A Survey on IEEE 802.11 Wireless LAN Standards and Physical Layer Issues

* Following is my work done as an assignment for Computer Networks.

 Wireless LAN s: 802.11

 

1.1        Introduction

In this chapter, we present on the evolution of wireless LANs and how it operates.

1.2        Wireless LANs: 802.11

Computer networks for the home and small business can be built using either wired or wireless technology but as the world is looking for “mobility” the use of wireless networks seems to be dominating the Ethernet. More than calling it “Wireless LANs” the common slang used for 802.11 standards is “wifi” which most of today world is quite familiar with.

1.2.1     IEEE 802.11 standard

At present IEEE 802.11 is the de facto standard for WLANs [2]. Both the medium access control (MAC) and the physical (PHY) layers for WLANs have been specified in this standard. This separation of 2 layers done in the standard allow a functional separation of the standard and more importantly allows a single data protocol to be used with several different Radio Frequency (RF) transmission techniques [4]. The scope of IEEE 802.11 working groups (WGs) is to propose and develop MAC and PHY layer specifications for WLAN to handle mobile as well as portable stations [3].

1.2.2     IEEE 802.11 Configurations

1.2.2.1       Wireless networking with a base station

In this mode, all communications go through the base station, called an “access point” in 802.11 terminology. Figure 1 depicts this scenario. This mode is also called infrastructure mode. Access points are used for all communications in these networks. If one mobile station needs to communicate with a second mobile station, the communication must take two hops. First, the originating mobile station transfers the frame to the access point and then the access point transfers the frame to the destination station.

The 802.11 standard places no limit on the number of mobile stations that an access point may serve. Implementation considerations may, of course, limit the number of mobile stations an access point may serve. Still practically the number of stations placed on a wireless network is limited by low throughput of wireless networks.

1

Figure 1: Wireless networking with a base station.[2]

1.2.2.2       Ad hoc networking

            Figure 2 shows ad hoc networking where the computers would just send to one another directly. The smallest possible 802.11 network is an ad hoc network with two stations. Typically, these are composed of a small number of stations set up for a specific purpose and for a short period of time. An example is to create a short-lived network to support a single meeting in a conference room where the participants create an ad hoc network to share data as the meeting begins and the network is dissolved as the meeting ends.2   Figure 2: Ad hoc networking [2]

IEEE 802.11 physical layer standards.

2.1        Introduction

In this chapter we will present on different standards being introduced under IEEE 802.11 and different signal transmission techniques used in them. Also some issues related to 802.11 PHY layer will be discussed.

2.2        Physical layer in 802.11 standard

Physical layer in 802.11 is divided into 2 sub layers as below.

5

 

 

As far as the 802.11 PHY layer is concerned, there are 5 standards available at present as

1)                  802.11a

2)                  802.11b

3)                  802.11g

4)                  802.11n

5)                  802.11ac

We can compare above different standards considering the modulation techniques used and also with respect to behavior in 802.11 MAC layer.           The physical layer of the original 802.11 standardized three wireless data exchange techniques:

1)                   Infrared (IR);

2)                   Frequency hopping spread spectrum (FHSS);

3)                   Direct sequence spread spectrum (DSSS).

Below we will present briefly on above 3 techniques.

2.2.1   Infrared (IR)

We could not find any successful implementation of this in 802.11 since it could not receive public acceptance since the connection was very slow.

2.2.2   Frequency hopping spread spectrum (FHSS)

The basic idea behind this modulation technique is to transmit on a given frequency for a very short time and switch to another frequency according to a pre-defined frequency hopping pattern known to both transmitter and receiver.

2.2.3        Direct sequence spread spectrum (DSSS).

The most commonly used technique at present is DSSS. In addition to Wireless LANs, it is also being used in cellular networks (CDMA systems), Global Positioning Systems (GPS). The idea behind this technique is to multiply the data being transmitted to a pseudo random binary sequence of a higher bit rate.

