The metal lines have been used for decades for the transport of the telephone speech with a bandwidth of 3200 Hz. They are used today for transport, at the same time as the telephone, broadband data packets. The xDSL environment is devoted to terrestrial broadband connections, we give here in an introduction.
A first important feature of this environment is in the telephone wiring. Between a telephone and the operator switch, a cable is totally dedicated to communication. This is to get each cable high bandwidth parallel to the telephone bandwidth. There is thus a type of communication channel circuit between the user and the operator switch, unlike what happens in networks of cable operators in which a significant portion of the wiring is common to all users. On this circuit, the ends emit frames which contain IP packets. The chosen frame was very long ATM type, but today Ethernet is preferred for its cost and greater adaptability.
XDSL modems allow the use of metallic pairs in the access network to achieve a high-speed local loop. The flow rate depends heavily on the quality of the cable used and the distance. Several categories of xDSL modems are sold, the letter x to differentiate. The x to specify the type of DSL modem.
ADSL modems (Asymmetric Digital Subscriber Line) are the most common. Their speeds are asymmetrical, slower between the terminal and the network in the other direction. Typically, the uplink direction is four times slower than the downward direction. The speeds on the downlink can reach 2 Mbit/s over a distance of about 5 km and exceed twenty megabits per second when it is less than a kilometer from the operator of the equipment. The ADSL modem uses a quadratic amplitude modulation, that is to say, 16 bits are carried on each signal. With a modulation rate of 340 kbaud and an attenuation of about thirty decibels, we reached more than 5 Mbit/s.
Before the success of the ADSL technology, derivatives have been developed, including the technique of varying the flow rate on the cable, which gave birth to the RADSL (Rate Adaptive DSL). For broadband, HDSL solutions (High bit rate DSL) and VDSL (Very high bit rate DSL) can be used successfully if the wiring often fiber optic permits. Measurements taken in the operators show that flows are becoming more symmetrical since the advent of peer-to-peer (P2P) applications, the stations become client server users. The SDSL technology (Symmetric DSL) will therefore become more and more common.
Speech and video over xDSL
We saw that in xDSL telephone speech was transported parallel to data on the lower part of the spectrum. This technology is well suited for incumbents, also known as ILEC (Incumbent Local Exchange Carrier). the newcomers or CLEC (Competitive Local Exchange Carrier), can now expect to challenge incumbents through deregulation of the local loop.
To support clients on the local loop of the incumbent, the Incoming operators can pass on the telephone speech DSL party. We call this solution ToDSL (Telephony over DSL). The transition from the floor on the given part is similar to Voice over IP technologies.
Speech packets to arrive at the receiver before 150 ms, it is necessary that their priority is applied. In this case, the ten kilobits per second of the compressed speech pass easily. It must, however, priority may be exercised not only on the modem part but also on parts network before and after the two modems. This implies, for the corporate network part, the application of technical and priority for the ISP network, the possibility of negotiating an SLA (Service Level Agreement) compatible with the maximum time after crossing end .
Another, less integrated but simpler to implement, is marketed by many ISPs to offer ToDSL telephone service. It consists in using a specific band modem, 4.3 MHz, giving a flow rate of 32 kbit / s. The disadvantage of this solution is that if the telephone speech is not used, the corresponding bandwidth is lost. However, as the bandwidth is very low, this is not really a problem.
The DSL line must also convey telephone signaling, which is the second difficulty after the time constraint. On the modem, instead of using the priority over data it is possible to use the AAL-1, which provides synchronization and priority features. This solution, called VoATM (Voice over ATM) is complementary to the ToDSL technology.
Television is a second application that is available to modem users ADSL. With the first generation of ADSL modems, a specific bandwidth is devoted to the television channel, generally 3 Mbit/s. When the TV is on, the channel that carry digital signals of the television image. When the television is off, the 3 Mbit/s is allocated to the bandwidth used for data transport. In the second generation, the TV image is integrated with other waves, and the packets carrying the image stream are easily identified by a specific address corresponding to the decision on the case on which is connected television. Finally, in the third generation the flood of packets from the TV is a fully integrated flow to other IP packets, specific labeling for recognizing the packets to give them priority for an acceptable quality of service necessary.
Video is another service that can be offered by DSL modems. While it is still unimaginable to see this system supplant the video widely distributed, video over DSL, VoDSL (Video over DSL), starts to be deployed by many ISPs for limited broadcasts and VoD services (Video on Demand).
Both solutions we reviewed for telephony are possible for the video: either we integrate video packets in the flow by giving a high priority if possible, or they are assigned a specific channel. In the latter case, the width of the bandwidth allocated to video differs operators to fly from 800 Kbit/s to a few megabits per second. For a TV to 800 Kbit/s, you can now get 25 of the 256 sub-bands, each carrying 32 Kbit/s.
In the case of multipoint, that is to say the limited distribution to a small number of users, the video is compressed into MPEG-4 or possibly MPEG-2 and issued using a multipoint protocol. The most powerful of these protocols is IP Multicast, because the packets are the IP origin. However, as much as possible to compress video data, the choice of video codec is crucial that the tide comes in time.
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