Ethernet Specification

1. About Ethernet
2. Ethernet at Data Link Layer
3. Ethernet at Physical Layer
4. IEEE standards
5. Features of ethernet
○ PoE
○ Port mirroring

Bob Metcalfe, first invented the Ethernet network on May 22, 1973 to
interconnect advanced computer workstations.
Ethernet is a network protocol referred as IEEE 802.3 protocol.It was
introduced in 1980 and then standardized in 1983 as IEEE 802.3
It is a family of computer networking technologies that controls how data is
transmitted over a LAN and MAN network
The protocol has evolved and improved over time and now it can deliver
speed up to gigabits per second.
It is used to connect computers and devices.It connect any type of computer
to its network if that device has an Ethernet adapter or network card.
Ethernet Works on Layer 1 and Layer 2 of OSI model.

How data is transmitted over Ethernet cable?
Ethernet is a shared medium for sending packets of data. There are rules to
avoid conflicts and to protect data integrity over Ethernet.
Device check when the network is available for sending packets.
It is possible that two or more devices at different locations will attempt to
send data at the same time. When this happens, a packet collision occurs.

In order to eliminate collisions Ethernet uses a protocol called Carrier Sense
Multiple Access/Collision Detection (CSMA/CD).
CSMA/CD is a protocol which defines how to respond when two devices
attempt to transmit packets simultaneously.Any device can try to send a
packet at any time in the network.
Each device senses the line that it is idle and available for use. Then the
device start to transmit its first packet.
If then another device tried to send at the same time, a collision is said to
occur and the packet is discarded. Each device then waits a random amount of
time and retries until successful in getting its transmission sent.

Ethernet at Data link Layer:
Ethernet at data link layer is responsible for ethernet addressing ,Hardware
addressing or MAC addressing.
At data link layer, Ethernet encapsulates packets that are coming from
Network Layer and form frames and then transmit over physical media.
Ethernet MAC addresses are in hexadecimal format. It consists of 6 byte (48
bits).Here first three bytes tells the identity of the vendor and the last three
bytes tells node identity.
At Data Link layer, no modification is required to the MAC address when going
between different physical layer interfaces, such as from Ethernet to Fast Ethernet

Ethernet Addressing
A MAC address (Media Access Control address) is a unique code which is
stored on network interface cards or ethernet cards. There are two types of
MAC addresses:
● Universal administered MAC addresses assigned to devices by their
manufacturers (burned-in).
● Organizationally Unique Identifiers (OUI) are administered by the IEEE
Standards Association

Ethernet Addressing: MAC address format
● The first three bytes tell about the manufacturer also known as the
Organizationally Unique Identifier (OUI).
● The 1st bit of the first byte is referred to as unicast or multicast bit.
If the bit is set to 0 (zero), the frame is meant to reach only one receiving
NIC. This type of transmission is called unicast.
If the bit is set to 1, the frame will still be sent only once to specific nodes
in network; This is called multicast addressing.
● The 2nd bit of the first byte is referred to as the U/L bit (Universal/Local)
which identifies how the address is administered i.e. Globally unique or
locally administered.

Ethernet address
Each NIC card/ethernet adapter contain a unique code which is provided by the manufacturer, that unique code is known as Media Access Control address or Ethernet Address.

Ethernet at Data link Layer
Example 1:
Mac address = 04-00-00-00-00-00
The first byte in binary form is 00000100, where the first bit is 0.
This define that the address is unicast address.
Here the second bit is also 0
This define that the address is a universally administered address.

Ethernet Frames:
The main function of data link layer is to convert bits into bytes and then bytes
into frames.
It encapsulates the packet that are coming from network layer to frame and
then transmit it to the physical layer for transmission.

