Introduction to Ethernet and lab3.3

Introduction to Ethernet and lab3.3
Magnus Lundberg

Nov 2001

Computation structures

1

Agenda
 

Introduction

 

History

 

Components (MAC, PHY)

 

Packet format

 

CRC

 

Bit order

 

Hubs, Switches and Routers

 

Questions

Magnus Lundberg

LTU


Computation structures

2

What is (the) Ethernet
 

Ethernet is a network BUS

 

Ethernet is packet based

 

Ethernet is available as 10 Mbit/s - 10 Gbit/s

 

Ethernet is based on the CSMA/CD (Carrier Sense Multiple Access
with Collision Detect access) protocol

Magnus Lundberg
Computation structures

LTU
3

Ethernet Basics
 

Ethernet Packets are from 64 bytes up to 1518 bytes

 

Since Ethernet is a “BUS” several “Attached Units” can talk on the
BUS at the same time

 

Collisions occur and the talking parties backoff for a time and then
try again

 

For each failed attempt the backoff time is increased exponentially

Magnus Lundberg

LTU


Computation structures

4

OSI model
OSI Model
Application

L7

Presentation
Session
Transport
Network
Datalink
Physical

L1

 

On The OSI model with 7 layers , Ethernet is in the bottom two

layers: Data Link Layer and Physical Layer

 

OSI (Open Systems Interconnection)

Magnus Lundberg
Computation structures

LTU
5

History of Ethernet
 

Developed by Xerox at their PARC Research Facility by Bob
Metcalfe and David Boggs

 

First version was 3 Mbit /s

 

Initial Ethernet standard developed by Digital , Intel and Xerox in the
DIX consortium

 

Standard for 10 Mbit/s called “The Blue book” was ready in 1980

 

Standard was then submitted to a newly formed working group in
IEEE called 802

 

IEEE splits the 802 working group in four parts 802.3 becomes the
Ethernet working group

Magnus Lundberg

LTU


Computation structures

6

History of Ethernet
 

Late 1982 DIX’s Bluebook and the IEEE 802.3 specifications merge
into one standard

 

Ethernet was already multi vendor and more company’s endorsed the
standard

 

Ethernet vs 802.3 CSMA/CD

 

Ethernet patents where inexpensive and easy to license

 

Ethernet was developed into several different mediums.
Coaxial/UTP/Optical

Magnus Lundberg

LTU

Computation structures

7

Components
PHY

MAC

FPGA

MII

MAU

MDI

✁

MAC Media Access Control

✁

PHY Physical layer device

✁

MAU Medium Attachment Unit (RJ45 connector)

✁

MII Media Independant Interface

✁

MDI Media Dependant Interface

Magnus Lundberg

LTU

Computation structures

8

MAC - Media Access Controller
 

Does statistical functions : Packet Lengths, Packet collisions etc.

 

Does the stripping and adding of the preamble bits and the CRC
calculations

 

Communicates with the rest of the design

 

Back off mechanism with random delay in case of collision

 

A MAC usually has two different datapaths one for internal clock
(Sending) and one external (Receiving)

Magnus Lundberg

LTU

Computation structures

9

Media Independent Interface - MII
 

This is the interface between the PHY and the MAC

 

Regardless of the underlying Media this interface always stays the
same

 

MII is for 10 and 100 Mbit Ethernet.
First bit from MAC

First Nibble
D0

MSB

D1

D2

Second Nibble

D3

D4

D5

D6

D7

D0
D1
D2

LSB

D3

Magnus Lundberg

LTU


Computation structures

10

Media Independent Interface - MII
 

The MII consists of 10 different signals

 

Transmit Clock (TX_CLK) which runs at either 2.5 Mhz for 10 Mbit
or 25 Mhz for 100 Mbit

 

Transmit Data (TXD <3:0>) data nibble to the PHY circuit from
MAC

 

Transmit Enable (TX_EN) is high when data on the TXD pins are
valid

 

Transmit Error (TX_ERR) goes high when the PHY is coding
incorrectly, not used in 10 Mbit

Magnus Lundberg
Computation structures

LTU
11

Media Independent Interface - MII
 

Receive Clock (RX_CLK) generated by the PHY for incoming data

 

Receive Data (RXD <3:0>) data from the sender decoded by the
PHY to MAC

 

Receive Data Valid (RX_DV) asserted when there is valid data on the
RXD path

 

Receive Error (RX_ER) asserted to indicate that something in the
coding layer is wrong etc

 

Carrier Sense (CRS) asserted by the PHY to indicate someone else is
sending data

 

Collision (COL) asserted by the PHY when a collision has taken/is
taking place, useless if working in full duplex mode

Magnus Lundberg

LTU


Computation structures

12

MII - Sending Packets
TX_CLK
TX_EN

TXD<3:0>

COL

CRS

Magnus Lundberg
Computation structures

LTU
13

MII - Receiving Packets
RX_CLK
RX_DV

RXD<3:0>

COL

CRS

Magnus Lundberg

LTU


Computation structures

14

PHY
 

Encodes/Decodes (Manchester coding for 10 Mbit/s)

 

Converts from parallel to serial

 

Detects collisions

 

Auto-negotiation

Magnus Lundberg

LTU

Computation structures

15

Ethernet Packet Format
Preamble
8 Bytes

Magnus Lundberg

Destination Addr.
6 Bytes

Source Addr.
6 Bytes

Etype/Length
2 Bytes

Data
46−1500 Bytes

FCS
4 Bytes

LTU

Computation structures

16

Preamble
 

Preamble is used by the PHY to “sync” its RX_CLK to the
transmitters TX_CLK.

