ADSL Primer

Introduction:

Asymmetric Digital Subscriber Line (ADSL) is a data communications technology that allows faster data transmission rates over copper telephone lines than conventional modems can achieve. One key difference of ADSL over xDSL is that the volume of data flow is greater in one direction than the other therefore the term asymmetric.

Conventional ADSL downstream rates start at 256 Kbps and typically reach 8 Mbps within 1.5 Km or 5000 ft of the central office (CO) or remote terminal (RT) Upstream rates start at 64 Kbps and can go as high as 1.024 Mbps

Signal to noise ratio, attenuation are defining characteristics, and can vary completely independently of distance. 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.

Standard name Standard type Downstream rate Upstream rate
ANSI T1.413 Issue 2 ADSL 8 Mbps 1.0 Mbps
ITU G.992.1 ADSL (G.DMT) 8 Mbps 1.0 Mbps
ITU G.992.2 ADSL Lite (G.Lite) 1.5 Mbps 0.5 Mbps
ITU G.992.3-4 ADSL2 12 Mbps 1.0 Mbps
ITU G.992.3-4 Annex J ADSL2 12 Mbps 3.5 Mbps
ITU G.992.3-4 Annex L RE-ADSL2 5 Mbps 0.8 Mbps
ITU G.992.5 ADSL2+ 24 Mbps 1.0 Mbps
ITU G.992.5 Annex L RE-ADSL2+ 24 Mbps 1.0 Mbps
ITU G.992.5 Annex M ADSL2+ 24 Mbps 3.5 Mbps

ADSL was initially conceived using two modulation methods CAP (carrier less amplitude phase) and DMT (discrete multi tone). CAP modulation was the de facto standard for xDSL installations until 1996 when it was deprecated in favor of the more robust DMT. DMT is superior to CAP in the sense that each tone can be assigned a different carrying capacity and in severe noise conditions each tone can be independently turned off.

In addition to POTS ADSL use two distinct bands for upstream and downstream. Annex A stipulated the frequency band of 25.875 kHz to 138 KHz to be used as the upstream channel while 138 kHz to 1104 kHz is used for downstream communications. POTS use the lowest 4 KHz portion of the ADSL band from 0 KHz to 4.3125 KHz. In Annex A the ADSL spectrum has been subdivided into 256 independent channels or tones, each tone is 4.3125 Khz wide. 26 tones have been assigned to the upstream channel and 224 tones for the downstream channel. Annex J and M shift the upstream/downstream frequency split up to 276 kHz to increase the upstream rate. Other implementations like the all digital loop use the unused voice band to boost the upstream rate by 256 Kbps.

Tone 16 and 64 are reserved for the upstream and downstream pilot tones. Pilot tones are used for continuous timing data. Tone 256 at the Nyquist frequency is not used for data. Each tone is QAM and use a 16 constellation matrix.

ADSL2+ a variant of ADSL doubles the bandwidth to 2.2 MHz from 1.1 MHz. This excel spreadsheet show the frequency allocation for ADSL and ADSL2+

When the ADSL modem is first connected to an ADSL network it goes through a fairly extensive initialization process.

Initialization:

The initialization process consists of four major phases. The first phase is a handshake using the G.994.1 or G.hs protocol. (G.hs is a precursor to the G.992.1 specific initialization and is used by other DSL and telecommunication devices) The remaining three phases - transceiver training, channel analysis, and exchange - are covered directly in the G.992 standard and apply specifically to standards-based ADSL networks.

The G.994.1 handshake is used to determine the nature and capabilities of the endpoints (such as an ADSL modem) and to indicate which protocol will be used for the remainder of the initialization. The signaling method used for the handshake interchange is designed to be robust. Biphase shift keying (BPSK) modulation is used to modulate multiple single-tone subcarriers, all carrying the same data.

The subcarriers used are selected based on the typical impairments likely to be present in a given global region. The handshake has several possible variants, but, fundamentally, the two endpoints exchange a message which contains information about the endpoint type, and a number of related subparameters such as the frequency range and number of DMT subcarriers supported.

The second phase of initialization is transceiver training. Receivers at each end of the line acquire the DMT symbol stream, adjust receiver gain, perform symbol timing recovery, and train any equalizers. There is an optional echo cancellation training step that can also be performed during this phase, but the specification does not define the training signal to be used.

The transmitter power at each end of the line is set to a predetermined level at this phase, allowing a preliminary estimate of loop attenuation by the receivers. The received upstream power level is reported back to the ATU-R transmitter to allow limited power level adjustment (attenuation), if needed, to meet spectral mask requirements. The training phase is conducted with all available upstream and downstream subcarriers modulated, using two of the four constellation points of a QPSK constellation.

In the third phase, the transceivers exchange capability information and perform detailed channel characterization. For example, the ADSL termination unit-CO (ATU-C) specifies the minimum SNR margin for the system and whether it can support functions such as trellis coding and echo cancellation. Similar information is exchanged about the ATU-R. Although some of these same parameters were exchanged during the G.994 handshake, the handshake message parameters are used to gather information only and are not necessarily used for the connection.

