24 Layers description of IEEE 802.11

Suchit Purohit

Learning objectives

  • IEEE 802.11 Physical layer entities
  • IEEE 802.11 MAC layer entities introduction and inter frame spaces
  • CSMA/CA
  • CSMA/CA with hidden terminal problem solution
  • Fragmentation and Reassembly
  • Synchronization and Power Saving
  • Mobility and Roaming

Introduction

 

In the previous modules we learnt about basic architecture of IEEE 802.11 standard. In this this module we will learn about basic layers of the standard. It specifies MAC and PHY layer. We will learn about different PHY layers, multiple access methods, need of fragmentation and reassembly, how synchronization and  power saving  is done.  We will learn  about types  of mobility how it is performed and how roaming is facilitated.

 

IEEE 802.11 layers

 

IEEE 802.11 is a set of (MAC) and physical layer (PHY) specifications for implementing wireless local area network (WLAN) computer communication . Standard defines single MAC which interacts with multiple PHYs namely(Fig. 1)

  • Direct Sequence Spread Spectrum
  • Frequency Hopped Spread Spectrum
  • Infrared
  • OFDM

Figure 1: MAC and PHY Layer

 

Physicallayer
Figure 1: MAC and PHY Layer

The Physical layer consists of two sub-parts namely

 

PLCP (Physical Layer Convergence Procedure)

  • Performs Carrier Sense during CSMA
  • Prepares MAC Protocol data Unit(MPDU) for transmission
  • Delivers incoming frames from wireless medium to the MAC layer
  • Provides asynchronous transfer of MPDU between the stations
  • Provides synchronization of receiving stations Physical layer with incoming packets

PMD (Physical medium dependent)

  • Provides actual transmission and reception of physical layers entities between stations
  • Provides modulation and demodulation of the transmission

FHSS PHY

  • Done using Frequency Hopping
  • 79 channels of 1 MHz each in 2.402 to 2.480 GHz band
  • Hopping sequence is set in access point, stations synchronize to it

Figure 2: FHSS PHY

DSSS PHY

 

Data is spreaded using PN code. Code is a sequence of chips or bits also known as chipping sequence. The code is also known as pseudo random because of its nature. To destined receivers it is deterministic for others it is noise therefore also as pseudo random noise. The size of the code determines amount of spreading. Different versions use different types of codes. For example:

  • 1 Mbps and 2 Mbps versions use 11 bit Barker code
  • 5.5 and 11 Mbps versions use complementary Code Keying(CCK)-set of 64 eight bits code words (802.11b)
  • 22 and 33 Mbps transmission uses PBCC-22, PBCC-33(802.11g)

 

Figure 3: DSSS PHY

 

Data is spreaded using PN code

1 Mbps and 2 Mbps versions use 11 bit Barker code

5.5 and 11 Mbps versions use complementary Code Keying(CCK)-set of 64 eight bits code words (802.11b)

22 and 33 Mbps transmission uses PBCC-22, PBCC-33(802.11g)

 

Infrared PHY

 

Diffuse infrared uses 1 and 2 Mbps transmission, 16-PPM and 4-PPM

 

Figure 4: Infrared PHY

 

OFDM PHY

 

Orthogonal Frequency Division Multiplexing :Divides a channel into sub-channels, thus using multiple carriers.Carriers may overlap, but received without the interference because they are orthogonal to each another. The modulation on each carrier is independent of each other. Bands used in different variants are:

 

2.4 GHz band (IEEE 802.11g D2.1 DSSS-OFDM, OFDM)

 

5 GHz band (IEEE 802.11a)

 

GHz OFDM + MIMO (IEEE 802.11n),(IEEE 802.11ac)

 

Figure 5: OFDM PHY

 

Layer2 MAC

 

Layer 2 or the MAC layer comprises of MAC entity which deals with basic access mechanisms and fragmentation and reassembly whereas MAC layer management entity deals with functions like synchronization,power management and roaming. The traffic services can be asynchronous data service which is mandatory whereas time bound services implemented using Point coordinate function are optional.