The MAC frame format for Ethernet, IEEE 802.3 used within the 10 and 100
Mbps systems is shown as

Ethernet at Data link Layer

The frame consists of seven parts divided into three main areas:-
1. Header
○ Preamble (PRE) – This is seven bytes long. It consists of a pattern of
alternating ones and zeros, and this informs the receiving stations
about the start of frame and enable synchronisation. (10 Mbps
○ Start Of Frame delimiter (SOF) – This consists of one byte. It 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 leftmost bit indicates whether the
destination is an individual address(0 bit) or a group address(1 bit).
○ Source Address (SA) – The source address consists of six bytes, and it
is used to identify the sending station.
○ Length / Type – This field is two bytes in length. It gives MAC
information and indicates the number of client data types that are
contained in the data field of the frame.
2. Payload
○ Data – This contains the payload data and it size ranges up to 1500
bytes. If the length of the field is less than 46 bytes, then the padding
data is added to fill the data block
3. Trailer
○ Frame Check Sequence (FCS) – This is four bytes long. It contains a 32
bit Cyclic Redundancy Check (CRC) that is generated over the
Destination Address, Source Address, Length / Type and Data fields.

Ethernet at Physical Layer

The Ethernet at physical layer defines the electrical or optical properties of the
physical connection between a device and the network or between network
It tells about the medium of transmission used between the network devices
It speed ranges from 1 Mbit/s to 100 Gbit/s.
The physical medium can be a coaxial cable , twisted pair cable or optical

Ethernet Evolution
1. Standard Ethernet (10mbps)
○ 10Base5 (Thick coaxial wire, Bus topology)
○ 10base2 (Thin coaxial wire, Bus Topology)
○ 10BaseT (UTP cable, Star Topology)
○ 10BaseF (Optical Fiber Cable, Star Topology)
2. Fast Ethernet (100mbps)
○ 100BaseTX (2 UTP wire)
○ 100BaseFX (2 Fiber wire)
○ 10Base T 4 (4 UTP wire)
3. Gigabit Ethernet (1gbps)
○ 1000Base SX (Short wave,Optical Fiber)
○ 1000Base LX (Long wave,Optical Fiber)
○ 1000Base CX (STP wire, 2 Copper wire)
○ 1000Base T (STP wire, 4 Copper wire)
4. Ten Gigabit Ethernet (10gbps)
To understand standard Ethernet code, we have to understand the meaning of
each code:
10 – at the beginning indicate the speed of network i.e. 10Mbps.
BASE – indicates the type of signaling used i.e. baseband.
2 or 5 – at the end indicates the maximum length of cable in meters.
T – at the end stands for twisted-pair cable.
X – at the end stands for full duplex-capable cable.
FL – at the end stands for fiber optic cable.

IEEE Standard:
IEEE 802.3 is a working group which define the physical layer and data link
layer (MAC) of wired Ethernet.
Ethernet is defined under a number of IEEE standards and each standard
define a different type of Ethernet.
Each of the Ethernet IEEE 802.3 standards can be uniquely identified.
These IEEE standards are being updated as a new Ethernet cable is evolved.
Some of them are also defined below.

Standards Year Description
802.3a 1985 10Base-2 (thin Ethernet)
802.3c 1986 10Mb/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)
802.3z 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)
802.3ae 2002 10-Gigabit Ethernet
802.3at 2005 Power over Ethernet Plus

Features of Ethernet

Power over Ethernet (PoE)
Power over Ethernet (PoE) is a networking feature which is defined by the IEEE
802.3af and 802.3at standards.
It is a solution in which electric power is pass along with data on twisted pair
Ethernet cabling. This allows a single cable to provide both data connection
and electric power to device such as wireless access points, IP cameras and
VoIP phones.
Thus there is no need of an extra AC power cord to be attached to device.
This minimizes the amount of cable required and reduce the difficulties and
cost of installing extra outlets
The IEEE standards for PoE require category 5 cable for high power levels and
allow using category 3 cable for low power level.
For example, a digital security camera which requires two connections to be
made when it is installed. One for network connection, in order to be able to
communicate with video recording and display equipment and other for power
supply. So here PoE is used.

Port Mirroring:
Port mirroring is used on a network switch to send a copy of network packets
seen on one switch port to a another switch port in network. This is used for
monitoring network traffic.
Port mirroring on a Cisco Systems switch is known as Switched Port Analyzer
(SPAN) or Remote Switched Port Analyzer (RSPAN). Other vendors have
different names for it, such as Roving Analysis Port (RAP) on 3Com switches.
Network engineers or administrators use port mirroring to analyze and debug
data or diagnose errors on a network.

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