Ethernet_Networks

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Ethernet Networks
Pawan Shriniwas Parande
6 September 2005
In this paper various networking concepts are
understood by studying the Ethernet technology
informally. Apart from studying the frame
formats, MAC hardware addresses and switching
schemes there are lot of comprehensive and
formal concepts that associate themselves with
this technology. I am compiling my study of this
technology in this paper.
Introduction to Ethernet
Ethernet is a wired LAN protocol developed by
Xerox, DEC and Intel in mid 1970s. This
technology was developed to interconnect
various office peripherals and achieve a decent
rate of data transfer among them. University of
Hawaii introduced the ALOHA networks in
1960s. This technology paved the platform for
the development of the Ethernet technology.
ALOHA networks demonstrated the usage of a
shared media for the first time and upheld the
importance of a new sub layer, called the
Medium Access Control layer (MAC), inside the
Datalink Layer. ALOHA provisioned any node
to transmit the data at any time and retransmit
the data once it detected collision. This collision
detection and retransmission scheme employed
in ALOHA was least efficient in nature and was
ill engineered. Ethernet appended Carrier Sense
Multiple Access/ Collision detection
(CSMA/CD) scheme to the already present
ALOHA networks.
As a standard, the Ethernet II (also read as
Ethernet Version-2) was published and accepted
as a de-facto in 1978-79. As this standard
supported majority of the American standards,
Europeans found it less compatible to implement
it. Hence IEEE took the task of providing a same
level of technology, with a promise of providing
worldwide compatibility and symmetrical
implementation of the technology everywhere.
This technology was named as IEEE 802.3
standard, on the name of the committee that took
up this operation.
Differences Between Ethernet/802.3 and
Ethernet II
Both these specifications are almost similar in
nature as IEEE took Ethernet II as the bases for
its development but there are some serious
technical differences between these technologies.
Cable Size: Ethernet II prescribes thick coaxial
cables with 0.395-inch diameter. The IEEE
specification proposes a thicker coaxial cable
with 0.405-inch diameter. This larger diameter
cable provided better electrical characteristics.
But this cable is compatible with Ethernet II
transceivers. But currently vendors have
manufactured transceivers, which can adapt to
both these cables.
Transceiver issues: Transceivers transmit and
receive the data simultaneously. They also sniff
the shared medium for collisions and share the
collision information to the MAC layer. Going
with Ethernet II specifications, the transceiver
keeps sending periodic signals on the medium to
indicate to all hosts that it is alive. These signals
are called “heartbeat signals”. The transceivers
prescribed by Ethernet 802.3 do not produce
these signals. This absence will prompt the
controller to conclude that the transceiver is
dead. But currently vendors have manufactured
transceivers, which have a switch that
automatically turns on/off depending on the
Ethernet network employed.

Different frames are shown below. (The start and stop delimiter bytes not shown)
Frame Type Designation: Ethernet II
Common name: Ethernet
Layout:
6 bytes 6 bytes 2 bytes Up to 1500 bytes
+-------------------+------------------+----------------+----------------------------------+
| Destination | Source | Type | Network Protocol Packet |
| MAC Address | MAC Address | | |
+-------------------+------------------+----------------+----------------------------------+
Frame Type Designation: Ethernet/802.3
Common name: "Raw" 802.3
Layout:
6 bytes 6 bytes 2 bytes Up to 1500 bytes
+-----------------+------------------+---------------+------------------------------+
| Destination | Source | Total packet | Packet Data |
| MAC Address | MAC Address | length | first two bytes: FF, FF |
+-----------------+------------------+---------------+------------------------------+
Frame Type Designation: Ethernet/802.2
Common Name: 802.3 (the 802.2 header is implied by the 802.3 standard).
Also Known As: 802.3/802.2, to distinguish from "raw" 802.3
Layout:
6 bytes 6 bytes 2 bytes 1 byte 1 byte 1 byte Up to 1497 bytes
+--------+--------+---------+--------+--------+----------+----------------------+
| Dest | Src | length | DSAP | SSAP | Control | Network Packet |
| Addr | Addr | | (E0) | (E0) | (03) | |
+--------+--------+---------+---------+-------+----------+----------------------+
Frame format difference: This difference is the
major difference between these two
technologies. The Ethernet II frame has a 2-byte
‘type’ field in it. This field indicates the type of
data that is present in the data field of the frame.
This field is of very much importance as packet
encapsulation process drives the major logic in
the protocol stack development. The
Ethernet/802.3 frame instead has a 2-byte
‘length’ field, which specifies the length, in bits,
of the data field.
To fix this faulty frame format, the IEEE 802
committee introduced a new functionality into
the Logical link Control (LLC) layer. The LLC
was giving an additional job to build a small
frame with three fields of 1 byte each. These
fields are Destination SAP field, Source SAP
field and the control field. This new frame was
called the IEEE 802.2 frame. The Ehternet/802.3
frame was later modified to accommodate 802.2
frame. The 802.2 frame served the purpose of the
type field of Ethernet II technology. This new
802.3 frame was hence called the de-facto
Ethernet/802.3 frame, while the earlier version of
the frame (the one with no compatibility to 802.2
frame) was called the “raw frame”.