Figure 3 depicts the relationship between above 3 RF transmission techniques.

3

Figure 3: A diagrammatic overview of the IEEE 802.11 standard. [4]

2.2.4      Comparison between different transmission techniques

As far as complexity is concerned, DSSS is more complicated than FHSS and former has a simpler MAC protocol. FHSS are considered to be cheaper than DHSS systems because with the increasing integration of digital hardware, it doesn’t cost much more to implement the specific MAC functionalities required for the FHSS system.

As far as bandwidth sharing and of resistance to interferences are considered, the two technologies perform really differently. Also DHSS systems tend to have a lower overhead on the air compared to FHSS [6].

2.3        Comparison Between different standards

Table 1 summarizes different parameter values for different PHY standards. The detailed explanation on these is given after.

Table 1: Comparison of different parameter values of different PHY standards

          StandardCharacteristic 802.11a 802.11b 802.11g 802.11n
Frequency/GHz 5 2.4 2.4 2.4/5
Speed (Mbps) 54 11 54 300
Modulation BPSK,QPSK,16 QAM,64 QAM (OFDM) DBPSK,DQPSK,CCK(DSSS,IR and FH) DBPSK,DQPSK,CCK(DSSS,IR and FH) BPSK,QAM,QPSK

2.3.1        802.11b

From all above the 802.11b standard is the most widely deployed WLANs [5]. The standard was a result of adoption of standard called Complementary Code Keying (CCK) to achieve 5.5Mbps and 11Mbps transmit rates.

802.11b included a new option to transmit PLCP header with a short (56 bits) preamble. In the short preamble mode only the Synchronization and Start Frame Delimiter fields are transmitted at 1Mbps. The rest of the PLCP header is transmitted at 2Mbps (using DSSS DQPSK) and the data payload at either the same 2Mbps, or using CCK at 5.5Mbps or 11Mbps.802.11b also standardized the procedure of adjusting the data transmission rate depending on the link quality by introducing the auto rate fallback mechanism missed in the original 802.11.

802.11b uses the same unregulated radio signaling frequency (2.4 GHz) as the original 802.11 standard. Though it is hence cheaper being unregulated, 802.11b gear can incur interference from microwave ovens, cordless phones, and other appliances using the same 2.4 GHz range. However, by installing 802.11b gear a reasonable distance from other appliances, interference can easily be avoided.

2.3.2  802.11a

A new modulation technique called Orthogonal Frequency Division Multiplexing (OFDM) was introduced by 802.11a which   allows higher data transmission rates in the smaller bandwidth. Besides proposing the new modulation method, 802.11a also switches from the rapidly getting overused 2.4GHz ISM band to 5GHz ISM band. The 5GHz ISM bandwidth is not continuous. There are two areas 5.15GHz – 5.35GHz and 5.725GHz – 5.825Ghz. Both areas are separated by 802.11a into 12 overlapping carriers (similar to 802.11 channels) spaced 20MHz.

802.11a supports bandwidth up to 54 Mbps and signals in a regulated frequency spectrum around 5 GHz. This higher frequency compared to 802.11b shortens the range of 802.11a networks. The higher frequency also means 802.11a signals have more difficulty penetrating walls and other obstructions.

2.3.3  802.11g

WLAN products supporting 802.11g appeared in the market in 2002 which attempted to combine the advantages of both 802.11a and 802.11b.It supports bandwidth up to 54 Mbps, and uses the 2.4 Ghz frequency for greater range. 802.11g is backwards compatible with 802.11b, meaning that 802.11g access points will work with 802.11b wireless network adapters and vice versa.

In the case of 802.11g, appliances may interfere on the unregulated signal frequency.

2.3.4  802.11n

802.11n tries to provide data transfer rates similar to those of the Fast Ethernet. The concept presented for achieving this is as follows.

