The document provides an overview of the IEEE 802.11 architecture for wireless local area networks (WLANs), detailing its components including stations, access points, and basic service sets. It describes the MAC protocol and physical specifications that allow for station mobility and communication, highlighting the differences between infrastructure and independent BSS. Additionally, it covers MAC addressing, frame formats, and the operational mechanisms used in IEEE 802.11, such as DCF and PCF for medium access coordination.

1. TELE COMMUNICATIONS
Assignment Topic:
IEEE 802.11 Architecture
Assignment Submitted To:
Miss Anila Majeed
Assignment Submitted By:
Shefa Idrees (101631049)
Assignment Submission Date:
20-May-2019
Post Graduate College for Women, Samnabad Lahore

2. IEEE 802.11 Architecture
What is IEEE 802.11?
In 1990, IEEE 802 Committee formed a new working group, IEEE 802.11, specifically devoted
to wireless LANs, with a charter to develop a MAC protocol and physical medium
specification.
WLANs
Wireless LANs are those Local Area Networks which use high frequency radio waves instead
of cables for connecting the devices in LAN. Users connected by WLANs can move around
within the area of network coverage. Most WLANs are based upon the standard IEEE 802.11
or WIFI.
The Architecture of IEEE 802.11
The IEEE 802.11 architecture consists of several components. These components interact to
provide a wireless LAN that supports station mobility transparently to the upper layers.
They include:
 STA - Station
 AP - Access Point
 BSS - Basic Service Set
 IBSS- Independent BSS
 ESS - Extended Service Set
 A set of Infrastructure BSSs.
 Connection of APs
 Tracking of mobility
 DS - Distribution System
 AP communicates with another
802.11Architecture

3. ESS
 WLAN is based on cellular architecture.
 Stations (STAs) comprise all the devices and equipment which are connected to the
WLAN. A station can be of two types:
o Wireless Access Points (WAP): WAPs or simply Access Points (AP) are
generally wireless routers which form the Base Stations or Access.
o Client: Clients are workstations, computers, laptops, printers, smartphones, etc.
 Basic Service Set (BSS) is a group of stations communicating at Physical Layer level.
BSS can be of two categories depending upon the mode of operation:
o Infrastructure BSS: Here, the devices communicate with other devices
through Access Points.
o Independent BSS (IBSS): Mobile Station (MS) in BSS with no connection to
other BSSs form IBSS. Here, the devices communicate in peer-to-peer basis in
an ad-hoc manner.
 Each cell/Basic Service Set (BSS) is controlled by a base station/Access Point (AP).
 Access Points are connected with backbone called Distribution System (DS).
 The whole interconnected WLAN through DS form Extended Service Set (ESS).
 AP functions as a bridge and a relay point.
 In BSS, MS communicate through the AP
 IBSS is since an ad hoc network, where station communicate directly.
 To integrate 802.11 with 802.2 (Wired LAN), a portal is used.
 Portal is a device such as bridge or router attached to DS.

4. IEEE LAN Standards
Layer 1 Physical Layer
Limitations:
 Cannot organize streams of bits.
 Cannot name or identify computers.
 Cannot communicate with the upper-level layers.
 Cannot decide which computer will transmit binary data.
Layer 2 Data Link Layer
Uses:
 Framing to organize or group the data bits.
 An addressing process to identify computers called flat addressing convention.
 Logical Link Control (LLC) to communicate with the upper-level layers.
 Media Access Control (MAC) to decide which computer will transmit binary data, from
a group in which all computers are trying to transmit at the same time.
Purpose of Sub-layers
 LLC or Logical Link Control serves to communicate upward to Network layer,
independent of the specific LAN technology used and Upper layer.
 MAC or Media Access Control serves to access and communicate downward to the
technology-specific Physical layer.

5. MAC Addressing
 Every computer has a unique way of identifying itself via MAC address or physical address.
 The physical address is located on the Network Interface Card (NIC).
 MAC addresses have no structure, and are considered flat address spaces.
 MAC addresses are sometimes referred to as burned-in addresses (BIAs) because they are
burned into read-only memory (ROM) and are copied into random-access memory (RAM)
when the NIC initializes.
MAC Frame Format
The IEEE 802.11 MAC accepts MSDUs (MAC Service Data Unit) from higher layers in the protocol
stack for the purpose of reliably sending those MSDUs to the equivalent layer of the protocol stack in
another station.
 MAC adds information to the MSDU in the form of headers & trailers to create an MPDU
(MAC Protocol Data Unit).
 MPDU is then passed on to the physical layer to be sent over the wireless medium to the other
station.
 MAC may fragment MSDUs into several frames.
 The maximum size of a MAC frame is 2048 octets.
Frame Fields
 Frame Control: Indicates the type of frame provides control information. Control
information includes whether the frame is to or from a DS and privacy information.
 Duration/ID: If used as a duration field, indicates the time (in microseconds) the
channel will be allocated for successful transmission of a MAC frame.
 Address 1: MAC address of wireless host or AP to receive this frame.
 Address 2: MAC address of wireless host or AP transmitting this frame.
 Address 3: MAC address of router interface to which AP is attached.
 Address 4: Used only in ad hoc mode.
 Sequence Control: Sequence number used to number frames sent between a given
transmitter and receiver.
 Frame Body or Payload: Contains the message.
 CRC: The last field contains error detection information.

6. MAC Modes
The 802.11 MAC protocol designed with two modes of communication
 Distributed Coordination Function (DCF)
The basic 802.11 MAC layer uses the Distributed Coordination Function (DCF) to share the
medium between multiple stations, based on Carrier Sense Multiple Access with Collision
Avoidance (CSMA/CA)
o All stations are equal
o "Listen before talk”
o Station waits for quiet period on network
o Begins to transmit data
o Detects possible collisions
 Point Coordination Function (PCF)
This is available only in "infrastructure" mode, where stations are connected to the network
through an Access Point (AP). This mode is optional, and only very few APs or Wi-Fi adapters
actually implement.
PCF defines two periods:
o The Contention Free Period (CFP) - In the CP, DCF is used.
o The Contention Period (CP) - In the CFP, the AP sends Contention-Free-Poll
packets to each station, one at a time, to give them the right to send a packet.
The AP is the coordinator.

7. LLC Services
The services of Logical Link Control (LLC) include:
 Unacknowledged connectionless service
o No flow- and error-control mechanisms
o Data delivery not guaranteed
 Connection-mode service
o Logical connection set up between two users
o Flow- and error-control provided
 Acknowledged connectionless service
o Cross between previous two
o Datagrams acknowledged
o No prior logical setup
LLC Operations
Type 1 Operation: Type 1 operation supports unacknowledged connectionless service. There
is no acknowledgment, flow control, or error control.
Type 2 Operation: Type 2 operation supports connection-mode service, using mechanisms
similar to HDLC.
Type 3 Operation: Type 3 operation supports acknowledged connectionless service. Each
transmitted PDU is acknowledged using a stop-and-wait technique.