Fig. 10.11 shows the topology of an ADSL system. To the access node different types of services such as Digital broadcast, broadband network, Narrow band network, Network management etc enteres. The access node provides interfacing of broadband services to ATU-C (ADSL-terminal unit central office).
ATU-C converts the data into ADSL format. The ADSL format fed to the splitter multiplexes them onto a single loop line. Telephone connection from the PSTN enter the system at the splitter level and are added to POTS-C (POTS-central office) area of the ADSL spectrum. The splitter in POTS-R (POTS-Remote) demultiplexes and transfers phone calls to the phones. The ADSL formats are transferred to ATU-R (ADSL-Terminating unit remote) which in turn converts to the orignal format and supplies to the terminal ends (TE).
ADSL Frame. The transport of data packets over ADSL requires link layer protocols. There are two ANSI stardards.
(a) Point-to-point protocol (PPP) variable length data units within and HDLC framed structure (RFC 1662).
(b) The ATM Forum's standard for ATM Frame UNI, also within an HDLC framed structure.
Fig. 10.12 shows the ADSL frame format. The frame begins with the standard HDLC flag (7E in hex) followed by a PPP address code field of FF and 03 hex. Two bytes of protocol ID identifies the payload type and the possible protocol that has been encapsulated in it. The frame check sequence (FCS) field uses CRC-16 for error detection and the frame ends with another 7E flag.
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