Study of LAN

1. Experiment No: 3
CLASS : BE E &TC SUB : CCN
Title:
Study,designandconfigurationof IEEE802.3 EthernetandIEEE 802.11 WirelessLANs.
Aim:
To studythe designandconfigurationof IEEE 802.3 and IEEE 802.11 standards.
Objectives:
• Idea on IEEE 802 project.
• IEEE 802 project Standards 802.3 and 802.11.
• Their design and configuration.
Relevance:
For configuring the IEEE 802.3 and 802.11 technologies.
Theory:
In 1985, the Computer Society of the IEEE started a project, called Project 802, to set standards to enable
intercommunication among equipment from a variety of manufacturers. It is designed to regulate the
manufacturing and interconnectivity between different LANs. Project 802 does not seek to replace any
part of the OSI or the Internet model. Instead, it is a way of specifying functions of the physical layer and
the data link layer of major LAN protocols. The standard was adopted by the American National
Standards Institute (ANSI). In 1987, the International Organization for Standardization (ISO) also
approved it as an international standard under the designation ISO 8802. The relationship of the 802
Standard to the traditional OSI model is shown in Figure 1. The IEEE has subdivided the data link layer
into two sub-layers: logical link control (LLC) and media access control (MAC). IEEE has also created
several physical layer standards for different LAN protocols. IEEE has defined specifications for Wired
LAN and Wireless LAN, called 802.3 and 802.11 respectively, which covers the physical and data link
layers.
ETHERNET Evolution:
The original Ethernet was created in 1976 at Xerox's Palo Alto Research Center (PARC). Since then, it
has gone through four generations: Standard Ethernet (10 Mbps), Fast Ethernet (100 Mbps), Gigabit
Ethernet (l Gbps), and Ten-Gigabit Ethernet (l0 Gbps).
Figure 1. IEEE standards for LANs
Wired Ethernet Frame Format:
The packet sent in an Ethernet LAN is called a frame. The Ethernet frame contains seven fields:
preamble, SFD, DA, SA, length or type of protocol data unit (PDU), upper-layer data, and the CRC.

2. Ethernet does not provide any mechanism for acknowledging received frames, making it what is known
as an unreliable medium. Acknowledgments must be implemented at the higher layers. The frame format
of the MAC frame is shown in Figure 2.
Figure 2. 802.3 MAC Frame
Preamble:The firstfieldof the 802.3 frame contains7 bytes(56 bits) of alternatingOsand1s that alerts
the receivingsystemtothe comingframe andenablesittosynchronize itsinputtiming.The pattern
providesonlyanalertanda timingpulse.The 56-bitpatternallowsthe stationstomisssome bitsatthe
beginningof the frame.The preamble isactuallyaddedatthe physical layerandisnot (formally) partof
the frame.
Start frame delimiter(SFD):The secondfield(l byte:10101011) signalsthe beginningof the frame.The
SFD warnsthe stationor stationsthat thisisthe last chance for synchronization.The last2 bitsis 11 and
alertsthe receiverthatthe nextfieldisthe destinationaddress.
Destinationaddress (DA):The DA fieldis6bytesand containsthe physical addressof the destination
stationor stationstoreceive the packet.
Source address(SA): The SA fieldisalso6 bytesandcontainsthe physical addressof the senderof the
packet.
Length or type: Thisfieldisdefinedasatype field orlengthfield.The original Ethernetusedthisfieldas
the type fieldtodefine the upper-layerprotocol usingthe MACframe.The IEEE standarduseditas the
lengthfieldtodefine the numberof bytesinthe data field.Bothusesare commontoday.
Data: Thisfieldcarriesdataencapsulatedfromthe upper-layerprotocols.Itisa minimumof 46 and a
maximumof 1500 bytes.
CRC: The lastfieldcontainserrordetectioninformation,inthiscase a CRC-32.
Wireless Ethernet Frame Format:
The MAC layer frame consists of nine fields, as shown in Figure 3.
FC D Address1 Address 2 Address 3 SC Address 4 Frame
Body
FCS
2bytes 2bytes 6bytes 6bytes 6bytes 2bytes 6bytes 0-2312
bytes
4bytes
Figure 2. Frame Format
Frame control (FC): The FC field is 2 bytes long and defines the type of frame and some control
information.Table 1describesthe subfields.
Field Explanation

