Mini Project- Implementation & Evaluation of Wireless LANs

Mini Project – Implementation and Evaluation of Wireless LANs BSc (Hons) Computer & Network technologies Author: University of Hertfordshire Date created: Date revised: 2009 Abstract The following resources come from the 2009/10 BSc in Computer and Network Technologies (course number 2ELE0072) from the University of Hertfordshire. All the mini projects are designed as level two modules of the undergraduate programmes. The objectives of this module are to Demonstrate within a private network environment: • The implementation of a wireless local are networks (WLANs) topology with diverse physical parameters • The real-time performance evaluation of the individual WLAN transmission characteristics in the presence of standard transport protocols. This mini-project involves the implementation of an “infrastructure” wireless network, the generation and transmission of packets and the measurement of network performance for TCP transport protocols by means of the “Wireshark” benchmarking tool. Parameters most likely to affect network performance such as the transmission medium’s signal-to-noise ratio, the propagating signal’s latency and jitter and the packet loss rate will be determined. © University of Hertfordshire 2009 This work is licensed under a Creative Commons Attribution 2.0 License .

Contents MiniProject Objectives Impact of wireless environment on networks Wireless networks The Wireless Spectrum Physical Impairments: Noise Physical Impairments: Interference Physical impairments: Fading Contention for the Medium Security WLANs : IEEE 802.11 Family IEEE 802.11 Standard WLAN characteristics TCP Flow Control Flow Control and throughput TCP Congestion Avoidance Address Resolution between IP and Underlying Networks Address Resolution protocol (ARP) Routing to another LAN RARP, BOOTP, DHCP protocols Data Link and Physical layers Mac Layer Functions Credits In addition to the resources found below there are supporting documents which should be used in combination with this resource. Please see: Mini Projects - Introductory presentation. Mini Projects - E-Log. Mini Projects - Staff & Student Guide. Mini Projects - Standard Grading Criteria. Mini Projects - Reflection. You will also need the ‘Mini Project Implementation and Evaluation of Wireless LANs’ text document. Wireless LANs MiniProject

MiniProject Objectives Impact of the wireless environment on networks Overview of current mobile wireless technologies Introduce the basic operation of IEEE 802.11 Wireless LANs MiniProject

Impact of wireless environment on networks The wireless spectrum Physical impairments Contention for the shared medium Security Wireless LANs MiniProject

Wireless networks IEEE 802.11 Characteristics Modes of operation Association, authentication and privacy Wireless LANs MiniProject

The Wireless Spectrum 30 MHz 30 GHz 3 GHz 300 MHz Broadcast TV VHF: 54 to 88 MHz, 174 to 216 MHz UHF: 470 to 806 MHz FM Radio 88 to 108 MHz Digital TV 54 to 88 MHz, 174 to 216 MHz, 470 to 806 MHz Wireless LANs MiniProject

The Wireless Spectrum (cont…) 30 MHz 30 GHz 3 GHz 300 MHz 3G Broadband Wireless 746-794 MHz, 1.7-1.85 GHz, 2.5-2.7 GHz Cellular Phone 800-900 MHz Personal Communication Service (PCS) 1.85-1.99 GHz Wireless LANs MiniProject

The Wireless Spectrum (cont..) 30 MHz 30 GHz 3 GHz 300 MHz Wireless LAN (IEEE 802.11b/g) 2.4 GHz Local Multipoint Distribution Services (LMDS) 27.5-31.3 GHz Bluetooth 2.45 GHz Wireless LAN (IEEE 802.11a) 5 GHz Wireless LANs MiniProject

Physical Impairments: Noise Unwanted signals added to the message signal May be due to signals generated by natural phenomena such as lightning or man-made sources, including transmitting and receiving equipment as well as spark plugs in passing cars, wiring in thermostats, etc. Sometimes modeled in the aggregate as a random signal in which power is distributed uniformly across all frequencies (white noise) Signal-to-noise ratio (SNR) often used as a metric in the assessment of channel quality Wireless LANs MiniProject

Physical Impairments: Interference Signals generated by communications devices operating at roughly the same frequencies may interfere with one another Example: IEEE 802.11b and Bluetooth devices, microwave ovens, some cordless phones CDMA systems (many of today’s mobile wireless systems) are typically interference-constrained Signal to interference and noise ratio (SINR) is another metric used in assessment of channel quality Wireless LANs MiniProject

