Define WLAN. Illustrate the system architecture of IEEE802.11 WLAN. Explain with diagram the protocol architectures and manage of IEEE 802.11.
WLAN (wireless local area network) is a local area network without wires. Mobile users can access information and network resources through WLAN as they are moving from one place to another. It is implemented as an extension of wired LAN within a building or campus.
The advantages of WLAN are:
In IEEE 802.11 WLAN, one cell or a or a BSS is controlled by one Base Station which is called the access points(AP). The access points in different cells are connected through a backbone called the distribution system(DS). The DS is some kind of Ethernet.
The entire interconnected wireless LAN, including the different cells, their respective Access points and the distribution system is seen as a single network to the upper layer of the OSI model and is called the extended Service set(ESS). 802.11 also defines the concept of portal. A portal is the logical integration between wired LANs and 802.11. It also can serve as the access point to the DS. All data going to an 802.11 LAN from an 802.X LAN must pass through a portal. It thus functions as bridge between wired and wireless.
For proper functioning of wireless LAN, neighbouring cells (BSS) are set up on different frequencies, so that wireless LAN cards in each cell do not interfere with one another when they transmit.
802.11 standards cover for both MAC(medium access control) and physical layer. The standard currently defines a single MAC, which interacts with three PHYs as follows:
1) frequency hoping spread spectrum
2)direct sequence spread spectrum
3) infrared
The 802.11 protocol stack is as follows:
|
802.2 |
Data link layer |
|||
|
802.11 MAC |
MAC layer |
|||
|
Frequency hoping |
Direct sequence |
infrared |
Physical layer |
PLCP sublayer |
|
PMD sublayer |
||||
Layer 1 is the physical layer:
It comprises of two sublayers:
1)PLCP(physical layer convergence procedure): PLCP sublayer is responsible for each carrier sense part of the CSMA/CA protocol.
2) PMD(physical medium dependent): it provides the actual transmission and reception of physical layer entities between stations through the wireless media. This sublayer provides the modulation and demodulation of the transmission.
FHSS(frequency hoping spread spectrum) physical layer: it carries the clocking information to synchronise the receiver clock of the transmitted packet. The field in the FHSS PLCP is as follows:
|
SYNC(synchronize clock of the receiver) |
START FRAME DELIMITER |
PLW(length of PLW) |
PSF(PLCP signalling) |
HEADER ERROR CHECK(contain CRC) |
VARIABLE PLCP SERVICE DATA UNIT |
Direct sequence spread spectrum(DSSS)physical layer:it is responsible for synchronizing and receiving the data correctly. The DSSS PLCP header is:
|
SYNC |
START FRAME DELIMITER |
SIGNAL |
SERVICE |
LENGTH |
FRAME CHECK SEQUENCE |
MPDU/p> |
Layer 2 is the MAC layer: the MAC layer defines two different access methods 1) distributed coordination function: Distributed coordinate function uses CSMA/CA.
2) point coordination function:Point coordination function has a centralized, control free polling access method. The AP performs polling for stations that are capable of being polled. The stations are polled one after another, sending any data they have to the AP.
What are hidden station problem and exposed station problem in WLAN? How are the problems solved?
What is meant by access control? What are the contention-based and contention free access control mechanisms?
What is multiple access technique? Do a comparative study among FDMA,TDMA and CDMA.
Write a C program to print all prime no.(1-100).
Write short notes on the following :
a) BFS
b) Tail recursion
What is WAP? Why it is used?
Difference between Statement coverage and Branch coverage.
What are the methodology for dialog design?
Write a C program to find a number is AMSTRONG or NOT.
