WLANS CHAPTER 5,6,7,8,9
Header Error Check
- a checksum for used by the receiver to verify the integrity of the frame header
Synchronization
- a sequence of alternating 0s and 1s that alerts the receiver that a frame is on its way
Signal Data Rate
- the transmission speed of the signal
40-MHz Channels
802.11n can use 20 MHz channels - but it can also use two adjacent 20 MHz channels "bonded" together In the resultant 40 MHz channel, one 20 MHz channel becomes primary and the second one is secondary This wider channel can use the small amount of space at the top and bottom of each 20 MHz channel - this space would be reserved in case of a 20 MHz channel is used Channel bonding increases probability of interference with other WLANs - especially in the ISM band with three non-overlapping channels (UNII - 23)
Wireless LAN Controllers (WLCs)
A WLC is very similar to a switch on a wired network WLC is sometimes called a wireless switch An enhanced feature of a WLC is network placement. A WLC can be placed at either: Core layer: backbone of the network Distribution layer: also called the workgroup layer because devices at this layer ensure frames are routed between subnets Access layer: where client nodes connect to the network (hubs and switches are located here)
Wireless Modulation Techniques
A band is comprised of many frequencies - divided further into smaller frequency ranges (channels) Every wireless network uses once channel at a time - with recent addition of combining several channels Devices operating a channel divide it into subcarriers - subchannels - that get modulated to carry data Two major approaches exist - narrowband and spread spectrum - that define how a channel is used
Orthogonal Frequency Division Multiplexing (OFDM
A major modulation method for modern wireless technologies - enables very high data rates Allows sending different data portions over available subchannels - uses them very efficiently Low power is used on each subchannel - minimizes multipath behavior, resulting in higher throughput In case higher power is used, reflections and other distortions would cause non-uniform symbol arrivals A receiver ensures quality reception of data by adding multiple copies arriving at its antennas
Distributed Coordination FunctION
A medium access method that implies contention - nodes may transmit when no carrier signal is detected Two nodes that start transmitting at the same time cause a collision - damages data and requires retransmission Requires a method that would lower collision rate and guarantees retransmission of damaged data Different methods are used on wired and wireless LANs due to wireless power, distance, and duplex mode issues
Point Coordination Function
A medium access method that implies no contention - nodes may send data only after polled by the AP
Self-Managing
A model that allows wireless devices to monitor the environment and compensate for changes Implemented via WNMS using lightweight APs - adjust the wireless network to changes automatically Relies on transmit power control (TPC) and similar technologies - not a substitute for a site survey Also referred to as automated RF resource manag
Site Survey Tools
A predictive site survey requires the site's floor plans and the software - with several Web sites available The software accepts site information in the form of a graphical image file with additional input data A variety of tools may be needed for a manual site survey - from generic to specialized survey tools A manual site survey tools are classified as wireless, specialized, and documentation tools Voice communications over the wireless network would require special VoWiFi tools for the site survey stage
L3 Roaming
A router may exist between the old and new access points - the APs are on different logical networks The roaming client's IP address must be changed with new association - connectivity may be lost temporarily Mobile IP was the first mechanism to allow for smooth mobility within subnetted IP environments Mobile IP allows internetwork travel to be transparent to users - with no interruption above the Network layer Major intended application - Internet sessions for cell phones roaming freely between wireless networks
Independent Basic Service Set (IBSS
A small wireless network without an access point - wireless clients communicate directly Implies no connection to other networks - also lacks multiple improvements an access point would provide Lacks centralized security management and has a very limited coverage area - for short temporary use only Also referred to as peer-to-peer or Ad Hoc Mode
802.11a
A subchannel width of 6 bits and 48 data subchannels supported result in the following: symbol size = (subchannel bit width) x (# subchannels) = = 6 x 48 = 288 bits The highest correction rate supported is 3/4 and thus the number of frame bits in each symbol is: symbol data size = (symbol size) x (correction rate) = = 288 x 3/4 = 216 data bits
