Computer Networks

1. Active Learning Assignment
Prepared by
Tejoy Vachhrajani(160110116057), Bhavik Vashi (160110116061),
Khyati Valera (160110116059)
Topic : Ethernet and Switches
Computer Networks (2140709)
2nd
Year B.E. (Information Technology) A.Y. 2017-18 Semester 4
G H Patel College of Engineering and Technology
Department of Information Technology
Date: 28th
Feb, 2018 Batch: 1D16Group No: D5

2. What Is Ethernet?
 A local-area network (LAN) protocol developed by Xerox.
 Corporation in cooperation with DEC and Intel in 1976.
 Ethernet uses a bus or star topology and supports data transfer rates of
10/100/1000 Mbps.
 The Ethernet specification served as the basis for the IEEE 802.3 standard, which
specifies the physical and lower software layers.
 Ethernet uses the CSMA/CD access method to handle simultaneous demands.
 Ethernet defines the lower two layers of the OSI Reference Model.

3. Ethernet Frame Structure

4. Ethernet Frame Structure
Preamble
Ethernet frame starts with 7-Bytes Preamble. This is pattern of
alternative 0’s and 1’s which indicates starting of the frame and allow
sender and receiver to establish bit synchronization.
Initially, PRE (Preamble) was introduced to allow for the loss of few bits
due to signal delays. But todays high-speed Ethernet don’t need
Preamble to protect the frame bits.

5. Ethernet Frame Structure
Start of frame delimiter (SFD)
This is a 1-Byte field which is always set to 10101011.
SFD indicates that upcoming bits are starting of frame, which is
destination address.
Sometimes SFD is considered the part of PRE, this is the reason

6. Ethernet Frame Structure
Destination Address – This is 6-Byte field which contains the MAC
address of machine for which data is destined.
Source Address – This is a 6-Byte field which contains the MAC
address of source machine.
As Source Address is always an individual address (Unicast), the least
significant bit of first byte is always 0..
Length – Length is a 2-Byte field, which indicates the length of entire

7. Ethernet Frame Structure
Destination Address – This is 6-Byte field which contains the MAC
address of machine for which data is destined.
Source Address – This is a 6-Byte field which contains the MAC
address of source machine.
As Source Address is always an individual address (Unicast), the least
significant bit of first byte is always 0..
Length – Length is a 2-Byte field, which indicates the length of entire

8. Ethernet Frame Structure
Data – This is the place where actual data is inserted, also known
as Payload.
Both IP header and data will be inserted here, if Internet Protocol is
used over Ethernet. The maximum data present may be as long as 1500
Bytes.
Cyclic Redundancy Check (CRC) – CRC is 4 Byte field. This field
contains 32-bits hash code of data, which is generated over Destination
Address, Source Address, Length and Data field.

9. Ethernet Frame Structure
Data – This is the place where actual data is inserted, also known
as Payload.
Both IP header and data will be inserted here, if Internet Protocol is
used over Ethernet. The maximum data present may be as long as 1500
Bytes.
Cyclic Redundancy Check (CRC) – CRC is 4 Byte field. This field
contains 32-bits hash code of data, which is generated over Destination
Address, Source Address, Length and Data field.

10. Ethernet Cabling
Twisted Pair Cables
– Pairs of wires are twisted around one another.
– Each pair consists of two insulated copper wires
twisted together.
– High quality twisted pair cables have about 1 to 3
twists per inch.
– Twisted pair cables are used with the following
Ethernet physical layers:
• 10Base-T, 100Base-TX, 100Base-T2, 100Base-T4, and
1000Base-T

11. Ethernet Cabling
Coaxial Cables
– A solid center conductor is surrounded by an
insulating spacer.
• Insulating spacer in turn is surrounded by a tubular outer
conductor (usually a braid, foil or both).
– The entire assembly is then covered with an
insulating and protective outer layer.
– Coaxial cables have a wide bandwidth and are
capable of carrying many data, voice, and video
conversations simultaneously

12. Ethernet Cabling
Fiber Optics Cable
– Fiber optic cabling is a technology where electrical signals are converted into optical signals,
transmitted through a thin glass fiber, and re-converted into electrical signals.
– It is used on FOIRL, 10Base-FL, 10Base-FB, 10Base-FP, 100Base-FX, 1000Base-LX, and
1000Base-SX.
– Fiber optic cabling is constructed of three concentric layers:
1. The core : Is the central region of an optical fiber through
which light is transmitted.
2. The cladding: Is the material in the middle layer.
3. The protective layer: serves to protect the core and
cladding from damage.

13. CSMA / CD
CSMA/CD signifies carrier-sense multiple
access with collision detection,describes how
the Ethernet protocol regulates communication
among nodes.
Before a station transmits, it checks if another
station is transmitting. If the medium is quiet, the
station recognizes that this is an appropriate time
to transmit.

