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WiFi and networking: dBm, DHCP, IP and DNS

Module 4 — IoT Platform Connectivity · Slides: slides.md · Module overview · Course page

Open up what happens while the board is “connecting”, from signal strength in dBm through joining the network, DHCP and the IP address, to why you ping two targets so you can say where the network breaks, not just that it broke.

Objectives

Section titled “Objectives”

By the end of this lesson you will be able to:

  1. Compare two signal strengths in dBm with the rule “every 10 dB is 10×, every 3 dB is about 2×” without a calculator, e.g. state that −50 dBm is 1000× stronger than −80 dBm, and place the board’s network against the field thresholds (better than −60, −67, below −80)
  2. Put the five steps from the board asking for access points to receiving an IP address, gateway and DNS in the right order, and say that steps 1–3 are WiFi while steps 4–5 are IP via DHCP (DORA), which fail in different ways
  3. Interpret a gateway ping paired with an 8.8.8.8 ping correctly in all three cases (both pass, gateway passes but internet fails, gateway fails) and say where the fix belongs
  4. Explain two limits of the board’s wifi module: wifi.ping() accepts IP numbers only because no resolver is exposed to Python, and netmask and gateway cannot be read, so code guesses the gateway as .1 and proves the guess with ping

Before you start

Section titled “Before you start”

Review lessons 1.4–1.6, where we called wifi.connect() and got online successfully, and picture lessons 3.7–3.9’s dashboard, because lessons 4.1–4.3 will attach a network status page to that same screen. This lesson has no code to write yet. Have your learning log ready to record numbers read off the screen, and have ready the name and password of the home WiFi or phone hotspot you will connect the board to (set up per the table in lesson 1.4: an English name with no spaces, a password of at least 8 characters, the 2.4 GHz band).

See it work first

Section titled “See it work first”

Open the Wi-Fi Setting menu that ships with the board (we first met it back in lessons 1.1–1.3), and watch three steps on screen. First, scanning shows network names with strength in dBm. Second, choosing a network and entering the password makes the screen sit still for a while — that is waiting, not hanging. Third, getting an IP address lights the icon in the top bar, meaning the board now has an address. These three steps are three lines in our own code: wifi.scan() · wifi.connect() · wifi.ip()

Concepts

Section titled “Concepts”

“Connected” and “usable” are not the same thing. Lessons 1.4–1.6 were a tour of the whole path; this set of lessons opens that same box back up to see what a line that ran successfully actually passed through, why it is sometimes alarmingly slow, and what the numbers it returns mean. The destination in lesson 4.3 is your team’s network status page: a table of SSID · dBm · channel · security, sorted strongest to weakest, two link-status lamps, a ping to two targets, a dBm gauge ranging −90 to −40, and a rescan button.

dBm is real power compared with 1 mW on a logarithmic scale. $P_{\text{dBm}} = 10\log_{10}(P / 1\ \text{mW})$. At −67 dBm, the antenna receives about 0.2 nanowatts, while 0 dBm is exactly 1 mW. Values on the board are therefore always negative. Remember just two rules: every 10 dB is 10×, and every 3 dB is about 2×. The Wi-Fi Setting screen uses five bars at thresholds of −50 / −60 / −70 / −80 / −90, while network technicians use better than −60 for comfortable everything, −67 as the limit for video or voice, and below −80 do not trust it. The 5 GHz band is not “better” than 2.4 GHz — it trades range for speed. At 10 metres, the FSPL of channel 36 (5180 MHz) is 6.5 dB more than channel 6 (2437 MHz), leaving about a quarter of the power, and a concrete wall costs another 10–15 dB. Scanning listens channel by channel one at a time, so wifi.scan() can block for up to 10 seconds — show “scanning” text before calling it, never after.

From radio wave to IP address takes five steps. Steps 1–3 are WiFi: ask who is around here · the AP answers with its name, strength and channel · ask to join, proving the password. Steps 4–5 are IP: ask for an address via DHCP (Discover · Offer · Request · Acknowledge), then receive an IP address with a gateway and DNS. This number is a temporary loan (a lease); power-cycling the board may give a different one, so code must read wifi.ip() fresh every time after connecting. wifi.connect() returns True only after all five steps succeed; failing at any of them gives the same False. If the network’s DHCP is full or broken, we join the WiFi but get no IP address at all, which looks exactly like a failed connection even though it is not.

