Is WiFi a communication protocol

WLAN transmission technology

4Transport Layer (TCP)TCP
3Network Layer (IP)IP
2Logical Link Control (LLC)802.2
Medium Access Control (MAC)CSMA, VCD
1Physical Layer Convergence Protocol (PLCP)DSSS, FHSS, infrared

The radio network standard IEEE 802.11 defines a common MAC layer (Medium Access Control) for three specific physical layers (PHY). Two of them are assigned to the wireless LANs and one to the infrared network. The frequency range used in the radio network is the ISM band (2.4 GHz) from 2.400 to 2.4835 GHz.
The infrared variant is practically unknown. It uses frequencies from 850 to 950 nanometers (light). The diffuse IR transmission used does not require an exact alignment of the transmitter and receiver. The line of sight, which is a maximum of 10 meters wide, should still be free of obstacles in order to avoid unnecessary interference with data transmission.

Radio technology provides several modulation methods that work with the spread spectrum method. The radio signal is divided over the broadest possible frequency spectrum. This method reduces the influence of narrowband and broadband interference.

WLAN frame according to IEEE 802.11

IEEE 802.11 has Ethernet as the basic technology and therefore also has its frame types and access methods. IEEE 802.11 knows three different frame types. Including control, management and data frames. Normal WLAN adapters only have to understand part of these frames. Some things are reserved for access points that have to master all services.

preamble802.11 headersIVSNAPEthernet frameChecksum
20 µs24 to 32 bytes4 or 8 bytes8 bytesmaximum 2304 bytes4 bytes

In principle, an Ethernet frame is transmitted embedded in a WLAN frame. The Ethernet frame can be significantly longer than with Fast Ethernet. While a normal Ethernet frame can have a maximum of 1518 bytes, the Ethernet frame via WLAN can be 2304 bytes. With longer frames, the number of headers can be reduced and the transmission rate increased.
So that the frames can be transmitted via WLAN, up to 64 bytes of headers and checksums are added and a preamble of 20 µs is prefixed. The preamble is used to synchronize the receiver. This is followed by the 802.11 header with up to 32 bytes. The sequence counter (IV) is required for encrypted packets and is 4 or 8 bytes. The LLC-SNAP header is required to transport Ethernet packets over non-Ethernet media. It needs 8 bytes. This is followed by the actual Ethernet frame with a maximum of 2304 bytes and the checksum with 4 bytes.

Power-saving functions / power-saving

WLAN is mainly used in mobile and therefore battery-operated devices. For example smartphones, tablets and notebooks. There are special power-saving and power management functions to extend the battery life of these devices.
The Traffic Indicator MAP (TIM) is a list that the access point creates in order to store all wireless stations there. To keep this list up to date, the access point regularly sends TIM signals (beacons) that wake up the wireless stations.
The Delivery-Traffic-Indicator-MAP (DTIM) is also a list that is maintained by the access point. The DTIM beacon is a broadcast signal that is sent with a greater time interval than the TIM beacon. As a rule, WLAN network cards are only woken up with the DTIM beacon in order to further increase the runtime of mobile devices.
For the TIM or DTIM beacon, there are often settings in the access point for how often it should be sent. As a rule, these settings are not used.

IEEE 802.11d - World Mode - Global Harmonization

The IEEE 802.11d standard falls under the term "Global Harmonization" and is also referred to as "World Mode". It regulates the technical differences in different countries and regions. This also includes the definition of the number and selection of channels that are approved for the use of WLAN in a country.
The selection of the basic technology, i.e. whether IEEE 802.11 a, h, b, g or n may be used, is also regulated. Thanks to IEEE 802.11d, it is irrelevant for the WLAN user which standard is used. All he has to do is enter his current location. The WLAN client then works with the respective approved standards.
In practice, it looks like a WLAN router or WLAN client has stored a country profile that it uses to make the necessary settings.

