Two features 802.11 makes possible without specifying, and a network so small it has no infrastructure at all — and is built almost entirely from parts you have already met.
Words you will meet
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Rate adaptation — changing the modulation as conditions change, using acknowledgements as the signal.
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Power management — sleeping between beacons, with the access point holding your frames.
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Piconet — a Bluetooth network of up to eight active devices, also called a wireless personal area network.
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Master and client — a piconet’s one coordinator, and the devices it admits.
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Parked — admitted but asleep, and unable to communicate until reactivated.
Why this matters
Both 802.11 features here are deliberately not fully specified. The standard provides the mechanisms — the acknowledgement, the power-management bit, the beacon — and leaves vendors to build the policy on top, presumably to compete on it.
That is worth noticing in itself. A standard can specify enough to interoperate while leaving room for one implementation to be better than another.
Then Bluetooth, which is the chapter’s smallest network and its best revision exercise. Almost everything in it is something chapters 3 and 6 already taught.
Rate adaptation
Twenty metres away, with a high signal-to-noise ratio. The station can use a modulation technique that gives a high rate while keeping the bit error rate low. One happy user.
Five steps, and the last is the point. This is section 3.7’s idea appearing at the physical layer, and neither mechanism knows the other exists.
Read all steps as text
- Close to the base station, everything is easy — Twenty metres away, with a high signal-to-noise ratio. The station can use a modulation technique that gives a high rate while keeping the bit error rate low. One happy user.
- They walk away, and the SNR falls — If the modulation does not change, the error rate climbs until eventually no frame is received correctly at all. The link does not degrade gracefully — it stops.
- Two unacknowledged frames → drop a rate — A missing acknowledgement is an implicit report of bit errors. Two in a row, and the transmission rate falls to the next lower one — trading speed for a modulation that survives worse conditions.
- Ten acknowledged in a row → try a higher one — Or a timer since the last fallback expires. Either way the rate goes up one step — a probe, to find out whether conditions have improved.
- Which is TCP congestion control, at the physical layer — The book draws the comparison itself. Push while things go well, back off when they stop, try again later. Its analogy is a child asking for more sweets until a parent says "Enough!" — and trying again once conditions have improved.
In plain words
Look at what the signal is. Nobody reports a bit error. The station infers one from an acknowledgement that did not arrive — silence, interpreted.
Two missing acknowledgements and the rate drops a step. Ten consecutive successes, or a timer expiring, and it climbs one. Push while things go well; back off when they stop; try again later.
That is TCP (Transmission Control Protocol) Transmission Control Protocol The Internet transport protocol that delivers data reliably and in order, with flow control and congestion control. introduced in ch. 1 congestion control from section 3.7, and the book says so outright: the same probing philosophy, four layers further down, with neither mechanism aware the other exists.
Power management
Power is precious on a mobile device, so 802.11 lets a node turn its sensing, transmitting and receiving circuitry off for most of its life.
Click any arrow to see what that message says and why it is sent.
A 100 ms gap between beacons, a 250 µs wake-up, and a similarly short check of the beacon. A node with nothing to send or receive can be asleep about 99 per cent of the time.
Read this diagram as text
- Node sends frame with power-management bit = 1 to AP (4 ms). One bit in the frame control field of section 7.3.3 announces the node is going to sleep. From here the AP knows not to send it anything.
- AP sends beacon — list of nodes with buffered frames to Node (4 ms). An AP typically beacons every 100 ms. Each beacon carries a list of the nodes whose frames it has been holding, so a waking node learns in one message whether anything is waiting.
- Node sends poll — send me my buffered frames to AP (4 ms). Only sent if the beacon named this node. Otherwise it simply goes back to sleep.
- AP sends buffered data to Node (6 ms).
Lifelines, left to right: Node (mobile device), AP (access point).
The mechanism is one bit and one list. A node sets the power-management bit in a frame header, and from then on the AP (access point) access point 802.11's word for a base station: the central device of a basic service set, which relays between its associated stations and the wired network behind it. introduced in ch. 7 knows to buffer anything addressed to it. The node sets a timer to wake just before the next beacon — an AP typically beacons every 100 ms — and waking takes about 250 µs.
The beacon carries a list of the nodes with frames waiting. If this node is not on it, back to sleep. If it is, it polls for them.
In plain words
Do the arithmetic: 250 µs of waking, plus a short check of the beacon, in every 100 ms. A node with nothing to send or receive is asleep about 99 per cent of the time.
And notice which side does the remembering. The sleeping node holds no state about what it missed — the AP holds it all, and announces it in one broadcast that every sleeping node can use.
Bluetooth: a network with no infrastructure
Bluetooth is a cable replacement: a keyboard, a mouse, earbuds, a watch, a car’s audio system. Short range — tens of metres or less — low power and low cost. Such networks are called wireless personal area networks, or piconets.
They share the unlicensed 2.4 GHz band — the Industrial, Scientific and Medical band — with microwaves, garage door openers and cordless phones.
