2.

1 Introduction
A LAN consists of shared transmission medium and a set of hardware and software for
interfacing devices to the medium and regulating the ordering access to the medium. These are used to
share resources (may be hardware or software resources) and to exchange information. LAN protocols
function at the lowest two layers of the OSI reference model: the physical and data-link layers.
Project 802:
In 1985, the computer society of the IEEE started a project, called Project 803, to set standards to
enable intercommunication between equipment from a variety of manufacturers. Project 802 does not
seek to replace any part of the OSI model. Instead it is a way of specifying functions of the physical
layer, the data link layer, and, to a lesser extent, the network layer to allow for interconnectivity of
major LAN protocols.
The strength of Project 802 is modularity. By subdividing the functions necessary for LAN
management, the designers were able to standardize those that can be generalized and to isolate those
that must remain specific. Each subdivision is identified by number.
The 802.1 sub layer gives an introduction to set of standards and gives the details of the interface
primitives. It provides relationship between the OSI model and the 802 standards.
The 802.2 sublayer describes the LLC (logical link layer), which is the upper part of the data link
layer. LLC facilitate error control and flow control for reliable communication. It appends a header
containing sequence number and acknowledgement number. And offers the following three types of
services:
Unreliable datagram service
Acknowledged datagram service
Reliable connection oriental service
The standards 802.3, 802.4 and 802.5 describe three LAN standards based on the CSMA/CD, token bus
and token ring, respectively. Each standard covers the physical layer and MAC sublayer protocols.

Ethernet - A Brief History

The original Ethernet was developed as an experimental coaxial cable network in the 1970s by Xerox
Corporation to operate with a data rate of 3 Mbps using a carrier sense multiple access collision
detection (CSMA/CD) protocol for LANs with sporadic traffic requirements. Success with that project
attracted early attention and led to the 1980 joint development of the 10-Mbps Ethernet Version 1.0
specification by the three-company consortium: Digital Equipment Corporation, Intel Corporation, and
Xerox Corporation.

The original IEEE 802.3 standard was based on, and was very similar to, the Ethernet Version 1.0
specification. The draft standard was approved by the 802.3 working group in 1983 and was
subsequently published as an official standard in 1985 (ANSI/IEEE Std. 802.3-1985). Since then, a
number of supplements to the standard have been defined to take advantage of improvements in the
technologies and to support additional network media and higher data rate capabilities, plus several new

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optional network access control features. From then onwards, the term Ethernet refers to the family of
local-area network (LAN) products covered by the IEEE 802.3 standard that defines what is commonly
known as the CSMA/CD protocol. Three data rates are currently defined for operation over optical fiber
and twisted-pair cables:

10 Mbps—10Base-T Ethernet
100 Mbps—Fast Ethernet
1000 Mbps—Gigabit Ethernet

Ethernet has survived as the major LAN technology (it is currently used for approximately 85 percent
of the world's LAN-connected PCs and workstations) because its protocol has the following
characteristics:
It is easy to understand, implement, manage, and maintain
It allows low-cost network implementations
It provides extensive topological flexibility for network installation
It guarantees successful interconnection and operation of standards-compliant products, regardless of
manufacturer

Ethernet:

IEEE 802.3 supports a LAN standard originally developed by Xerox and later extended by a
joint venture between Digital Equipment Corporation, Intel corporation, and Xerox. This was called
Ethernet.

IEEE 802.3 defines two categories such as baseband and broadband as shown below.

The word base specifies a digital signal (in this case, Manchester encoding). The word broad
specifies an analog signal (in this case PSK encoding). IEEE divides baseband category into five
different standards 10Base 5,10Base 2,10Base T, 1Base 5 and 100Base T. The first number (10, 1 or
100) indicates the data rate in mbps. The last number or letter (5, 2, T) indicates the maximum cable
length or the type of the cable.

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 IEEE defines only one specification for broadband category 10Broad 36 where first number
indicates data rate and last number defines the maximum length. However, the maximum cable length
restriction can be changed using networking devices such as repeaters or bridges.

Access Method:

CSMA/CD:

Whenever multiple users have unregulated access to a single line , there is a danger of
signals overlapping and destroying each other such overlaps which turns the signal into unusable noise
are called collisions as traffic increases on a multiple access link so the collisions also increases. A
LAN therefore needs a mechanism to coordinate traffic, minimize the number of frames that are
delivered successfully. The access mechanism used in an Ethernet is called carrier sense multiple access
with collision detection (CSMA/CD).

CSMA/CD is the result of an evolution from multiple access (CSMA) and finally to carrier
sense multiple access with collision detection (CSMA/CD):

The original design was a multiple access method in which every workstation had equal access to a
link.

