Local Area Network (LAN Technology) by hiy10027

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									         Local Area
         Network
         (LAN Technology)
         (LAN Technology)
         Computer Network & Distributed
         Rachphat Sriprom
         Rajabhat Chandrakasem Institute




What is LAN ?
 A local area network (LAN) is a data
communication system that allows a
number of independent devices to
communicate directly with each other in
a limited geographic area




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LAN characteristics




                      3




Transmission media




                      4
Topology




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LAN Topologies




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Topology : Bus/Tree
Multipoint medium
Transmission propagates throughout medium
Heard by all stations
  Need to identify target station
    Each station has unique address
Full duplex connection between station and tap
  Allows for transmission and reception
Need to regulate transmission
  To avoid collisions
  To avoid hogging
     Data in small blocks - frames
Terminator absorbs frames at end of medium
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Topology : Bus




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Frame Transmission - Bus LAN




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Topology : Ring
Repeaters joined by point to point links in closed
loop
  Receive data on one link and retransmit on another
  Links unidirectional
  Stations attach to repeaters
Data in frames
  Circulate past all stations
  Destination recognizes address and copies frame
  Frame circulates back to source where it is removed
Media access control determines when station
can insert frame
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Frame
Transmission
Ring LAN




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Topology : Star

Each station connected directly to central node
  Usually via two point to point links for transmitting
  and receiving
Central node can broadcast
  Physical star, logical bus
  Only one station can transmit at a time
Central node can act as frame switch




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Frame Transmission – Star LAN




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IEEE 802 reference model

In 1985, the Computer Society of the IEEE
(Institute of Electrical and Electronics Engineers)
started a project, called Project 802, 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.
It is a way of specifying functions of the physical
layer, the data link layer and part of the network
layer to allow for interconnectivity of major LAN
protocols

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IEEE 802 reference model

The IEEE has subdivided the datalink layer
into two sublayers :
  LLC (Logical Link Control Sublayer)
  MAC (Medium Access Control Sublayer)
So, IEEE 802 reference model are
  Physical
  Logical link control (LLC)
  Media access control (MAC)


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IEEE 802 v OSI




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IEEE 802

               Other                     Other
               layers                    layers

    802.1 Overview (Internetworking)    Network

    802.2 Logical link control (LLC)
                                        Data link



  802.3       802.4       802.5         Physical
 CSMA/CD    Token Bus   Token Ring



            Project 802                OSI Model
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802 Layers - Physical

Encoding/decoding of signals
Preamble generation/removal (for
synchronization)
Bit transmission/reception
Transmission medium and topology




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802 Layers -
Media Access Control
Control access to the transmission medium
On transmission, assemble data into a frame
with address and error detection fields
On reception, disassemble frame and perform
  Address recognition
  Error detection
Govern access to transmission medium
For the same LLC, several MAC options may be
available


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802 Layers -
Logical Link Control
Controlling the exchange of data between
two users
Provide an interface to higher levels
Provide Flow and error control




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Relationship between the levels
of the architecture
Higher-level data are passed down to LLC
LLC creating an LLC protocol data unit (PDU)
by appends control information as a header,
this control information is used in the
operation of the LLC protocol
The entire LLC PDU is then passed down to
the MAC layer, which appends control
information at the front (header) and the
back (trailer) of the packet, forming a MAC
frame ,the control information in the frame is
needed for the operation of the MAC protocol

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LAN Protocols in Context
diagram




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Typical Frame Format




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MAC Frame Format
MAC layer receives data from LLC layer
For making MAC frame, the follows fields are
added
•   MAC control
•   Destination MAC address
•   Source MAC address
•   LLC Data
•   CRC
MAC layer detects errors and discards frames
LLC optionally retransmits unsuccessful frames

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MAC Address




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Logical Link Control
Transmission of link level PDUs between two
stations
Must support multi-access, shared medium
Relieved of some link access details by MAC
layer
Addressing involves specifying source and
destination LLC users
  Referred to as service access points (SAP)
    DSAP
    SSAP


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DSAP and SSAP
 The DSAP (destination service access point) and
 SSAP (source service access point) are addresses
 used by the LLC to identify the protocol stacks on
 the receiving and sending machines that are
 generating and using the data
 The first bit of the DSAP indicates whether the
 frame is intended for an individual (0) or a group
 (1)
 The first bit of the SSAP indicates whether the
 communication is a command (0) or response (1)
 PDU


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LLC Services

Unacknowledged connectionless service
• The delivery of data is not guaranteed
Connection Oriented mode service
• Make connection before transfer data
• ARQ Error / Flow control
Acknowledged connectionless service
• Don’t make connection before transfer data




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Overview of LAN Standard




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LAN Frame Format Summary




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