Consider a conventional 802.11g adapter radio. One of the major problems that radio receiver has to deal with is the multipath propagation. Essentially the receiver has to filter various echo of the main signal it is tuned to, which is similar to having multiple transmitters of that signal in the environment with no echo.

The receivers successfully do the job and hence it must be possible to install two transmitters, then install two receivers and tune each of them to the individual transmitter. Data can be sent through two channels and the 54Mbs rate of the standard 802.11g equipment is doubled if this succeeds.

Figure 4depicts this scenario.

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Figure 4: The concept behind 802.11n [7]

2.4        Physical layer issues

2.4.1     Interferences and noise

One of the factors that affect the performance of wireless system is interferences and background noises.

2.4.1.1       Microwave oven and other interferers

Since Wireless LANs tend to be tend to be implemented in the unlicensed bands which is not the case for vast majority of Wireless systems like cellular phone, telecoms, aviations etc. ,they don’t benefit from absence of interferers in the band they are using and hence have to cope with the emissions of other systems.Therefore other communication systems like other Wireless LANs and cordless phones can create interferences.

Also, microwave ovens which operates using 2.4GHz band can create interferences. The impact of these interferences is that it could make packets corrupted since packets may get collided with interferences signal. If the Signal to Noise Ratio (SNR) between the packet and the interferer is high enough the receiver can “capture” the packet, otherwise it would get corrupted. Although most Wireless LANs can cope well with interferences, they may reduce performance.

2.4.1.2       Fading

Fading refers to temporal variations of the signal attenuation due to its propagation in environment. The environment being not static and things being moving, the propagation between two nodes may alternate from poor to good on a packet basis.

The transmission errors caused by fading are needed to be overcome by the system which would add overhead. Also the impact of fading would increase with the operating distance between 2 nodes.

The technique used for overcoming the effect of fading is antenna diversity which basically means that radio device has 2 or more antennas.  One antenna may give a poor signal and the other a good one, and later it might be the reverse. So, before receiving each packet, the receiver chooses the best antenna of the two by comparing the signal strengths, and so can avoid most of the fade out periods.

2.4.1.3       Forward Error Correction (FEC)

Although the packet would be error free when signal is strong, when signal is weak the packet contains lots of error and hence we have to use some error control mechanism like FEC. To correct all those errors in corrupted packets, it would require a very strong FEC code. Still, this code would add a lot of redundancy bits, resulting in a lot of overhead. In the meantime normal FEC code would add less overhead, but be useless with the correct packets and inefficient with the highly corrupted packets.

Therefore retransmission (just retransmit the original packet in case of errors – some form of packet scheduling and retransmission has been proven to be nearly optimal in Rayleigh fading channels) is considered to as a better solution. This is usually implemented at the MAC level.

Conclusion

In this we saw different IEEE 802.11 standards and techniques used in them. Issues encountered in Physical layer were also discussed. We observe that new techniques for transmitting signals with fewer errors and also for error controlling are being introduced. Each of these standards has its own pros and cons. These standards also try to address several issues faced in 802.11 physical layer.

References

 

         [1]               IEEE 802.11 WG. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications, 1999.

            [2]            http://itlaw.wikia.com/wiki (2012 -05-10)

            [3]            “Hands-On Exercises:IEEE 802.11  Standard”  Mohammad Hossein Manshaei and Jean-Pierre Hubaux  (Laboratory for Computer Communications andApplications (LCA))

            [4]            “A Brief Tutorial on the PHY and MAC layers of the IEEE 802.11b  standard” Benjamin E. Henry july 12, 2011

            [5]            IEEE 802.11 WG part 11b. Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifcations, Higher Speed PHY Layer Extension in the 2.4 GHz Band, 1999.

            [6]            “The radio modem (Physical layer)”http://www.hpl.hp.com/personal/Jean_Tourrilhes/Linux/Linux.Wireless.modem.html (2012-5-10)

            [7]            Evolution of 802.11 (physical layer)https://www.okob.net/texts/mydocuments/80211physlayer/ (2012-05-10)

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