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در باب تفاوت های ADSL و VDSL :
Why VDSL?
With downstream speeds of up to a blazing 52 Mbits/s, VDSL (Very high speed Digital Subscriber Line), is the next step up the speed ladder beyond ADSL. However, the price paid for VDSL's increased speed is a shorter distance range. Additionally, VDSL comes in two variants, a symmetrical version and an asymmetrical version. Here is one of the key differences from ADSL, which usually only comes in a single asymmetrical version although some telcos may offer a 384 Mbits/s or 500 kbits/s symmetrical version.
VDSL technology will provide the final delivery of data over the existing copper drop, most likely fed by a high capacity fibre optic link from a central site but occasionally directly from a central office or switch.
Over short ranges, VDSL can offer up to 52 Mbits/s downstream capacity compared to the ADSL capacity of up to 8 Mbits/s; upstream, the asymmetrical versions of VDSL offers a slower data rate but still proportionately higher than for ADSL.
Why Develop VDSL?
ADSL is a great, always-on, service; however, the maximum speed available downstream is around 8 Mbits/s with a lesser upstream speed. VDSL extends this downstream speed to a potential 52 Mbits/s with a proportionately lower upstream speed but at the price of a shorter distance range than ADSL.
Ideal applications for VDSL include:
provision of multiple TV channels within apartment blocks;
ultra-high speed data access;
campus style data distribution over short distances;
video conferencing;
streaming video;
combined data and video over the same line;
the final drop for a full service network architecture.
When Will VDSL Become Available?
Today, it is impossible to tell when VDSL will become widely available. However, with the current rate of technical progress it seems likely that there will be major trials taking place before the end of 1999 and limited commercial services introduced before the end of 2000.
Since VDSL comes in two forms - asymmetrical and symmetrical - unlike ADSL which is usually only asymmetrical, one needs to look separately at these two different flavours of service. Most early uses of the symmetrical service seem likely to be in the corporate sector while early users of the asymmetrical services are likely to be in the multi-dwelling or campus environment. In the former case, VDSL can offer a cheap way to use the existing copper infrastructure to transmit data to / from the final drop, while in the latter case, the service is more likely to centre on a mixture of Internet / remote LAN access and multiple TV delivery.
What Will VDSL Do?
VDSL will open up new possibilities in two key areas:
corporate data distribution - symmetric;
very high speed downstream capacity for VoD etc. for users, typically in multi-dweller apartment blocks.
In both cases, VDSL will use the existing copper telephone lines for the final drop; these lines will be fed by high capacity fibre optic connections from the host or remote site. Over short distances, VDSL will offer a speed increase of around 5 times over ADSL. Over longer distances, VDSL will offer a speed increase of 2 to 3 times but VDSL's distance range will always be less than that for ADSL.
ADSL Forum Objectives With VDSL?
There is a high degree of synergy between ADSL and VDSL; the same companies are working on both technologies and the same individuals within those companies are also involved. Thus, it is sensible that the ADSL Forum should address the development and deployment of VDSL; this will make the best use of resources, people and time.
Additionally, the VDSL development time-line runs behind that for ADSL; as a result, as the work-load on ADSL falls off, so the work-load on VDSL will continue building. It is also important to ensure that there is the maximum re-use of technology and solutions passing from ADSL to VDSL.
The overall objective of the ADSL Forum adopting VDSL is to consider the architectural and VDSL systems aspects of a hybrid fibre / copper VDSL-based broadband access solution.
What Are the Differences Between ADSL and VDSL?
Speed is the main differentiation between ADSL and VDSL, the latter being significantly faster over short distances. Both ADSL and VDSL offer high speed data transmission over existing copper telephone lines. ADSL, as its name implies, offers asymmetrical data transmission with a ratio of about 10:1, downstream to upstream capacity. ADSL also offers a maximum range of up to 18,000ft (say 5.5 km) albeit at a relatively slow downstream data rate of around 2 Mbits/s; below about 9,000ft (say 2.7 km) a data rate of 6+ Mbits/s is achievable.
