什么是ADSL?英文版
Asymmetric Digital Subscriber Line
ADSL standards
ADSL
ANSI T1.413-1998 Issue 2
G.DMT
ITU G.992.1
G.Lite
ITU G.992.2
ADSL2
ITU G.992.3/4
ITU G.992.3/4 Annex J
ITU G.992.3/4 Annex L
ADSL2+
ITU G.992.5
ITU G.992.5 Annex L
ITU G.992.5 Annex M
Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide.
The distinguishing characteristic of ADSL over xDSL is that the volume of data flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market ADSL as a service for people to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require bandwidth in the other direction.
There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close together) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL.
For conventional ADSL, downstream rates start at 256 kbit/s and typically reach 8 Mbit/s within 1.5 km (5000 ft) of the DSLAM equipped central office or remote terminal. Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 1024 kbit/s. The name ADSL Lite is sometimes used for the slower versions.
Note that distances are only approximations aimed at consumers of ADSL services. Signal attenuation and Signal to Noise Ratio are defining characteristics, and can vary completely independently of distance (e.g., non-copper cabling, cable diameter). Real world performance is also dependent to the line impedance, which can change dynamically either dependent on weather conditions (very common for old overhead lines) or on the number and quality of joints or junctions in a particular cable length.
A newer variant called ADSL2 provides higher downstream rates of up to 12 Mbit/s for spans of less than 2.5 km (8000 ft). Higher symbol rates and more advanced noise shaping are responsible for these increased speeds. ADSL2+, also referred to as ITU G.992.5, boosts these rates to up to 24 Mbit/s for spans of less than 1.5 km (5000 feet). ADSL2+ also offers seamless bonding options, allowing lines with higher attenuation or lower signal to noise (SNR) ratios to be bonded together to achieve theoretically the sum total of the number of lines (i.e., up to 50 Mbit/s for two lines, etc.), as well as options in power management and seamless rate adaptation - changing the data rate used without requiring to resynchronize.
Because of the relatively low data-rate (compared to optical backbone networks), ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as web traffic; ADSL is commonly deployed with ATM to ensure that this remains a possibility. In a triple play scenario, different ATM virtual circuits (VCs) may be allocated for different services.
More recently, network operators are increasingly moving away from ATM, and towards Ethernet-based solutions, where 802.1Q and/or VPLS offer multiplexing solutions. The main reason for this switch is cost savings and the possibility of removing the older and more expensive ATM network.
ADSL service providers may offer either static or dynamic IP addressing. Static addressing is preferable for people who may wish to connect to their office via a virtual private network, for some Internet gaming, and for those wishing to use ADSL to host a Web server.
How ADSL works
On the wire
ADSL uses two separate frequency bands. With standard ADSL, the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz - 1104 kHz is used for downstream communication.
Frequency plan for ADSL
Each of these is further divided into smaller chunks of 4.3125 kHz. During initial training, the ADSL modem tests which of the available chunks have an acceptable signal-to-noise ratio. The distance from the telephone exchange, or noise on the copper wire, may introduce errors on some frequencies. By keeping the chunks small, an error on one frequency thus need not render the line unusable: the chunk will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connection.
Vendors may support usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk issues that affect other lines in the same bundle.
There is a direct relationship between the number of chunks available and the throughput capacity of the ADSL connection. The exact data capacity per chunk depends on the modulation method used.
A common error is to attribute the A in ADSL to the word asynchronous. The separated frequencies of ADSL used for download and upload channels operate simultaneously; they are thus synchronous.
Modulation
ADSL initially existed in two flavors (similar to VDSL), namely CAP and DMT. CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs at the time. However, DMT was chosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called G.dmt and G.lite respectively). Therefore, all modern installations of ADSL are based on the DMT modulation scheme.