OSI MODEL FOR NETWORKING
A universal framework for digital data communications that is based on open specifications - developed by ISO Divides the large problem of networking into seven smaller layers that facilitate services and protocols
Predictive
A virtual site survey that uses modeling techniques to design the wireless network Implies loading building floor plans into the special software (predictive analysis simulation application) Such software application can account for building materials, square footage, the number of wireless users, types of application used, antenna models, etc. The outcome is a wireless plan for the site - with any desired changes analyzed very quickly
Network Layer (L3
Accepts data from the Transport layer - wraps this data into packets by adding protocol-specific headers Defines protocol-specific logical addressing schemes -uniquely identify networks and nodes within networks Establishes the best delivery path for each packet - accounts for multiple factors along the path (routing) Large segments and datagrams may be broken into smaller fragments - according to the network's capacity Accepts packets from the Data Link layer, processes the headers, and forwards payloads to the Transport layer
Need for a Site Survey WLAN Design Goals
Adequate wireless coverage for the entire target site - without dead spaces or low-throughput locations Minimum amount of wireless signal beyond the limits of the intended wireless site Optimal AP placement - with interference taken into account (other APs, machinery, electronic devices, etc) Minimum obstructions to the signal - with surrounding materials, doors, aquariums, office partitions identified Best possible performance - for users in all locations and with all existing applications considered
Modulation and Coding Scheme (MCS
An outline of the different combinations of factors - channel width, modulation, number of partial streams, guard interval, etc. - that affect wireless configuration The standard supports up to 4 partial streams, 52/108 subchannels for 20/40 MHz channels, BPSK/QPSK and 16/64-QAM, 800/400 ns GI, etc. All APs are required to support at least MCS 0-15 and all wireless clients must support MCS 0-7 - with 77 various MCS available
MAC Frame Types Control
Assist with delivery of data frames over the existing wireless connection - mainly used for data flow control The Receiver/Transmitter Address fields provide the receiving and sending nodes' addresses, respectively
CHAPTER 6
CHAPTER 6
CHAPTER 7
CHAPTER 7
CHAPTER 8
CHAPTER 8
CHAPTER 9
CHAPTER 9
Captive Portal APs
Captive portal AP: uses a standard Web browser to provide information, give the wireless user the opportunity to agree to a policy, or present valid login credentials Typically used for public WLAN access Usually requires the user to read and accept an Acceptable Use Policy (AUP) Captive portal AP is not a required part of a controller-base architecture, however, this is the architecture in which it is often foun
MAC Frame Types Data
Carry application data transmitted over the WLAN - their content is processed by upper OSI layers Rely on four Address fields for flow between networks
Controller-Based Architectures
Controller-based architectures: rely upon a wireless LAN controller (WLC) at the heart of the network WLC can be centrally configured, the settings can then be automatically distributed to all APs APs used in controller-based architectures are different from APs used in an autonomous access point WLAN
Direct Sequence Spread Spectrum (DSSS
Converts each data bit to a series of bits - an expanded redundant code - before it's sent over a subchannel Replaces each original 1 and 0 with by a predefined bit pattern - chipping code - and its inverse, respectively FCC mandates 10-bit or longer chipping codes - with 11- or 22-bit codes actually used on WLANs Applies the XOR operation to the original bit and its replacement code - transmits the result on the channel Separates consecutive 1/0s with an extra 0/1 - placed between the two codes and inverted for transmission
UNII Band
Currently, comprised of four subbands - UNII-1, UNII-2, UNII-2 Extended, and UNII-3 In the US, FCC mandates how each subband is to be used and limits power that may be used in each one Makes 23 nonoverlapping channels usable in US, 12 original ones and 11 channels that were added later US uses 4, 4, 11, and 4 channels in the UNII-1, UNII-2, UNII-2 Extended, and UNII-3, respectively Higher number of channels allows for more flexible WLAN management - more networks can coexist Each channel has width of 20 MHz - can be combined together for 40, 80, and 160 MHz width effectively Recommended and popular band - new component bands and nonoverlapping channels are being added