Ping two targets, and read the results as a pair. wifi.ping(ip, timeout_ms) returns the round-trip time in milliseconds, or −1 when there is no answer. Gateway passing and internet passing is normal · gateway passing but 8.8.8.8 silent means the problem is upstream of our router, not on our side · gateway failing points to the WiFi link or a wrongly guessed gateway number. MicroPython on this board only gives wifi.ip() — there is no netmask or gateway to read. So code guesses the gateway from the IP’s first three octets followed by .1, which is correct on most home networks but not all, and wifi.ping() only accepts an IP number — passing "google.com" raises ValueError, because no name resolver is exposed to Python yet. This is a gap in the module, not the hardware. We choose 8.8.8.8 because it is easy to remember and almost never goes down.

Our data is wrapped layer by layer before it goes on air, and each layer adds size and can fail on its own — good radio signal but no IP really can happen. wifi.connect() handles the lower layers for us; wifi.ping() measures the middle layer; and in lessons 4.4–4.6 we will write the top layer ourselves, with MQTT.

Check your understanding

Section titled “Check your understanding”

The same questions are in quiz.yaml for automatic marking.

  1. Network A measures −50 dBm, network B measures −80 dBm. About how many times more power does the antenna receive from A than from B? (choose one · objective 1)

    • A) 30 times, because the numbers differ by 30
    • B) 1000 times
    • C) About 1.6 times
    • D) 3 times
    Solution

    B — The difference is 30 dB, and every 10 dB is 10×, so 10 × 10 × 10 = 1000 times. dBm is a logarithmic scale, so a small difference in the number can mean a huge difference in real power.

  2. Put what happens while wifi.connect() runs in order, from first to last. (order · objective 2)

    • A) The board asks to join the network, proving the password
    • B) The router gives an IP address, with a gateway and DNS
    • C) The board asks which access points are around here
    • D) The board asks for an address via DHCP (DISCOVER / REQUEST)
    • E) The AP answers with its name, strength and channel
    Solution

    C → E → A → D → B — The first three steps are WiFi, and by the end of step 3 the board still has no IP address. The last two steps are IP via DHCP. Whichever step fails, connect() returns the same False, which is why ping is needed to tell apart where it actually failed.

  3. The status page shows the gateway ping at 3 ms, but the 8.8.8.8 ping times out. Which conclusion is correct? (choose one · objective 3)

    • A) The board’s WiFi link dropped; reconnect
    • B) The problem is upstream of the router, not on our board
    • C) The guessed gateway of .1 is wrong
    • D) You should ping “google.com” instead of 8.8.8.8
    Solution

    B — The gateway answering means the WiFi link and the guessed gateway number both work. Silence from a destination outside the home means the break is beyond the router. A silent gateway, on the other hand, points to the WiFi link or a wrongly guessed gateway.

  4. Which statements about the wifi module on this board are correct? Choose every correct one. (choose all that apply · objective 4)

    • A) wifi.ping(“google.com”) raises ValueError, because no name resolver is exposed to Python yet
    • B) Code can read the real netmask and gateway from the firmware
    • C) The gateway in the code is a guess from the IP address (the first three octets followed by .1), so it must be proven with ping
    • D) The domain-name limitation exists because the radio chip cannot speak DNS
    Solution

    A, C — The board only gives wifi.ip(); there is no netmask or gateway to read, so the code guesses the gateway and fires a ping to prove it. The DNS matter is a gap in the module — the chip itself can speak DNS; nobody has opened that door on the Python side yet.

  5. The board has joined the WiFi network successfully, but the network’s DHCP is full. What symptom would you see? (choose one · objective 2)

    • A) WiFi connects, but there is no IP address, which looks like a failed connection
    • B) wifi.scan() finds no networks at all
    • C) It gets a normal IP address, but the 8.8.8.8 ping fails
    • D) The RSSI value becomes positive
    Solution

    A — DHCP belongs to steps 4–5, which are about IP, not WiFi. The radio signal is good and joining succeeds, but nobody hands out an address, so it cannot talk to anyone outside the room, even though it looks like the connection failed.

