The OSI Model with Layer Description by lzg15357


									               The OSI Model

  Node A                              Node B

Application                         Application
                   Peer Protocols
Presentation                        Presentation
                   Peer Protocols
  Session                             Session
                   Peer Protocols
 Transport                           Transport
                   Peer Protocols
  Network                             Network
                   Peer Protocols
 Data Link                           Data Link
                  Peer Protocols
  Physical                            Physical

     Packet creation process
                                      Data                       Data


                                              Application Protocol Data Unit
        Application Layer                                                          Application Layer

     Presentation Service Data Unit

        (IDU=PDU+PCI+ICI)                    Presentation Protocol Data Unit
                                                                                   Presentation Layer
       Presentation Layer
       Session Service Data Unit

        (IDU=PDU+PCI+ICI)                      Session Protocol Data Unit
                                                                                     Session Layer
          Session Layer
      Transport Service Data Unit

        (IDU=PDU+PCI+ICI)                      Transport Protocol Data Unit
                                                                                    Transport Layer
         Transport Layer
       Netw ork Service Data Unit

        (IDU=PDU+PCI+ICI)                       Network Protocol Data Unit
                                                                                     Network Layer
          Network Layer
      Data Link Service Data Unit

                                               Data Link Protocol Data Unit
                                                                                    Data Link Layer
         Data Link Layer
       Physical Service Data Unit

        (IDU=PDU+PCI+ICI)                       Physical Protocol Data Unit
                                                                                     Physical Layer
          Physical Layer


     OSI Layer                                        Function
                     Serves as a window or user interface for applications to access
                     standardized network services. Initiates or accepts a request from an
                     application (such as Microsoft Word) through a mechanism written into
                     the application called an Application Programming Interface (APIs).
Application Layer
                     Handles general network access, flow control and error recovery.
                     LAN Connectivity Components: Gateways
                     Internet Protocols: DHCP; DNS; WINs; ARPA Services including HTTP,
                     FTP, TFTP, Telnet, and SMTP; SNMP Services
                     Serves as the network's translator (architecture independent data transfer
                     format). Determines format for data by converting ASCII code, pictures
                     (JPEG GIF BMP) or video (MPEG) into a form that can be sent across the
                     network (binary 1s and 0s). Responsible for data encryption and
Presentation Layer   decryption and data compression and decompression. Responsible for
                     protocol conversion. The redirector (a software mechanism that
                     determines where to send the data; either to the local computer or the
                     network) operates here.
                     LAN Connectivity Components: Gateways, Redirectors
                     Allows applications on two PC's to connect, establish, maintain and end a
                     session. Provides synchronization between communicating computers by
                     placing checkpoints in the data stream so that if a session is interrupted
                     the two computers can re-establish the connection at the point of
Session Layer
                     interruption. The “Traffic Cop” of the network, it determines who can
                     communicate first and for how long before it must stop and allow a
                     LAN Connectivity Components: Gateways
                     Responsible for packet handling. Takes the datagram received from the
                     upper three layers and segments it into the maximum transmission unit
                     size (MTU) that the network in use can accept. Ensures error-free
                     delivery. There are two ways that data can be transported across the
                     network. The first way is to use a connection-oriented transport protocol
                     such as TCP or SPX which requires a connection be established between
                     the sender and receiver before communication can take place. TCP or
                     SPX then assigns a sequence number to each segment so that they can
                     be put back together at the receiving end if they arrive out of order. The
                     receiving computer must then acknowledge each segment it receives. If
                     absolute integrity of the data transfer is required such as saving a file or
                     printing then a connection-oriented transport protocol is needed. The
Transport Layer      second way data can be transported is to use a connection-less protocol
                     such as UDP which requires no connection between sender and receiver.
                     UDP does not sequence the data nor acknowledge the data received.
                     UDP also provides no guarantee of delivery. UDP is better suited for real-
                     time applications such as video and audio transmissions or for network
                     broadcasts. The Transport layer also takes care of error correction. If a
                     packet is received and has become corrupted in transmission the
                     receiver’s Data Link layer checks the packet for accuracy and will toss it
                     out. The receiver’s Data Link layer will then alert it’s Transport layer to
                     request that packet’s sequence number be retransmitted.
                     LAN Connectivity Components: Gateways
                     Internet Protocols: TCP (maps to DHCP, DNS, LDAP, NetBIOS, and
                     ARPA Services); UDP (maps to DHCP, DNS, and SNMP) SPX; NetBEUI
                     Takes the segments received from the Transport layer and converts them
                     to packets by assigning logical addressing information through routable
Network Layer        protocols (IP or IPX). Determines routes for sending data, packet
                     switching, routing, fragmentation and reassembling of data between
                     networks by using routing protocols (RIP OSPF EIGRP BGP). Resolves

                  logical addresses to physical addresses (ARP) Also responsible for
                  troubleshooting network connectivity problems through the use of the
                  ping and tracert utilities (ICMP).
                  LAN Connectivity Components: Routers, BRouters, Layer 3
                  Switches, Gateways
                  Internet Protocols: RIP, OSPF, IP, ICMP, ARP - Address Resolution
                  Protocol - each NIC has an ARP cache that is used to map IP
                  addresses to the NIC
                  Other Protocol Suite Components: IPX, NWLink, NetBEUI
                  Takes packets received from the Network layer and converts them into
                  the proper frame type for the network by encapsulating them with a
                  header containing the source and destination physical (MAC) addresses
                  and by adding a trailer called a frame check sequence (FCS) which
                  contains and error checking algorithm called the cyclic redundancy check
                  (CRC). The CRC basically takes a snapshot of the data at the sending
                  end and the Data Link layer at the receiving end compares the snapshot
                  to the actual data received to determine it’s accuracy. If the two snapshots
                  don’t match the packet is tossed and the Data Link layer alerts the
Data Link Layer   Transport layer to request a re-send of the data.
                  The Data Link layer is divided into two sublayers:
                           Logical Link Control (LLC) – Builds the correct frame type
                            (802.3 802.5 802.11 etc.) LLC defines use of Service Access
                            Points (SAPs)
                           Media Access Control (MAC) – Appends the MAC address and
                            the FCS to the frame. Direct communication with the NIC.
                  LAN Connectivity Components: NICs, Bridges (MAC), Switchs,
                  Brouter, Gateways

                  Packets sent as a bit stream. Binary 1s and 0s are converted by the
                  network card into precise voltages, light pulses, or radio waves.
                  LAN Connectivity Components: NICs, Modems, Repeaters, Hubs,
                  Amplifiers, Multiplexers, Cable and Media, Connectors
Physical Layer
                  LANs: Protocols
                    Ethernet: 10BaseT, 10Base2, 10Base5, 10BaseF, 10Broad36,
                    100BaseT, 100BaseF,1000BaseT, 1000BaseF
                    Token Ring: STP 4/16 mbps; UTP 4/16 mbps; Fiber Optic
                  WANs: ATM, ISDN, Modems and Standards


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