This is the chapter’s best revision exercise
The book lists what Bluetooth is built from, and every item has a section number attached. A small, simple, self-organising network turns out to contain most of two earlier chapters.
| Technique | First met in | What Bluetooth does with it |
|---|---|---|
Cells marked ⓘ have an explanation — click to read it.
The book lists these itself, with section numbers. A small, simple network turns out to contain most of two chapters. Click a row.
The frequency hopping deserves its own sentence. The channel is time-divided into 625 µs slots, and in each slot the sender transmits on one of 79 channels, changing in a known but pseudo-random sequence. So interference from a microwave spoils a subset of the slots rather than the conversation. Rates reach about 3 Mbps.
The piconet
Drag any device to rearrange the picture. Hover a link to see its rate, delay and length.
Up to eight ACTIVE devices — one master and seven clients — plus as many as 255 parked ones. Only the clients have links drawn, because a parked device cannot communicate at all until the master reactivates it.
Read this diagram as text
- M — master — wireless link — C — client
- M — master — wireless link — C — client
- M — master — wireless link — C — client
Up to eight active devices: one master and seven clients. Plus as many as 255 parked devices, asleep to save energy and waking on the master’s schedule to hear its beacons. A parked device cannot communicate at all until the master changes its status back to active.
In plain words — the master really does rule
The book’s phrase is that the master “truly rules the piconet”, and the list justifies it. The master’s clock determines time in the piconet, including where the slot boundaries fall. It sets the frequency-hopping sequence. It controls who joins. It sets the power each client transmits at — 100 mW, 2.5 mW or 1 mW. And it polls polling protocol Simple A master node invites each node to speak in turn. Precise One node is designated master and polls the others in round-robin order, telling each that it may transmit up to some maximum number of frames. It eliminates collisions and empty slots, at the cost of a polling delay and a single point of failure. Bluetooth is an example. introduced in ch. 6 — open in glossary clients for permission to transmit.
So a piconet is a taking-turns network with a master node, in section 6.3.3’s exact sense — and it inherits exactly the single point of failure that section warned about.
Bootstrapping, with nothing to bootstrap from
With no infrastructure, a would-be master must first find out who is nearby. It broadcasts a series of 32 inquiry messages, each on a different frequency channel, and repeats the whole sequence up to 128 times.
A network with no infrastructure has to invent its own, and these four steps are how. Compare them with section 7.3.1’s active scanning: broadcast, listen, back off, associate.
Read all steps as text
- Neighbour discovery — the master shouts on 32 frequencies — With no infrastructure, a would-be master must first find out who is nearby. It broadcasts a series of 32 inquiry messages, each on a different frequency channel, and repeats the whole sequence up to 128 times.
- A client listens on one frequency, and waits — Each client picks a frequency and hopes to hear one of the master’s inquiries on it. The repetition is what makes the meeting likely: neither side knows the other’s choice.
- It backs off 0–0.3 s, then answers — A random wait before replying, so that two clients hearing the same inquiry do not answer at once. The book calls it reminiscent of Ethernet’s binary backoff — and it is the same idea for the same reason.
- Paging — the master invites the ones it wants — Having found everybody in range, the master invites the clients it wishes to admit. The book calls this reminiscent of 802.11 clients associating with a base station, and the resemblance is exact: discovery, then a chosen relationship.
Everyday picture
Two people trying to meet in a huge building with no phones and no agreed meeting point. One walks the corridors calling out, over and over. The other stands in a single room and listens.
They meet only when the caller reaches the right room — which is why the caller repeats the whole circuit many times rather than once.
Where the picture stops. The listener also has to wait a random moment before answering, because someone else may be listening in the same room. That has no natural equivalent in the picture, and it is section 6.3.2’s randomised backoff arriving in a network with no carrier sensing at all.
Check yourself
Check yourself
0 of 7 answered1.How does a station know it should drop to a lower transmission rate?
2.The book compares 802.11 rate adaptation to something you met in chapter 3. What, and why?
3.predictA node sets the power-management bit and sleeps. What does the AP do with frames addressed to it?
4.predictBeacons every 100 ms, a 250 µs wake-up, and a short beacon check. Roughly what fraction of the time can an idle node sleep?
5.Why does Bluetooth hop pseudo-randomly among 79 channels?
6.In what sense does the master "truly rule" a piconet?
7.predictA Bluetooth master broadcasts 32 inquiry messages on different frequencies, and repeats up to 128 times. Why so many?
What to remember
- Both 802.11 features here are unspecified on purpose. The standard gives the mechanisms; vendors build the policy. Rate adaptation infers errors from missing acknowledgements — TCP’s probing philosophy at the physical layer.
- Power management is one bit and one list: the AP buffers, the beacon announces, the node polls. Asleep 99 per cent of the time.
- Bluetooth is TDM, frequency division, randomised backoff, polling, error detection and reliable transfer with NAK (negative acknowledgement) negative acknowledgement A message saying that data arrived damaged and must be resent. Used by rdt2.1; removed in rdt2.2; TCP has none, and uses three duplicate ACKs instead. introduced in ch. 3 s. Five sections of earlier chapters, in one small network of up to 8 active devices.