Data Communication Network Basics · Topic 1 of 3
Networks, Devices and the Communication Process
Learn what a data communication network is, what switches, routers, firewalls, ACs and APs do, how data travels, and the network engineer role for H12-811.
- 5 min
- Switches forward frames by MAC address. Routers forward packets by IP address and split broadcast domains.
- Each switch port is a separate collision domain. All ports in one VLAN share one broadcast domain.
- Encapsulation adds headers top-down at the sender. Decapsulation removes them at the receiver.
What it is
A data communication network is a set of devices joined by links that carries data between terminals such as PCs, phones, servers and cameras. Routers, switches, firewalls, wireless access controllers (ACs) and access points (APs) form the network, and the terminals are the endpoints that send and receive data.
The communication process is how one host's data is wrapped, carried hop by hop and unwrapped by the receiver. Network engineering is the work of planning, building and running that network, done by network engineers.
How it works
Network scope and topology
| Type | Coverage | Typical example |
|---|---|---|
| LAN | One building or campus | Office floor switches |
| MAN | A city | City operator ring |
| WAN | Countries or continents | Carrier backbone, the Internet |
Common topologies are bus, star, ring, tree, partial mesh and full mesh. Star is the usual campus access design because one failed cable affects only one terminal. Full mesh gives the most redundancy but needs n(n-1)/2 links for n nodes, so it is costly.
The devices and what each does
| Device | Works mainly at | Main job |
|---|---|---|
| Switch | Data link layer (Layer 2) | Forwards frames by destination MAC address inside a LAN; each port is its own collision domain |
| Router | Network layer (Layer 3) | Connects different networks, selects paths by IP address, separates broadcast domains |
| Firewall | Network layer and above | Controls traffic between security zones by policy |
| AC | Management of WLAN | Manages and configures Fit APs centrally |
| AP | Wireless access | Gives wireless terminals access to the wired network |
A Layer 3 switch combines switching with IP routing between VLANs. The exam usually asks which device isolates broadcast domains (the router) and which forwards by MAC address (the switch).
Collision and broadcast domains
A collision domain is the set of devices whose transmissions can collide. A hub puts every port in one collision domain, while a switch gives each port its own.
A broadcast domain is the set of devices that receive a broadcast frame. A plain switch forwards broadcasts out every port in the same VLAN, so a router or VLAN boundary is needed to split it.
How data travels
The sender's application creates data, and each layer adds its own header on the way down. This is encapsulation. Each device on the path reads only the headers it needs, and the receiver removes them layer by layer, which is decapsulation.
On the way, a switch reads the frame's MAC addresses. A router removes the old Ethernet header, reads the IP header, chooses the next hop, and builds a new frame for the next link. The source and destination IP addresses stay the same end to end, while MAC addresses change at every Layer 3 hop.
Network engineering and network engineers
Huawei describes a network project as a life cycle of phases.
| Phase | What happens |
|---|---|
| Planning and design | Gather requirements, choose topology, devices and addressing |
| Deployment and implementation | Install, cable and configure devices, then test |
| Operations and maintenance (O&M) | Monitor, back up configurations, handle faults |
| Optimization | Improve performance and adjust as the business changes |
A network engineer needs skills in theory, device configuration, troubleshooting and documentation. The Huawei certification path for datacom runs HCIA (associate), then HCIP (professional), then HCIE (expert).
A worked example
PC1 (192.168.1.10) on a switch sends a web request to a server on another network. PC1 builds a frame with the router interface's MAC as the destination, because the server is not on its subnet.
The switch forwards the frame by MAC to the router. The router strips the frame, checks its routing table, and sends a new frame on the next link. The server finally decapsulates the frame, packet and segment and hands the HTTP data to its web service.
How the exam asks about this
- "Which device can isolate broadcast domains?" → The router (or a Layer 3 interface), not a Layer 2 switch.
- "Which of the following statements about forwarding across a router is true?" → The IP addresses stay the same and the MAC addresses change at each hop.
- "Which topology provides the highest reliability but the highest cost?" → Full mesh.
What to remember for the exam
- Switches forward frames by MAC address. Routers forward packets by IP address and split broadcast domains.
- Each switch port is a separate collision domain. All ports in one VLAN share one broadcast domain.
- Encapsulation adds headers top-down at the sender. Decapsulation removes them at the receiver.
- The AC manages Fit APs. The AP gives wireless terminals access.
- Huawei certification levels in order: HCIA, HCIP, HCIE.
Do not say a router changes the destination IP address of a normal routed packet. Only the Layer 2 header is rebuilt at each hop, unless NAT is in use.
Common mistakes
Mixing up collision and broadcast domains. A switch splits collision domains but not broadcast domains. A router or VLAN boundary is needed for that.
Thinking a frame crosses the whole path unchanged. The frame lives only on one link. The router builds a new frame with new MAC addresses for the next link.
Treating the firewall as a switch. A firewall filters traffic between security zones by policy. Its job is control, not fast LAN switching.
Check yourself
Which device forwards data by MAC address inside a LAN?
The Layer 2 switch, which looks up the destination MAC address in its MAC address table.
What changes when a packet passes through a router: the IP addresses or the MAC addresses?
The MAC addresses change on each link, while the source and destination IP addresses stay the same.
In which project phase are devices monitored and faults handled?
The operations and maintenance (O&M) phase, after deployment and before optimization.
Topic 1 of 3 in Data Communication Network Basics
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