ADSL是“非對稱數(shù)字用戶線路”（Asymmetric Digital Subscriber Line）之簡稱，亦可稱作“非對稱數(shù)字用戶環(huán)路”（Asymmetric Digital Subscriber Loop）。
介紹
ADSL因為上行（從用戶到電信服務提供商方向，如上傳動作）和下行（從電信服務提供商到用戶的方向，如下載動作）速率不對稱（即上行和下行的速率不相同）因此稱為非對稱數(shù)字用戶線路。它采用頻分復用技術把普通的電話線分成了電話、上行和下行三個相對獨立的信道，從而避免了相互之間的干擾。通常ADSL在不影響正常電話通信的情況下可以提供512Kbps——1Mbps的上行信道和1.5——8Mbps的下行信道。
設備
ADSL是一種異步傳輸模式（ATM）。
在電信服務提供商端，需要將每條開通ADSL業(yè)務的電話線路連接在數(shù)字用戶線路訪問多路復用器（DSLAM）上。而在用戶端，用戶需要使用一個ADSL終端（因為和傳統(tǒng)的調制解調器（Modem）類似，所以也被稱為“貓”）來連接電話線路。由于ADSL使用高頻信號，所以在兩端還都要使用ADSL信號分離器將ADSL數(shù)據(jù)信號和普通音頻電話信號分離出來，避免打電話的時候出現(xiàn)噪音干擾。
通常的ADSL終端有一個電話Line-In，一個以太網(wǎng)口，有些終端集成了ADSL信號分離器，還提供一個連接的Phone接口。
標準
傳輸標準
由于受到傳輸高頻信號的限制，ADSL需要電信服務提供商端接入設備和用戶終端之間的距離不能超過5千米，也就是用戶的電話線連到電話局的距離不能超過5千米。
ADSL設備在傳輸中需要遵循以下標準之一：
ITU-T G.992.1(G.dmt)
G.dmt：全速率，下行8Mbps，上行896Kbps
ITU-T G.992.2(G.lite)
G.lite：下行1.5Mbps，上行512Kbps
ITU-T G.994.1(G.hs)
可變比特率（VBR）
ANSI T1.413 Issue #2
下行8Mbps，上行896Kbps
還有一些更快更新的標準，但是目前還很少有電信服務提供商使用：
ITU G.992.3/4
ADSL2 下行12Mbps，上行1.0Mbps
ITU G.992.3/4
Annex J ADSL2 下行12Mbps，上行3.5Mbps
ITU G.992.5
ADSL2+ 下行24Mbps，上行1.0Mbps
ITU G.992.5
Annex L ADSL2+ 下行24Mbps，上行3.5Mbps
當電信服務提供商的設備端和用戶終端之間距離小于1.3千米的時候，還可以使用速率更高的VDSL，它的速率可以達到下行55.2Mbps，上行19.2Mbps。
網(wǎng)絡登錄標準
ADSL通常提供三種網(wǎng)絡登錄方式：
橋接，直接提供靜態(tài)IP
PPPoA，基于ATM的端對端協(xié)議
PPPoE，基于以太網(wǎng)的端對端協(xié)議
其中后兩種通常不提供靜態(tài)IP，而是動態(tài)的給用戶分配網(wǎng)絡地址。
Asymmetric Digital Subscriber Line
ADSL standards
ADSL
ANSI T1.413-1998 Issue 2
G.DMT
ITU G.992.1
G.Lite
ITU G.992.2
ADSL2
ITU G.992.3/4
ITU G.992.3/4 Annex J
ITU G.992.3/4 Annex L
ADSL2+
ITU G.992.5
ITU G.992.5 Annex L
ITU G.992.5 Annex M
Asymmetric Digital Subscriber Line (ADSL) is a form of DSL, a data communications technology that enables faster data transmission over copper telephone lines than a conventional modem can provide.
The distinguishing characteristic of ADSL over xDSL is that the volume of data flow is greater in one direction than the other, i.e. it is asymmetric. Providers usually market ADSL as a service for people to connect to the Internet in a relatively passive mode: able to use the higher speed direction for the "download" from the Internet but not needing to run servers that would require bandwidth in the other direction.
There are both technical and marketing reasons why ADSL is in many places the most common type offered to home users. On the technical side, there is likely to be more crosstalk from other circuits at the DSLAM end (where the wires from many local loops are close together) than at the customer premises. Thus the upload signal is weakest at the noisiest part of the local loop, while the download signal is strongest at the noisiest part of the local loop. It therefore makes technical sense to have the DSLAM transmit at a higher bit rate than does the modem on the customer end. Since the typical home user in fact does prefer a higher download speed, the telephone companies chose to make a virtue out of necessity, hence ADSL.
For conventional ADSL, downstream rates start at 256 kbit/s and typically reach 8 Mbit/s within 1.5 km (5000 ft) of the DSLAM equipped central office or remote terminal. Upstream rates start at 64 kbit/s and typically reach 256 kbit/s but can go as high as 1024 kbit/s. The name ADSL Lite is sometimes used for the slower versions.
Note that distances are only approximations aimed at consumers of ADSL services. Signal attenuation and Signal to Noise Ratio are defining characteristics, and can vary completely independently of distance (e.g., non-copper cabling, cable diameter). Real world performance is also dependent to the line impedance, which can change dynamically either dependent on weather conditions (very common for old overhead lines) or on the number and quality of joints or junctions in a particular cable length.
A newer variant called ADSL2 provides higher downstream rates of up to 12 Mbit/s for spans of less than 2.5 km (8000 ft). Higher symbol rates and more advanced noise shaping are responsible for these increased speeds. ADSL2+, also referred to as ITU G.992.5, boosts these rates to up to 24 Mbit/s for spans of less than 1.5 km (5000 feet). ADSL2+ also offers seamless bonding options, allowing lines with higher attenuation or lower signal to noise (SNR) ratios to be bonded together to achieve theoretically the sum total of the number of lines (i.e., up to 50 Mbit/s for two lines, etc.), as well as options in power management and seamless rate adaptation - changing the data rate used without requiring to resynchronize.
Because of the relatively low data-rate (compared to optical backbone networks), ATM is an appropriate technology for multiplexing time-critical data such as digital voice with less time-critical data such as web traffic; ADSL is commonly deployed with ATM to ensure that this remains a possibility. In a triple play scenario, different ATM virtual circuits (VCs) may be allocated for different services.