 

The Preamble consists of 8 bytes of alternating ones and zeros

 

The two last bits of the preamble field are both high and are called
the synch sequence, it is used to signal that the preamble ends and the
Destination address begins.

Magnus Lundberg
Computation structures

LTU
17

Destination and Source address
 

Both Addresses are 6 bytes each and consists of two parts:
– 3 Bytes Vendor code (0x00003F = Syntrex Inc)
– 3 Bytes Serial number (0x0001000)

 

This address are sometimes callad MAC-address

 

Stored in ROM in a NIC

 

0xFFFFFFFFFFFF is the broadcast address.

Magnus Lundberg

LTU


Computation structures

18

Type/Length
 

Length / Etype field is either a type field or length.

 

if the value is smaller than 0x0600 it contains length, otherwise it is
an Ethernet Type
– 0x0800 is IPv4 Datagram
– 0x0806 is ARP
– 0x81DD is IPv6 Datagram
– 0x0101 is experimental(below 0x0600 ?)

Magnus Lundberg
Computation structures

LTU
19

Data
 

Minimum data size 46 Bytes

 

Maximum data size 1500 Bytes

 

If actual data is less than 46 Bytes the packet is padded with zeros

Magnus Lundberg

LTU


Computation structures

20

Frame Check Sequence (FCS)
 

4 Bytes used to detect bit faults in the packet

 

Computed with the CRC-32 algorithm

 

Computed on the Destination Address, Source Address, type and
data fields.

Magnus Lundberg

LTU

Computation structures

21

CRC theory

✞

(1)

✄
✆
✡

☎

✂
✝
✠

M

DA, SA, type and data fields, k bits long.

F

The FCS field, n bits long.

T

M and F concatenated.(k+n bits long).

P

The CRC Polynomial. A pattern of n+1 bits.
✞

✆

☎

(2)

✝
✌

✄
✠

☞
☛

☛
✞

✄
✆

☎

✂
✝
✌
✠

Magnus Lundberg

(3)

LTU


Computation structures

22

✞

✞
✆

✆
☎

☎

(4)

✂
✝

✝
✌

✌

✌

✌
✌

✌
✠

✠
✄

✄

✄

✄
✠

✠
✠

✠
☞

☞
☛

☛

☛

☛

☛

☛

☛

But any binary number added to itself in a modulo 2 field yields zero so:
✄
✍

✌

✌
✠

✂

(5)

✄
☞

☛

Magnus Lundberg

LTU

Computation structures

23

CRC polynomial
The CRC polynomial for Ethernet is:
CRC =
✓

✙

✓

✓
✑

✓

✓

✓✑

✑

✑

✑

✑

✑
✘

✗
✝

✝

✝

✕

✒

✎

✒

✖

✎
✚

✝

✝

✝

✝

✝

✝

✝

✝

✝

✝

✝
✠

✠

✠

✠

✠

✠

✠

✠

✠

✠

✠

✠

✠

✠

Magnus Lundberg

LTU


Computation structures

24

CRC in hardware
✙

The VHDL code for the 1-bit wide data bus implementation with
is shown below:
polynomial
✑

✘

✄

☛

✚

✝

✝

✝

✝

✠

✠

✠

✜

✛

crc(0) <= data_in xor crc(4);
crc(1) <= crc(0);
crc(2) <= data_in xor crc(1) xor crc(4);
crc(3) <= crc(2);

crc(4) <= data_in xor crc(3) xor crc(4);

Magnus Lundberg

LTU

Computation structures

25

CRC in hardware
Data In

5

x
P(x) = x 5 +

Q D

Q D

Q D

4

x
x 4+

x

3

Q D

x

2
2

x +

Q D

x

1

x

0

1

 

Load the CRC register with 0xFFFFFFFFFF initially

 

The resulting CRC must be inverted and reflected.

Magnus Lundberg

LTU


Computation structures

26

Bit Order
Byte-order
Bit-order

Bit-order
Highest Byte . . . . . .
MSB

Lowest Byte
LSB

MSB

LSB

Preamble

Synch

DA

SA

Type

Data

62 Bits

2 Bits

6 Bytes

6 Bytes

2 Bytes

46-1500 Bytes

FCS
4 Bytes

 

Note that the Bit order is reversed compared to the Byte order

Magnus Lundberg

LTU

Computation structures

27

Bit Order example
 

When sending the two Bytes 0x3F01 the following bit order is used.
0
0

0

1
0

0

0

0

3
0

1

0

0

F
1

1

1

1

Tx_d(3)

Magnus Lundberg

1

1
Tx_d(0)

LTU


Computation structures

28

Bit Order CRC
 

When sending 0xD0D5 the following bit order is used.

 

Note that the CRC is not sent bytewise.
D
1

1

0
0

1

0

0

D
0

0

1

1

0

5
1

0

1

Tx_d(3)

0

1
Tx_d(0)

Magnus Lundberg
Computation structures

LTU
29

Hubs & Switches & Routers
 

Hubs = Is an extension cord for networks, all signals are sent to all
computers on network, all users share the same bandwidth

 

Switches = Each signal is connected to a specific link to a switch.
The switch receives the packet looks where it is heading and sends it

to the appropriate port

 

Router = Is working on higher level than the Ethernet Protocol, it
usually works on IP level and looks at the IP address instead of the
Ethernet Address

Magnus Lundberg

LTU


Computation structures

30

Clip-art
Every presentation needs some clip-art
Ethernet
Switch

✥
✥

✦

✣✤

✦

NIC

✩

✪
✩

✪

★

✧★

✧

✫

✬

✫

✬

✫

✬

✫

✬
✭✮

Magnus Lundberg

LTU

Computation structures

31

Questions ?

Magnus Lundberg

LTU