Chapter 6. Single-Pair High-Speed Digital Subscriber Line (SHDSL)
6.3 System Functional Reference Model
6.2 Standards for Multirate SHDSL
ITU-T Recommendation G.991.2 (also referred to as G.shdsl) defines the interface specification for SHDSL transceivers. G.shdsl defines a multirate single-pair symmetric DSL transport using 16-level trellis coded PAM. The main body of Recommendation G.991.2 defines the interworking
specification that is independent of any regional requirements. Annex A of G.991.2 defines requirements and objectives that are specific to North America; similarly, Annex B of G.991.2 defines requirements and objectives that are specific to the European community.
Japan does not use TC-PAM for SHDSL. Because the Japan network has an abundance of time compression multiplexed ISDN (TCM-ISDN), the line code chosen for SHDSL applications in the Japan access network is DMT as defined in Annex H of G.992.1.
G.shdsl defines symmetric data rates from 192 kb/s up to 2,312 kb/s in increments of 8 kb/s for transmission on a single wire pair. The same Recommendation also defines an option for operation on two wire pairs. This option may be used to either increase the reach by operating on two wire pairs at half the payload rate on each pair, or to simply double the capacity on a given link.
ETSI TM6 has developed ETSI TS 101 524 [2] that defines SDSL. This regional standard is the primary source of content to Annex B of G.991.2.
Committee T1 has defined T1.422 [4], which is a regional standard that points to the ITU-T Recommendation G.991.2, with specific reference to Annex A, which contains regional specific requirements for North America.
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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:
z Network or customer interfaces
z TPS-TC: transmission protocol specific–transmission convergence layer
z PMS-TC: physical medium specific–transmission convergence layer
z PMD: physical medium dependent 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:
z Modulation and demodulation
z Bit clock and symbol clock generation and recovery
z Trellis coding and decoding
z Echo-cancellation
z Channel equalization
z Initialization and training
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 α-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 β-interface.
The connection between the interfaces and TPS-TC blocks is called the γ-interface. The γ-interface in the central office unit is referred to as the γC interface, whereas that in the customer premises unit is referred to as the γR interface. In general, the γ-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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s="docText">NAK-NR 0
0 1 0 0 0 0 1
NAK-NS 0
0 1 0 0 0 1 0
NAK-CD 0
0 1 0 0
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0 1 1
REQ-MS 0
0 1 1 0 1 0 0
REQ-MR 0
0 1 1 0 1 1 1
REQ-CLR 0
0 1 1 0
1 1 1
The Par(1) parameters in the standard information field contain one NPar(1) octet, three SPar(1) octets, and 20 NPar(2) octets defining the subparameter values in support of the parameters
identified in the three SPar(1) octets. The only parameter defined in the NPar(1) octet is to identify use of the nonstandard information field. The SPar(1) octets identify parameters such as net
upstream and downstream data rates, upstream and downstream data flow characteristics, and central office and customer premises splitter information. The 20 NPar(2) octets in support of the SPar(1) parameters are distributed as follows:
z Net upstream data rate—3 octets
z Net downstream data rate—3 octets
z Upstream data flow characteristics—2 octets
z Downstream data flow characteristics—2 octets
z Customer premises splitter information—1 octet
z Central office splitter information—1 octet
z Relative power level/carrier for upstream carrier set A43—1 octet
z Relative power level/carrier for downstream carrier set A43—1 octet
z Relative power level/carrier for upstream carrier set B43—1 octet
z Relative power level/carrier for downstream carrier set B43—1 octet
z Relative power level/carrier for upstream carrier set C43—1 octet
z Relative power level/carrier for downstream carrier set C43—1 octet
z Relative power level/carrier for upstream carrier set B4—1 octet
z Relative power level/carrier for downstream carrier set B4—1 octet
padding="0">Topd of responding with ACK(1) as in basic transaction A, the CO unit requests that the capabilities list be exchanged. Once the two units exchange capabilities and negotiate mode of operation, a follow-up transaction is needed for mode selection.