VDSL, although not yet standardised, will certainly offer both symmetrical and asymmetrical versions; within these two versions, there will be a variety of speeds. Like ADSL, there is a trade-off between speed and range - the higher the speed, the shorter the range. The ultimate form of asymmetrical VDSL will offer up to 52 Mbits/s downstream and up to 6.4 Mbits/s upstream, only over some 1,000 ft or 300 m, but as the data rate requirement reduces, then the range would increase. On a symmetrical basis, speeds of up to 34 Mbits/s in both directions are possible, with the same range restrictions as for the asymmetrical version.
Because VDSL gains in speed but loses in distance or range, there is a need for a high capacity feed to bring the service to within say a maximum of 2 km from the user - this feed is a fibre optic link terminating in an ONU (Optical Network termination Unit). At the ONU, the signal is transferred from the fibre to the final copper drop to the subscriber or user.
VDSL technology will provide the final delivery of data over the existing copper drop, most likely fed by a high capacity fibre optic link from a central site but occasionally directly from a central office or switch.
What Is The Current Status of VDSL Standards?
Today, there are no accepted VDSL standards. However, ETSI and ANSI, the European and American standards bodies, are both actively working towards a standard for VDSL; both bodies are actively co-operating in this effort. In addition, DAVIC (the Digital Audio VIsual Communication forum) had been actively working to define a specific physical layer implementation of VDSL.
There is also an active group of telcos working under the nomenclature of FSAN (Full Services Access Network) which has developed a VDSL requirements specification. While this specification does not amount to a standard, such an influential group of telcos will undoubtedly have a major influence on the formal standards bodies. Also, since the FSAN group incorporates many of the major purchasers of VDSL, their specified requirements amount - de facto - to the minimum standard their vendors much achieve.
و همچنین :
Types of DSL - Compare ADSL & SDSL Internet Access
Digital Subscriber Line (DSL) brings broadband information to homes and businesses over ordinary copper telephone lines. DSL provides a high-speed broadband conduit for instant transmission of data for information, entertainment, and much more.
Symmetric (SDSL) vs. Asymmetric (ADSL)
There are two main types of DSL. Symmetric DSL refers to DSL technologies that provide the same bandwidth upstream (sending) and downstream (receiving). Symmetric DSL services provide identical data rates whether you are sending information, like an email, or receiving information, downloading a file or accessing a web site. Asymmetric DSL provides relatively lower rates upstream but higher rates downstream. When considering your broadband needs, the choice between ADSL and SDSL largely hinges on price. SDSL is more expensive.
Asymmetric Digital Subscriber Line
ADSL is ideal for most homes and small businesses. Most of its two-way or duplex bandwidth is devoted to the downstream direction, sending data to the user. Only a small portion of bandwidth is available for upstream or user-interaction messages. Most broadband data (graphics and multi-media, for example) need lots of downstream bandwidth. At the same time, the average homes or small business DSL generally requires little upstream bandwidth. Using ADSL, up to 6.1 megabits per second of data can be sent downstream and up to 640 Kbps upstream. The high downstream bandwidth means that your telephone line will be able to bring motion video, audio, and 3-D images to your computer or hooked-in TV set. In addition, a small portion of the downstream bandwidth can be devoted to voice rather data, and you can hold phone conversations without requiring a separate line.
Unlike a similar broadband service over your cable TV line, ADSL won't compete for bandwidth with neighbors. Although existing telephone lines will probably work with ADSL, in some areas, they may need upgrading to support broadband communication.
ADSL Varieties
There are several sub-categories of ADSL. RADSL (Rate-Adaptive DSL) is an ADSL technology in which software is able to determine the rate at which signals can be transmitted on a given customer phone line and adjust the delivery rate accordingly.