Access methods

 

Regardless of the physical layer used, all IEEE 802.11 wireless LAN clients use the same channel to transmit on. This Demands for precise multiple access mechanisms. The most basic of these is the Carrier Sense Multiple Access with Carrier Avoidance (CSMA/CA) mechanism. This mechanism is defined as part of the Distributed Coordination Function (DCF) of the IEEE 802.11b standard and is mandatory. In CSMA users first sense the transmission medium to see if anyone is transmitting just before transmitting a packet of data. When two or more packets are transmitted simultaneously a” collision” occurs. Wired Ethernet can listen to these collisions also but wireless LANs cannot because in WLANs when a wireless LAN card is transmitting it cannot listen to detect if packets collide. To handle this problem, the IEEE standard devised mechanisms to avoid collisions using waiting periods. These waiting periods ensure a channel has been idle for a certain period of time before transmitting. The waiting periods are known as Interframe spaces. Fig. shows different types of interframe spaces.

 

 

This implementation does not handle hidden terminal problem. Hidden terminal problem occurs when a mutual receiver is in range of two transmitters which are out of range with each other. The two transmitters cannot detect one another’s transmissions and their packets collide at mutual receiver. To handle this additional mechanisms are applied using control packets.

 

Another optional protocol that is part of the IEEE 802.11 standard is the Point Coordination Function (PCF). This function allows time critical or delay sensitive packets to be given priority over regular data transmissions. The PCF uses a polling procedure to setup a contention free period which takes priority over the DCF procedure. During the PCF established contention free period, a single host poles clients and allows them to transmit. In this way delay sensitive packets such as voice or video can be given priority over other data.

 

Both protocols are together known as Distributed foundation wireless Medium Access Control (DFWMAC) . The organization is shown below

 

Next section describes CSMA/CA and CSMA/CA with collission avoidance in detail.

 

CSMA/CA

 

The CSMA/CA works as follows(Fig. 6)

Sender

  1. Device wishing to transmit, senses channel to be idle . If medium is free for DIFS then transmit entire frame .
  2. if it sense channel busy then Back offs(No transmission) .It Initiates a random backoff time. Again senses the medium and continues to initiate timeouts if medium busy.

If after transmission it does not get ACK, it increase random backoff interval, repeat 2

Receiver

It checks CRC of the packet . Wait for SIFS and return ACK

 

Figure 6: Receipt of ACK indicates NO COLLISION

 

Virtual Carrier Sense

 

In this mechanism, transmitter relays short Ready to Send (RTS) packet to request the use of the medium. If this succeeds, the receiver waits for SIFS period and sends a Clear. After this, the actual transmission takes place. RTS and CTS broadcast duration of transmission .Other stations set a NAV or network allocation vector timer for this duration. For this duration they are deferred to access to medium. In this way the transmitter does not need to continue to sense the state of the channel until the NAV timer has expired. Hence the name virtual carrier sense. The steps are as follows:

  • Station send RTS with duration parameter after waiting for DIFS (duration determines amount of time the data packet needs the medium)
  • If ready to receive, Receiver sends CTS after SIFS;
  • Stations receiving RTS and CTS set their Network Allocation vector for the given duration
  • Stations use NAV along with physical carrier sense
  • Sender sends data; Receiver send ACK

The schematic diagram of the method is provided in Fig. 7

 

Figure 7 : CSMA / CA with RTS & CTS

 

Figure 8: Virtual Carrier Sense

 

Fragmentation and Reassembly Smaller packets are preferred for WLAN for the reasons given below:

  • Due to high bit error rate, probability of corruption increases with packet size
  • Smaller the packet less overhead to transmit it
  • Smaller the packet, smaller chance of postponed transmission after dwell time

 

But Ethernet is backbone for most of WLANs. Ethernet can handle packets 1518 bytes long. Therefore WLANs should be able to handle big packets. For trade-off, bigger packets from Ethernet are fragmented and smaller packets from WLAN are reassembled. It is implemented using Send-and-wait Algorithm. New fragment is transmitted only when acknowledgement for previous fragment is received or when after several retransmissions whole frame is dropped. Below diagram shows virtual carrier sense with fragmentation.