Manchester Encoding
Topological differences: The Ethernet/802.3
supports star and bus topologies while Ethernet
II supports only the bus topology.
Band support difference: The Ethernet/802.3
supports both baseband and broadband
technologies while Ethernet II renders its support
only to the baseband technology.
Although these technologies pose many
differences it does not mean that one is superior
to another. But the support for Ethernet/802.3
standard is far more compared to the Ethernet II
in the networking field.
Ethernet/802.3 Physical layer highlights
The physical layer of Ethernet /802.3 addresses
issues like cable type, cable length and
connectors. I have compiled detailed
information on these issues in the paper
“Hardware Components of Layer 1 and Layer
2”. The Ethernet/802.3 physical layer also
1. Instead of high representing digital one
and low representing digital zero, in
Manchester encoding the timing interval
is used to measure the high to low
transitions.
2. Instead of the times transmission period
being ‘all high’ or ‘all-low’ for either 1 or
0, a 1 is sent as a ‘half time period low’
followed by ‘half time period high’ and a
0 is sent as a ‘half time period high’
followed by ‘half time period low’.
The diagram below shows graphically how
Manchester encoding operates. The example at
the bottom of the diagram indicates how the
digital bit stream 10110 is encoded. The main
benefit of this is in error recovery. Although, if
during signal transition some of the signals get
distorted, we still have the information of the
timing interval to determine if the signal was
rising or falling in nature.
encodes the data in the “Manchester
Encoding” format before
transmission. The sole purpose of
encoding the data is to make sure that
transmission failures are detected on
the receiving side. There are two
differences between normal digital
transmission and Manchester
encoding.