Lab

Section titled “Lab”

Read the network around you as numbers (about 15 minutes). Do this on a real board, and record in your learning log.

  • Open Wi-Fi Setting and record the name and dBm value of the three strongest networks, and the weakest one you can see
  • Compute how many dB apart the strongest and weakest networks are, and how many times the power differs, using the rule 10 dB = 10×, 3 dB ≈ 2×
  • Place the network the board connects to against the field thresholds: better than −60 · around −67 · below −80
  • Connect to your home WiFi or hotspot and record the IP address you get, then write down the gateway you “guess” (the first three octets followed by .1), to prove it with ping in lesson 4.3
  • Build a three-row table in your learning log: gateway ping result · 8.8.8.8 ping result · where the problem lies, covering all three cases

Going further

Section titled “Going further”

Lesson 4.2 reads through the network status page’s code move by move, along with the wifi module’s eight names, and the values on the Wi-Fi Setting page that are not from a real measurement.

Next lesson: Lesson 4.2 — The network status screen: reading the wifi code

Reflect

Section titled “Reflect”
  • When a teammate says “the signal is just a little weak”, what number would you ask back for, and what does a 20 dB difference mean in multiples?
  • A shed 300 metres from the house — should it use 2.4 or 5 GHz, and which formula helps decide before buying anything?
  • How is a system that can say “where it broke” worth more than one that only says “it’s broken”, in the work you want to do?

Review questions

Answer on your own first, then open the answer.

  1. Network A reads −50 dBm and network B reads −80 dBm. Roughly how many times more power does the antenna receive from A than from B? (Objective 1)

    1. 30 เท่า เพราะตัวเลขต่างกัน 30
    2. 1000 เท่า
    3. ราว 1.6 เท่า
    4. 3 เท่า
    Show answer

    Answer: B. 1000 เท่า

    ต่างกัน 30 dB และทุก 10 dB คือ 10 เท่า จึงเป็น 10 × 10 × 10 = 1000 เท่า dBm เป็นสเกลลอการิทึม ตัวเลขที่ต่างกันเล็กน้อยจึงอาจหมายถึงกำลังที่ต่างกันมหาศาล

  2. Order what happens while wifi.connect() runs, from first to last. (Objective 2)

    1. บอร์ดขอเข้าร่วมวงและพิสูจน์รหัสผ่าน
    2. เราเตอร์ให้เลข IP มา พร้อมเกตเวย์และ DNS
    3. บอร์ดขอดูว่ามี AP ไหนอยู่แถวนี้บ้าง
    4. บอร์ดขอเลขที่อยู่ผ่าน DHCP (DISCOVER / REQUEST)
    5. AP ตอบกลับด้วยชื่อวง ความแรง และช่องสัญญาณ
    Show answer

    Correct order: C. บอร์ดขอดูว่ามี AP ไหนอยู่แถวนี้บ้าง → E. AP ตอบกลับด้วยชื่อวง ความแรง และช่องสัญญาณ → A. บอร์ดขอเข้าร่วมวงและพิสูจน์รหัสผ่าน → D. บอร์ดขอเลขที่อยู่ผ่าน DHCP (DISCOVER / REQUEST) → B. เราเตอร์ให้เลข IP มา พร้อมเกตเวย์และ DNS

    สามขั้นแรกเป็นเรื่องของ WiFi และตอนจบขั้นที่สามบอร์ดยังไม่มีเลข IP สองขั้นหลังเป็นเรื่องของ IP ผ่าน DHCP ติดขั้นไหนก็ตาม connect() คืน False เหมือนกันหมด จึงต้องมี ping ไว้แยกว่าติดตรงไหน

  3. The status page shows the gateway ping at 3 ms but the 8.8.8.8 ping times out. Which conclusion is correct? (Objective 3)

    1. ลิงก์ WiFi ของบอร์ดหลุด ต้องต่อใหม่
    2. ปัญหาอยู่เหนือเราเตอร์ ไม่ใช่ที่บอร์ดของเรา
    3. เลขเกตเวย์ที่เดาเป็น .1 ผิด
    4. ต้องเปลี่ยนไป ping "google.com" แทน 8.8.8.8
    Show answer

