to summarize, one of the main backward compatibility requirements for DSL Broadband networks is support for both ATM and Ethernet technologies.

Photo Credit: Daniel Boros

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