Ethernet tutorial
- an overview or tutorial of the Ethernet standard, IEEE802.3 used widely
for local area network, LAN applications.
Ethernet, defined under the standard IEEE 802.3, is one of
today's most widely used data communications standards, and it finds its major
use in Local Area Network (LAN) applications. With versions including 10Base-T,
100Base-T and now Gigabit Ethernet, it offers a wide variety of choices of
speeds and capability. Ethernet is also cheap and easy to install. Additionally
Ethernet offers a considerable degree of flexibility in terms of the network
topologies that are allowed. Furthermore as it is in widespread use in LANs, it
has been developed into a robust system that meets the needs to wide number of
networking requirements.
History
The Ethernet standard was first developed by the Xerox
Corporation as an experimental coaxial cable based system in the 1970s. Using a
Carrier Sense Multiple Access / Collision Detect (CSMA/CD) protocol to allow
multiple users it was intended for use with LANs that were likely to experience
sporadic use with occasional heavy use.
The success of the original Ethernet project lead to a joint
development of a 10 Mbps standard in 1980. This time three companies were
involved: Digital Equipment Corporation, Intel and Xerox. The Ethernet Version 1
specification that arose from this development formed the basis for the first
IEEE 802.3 standard that was approved in 1983, and finally published as an
official standard in 1985. Since these first standards were written and
approved, a number of revisions have been undertaken to update the Ethernet
standard and keep it in line with the latest technologies that are becoming
available.
Ethernet standard releases
The Ethernet standard has undergone many releases and updates
as a result of the continual development of the technology. In this way,
Ethernet has been able to meet the ongoing needs of the industry.
Standard
Supplement |
Year |
Description |
| 802.3a |
1985 |
10Base-2 (thin Ethernet) |
| 802.3c |
1986 |
10 Mb/s repeater
specifications (clause 9) |
| 802.3d |
1987 |
FOIRL (fiber link) |
| 802.3i |
1990 |
10Base-T (twisted pair) |
| 802.3j |
1993 |
10Base-F (fiber optic) |
| 802.3u |
1995 |
100Base-T (Fast Ethernet and
auto-negotiation) |
| 802.3x |
1997 |
Full duplex |
| 802.3z |
1998 |
1000Base-X (Gigabit Ethernet) |
| 802.3ab |
1999 |
1000Base-T (Gigabit Ethernet
over twisted pair) |
| 802.3ac |
1998 |
VLAN tag (frame size extension
to 1522 bytes) |
| 802.3ad |
2000 |
Parallel links (link
aggregation) |
| 802.3ae |
2002 |
10-Gigabit Ethernet |
| 802.3as |
2005 |
Frame expansion |
| 802.3at |
2005 |
Power over Ethernet Plus |
Ethernet standards supplements and releases
Ethernet terminology
There is a convention for describing the different forms of
Ethernet. For example 10Base-T and 100Base-T are widely seen in the technical
articles and literature. The designator consists of a three parts:
- The first number (typically one of 10, 100, or 1000) indicates the
transmission speed in megabits per second.
- The second term indicates transmission type: BASE = baseband; BROAD =
broadband.
- The last number indicates segment length. A 5 means a 500-meter (500-m)
segment length from original Thicknet. In the more recent versions of the
IEEE 802.3 standard, letters replace numbers. For example, in 10BASE-T, the
T means unshielded twisted-pair cables. Further numbers indicate the number
of twisted pairs available. For example in 100BASE-T4, the T4 indicates four
twisted pairs.
Elements
The Ethernet LAN can be considered to consist of two main
elements: the interconnecting media, and the network nodes.
The network nodes themselves fall into two categories. The
first is the Data Terminal Equipment (DTE). These devices are either the source
or destination of the data being sent. Devices such as PCs, file servers, print
servers and the like fall into this category. The second category of devices are
known as Data Communications Equipment (DCE). Devices that fall into this
category receive and forward the data frames across the network, and they may
often be referred to as 'Intermediate Network Devices' or Intermediate Nodes.
They include items such as repeaters, routers, switches or even modems and other
communications interface units.
The media through which the signals propagate are just as
important. Initially coaxial cable with a single inner connector were used. Now
either an Unshielded Twisted Pair (UTP) or a Shielded Twisted Pair (STP) are
normally used. There are also optical fibre options, and these are often used
for the much higher data rate systems.
Network topologies
There are several network topologies that can be used for
Ethernet communications. The actual form used will depend upon the requirements.
Point to point - This is the simplest configuration as
only two network units are used. It may be a DTE to DTE, DTE to DCE, or even a
DCE to DCE. In this simple structure the cable is known as the network link.
Links of this nature are used to transport data from one place to another and
where it is convenient to use Ethernet as the transport mechanism.
Coaxial bus - This type of Ethernet network is rarely
used these days. The systems used a coaxial cable where the network units were
located along the length of the cable. The segment lengths were limited to a
maximum of 500 metres, and it was possible to place up to 1024 DTEs along its
length. This form of network is not used these days, although a very few legacy
systems might just still be in use.
