4.8 WiFi multiple access

This animation consists of six sections.

Section 1 starts with a pictorial diagram of five devices. One is a rectangular box with two antennas at the back, which is labelled ‘access point (AP)’. The other four are a laptop computer, a tablet device, a smartphone and a wireless printer.

A diagram of the 3G and 4G networks is then shown to illustrate cellular mobile communications. This is the same as Figure 2.1 in the Block 3 text. The description of that figure is repeated below:

The figure shows 3G and 4G networks, with the 3G network on the left of the diagram and the 4G network on the right. Each network is divided by a horizontal dotted line, with the core network above the line and the access network below the line.

In the 3G network there is a series of base stations at the bottom of the diagram in the access network area, called Node Bs. Above these, but still in the network access area, are four radio network controllers (RNCs), represented by rectangles. Each RNC has two or more Node Bs, or base stations, connected to it.

Above the RNCs, in the core network area, are two rectangles labelled ‘serving GPRS support node (SGSN)’. There are various connections between the RNCs and the SGSNs, shown as blue lines. From the top of each RNC, there are two connections, one labelled ‘voice’ and one labelled ‘data’. The data connections go to the SGSN that is closest to the RNC in question. All the voice connections go to a rectangle labelled ‘voice switching’. This is in the core network.

Above the SGSNs, also in the core network area, is a further rectangle labelled ‘gateway SGSN (GGSN)’. A curved arrow at the top of this rectangle indicates a connection to other networks.

In the 4G network there is a series of base stations at the bottom of the diagram in the access network area, called eNode Bs. Above these, in the core network area, are two rectangles labelled ‘serving gateway (S-GW)’. To the left of these is a further rectangle, labelled ‘mobility management entity’. This has a direct connection, shown by a blue line, to one of the SGSNs in the 3G network.

Above the two serving gateway rectangles is a further rectangle, labelled ‘packet data network’. A curved arrow at the top of this rectangle indicates a connection to other networks.

All of the 4G network elements are within a cloud labelled ‘IP network’. (End of figure description.)

The animation then returns to the five devices shown at the beginning. All five are labelled as nodes. After this, double-headed arrows are shown between the access point and each of the other four devices, indicating how this group of nodes could be a Basic Service Set.

The access point is then shown by itself. Curved lines radiate out from each of the two antennas, labelled ‘Sarah’s network’.

Section 2 of the animation shows an access point, a smartphone and a laptop computer, all with curved lines radiating from them at the same time. The curved lines overlap and clash with each other.

The text ‘CSMA/CA’ and ‘Carrier Sense Multiple Access with Collision Avoidance’ is then shown on screen when this is mentioned by the narration.

Next, four devices are shown on screen: an access point, two laptop computers and a smartphone. One laptop is shown with curved lines radiating out from it towards the access point. The other laptop and the phone are labelled as ‘monitoring the radio channel’.

A laptop computer and a smartphone are then shown by themselves, each with a clock showing 00:00:12. They count down to 00:00:00 simultaneously. After this, new amounts of time are shown on each clock, but this time the two times are different: the phone has 00:00:25 and the laptop has 00:00:15. Again, they begin counting down together, but this time the laptop reaches 00:00:00 when the phone has only got down to 00:00:10.

The laptop and phone are then shown next to the access point. The laptop, which has 00:00:00 on its clock, has concentric circles radiating from it to show that it is communicating with the access point. The phone, which has 00:00:10 on its clock, does nothing.

Finally, the phone with 00:00:10 on its clock is shown by itself and counts down to 00:00:00, indicating that it would continue to count down from 00:00:10 next time round rather than start again with a new number.

Section 3 of the animation starts by showing the laptop computer and smartphone from the previous section, each with a clock showing 00:00:12.

After this, an access point and laptop computer are shown, with curved lines radiating out from the access point towards the laptop. A rectangle marked ‘frame’ is shown moving across from the access point to the laptop. The same access point and laptop are then shown with two other devices, another laptop and a smartphone. The original laptop has a clock showing 00:00:05, whereas the other laptop and the phone have clocks showing 00:00:12. They all start counting down simultaneously, but the first laptop reaches 00:00:00 when the other two have only got down to 00:00:07. Curved lines are then shown radiating from the first laptop to the access point, and a rectangle marked ‘ACK’ is shown moving across from the laptop to the access point.

In Section 4, two houses are shown side by side. House 1 contains an access point, a smartphone and a laptop computer. House 2 contains an access point, a laptop computer and a wireless printer. Curved lines radiate from one device to another, as follows:

• the laptop to the access point in house 1
• the access point in all directions in house 2
• the printer to the access point in house 2
• the access point in all directions in house 1.

Section 5 shows a screenshot of all the WiFi signals detected in a shopping centre. The signals are shown on a graph where the horizontal axis is labelled channel number from 1 to 11. The vertical axis is labelled signal strength in dBm, and ranges from minus 30 at the top of the axis to minus 90 at the bottom. Approximately 25 signals are shown, each with its own network address. The majority of the signals are clustered in three groups: one centred on channel 1, one centred on channel 6 and one centred on channel 11. There is little to no overlap between the three groups. Most of the signals have a flat top and very steeply sloping sides, but a few have sides that rise less steeply to a rounded peak.

Another, similar graph is then shown, with channel numbers from 1 to 6 on the horizontal axis, and signal strength in dBm from minus 40 down to minus 100 on the vertical axis. This time only one WiFi signal is shown, centred on channel 1. It has a flat top and sides that are vertical nearer the top, then slope outward. A remote control signal then appears. This is centred on channel 3 and has the shape of a gently sloping peak. Between channel 1 and channel 3, the two signals overlap.

Section 6, the final section of the animation, shows a horizontal axis marked with time in slots. The end of one frame is shown as a blue box at the left-hand end of the axis. The interval covering the next six time slots is labelled ‘check period’. The interval covering the 14 slots after that is labelled ‘contention window’ and the slots within it are labelled ‘back-off slots’. Finally, another blue box covering the next seven slots is labelled ‘next transmission’ and the left-hand edge of this box is labelled ‘data transmission begins’.

Three frames are then shown, each represented as a grey rectangle. The three rectangles join together to make one long rectangle, and this is labelled ‘aggregated frame’.

Finally, the animation returns to the diagram shown at the very beginning of Section 1, in which there are five nodes: an access point, a laptop computer, a tablet device, a smartphone and a wireless printer. Curved lines radiate out from the access point to all four of the other devices, representing infrastructure mode. Then curved lines radiate out from the tablet to the laptop, and from the phone to the printer, representing ad hoc mode.