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Wireless Networks Presentation
Transcript of Wireless Networks Presentation
Benefits of WLAN
Standardization and Frequency Band
Applications Introduction to WLAN
WLAN- Wireless Local Area Network, it provides high bandwidth to users in a limited geographical area using wireless techniques.
What is the Range of Local Area in WLAN?
indoor range 20m - 70m
outdoor range 100m - 250m IEEE 802.11 Maiwulan - Introduction to WLAN - Challenges in WLAN - IEEE 802.11 standard IEEE 802.11 Topology
Physical layer What is WLAN? History of WLAN The basis for the technology that is used in WLANs today was developed in World War II by U.S military as a way for securing the safe, private delivery of voice communication without eavesdropping by the enemy. Several new techniques: spread spectrum, infrared, UHF (narrow band), and orthogonal frequency division multiplexing (OFDM)... First WLAN products appeared in the market around 1990. ISM band (industrial, scientific and medical radio bands) released worldwide. It provided unlicensed spectrum at 2.4 GHz . This Increased the design for WLAN. Standardization started by IEEE (Institute of Electrical and Electronic Engineers) in 1990. IEEE 802.11 standard is ratified as an international standard for WLAN in 1997. Various versions of IEEE 802.11 added later, such as 802.11a, 802.11b, and so on... Benefits of WLAN Mobility Cost Saving Wire-free. There is no need for cable anymore. Mobility allows users to physically move by the time using a wireless application such as Laptops, Tablets, mobile phones or data collector within the connection area. It costs a lot of money to establish wired networks. WLAN applications save money in this matter. Other benefits: Easy of installation. WLAN has competitive advantage when connecting to the network for a short period of time, such as in conferences. When we consider the difficult wiring environment, for example, historical buildings or older buildings. Standardization and Frequency Band Two standards: IEEE 802.11 HIPERLAN-High Performance European Radio LAN European Telecommunications Standards Institute developed by IEEE 802.11 IEEE 802.11 is a set of standards for implementing wireless local area network (WLAN) computer communication. It works on 2.4, 3.6, 5 and 60 GHz frequency bands. 802.11-1997 was the first wireless networking standard, 802.11b and 802.11g developed later on, but 802.11a was the first widely accepted one. Other standards in the family (c–f, h, j) are extensions or corrections to the previous specifications. 802.11n used new multi-streaming modulation technique. Source: http://en.wikipedia.org/wiki/IEEE_802.11 A figure on 802.11 standard development Applications What is in this part? IEEE 802.11 Topology Physical layer Medium Access Control sublayer IEEE 802.11 Topology The 802.11 standard supports two topologies:
Extended service set (ESS) networks. also known as Ad hoc network Minimal configuration and quick deployment make ad hoc networks suitable for emergency situations like natural disasters or military conflicts. Advantages and Applications: Basic service set (BSS) is the fundamental building block of the IEEE 802.11 architecture. All stations in a BSS can communicate directly with all other stations in a BSS. A group of stations into a single BSS for the purpose of communications without the aid of an infrastructure network is considered as an ad hoc network. Comparing to Ad hoc network, ESS network provides wireless users with specific services and range extension. Infrastructure networks are established using APs (Access Point). AP supports geographical range extension by providing the integration points necessary for network connectivity between multiple BSSs, thus forming an ESS. A figure of Extension service set. An ESS also provides gateway access for wireless users into a wired network such as the Internet. Physical layer There are four different implementations in physical layer of IEEE 802.11 standard:
Frequency hopping spread spectrum (FHSS)
Direct sequence spread spectrum (DSSS)
OFDM Direct sequence spread spectrum (DSSS) DSSS uses the 2.4-GHz ISM band, it gives data rates of 1 and 2 Mbps and extension rate of 5.5 and 11 Mbps. Following figure shows the DSSS packet structure: The first segment Preamble is used for signal detection and synchronization. The second segment Header contains data-rate and packet-length information. The third segment MPDU contains the information bits. The complete packet PPDU has three segments: The FSSS utilizes the 2.4-GHz ISM band (i.e., 2.4000-2.4835 GHz). In the United States, a maximum of 79 channels are specified in the hopping set. Different hopping sequences enable multiple BSSs to coexist in the same geographical area, which may become important to mitigate congestion and maximize the total throughput in a single BSS. Infrared The IR band is designed for indoor use only. It is enable stations to receive line-of-site and reflected transmission. The IR specification identifies a wavelength range from 850 to 950 nm. Characteristics of IR:
Highest tolerent to interference
Accepted worldwide without frequency regulation Medium Access Control (MAC) sublayer Responsibilty of MAC sublayer: Channel allocation procedures
Protocol data unit (PDU) addressing
Fragmentation and reassembly Medium can alternate between contention period (CP), and a contention-free period (CFP). Three different types of frames in MAC sublayer: Management frame
Data frame Management frames are used for station association and disassociation with the AP, timing and synchronization. Control frames are used for handshaking during the CP, for positive acknowledgments during the CP, and to end the CFP. Data frames are used for the transmission of data during the CP and CFP. The following figure shows IEEE 802.11 frame format: The 2 duration bytes indicate the time (in microseconds) the channel will be allocated for successful transmission of a MAC protocol data unit (MPDU).
