FREE ESSAY ON HOW ETHERNET WORKS

HOW ETHERNET WORKS

What Is Ethernet and How Is It Used?
ITSK2511
Cecil Jackson
Invention of Ethernet
A gentlemen by the name of Bob Metcalfe realized that he could improve on a system called
the Aloha System which arbitrated access to a shared communications channel. He developed
a new system that included a mechanism that detects when a collision occurs (collision
detect). The system also includes listen before talk, in which stations listen for
activity (carrier sense) before transmitting, and supports access to a shared channel by
multiple stations (multiple access). Put all these components together, and you can see
why the Ethernet channel access protocol is called Carrier Sense Multiple Access with
Collision Detect (CSMA/CD). Metcalfe also developed a much more sophisticated backoff
algorithm, which, in combination with the CSMA/CD protocol, allows the Ethernet system to
function all the way up to 100 percent load. 
In late 1972, Metcalfe and his Xerox PARC colleagues developed the first experimental
Ethernet system to interconnect the Xerox Alto. The Alto was a personal workstation with
a graphical user interface, and experimental Ethernet was used to link Altos to one
another, and to servers and laser printers. The signal clock for the experimental
Ethernet interfaces was derived from the Alto's system clock, which resulted in a data
transmission rate on the experimental Ethernet of 2.94 Mbps. 
Metcalfe's first experimental net was called the Alto Aloha Network. In 1973 Metcalfe
changed the name to Ethernet, to make it clear that the system could support any
computer, and not just Altos, and to point out that his new network mechanisms had
evolved well beyond the Aloha system. He chose to base the name on the word ether as a
way of describing an essential feature of the system: the physical medium (cable) carries
bits to all stations, much the same way that the old luminiferous ether was once thought
to propagate electromagnetic waves through space. Physicists Michelson and Morley
disproved the existence of the ether in 1887, but Metcalfe decided that it was a good
name for his new network system that carried signals to all computers. Thus, Ethernet was
born. 
The Ethernet System
This is a brief tutorial on the Ethernet system. We'll begin with the origins of Ethernet
and the Ethernet standards, and then describe the essential features of Ethernet
operation. 
Ethernet is a local area network (LAN technology that transmits information between
computers at speeds of 10 and 100 million bits per second (Mbps). Currently the most
widely used version of Ethernet technology is the 10-Mbps twisted-pair variety. 
The 10-Mbps Ethernet media varieties include the original thick coaxial system, as well
as thin coaxial, twisted-pair, and fiber optic systems. The most recent Ethernet standard
defines the new 100-Mbps Fast Ethernet system which operates over twisted-pair and fiber
optic media. 
Ethernet is a Vendor-Neutral Network Technology
There are several LAN technologies in use today, but Ethernet is by far the most popular.
Industry estimates indicate that as of 1994 over 40 million Ethernet nodes had been
installed worldwide. The widespread popularity of Ethernet ensures that there is a large
market for Ethernet equipment, which also helps keep the technology competitively priced.

From the time of the first Ethernet standard, the specifications and the rights to build
Ethernet technology have been made easily available to anyone. This openness, combined
with the ease of use and robustness of the Ethernet system, resulted in a large Ethernet
market and is another reason Ethernet is so widely implemented in the computer industry.

The vast majority of computer vendors today equip their products with 10-Mbps Ethernet
attachments, making it possible to link all manner of computers with an Ethernet LAN. As
the 100-Mbps standard becomes more widely adopted, computers are being equipped with an
Ethernet interface that operates at both 10-Mbps and 100-Mbps. The ability to link a wide
range of computers using a vendor-neutral network technology is an essential feature for
today's LAN managers. Most LANs must support a wide variety of computers purchased from
different vendors, which requires a high degree of network interoperability of the sort
that Ethernet provides.
Development of Ethernet Standards
Ethernet was invented at the Xerox Palo Alto Research Center in the 1970s by Dr. Robert
M. Metcalfe. It was designed to support research on the office of the future, which
included one of the world's first personal workstations, the Xerox Alto. The first
Ethernet system ran at approximately 3-Mbps and was known as experimental Ethernet. 