More recently, network operators are increasingly moving away from ATM, and towards Ethernet-based solutions, where 802.1Q and/or VPLS offer multiplexing solutions. The main reason for this switch is cost savings and the possibility of removing the older and more expensive ATM network.
ADSL service providers may offer either static or dynamic IP addressing. Static addressing is preferable for people who may wish to connect to their office via a virtual private network, for some Internet gaming, and for those wishing to use ADSL to host a Web server.
How ADSL works
On the wire
ADSL uses two separate frequency bands. With standard ADSL, the band from 25.875 kHz to 138 kHz is used for upstream communication, while 138 kHz - 1104 kHz is used for downstream communication.
Frequency plan for ADSL
Each of these is further divided into smaller chunks of 4.3125 kHz. During initial training, the ADSL modem tests which of the available chunks have an acceptable signal-to-noise ratio. The distance from the telephone exchange, or noise on the copper wire, may introduce errors on some frequencies. By keeping the chunks small, an error on one frequency thus need not render the line unusable: the chunk will not be used, merely resulting in reduced throughput on an otherwise functional ADSL connection.
Vendors may support usage of higher frequencies as a proprietary extension to the standard. However, this requires matching vendor-supplied equipment on both ends of the line, and will likely result in crosstalk issues that affect other lines in the same bundle.
There is a direct relationship between the number of chunks available and the throughput capacity of the ADSL connection. The exact data capacity per chunk depends on the modulation method used.
A common error is to attribute the A in ADSL to the word asynchronous. The separated frequencies of ADSL used for download and upload channels operate simultaneously; they are thus synchronous.
Modulation
ADSL initially existed in two flavors (similar to VDSL), namely CAP and DMT. CAP was the de facto standard for ADSL deployments up until 1996, deployed in 90 percent of ADSL installs at the time. However, DMT was chosen for the first ITU-T ADSL standards, G.992.1 and G.992.2 (also called G.dmt and G.lite respectively). Therefore, all modern installations of ADSL are based on the DMT modulation scheme.
ADSL是“非對稱數(shù)字用戶線路”（Asymmetric Digital Subscriber Line）之簡稱，亦可稱作“非對稱數(shù)字用戶環(huán)路”（Asymmetric Digital Subscriber Loop）。
介紹
ADSL因為上行（從用戶到電信服務提供商方向，如上傳動作）和下行（從電信服務提供商到用戶的方向，如下載動作）速率不對稱（即上行和下行的速率不相同）因此稱為非對稱數(shù)字用戶線路。它采用頻分復用技術把普通的電話線分成了電話、上行和下行三個相對獨立的信道，從而避免了相互之間的干擾。通常ADSL在不影響正常電話通信的情況下可以提供512Kbps——1Mbps的上行信道和1.5——8Mbps的下行信道。
設備
ADSL是一種異步傳輸模式（ATM）。
在電信服務提供商端，需要將每條開通ADSL業(yè)務的電話線路連接在數(shù)字用戶線路訪問多路復用器（DSLAM）上。而在用戶端，用戶需要使用一個ADSL終端（因為和傳統(tǒng)的調制解調器（Modem）類似，所以也被稱為“貓”）來連接電話線路。由于ADSL使用高頻信號，所以在兩端還都要使用ADSL信號分離器將ADSL數(shù)據(jù)信號和普通音頻電話信號分離出來，避免打電話的時候出現(xiàn)噪音干擾。
通常的ADSL終端有一個電話Line-In，一個以太網(wǎng)口，有些終端集成了ADSL信號分離器，還提供一個連接的Phone接口。
標準
傳輸標準
由于受到傳輸高頻信號的限制，ADSL需要電信服務提供商端接入設備和用戶終端之間的距離不能超過5千米，也就是用戶的電話線連到電話局的距離不能超過5千米。
ADSL設備在傳輸中需要遵循以下標準之一：
ITU-T G.992.1(G.dmt)
G.dmt：全速率，下行8Mbps，上行896Kbps
ITU-T G.992.2(G.lite)
G.lite：下行1.5Mbps，上行512Kbps
ITU-T G.994.1(G.hs)
可變比特率（VBR）
ANSI T1.413 Issue #2
下行8Mbps，上行896Kbps
還有一些更快更新的標準，但是目前還很少有電信服務提供商使用：
ITU G.992.3/4
ADSL2 下行12Mbps，上行1.0Mbps
ITU G.992.3/4
Annex J ADSL2 下行12Mbps，上行3.5Mbps
ITU G.992.5
ADSL2+ 下行24Mbps，上行1.0Mbps
ITU G.992.5
Annex L ADSL2+ 下行24Mbps，上行3.5Mbps
當電信服務提供商的設備端和用戶終端之間距離小于1.3千米的時候，還可以使用速率更高的VDSL，它的速率可以達到下行55.2Mbps，上行19.2Mbps。
網(wǎng)絡登錄標準
ADSL通常提供三種網(wǎng)絡登錄方式：
橋接，直接提供靜態(tài)IP
PPPoA，基于ATM的端對端協(xié)議
PPPoE，基于以太網(wǎng)的端對端協(xié)議
其中后兩種通常不提供靜態(tài)IP，而是動態(tài)的給用戶分配網(wǎng)絡地址。