The type bits identify the frame as either control, management, or data. The subtype bits further identify the type of frame (e.g., Clear to Send control frame).
For error detection, a 32-bit cyclic redundancy check (CRC) is used. Challenges in WLAN Interference and Reliability
Throughput Interference and Reliability Simultaneous transmissions (i.e., collisions) by two or more sources sharing the same frequency band. Multi-path fading Interference: Reliability: In packetized voice, packet loss rates on the order of are generally acceptable. In encoded data, a BER of is regarded as acceptable. Automatic repeat request (ARQ)
Forward error correction (FEC) There are two methods used to increase reliability: Security Wired LAN:
The information is running inside a cable. Transmission medium can be physically secured, and access to the network is easily controlled. WLAN:
Transmission medium is open to anyone within the geographical range of a transmitter. It is difficult to secure. Power Consumption As we know WLAN devices are battery powered. Energy-efficient is very important to make battery last longer. There are some Slolutions for energy-efficiency: Switch wireless NICs (network interface card) to lower power standby mode when there is not data transmission. Low power display MAC layer implements power management functions by putting the radio to sleep when there is no transmission activity occurs for a specific or user-definable time period. Human Safety Research is ongoing to determine whether radio frequency (RF) transmission from WLAN is linked to human illness. Does WLAN cause human illness? How is the capacity of WLANs comparing to the wired LANs? Throughput Physical limitation and limited available bandwidth. v98982 THANK YOU!! ? We expect the capacity of WLAN approaches to the wired LAN. In communication networks, such as Ethernet or packet radio, throughput or network throughput is the average rate of successful message delivery over a communication channel. What is throughput? Example: with no transmission errors and 1460 byte sized packets, the throughput of an 10 Mbps system is just 6.1 Mbps. The efficiency is significantly lower for smaller packet sizes. The efficiency of IEEE 802.11 is in sharp contrast to wired technologies where, for example, a 10 Mbps Ethernet (802.3) link offers the users almost 10 Mbps. The reason for low efficiency might related to : There are two basic aspects to wireless LAN security:
Encryption. Authentication A system uses authentication to check a user's credentials and determine if the user should be given access to the data and resources. The supplicant is the client (PC or laptop computer, for example) who would like to gain access to network resources through the WLAN.
The authenticator, which for a WLAN is usually an access point (AP), plays the role of gatekeeper.
The authentication server, which connects to the AP over a wired network, handles the authentication procedure. The authentication procedure involves three basic steps: Encryption Encryption is the process of encoding messages (or information) in such a way that eavesdroppers or hackers cannot read it, but that authorized parties can. It is one of the most crucial aspects of WLAN technology. As illustrated in the schematic, wireless encryption has devolved from WEP, which was released in 1999, to the 802.11i standard, more commonly referred to as WPA2. Cellular phones 600 milliwatts - 3 watts WLAN 50 -100 milliwatts However, WLANs should be designed to minimize the power transmitted by network devices. But at least it is safer then cellular phone network: A topology explains necessary physical components of a network, but the logical architecture defines the network's operation. The logical architecture of the 802.11 standard that applies to each station consists of a single MAC layer and one of multiple PHY. In the seven-layer OSI model of computer networking, media access control (MAC) data communication protocol is a sublayer of the data link layer. This figure shows the location of Physical layer and MAC in seven-layer OSI model. Frequency hopping spread spectrum (FHSS) A simple internet speed test related to throughput: Wired LAN network speed test result Wireless LAN network speed test result A simple internet speed test related to throughput: Test environment: Same laptop
Same local area
Independent basic service set (IBSS) networks. What is Ad hoc network? Any station in a Ad hoc network can establish a direct communication session with any other station in the BSS, without the requirement of channeling all traffic through a centralized AP. Extension service set (ESS) is extension of Ad hoc network. We have talked about two different topology that used in WLAN: IBSS and ESS. What kind of challenges are we facing in Wireless local area network? Security is a main issue in WLAN, because.. WEP - Wired Equivalent Privacy. It is a security algorithm for IEEE 802.11 wireless networks. WPA - Wi-Fi Protected Access. WPA and Wi-Fi Protected Access II (WPA2) are two security protocols and security certification programs developed by the Wi-Fi Alliance to secure wireless computer networks.