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This set of PSDs (}{\b\i\f18\fs24\cgrid0 Fig.2}{\f17\fs24\cgrid0 ) has been designed to fit three major requirements. Firstly, these masks fit under the PSD masks for HDSL 3-pair, HDSL 2-pair or ISDN. This provides an excellent spectral compatibility because at a comparable data rate the amplitude of the appropriate HDSL mask is filled but not the bandwidth. Secondly these masks provide best performance on all defined loops for the FSAN (Full Service Access Network) noise models. This includes a scenario with 100% self-NEXT (Near End Cross Talk). In this way there is a no limitations guarantee on the deployment of SHDSL in terms of number of lines in a bundle. The third goal was to create a smooth PSD that allows low power consumption in the line driver in order to address requirements like line card density and remote feeding. If one is using SHDSL to replace HDSL 2-pair systems, it is desirable to achieve the same loop reach of 8.000 kfeet at 2.048 kbps, as defined in TS 101 135. By using the symmetric mask as described above this goal is missed by approximately 500\endash 1000 feet depending on implementation. Therefore the standardization bodies have decided to allow asymmetric PSDs as an option. However due to the higher bandwidth and the higher transmission power required, this causes significantly greater power consumption and complexity of the DSP. In its North American annex the ITU also allows the use of the OPTIS shaping of HDSL2 at T1 rate. In this way the appropriate ANSI standard is incorporated. Other optional templates for certain data rates are still being debated. Since SHDSL is a pure digital technology, the voice channels are inserted in the data stream. This way the valuable base band of the transmission line is not wasted, as is the case with ADSL for example, but is used to achieve higher loop lengths. On longer loops the lower frequencies are much less attenuated than the higher frequencies. VoDSL is therefore an integrated feature of SHDSL and can be provided via PCM (Pulse Code Modulation) as is the case with ISDN or by means of ATM (Asynchronous Transfer Mode) as currently defined in the DSL-Forum. The payload of an SHDSL frame consists of three to 36 B-channels with 64 kbps each and up to 7 auxiliary channels also referred to as Z-channels with 8 kbps each. An 8 kbps overhead channel is added to provide synchronization, as well as an EOC (Embedded Operation Channel) and the stuffing bits required for plesiochronous operation. As a consequence the line bit rate can be calculated to be 8+i*8+n*64 kbps with i being the number of 8 kbps Z-channels and n being the number of 64 kbps B-channels. The channel architecture makes it possible to transmit different services independently and in parallel e.g. a combination of TDM (Time Division Multiplexing) voice channels with an ATM bit stream. Standardized mappings of different services like ATM, ISDN and POTS into the SHDSL frame are currently being defined by the standardization bodies. A detailed description of the SHDSL frame is provided in }{\b\i\f18\fs24\cgrid0 Fig 3}{\f17\fs24\cgrid0 . }}


ITU G.handshake has been chosen as the protocol for the initialization of the connection. This enables a pre-activation negotiation of bit rates and protocols for example. G.handshake uses a very simple transmission method called DPSK (Differential Phase Shift Keying) at a low data rate. It doesn\rquote t need an initial training. Moreover some sort of lifeline service has to be defined in order to ensure the availability of at least one voice channel during a power cut. To achieve this, low power consumption is crucial because the power for the subscriber module has to be provided by remote feeding. }{\b\f16\fs28\cgrid0 SHDSL in the Network }{\f17\fs24\cgrid0 Although the symmetric DSL technologies were originally developed to be used mainly in infrastructure, subscriber access is expected to become the biggest portion of the market. Due to their demand for multiple voice channels, business customers are likely to adopt this technology very soon. Subsequently residential customers are expected to follow that trend, because of the possibility of having more than one voice channel combined with high speed Internet access. Depending on the ratio between voice and data SHDSL can be deployed in a switch as well as in a DSLAM. In an ATM based network on the customer side, an IAD (Integrated Access Device) is installed to convert voice and data into ATM cells. An IAD can also contain some routing functionality. Data is converted using AAL5 (ATM Adaptation Layer); while voice requires AAL1 (without compression) or AAL2 (with compression and micro cells). These cells are mapped together in the SHDSL frame and recovered later on in the DSLAM. An ATM switch routes the cells either to an ISP (Internet Service Provider) or to a voice gateway that translates the voice cells back into the TDM world. A complete ATM network architecture is shown in }{\b\i\f18\fs24\cgrid0 Fig 4}{\f17\fs24\cgrid0 . }}


In a TDM network the subscriber access can be done by means of a DLC (Digital Loop Carrier) or Access Network. The voice part of the SHDSL frame will be treated in a similar fashion to normal ISDN or POTS services. The data needs to be converted into ATM. This can be done either in an IAD, resulting in a mix of TDM and ATM on the SHDSL line, or at the central office side. In the second case it is necessary to protect the data on the line. This can be easily done by an HDLC protocol. The division between voice and data should be done in the loop carrier so that the ATM cells can be sent directly to the ATM backbone in order not to congest the PSTN network. This approach has the advantage of being more bandwidth efficient since the HDLC overhead is smaller than the ATM overhead. Additionally the SAR (Segmentation And Reassembly) functionality can be centralized in the DLC. However, since an IAD normally uses Ethernet to connect to a LAN some intelligence is required at the subscriber side; to process the Ethernet MAC and also have SAR functionality. }{\b\i\f18\fs24\cgrid0 Fig 5 }{\f17\fs24\cgrid0 shows DLC based subscriber access architecture. }}