6.9 Application Specific Framing (TPS-TC)

   


Earlier in the chapter, we described some possible applications for SHDSL transceivers. In order to support we need a TPS-TC and corresponding interface(s) with definitions that are in line with the application. Annex E of ITU-T Recommendation G.991.2 provides definitions of various types of TPS-TC that may be supported by SHDSL transceivers. Depending on the application being supported, other types of TPS-TCs may be defined as needed. Following are some example TPS-TC definitions.

6.9.1 Clear Channel Data

In transporting clear channel data, there is no special relationship between the structure of the user data and that of the core SHDSL frame. For this application, use of the core SHDSL frame as defined in Figure 6.19 is sufficient. The user payload data is inserted directly into the payload data blocks.

This same core frame structure may be used to transport T1 (1.544 Mb/s) or E1 (2.048 Mb/s) payloads. Although the T1 and E1 signals have a frame structure defined, the data may be placed directly into payload data blocks without regard for the boundaries of the T1 or E1 frames . External circuits may be used to determine the T1 or E1 frame boundaries carried in the payload bit sequences.

6.9.2 Fractional T1/E1 Transport

Figure 6.22 shows the subchannel frame structure for supporting fractional T1 or E1 services. Each active time slot from the T1 or E1 payload is assigned appropriate locations in the subblock data fields of the core framer's data block.

Figure 6.22. Fractional T1/E1 frame structure.

graphics/06fig22.gif

The transport of fractional T1 or E1 signals means that the SHDSL transceiver transports only the active time slots in the T1 or E1 payload. In a fractional T1 configuration, for example, both the customer and network interfaces in the system reference model of Figure 6.6 are T1 AMI signals. This interface operates at the full rate of 1.544 Mb/s; however, not all of the time slots are active. The TPS-TC block of the fractional T1 SHDSL transceiver takes only the active time slots from the T1 payload and inserts them into the core SHDSL payload data blocks as shown in Figure 6.22. At the receive end of the line, the TPS-TC block inserts the active time slots from the core frame into their corresponding time slot locations in the T1 frame. The operations are similar for the fractional E1 example.

6.9.3 Dual Bearer Transport

An example of a dual bearer application is in the simultaneous transport of a high-speed data channel together with one or more digitized voice channels. This application example is shown in the work-at-home application diagram of Figure 6.5. The customer interface for the high-speed data channel may be an Ethernet 10/100 Base-T interface, and those for the analog voice channels may consist of a codec and a subscriber line interface circuit (SLIC) for provisioning of analog/digital conversion and phone channel signaling. Details of the interface circuits are outside the scope of this chapter and are not described here.

The digitized voice channels may be transported through the network hierarchy in a bit synchronous T1 signal. The data signals would be transported through data network that is separate from the voice traffic.

Figure 6.23 shows the framing structure for dual bearer framing taken from Annex E.10 of G.991.2. Inside this frame, the digitized voice samples would be transported with proper time slot assignments in one of the bearers designated as the synchronous bearer. The data traffic would be transported without any special frame boundary alignment in the second bearer designated as the asynchronous data bearer channel.

Figure 6.23. Dual bearer subchannel framing.

graphics/06fig23.gif

Note that in the dual bearer mode, the transceiver would most likely need to operate in the plesiochronous timing mode with bit stuffing enabled. This may be required in support of the bit synchronous traffic of the voice channels. Alternatively, the transceiver line clock may be frequency lock to payload data clock of the digitized voice traffic.


   
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DSL Advances
DSL Advances
ISBN: 0130938106
EAN: 2147483647
Year: 2002
Pages: 154

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