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2.3 ISO/OSI Layer Model

According to the pure doctrine the ISO/OSI layer model, technically separates a computer network into seven layers (see Fig. 2.4).

Table 2.1 OSI layer

OSI layer

Layer name

Task

1

Physical

Cables, Antennas, etc.

2

Data-Link

Creates a point-to-point connection between two computers

3

Network

Provides for addressing of the destination system

4

Transport

Takes care that the data is received in the

right order and enables retransmission on packet loss

5

Session

Used to address single applications (e.g. using ports)

6

Presentation

Conversion of data formats (e.g. byte order, compression, encryption)

7

Application

Protocols that define the real service like HTTP

Each layer has a clearly defined task and each packet passes them one after another in the operating systems kernel up to the layer it's operating on (Table 2.1).

2.4 Ethernet

Have you ever bought a “normal” network cable or card in a shop? Than the chance is nearly 100 % that you own ethernet hardware, because Ethernet is with huge margin the most used network technology today. You will see network components with different speed limits like 1, 10, 100 MBit or gigabit and an ethernet can be constructed with different cable types like coaxial (old school), twisted pair (common) or glass fiber (for data hungry guys).

Twisted pair cables can be divided into to the variations STP (Single Twisted Pair) and UTP (Unshielded Twisted Pair) as well as patchand crossover cables.

The difference between STP and UTP cables is that the fibers of the UTP cables are unshielded and therefore they have a lower quality compared to STP cables. Nowadays new cables in a shop should all be STP.

Patch and cross cables can be separated from each other by looking at the plugs of the cable. If the colors of the fibers are in the same order than its a patch otherwise a cross cable. A cross cable is used to directly connect two computers, a patch cable is used to connect a computer to a hub or switch. Modern network cards can automatically cross the fibers so cross cables are a dying race.

Every network card in an Ethernet network has a MAC address that's worldwide unique and are used to address devices on the net. The MAC address consists of six two digit hexadecimal numbers, which are separated by colons (e.g. aa:bb:cc:11:22:33).

Its a common misbelief that a computer in a local TCP/IP network is reached over

its IP address; in reality the MAC address is used for this purpose. Another common misunderstanding is that the MAC address cannot be spoofed. The operating system is responsible to write the MAC into the Ethernet header and systems like GNU/Linux or *BSD have possibilities in their base system to change the MAC with one command.

ifconfig eth0 hw ether c0:de:de:ad:be:ef

Fig. 2.5 Ethernet header

Fig. 2.6 VLAN header

Beside the source destination MAC address an Ethernet header (see Fig. 2.5) consists of a type field and a checksum. The type field defines the protocol that follows Ethernet e.g. 0x0800 for IP or 0x0806 for ARP.

Last but not least the term CSMA/CD should be explained. CSMA/CD stands for Carrier Sense Multiple Access/Collision Detect and describes how a computer sends data over an Ethernet. First of all it listens on the wire if someone is currently sending something. If that's the case it just waits a couple of random seconds and tries again. If the channel is free it sends the data over the network. Should two stations be transmitting data at the same data a collusion will result, therefore every sending station must listen afterwards to detect a collusion, than randomly wait some seconds and retransmit the data.

 
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