Ethernet/802.3 Datalink layer highlights
The IEEE 802.3 Datalink layer is based on 1
persistent CSMA/CD, but this uses exponential
back off algorithm for calculating the wait time
period. The initial wait time period is set to some
random value here, the subsequent wait time
periods will be sequential multiples of this base
wait time. The Datalink layer supports a
minimum frame size of 64 bytes and a maximum
frame size of 1518 bytes. Apart from CRC and
padding fields the address field indicates a 48-bit
address. This address is represented by 12
hexadecimal digits, which are petitioned into 6
groups. The higher-order three bytes (the
leftmost six hexadecimal digits) represent the
manufacturer of the Ethernet device; the lower-
order three bytes represent the serial number of
the device. For example, the address
08:00:20:01:D6: 2A corresponds to an
Ethernet/802.3 device manufactured by Sun
Microsystems (as indicated by Sun's code
08:00:20) that has the serial number 01:D6: 2A.
The IEEE is responsible for assigning Ethernet
addresses.
IEEE Ethernet 10 Mbps specifications
The convention used in giving various Ethernet
designations is very intuitive. Whenever we say
10BASE-T it means that the network is a 10
Mbps baseband network that uses twisted pair
(UTP) cables. In the similar fashion 10BASE-2
means that the network is a 10 Mbps baseband
network with 200-meter segment length. In a
nutshell, the baseband and broadband IEEE
802.3 LANs are by bandwidths in Mbps
followed by the type of the cable it supports or
the length of the segment in meters.
IEEE 802.3 proposes 6 specifications for 10
Mbps Ethernet. They are
3. 10BASE-5 (Thick wire),
4. 10BASE-2 (Thin wire),
5. 10BASE-T (UTP Ethernet),
6. 10BASE-FL (Fiber link),
1. 10BASE-FP (Fiber passive), and
2. 10Base-FB (Fiber backbone).
All these Ethernet specifications are
characterized by some basic properties. The
properties are listed below here.
1. Type of cable,
2. Topological support,
3. Maximum segment length,
4. Maximum nodes per segment,
5. Cable diameter, and
6. Cable characteristics (like ohm support
and grounding features).
The cables employed are thin coax, thick coax,
3,4,5 UTP and fiber cables. One of these cables
adheres to one particular specification. Coming
to connectors, if transceivers themselves act as
the connectors then an additional specification
about the AUI and type of tap used, is provided.
Other wise the use of connectors like RJ45, BNC
etc. are provided.
The segment length defines the maximum
allowable LAN length. With the information
about the maximum number of nodes allowed,
we can calculate the inter-space length between
two nodes.
As an example the 10Base5,thick wire topology
uses an external transceiver’ to attach to the
network interface card. The NIC attaches to the
external transceiver by ‘AUI cable’ to the ‘DIX
(DEC-Intel-Xerox) connector’ on the card. The
external transceivers clamp to the thick net cable
with the vampire tap, or BNC or N-series barrel
connectors. Here the each network segment is
terminated at both ends with one end using a
grounded terminator. The components of a thick
wire network are shown in the figure below.

The 10BASE5-cabling scheme and components are shown below.

In the above figure the diameter is 100m +100m = 200 meters.
In the above figure the diameter is 100m +100m +50m + 50m= 300 meters.
IEEE Ethernet 10 Mbps performance
issues
Diameter:
The diameter in the context of Ethernet
technology describes the overall length of the
The variation of these diameters for the various
types of LAN solely depends on the type of the
cable used for establishing the LAN. The cable
characteristics like electrical attenuation, data
fading, inter-wire-insulation etc also determine
the maximum diameter for the LAN.
10BASE–T Repeater Hub
100 meters 100 meters
Diameter = 200 meters
10BASE- T
Repeater Hub
10BASE- T
Repeater Hub
10BASE- T
Repeater Hub
100 meters 100 meters
50 meters 50 meters
LAN between the two most remote workstations
(nodes). This is a term borrowed from graph
theory. This diameter is always expressed in its
maximum value. Repeaters increase the diameter
of a network. Consider the figures above.
5-4-3 repeater placement rule
In order to understand this rule, initially let me
discuss some basics. This information will
eventually justify the need for this rule in
architectural level decision-making. Consider the
following figure

