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ETHERNET
Yash Vaidya
Introduction
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Ethernet is a family of computer networking technologies
for local area networks (LANs). Ethernet was commercially
introduced in 1980 and standardized in 1985 as IEEE Standard
Project 802.3. Ethernet has largely replaced competing wired
LAN technologies.
The Ethernet standards comprise several wiring and signaling
variants of the physical layer in use with Ethernet. The
original 10BASE5 Ethernet used coaxial cable as a shared
medium. Later the coaxial cables were replaced with twisted
pair and fiber optic links in conjunction with hubs or switches.
Data rates were periodically increased from the original
10 megabits per second to 100 gigabits per second.
Systems communicating over Ethernet divide a stream of data
into shorter pieces called frames. Each frame contains source
and destination addresses and error-checking data so that
damaged data can be detected and re-transmitted.
As per the model Ethernet provides services up to and
including the data link layer which is divided into 2 sublayers:
LLC & MAC.
 Logic Link Control: Flow control, error control & part of the
framing duties are collected here. Provides interconnectivity
between different LANs.
 Media Access Control: It is a unique identifier that defines a
specific access method for each LAN. For Ethernet LANs its
CSMA/CD. It frames data received from the upper layer &
passes to the physical layer.
 Physical Layer: The first(lowest) layer consisting of basic
networking hardware transmission technology. It defines
transmission of raw bits rather than data packets.
 Since its commercial release, Ethernet has retained a good
degree of compatibility.
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Preamble: It contains 7 bytes of alternating 0s & 1s. This alerts
the receiving system to the incoming frame & enables it to
synchronize its input timing.
Start Frame Delimiter: It signals the beginning of the frame.
This warns the stations that this is the last chance for
synchronization. Contains 1 byte (10101011).
Destination Address: Contains 6 bytes & the physical address
of destination station to receive the packet.
Source Address: Contains 6 bytes & physical address of the
sender of the packet.
Length/Type: Primitively the type field was used to define the
upper layer protocol using MAC frame. Now the length field
is used to define the number of bytes in the data field.
Data & Padding: It carries data encapsulated from upper layer
protocols. It’s a minimum of 46 & maximum of 1500 bytes.
Frame Check Sequence: Also called Cyclic Redundancy
Check. This contains error detection information.
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Frame Length is minimum of 64 bytes & maximum of 1518
bytes.
Addressing in an Ethernet network is written in hexadecimal
notation.
If the LSB of the first byte in a destination address is 0, the
address is Unicast(one-to-one) else Multicast(one-to-many).
The broadcast destination address is a special case of multicast
address in which all bits are 1s.
Maximum Length= Propagation Speed x (Slot Time)/2.
Where
Maximum Length is a length of the cable to which different
workstations are connected.
Propagation Speed is 2x10 raised to 8 m/s in most
transmission media.
Slot time is the time required for a frame to travel from one
end of a maximum length network to the other.
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Thick Ethernet/Thickknet: Max Length of cable=500m. Too
stiff to bend with hands. This is the first Ethernet specification
using a bus topology.
Thin Ethernet/Cheapernet: Max Length of cable is roughly
185m. Very flexible. Uses a bus topology too.
Twisted Pair Ethernet: Workstations connected to a hub via 2
twisted cables. Uses a star topology. Max Length of
cable=100m.
Fiber Ethernet: Workstations connected to a hub via 2 fiber
optic cables. This is the most common today. Uses a star
topology. Max Length of cable=2000m.
Summary:
Common Fast
Ethernet
Implementations
100 BASE-TX
100 BASE-FX
100 BASE-T4
The goals of the above are:
 Upgrade the data rate to 100Mbps.
 Make it compatible with Standard Ethernet.
 Keep the same 48-bit address.
 Keep the same frame format.
 Keep the same minimum & maximum frame lengths.
A new feature added here is Autonegotiation. It allows a
station/hub a range of capabilities. Allows 2 devices to
negotiate the data rate operation.
 100Base-TX uses 2 Twisted Pair cables. Good Bandwidth.
 100Base-FX uses 2 pairs of Fiber optic cables. Higher
Bandwidth.
 100Base-T4 uses 4 pairs of Unshielded Twisted Pair cables.
This is cost efficient than the above two.
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Summary:
Gigabit Ethernet
Implementations
1000 BASESX
1000 baseLX
1000BASECX
1000 BASE-T
The goals of the above are:
 Upgrade the data rate to 1Gbps.
 Make it compatible with Standard or Fast Ethernet.
 Use the same 48-bit address.
 Use the same frame format.
 Keep the minimum & maximum frame lengths.
 To support autonegotiation as defined in Fast Ethernet.
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Gigabit Ethernet has distinctive approaches for medium access
& those are: Full-Duplex Mode, Half-Duplex Mode,
Traditional Mode, Carrier Extension Mode & Frame Bursting
Mode.
Summary:
Ten-Gigabit Ethernet
Implementation:
 Upgrade the data rate to 10Gbps.
 Make it compatible with Standard, Fast & Gigabit Ethernet.
 Use the same 48-bit address.
 Use the same frame format.
 Keep the same minimum & maximum frame lengths.
 Allow the interconnection of existing LANs into a
Metropolitan Area Network (MAN) or a Wide Area Network
(WAN).
 Make Ethernet compatible with technologies like Frame Relay
& ATM.
Summary:
Ethernet Application
References
Data Communications & Networking by Behrouz A Forouzan.
 Wikipedia.
 www.froydwess.com.
 www.precicon.com.
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