 

Figure 9: Virtual Carrier Sense with Fragmentation

 

Synchronization

 

Stations synchronize themselves with AP’s clock.The clock of AP is known by transmission of Beacon frames. This is the time when actual transmission takes place. Receiving stations synchronize their clocks with AP when the signal is received

 

Power Saving

 

Power is a scarce resource for wireless resource for wireless devices. IEEE 802.11 provides mechanisms for power saving. It provides provision for devices to go into sleep mode. AP maintains list of devices in sleep mode. AP buffers packets intended for devices in sleep mode. AP transmits information that which power saving devices have packets buffered at AP. Stations request packets either by polling or changing their mode to active.

 

Mobility

 

IEEE 802.11 standard defines the following mobility types (Fig. 10):

  • No-transition: no movement or moving within a local BSS
  • BSS-transition: station moves from one BSS in one ESS to another BSS within the same ESS
  • ESS-transition: station moves from a BSS in one ESS to a BSS in a different ESS

Figure 10: Types of Mobility

 

When a station wants to access an existing BSS either after power up, sleep mode or by physically entering into the BSS area, the station needs synchronization information from the AP. This information can be obtained via scanning process which may be active or passive. Once the access point is located, device decides to join the network, it goes through authentication process by exchange of authentication information between AP and device. Once authenticated, it starts association process by exchanging information about stations and BSS capabilities. Then and only then the station can transmit and receive data frames. The below diagram describes the whole process

 

Roaming

 

On a packet-based LAN system, when there is transition from cell to cell it may be performed between packet transmissions. In telephony environment, the transition may occur during a phone conversation. On a voice system, a temporary disconnection during handoff does affect the conversation. But, in a packet-based environment it significantly  reduces performance because retransmission is performed by the upper layer protocols. IEEE 802.11 standard does not specify how roaming should be performed but defines basic tools:

 

IAPP (802.11f): Common roaming protocol enabling the wireless stations to move across multivendor access points

  • Announce Protocol. It Provides coordination information between access points
  • Handover protocol. It allows APs to coordinate with each other and find status of the station. Frames buffered at old AP are forwarded to the new AP. New AP updates the necessary tables

Another protocol is Mobile IP which provides seamless connectivity between different types of networks.

 

Summary

  • IEEE 802.11 specifies a common MAC with multiple Physical layers namely Infra-red, DSSS, FHSS and OFDM
  • Distributed coordinate function and point coordinate function are basic access methods
  • DCF is mandatory and includes CSMA/CA and CSMA/CA with hidden terminal problem solving
  • PCF is for time bound services and is optional
  • Synchronization is achieved via access point in IEEE 802.11 LAN
  • To enable power saving, devices go in sleep mode and AP buffers the packets intended for them
  • WLAN handles smaller packets to reduce probability of error and reduce overhead during retransmission. To exchange packets with Ethernet, fragmentation and reassembly is done
  • To enable mobility scanning, authentication and authorization processes are followed
  • Roaming is supported by protocols like IAPP and Mobile IP

Suggested Reading:

  1. Mobile Communication 2nd edition by Jochen Schiller, Pearson education
  2. Mobile Computing by Asoke Talukder, Roopa Yavagal (Tata McGraw Hill)
  3. “Wireless communication and networking” by William Stallings
  4. Mobile Cellular Telecommunications — W.C.Y. Lee, Mc Graw Hill
  5. Wireless Communications – Theodore. S. Rapport, Pearson Education
  6. Reza B’Far (Ed), “Mobile Computing Principles”, Cambridge University Press.