Formal specifications for Ethernet were published in 1980 by a multi-vendor consortium
that created the DEC-Intel-Xerox (DIX) standard. This effort turned the experimental
Ethernet into an open, production-quality Ethernet system that operates at 10-Mbps.
Ethernet technology was then adopted for standardization by the LAN standards committee
of the Institute of Electrical and Electronics Engineers (IEEE 802). 
The IEEE standard was first published in 1985, with the formal title of IEEE 802.3
Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and
Physical Layer Specifications. The IEEE standard has since been adopted by the
International Organization for Standardization (ISO), which makes it a worldwide
networking standard. 
The IEEE standard provides an Ethernet like system based on the original DIX Ethernet
technology. All Ethernet equipment since 1985 is built according to the IEEE 802.3
standard, which is pronounced eight oh two dot three. To be absolutely accurate, then, we
should refer to Ethernet equipment as IEEE 802.3 CSMA/CD technology. However, most of the
world still knows it by the original name of Ethernet, and that's what we'll call it as
well. 
The 802.3 standard is periodically updated to include new technology. Since 1985 the
standard has grown to include new media systems for 10-Mbps Ethernet (e.g. twisted-pair
media), as well as the latest set of specifications for 100-Mbps Fast Ethernet.
Elements of the Ethernet System
The Ethernet system consists of three basic elements: 1. the physical medium used to
carry Ethernet signals between computers, 2. a set of medium access control rules
embedded in each Ethernet interface that allow multiple computers to fairly arbitrate
access to the shared Ethernet channel, and 3. an Ethernet frame that consists of a
standardized set of bits used to carry data over the system. 
The following chapters describe the configuration rules for the first element, the
physical media segments. Next we'll take a quick look at the second and third elements;
the set of medium access control rules in Ethernet, and the Ethernet frame
Operation of Ethernet
Each Ethernet-equipped computer, also known as a station, operates independently of all
other stations on the network: there is no central controller. All stations attached to
an Ethernet are connected to a shared signaling system, also called the medium. Ethernet
signals are transmitted serially, one bit at a time, over the shared signal channel to
every attached station. To send data a station first listens to the channel, and when the
channel is idle the station transmits its data in the form of an Ethernet frame, or
packet. 
After each frame transmission, all stations on the network must contend equally for the
next frame transmission opportunity. This ensures that access to the network channel is
fair, and that no single station can lock out the other stations. Access to the shared
channel is determined by the medium access control (MAC) mechanism embedded in the
Ethernet interface located in each station. The medium access control mechanism is based
on a system called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). 
I. The CSMA/CD Protocol
The CSMA/CD protocol functions somewhat like a dinner party in a dark room. Everyone
around the table must listen for a period of quiet before speaking (Carrier Sense). Once
a space occurs everyone has an equal chance to say something (Multiple Access). If two
people start talking at the same instant they detect that fact, and quit speaking
(Collision Detection.) 
To translate this into Ethernet terms, each interface must wait until there is no signal
on the channel, then it can begin transmitting. If some other interface is transmitting
there will be a signal on the channel, which is called carrier. All other interfaces must
wait until carrier ceases before trying to transmit, and this process is called Carrier
Sense. 
All Ethernet interfaces are equal in their ability to send frames onto the network. No
one gets a higher priority than anyone else, and democracy reigns. This is what is meant
by Multiple Access. Since signals take a finite time to travel from one end of an
Ethernet system to the other, the first bits of a transmitted frame do not reach all
parts of the network simultaneously. Therefore, it's possible for two interfaces to sense
that the network is idle and to start transmitting their frames simultaneously. When this
happens, the Ethernet system has a way to sense the collision of signals and to stop the
transmission and resend the frames. This is called Collision Detect. 
The CSMA/CD protocol is designed to provide fair access to the shared channel so that all
stations get a chance to use the network. After every packet transmission all stations
use the CSMA/CD protocol to determine which station gets to use the Ethernet channel
next.
