On-line convergence
On-line convergence involves sending video broadcast services on tradi- tional twisted-pair telephone lines. There is nothing new about video streaming, sending video clips down the line to be downloaded on a PC. However, sending live TV broadcasts down the line, usually known as IPTV, is of a qualitatively different scale. These services are often called Broadband TV, ADSL TV, DSL TV or IPTV. First let us look at the telephone system and at the technique known as ADSL.
In the UK alone there are over 30 million twisted-pair phone lines in operation between British Telecom exchanges and individual subscribers’ premises. Originally the telephone lines were designed to carry simple command (dialling) pulses and frequency-restricted (300–3500 Hz) base- band voice signals. The remaining bandwidth of a copper wire was left unused. With the introduction of electronic exchanges, touch-tone dialling and routing functions became possible. The next step was to use a full dig- ital system where the copper wires from each subscriber terminated in an interface or ‘line card’ containing ADCs and DACs. This made a wide band of frequencies available to be divided into 4 kHz ‘telephone channels’. The sampling rate of 8 kHz and 8-bit quantisation retained the traditional ana- logue bandwidth resulting in a 64 kbps per telephone channel. First, time- division multiplexing was used in what is known as integrated services digital network (ISDN) and by combining a number of 64 kbps channels, high bit rates were reached. One of the problems of ISDN is that the bit rate is limited and a new cabling from the subscriber to the exchange was needed. Asymmetrical digital subscriber line (ADSL) solved that problem by using frequency-division techniques and dynamic control of the bit rate.
ADSL
ADSL works on frequency-division multiplexing using 4 kHz wide chan- nels (Figure 24.6). Twenty five channels are dedicated for the upstream or back channel (subscriber to provider) and 249 for the upstream (provider to subscriber). The different bandwidth allocation for the two streams is the ‘A’ for asymmetrical part of ADSL. As can be seen from Figure 24.6, the traditional analogue telephone is retained with the upstream occupying the frequency band above it and the downstream occupying the highest
frequency bands. To accommodate the varying quality of the telephone lines, ADSL constantly monitors the conditions of each channel and if a given channel has adequate level and low noise, the full bit rate is used. However, where a channel suffered from attenuation and noise, the bit rate is reduced. By independently coding the channels in terms of bit rate, the optimum data throughput for each telephone cable is obtained. Each ADSL channel is modulated using discrete multitone technique (DMT) in which combinations of discrete sub-carriers are used in a similar way to the OFDM scheme described in Chapter 8.
In the downstream, 249 sub-carriers are used, with each sub-carrier QAM-modulated with 0–15 bits. With a symbol rate of 4000, the maximum theoretical downstream bit rate = 15 X 249 X 4000 = 14.94 Mbps. However, in practice, the maximum achievable downstream bit rate is 8.128 Mbps which itself is affected by the length of the telephone line from the sub- scriber to the exchange. Higher bit rates are obtained with other tech- niques such as ADSL2.
At the subscriber’s home is a band splitter and a modem, and at the telephone exchange a modem and a subscriber access multiplexer (SAM), which sends the signal into a cable network on its way to the service provider. Unlike the ordinary telephone system with its line-grabbing and dial-up processes, ADSL is continuously alive and capable of two- way communication; data routing is directed by addressing information within the datastream, and each subscriber’s terminal has an individual address.
IPTV
Internet protocol television (IPTV), also known as broadband television (BTV) involves accessing multimedia content via a broadband connection and viewing it on a normal TV. IPTV is not the same as Internet TV which accesses TV via a PC. IPTV is sometimes called ADSL TV or DSL TV.
The IP as mentioned above is a packet delivery system operating at the Network Layer 3 of the OSI model in which the data load (payload) is encapsulated into a packet with an IP header containing various informa- tion and control bits including the destination and source addresses. Since IP networks are bi-directional, IPTV can deliver not only live television but also interactive and on-demand TV. Telecom operators who have been tra- ditionally interested in providing communication services between clients find their role is being expanded to provide what is known as Triple Play:
● communication services (including Voice over IP, VoIP);
● a high-speed internet connection;
● IP-based television and video-on-demand services
At the receiving end, playback requires only an Internet connection and an Internet-enabled device such as a personal computer, iPod, set-top-box connected to a TV receiver or even a 3G cell/mobile phone to watch the IPTV broadcasts. Apple’s iPhone uses mobile phone Global System for Mobile Communications (GSM) quad-band (900-MHz, 1800 MHz for Europe and 850 and 1900-MHz for the Americas) to provide images and television shows and films, internet, email and text messages as well as mobile phone facility.
Bit rates
One of the main issues concerning broadband services delivered over the telephone network is their reach or coverage area. Generally, the higher the bitrate of the signal, the lower is the reach of the service. As the band- width required for minimum TV quality is relatively high when compared with broadband internet (2–4 Mbps in the case of MPEG-2), the TV reach is more limited than the traditional reach of broadband internet connec- tions. The operators have several options to cope with this problem:
● They can reduce the bit rate and with it picture quality;
● they can use more advanced encoding schemes such as MPEG-4 AVC/H.264 or VC-1 (this will also help to pave the way to HDTV transmission), or
● they can upgrade their networks by introducing more efficient trans-mission technologies such as ADSL2+ or VDSL (very high bit rate DSL).
Upgrading the infrastructure requires significant capital investment and takes time to implement which leaves the second option as the favourite since reducing picture quality is not acceptable for the majority of customers.
The bandwidth problem becomes even more severe if more than one TV stream to the home is required. Such a need may arise if there are a number of TV sets in a house (e.g. one in the living room and another in a child’s bedroom), each requesting a different TV programme at the same time. More than one TV stream is also required if there is a local personal video recorder (PVR) with one stream being recorded locally while another is being watched.
One of the main applications of IPTV is ‘network-based private video recorder’ (NPVR). Network Personal Video Recording is the ultimate time- shifted viewing where real-time broadcast television is captured in the network on a server allowing the end user to access the recorded pro- grams on the schedule of their choice, rather than being tied to the broad- cast schedule.
The drawback of IPTV is the inclination of IP connection towards packet loss and delays in cases where the IP link is not fast enough. While this may be overcome by the inclusion of a video buffer at the receiving end in which case, lost packets may be re-transmitted, to ensure sufficient picture and sound quality, IPTV requires a reliable network with a robust Quality-of- Service (QoS) mechanism. The required QoS can only be met by providers that are able to control all elements of the transmission path from the source to the user’s premises. This is the reason why open Internet is not able to offer IPTV services, as it cannot guarantee QoS. In addition, stream- ing over open Internet would require some technical measures that address piracy, spoofing and network congestion. A comparison of the properties of IPTV and Internet video streaming is outlined in Table 24.3.