ECE442 Communications Lecture 3. Wireless Local Area Networks
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1 ECE442 Communications Lecture 3. Wireless Local Area Networks Husheng Li Dept. of Electrical Engineering and Computer Science Spring, 2014
2 Wireless Local Networks 1 A WLAN links two or more devices using some wireless distribution method and usually provide a connection through an access point to the wider Internet. 2 WLAN becomes popular in the home due to ease of installation and in commercial complexes offering wireless access to their customers. 3 A typical WLAN is IEEE (WiFi).
3 History of IEEE
4 IEEE n 1 The basic service set (BSS) is the basic building block of an n LAN. There is a central station dedicated to managing the BSS, which is called access point (AP). A BSS built around an AP is called an infrastructure BSS. Infrastructure BSSs may be interconnected via their APs through a distributed system (DS). 2 The BSSs interconnected by a DS form an extended service set (ESS).
5 BSS, DS and ESS
6 Illustration of Environments Three typical environments: residential, enterprise and hotspot.
7 Features of IEEE n In contrast to previous protocols, n has various mandatory or optional features.
8 Channel Models There are A to F models for the wireless channels in n, with different time spreads.
9 PHY Layer of IEEE n 1 OFDM is used in n, built upon the a. 2 In a, the fundamental sampling rate is 20MHz, with a 64-point FFT/IFFT. The Fourier transform symbol period, T, is 3.2us in duration and F is 312.5kHz. 3 Of the 64 subcarriers in a, there are 52 populated subcarriers. Other 12 subcarriers are not used.
10 Comparison of OFDM and Single Carrier In contrast to previous protocols, n has various mandatory or optional features.
11 MIMO in n A key feature of n is MIMO/SDM.
12 MIMO Receiver Structure
13 Receive Diversity Maximal ratio combining or selection diversity is used for the receive diversity.
14 Transmit Diversity In n, space-time coding (e.g., the Alamouti scheme) is also used.
15 LDPC Coding
16 MAC Layer of IEEE n 1 MAC layer provides addressing and channel access control that makes it possible for multiple stations on a network to communicate. 2 IEEE makes use of the IEEE bit global address space, making it compatible with Ethernet at the link layer. 3 IEEE supports CSMA/CA.
17 Protocol Layering Different layers exchange different types of PDUs.
18 Management Functions: Beacon The AP in an infrastructure BSS periodically broadcasts Beacon frames.
19 Management Functions: Scanning Scanning is a process by which a station discovers a BSS and the attributes associated with that BSS. Scanning could be either passive or active. A probe request frame includes (a) SSID, (b) BSSID and (c) DA.
20 Management Functions: Others Authentication Association Reassociation Disassociation
21 Distributed Channel Access The CSMA/CA mechanism in MAC is referred to as the distributed coordination function (DCF). A station that wishes to transmit first performs a clear channel assessment (CCA) by sensing the medium for a fixed duration (DIFS). If the medium is idle, the station begins a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS and waits for a further random back off period. Once a station has gained access to the medium, it maintains control of the medium by keeping a minimum gap (SIFS).
22 Channel Access Timing The different inter-frame space (IFS) durations effectively provide access to the wireless medium at different priority levels.
23 Random Backoff Time The random backoff count is selected as a pseudo-random integer drawn from a uniform distribution over [0, CW ]. CW means contention window. CW doubles on each unsuccessful MPDU transmit, within [CWmin, CWmax]. CW, CWmin and CWmax may take values that are a power of 2 less 1.
24 Backoff Procedure When multiple stations are deferring and go into random back off, the station selecting the smallest back off count will win the contention and transmit first.
25 Data/ACK Frame Exchange If the station sending the data frame does not receive the ACK frame, it assumes the data frame was not received and may retransmit it.
26 Fragmentation Fragmentation is used to break up large MSDUs to improve the chance that the MSDU will be received correctly and to reduce the overhead of retransmission.
27 Duplicate Detection It is possible that a transmitted data frame is received twice (e.g., the ACK message is missed). To detect duplicate frames, the receiving station keeps track of the sequence number and fragment numbers of the last MSDU or management frame.
28 Overhead and Fairness The overhead includes the contention period during which the medium is essentially idle, the overhead associated with the transmission, the ratio turnaround time and the time for ACK.
29 Hidden Node Problem A serious challenge in CSMA is the hidden node problem, because the different locations of transmitter and receiver.
30 How to Tackle the Hidden Node Network allocation vector (NAV) can be used to handle the hidden nodes. RTS/CTS can also be used to handle the hidden nodes.
31 How EIFS Works? EISF assists NAV.
32 Enhancement: EDCA QoS is supported. The EDCA mechanism defines four access categories.
33 Enhancement: EDCA If two or more instances of the EDCA access function gain access simultaneously, the interval collision is resolved by the highest priority AC gaining access and the other AC behaving as if an external collision occurred by doubling its contention window.
34 Enhancement: EDCA Priority
35 Block Acknowledgement The block acknowledgement protocol, introduced with the e amendment, improves efficiency by allowing for the transfer of a block of data frames that are acknowledged with a single BA frame.
36 Throughput vs. PHY Rate The poor scaling of throughput above the MAC with PHY data rate is illustrated above.
37 802.11n Enhancement The n amendment developed a number of simple enhancements to the e MAC that significantly improved efficiency.
38 Throughput vs. PHY Rate in n
39 Aggregation Three techniques were proposed for aggregation, two of which were ultimately adopted in the standard.
40 Aggregate MPDU With A-MPDU, fully formed MAC PDUs are logically aggregated at the bottoms of the MAC layer.
41 Immediate and delayed block ACK differ in the handling of the BAR and BA frames. Block ACK
42 HT-immediate Block ACK HT-immediate black ACK is introduced in n. It is used between HT stations.
43 PCF With PCF, the point coordinator (PC), which resides in the AP, establishes a periodic contention free period during which con tention free access to the wireless medium is coordinated by the PC.
44 Data Transfer in PCF The contention free transfer protocol is based on a polling scheme controlled by the PC in the AP of the BSS.
45 HCCA Hybrid coordination function (HCF) controlled channel access (HCCA) is a channel access technique that is centrally coordinated by the hybrid coordinator residing in the AP.
46 Reverse Direction Protocol Many traffic patterns are highly asymmetric. The reverse direction protocol is an optional feature to tackle this.
47 PSMP Power-save multi-poll (PSMP) is a scheduling technique introduced in n to optimize channel access for devices that receive and transmit small amounts of data periodically.
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