802.11n MAC Improvements
Data Unit Terms: A protocol data unit (PDU) is the OSI layer data that is ready to be sent to the peer layer at the receiving device A service data unit (SDU) is a specific data unit that has been passed from the OSI layer above this one - but has not yet been encapsulated by that lower layer Example - a segment is the L4 PDU which becomes an SDU at the L3 (before becomes a packet) Example - a packet is the L3 PDU which becomes an SDU at the L2 (before becomes a frame) This also applies to sublayers that encapsulate data
PMD Sublayer
Defines characteristics of the wireless medium and defines the transmission and receiving methods Works directly with the wireless medium on one end - and with the PLCP sublayer on the other end
Wi-Fi Multimedia (WMM
Defines four levels of prioritization for WLAN QoS - not used as widely as the IEEE solution WMM Background - frames of the lowest priority (print jobs, file transfers, backups, etc.) WMM Best Effort - frames submitted by not time-sensitive applications WMM Voice and WMM Video - frames of the highest and second highest priority, respectively
PSK Modulations
Differential binary phase shift keying (DBPSK) allows for transmitting one data bit via a phase change DBPSK encodes bit 0 into the phase change of zero degrees - and bit 1 into the change of 180 degrees Differential quadrature phase shift keying (DQPSK) allows for transmitting two data bits via a phase change DQPSK encodes combinations 00, 01, 10, and 11 using two different phase changes for two amplitude levels Uses a 16/64-level quadrature amplitude modulation (16/64-QAM) for the 24/54 Mbps rates, respectively
ISM Band
Divided into 14 channels in the range from 2.412 to 2.484 GHz - each one has width 22 MHz Frequencies within +/-11 MHz from the channel center frequency are used for transmissions on each channel Transmissions can affect frequencies up to 30 MHz from the channel center - or two channels on each side Recent WLAN standards used frequencies within 20 MHz of each channel's center, not 22 MHz FCC mandates use of channels 1-11 only for wireless LANs in the US - can be different in other countries
Peer Communication
Each layer is unaware of activities at all other layers - thus doesn't acknowledge their services Each layer only communicates logically to the identical layer on the other side - via headers and trailers Headers and trailers added at the sending layer will be read and removed at the identical layer on the other side Protocol suites combine protocols running at different layers together to enable network communications
RTS/CTS Evaluation
Effectively minimizes collision damage and solves the hidden node problem - helped by using the access point Only small RTS frames can be damaged by collisions - saves retransmission overhead of very large data frames Without the RTS/CTS, busy networks with many client devices would experience performance degradation May result in very high and thus damaging overhead in case of frequent transmissions of very short data frames Can be avoided entirely by keeping wireless devices very close to each other - able to sense each other
CSMA/CA
Employs slot times for reducing collision rates and uses acknowledgements for detecting collisions Upon detecting no carrier, each node selects a random number from a contention window interval (0-16) The node then multiplies the random number drawn by duration of a slot time to compute a backoff interval Slot time duration is defined within the IEEE standard used - 802.11a/b/g use 9μs/20 ms/9μs, respectively Upon expiration of the backoff interval, a wireless node senses the carrier again - may transmit if not detec
Data Link Layer (L2
Encapsulates packets received from the Network layer into frames - complete packages to be transmitted Defines the format of the header and/or trailer added to each packet - depends on the network type in use Performs verification of data integrity using a checksum mechanism - enables detection of transmission errors Implies error correction upon the receiver's request for retransmission in case a frame is dropped or altered Maximum size of a frame is dictated by the carrying capacity of the network medium The sender computes a cyclic redundancy check (CRC) checksum of all of each frame's fields before it is sent This unique 4-byte frame check sequence (FCS) code is attached to the frame - to be detached by the receiver The receiver computes the checksum again - compares it to the one received and assumes no error if identical An error is assumed if the two codes don't match - a retransmission request may be issued in such case
Wireless LAN Issues