14. CSMA / CD

15. Limitations of Ethernet
Electrical signals propagate along a cable very quickly,
but they weaken as they travel, and electrical
interference from neighboring devices can scramble the
signal.
Distance limitation on the maximum separation between
two devices (called the network diameter) on an
Ethernet network.
In CSMA/CD only a single device can transmit at a given
time.

16. Interconnecting Devices
HUBS
- Connects two or more devices without having to connect to
each other directly.
-uses twisted pair cable.
-basic function: forward the data it received from a connected
device to all other connected devices regardless of whether the
data is destined for the device.
-comes in variety of shapes and sizes:

17. Interconnecting Devices
HUBS
Are of three types:
1. Passive Hubs:
provides a pathway for signals to travel
does not need power to operate
2. Active Hubs:
provides a pathway for signals to travel
regenerates the signals before passing it on
requires power to operate
3. Intelligent Hubs:
performs all the jobs of active and passive
helps in troubleshooting by pointing the actual
location of the problem.

18. Interconnecting Devices
HUBS

19. Interconnecting Devices
SWITCHES
• forwards data packages to only the destined device
• checks the MAC address found in the NIC to do so
• by forwarding the data to only the destined device, it reduces the number
of
collision in the network
• uses three methods to deal with the data’s as they arrive:
1. Cut through:
o Begins to forward the package as soon as it arrives
o No error checking performed
o Packet is moved quickly
2. Store and forward
o Waits to receive the entire package before forwarding it
o Basic error checking is performed
3. Fragment Free
o Built on the speed advantage of cut through
o Forwards package in fragments

20. Interconnecting Devices
SWITCHES

21. Interconnecting Devices
ROUTERS
• most commonly used to connect a home computer to an ISP
• derives it’s name from the fact that it can route data from onenetwork to
another.
• when it receives a packet of data, it reads the header of the
packet to determine the destination address.
• when address is determined, it looks in it’s routing table to see whether it knows
how to reach the destination.
•If it does know how to reach the destination it then forwards the data to the
next hop on the route.The next hop could be the final destination or another
router

22. Interconnecting Devices
ROUTERS

23. Switches:-
• A switch is a device in a computer network that connects
together other devices.
• Multiple data cables are plugged into a switch to enable
communication between different networked devices.
• Switches manage the flow of data across a network by
transmitting a received network packet only to the one or
more devices for which the packet is intended.
• Each networked device connected to a switch can be
identified by its network address, allowing the switch to
direct the flow of traffic maximizing the security and
efficiency of the network.

24. Switches:-
• Layer 1 Switch:-
• An Ethernet hub is a simple layer 1 network device
that does not manage any of the traffic coming
through it.
• Any packet entering a port is repeated to the output of
every other port except for the port of entry.
• A repeater hub can therefore only receive and forward
at a single speed.
• Since every packet is repeated on every other port,
packet collisions affect the entire network, limiting its
overall capacity.

25. Switches:-
• Layer 2 Switch:-
• Layer 2 switch performs at the physical and data link
layer.
• It is a bridge with many ports and a design that allows
better performance.
• Layer 2 switch operates using physical network
address identify individual devices.

26. Switches:-
• Layer 3 Switch:-
• Layer 3 switches use IP address that identifies location
on the network.
• They identifies the network/IP address as well as
physical devices.
• It finds the best way to send the packet to destination.

27. Switches:-
• Layer 4 Switch:-
• Layer 4 of the OSI model co-ordinates communication
between system.
• Layer 4 switch are capable of identifying which
application protocol (http , smtp ,ftp).
• Layer 4 checks the port number and supply the data
to next level.

28. Switches:-
• Layer 7 Switch:-
• Layer-7 switches may distribute the load based on uniform
resource locators (URLs), or by using some installation-
specific technique to recognize application-level
transactions.
• A layer-7 switch may include a web cache and participate
in a content delivery network (CDN).

29. Switches:-
ADVANTAGES OF SWITCHES
Switches increase available network bandwidth.
Switches reduce the workload on individual computers.
Switches increase network performance.
Networks that include switches experience fewer frame collisions
because switches create collision domains for each connection (a
process called micro segmentation).
Switches connect directly to workstations.

30. Switches:-
DISADVANTAGES OF SWITCHES
They are more expensive compare to network bridges.
Network connectivity issues are difficult to be traced through the network switch.
Broadcast traffic may be troublesome.
If switches are in promiscuous mode, they are vulnerable to security attacks e.g.
spoofing IP address or capturing of ethernet frames.
Proper design and configuration is needed in order to handle multicast packets.
While limiting broadcasts, they are not as good as routers.

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