II. Collisions
If more than one station happens to transmit on the Ethernet channel at the same moment,
then the signals are said to collide. The stations are notified of this event, and
instantly reschedule their transmission using a specially designed backoff algorithm. As
part of this algorithm the stations involved each choose a random time interval to
schedule the retransmission of the frame, which keeps the stations from making
transmission attempts in lock step. 
It's unfortunate that the original Ethernet design used the word collision for this
aspect of the Ethernet medium access control mechanism. If it had been called something
else, such as stochastic arbitration event (SAE), then no one would worry about the
occurrence of SAEs on an Ethernet. However, collision sounds like something bad has
happened, leading many people to think that collisions are an indication of network
failure. 
The truth of the matter is that collisions are absolutely normal and expected events on
an Ethernet, and simply indicate that the CSMA/CD protocol is functioning as designed. As
more computers are added to a given Ethernet, and as the traffic level increases, more
collisions will occur as part of the normal operation of an Ethernet. 
The design of the system ensures that the majority of collisions on an Ethernet that is
not overloaded will be resolved in microseconds, or millionths of a second. A normal
collision does not result in lost data. In the event of a collision the Ethernet
interface backs off (waits) for some number of microseconds, and then automatically
retransmits the data. 
On a network with heavy traffic loads it may happen that there are multiple collisions
for a given frame transmission attempt. This is also normal behavior. If repeated
collisions occur for a given transmission attempt, then the stations involved begin
expanding the set of potential backoff times from which they chose their random
retransmission time. 
Repeated collisions for a given packet transmission attempt indicate a busy network. The
expanding backoff process, formally known as truncated binary exponential backoff, is a
clever feature of the Ethernet MAC that provides an automatic method for stations to
adjust to traffic conditions on the network. Only after 16 consecutive collisions for a
given transmission attempt will the interface finally discard the Ethernet packet. This
can happen only if the Ethernet channel is overloaded for a fairly long period of time,
or is broken in some way.
III. Best Effort Data Delivery
This brings up an interesting point, which is that the Ethernet system, in common with
other LAN technologies, operates as a best effort data delivery system. To keep the
complexity and cost of a LAN to a reasonable level, no guarantee of reliable data
delivery is made. While the bit error rate of a LAN channel is carefully engineered to
produce a system that normally delivers data extremely well, errors can still occur. 
A burst of electrical noise may occur somewhere in a cabling system, for example,
corrupting the data in a frame and causing it to be dropped. Or a LAN channel may become
overloaded for some period of time, which in the case of Ethernet can cause 16 collisions
to occur on a transmission attempt, leading to a dropped frame. No matter what technology
is used, no LAN system is perfect, which is why higher protocol layers of network
software are designed to recover from errors. 
It is up to the high-level protocol that is sending data over the network to make sure
that the data is correctly received at the destination computer. High-level network
protocols can do this by establishing a reliable data transport service using sequence
numbers and acknowledgment mechanisms in the packets that they send over the LAN.
IV. Ethernet Frame and Ethernet Addresses
The heart of the Ethernet system is the Ethernet frame, which is used to deliver data
between computers. The frame consists of a set of bits organized into several fields.
These fields include address fields, a variable size data field that carries from 46 to
1,500 bytes of data, and an error checking field that checks the integrity of the bits in
the frame to make sure that the frame has arrived intact. 
The first two fields in the frame carry 48-bit addresses, called the destination and
source addresses. The IEEE controls the assignment of these addresses by administering a
portion of the address field. The IEEE does this by providing 24-bit identifiers called
Organizationally Unique Identifiers (OUIs), since a unique 24-bit identifier is assigned
to each organization that wishes to build Ethernet interfaces. The organization, in turn,
creates 48-bit addresses using the assigned OUI as the first 24 bits of the address. This
48-bit address is also known as the physical address, hardware address, or MAC address. 