    Answer: B. ปัญหาอยู่เหนือเราเตอร์ ไม่ใช่ที่บอร์ดของเรา

    เกตเวย์ตอบแปลว่าลิงก์ WiFi และเลขเกตเวย์ใช้ได้ ปลายทางนอกบ้านเงียบจึงแปลว่าขาดหลังเราเตอร์ ถ้าเกตเวย์ไม่ตอบต่างหากที่ชี้ไปที่ลิงก์ WiFi หรือเกตเวย์ที่เดาผิด

  4. Which statements about the board's wifi module are true? Choose all that apply. (Objective 4)

    1. wifi.ping("google.com") ได้ ValueError เพราะยังไม่มีตัวแปลชื่อเปิดให้ Python เรียก
    2. โค้ดอ่าน netmask และ gateway จริงจากเฟิร์มแวร์ได้
    3. เกตเวย์ในโค้ดเป็นการเดาจากเลข IP (สามช่องแรกต่อด้วย .1) จึงต้องพิสูจน์ด้วย ping
    4. ข้อจำกัดเรื่องชื่อโดเมนเกิดจากชิปวิทยุคุย DNS ไม่ได้
    Show answer

    Answer: A. wifi.ping("google.com") ได้ ValueError เพราะยังไม่มีตัวแปลชื่อเปิดให้ Python เรียก · C. เกตเวย์ในโค้ดเป็นการเดาจากเลข IP (สามช่องแรกต่อด้วย .1) จึงต้องพิสูจน์ด้วย ping

    บอร์ดให้แค่ wifi.ip() ยังไม่มี netmask และ gateway ให้อ่าน โค้ดจึงเดาเกตเวย์แล้วยิง ping พิสูจน์ ส่วนเรื่อง DNS เป็นช่องว่างของโมดูล ตัวชิปคุย DNS ได้ แค่ยังไม่มีใครเปิดประตูฝั่ง Python ให้

  5. The board has joined the WiFi network, but the network's DHCP pool is full. What symptom do you see? (Objective 2)

    1. ต่อ WiFi ติด แต่ไม่มีเลข IP ซึ่งดูเหมือนต่อไม่ติด
    2. wifi.scan() ไม่เจอวงใดเลย
    3. ได้เลข IP ปกติ แต่ ping 8.8.8.8 ไม่ผ่าน
    4. ค่า RSSI กลายเป็นเลขบวก
    Show answer

    Answer: A. ต่อ WiFi ติด แต่ไม่มีเลข IP ซึ่งดูเหมือนต่อไม่ติด

    DHCP อยู่ในขั้นที่ 4–5 ซึ่งเป็นเรื่องของ IP ไม่ใช่ WiFi คลื่นแรงดีและเข้าร่วมวงได้ แต่ไม่มีใครแจกเลขให้ จึงคุยกับใครนอกห้องไม่ได้ ทั้งที่ดูเหมือนต่อไม่ติด

Cite this lesson

If you teach from this lesson or reuse it in slides or documents, credit it with the text below. If you changed it, add (adapted) after the title.

"WiFi and networking: dBm, DHCP, IP and DNS" from TESA Open Knowledge by the Thai Embedded Systems Association (TESA), https://github.com/tesaiot/tesa-qualification-program, licensed under CC BY-NC 4.0

Thai attribution: "WiFi และเครือข่าย: dBm DHCP IP และ DNS" จาก TESA Open Knowledge โดยสมาคมสมองกลฝังตัวไทย (Thai Embedded Systems Association: TESA) https://github.com/tesaiot/tesa-qualification-program สัญญาอนุญาต CC BY-NC 4.0

Lesson link: https://tesaiot.github.io/tesa-qualification-program/en/courses/aiot-micropython/m04-iot-connectivity/l01-wifi-networking/

This lesson adapts the source below; keep its credit too.
https://github.com/Advance-Innovation-Centre-AIC/embedded-systems-for-aiot-developer/blob/a80bbe88a34bcb9bb8d991f42f9252b77cdab079/session-09.html (slides 1–16)

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TESA Open Knowledge · © 2026 สมาคมสมองกลฝังตัวไทย (TESA) · CC BY-NC 4.0

Content is licensed CC BY-NC 4.0. Reuse it non-commercially and credit the Thai Embedded Systems Association (TESA) every time. · How to cite TESA