Network Basics 1: the OSI-model
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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, ...? 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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