During this third phase, both transceivers attempt to measure specific channel characteristics such as unusable subcarriers, loop attenuation on a per subcarrier basis, SNRs, and any other channel impairments that would affect the potential transmitted bit rates. Based on the discovered channel characteristics, the ATU-C makes the first offer of the overall bit rates and coding overhead that will be used for the connection.

Four possible rates are offered, in decreasing order of preference. In the current release of the ADSL standard, the ATU-C completely controls the final bit rate. All subcarriers are modulated simultaneously with the same information. The primary tool for channel measurement is a pseudo-random bit sequence.

Setting the rates:

The last phase of the initialization sets the final overall transmission rates in both the upstream and downstream directions for the connection. These final rates are determined based on calculated channel parameters measured during the channel analysis phase, and are not necessarily the same as the preliminary rates offered during that phase.

As the ATU-C controls data rates, if the ATU-R cannot support any of the offered rates, both terminals will return to the beginning of the initialization process. Otherwise the ATU-R responds with the rate it can support.

Since ADSL uses multiple orthogonal subcarriers, each subcarrier can be assigned a modulation format (number of bits per subcarrier) and relative gain independently. The ATU-C assigns bits and gains for the downstream direction while the ATU-R assigns the upstream parameters.

The resulting assignment maximizes the amount of traffic that can be carried over the PHY layer. The last part of the exchange phase is a synchronized transition from the highly robust BPSK and QPSK modulations used during the initialization to the full traffic rate modulations (such as higher-order QAM) assigned during the exchange phase. At the conclusion of the initialization steps, the system is ready to pass higher-layer traffic.

Along with TCP/IP, ATM, with its key layers and its relationship with ADSL, is one of the key enabling technologies for broadband applications using ADSL.

ADSL and ATM:

ADSL uses a framed transport structure in which frames are encoded and modulated into DMT symbols. The ADSL frames can be further grouped into superframes, each consisting of 68 frames. An ADSL frame is sent every 250 microseconds, therefore a superframe is sent every 17 ms.

In full-rate ADSL, a frame can be broken down further into two parts, each being 125 microseconds. These two parts can be classified as the fast data path and the interleaved data path. The fast data path could have a higher bit error rate (BER) because the interleaver is not used to mitigate the effects of impulse noise. However, the removal of the interleaver significantly reduces the latency of this data path (which is well suited for time-sensitive information such as interactive audio and video).

ATM, which allows data to be sent asynchronously, uses cells consisting of 53 B of information. The cells' small size allows the efficient multiplexing of data from multiple sources. ATM is connection- oriented - once a connection is established, the connection carries traffic that meets the quality of service (QoS) requirements requested by the source and destination.

The ATM and ATM adaptation layer (AAL) are the two most important of the several layers in the ATM protocol stack. The ATM layer is responsible for the definition of logical connections through the network. Logical connections in ATM are known as virtual circuits (VCs).

VCs represent fundamental ways of switching in an ATM network; a VC is established between two end users on the network. Bundles of VCs are called virtual paths (VPs) and they share the same end-point. A single VP carries the cells from multiple VCs, and the cells are subsequently switched together.

The AAL is responsible for inserting higher-layer information into cells to be transported over the network. It consists of two sublayers called the segmentation and re-assembly (SAR) sublayer and the common part convergence sublayer (CPCS). The SAR sublayer segments the upper-layer protocol data units (PDUs) into 48-B cell payloads (SAR PDUs). The SAR PDUs are then passed to the ATM layer to form a complete cell.

The CPCS is responsible for performing functions for different classes of service. These are referred to as AAL1-5. A summary of the types of services is shown in Figure 1.

AAL5 is most often used for connectionless Internet traffic, as it allows the entire 48 B of cell payload to transport data with minimal overhead. A 10% overhead is typical when transporting Internet traffic over ATM.

Figure 2 illustrates a basic ADSL network architecture representing the connection between the service provider and the customer. An ATM connection is set up between the ADSL modem and a termination point in the back-end network. This connection is referred to as a permanent virtual circuit (PVC).

The deployment model for ADSL is based on the current dial-up system, which uses point-to-point protocol (PPP) to support network services such as authentication and client addressing. The ADSL Forum recommends this PPP-over-ATM-over-ADSL model, and it has become the standardized method for accessing data networks over ADSL. For the ever-popular encapsulation method of transmission, the Internet Engineering Task Force (IETF) has defined a method called RFC-2364 for PPP encapsulation over AAL5.

A DSLAM is used in all instances of ADSL deployments to aggregate traffic. With this device, the data from the ADSL modems is statistically multiplexed onto a common upstream link that interfaces to an ATM network. The ATM switch then routes the cells to their destination based on the cell header information. This destination is an IP router that reassembles the data cells into packets for transmission across the Internet.


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Last Update: December 2, 2014