10BASE-T Repeater Hubs
100 m
A
B C D
E
100 m100 m
100 m 100 m
Collision Domain
In this figure there are 5 segments (labeled A
through E), four repeaters, and at most only three
segments populated with end nodes. This
network is compliant with 5-4-3 repeater
placement rule. While configuring the Ethernet
LAN, this rule acts as a rule of thumb. In any
Ethernet/802.3 LAN we cannot avoid collisions.
Supplementing to this, the Ethernet/802.3
supports CSMA/CD. If a LAN contains only a
single segment or a single UTP router hub then
5-4-3 repeater placement rule holds no
significance. But however, if the LAN segment
is extended using many repeaters then this rule
becomes extremely critical. The Ethernet/802.3
LANs comprise of collision domains. A collision
domain is an area where there are chances data
from two different workstations to collide. In a
LAN with a single segment, there is only one
collision domain, that is the segment itself, but in
a multi-segmented Ethernet/802.3 LAN collision
domain the collision domain comprises of
various segments and connecting repeaters.
Physically the network appears to contain many
segments but electrically they form a single
collision domain. A single collision domain
cannot contain 1024 nodes and its length is
restricted to 2500 meters.
Now CSMA/CD design rule says that, whenever
a node’s transmission results in a collision, then
the transmitting node should get the jamming
signal before it finishes its transmission. This is
because, once the node fails to intercept the
jamming signal, before it finishes its
transmission, then the node believes that it has
successfully performed the transmission and
starts buffering the data ahead. But if it intercepts
a jamming signal, then it can follow the ARQ
scheme and get ready for retransmission. Hence
on an Ethernet/802.3 LAN timing is everything.
We know that a signal takes a definite amount of
time to travel form one node to another. From
the studies and experiments it has been
concluded that it takes a minimum of 0.1 µ
second for a bit to be transmitted. This is called
the “bit time”. A smallest Ethernet/802.3 frame
will be 64 bytes in size. This is equivalent to 512
bits. Therefore in order to transmit 512 bits, we
will take 51.2 µ seconds. Incase a collision
happens while transmitting this data; it has to
happen within this time period. Also the
jamming signal should reach the transmitting
node within the same time period.
We also can guess that, a jamming signal, in
order to reach the transmitter should pass via

various devices like routers, bridges, NICs etc.
All these devices introduce a delay in the
propagation of the signal. Hence it is important
to for a network designer to take into
consideration all these delays during the timing
calculations for detecting the collisions and
generating the jamming signals. Now these
delays are vendor specific in nature. Hence
performing the timing calculations every time is
a futile effort. 5-4-3 repeater placement rule
generalize these calculations.
The rule says
a. There can be no more than 5 segments in
a LAN, which extend up to 500 meters.
b. There can be no more than 4 repeaters in
this LAN. And
c. There can be no more than 3 segments
whose end nodes are connected to these 4
repeaters.
This rules introduces a constraint on the number
of hardware equipments that can become a part
of the network. This constraint shall help us
easily achieve the goal of transmitting the
jamming signal, so that it reaches the data
transmitter within 51.2 µ seconds. With this type
of architecture we can achieve our goal by using
hardware from any vendor.
Hence
ÿ (Hardware and cable latency) * 2 <= 51.2 µ
seconds, then the network is CSMA/CD
compliant else it is not. [2 => Round trip time]
After having discussed the 5-4-3 repeater
placement rule, there is still contemplation in my
mind, about its practical applicability. In the real
time LAN deployment we see far more network
elements than prescribed by the rule and still the
network is perfectly compliant of CSMA/CD.
This means that there are ways to bypass this
rule and still adhere to Ethernet/802.3
specifications. Let me discuss how this is done.
The chassis based repeater hub and a daisy
wheel connected repeater stack hubs basically
cheat the above rule. In the former there is a
single backplane to which many of the Ethernet
interface boards are connected. Each Ethernet
interface board in turn handles many segments
summing a collision domain. As the backplane
acts as only one repeater for all these interface
boards, we can serve many more collision
domains. In case of the stackable repeaters, the
daisy wheel sort of inter-repeater connection acts
as a “pseudo backplane”. This again renders the
same performance as in the previous case.
But there is again a limit to this. In both the cases
we cannot stack more that 12 repeaters, or sneak
more than 12 Ethernet interfaces on a backplane.
It is also important to note that this rule applies
only to repeaters and not to switches, bridges or
routers. The justification is simple. Repeater is a
layer 1 device and it is solely responsible to
intercept the collisions, generate the jamming
code and transmit it. Also we know that bridges,
switches and routers are layer2 or layer 3
devices. Although they will intercept the
jamming signal but they will never propagate
these collision signals from one network area to
another. The electrical isolation nature of these
layer 2 devices will basically divide a collision
domain into two collision domains.
Performance monitoring of Ethernet LAN
networks
From the previous section, we understand that a
chassis based repeater hub or a daisy wheel
connected stackable repeaters encourage adding
more than the permissible number of nodes into
a collision domain. Although they are successful
in upholding the CSMA/CD scheme, but on a
larger account they also take the responsibility
for the degradation of the networks operational
characteristics. The deployment of these devices,
therefore, demands a proper and scientific study
of the network performance.