A unique 48-bit address is commonly pre-assigned to each Ethernet interface when it is
manufactured, which vastly simplifies the setup and operation of the network. For one
thing, pre-assigned addresses keep you from getting involved in administering the
addresses for different groups using the network. And if you've ever tried to get
different work groups at a large site to cooperate and voluntarily obey the same set of
rules, you can appreciate what an advantage this can be. 
As each Ethernet frame is sent onto the shared signal channel, all Ethernet interfaces
look at the first 48-bit field of the frame, which contains the destination address. The
interfaces compare the destination address of the frame with their own address. The
Ethernet interface with the same address as the destination address in the frame will
read in the entire frame and deliver it to the networking software running on that
computer. All other network interfaces will stop reading the frame when they discover
that the destination address does not match their own address.
V. Multicast and Broadcast Addresses
A multicast address allows a single Ethernet frame to be received by a group of stations.
Network software can set a station's Ethernet interface to listen for specific multicast
addresses. This makes it possible for a set of stations to be assigned to a multicast
group which has been given a specific multicast address. A single packet sent to the
multicast address assigned to that group will then be received by all stations in that
group. 
There is also the special case of the multicast address known as the broadcast address,
which is the 48-bit address of all ones. All Ethernet interfaces that see a frame with
this destination address will read the frame in and deliver it to the networking software
on the computer. 
VI. High-Level Protocols and Ethernet Addresses 
Computers attached to an Ethernet can send application data to one another using
high-level protocol software, such as the TCP/IP protocol suite used on the worldwide
Internet. The high-level protocol packets are carried between computers in the data field
of Ethernet frames. The system of high-level protocols carrying application data and the
Ethernet system are independent entities that cooperate to deliver data between
computers. 
High-level protocols have their own system of addresses, such as the 32-bit address used
in the current version of IP. The high-level IP-based networking software in a given
station is aware of its own 32-bit IP address and can read the 48-bit Ethernet address of
its network interface, but it doesn't know what the Ethernet addresses of other stations
on the network may be. 
To make things work, there needs to be some way to discover the Ethernet addresses of
other IP-based stations on the network. For several high-level protocols, including
TCP/IP, this is done using yet another high-level protocol called the Address Resolution
Protocol (ARP). As an example of how Ethernet and one family of high-level protocols
interact, let's take a quick look at how the ARP protocol functions.
VII. Operation of the ARP Protocol
The operation of ARP is straightforward. Let's say an IP-based station (station A) with
IP address 192.0.2.1 wishes to send data over the Ethernet channel to another IP-based
station (station B) with IP address 192.0.2.2. Station A sends a packet to the broadcast
address containing an ARP request. The ARP request basically says Will the station on
this Ethernet channel that has the IP address of 192.0.2.2 please tell me what the
address of its Ethernet interface is? 
Since the ARP request is sent in a broadcast frame, every Ethernet interface on the
network reads it in and hands the ARP request to the networking software running on the
station. Only station B with IP address 192.0.2.2 will respond, by sending a packet
containing the Ethernet address of station B back to the requesting station. Now station
A has an Ethernet address to which it can send data destined for station B, and the
high-level protocol communication can proceed. 
A given Ethernet system can carry several different kinds of high-level protocol data.
For example, a single Ethernet can carry data between computers in the form of TCP/IP
protocols as well as Novell or AppleTalk protocols. The Ethernet is simply a trucking
system that carries packages of data between computers; it doesn't care what is inside
the packages. 
VII. Signal Topology and Media System Timing
When it comes to how signals flow over the set of media segments that make up an Ethernet
system, it helps to understand the topology of the system. The signal topology of the
Ethernet is also known as the logical topology, to distinguish it from the actual
physical layout of the media cables. The logical topology of an Ethernet provides a
single channel (or bus) that carries Ethernet signals to all stations. 
Multiple Ethernet segments can be linked together to form a larger Ethernet LAN using a
signal amplifying and retiming device called a repeater. Through the use of repeaters, a
given Ethernet system of multiple segments can grow as a non-rooted branching tree. This
means that each media segment is an individual branch of the complete signal system. Even
though the media segments may be physically connected in a star pattern, with multiple
segments attached to a repeater, the logical topology is still that of a single Ethernet
channel that carries signals to all stations. 
