Hi there,
This is the very first of our HCIA-Datacom authentication course. We would like to introduce to you some Datacom concepts, which would help you understand what the datacom is.
Datacom, or data communication, is the combination of communication technology and computer technology. Assimilate the information or data to the vehicles between cities, while the datacom would be focusing on building the highway to vehiclery these vehicles.
As the cities interconnection above the picture, we could draw the same connection network as the figure below:
We'll introduce the concepts compare to figure 1.
Bandwidth:
We usually define the bandwidth as the net bit rate. The bandwidth is measured in bit/s, or more commonly used Mbit/s. The bandwidth defines the total data that could pass through simultaneously. Unlike the data rate, bandwidth indicates the maximum throughput of the link, while the data rate represents the current using bandwidth. Compared to figure 1, bandwidth is the whole lane of the road, while the data rate is the current using lanes. For example, there are two lanes between city A and city C, if one lane is occupied, then, the bandwidth would be 2, and the data rate is 1. What if all lanes are occupied, then the data rate comes to the same as bandwidth, which is 2.
More bandwidth can carry more packets simultaneously
Packets:
Packets are the minimum unit of the transferred data. It's like the vehicles on the road. As the vehicles are in different sizes, packets also have different sizes, this is due to the payload carried in the packet, same as the different types of goods claim the different types of vehicles.
Packet header:
Each packet can be divided into two parts, header, and payload. The packet header has a fixed structure to identify the packet from other packets. The payload is the data needed to be transferred. Packet header could be considered as the vehicle itself, and the payload is the goods that are carried on the vehicle.
Port:
As we know, we could identify the different vehicles through the license plate number. Then how can the network devices identify the packets? Port is introduced to help to do that. Usually, the port is carried in the packet header, and it's a number range from 1 to 66535. To mark the well-known protocols, we retain port ranges 1 to 1023 for them. For example, for the HTTP service, we use TCP 80, TCP 443 for HTTPS, 22 for the SSH, and so on.
Route:
To transfer the goods between cities, we need to know how to get to the destination city. Usually, we could learn that from a map. The map is the route in datacom. A route indicates where the packet should be delivered so that it could get the correct destination. A set of routes is the routing table. In the routing table, all available routes are stored, so that the devices could forward the packets normally. If there are no routes for some specific packets, then the device would drop these packets, just as you stop your vehicle when don't which road should you go.
Data plane & control plane:
In the real-life, map draws the way and vehicles carry the goods based on the selected way on the map. The same as that relationship, we use the data plane and control plane to perform the different functions. The control plane works on learning the routes, and the data plane focus on data transferring. What should be paid attention, the direction of data plane flow and control plane flow is opposite. That's easy to understand, before we traveling between cities, we should know how to get to the destination firstly. In the datacom, the destination devices will advertise their location so that the rest could learn how to reach them, while transferring data, the data will flow in opposite direction. Just as the figure below draws.