FSPL results in signals that are received by an antenna having much lower power than signals it's sending Power used by an antenna for transmissions masks all other signals on the air - complicates possible detection A wireless transmitter can't listen while transmitting - a single antenna always operates in half-duplex mode These issues - together with hidden node problem - make any CSMA/CD-like mechanism impractical The CSMA/CA (Collision Avoidance) solution is used
Feature Sets
Features found in APs vary Many are designed to enhance the configuration, installation, and management of the AP Typical feature sets: Ability to add 5-GHz 802.11a support to a 2.5-GHz 802.11g AP Built-in security and manageability features External antenna connection Support for Wireless Distribution Systems (WDS) Support for large numbers of wireless stations Auto network connect and dynamic rate shifting
Extended Service Set (ESS
Formed by connecting two or more BSS networks together - multiple APs with overlapping BSAs A mobile user roams between APs - connects to the AP with the strongest signal at every specific moment Roaming resembles cellular structure - with the access points connected through the wired network Provides larger coverage area and accommodates more wireless clients - at the price of additional access points Also referred to as infrastructure mode (includes B
WLAN Service Sets Compared
IBSS is the least expensive and easiest to setup - can be put in place by personnel with lower levels of expertise BSS enables connection to the wired network and offers centralized security management, larger BSA, and multiple communications improvements ESS is required for accommodating large numbers of users, extending area of coverage, and having persistent high data rates - at the expense of more equipment IBSS is preferable for short meetings and classrooms - while BSS and ESS are used for permanent setu
Data Link Sublayers
IEEE split the Data Link layer into two sublayers - LLC and MAC - that perform entirely different tasks The purpose of such division is to allow L3 protocols to interact with L2 ones without regard for L1 specs LLC sublayer builds and destroys frames, provides common interface, reliability, and flow control MAC sublayer facilitates MAC addresses - a unique 48-bit identifier of every network adapter
Common Tasks
Identification of all the issues that may affect the overall performance of the WLAN Prediction of the network's throughput and coverage - and of how future changes affect them Implementation of security policy - with a balance searched for between performance and security Determination if the network is going to function as needed upon deployment Finding optimal locations and power levels for APs
Frequency Hopping Spread Spectrum (FHSS
Implies short data bursts sent over each subchannel - with a different subchannel used for the next burst Hopping code - the sequence of subchannels used for transmission that has to be known to both sides Dwell time - the amount of time that a transmission occurs on a specific subchannel (ms) Hop time - the overhead amount of time that is needed for switching to the next subchannel (μs) Subchannels are rotated continuously until the entire data is transmitted - hopping causes overhead Benefit - sends a short burst only via each subchannel, thus resistant to interference and does not affect others Benefit - if damaged on one subchannel, data can be retransmitted over the next available subchannel Limitation - ineffective on WLANs (used in 802.11 and 802.11b, resulted in throughput up to 3 Mbps
Spread Spectrum
Implies spreading data transmissions across multiple subchannels - a more flexible and effective approach Benefit - resistant to narrowband interference which affects only a small portion of the signal Benefit - radio sending unit only needs a fraction of power used for narrowband transmissions Benefit - reduced multipath distortion due to lower power levels used for transmissions Benefit - no interference with other spread-spectrum transmissions due to wide variety of signal codes used Benefit - more data bits can be sent at a time than in case of a narrowband transmission Benefit - increased security since other radio receivers see the transmission beneath the noise level as noise Two methods - frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) Orthogonal frequency division multiplexing (OFDM) is frequently classified as a spread spectrum technique too
Narrowband
Implies transmissions over a very tight radio frequency range ("single frequency") - used by radio stations Benefit - the most efficient scheme since only a small portion of the spectrum is used for each transmission Limitation - vulnerable to interference from other radio signals transmitted at or near the frequency used Limitation - requires higher power in order to exceed the outside noise level by a substantial margin Suitable for broadcast radio stations that are tolerant to interference - not suitable for IEEE wireless networks