G.lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially a slower ADSL that provides a data rate from 1.544 Mbps to 6 Mpbs downstream and from 128 Kbps to 384 Kbps upstream. G.Lite is expected to become the most widely installed form of DSL.
Symmetric Digital Subscriber Line
SDSL is generally used for wideband digital transmission within a corporate site and between the telephone company and a customer. The main characteristic of SDSL is that it is balanced: an equal amount of bandwidth is available in both directions and the data rate is the same in both directions. SDSL can carry as much on a single wire of twisted-pair cable as a T1 line (up to 1.544 Mbps) but over a somewhat longer range.
اما در باب اینکه اصولا VDSL چیست !؟ خدمت شما دوست عزیز :
What is VDSL?
Very-High-Data-Rate Digital Subscriber Line (VDSL) is an emerging broadband technology that promises much higher data rates over relatively short distances (between 51 and 55 Mbps over lines up to 4,000 feet in length).
VDSL is seen by many as the next step in providing a complete home-communications and entertainment package. For example, the broadband internet access VDSL affords will support high definition TV. Some companies currently offer VDSL service in selected areas. VDSL provides an incredible amount of bandwidth compared to a maximum speed of 8 to 10 Mbps for ADSL or cable modem and clearly the move from current broadband technology to VDSL could be as significant as the shift from a 56K modem to broadband.
Fiber Remotes
The key to VDSL availability is the deployment of fiber remotes to shorten loop length.
Telephone companies are replacing many of their main feeds with fiber-optic cable. In fact, many phone companies are planning Fiber to the Curb (FTTC), which means that they will replace all existing copper lines right up to the point where your phone line branches off at your house. At the least, most companies expect to implement Fiber to the Neighborhood (FTTN). Instead of installing fiber-optic cable along each street, FTTN has fiber going to the main junction box for a particular neighborhood. By placing a VDSL transceiver in your home and a VDSL gateway in the junction box, the distance limitation can be economically overcome.
VDSL Obstacles
As important as fiber remotes would be to VDSL availability, they aren’t the whole story. VDSL and any other ultra high bandwidth technologies need to co-exist with other communications media and applications. VDSL data traffic exists in very high frequency bands and any high speed technology has to contend with cross talk and interference.
Another factor is the bandwidth between the remote and carrier serving offices-the "uplink." If each user has 58Mbits/sec, there won't be very many users per fiber remote. This would make the cost per user unacceptably high. While costs could be reduced by assuming that users shared the uplink for non-voice traffic, bandwidth oversubscription would reduce the chance that VDSL loops could support video applications.
Finally, running a fiber connection to each remote is extremely expensive. The answer is a hierarchical network. The architecture that shows the greatest promise is Passive Optical Networking (PON). PON fiber networks don't require a device at each fiber junction. Instead, the fiber is installed like a tree with "branches" from the "trunk" and branches from other branches. A very large service area can be covered by a PON at a very low equipment cost. One of the most promising targets for this broadband delivery system is the huge number of multi-tenanted buildings with business and residential occupants in urban areas
بطور خلاصه تر :
VDSL Last modified: Monday, August 27, 2001
Very High Speed Digital Subscriber Line transmits data in the 13 Mbps - 55 Mbps range over short distances, usually between 1000 and 4500 feet (300 - 1500 meters), of twisted pair copper wire. The shorter the distance, the faster the connection rate.
As the final length of cable into the home or office, VDSL connects to neighborhood Optical Network Units (ONUs), which connect to the central office's main fiber network backbone. This architecture will allow VDSL users to access the maximum bandwidth available through normal phone lines.
VDSL is currently going through a standards issue, so it isn't widely deployed yet. The VDSL alliance favors a line coding scheme based on Discrete Multitone (DMT), a multi-carrier system that is more compatible with existing ADSL technology. The VDSL coalition favors a line coding scheme based on Quadature Amplitude Modulation (QAM), a single-carrier system that is less expensive and consumes less power.
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