What is HDTV?

High Definition Television (HDTV) refers to a set of digital television broadcasting and display standards that provide high quality images. Due to current video compression standards as well as sufficient bandwidth available in access networks, the use of HDTV has increased in various regions of the world and multiple channels are actually broadcast in HDTV.

Three important parameters that define HDTV:

Frame size defined in number of horizontal pixels x number of vertical pixels

Scanning system referred to as i for interlaced (half the number of lines on alternating frames) or p for progressive (all lines on one frame). Progressive scanning requires more bandwidth than interlaced.

Frame rate defining the number of frames per second

Technically HDTV has twice the resolution of the previous SDTV (Standard Definition Television) and hence shows greater detail of the image.

HDTV parameters are defined by ITU recommendation BT.709 . HDTV broadcast transmission is governed by ATSC (Advanced Television Standards Committee) in the US, DVB (Digital Video Broadcasting) in Europe and ISDB-T ( Integrated Services Digital Broadcasting) nd other regions of the world adopt one of these standards.

Wikipedia reference: HDTV

What is DSL?

DSL or Digital Subscriber Line is the technology that allows data transmission over the local telephone line. Bellcore is credited with the discovery of DSL in 1988. The fundamental concept that allows both voice and data to be transmitted over the same phone line is the fact that voice is carried in low frequency bands and data in high frequency bands. Before the advent of DSL, the high frequency bands were simply unused on the phone lines as they were filtered out by the phone instruments. By using a splitter to separate out the high and low frequency bands and placing a DSL modem in front of the splitter (the traditional phone is also placed in front of the splitter) it is possible to have simultaneous voice and data communications over the existing phone infrastructure.

At the exchange, the phone line is connected to a DSLAM (DSL Access Multiplexer) which aggregates lines from a neighbourhood at one side and splits off the voice circuits to traditional voice switching systems & the data circuits to switches and routers to connect to the internet on the other side.

The most prevalent form of DSL is ADSL or Asynchronous DSL that allows for higher bandwidths downstream towards the user and lower bandwidths upstream towards the exchange – properties that make it eminently suitable for residential internet access. With the advent of peer to peer technologies this “suitability” factor is changing over time.

One important consideration with DSL is distance from the exchange – at high frequencies the signal attenuates with distance and hence the quality of the signal needs to be verified before installing DSL services. Another consideration is that of contention – in order to preserve bandwidth, the service providers over-subscribe the existing bandwidth under the assumption that all customers shall not simultaneously use the connection. In India, TRAI defines the contention ration to be 1:50 for residential subscribers and 1:30 for commercial subscribers.

The advent of triple play services and the considerations for offering triple play over DSL access have greatly changed the parameters of the DSL connection and the service providers begin to appreciate that higher bandwidth, better quality of signal and lower contentions are necessary to offer a good quality of experience for the triple play subscriber.

The Wikipedia reference is here: DSL
The Broadband forum (previously DSL forum) is the definitive technical reference for DSL and related technologies. Their first technical report defined the network reference model for ADSL networks. They now have over a hundred reports on topics related to DSL and services over DSL.

What is DOCSIS?

Data Over Cable Service Interface Specification is a standard published by Cablelabs which allows for the transfer of high-speed data over a Cable TV infrastructure (HFC or hybrid fibre coaxial network). The implementation of DOCSIS 1.0 allowed cable operators to start cable-internet services in the late nineties. DOCSIS 2.0 in the first half of this decade permitted broadband cable-internet and allowed for synchronous communication (addition of return path) enabling VOIP. DOCSIS 3.0 defined in 2006 allows greater speeds (both downstream and upstream) and is considered the basic requirement for IPTV over cable which will permit a full-fledged personalized and interactive TV service with blended applications.

DOCSIS is basically a communication protocol defined between a cable modem situated at the customer premise and a cable modem termination system (CMTS) at the operator’s point of presence. The physical and data link layer characteristics are defined as well as the upper layer protocol support as well as security and management.

As the cable modem gives way to more sophisticated set top boxes the DOCSIS Set top Gateway (DSG) standards have also been defined to standardize the mechanism of delivery of out of band data (channel lineups, program guides, software and configuration updates) to the user.

Motorola has been a proponent of DIBA (DOCSIS IPTV Bypass Architecture) which has not been accepted into DOCSIS 3.0. The proposal was to take the load of IPTV distribution away from the CMTS via a bypass point at the edge of the network. The debate on this topic still continues as is evidenced by this recent LR article which also discusses vendor positions and some deployments.

Wikipedia entries are here:
DOCSIS
DSG