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Thread: Network Basics 1: the OSI-model

  1. #1
    Join Date
    Aug 2001

    Post Network Basics 1: the OSI-model

    The OSI-model.

    1. General.

    One of the most important networking standardisations. It makes abstraction from the communication between transmitter and receiver (meaning it can be implemented for most of existing networking protocols).

    2. 7 Layers.

    The OSI-model contains 7 Layers --> 7 Layers on the T(x) side, and the same 7 Layers on the R(x) side.

    Application Services (the application determines which functionality is adressed):

    7. Application Layer
    6. Presentation Layer
    5. Session Layer
    4. Transport Layer
    Communication Services (independent from application)

    3. Network Layer
    2. Datalink Layer
    1. Physical Layer.

    Their most important functions

    Layer 7: - Interface between user application and network services
    Layer 6: - Exchange of usefull data (data that can be used by applications)
    Layer 5: - To make sure a request for a special service is responded to correctly.
    Layer 4: - Flow control, multiplexing (multiple applications can use the same connection), error checking (transmissionproblems, cfr. Ack) and error recovery (initiate actions to resolve the problem). Splits the data into frames.
    Layer 3: - Logical adressing (IP-adress!), packet switching (every packet is assigned the same destination adress, and every packet is optimally routed. Packet sequence numbers are used to reassemble the packets).
    Layer 2: - Fysical adressing (MAC-adress); this is where the network topology is situated (Ethernet, FDDI, ...).
    Layer 1: - Encoding / decoding (for digital communication), modulation / demodulation (for analog communication).

    So let's say you want to open a Word-document located on a server in your network.
    Your request goes from Layer 7 on your side (Tx) down to Layer 1 on your side. From there, it goes over the cable to Layer 1 from the server side (Rx), up to Layer 7 (Rx). The information requested is then sent back the opposite way.

    This proves the use of the OSI-model: different protocols (TCP/IP, IPX/SPX,...) can be used on the topology (ethernet, ATM, FDDI,...) of your choice.

    70 % of todays LAN networks being TCP/IP over ethernet, I'll discuss those in further detail.

    3. Ethernet (Datalink Layer topology).

    Based on broadcastst and the CSMA/CD-principle (Carrier Sense, Multiple Access / Collision Detect):

    Carrier Sense: your NIC checks to see if there's a heartbeat (a carrier wave) on the network.
    Multiple Access: Ethernet uses a bus-structure (meaning every host on the network sees all packets on that network). Every communication is proceeded by a broadcast to get to know the R(x) IP-adress. --> T(x) sends a MAC-broadcast requesting for the host with the IP-adress of his choice.
    Collision Detect: in case of collision, hosts involved will retransmit after a random period of time.

    4. TCP/IP (Network Layer protocol).

    I know TCP/IP has been dealt with in another thread, but here it is, implemented on the OSI-model.

    TCP - Transport Layer: Transmission Control Protocol, connection oriented.

    Conection oriented meaning there's a guarantee on data exchange via Acks: T(x) sends a SYNchronous Sequence Number to R(x). R(x) responds with a SYN ACK(nowledge). T(x) responds with an ACK to R(x), along with the first data --> Three-way handshake. The real datatransmission begins, T(x) regularly sending an ACK. Communication is closed by a CLOSE request, acknowledged by FIN.

    This is in contrast to connection less (UDP for example). UDP (Transport Layer) also is a TCP/IP-suite protocol.

    IP - Network Layer: the IP-protocol encapsulates TCP or UDP segments into IP-packets: the IP-header contains both the T(x) and R(x) IP-adresses. IP is a packet delivery service: it uses higher layers to check reception of the packets, and lower layers to actually send the packets. IP is connection less.

    Other TCP/IP-suite protocols:

    Adress Resolution Protocol (ARP): link between MAC-adress and IP-adress.

    Internet Control Message Protocol (ICMP): protocol for control and errormessages. Network devices use an IP-packet (carrying ICMs) to inform each other about congestions, rerouting,...). Ping (echo request and echo reply) is an example of it.

    Suggestions for upcoming tutorials, contributions, ...? Negative@skynet.be or PM me...

  2. #2
    Join Date
    Jul 2001
    Nice post, i was just about to post a little background on the OSI model but you beat me to it hmm what to do now...

  3. #3
    Join Date
    Aug 2001
    LOL ! Maybe you can start with the networkcomponents (routers, bridges, hubs,...). I'm working on that, too, so here's your chance to beat me

  4. #4
    Junior Member
    Join Date
    Nov 2003

    good read

    Like the post it's alot more simpler then the way cisco explains it. Nice document.

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