Two assumptions have held since chapter 1: that a link is a wire, and that a host stays where it is. This chapter removes them one at a time.
Words you will meet
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Wireless host wireless host Simple An end system that connects over radio. Precise An end-system device running applications, connected to the network by a wireless link. It may or may not be mobile — chapter 7 is explicit that being wireless and being mobile are different properties, and that the hard problems each raises are different. introduced in ch. 7 — open in glossary — an end system connected by radio. It may or may not move.
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Base station base station Simple The infrastructure a wireless host talks to. Precise A key part of the wireless network infrastructure, with no obvious counterpart in a wired network. It sends and receives data to and from the hosts associated with it, often coordinates their transmissions, and acts as a link-layer relay between them and the rest of the network. introduced in ch. 7 — open in glossary — the infrastructure a host talks to. The one element with no wired counterpart.
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Infrastructure mode infrastructure mode Simple The network provides the services; you just associate with it. Precise Operating while associated with a base station, so that address assignment, routing and other traditional network services come from the network behind it. Contrasted with an ad hoc network, where the hosts must provide those services for themselves. introduced in ch. 7 — open in glossary — the network provides the services; contrast an ad hoc network, where the hosts must.
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Handoff handoff Simple Changing which base station you are attached to. Precise What happens when a mobile host moves beyond one base station's range and into another's, changing its point of attachment to the larger network. Also called handover. introduced in ch. 7 — open in glossary — changing which base station you are attached to. Also called handover.
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Coverage area — how far one base station reaches.
Why this matters
Every chapter so far assumed a link is a wire. A wire has a bit error rate that is small and roughly constant, it reaches exactly the machines plugged into it, and it does not care where you are standing.
Radio has none of those properties. And a host that moves breaks something else entirely: not the link, but the assumption that an address tells you where to send things.
These are two separate problems, and the most useful thing this page can do is keep them apart.
The pieces
Drag any device to rearrange the picture. Hover a link to see its rate, delay and length.
Four elements, and only one of them has no counterpart in a wired network. The wired links from each access point back to the infrastructure are solid; every wireless link is drawn dashed and pink, as it is everywhere on this site. The host in motion is at a boundary — which is where the second half of this chapter begins.
Read this diagram as text
- network infrastructure — wired link — access point
- network infrastructure — wired link — access point
- network infrastructure — wired link — access point
- network infrastructure — wired link — access point
- access point — wireless link — wireless host
- access point — wireless link — wireless host
- access point — wireless link — wireless host
- access point — wireless link — wireless host
- access point — wireless link — wireless host
- access point — wireless link — host in motion →
- access point — wireless link — wireless host
Four elements, and the book is careful about one of them.
Wireless hosts are end systems running applications — a phone, a tablet, a laptop, or an IoT (Internet of Things) Internet of Things Everyday devices with network connections — the Mirai botnet was built from about 100,000 of them. introduced in ch. 2 device such as a sensor, an appliance or a car. They may or may not be mobile.
Wireless links connect a host to a base station or to another host. They vary enormously in rate and reach, which is the next table.
Network infrastructure is the larger network the host wants to reach.
And base stations, which the book singles out:
In plain words
A base station has no obvious counterpart in a wired network. Nothing in chapters 1 to 6 does its job.
It sends and receives on behalf of the hosts associated with it, and often coordinates their transmissions. Being associated means two things: the host is within communication distance, and it uses that base station to relay between itself and the larger network.
Cell towers in cellular networks and access points in 802.11 wireless LAN (Local Area Network) Local Area Network A network covering one building, campus or home. introduced in ch. 1 s are both base stations.
Hosts associated with a base station are in infrastructure mode, because the network provides the traditional services — address assignment, routing, and the rest. In an ad hoc network there is no such infrastructure, and the hosts themselves have to provide routing, addressing, name translation and everything else.
When a moving host passes out of one base station’s range and into another’s, it changes its point of attachment. That is a handoff, and it is the seam between the two halves of this chapter.