In multiple access there was no provision for traffic coordination. Access to the line was open
to any node at any time with the assumption that the odds of two devices competing for access at the
same time where small enough to be unimportant. Any station wishing to transmit did so, then relied on
acknowledgements to verify that the transmitted frame had not been destroyed by other traffic on the
line.

In the CSMA system, any workstation wishing to transmit must first listen for existing traffic
on the line a device listens by checking for the voltage. If no voltage is detected, the line is considered
idle and the transmission is initiated. CSMA cuts down on the number of collisions but does not
eliminate them. Collisions can still occur. If another station has transmitted too recently for its signal to
have reached the listening station, the listener assumes the line is idle and introduces it’s own signal on
the line.

The final setup is the addition of collision detection. In CSMA/CD the station wishing to
transmit first listens to make certain the link is free, then transmits it’s data, then listens again. During
the data transmission, the station checks the line for the extremely high voltages that indicate a
collision. If a collision is detected a station quits the current transmission and waits a predetermined
amount of time for the line to clear, then sends it’s data again.

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Collision in CSMA/CD:

Addressing:

Each station on a Ethernet network such as a pc, workstation or printer has it’s own network
interface card (NIC). The NIC usually fits inside the station and provides the station with a six
byte physical address. The number on the NIC is unique.

Electrical Specification :

Signalling:

The baseband systems use Manchester digital encoding

Data Rate: Ethernet LANs can support data rates between 1 and 100 Mbps.

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Ethernet Frames

In Ethernet, both the data link and the physical layers are involved in the creation and
transmission of frames. The physical layer is related to the type of LAN cabling and how the bits are
transmitted and received on the cable. The data link layer is divided into sublayers, the Logical Link
Control (LLC) and the Media Access Control layers (MAC). The frames created by these layers contain
several fields that are processed by Network Interface Cards (NICs) in the sending and receiving
devices.
The MAC sublayer address is the physical hardware address of the source and destination
computer. It is called the MAC layer address and should not be confused with the network address. All
devices on a LAN must be identified by a unique MAC address. This sublayer controls which
computer devices send and receive the data and allows NICs to communicate with the physical layer.
IEEE 802.3 protocols control the format of the MAC sublayer frame fields.The next level of processing
is the LLC sublayer. It is responsible for identifying and passing data to the network layer protocol.
Two LLC protocols are IP and Novell’s IPX.
IEEE 802.3 Frame Structure
Length Data
Start of Frame Dest. Source
Preamble &Padding
Delimiter Address Address PDU CRC
(7 bytes) (46-1500
(1 byte) (2/6 bytes) (2/6 bytes) (2 bytes) bytes)

Preamble :Each frame starts with a preamble of 7 bytes, each byte containing the bit pattern
10101010. Manchester encoding is employed here and this enables the receiver's clock to synchronize
with the sender's and initialise itself.

Start of Frame Delimiter :This field containing a byte sequence 10101011 denotes the start of the
frame itself.

Dest. Address :The standard allows 2-byte and 6-byte addresses. Note that the 2-byte addresses are
always local addresses while the 6-byte ones can be local or global.

Source Address :Refer to Dest. Address. Same holds true over here.

Length : The Length field tells how many bytes are present in the data field, from a minimum of 0
to a maximum of 1500. The Data and padding together can be from 46bytes to 1500 bytes as the valid
frames must be at least 64 bytes long, thus if data is less than 46 bytes the amount of padding can be
found out by length field.

Data :Actually this field can be split up into two parts - Data(0-1500 bytes) and Padding(0-46
bytes).
Reasons for having a minimum length frame :

1. To prevent a station from completing the transmission of a short frame before the first bit has
even reached the far end of the cable, where it may collide with another frame. Note that the
transmission time ought to be greater than twice the propagation time between two farthest
nodes.

transmission time for frame > 2*propagation time between two farthest nodes

2. When a transceiver detects a collision, it truncates the current frame, which implies that stray
bits and pieces of frames appear on the cable all the time. Hence to distinguish between valid
frames from garbage, 802.3 states that the minimum length of valid frames ought to be 64 bytes
(from Dest. Address to Frame Checksum).
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 Frame Checksum : It is a 32-bit hash code of the data. If some bits are erroneously received by the
destination (due to noise on the cable), the checksum computed by the destination wouldn't match with
the checksum sent and therefore the error will be detected. The checksum algorithm is a cyclic
redundancy checksum (CRC) kind. The checksum includes the packet from Dest. Address to Data field.