TX FEC
The Integrated KP4 Reed-Solomon Forward Error Correction (RS-FEC) provides a robust multi-bit
error detection/correction algorithm that protects up to 2 x 58 Gb/s or 1 x 116 Gb/s electrical
and optical links. This section describes the operation of the Integrated KP4 RS-FEC within the
Ult™ device GTM transceivers.
KP4 FEC is based on the RS(544,514) code, which encodes message blocks of 5140 bits to
produce codewords of 5440 bits. For a detailed description of the RS-FEC sublayer in Ethernet,
including the definition of the KP4 FEC code, refer to clause 91 of the IEEE Standard for Ethernet
(IEEE Std 802.3-2015). The same FEC code is used in other standards such as OTN FlexO and
Interlaken.
The Integrated KP4 RS-FEC for each GTM dual is composed of two logical slices for each
channel. These can operate as two independent RS-FEC processing units at up to 58 Gb/s each,
or as one unified unit at up to 116 Gb/s. When operating at up to 116 Gb/s, data is transmitted
and received over four virtual FEC lanes as described in IEEE 802.3-2015 clause 91. When
operating as 2 x 58 Gb/s, data can be transmitted and received over two virtual FEC lanes per
58 Gb/s channel, or as a raw data stream (one virtual lane) with optional PN scrambling for
backplane operations. The general principle of operation of the FEC is the same whichever mode
is chosen.
When RS-FEC is enabled in the transmit direction, data to be FEC-encoded and transmitted is
provided from the fabric to the input of the GTM transceiver. The pre-FEC data must be pre-
formatted to contain zero padding regions where the parity will be inserted. The Integrated KP4
RS-FEC performs RS encoding to fill in the parity space, and (except in raw mode) also performs
symbol distribution operations according to the 802.3bj clause 91 specification. The encoded
output data from the Integrated KP4 RS-FEC is then presented to the GTM PCS for transmission.
The Integrated KP4 RS-FEC does not natively perform transcoding, alignment marker removal,
alignment marker mapping, or alignment marker insertion operations in the transmit directions.
To support protocols such as 100G Ethernet (IEEE 802.3 clause 91) and 50G Ethernet (IEEE
802.3 clause 134) which require 257b transcoding and alignment marker processing, the GTM
Wizard IP can optionally include soft logic blocks for these functions.
Ports and Attributes
The following table shows the TX FEC-related ports for the GTM dual.
Table 32: TX FEC Ports
Ports
Dir
Clock Domain
Description
CH[0/1]_TXFECRESET
In
Async
Component reset port for TX FEC.
Chapter 3: Transmitter
UG581 (v1.0) January 4, 2019
Virtex Ult GTM Transceivers
63