What is Wifi (IEEE 802.11 Wireless Lan Standard) ?

We all are nothing but a social animal trying to access the internet to fulfill our basic needs (sharing pics in social media, watching web series , eCommerce etc). Most of the time we use data in our mobile or use wifi for wireless communication. Wireless communication is now becoming the top choice for accessing the internet (unlike the case of wired LAN cables).

So in this post, we will understand the in-depth protocol architecture of IEEE 802.11 wireless LAN standard (wifi). The other WLAN (wireless LAN) is the Bluetooth (see you in the next post) . Wifi is very common in public places like offices, colleges etc.

What is wifi (Wireless LAN) ?

IEEE has defined the specifications for a wireless LAN , called IEEE 802.11, which covers the physical and data link layers. Let us now see the various standards with their frequency range and supported  data rates :

 

 

A wifi standard 802.11n  uses multiple input multiple-output (MIMO) antennas i.e two or more antennas on the sending side and receiving side that is transmitting/receiving different signals . Throughput of over 200 Mbps is possible with this standard of WLAN.

IEEE 802.11 (wifi) Architecture !!

The fundamental building block of the 802.11 architecture is mainly the Basic Service Set (BSS) . A BSS contains one or more wireless stations and an optional base station known as the access point (AP). In a typical home network , there is one wifi router (access point + router) that connects BSS to the internet.

A BSS without  AP is called an Ad-hoc network. An ad-hoc network might be formed when people with laptops get together in a conference room , a train, or a car and want to exchange data in the absence of a centralized AP.

In this mode, the client is associated with an Access Point (AP) which is in turn connected to the other network as shown.The client sends and receives its packets via AP. Many such APs are connected together to form an extended 802.11 network.

 

A BSS with AP is called a Infrastructure network.In this mode, a group of computers can communicate to each other directly without any access point (AP) in between.

 

In IEEE 802.11, a set of BSS’s can be interconnected by a Distribution system (DS)  to form an Extended Service Set (ESS). The AP can be identified by Service set identifier (SSID) .

IEEE 802.11 (Wifi) Station Types 

No transition mobility station : Stationary station or moving only inside a BSS.

BSS transition mobility station : This station can move from one ESS to another , however, IEEE 802.11 doesn’t guarantee that communication is continuous during that move.

A station is portable if it can move from one location to another but remains fixed while in use. A mobile station can move while in use.

Active and Passive Scanning for AP

The 802.11 standard requires that an AP periodically send beacon frames each of which includes the AP’s SSID and MAC address. The host selects one of the AP’s for association. This is called passive scanning.

In active scanning , the host broadcasts a probe frame that will be received by all AP’s within the wireless host’s range. Let us understand the main steps involved in this :

• Probe requests frame from host (broadcasts).
•  Probe response frame sent from AP’s .
• Association request frame (host to selected AP)
• Association response frame (selected AP to host)

IEEE 802.11 Reference Model (protocol architecture)

Below is the high-level diagram of the complete IEEE 802.11 architecture (wifi model) :

 

Distributed Coordination Function 

One of the two protocols defined by IEEE at the MAC sublayer is called the distributed coordination function (DCF) . DCF uses CSMA/CA as the access method (already covered in the previous post) . Before sending any frame, the source station senses the medium by checking the energy level at the carrier frequency.

Point Co-ordination Function (PCF)

The PCF is an optional access method that can be implemented in an infrastructure network (not in an ad-hoc network ) . PCF has a centralized contention-free polling access method.  The AP performs polling for stations that are capable  of being polled.

The polling procedure is performed by the point coordinator (PC) in the AP within a ESS. PCF is implemented on top of the DCF and is used mostly for time-sensitive transmission.

IEEE 802.11 (wifi) Frame Format

Let us first decode the fields present in it :

FC : The FC field is 2 bytes long and it defines the type of frame and some control information.

D : In all frame types except one, this field typically defines the duration of the transmission .

Addresses : There are four address fields, each of 6 bytes long.

Sequence control :  This field defines the sequence number of a frame to be used in flow control.

Frame body : This field can be between 0 and 2312 bytes, contains information based on the type.

FCS :  The FCS field is 4 bytes long and contains a CRC-32 error detection sequence .

IEEE 802.11 supports 3 types of frames : management frames , control frames and data frames.

Management frames are used for the initial communication between station and AP’s . It is used  for station association and disassociation  with the AP’s , timing and synchronization and authentication procedure.

Control frames are used for accessing the channel and acknowledging the frames. Data frames are used for data transmission .

Addressing Mechanism in IEEE 802.11 (Wifi) protocol 

The IEEE 802.11 addressing mechanism basically specifies four cases, defined by the value of the two flags in the FC field.

Address 1 is always the address of the next device. Then Address 2 is always the address of the previous device. Address 3 is the address of the final destination station (host) if it is not defined by address 1. Address 4 is the address of the original source station which is not the same as address 2 .

Different specifications of  IEEE 802.11 (wifi WLAN)

Let us now understand the other specs of IEEE 802.11 reference model :

1. IEEE 802.11a

IEEE 802.11a OFDM explains the orthogonal frequency-division multiplexing (OFDM) method for signal generation in a 5-GHz ISM band.

Sources contend with one another at the data link layer for access. The band is divided into 52 sub-bands, with 48 sub-bands for sending 48 groups of bits at a time and 4 sub-bands for control information.

OFDM uses PSK and QAM for modulation.

2. IEEE 802.11b 

IEEE 802.11b DSSS describes the high-rate direct sequence spread spectrum (HR-DSSS) technique for signal generation in the 2.4-GHz ISM band.

HR-DSSS is the same as the DSSS except for the encoding method, which is called complementary code keying (CCK). CCK encodes 4 or 8 bits to one CCK symbol.

3. IEEE 802.11g

This new specification simply defines forward error correction and OFDM using the 2.4-GHz ISM band.

The modulation technique achieves a 22- or 54-Mbps data rate. It is backward compatible with 802.11b, but the modulation technique is OFDM.

In the next post , we will further look into the next widely used WLAN type i.e the Bluetooth functionality. So stay tuned.