The support to various types of applications,
QoS and many dataflow characteristics, at any
moment of time, we will see that frames of
different sizes floating around the network at any
moment of time. Under such a situation it
becomes compulsory to engineer the study of
network performance. Here is an explanation of
some of the scientific measurements for
evaluating the network performance.
The efficiency of an Ethernet802.3 LAN can be
directly measured by the “size of the frames”
that are floating. In order to utilize the
maximum bandwidth of the network we must
float the frames with maximum size.
Experiments show that if all the frames bear a
maximum size of 1518 bytes, then we utilize
about 95% of the network bandwidth. But if we
are floating frames with just 64 bytes, with one
byte of real data and 45 bytes of padding
information then we just utilize 2% of the
network bandwidth.
“Utilization” is another factor, which helps us to
measure the efficiency of the network. This is the
amount of time spent by the LAN in transferring
the data successfully. If we say that an average
utilization is 45% for a LAN then it means that,
in a given amount of time (say 100ms or 200ms)
only 45% of the transmissions were successful.
There is one more interpretation for this.
Whenever a performance evaluation statement
says that the peak utilization is 45%, then it
means that at any particular time a minimum of
45% of the network bandwidth is utilized.
“Throughput”, the measure of amount of data
transmitted between two nodes in a given time,
also reflects the network performance.
Schemes to Enhance Ethernet Network
Performance
There are various schemes, which enhance the
performance of the Ethernet/802.3 networks’
performance. Following the practices discussed
below we can see better performance results in
terms of utilization time and throughput.
Partitioning is a fissure scheme. Here a network
is split into various interconnected segments by
introducing many bridges or a multiport bridge.
Basic bridges basically continue to use the same
backbone network, but can electrically isolate a
group of nodes into smaller clusters of nodes. A
multiport Bridge achieves the same task but it
incorporates the backbone network within itself.
With this scheme,
a. Network performance gets enhanced,
b. Traffic flow in the network is sub-
channelized,
c. Security management becomes easy,
d. Throughput increases, and
e. Reliability of the network also increases.
The act of bridging successfully diverts a
minimum of 20% of the traffic from the main
stream towards the bridges. But there is another
strategy of partitioning that helps is achieving
high level of network efficiency. This is called
the “physical partitioning”. Here, initially, highly
intercommunicating nodes are identified and
physically they are interconnected. Many such
interconnections are later connected via a
backbone. In this scheme the traffic is localized
among local segments and there will be only
about 20% of traffic on the backbone.
This physical partitioning strategy demands that
people working on workstations, under a
segment, should be located within the limits of
that segment. But practically thinking, in the real
time scenarios we see people spread across
various buildings. And all of them fall under a
single segment. So how do we achieve this
dynamic workgroup setting? Let me reframe the
problem here. We want to have a network that
works as efficiently as a physically partitioned
network does and we also do not want to localize
the nodes within a particular area. Well, the only
solution for this problem is implementing “
Virtual LANs (VLAN)”.