Physical impairments: Fading Strength of the signal decreases with distance between transmitter and receiver: path loss Slow fading (shadowing) is caused by large obstructions between transmitter and receiver Fast fading is caused by scatterers in the vicinity of the transmitter Wireless LANs MiniProject

Contention for the Medium If A and B simultaneously transmit to C over the same channel, C will not be able to correctly decode received information: a collision will occur Need for medium access control mechanisms to establish what to do in this case (also, to maximize aggregate utilization of available capacity) A packets B C Wireless LANs MiniProject

Security Safeguards for physical security must be even greater in wireless communications Encryption: intercepted communications must not be easily interpreted Authentication: is the node who it claims to be? Wireless LANs MiniProject

WLANs: IEEE 802.11 Family 802.11 working group Specify an open-air interface between a wireless client and an access point or among wireless clients IEEE 802.11a Up to 54 Mbps in the 5 GHz band Uses orthogonal frequency division multiplexing (OFDM) IEEE 802.11b (Wi-Fi) 11 Mbps (with fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band IEEE 802.11g 20+ Mbps in the 2.4 GHz band Wireless LANs MiniProject

IEEE 802.11 Standard Final draft approved in 1997 Operates in the 2.4 GHz industrial, scientific and medical (ISM) band Standard defines the physical (PHY) and medium access control (MAC) layers Note that the 802.11 MAC layer also performs functions that we usually associated with higher layers (e.g., fragmentation, error recovery, mobility management) Initially defined for operation at 1 and 2 Mbps Extensions (IEEE 802.11b, IEEE 802.11a, etc.) allow for operation at higher data rates and (in the case of 802.11a) different frequency bands Wireless LANs MiniProject

WLAN characteristics Wireless LANs MiniProject Wireless PANs, LANs and WANs

WLAN Basic Infrastructure 3ELE0049 Optical Communication Systems

Wireless LANs MiniProject IEEE 802.11 Based Architecture

TCP Flow Control TCP inherently supports flow control to prevent buffer overflow at the receiver Useful for fast sender transmitting to slower receiver Receiver advertises a window ( wnd ) in acknowledgements returned to the sender Sender cannot send more than wnd unacknowledged bytes to the receiver © Virginia Tech. taken from a course entitled ‘Wireless Networks and Mobile Systems’ by Scott f Midkiff, Luiz DaSilva & Ing-Ray Chen Src Dest Limits amount of data that destination must buffer

TCP Flow Control Example Wireless LANs MiniProject © Virginia Tech. taken from a course entitled ‘Wireless Networks and Mobile Systems’ by Scott f Midkiff, Luiz DaSilva & Ing-Ray Chen Sender Receiver wnd = 1200 500 bytes 500 bytes wnd = 200 200 bytes wnd = 500 500 bytes

Flow Control and throughput Let rtt be the round-trip time, i.e., the time from sending a segment until an acknowledgement (ACK) is received Let t = wnd / b be the time to transmit a full “window” of data, where b is link bandwidth For a link with a high delay-bandwidth product ( rtt  b ), the flow control window can limit throughput for the connection In this case, t  rtt Throughput is wnd / rtt Wireless LANs MiniProject © Virginia Tech. taken from a course entitled ‘Wireless Networks and Mobile Systems’ by Scott f Midkiff, Luiz DaSilva & Ing-Ray Chen Sender Receiver t rtt wnd bytes

TCP Congestion Avoidance Congestion avoidance (control) was added to TCP in an attempt to reduce congestion inside the network A much harder problem … Requires the cooperation of multiple senders Must rely on indirect measures of congestion Implemented at sender Wireless LANs MiniProject © Virginia Tech. taken from a course entitled ‘Wireless Networks and Mobile Systems’ by Scott f Midkiff, Luiz DaSilva & Ing-Ray Chen Src Dest Attempts to reduce buffer overflow inside the network

Address Resolution between IP and Underlying Networks Most hosts attached to a LAN by an interface board that only understands LAN addresses. E.g. every Ethernet board is equipped with a 48-bit Ethernet address. The boards send and receive frames based on 48-bit Ethernet (MAC) addresses. They know nothing about the 32-bit IP addresses. Address Resolution Protocol (ARP) maps the IP addresses onto data link layer addresses (e.g., MAC address being t he hardware address that is sent back to the host. Every hardware devices’ MAC address can be found on the network interface card (NIC), located inside the host. The MAC address is hard coded, which means that it cannot (usually) be altered by software  University of Illinois, produced by M. T. Harandi, J. Hou, I. Gupta, N. Vaidya and K Nahrstedt Wireless LANs MiniProject

