The use of high-speed lines to access the Internet has grown greatly in recent years. Cable modems and ADSL lines are two different approaches in this field. The above technologies allow users to access the Internet at optimal speed. Internet is growing rapidly in all dimensions. Digital televisions and broadcasting of video images are two new applications on the Internet that have attracted many enthusiasts. In order to provide the above services and other similar services, Internet users need to use much faster lines than the current situation. Cable modems or ADSL lines, despite providing the appropriate speed in the mentioned applications, lack the required speed.
Recently, many organizations and companies have proposed VDSL (Very high bit-rate DSL) technology. Some companies have tried to provide the above service in some parts of America. VDSL offers a very high bandwidth and the data transfer speed is 52 megabits per second. The above speed is very high compared to DSL (maximum speed of 8 to 10 megabits per second) or cable modems, and it will definitely be a turning point in the field of access to the Internet in terms of speed. The previous turning point was the transition from using modems with a capacity of 56 kilobits per second to broadband (cable modems and DSL lines).
Basics of DSL
When installing a telephone (standard) in most countries, a pair of copper cables is used. Copper cable has much more width than what is used in telephone conversations (most of the bandwidth capacity is not used). DSL uses unused bandwidth without negatively affecting the quality of voice calls. (adjustment of special frequencies in order to perform special operations)
In order to understand how DSL works, it is necessary to get more familiar with a normal telephone line. Most of the telephone lines and related equipment have frequency limitations in connection with the switch, telephone and other equipment that are somehow involved in the signal transmission process. The human voice (in a normal voice conversation) can be transmitted by signals with a frequency between zero and 3400. The above range is very small (for example, most stereo speakers have a range between 20 and 20,000 Hz). The cable used in the telephone system is capable of transmitting signals with a capacity of several million hertz. In this way, only a very limited part of the available bandwidth is used in voice conversations.By using the unused bandwidth, in addition to exploiting the existing potentials, it is possible to act in such a way that the quality of voice conversations does not suffer. Advanced devices that send information digitally are able to fully use the capacity of telephone lines. DSL pursues such a goal.
ADSL uses two special devices. One of the devices is installed at the subscriber’s place and the other device is installed for the ISP, telephone company or organizations providing DSL services. A DSL transceiver is used at the subscriber’s location. The company providing DSL services uses a DSL Access Multiplexer (DSLAM). The above device is used to receive subscriber connections.
Most DSL subscribers call the DSL transceiver a DSL modem. Engineers and experts of telephone companies call the above device ATU-R. Regardless of any name used for it, the above device is the point of communication between the user’s computer or the network to the DSL line. The transceiver is connected to the subscriber’s device using different methods. The most common method is to use USB or Ethernet connections.
The above device can be installed in DSL service provider centers and provides the possibility of providing DSL-based services. DSLAM takes the connections of a number of subscribers and turns them into a high-capacity connection for sending over the Internet. DSLAM devices have the flexibility to use different DSL lines, different protocols and different modulation (Cap, DMT). In some of the above models, it is possible to perform special operations such as dynamically assigning IP addresses to subscribers.
One of the important differences between ADSL and cable modems is the handling and behavior of DSLAM. Cable modem users use a shared closed network. In such cases, as the number of users increases, their efficiency will decrease. ADSL creates a dedicated connection for each user and connects it to DSLAM. In this way, with the increase of users, the relevant efficiency will not decrease. The above situation will continue until the users have not used all the available capacity of the Internet connection line. In case of using the full capacity of the Internet communication line, the DSL service provider centers can upgrade the Internet communication line so that all subscribers connected to the DSLAM have optimal efficiency in terms of using the Internet.
VDSL speed
The performance of VDSL is similar to ADSL in most cases. Despite the similarities in this field, there are also many differences. VDSL is capable of providing a speed of 52 megabits per second to send information from the Internet to the user (Downstream) and 16 megabits per second to send information to the user on the Internet (Upstream). The above speeds are much higher than ADSL. In ADSL, the maximum speed of sending information from the Internet to the user is 8 megabits per second, and the speed of sending information from the user to the Internet is 800 kilobits per second. VDSL owes its high speed to narrowing the distance between subscribers and the service provider. The maximum available distance is 4,000 feet (1,200 meters).
Telephone companies are replacing most of the equipment (related to feeding information) to optical fiber. Most of the telephone companies use FTTC (Fiber to the Curb) technology. The above companies intend to replace all the existing copper lines up to the place where they are branched off and distributed to the subscribers’ homes. Telephone companies are trying to implement the Fiber To the Neighborhood (FFTN) system. In the above method, instead of installing optical fiber cable in every street, FFTN has the desired fiber up to the junction box (branches) for a particular neighbor (subscriber).
By installing a VDSL transceiver at home and a VDSL gateway in the connection distribution box, the distance limitation will be reduced.
The gateway takes care of the analog to digital and digital to analog conversions that disable ADSL on fiber optic lines. Gateway converts the incoming and received data from the transceiver into light pulses so that they can be sent through optical fiber. When the data is sent to the user’s computer (return data), the Gateway converts the incoming signals from the optical fiber and sends them to the user’s transceiver. The above process will be repeated millions of times per second.
ADSL and VDSL are just two examples of technologies related to DSL spectrum. In the following, we will refer to the review of other models related to the above technology.
