What Every Beginner Must Know About Routers, Switches, Modems, and Wi‑Fi

Introduction

Home networks typically consist of an optical modem (光猫), a router, a switch, and Wi‑Fi. Many beginners cannot distinguish these devices, so this article provides a clear, step‑by‑step explanation of each component and how they work together.

Network Layer Overview

Data transmission follows the OSI model. When a packet travels from computer A to computer B, it passes through multiple layers that encapsulate and later decapsulate the data. The physical layer is represented by the Ethernet cable that connects the two devices.

Router

A router (Router) connects different networks (LAN, WAN, Internet) and selects the optimal path for each packet. Internally it has a control plane (software) and a data plane (hardware). Incoming electrical signals are converted by the PHY module, then by the MAC module into digital frames, which are checked for errors and addressed. If the destination MAC address matches the router, the frame is stored in memory; otherwise it is discarded.

The software routing processor looks up the routing table to decide the outgoing interface. If no matching entry is found, the router drops the packet and sends an ICMP message back to the sender. Routers also perform address translation and can be combined with switching functions in modern “switch‑router” devices.

Router vs. Optical Modem

An optical modem (光猫, also called a modem) converts optical signals from the fiber link into electrical signals that can be processed by the router. It does not perform routing; it simply demodulates the incoming light signal and modulates outgoing data back onto the fiber.

Switch

A switch (Switch) operates at the data link layer (Layer 2). It provides many ports for connecting devices in a star topology, learns MAC addresses, and forwards frames only to the appropriate port, reducing collisions and increasing bandwidth.

Switches can be classified by scale and application:

Enterprise‑grade switches (rack‑mount, high port count)

Department‑grade switches (mid‑range, may be rack‑mount or fixed)

Workgroup switches (desktop‑style, limited features)

In essence, a switch is a multi‑port bridge, while a bridge is a two‑port device. Modern switches also support VLANs, link aggregation, and other advanced features.

Optical Modem (光猫)

The optical modem, also known as a fiber‑to‑the‑home (FTTH) modem, is a type of modem that converts optical signals to electrical signals and vice versa. It performs modulation (digital‑to‑analog conversion) for transmission over fiber and demodulation (analog‑to‑digital conversion) for reception. The modem connects to the router via Ethernet, allowing the router to handle IP routing and network services.

Wi‑Fi

Wi‑Fi is a wireless LAN technology defined by the IEEE 802.11 family. It operates mainly in the 2.4 GHz and 5 GHz ISM bands. Different amendments (b, a, g, n) specify frequency ranges, channel widths, modulation schemes, and maximum data rates.

IEEE 802.11 Overview

802.11b – Released Sep 1999 – 2.4 GHz – 11 Mbps PHY rate – 3 non‑overlapping channels – CCK/DSSS modulation.

802.11a – Released Sep 1999 – 5 GHz – 54 Mbps – 24 non‑overlapping channels – OFDM modulation.

802.11g – Released Jun 2003 – 2.4 GHz – 54 Mbps – 3 non‑overlapping channels – OFDM/CCK/DSSS.

802.11n – Released Sep 2009 – 2.4 GHz/5 GHz – up to 600 Mbps – 20/40 MHz channels – MIMO‑OFDM.

2.4 GHz Channel Layout (China)

The 2.4 GHz band is divided into 13 channels, each 22 MHz wide, with only channels 1, 6, and 11 being non‑overlapping. The table below lists the center frequency and the lower/upper edge frequencies for each channel.

Channel  Center (MHz)  Low/High (MHz)
1        2412          2401/2423
2        2417          2406/2428
3        2422          2411/2433
4        2427          2416/2438
5        2432          2421/2443
6        2437          2426/2448
7        2442          2431/2453
8        2447          2436/2458
9        2452          2441/2463
10       2457          2446/2468
11       2462          2451/2473
12       2467          2456/2478
13       2472          2461/2483

Receiver Sensitivity (802.11b/g/n)

Typical minimum signal levels required for different data rates (PER ≤ 8 %):

Rate   Min Signal (dBm)
6 Mbps   -82
9 Mbps   -81
12 Mbps  -79
18 Mbps  -77
24 Mbps  -74
36 Mbps  -70
48 Mbps  -66
54 Mbps  -65

SSID and BSSID

A Basic Service Set (BSS) is a group of stations that can communicate directly. An Extended Service Set (ESS) consists of multiple BSSs that share the same SSID, allowing roaming between access points. The SSID identifies the network (e.g., "TP_Link_1201"). The BSSID is the MAC address of the access point that uniquely identifies each BSS.

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