Address Resolution protocol (ARP) The ARP protocol operates between the network layer and the data link layer in the Open System Interconnection (OSI) model. The phrase “address resolution” refers to the process of finding a MAC address of a host (computer) on a network. The address is resolved using a protocol in which a short frame (data link layer “packet”) is broadcast on the local network by the host attempting to transmit data (client). The server on the receiving end processes the frame. The address resolution procedure is completed when the client receives from the server, a response containing the server’s address.  University of Illinois, produced by M. T. Harandi, J. Hou, I. Gupta, N. Vaidya and K Nahrstedt Wireless LANs MiniProject

RARP, BOOTP, DHCP protocols ARP: Given an IP address, return a hardware address RARP: Given a hardware address, give me the IP address DHCP, BOOTP: Similar to RARP Hosts (host portion): hard-coded by system admin in a file DHCP: D ynamic H ost C onfiguration P rotocol: dynamically get address: “plug-and-play” host broadcasts “ DHCP discover ” msg DHCP server responds with “ DHCP offer ” msg host requests IP address: “ DHCP request ” msg DHCP server sends address: “ DHCP ack ” msg Wireless LANs MiniProject

Data Link and Physical layers Wireless LANs MiniProject Medium Access Control (MAC) sublayer Physical Layer convergence procedure (PLCP) sublayer Physical medium Dependent (PMD) sublayer MAC sublayer management PHY sublayer management station management Data Link Layer Physical Layer

Mac Layer Functions 802.11 MAC Layer Functions The following summarizes primary 802.11 MAC functions, especially as they relate to infrastructure wireless LANs: Scanning: The 802.11 standard defines passive and active scanning methods by which station scans individual channels to find searches for access points. In passive scanning the station scans individual channels to find the best access point signal. The access points periodically broadcasts a beacon, and the station receives these beacons while scanning and takes note of the corresponding signal strengths. The beacons contain information about the access point, including service set identifier (SSID), supported data rates, etc. The station can use this information along with the signal strength to compare access points and decide upon which one to use. Passive scanning is mandatory. Wireless LANs MiniProject

Mac Layer Functions (cont..) In active scanning the station initiates the process by broadcasting a probe frame and all access points within range respond with a probe response. Active scanning enables a station to receive immediate response from access points, without waiting for a beacon transmission. The issue, however, is that active scanning imposes additional overhead on the network because of the transmission of probe and corresponding response frames. Wireless LANs MiniProject

Mac Layer Functions (cont..) Authentication: Authentication is the process of proving identity, and the 802.11 standard specifies two forms: Open system authentication and shared key authentication. Open system authentication is mandatory, and it's a two step process. A station first initiates the process by sending an authentication request frame to the access point. The access point replies with an authentication response frame containing approval or disapproval of authentication indicated in the Status Code field in the frame body. Wireless LANs MiniProject

This resource was created by the University of Hertfordshire and released as an open educational resource through the Open Engineering Resources project of the HE Academy Engineering Subject Centre. The Open Engineering Resources project was funded by HEFCE and part of the JISC/HE Academy UKOER programme. Slides 19 to 22 are reproduced with permission. © Virginia Tech. taken from a course entitled ‘Wireless Networks and Mobile Systems’ by Scott f Midkiff, Luiz DaSilva & Ing-Ray Chen Slides 23 & 24 are reproduced with permission.  University of Illinois, produced by M. T. Harandi, J. Hou, I. Gupta, N. Vaidya and K Nahrstedt The remaining material: © University of Hertfordshire 2009 This work is licensed under a Creative Commons Attribution 2.0 License . The name of the University of Hertfordshire, UH and the UH logo are the name and registered marks of the University of Hertfordshire. To the fullest extent permitted by law the University of Hertfordshire reserves all its rights in its name and marks which may not be used except with its written permission. The JISC logo is licensed under the terms of the Creative Commons Attribution-Non-Commercial-No Derivative Works 2.0 UK: England & Wales Licence. All reproductions must comply with the terms of that licence. The HEA logo is owned by the Higher Education Academy Limited may be freely distributed and copied for educational purposes only, provided that appropriate acknowledgement is given to the Higher Education Academy as the copyright holder and original publisher. Wireless LANs MiniProject

Mini Project- Implementation & Evaluation of Wireless LANs

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