The notion of tree is just a formal name for systems like this, and a typical network
design actually ends up looking more like a complex concatenation of network segments. On
media segments that support multiple connections, such as coaxial Ethernet, you may
install a repeater and a link to another segment at any point on the segment. Other types
of segments known as link segments can only have one connection at each end. This is
described in more detail in the individual media segment chapters. 
Non-rooted means that the resulting system of linked segments may grow in any direction,
and does not have a specific root segment. Most importantly, segments must never be
connected in a loop. Every segment in the system must have two ends, since the Ethernet
system will not operate correctly in the presence of loop paths.
The caption box shows several media segments linked with repeaters and connecting to
stations. A signal sent from any station travels over that station's segment and is
repeated onto all other segments. This way it is heard by all other stations over the
single Ethernet channel. 
As shown here, the physical topology may include bus cables or a star cable layout. The
three segments connected to a single repeater are laid out in the star physical topology,
for example. The point is that no matter how the media segments are physically connected
together, there is one signal channel delivering frames over those segments to all
stations on a given Ethernet LAN.
IX. Round Trip Timing
In order for the media access control system to work properly, all Ethernet interfaces
must be capable of responding to one another's signals within a specified amount of time.
The signal timing is based on the amount of time it takes for a signal to get from one
end of the complete media system and back, which is known as the round trip time. The
maximum round trip time of signals on the shared Ethernet channel is strictly limited to
ensure that every interface can hear all network signals within the specified amount of
time provided in the Ethernet medium access control system. 
The longer a given network segment is, the more time it takes for a signal to travel over
it. The intent of the configuration guidelines is to make sure that the round trip timing
limits are met, no matter what combination of media segments are used in the system. The
configuration guidelines provide rules for combining segments with repeaters so that the
correct signal timing is maintained for the entire LAN. If the specifications for
individual media segment lengths and the configuration rules for combining segments are
not followed, then computers may not hear one another's signals within the required time
limit, and could end up interfering with one another. 
The correct operation of an Ethernet LAN depends upon media segments that are built
according to the rules published for each media type. More complex LANs built with
multiple media types must be designed according to the multi-segment configuration
guidelines provided in the Ethernet standard. These rules include limits on the total
number of segments and repeaters that may be in a given system, to ensure that the
correct round trip timing is maintained. 
X. Extending Ethernets with Hubs
Ethernet was designed to be easily expandable to meet the networking needs of a given
site. To help extend Ethernet systems, networking vendors sell devices that provide
multiple Ethernet ports. These devices are known as hubs since they provide the central
portion, or hub, of a media system. 
There are two major kinds of hub: repeater hubs and switching hubs. As we've seen, each
port of a repeater hub links individual Ethernet media segments together to create a
larger network that operates as a single Ethernet LAN. The total set of segments and
repeaters in the Ethernet LAN must meet the round trip timing specifications. The second
kind of hub provides packet switching, typically based on bridging ports as described in
Chapter 15. 
The important thing to know at this point is that each port of a packet switching hub
provides a connection to an Ethernet media system that operates as a separate Ethernet
LAN. Unlike a repeater hub whose individual ports combine segments together to create a
single large LAN, a switching hub makes it possible to divide a set of Ethernet media
systems into multiple LANs that are linked together by way of the packet switching
electronics in the hub. The round trip timing rules for each LAN stop at the switching
hub port. This allows you to link a large number of individual Ethernet LANs together. 
A given Ethernet LAN can consist of merely a single cable segment linking some number of
computers, or it may consist of a repeater hub linking several such media segments
together. Whole Ethernet LANs can themselves be linked together to form extended network
systems using packet switching hubs. While an individual Ethernet LAN may typically
support anywhere from a few up to several dozen computers, the total system of Ethernet
LANs linked with packet switches at a given site may support many hundreds or thousands
of machines.