VLANs are created using the “Ethernet-
Switches”. Switch basically localizes the traffic.
Whenever the traffic gets localized then, every
node need not sniff every frame in the network.
In this manner by introducing the switches in the
Ethernet802.3 network, we will transform the
broadcast nature of the network into a point-to-
point type of network. In this situation all the
connected nodes do not bother about the traffic
stipulated between a source node and a
destination node.
We know that there are two kinds of switches.
First a store-and-forward switch and second a
Cut-through Switch. This classification of
switches is made looking at its internal
architecture, but on the deployment scenario, we
can classify switches into 3 types.
a. Workgroup switches,
b. Private switches, and
c. Backbone switches.
A workgroup switch partitions a single shared
medium into multiple shared media. In this type
of a switch, every single port will support about
1024 MAC addresses. Suppose there is an
Ethernet/802.3 LAN of 10mbps with 100 nodes
in it. Then every node at any moment of type,
will typically occupy 1/100th
of the entire
bandwidth in a FDM (frequency division
multiplexing) scheme. This means that at any
moment of time every node can transmit a
maximum of 0.1mbps of data simultaneously. If
we introduce a 5 port Ethernet Switch on this
network, with all the ports falling under a switch
mesh, then we can partition the group of 100
nodes to groups of 10, and allocate 20 MAC
addresses per port. This means that every single
node will occupy 1/10 of the bandwidth. This
chunk is 10 times of what we achieved in the
previous case.
Private switches work on the same lines that of
the workgroup switches, but their ports will
always support a single MAC address. This will
ensure that every node will have the entire
bandwidth of the shared medium, but this will
also complicate the internal structure of the
switch as it has to incorporate large number of
ports. Whenever, every node of the network
demands high bandwidth for a longer interval of
time, then deploying private switches becomes
an ideal solution. Private switches divide some
of their internal ports as input ports and the rest
(always smaller in number) as output ports. The
interconnection of various input ports to output
ports will make up for the bandwidth imbalance
between the ports.
Follow the figure below for different types of
Private 10Mbps Ethernet Switch
In the above figure, the 10Mbps Ethernet Switch
looks like a Hub, but unlike a hub this private
switch dedicates a full 10Mbps line to each port.
Now each port supports only one MAC address.
This implies each node is a segment in its own
terms, with its own collision domain.
Private 10/100Mbps Ethernet Switch

A 10/100 Mbps Ethernet Switch is deployed in a
client/server Scenario. The port that connects to
the server provides 100Mbps bandwidth, while
the other ports provide just 10 Mbps. This helps
the server to cater to all the demands of the
clients without letting bandwidth imbalance
creep in.
Backbone Switches are incorporated in a
network to act like the network backbone itself.
Any network topology where a switch is
deployed to act as a backbone is called as a
“collapsed backbone topology”.
fault tolerance and redundancy.
Full-duplex Ethernet
From the above discussion of the Private
Ethernet Switches, we see that almost the entire
contention gets eliminated. This makes it
possible for nodes to simultaneously transmit
and receive the data. This kind of an Ethernet
LAN is called the Full-duplex Ethernet. The
switches that are used in a Full-duplex Ethernet
LAN incorporate Full-duplex ports and Full-
These switches are chassis-based and have
gigabit per second backplanes. They also support
multiple media types, shared or dedicated
segments, and both 10-Mbps and 100- Mbps
segments. To compensate for a single source of
failure, backbone switches have provisions for
-duplex NICs. As collision detection is switched
off in this type of switches achieve data
transmission at almost twice the allotted
bandwidth. It is also mandatory that all the nodes
connected to such a Switch, should support
multi-threaded operating system like UNIX or

Windows NT.
Virtual LAN
Virtual Local area network hosts many nodes
connected to each other with a difference. Here
the nodes are not connected via any physical
media. There are connected in a virtual sense
using specially designed software that groups
several ports in a switch into a single work
group. Nodes connected to these ports become a
part of the workgroup.
The backbone switches may work on Layer-2 or
on Layer-3. If the switch is designed to identify
IP address then it works on Layer-3 and if the
switch is designed to identify MAC address then
it works on Layer-2.
There is an advantage in using the IP based
(Layer 3) Ethernet backbone switch. In such a
switch the workgroups are created using nodes
IP addresses. Here the individual node can be
assigned more than one virtual sub networks at
the same time. This will enable more than one
work group share a single Server in client server
based architecture. The Switches supporting
MAC addresses cannot achieve this.
VLAN1
VLAN2
VLAN3
A further study of Fast Ethernet schemes, 100
Mbit Ethernet, should be appropriate to conclude
this paper.

Ethernet_Networks

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