Site Survey Tools Wireless
Included with the access point or wireless NIC - allow for a basic site survey and should not be used alone Access point - allows for power adjustment, DC-to-AC converter for running on batteries, amplifiers, etc. Wireless client device - can be used to measure the AP's signal strength and data rate throughout the site Most OS provide imprecise signal measurement methods only - RSSI value, percentage, etc. Use of several identical client devices saves time and increases effectiveness of the site survey
PMD Power Features
Incorporates transmission power control (TPC) - an effective WLAN power regulation technology An AP advertises TPC support to clients - tells the maximum allowed WLAN power and its current power Clients respond with their own transmit power ability - the AP determines the maximum power for the segment The radio power can be adjusted dynamically in order to reduce interference with surrounding devices
Site Survey Types ManuaL
Involves walking through the site while carrying a portable wireless client device - a laptop or a tablet Software installed on the client allows for taking and recording measurements during the motion Passive - implies that the client is only listening to frames as they are sent and processing them A passive survey allows for determination of signal strengths, noise levels, signal-to-noise ratios, etc.
Lightweight APs
Lightweight AP: does not contain management and configuration functions Only have simplified radios for wireless communication and a media converter for accessing the wired network Split MAC: division in which lightweight APs only handle the real-time layer functions while MAC functionality is processed by the WLC Benefit to lightweight APs is a decrease in the total cost of ownership (TCO) TOC: total cost of ownership includes acquisition, setup, support, ongoing maintenance, service, and all operating expenses
MAC Frame Types
MAC frames are encapsulated into PLCP frames' payload section - carry their own MAC header All MAC frames carry source and destination MACs, duration, and frame check sequence (FCS) fields All frames include the frame control field - identifies the 802.11 protocol version, frame type, encryption used, and various WLAN configuration flags IEEE WLANs use MAC frames of three types - data, management, and control - for different purposes Data frame content is handled by upper OSI layers
Outdoor
Make obtaining accurate data more difficult - due to investigation of multiple larger areas required Data usually needed includes the following: Height and materials of the buildings and poles that are to host an access point(s) Availability of power at each building, light pole, or outbuilding to power the access point(s) Location, size, and density of obstacles Types of applications or the desired bandwidth Availability of data connectivity at each location
Lightweight Mesh APs
Mesh access point: communicates with the next closes mesh access point does not have to be connected by a cable to the existing wired network Only one mesh access point must be physically connected to the wired network Lightweight mesh AP: a mesh AP that is centrally configured and managed through a WLC
Wireless Roaming L2 Roaming
Mobile clients scan the band periodically looking for an AP with better signal strength or packet error rates Upon finding a better AP, a station would switch to its radio channel if accepted by the AP (handoff) IEEE standards don't specify how a handoff should take place - implementation is left up to vendors Handoff should be seamless to users - smoother when using access points from the same vendor L2 roaming implies handoff without changing the
Autonomous Access Point Architectures
Most common WLAN architecture is one with a "stand-alone" AP These APs are known as autonomous access points All of the "intelligence" for wireless management, authentication, and encryption is contained within the AP Because everything is self-contained they are also called fat access points Autonomous access point architectures include network connectivity with lots of features
Multiple-Channel Architecture (MCA
Multiple-Channel Architecture (MCA): a wireless architecture in which more than one channel is used in the wireless network Cochannel interference: reduced throughput caused as a result of all of the APs set to same channel MCA helps to eliminate this interference
Network Connectivity
Network connectivity of WLANs depends on its type of service set Three WLAN service set configurations: BSS ESS IBSS ESS is composed of two or more BSS networks Distributed WLAN architecture: multiple APs form a non-centralized network through a wireless connection
MAC Addresses
