MTC20455

14/23

Generic TC Layer Functions

These functions relate to byte oriented data streams. They are completely described in ANSI T1.413. Ad-

ditions described in the Issue 2 of this specification are also supported. The data received from the demap-

per is split into two paths, one dedicated to an interleaved data flow, the other one for a non-interleaved

data flow. These data flows are also referred to as slow and fast data flows. The interleaving/-de-interleav-

ing is used to increase the error correcting capabilityof block codes for error bursts. After de-interleaving

(if applicable), the data flow enters a Reed-Solomon error correcting code decoder, able to correct a num-

ber of bytes containing bit errors. The decoder also uses the information of previous receiving stages that

may have detected the errored bytes and have labelled them with an “erasure” indication. Each time the

RS decoder detects and corrects errors in a RS codeword, an RS correction event is generated. The oc-

currence of such events can be signalled to the management layer. After leaving the RS decoder, the cor-

rected byte stream is descrambled in the PMD (Physical Medium Dependent) descramblers.

Two descramblers are used, for interleaved and non-interleaved data flows. These are defined in ANSI

T1.413. After descrambling, the data flows enter the Deframer that extracts and processes bytes to sup-

port physical layer-related functions according to ANSI T1.413. The ADSL frames indeed contain physical

layer-related information in addition to the data passed to the higher layers. In particular, the deframer ex-

tracts the EOC (Embedded Operations Channel), the AOC (ADSL Overhead Control) and the indicators

bits and passes them to the appropriate processing unit (e.g. the transceiver controller). The deframer also

performs a CRC check (Cyclic Redundancy Check) on the received frame and generates events in case

of error detection. Event counters can be read by management processes. The outputs of the deframer

are an interleaved and a fast data stream. These data streams can either carry ATM cells or another type

of traffic. In the latter case, the ATM specific TC layer functional block, described hereafter, is bypassed

and the data stream is directly presented at the input of the interface module.

Figure 6. Generic TC Layer Functions

ATM Specific TC Layer Functions

The 2 byte streams (fast and slow) are received from the byte-based processing unit. When ATM cells are

transported, this block provides basic cell functions such as cell synchronisa-tion, cell payload descram-

bling, idle/-unassigned cell filter, cell Header Error Correction (HEC) and detection. The cell processing

happens according to ITU-T I.163 standard. Provision is also made for BER measurements at this ATM

cell level. When non cell oriented byte streams are transported, the cell processing unit is not active.

From

Demapper

To

ATM TC

RS

coding

PMD

descrambler

PMD

descrambler

Detramer

Indications bits

AOC

EOC

De-inter-

leaver

FAST

AST

F

I