Preamble, start, and sfd insertion, Address insertion, Length/type field processing – Altera Low Latency 40-Gbps Ethernet MAC and PHY MegaCore Function User Manual
Page 49
Figure 3-2: Typical Client Frame at the Transmit Interface
Illustrates the changes that the TX MAC makes to the client frame. This figure uses the following
notational conventions:
•
= payload size, which is arbitrarily large.
• = padding bytes = 0–46 bytes.
•
MAC Frame
Added by MAC for TX packets
Destination
Addr[47:0]
SFD[7:0]
Preamble
[47:0]
CRC32
[31:0]
IPG
[
PAD []
Source
Addr[47:0]
Type/
Length[15:0]
Payload
[
Start
EFD[7:0]
Added by MAC for TX packets
Payload Data from Client
The following sections describe the functions that the TX module performs:
Preamble, Start, and SFD Insertion
on page 3-4
Frame Check Sequence (CRC-32) Insertion
on page 3-5
Inter-Packet Gap Generation and Insertion
on page 3-5
Error Insertion Test and Debug Feature
Preamble, Start, and SFD Insertion
In the TX datapath the MAC appends an eight-byte preamble that begins with a Start byte (0xFB) to the
client frame. If you turn on Enable link fault generation, this MAC module also incorporates the
functions of the reconciliation sublayer.
The source of the preamble depends on whether you turn on the preamble pass-through feature by
turning on Enable preamble passthrough in the Low Latency 40-100GbE parameter editor.
If the preamble pass-through feature is turned on, the client provides the eight-byte preamble (including
Start byte) on the data bus. The client is responsible for providing the correct Start byte.
Address Insertion
The client provides the destination MAC address and the source address of the local MAC.
Length/Type Field Processing
This two-byte header represents either the length of the payload or the type of MAC frame. When the
value of this field is equal to or greater than 1536 (0x600) it indicates a type field. Otherwise, this field
3-4
Preamble, Start, and SFD Insertion
UG-01172
2015.05.04
Altera Corporation
Functional Description