FHSS - Frequency Hopping Spread Spectrum


FHSS is part of the original IEEE 802.11 standard. It describes how the frequency spectrum is divided. The sender and receiver use the 79 channels in the 2.4 GHz band for transmission and divide the data packets into small pieces. By assigning a certain hopping sequence, the channels are changed according to a random pattern. The specified minimum jump distance is 6 channels, i.e. 6 MHz. Overall, this frequency range can be operated with 26 participants without having to share the transmission rate.
This technique is very susceptible to interference, especially when disturbed frequencies are left out of the hopping pattern. If two transmissions collide on one channel, these data packets are simply transmitted again. Since the collisions cannot be detected in a radio network, a collision avoidance procedure is used (CSMA / CA).
FHSS is relatively inexpensive and energy efficient, which is a big advantage for small mobile devices. However, the enormous administrative effort involved in frequency hops depresses the user data rate, complicates roaming between multiple access points and has only a limited range.
Frequency hopping has one major disadvantage. It can only achieve a maximum of 2 Mbit / s. WLAN according to IEEE 802.11b therefore uses DSSS as the modulation method and thus bridges larger distances with a faster data transmission rate.

DCF - Distributed Coordination Function

The Distributed Coordination Function (DCF) distributes the access rules to the stations. The MAC protocol CSMA / CA (Carrier Sense Multiple Access / Collision Avoidance) is used in the DCF.
Between the data packets, waiting times of different lengths coordinate access to the radio medium. The DIFS (Distributed Coordination Function Interframe Space) marks the backoff time in which a station can recognize the free radio medium. The SIFS (Short Interframe Space) identifies the ACK packet. This is the recipient's confirmation packet for the sender. The ACK packet is followed by another DIFS.

CSMA / CA - Carrier Sense Multiple Access / Collision Avoidance

The transmission technology of IEEE 802.11 provides for the uncoordinated, unplanned and spontaneous operation of a WLAN. In practice, it means that anyone can put a WLAN base station into operation without having precise technical knowledge of how it works. This means that the radio systems operated in this way must master techniques and measures for coexistence.
CSMA (Carrier Sense Multiple Access) is a multiple access method. It stipulates that each participant must check whether the transmission medium is free before sending. Transmission is only permitted when no other participant is sending anything.

PCF / Point Coordination Function

The WLAN transmission technology works with CSMA / CA, a listen-before-talk process. When transmitting time-critical data, a specific point in time for the transmission cannot be guaranteed.
However, there are also approaches to expand the access protocol to include a centrally coordinated procedure. For example PCF (Point Coordination Function) and HCFCCA (Hybrid Coordinator Function Controlled Channel Access). However, both methods are rarely found in the devices on the market.

The PCF extension provides an access regulation in which every participant is addressed by the access point in a specific time window. In this way, a certain characteristic is guaranteed in the transmission for certain communication requirements.
With PCF, an access point assigns broadcasting rights to the participants by means of a channel reservation. This process is known as polling. The access point asks the participants within its cell one after the other whether they want to send data. In this way, time-critical data traffic can be handled optimally.
The usual DCF and PCF can also be used in parallel. PCF then has a higher priority.

With PCF, IEEE 802.11 gets real-time capability through a coordinated multiple access procedure on the MAC layer and thus supports Quality of Service (QoS).

Request-to-Send (RTS) and Clear-to-Send (CTS)

In order to avoid the risk of multiple radio interface occupancy and transmission collisions, each station must explicitly reserve the radio interface before it can be occupied. The RTS / CTS procedure is used for this.
For collision avoidance, there is a Virtual Collision Detection (VCD) mode in the MAC layer, which contains the Request-to-Send (RTS) and Clear-to-Send (CTS) frames. Before any data is sent, the following sequence takes place:

  1. The WLAN station requires a free channel.
  2. The WLAN station identifies a free channel.
  3. The WLAN station sends an RTS on this channel.
  4. The access point (AP) sends a CTS.
  5. The WLAN station sends the data.
  6. The access point (AP) sends an acknowledgment (ACK) to confirm receipt.

