Chapter
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1-7
ZXDSL 9210 employs two kinds of channels with different delay and reliability: Fast
channels (Fastonly) and interleaved channels (Interleave). The function of the
“Fastonly” channel is to make bit stream sent at the transmitting end reach the
receiving end within the shortest period of time via this path. The necessary waiting
time is 2ms. However, data correctness is not always so important. Normally, these are
some voice packets. Therefore, conversation quality will not be affected greatly even if
some packets are lost. An interleaved channel ensures that data reach correctly the
receiving end. The ADSL access network requires that the waiting time of an
interleaved channel should be 20 ms, that is, an interleaved channel trades the waiting
time for correctness of transmitted data.
1.3 Related Basic Concepts
1.3.1 ATM Technology
When the ADSL technology is adopted, there can be cell transmission based on
Asynchronous Transfer Mode (ATM) between subscriber premises equipment and
central office equipment. Essentially, ATM is a fast packet switching mode, so it
features flexible adaptability to new services and high resource utilization. Without
link-by-link error control, but with fixed-length packet data structure, it simplifies
protocols greatly as compared with other packet switching modes. Since packets are
processed by hardware, the processing capability is enhanced and the switching delay
reduced, thus making ATM suitable for real-time services.
The ATM technology splits digital information into 53-byte units - ATM Cells, also the
basic units for ATM information transmission/switching/multiplexing. Each cell
consists of the header and payload, as shown in Fig. 1.3-1.
48 bytes
5 bytes
Cell header
Cell payload
Fig. 1.3-1 ATM Cell
The 5-byte ATM cell header is used to store the information like flow control, virtual
Circuit Identity Code (CIC), and data payload. The remaining 48 bytes are for data
payload. ATM realizes data switching by means of the virtual circuit technology, and
transmits multiple types of data, including voice, video, and data. In addition, its
Содержание ZXDSL 9210 (V3.1)
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