Rate against range
| Standard | Rate | Range |
|---|---|---|
Cells marked ⓘ have an explanation — click to read it.
The rates and ranges the book gives. Sortable — sort by rate and then by range, and the trade-off appears on its own.
Why this is a table and not a chart
The book draws this as a graph, and adds a note to its own caption: the axes are not linear.
Redrawing it faithfully would mean reproducing a distortion. 802.11ax at 14 Gbps would sit only a little above 5G at 10 Gbps, while 11 Mbps and 2 Mbps would be pushed apart. Redrawing it honestly, on log axes, would produce a different picture from the one in the book.
So the numbers are here as numbers. Sort by rate, then by range, and the trade-off is visible without either distortion: the fastest standards reach tens of metres, and the ones that reach kilometres are slower. 5G is the interesting exception, and its cost is base stations packed far more densely.
The book also warns that the figure is only a rough guide. Some of these networks are still being deployed, and real rates move with distance, channel conditions and how many people are using them.
Four kinds of wireless network
Two questions sort every wireless network. Does a packet cross one wireless hop or several? And is there a base station or not?
| Infrastructure-basedthere is a base station | Infrastructure-lessthere is not | |
|---|---|---|
| Single hopa packet crosses one wireless link | ||
| Multiple hopsnodes relay for each other | ||
| Does this chapter cover it? |
Cells marked ⓘ have a reason behind them — click to read it.
Two questions, so four answers: does a packet cross one wireless hop or several, and is there a base station? Click any cell.
The book then says which box it lives in: mostly single-hop, and mostly infrastructure-based. That is the top-left cell, and it is also where almost all of your own daily use sits.
Wireless is not the same as mobile
A desktop machine on the office WiFi. Its link is radio, with everything that implies — interference, varying error rates, a shared channel. It never moves, so nothing ever has to find it.
Keep these apart while reading. Almost every confusion in this chapter comes from treating them as one thing.
Read all steps as text
- Wireless, not mobile — A desktop machine on the office WiFi. Its link is radio, with everything that implies — interference, varying error rates, a shared channel. It never moves, so nothing ever has to find it.
- Mobile, not wireless — A laptop carried between two offices and plugged into Ethernet at each. Its link is a wire and behaves perfectly. But it changed its point of attachment, so anything trying to reach it has a genuine problem.
- So the chapter is really two chapters — Sections 7.1 to 7.4 are about wireless links — what radio is like, and what 802.11 and 4G do about it. Sections 7.5 and 7.6 are about mobility, and never mention radio. Section 7.7 is where the two meet again, and where chapters 3 and 6 get their answer.
Everyday picture
Two different problems, and it helps to name them as two.
Wireless is like talking in a crowded, echoing room. Your voice fades with distance, other conversations interfere, and sound bounces off the walls so people hear you twice.
Mobility is like moving house. Nothing about your voice changed. But everyone who wants to reach you now has stale information, and something has to arrange for your post to follow you.
Where the picture stops. You can tell people you have moved. A mobile device often cannot: its correspondents are anonymous machines that were never told anything in the first place. That is exactly the problem section 7.5 has to solve.
Check yourself
Check yourself
0 of 6 answered1.Which element of a wireless network has no obvious counterpart in a wired one?
2.A host is "associated" with a base station. What does that mean?
3.A desktop machine sits permanently on the office WiFi. Which problems does it have?
4.predictA laptop is carried between two offices and plugged into Ethernet in each. What kind of network problem is that?
5.Bluetooth connecting a keyboard to a laptop is which kind of network?
6.predict5G reaches nearly the rate of the fastest WiFi over kilometres rather than metres. What does that cost?
What to remember
- Four elements: wireless hosts, wireless links, base stations, network infrastructure. A base station has no counterpart in a wired network — cell towers and 802.11 access points are both examples.
- Associated means two things: within range, and using that base station as a relay. Infrastructure mode means the network provides the services; an ad hoc network means the hosts must.
- Wireless and mobile are separate properties. Keep them apart, or nothing in this chapter will make sense.