In transaction B:C, the CP unit requests that the CO unit select the mode of operation via a MR message, but instead of the expect MS response of basic transaction B, the CO unit requests a capabilities list request, where both units exchange their capabilities list and negotiate the mode
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selection. A follow-up transaction is needed for the mode selection.
In transaction D:C, the CP unit proposes a mode of operation via the MP message and requests that the CO unit select the operating mode. Instead of the expected MS message from the CO unit of basic transaction D, the CO unit requests a capabilities list request, where both units exchange their capabilities list and negotiate the mode selection. A follow-up transaction is needed for the mode selection.
5.11.3 Message Segmentation
Note that in the above transactions, some messages can become rather large when passing messages that contain the identification and standard information parameter fields. Excluding the two FCS octets and any octets inserted for transparency, the maximum message length in a frame is 64 octets.
If a message is longer than 64 octets, then it must be segmented into two or more messages. The message types that can be segmented are those that contain the parameter octets, namely, CL, CLR, MP, and MS.
When a receiving station is parsing a segmented message, the receiving station sends an ACK(2), which indicates to the sending station that it is ready to receive the remainder of the message. Once the complete message is received, the receiving station responds with an ACK(1) or other
appropriate response.
5.11.4 Example Transactions
In this section, we provide some example G.994.1 sessions to demonstrate the handshake process.
Example 1: Table 5.7 shows the use of a transaction sequence that combines basic transaction C with A. The CP unit first initiates a capabilities list request where the two units exchange and negotiate capabilities via the CLR and CL messages. The CP unit then selects the mode of operation via the MS command (basic transaction A).
Example 2: Table 5.8 shows an example transaction that combines extended transaction A:C with basic transaction A. First the CP unit selects a mode of operation and requests that the CO select this mode. Instead the CO unit requests the CP unit for a capabilities list request. The two modems then exchange capabilities lists and negotiate operating modes. Once the exchange and negotiation are complete, the CP unit selects the mode of operation via the MS message.
Table 5.7. Example 1—Basic Transaction C Followed by Basic Transaction A
CP Unit CO Unit CP Unit CP Unit CO Unit
CLR_ CL_ ACK(1) MS_ ACK(1)
Table 5.8. Example 2 – Extended Transaction A:C Followed by Basic Transaction A
CP Unit CO Unit CP Unit CO Unit CP Unit CP Unit CO Unit
MS_ REQ-CLR_ CLR_ CL_ ACK(1) MS_ ACK(1)
6.4 HDSL4
An example of a system that uses the two wire pair option of SHDSL technology is HDSL4.
HDSL4 may seem to be an odd technology to develop following the success of HDSL2 because HDSL4 uses four wires for DS1 transmission while the primary goal of HDSL2 was to reduce cost by reducing the number of wires from four to two. Each technology has its place. HDSL2 is the technology of choice for DS1 (1.544 Mb/s) transmission for lines within the carrier serving area design rules (CSA, up to 9 kft of 26 AWG wire). HDSL2's use of only two wires reduces copper-line costs and also reduces the cost of the transceiver because a two-wire system requires one-half as many line interfaces. However, extending HDSL2 beyond CSA length lines is not attractive due to the high cost of midspan repeaters and spectral incompatibility with other systems.
HDSL4's role is to serve lines beyond HDSL2's reach. The use of four wires permits the transmitted signal PSD to avoid the higher frequencies used by HDSL2. This enables nonrepeatered HDSL4 to reach 11 kft of 26 AWG, whereas HDSL2 can reach only 9 kft. Also, the lower frequency
characteristic of HDSL4 enables HDSL4 to remain spectrally compatible with other systems in the same cable even when HDSL4 is repeatered to virtually any distance. HDSL4 makes the trade-off of using an additional pair of wires to reduce the number of required repeaters, and provided that spare pairs are available, this is generally a very cost-effective trade-off. The combination of HDSL2 for shorter lines and HDSL4 for longer lines makes T1 and HDSL technology obsolete. For example, a 20 kft all 26 AWG loop would require two repeaters using first-generation HDSL technology, but HDSL4 would require only one repeater and the HDSL4 would have much better spectral
compatibility.
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