6.3 System Functional Reference Model
| Figure 6.6 shows the system reference model for SHDSL as defined in G.991.2. The reference model shows one unit located in the CO and the other unit located at the customer premises (CP) location; each unit terminates one end of the subscriber line. Each end unit is typically referred to as an SHDSL transceiver unit (STU). The unit at the central office is labeled STU-C and that at the customer premises is labeled STU-R, where R stands for "remote" location. Figure 6.6. System reference model. The reference model in Figure 6.6 shows the different layers of processing required in each STU. At the CO end, the STU provides connection between the subscriber line and the network interface(s); at the CP side, the STU provides connection between the subscriber line and the customer interface(s). The different layers of processing convert the signals from the network or customer interfaces into a form suitable for transmission on the digital subscriber line. The different layers of processing are identified as follows from highest to lowest layer:
The PMD layer is the lowest layer-processing block in the SHDSL transceiver unit; hence, it is the block that is least dependent on the supporting application. The PMD is the core modem of the STU in that it provides the modulation and demodulation operations at the bit level. The functions of the PMD layer include the following:
The PMD block processes the bit stream as though it is a random bit sequence in that it does not require knowledge of the meaning or relation of any of the bits that are transmitted. Any bit errors in the PMD are passed on to the higher layers. One layer above the PMD is the PMS-TC layer, which contains the framing and frame synchronization functions. This layer needs to know the relation of the payload bits to each other for proper identification in the frame. The framing at this level primarily separates the payload bits from the overhead bits. The overhead channel in the PMS-TC provides the following functions: frame boundary identification, performance monitoring using a 6 bit cyclic redundancy check (CRC-6), function indicator bits, an embedded operations channel, and optional bit stuffing for support of synchronous timing or rate adaptation functions. So the PMD and PMS-TC layers together provide the capability of transmitting and recovering a payload bit stream and supporting the required operations and maintenance functions via processing of the overhead channel. These blocks together can support the widest range of applications and hence they are seen to be application invariant in the system reference model. The TPS-TC is more application specific in that it provides any subchannel separation and identification needed in support of the SHDSL based service offering. To do so, the TPS-TC works together with interfaces block for transport of the different payload channels. For example, the SHDSL service may be configured to support one high-speed data channel and two digitized voice channels. The TPS-TC provides the framing of the three subchannels in the payload bit sequence that connects to the PMS-TC block. Correspondingly, the interfaces block provides the physical interfaces required in support of the data and digitized voice subchannels . The connection between the TPS-TC and PMS-TC blocks in the STU-C is termed the a -interface. This is a logical interface that defines the subchannel frame structure of the payload bits to be transmitted in the PMS-TC. In the STU-R, the corresponding interface is termed the b -interface. The connection between the interfaces and TPS-TC blocks is called the g -interface. The g -interface in the central office unit is referred to as the g C interface, whereas that in the customer premises unit is referred to as the g R interface. In general, the g -interface is a logical interface, and its definition is totally dependent on the application being supported. As mentioned earlier, the maximum distance that an SHDSL access circuit can be deployed depends on the bit rate of the channel. In some cases the desired range of deployment for the given line bit rate is greater than the specified range for that bit rate. To address these extended reach applications, the SHDSL recommendation defines two options: (1) the use of repeaters and (2) an alternative two-pair operation. Figure 6.7 shows the reference model for deployment of SHDSL link with repeaters to extend the reach of operations. The repeaters are identified by the term SHDSL repeater unit (SRU). The issues generally associated with repeaters are the powering of the repeater units and the spectral compatibility with other service deployed in the same cable that are not served with repeaters but served directly from the CO. Figure 6.7. Reference model for deployment of SDHSL with repeaters. Figure 6.8 shows the system reference model for deployment of SHDSL using two wire pairs. Because the reach of SHDSL is longer for smaller bit rates, this option may be used to increase the reach by provisioning the service on two wire pairs, where each wire pair transports half the payload rate. Alternatively, for a given reach, two-pair operation may be used to simply double the capacity on the given link. Figure 6.8. Reference model for alternative two-pair deployment. |
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