Computer Networks Course - Class 03

Hi, let's go to the next topics: 

 

1 - What are the main network equipment? 

2 - Collision domain? Broadcast? 

3 - What is the OSI model? 

 

1 - Starting with network equipment, the equipment we have available is: 

- Network Cards 

- Concentrators or Hubs 

- Repeater 

- Switches 

- Bridge 

- Routers 

- Gateways

 

Starting with the network cards wikipedia tells us: 

 - A network card (also called a network adapter or NIC, for Network Interface Card) is a hardware device responsible for a computer's communication on a computer network. The network card controls the sending and receiving of data from a computer connected to a network, through electromagnetic waves (Wi-Fi), metallic cables or fiber optic cables. 

Each board has a unique physical identifier or address called a MAC. This address is made up of 48 bits and is expressed in hexadecimal form, for example: 24:6E:2A:91:41:D1. The first half of the address identifies the board manufacturer and the second half a serial number. They basically act on the second layer of the OSI model. 

 

- Hubs are network interconnection devices, they form a star-type physical topology but a bus-type logical topology because all the machines connected to the hub are in the same collision domain, which we will see later. Therefore, communication using a Hub is Half-Duplex and packets sent in a communication will be forwarded to all nodes connected to the hub. Hubs can be active, when they amplify the signal, or passive, when they just retransmit the signal. 

 

- The repeater is the device that amplifies the signal in a network allowing to extend the network to greater distances. 

 

- A switch is a device that simply connects all elements of your network. It acts as a bridge or control unit so that computers, printers, servers and all other types of devices can communicate. 

The switch segments the collision domains, each of its ports represents a collision domain, when it comes to a switch it usually means a layer 2 switch, in the OSI model, the layer 2 switch works with MAC addresses, each NIC of a computer that is connected to a port on the swich has a MAC address and when the computer sends a packet that passes through the switch, the switch "learns" this MAC address, it has an internal table where it associates a MAC address for each port yours, and then when there is communication with a computer that has the MAC of your network card already registered in the switch, the switch does not send the packet in broadcast form, it only sends to the port and MAC of this port, so each port has only one collision domain. 

The switch generates 3 types of traffic, broadcast, multicast and unicast. The table where the switch keeps MAC addresses is called CAM (Content Addressable Memory). There are switches that operate at OSI layer 3 that are capable of routing packets. There is also the concept of VLAN where the switch is able to segment the broadcast domains, we will deal with this concept later. 

 

- A bridge has some motivations for its use, such as separating the traffic between the two network segments and matching different network standards that may be used. That is, traffic will only pass from one segment to another if there is communication between devices from different segments. 

 

- Routers are layer 3 devices, they perform routing based on the IP address, which we will talk about later, they have a smaller number of ports compared to the switch. Routers have the ability to connect different networks and do this through routes that are manually placed in the router or can dynamically learn routes to other networks where a routing algorithm such as the router must then be configured in the router or group of routers. OSPF, RIP and more. The router segments the collision and broadcast domains on each of its ports. It can interconnect networks with different technologies, for example on one port there is ethernet technology and on another token ring and it can make the networks of each of these ports communicate. 

 

- Gateways act on layer 7 of the OSI model and their basic function is to specifically address the services offered in the application layer. It is used for communication between different protocols and existing platforms. It is built specifically for each application, thus having very specific name terminology, such as voice gateways (interconnecting TDM and IP telephony networks).

 

 

 

 

2 - Collision domain and broadcast domain 

 

COLLISION DOMAIN: It is a domain in which only one can transmit at a time and all receive the information that is sent on the network even if it is not intended for it. Since it can be transmitted one at a time, then if “A” is transmitting to “B”, “C” cannot transmit anything to “D”, which could lead to frame collisions. The bandwidth available for the medium is shared by everyone and always limited by the one with the lowest speed. It has the perspective up to the second layer of the OSI model. 

 

BROADCAST DOMAIN: It is a logical network domain that identifies devices belonging to the same network. In this situation, there is no need for inter-network routing and packets will only be routed to all others if there is an express command for such activity. A cluster of collision domains is formed.

  

 

3 - What is the OSI model? 

 

The OSI (Open System Interconnection) model is an ISO reference model divided into layers and functions and aims to be a standard for computer network communication protocols. The model divides the network communication process into 7 layers where each layer has its functions and the model says what each layer should do. Each layer represents an abstraction. 

Each layer has its own protocols that define methodologies and standards so that two computer systems can communicate.

 

The OSI model was created under three well-defined main concepts: 

• Services – Each tier provides services to the upper tier by providing certain features. In this sense, it defines which services will be provided. 

• Interfaces – Determines the form of interaction between layers, that is, how the upper layer can access the lower layer to use services. Parameters to be provided are defined, as well as the expected results according to the offered resources. 

• Protocols – It is the implementation of the services itself. Protocols can fully or partially implement certain services. 

 

The model was defined in order to define generic characteristics or directives so that computer networks could be built and developed according to their diverse needs and specificities. 

 

The layers of the OSI model are: 

•  Application 
•  Presentation 
•  Session 
•  Transport 
•  Network 
•  link 
•  Physics

 

We can say that the upper 4 layers are considered host layers and the lower 3 layers are considered transmission media layers. The lower layers use encapsulation techniques in order to abstract the information from the upper layers. This set is given by the portion of data from the upper layer plus the header of the respective layer, we call PDU (protocol data unit – protocol data unit). 

 

Below is an example image of the model:

 

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