After recognizing a free channel, transmitter A sends an RTS signal to receiver B. If receiver B recognizes the channel as free, it sends a CTS signal. This signal is heard by all stations that are in contact with the radio cell of receiver B. This means that this channel is reserved for a certain transmission time from transmitter A to receiver B.
The acknowledgment (ACK), the confirmation of receipt after the data transmission, is another part of the CSMA / CA. When the package arrives, the recipient sends the sender an acknowledgment of receipt. If the sender does not receive this, he sends the package again. Without ACK, the transmitter has the right to use the radio medium again. Brief disturbances (interference) on the radio medium are avoided without the user noticing. Long-term interference from other radio applications in the same frequency range only causes the transmission rate to drop. If the interference cannot be avoided, the wireless network collapses.

Full duplex WLAN / Full Duplex Communication (FDC)

The advantage of a copper cable is the simultaneous sending and receiving on one channel. This is known as full duplex. Typically, the channel separation (sending / receiving) is done by echo cancellation. What works well on a cable is hardly possible in wireless technology. The difficulty is the "selectivity" to subtract the transmitted signal from the received signal.
Because of this difficulty, in radio technology the channels for sending and receiving are transmitted alternately on the same frequency (TDD) or simultaneously on two different frequencies (FDD).

Beacons

WLAN access points (WLAN AP) send beacons with general information about the offered WLAN at regular intervals. The WLAN beacons are special WLAN packets with which WLAN clients that are within range are informed about this WLAN.

OFDMA - Orthogonal Frequency Division Multiple Access

Transmission with OFDMA enables a variable number of subcarriers (frequencies) and individual modulation levels. OFDMA thus contributes to the optimal utilization of the available capacities in the frequency range. By assigning individual or multiple orthogonal subcarriers, second slots and subchannels to different users, the available transmission capacity can be adapted to user requirements.

MIMO - Multiple Input Multiple Output

MIMO is the generic term for processes that improve radio connections with several antennas used in parallel. This includes:

  • greater reception performance (group gain)
  • Interference suppression (interference suppression gain)
  • better connection quality (diversity gain)
  • higher transmission rates (multiplex gain)

Several antennas provide a better reception signal, increase the possible distance or increase the overall data throughput.

MU-MIMO - Multi-User MIMO

Multi-User-MIMO (MU-MIMO) is an extension to send different data streams on several antennas to individual WLAN clients at the same time. However, this only makes sense if there are three or more antennas in the base stations. Because only then can multiple data streams be sent to multiple clients at the same time. An access point with three antennas can send to three clients at the same time at 433 Mbit / s each (with IEEE 802.11ac). Or a total of 1,300 Mbit / s to a client. The client to be received also needs three antennas, but this should not be the case very often. Nevertheless, the total throughput of a WLAN increases with MU-MIMO, provided that at least a few MU-MIMO-capable WLAN clients are available.

Future WLAN developments

Speed ​​gain through

  • more bits per transmission step, thanks to higher-level modulation
  • more bits through a wider radio channel, 40 or 80 MHz instead of just 20 MHz
  • more bits through spatially separated transmission paths or directions, through MIMO and MU-MIMO

Work is also being carried out on being able to transmit and receive at the same time, which would double the cell efficiency in one fell swoop.

IEEE 802.11 problems

Although IEEE 802.11 works independently of the protocol, problems can arise in practice with some protocols and applications. The decisive factors are the higher bit error rate (BER) and the greater delay in the transmission of data. It is in the nature of a wireless LAN that the time required for transmission is longer than in a wired LAN. A simple ping has a round trip time of less than a millisecond in a wired LAN. In the wireless LAN, the time for a ping is up to four milliseconds.
Applications that require a short delay between sending and receiving (delay) may have difficulties with a wireless LAN.

Overview: WLAN technology

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