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General
On June 27 1997 the IEEE approved the 802.11 standard for Wireless Local Area Networks. The standard consists of a general MAC-layer and three different PHY-layers. The MAC-layer is the same for all the PHY-layers.
The standard defines two types of wireless networks one is called BSS (Basic Service Set). The BSS is completely wireless network and is normally built up of PCs (or notebooks) with a wireless network card.
The other type is the ESS (Extended Service Set) and is also called an 'Ad Hoc' network and connects the wireless stations to a wired network through an one or more access points. The access point is actually a bridge between a wired network and a wireless network.
A BSS network is limited in its range. All stations need to 'see' or 'hear' eachother.
An ESS network makes up a network with an extended range. A wired backbone could provide links between several access points. If the range of the different access points overlap eachother it is possible to move from one cell to another without losing network connection. For this to happen roaming should be enabled. A cell is defined as the coverage area of one access point.
In this example network you see three different cells where roaming is possible between cells 1 and 2. Cell 3 could be roaming enabled but someone walking from cell 2 to cell 3 will lose his network connection in between.
Roaming
Roaming is the feature which makes it possible to walk around a site without losing the network connection. The access points signal the wireless station (labtop) their signal strength and as soon as another access point has a stronger signal than the one connected to, the station decides to switch to the new access point. All this happens without the user knowing it.
Overview of 802.11 and OSI
| 2 | Data Link | LLC or LLC + SNAP | |
| MAC Sublayer | MAC Layer Management | ||
| 1 | Physical | PLCP Sublayer | PHY Layer Management |
| PMD Sublayer |
The MAC-Layer
The MAC protocol uses CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance). Next to this the MAC-layer is also responsible for fragmentation and encryption.
Each BSS has a unique 48 bit MAC-address, where every ESS has a variable length address.
The MAC-frame:
| Octets | 2 | 2 | 6 | 6 | 6 | 2 | 6 | 0-2312 | 4 |
| Frame Control | Duration ID | Address 1 | Address 2 | Address 3 | Sequence Control | Address 4 | Frame Body | CRC |
CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance)
The CSMA/CA protocol is similar to that in Ethernet. The station that has something to send listens to the medium and when there is no station active it, and that's the difference with CSMA/CD, waits a random time before sending its data, while still monitoring the medium.
This way the change that two stations will send at the same time is very rare. The loss in speed due to the random waiting time is compensated by the lesser retransmissions. The more stations there are in a network the more benefit you have of this technique.
The MAC-Layer Management
Is responsible for the synchronisation, power management, roaming and the MAC-MIB.
The PHY-Layer
The physical layer consists of two sub-layers called PLCP (Physical Layer Convergence Protocol) and PMD (Physical Medium Dependent). There are three types of physical layers. The PLCP is the layer responsible for the CS part of the CSMA/CA protocol. The PMD is the layer responisble for the modulation and encoding of the signal. Two use a radio-frequency and one is for infra-red.
Frequency hopping uses 79 frequency channels between 2.4 GHz and 2.438 GHz with a 1 MHz channel spacing. This has to do with local regulations for using radio frequency without a license. This also gives you the posibility to setup 26 different networks in the same area, which would not see eachother.
The different speeds are created by a different for of Freqency Shift Keying. The 1 Mbps version uses a 2 level Gaussian FSK and the 2 Mbps uses a 4 level Gaussian FSK.
The frame format:
| 80 bit preample | SFD 16 bits | LENGTH 12 bits | PSF 4 bits | CRC 16 bits | |
| PLCP Preample | PLCP Header | Payload MPDU | |||
| PPDU | |||||
The 80 bit preample has a 0101 sync format and is used for signal detection.
The SFD stands for Start of Frame Delimiter
The LENGTH field indicates the Payload length in bytes.
The PSF stands for Payload Signaling Field and indicates the rate used and some bits for future use.
The hopping rules say that there are 79 hopping channels and that the minimum hop should be 6 channels. The transmitter should settle on the new channel within 224 microseconds.
The different speeds are created by using a different form of Phase Shift Keying. The 1 Mbps version uses a Differential Binary PSK and the 2 Mbps uses a Differential Quadrant PSK.
The frame format:
| 128 bit preample | SFD 16 bits | SIGNAL 8 bits | SERVICE 8 bits | LENGTH 16 bits | CRC 16 bits | |
| PLCP Preample | PLCP Header | Payload MPDU | ||||
| PPDU | ||||||
The 128 bit preample is used for signal detection.
The SFD stands for Start of Frame Delimiter
The SIGNAL field indicates the speed used.
The SERVICE field is reserved for future use and now contains 00.
The LENGTH field indicates the Payload length in bytes.
The physical layer consists of two parts: the PLCP and the PMD. Both will be covered separatly.
Physical Layer Convergence Procedure Sublayer (PLCP)
This layer maps the MPDU (MAC Protocol Data Unit) into a suitable PDU (Physical Data Unit).
| PLCP Preample | PSDU (variable number of octets) | |||||
| SYNC | SFD | DR | DCLA | LENGTH | CRC | |
The SYNC field length ranges between 57 L-PPM slots to 73 L-PPM slots and is not L-PPM modulated. This field provides synchronization (clock recovery), automatic gain control (optional), signal-to-noise ratio estimate (optional) and diversity selection (optional).
The SFD (Start Frame Delimiter) field consists of 4 L-PPM slots with a hex symbol of 1001. This field indicates the start of the PLCP preample and performs bit and symbol synchronization.
The DR (Data Rate) field uses 3 L-PPM slots and indicates the speed used:
The DCLA (DC Level Adjustment) field is used for DC level stabilization. It's a 32 L-PPM slot and looks like this:
The LENGTH field gives you the number of octets that is transmitted in the PSDU and is given in a 16-bit integer.
The PSDU field is the actual data coming from the MAC layer. The maximum length is 2500 octets and minimum is 0.
Physical Medium Dependent Sublayer (PMD)
Here the actual bit-stream is formed that is transmitted. The bits are L-PPM encoded in a 16 or 4 code version. For both the versions a table is presented below:
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The power transmitted is due to safety regulations very strickt. According to the IEEE802.11:
| Emitter Radiation Pattern Mask | Peak Optical Power (averaged over the pulse width) |
|---|---|
| MASK1 | 2W ± 20% |
| MASK2 | 0.55W ± 20% |
The PHY-Layer Management
Is responisble for channel tuning and the PHY-MIB.