Star network - This type of Ethernet network has been
the dominant topology since the early 1990s. It consists of a central network
unit, which may be what is termed a multiport repeater or hub, or a network
switch. All the connections to other nodes radiate out from this and are point
to point links.
Data frame format
There is a basic data frame format that is defined in the
802.3 standard. This provides the basic frame that is normally used, although
there are additional formats defined that can be used to extend the performance
of the system should this be needed. With the high speeds and variety of media
used, this basic format is sometimes adapted to meet the individual requirements
of the transmission, but still specified within the amendment / update for that
given Ethernet variant.
| PRE |
SOF |
DA |
SA |
Length / Type |
Data payload |
FCS |
| 7 |
1 |
6 |
6 |
2 |
46 - 1500 |
4 |
Basic Ethernet Data Frame Format
The basic frame consists of seven elements split between
three main areas:-
Header
Preamble (PRE) - This is seven bytes long and it consists of a pattern of
alternating ones and zeros, and this informs the receiving stations that a frame
is starting as well as enabling synchronisation. (10 Mbps Ethernet)
Start Of Frame delimiter (SOF) - This consists of one
byte and contains an alternating pattern of ones and zeros but ending in two
ones.
Destination Address (DA) - This field contains the
address of station for which the data is intended. The left most bit indicates
whether the destination is an individual address or a group address. An
individual address is denoted by a zero, while a one indicates a group address.
The next bit into the DA indicates whether the address is globally administered,
or local. If the address is globally administered the bit is a zero, and a one
of it is locally administered. There are then 46 remaining bits. These are used
for the destination address itself.
Source Address (SA) - The source address consists of
six bytes, and it is used to identify the sending station. As it is always an
individual address the left most bit is always a zero.
Length / Type - This field is two bytes in length. It
provides MAC information and indicates the number of client data types that are
contained in the data field of the frame. It may also indicate the frame ID type
if the frame is assembled using an optional format.(IEEE 802.3 only)
Payload
Data - This block contains the payload data and it may
be up to 1500 bytes long. If the length of the field is less than 46 bytes, then
padding data is added to bring its length up to the required minimum of 46
bytes.
Trailer
Frame Check Sequence (FCS) - This field is four bytes
long. It contains a 32 bit Cyclic Redundancy Check (CRC) which is generated over
the DA, SA, Length / Type and Data fields.
Half-duplex transmission
This access method involves the use of CSMA/CD and it was
developed to enable several stations to share the same transport medium without
the need for switching, network controllers or assigned time slots. Each station
is able to determine when it is able to transmit and the network is self
organising.
The CSMA/CD protocol used for Ethernet and a variety of other
applications falls into three categories. The first is Carrier Sense.
Here each station listens on the network for traffic and it can detect when the
network is quiet. The second is the Multiple Access aspect where the
stations are able to determine for themselves whether they should transmit. The
final element is the Collision Detect element. Even though stations may
find the network free, it is still possible that two stations will start to
transmit at virtually the same time. If this happens then the two sets of data
being transmitted will collide. If this occurs then the stations can detect this
and they will stop transmitting. They then back off a random amount of time
before attempting a retransmission. The random delay is important as it prevents
the two stations starting to transmit together a second time.
Note: According to section 3.3 of the IEEE 802.3
standard, each octet of the Ethernet frame, with the exception of the FCS, is
transmitted low-order bit first.
Full duplex
Another option that is allowed by the Ethernet MAC is full
duplex with transmission in both directions. This is only allowable on
point-to-point links, and it is much simpler to implement than using the CSMA/CD
approach as well as providing much higher transmission throughput rates when the
network is being used. Not only is there no need to schedule transmissions when
no other transmissions are underway, as there are only two stations in the link,
but by using a full duplex link, full rate transmissions can be undertaken in
both directions, thereby doubling the effective bandwidth.
Ethernet addresses
Every Ethernet network interface card (NIC) is given a unique
identifier called a MAC address. This is assigned by the manufacturer of the
card and each manufacturer that complies with IEEE standards can apply to the
IEEE Registration Authority for a range of numbers for use in its products.
The MAC address comprises of a 48-bit number. Within the
number the first 24 bits identify the manufacturer and it is known as the
manufacturer ID or Organizational Unique Identifier (OUI) and this is assigned
by the registration authority. The second half of the address is assigned by the
manufacturer and it is known as the extension of board ID.
The MAC address is usually programmed into the hardware so
that it cannot be changed. Because the MAC address is assigned to the NIC, it
moves with the computer. Even if the interface card moves to another location
across the world, the user can be reached because the message is sent to the
particular MAC address.
Summary
Despite the fact that Ethernet has been in use for many
years, it is still a growing standard and it is likely to be used for many years
to come. During its life, the speed of Ethernet systems has been increased, and
now new optical fibre based Ethernet systems are being introduced. As the
Ethernet standard is being kept up to date, the standard is likely to remain in
use for many years to come.
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