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1/11

IEE 802.11

System ArchitectureProtocol Architecture As indicated by the standard number, IEEE 802.11 fits seamlessly into the other

802.x standards for wired LANs

Applications should not notice any difference apart from the lower bandwidth andperhaps higher access time from the wireless LAN. The WLAN behaves like a

slow wired LAN. Consequently, the higher layers (application, TCP, IP) look the

same for wireless nodes as for wired nodes.

The upper part of the data link control layer, the logical link control (LLC), coversthe differences of the medium access control layers needed for the different media.

The IEEE 802.11 standard only covers the physical layerPHY and medium accesslayerMAC like the other 802.x LANs do

The physical layer is subdivided into the physical layer convergence protocol(PLCP) and the physical medium dependent sublayerPMD.

The basic tasks of the MAC layer comprise medium access, fragmentation of userdata, and encryption.


2/11

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3/11

Apart from the protocol sublayers, the standard specifies management layers andthe station management. The MAC management supports the association and re-

association of a station to an access point and roaming between different access

points. It also controls authentication mechanisms, encryption, synchronization of

a station with regard to an access point, and power management to save battery

power. MAC management also maintains the MAC management information base(MIB).

The main tasks of the PHY management include channel tuning and PHY MIBmaintenance. Finally, station management interacts with both management layers

and is responsible for additional higher layer functions (e.g., control of bridging

and interaction with the distribution system in the case of an access point).

Physical Layer:

IEEE 802.11 supports three different physical layers: one layer based on infra red andtwo layers based on radio transmission (primarily in the ISM band at 2.4 GHz, which is

available worldwide). All PHY variants include the provision of

the clear channel assessment signal (CCA). This is needed for the MAC mechanisms

controlling medium access and indicates if the medium is currently idle.

The PHY layer offers a service access point (SAP) with 1 or 2 Mbit/s transfer rate to the

MAC layer (basic version of the standard).

Frequency hopping spread spectrumFHSS which allows for the coexistence of multiple networks in the same area by

separating different networks using different hopping sequences . The original standarddefines 79 hopping channels for North America and Europe, and 23 hopping channels for

Japan (each with a bandwidth of 1 MHz in the 2.4 GHz ISM band). The selection of a

particular channel is achieved by using a pseudo-random hopping pattern. National

restrictions also determine further parameters, e.g., maximum transmit power is 1 W in

the US, 100 mW EIRP (equivalent isotropic radiated power) in Europe and 10 mW/MHz

in Japan.

The standard specifies Gaussian shaped FSK (frequency shift keying), GFSK, as

modulation for the FHSS PHY. For 1 Mbit/s a 2 level GFSK is used (i.e., 1 bit is mapped

to one frequency), a 4 level GFSK for 2 Mbit/s (i.e., 2 bits are mapped to one frequency).While sending and receiving at 1 Mbit/s is mandatory for all devices, operation at 2

Mbit/s is optional. This facilitated the production of low-cost devices for the lower rate

only and more powerful devices for both transmission rates in the early days of 802.11.


4/11

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5/11

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6/11

The IEEE 802.11 Medium Access Control Layer

MSs in an IEEE802.11network have to share the transmission medium, which isair.

If two MSs transmit at the same time and the transmissions are both in range of thedestination, then they may collide, resulting in the frames being lost.

The MAC layer is responsible for controlling access to the medium and ensuringthat MSs can access the medium in a fair manner with minimal collisions.

The medium access mechanism is based on CSMA, but there is no collisiondetection, unlike the wired equivalent LAN standard (IEEE 802.3).

In IEEE 802.3, sensing the channel is very simple. The receiver reads the peakvoltage on the wire or cable and compares that against a threshold.

Collisions are extremely hard to detect in RF because of the dynamic nature of thechannel.

Detecting collisions also results difficulties in hardware implementation becausean MS has to be transmitting and receiving at the same time.

Instead, the strategy adopted is to avoid collisions to the greatest extent possible. In IEEE 802.11, there are two types of carrier sensing: physical sensing of energy

in the medium and virtual sensing.

Physical sensing is through a clear channel assessment (CCA) signal produced bythe physical layer convergence protocol (PLCP) in the physical layer of the

IEEE802.11.

The CCA is generated based on real sensing of the air interface, either bysensing the detected bits in the air or by checking the RSS of the carrier against athreshold.

Decisions based on the detected bits are made slightly slower, but they are morereliable.

Decisions based on the RSS may create false alarms caused by high interferencelevels.

The best designs take advantage of both carrier sensing and detected data sensing. In addition to physical sensing, IEEE 802.11 also provides for virtual carrier

sensing.

Virtual sensing is implemented by decoding a duration field in the 802.11 framethat allows an MS to know the time for which a frame will last.

A length field in the MAC layer is used to specify the amount of time that mustelapse before the medium can be freed.

This time is stored in a network allocation vector (NAV) that counts down to zeroto indicate when the medium is free again.


7/11

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8/11

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9/11

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10/11

C at the same time, the frames will collide. This problem is called the hidden

terminal problem.

There is a dual problem called the exposed terminal problem. In this case, MS-Ais transmitting a frame to MS-D. This transmission is heard by MS-C, which then

backs off. However, MS-C could have transmitted a frame to MS-B and the two

transmissions would not interfere or collide. In this case, MS-A is called anexposed terminal.

Both hidden and exposed terminals cause a loss of throughput.To reduce the possibility of collisions due to the hidden terminal problem, the

IEEE 802.11 MAC has an optional mechanism at the MAC layer, as given

below.

Suppose MS-A wants to transmit a frame to MS-C. It will first transmit a shortframe called the RTS frame. The RTS frame is heard in the transmission range of

MS-A and includes MS-C and MS-D, but not MS-B. Both MS-C and MS-D are alerted to the fact that MS-A intends to transmit a

frame and they will not attempt to simultaneously use the medium.

This is achieved by the virtual carrier sensing process that sets the NAV to a valueequal to the time it will take to complete the exchange of frames successfully.

In response to the RTS frame, MS-C will send a clear-to-send (CTS) frame thatwill be heard by all MSs in its transmission range. This includes MS-B and MS-A

but not MS-D.

The CTS frame lets MS-A know that MS-C is ready to receive the data frame. Italso alerts MS-B to the fact that there will be a transmission from some MS to

MS-C. Consequently, MS-B will defer any frames that it wishes to transmit inanticipation of the completion of the communication to MS-C.

This way, even though MS-B is outside the transmission range of MS-A, the CTSmessage can be used to extend the carrier sensing range, thereby reducing the

hidden terminal problem.

Of course, it is quite possible that the RTS frame itself collided with atransmission from MS-B. In such a case, both MS-A and MS-B will have to enter

the back-off process and retransmit their frames.


11/11

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