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.
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