3. Version Current version is 0
Type Type of information: management (00), control (01), or data (10)
Subtype Subtype of each type (RTS, CTS, ATS)
To DS Defined later
From DS Defined later
More flag When set to 1, means more fragments
Retry When set to 1, means retransmitted frame
Pwr mgt When set to 1, means station is in power management mode
More data When set to 1, means station has more data to send
WEP Wired equivalent privacy (encryption implemented)
Rsvd Reserved
Table 1. Sub fields in FC Field.
D: Inall frame typesexceptone,thisfielddefinesthe durationof the transmissionthatisusedtoset the
value of NAY.In one control frame,thisfielddefinesthe IDof the frame.
Addresses:There are four addressfields,each6byteslong.The meaningof eachaddressfielddepends
on the value of the To DS and From DS subfields.
Sequence control:This fielddefinesthe sequence numberof the frame tobe usedinflow control.
Frame body: Thisfield,whichcanbe between0and2312 bytes,containsinformationbasedonthe type
and the subtype definedinthe FCfield.
FCS: The FCS fieldis4 byteslongandcontainsa CRC-32 error detectionsequence.
Frame Types
A wirelessLAN definedbyIEEE802.11 has three categoriesof frames:managementframes,control
frames,anddata frames.
ManagementFrames: Managementframesare usedforthe initial communicationbetweenstations
and accesspoints.
Control Frames:Control framesare usedforaccessingthe channel andacknowledging
frames.
Data Frames: Data framesare usedforcarryingdata and control information.
AddressingMechanism:
The IEEE 802.11 addressingmechanismspecifiesfourcases,definedbythe value of the twoflagsinthe
FC field,ToDSand From DS. Each flagcan be either0or I,resultinginfourdifferentsituations.The
interpretationof the fouraddresses(addressItoaddress4) in the MAC frame dependsonthe value of
these flags,asshowninTable 2.
To DS From DS Address1 Address2 Address3 Address4
0 0 Destination Source BSS ID N/A
0 1 Destination Sending AP Source N/A
1 0 Receiving AP Source Destination N/A
1 1 Receiving AP Sending AP Destination Source
Table 2. Addresses
Note that address1 isalwaysthe addressof the nextdevice.Address2isalwaysthe addressof the
previousdevice.Address3isthe addressof the final destinationstationif itisnotdefinedbyaddressI.
Address4 isthe addressof the original source stationif itisnot the same as address2.
Case 1: 00 Inthiscase, To DS = 0 and From DS = O. Thismeansthat the frame is notgoingto a
distributionsystem(ToDS= 0) andis notcomingfrom a distributionsystem(FromDS=0). The frame is
goingfromone stationin a BSS to anotherwithoutpassingthroughthe distributionsystem.The ACK
frame shouldbe sentto the original sender.

4. Case 2: 01 Inthiscase, To DS = 0 and From DS = 1. Thismeansthat the frame iscomingfrom a
distributionsystem(From DS= 1). The frame is comingfroman APand goingto a station.The ACK
shouldbe sentto the AP.Note that address3 containsthe original senderof the frame (inanotherBSS).
Case 3: 10 Inthiscase, To DS =1 and FromDS =O. This meansthatthe frame isgoingto a distribution
system(ToDS = 1). The frame is goingfroma stationto an AP.The ACKissentto the original station.
Note that address3 containsthe final destinationof the frame (inanotherBSS).
Case 4:11 In thiscase,To DS =1 andFrom DS =1. Thus isthe case inwhichthe distributionsystemisalso
wireless.The frame isgoingfromone APto anotherAPin a wirelessdistributionsystem.We donot
needtodefine addressesif the distributionsystemisawiredLAN because the frame inthese caseshas
the format of a wiredLAN frame (Ethernet,forexample).Here,we needfouraddressestodefine the
original sender,the finaldestination,andtwointermediate APs.The addressesare showninfigure 3.
Figure 3. Addressing Mechanisms.
Configuration:
Figure 4. Static IP Configuration of Ethernet LAN (Wired and Wireless).
Conclusion:Thuswe understandthe IEEE 802.3 and 802.11 standardsfor WiredandWirelessLANs.