Non-replaceable burned-in addresses (BIA) - mostly represented using twelve hexadecimal characters Consist of two 24-bit portions - a block ID and a device ID - identify the vendor and the adapter, respectively IEEE assigns block IDs - or Organizational Unit Identifiers (OUIs) - large vendors can get several IDs Vendors manage and assign device IDs - also called serial numbers - to their manufactured products Many L2 protocols - such as Ethernet - require every frame to carry destination and source MAC addresses
Basic Service Set (BSS
One or more wireless devices (stations) served by a single AP - connected to a wired network or isolated Each station sends frames to and receives frames from the access point only - resembles star topology Mobile clients are only able to communicate with others configured for the same Service Set Identifier (SSID) SSID is a 2-32 character-long alpha-numeric string - serves as a MAC identifier of the access point SSID is preconfigured on the access point - clients that join the network supply it as simple security credential
Physical Layer (L1
Operates the medium - transmits and receives medium-specific pulses and defines the actual data rates Medium pulses include voltages (copper), light pulses (fiber optics), or energy intensities (EM waves) Defines mechanical and electrical characteristics of networking hardware and the medium At the sending end - accepts frames from the Data Link layer and turns their bits into the medium pulses At the receiving end - transforms pulses to bits and passes them to the Data Link layer that reads the frame Lacks intelligence - does not interpret data, adds no header or trailer, and performs no error correction Any problems that occur at this layer directly affect the ability of this host to communicate on the network
Appropriate Times
Prior to installing a wireless network - optimize the access point placement, locate security breaches, etc. After making physical changes to the building - to eliminate possible new sources of interference When changing an existing wireless network - to re-evaluate AP and antenna placement due to changes When there are changes in network needs - to account for various types of network usage by users When there are significant changes in personnel - to adjust the network to added or moved employees
Need for Fragmentation
Properly made wired media have very high signal-to-noise ratios and thus very low error rates That is why likelihood of successful transmission of frames of any size is very high on wired LANs RF medium has a much higher error rate than a wire - long frames are subject to medium-related errors Probability of successful wireless transmission of a long frame may be even close to 0 in case error rate is high A long frame that contains only one error needs to be retransmitted entirely - lowers throughput substantially
Quality of Service (QoS)
Quality of Service (QoS): Capability to prioritize different types of frames QoS has become increasingly important due to widespread adoption of Voice over IP (VoIP)
PMD Sublayer
Receives reformatted frames from PLCP - translates their bits into radio signals to be transmitted Uses the ISM band - 14 channels total, with 11 ones usable in the US Uses DSSS as the main spectrum spread method -improvement over FHSS towards higher data rates Relies on a single 11-bit chipping code - Barker code - for transmitting each bit at 1 or 2 Mbps via DSSS
PLCP Sublayer
Reformats frames received from L2 into PMD frames and passes them to the PMD sublayer for transmission Each PMD frame consists of three parts - the preamble, the header, and the data payload The 96-bit preamble prepares the receiving device for the rest of the frame - not considered part of the frame The 40-bit header carries information about the frame itself - includes several frame-specific fields The preamble and header are transmitted at a 1 Mbps - in order to accommodate connections to slower device
PLCP Sublayer
Reformats frames received from the 802.11 L2 into a frame that the PMD sublayer can transmit Listens to the medium to determine when data can be sent - wireless transmissions are contention-based
OFDM
Related Terms: Number of subchannels - the number of RF subcarriers used within each channel, determined by the IEEE Subchannel bit width - the number of bits that can be carried on each subchannel at once This width depends on a bit-to-clock coding scheme used, which varies between standards and rates Symbol size - the number of bits that each OFDM symbol is comprised of Correction rate - the fraction of a symbol carries frame data bits, determined by the IEEE standard
Single-Channel Architecture (SCA)
Single-channel architecture (SCS): all of the APs use the same channel Each AP has overlapping coverage that forms a continuous region on a single channel Advantages: Smoother handoffs Easy setup - all APs are set to the same RF channel Cochannel interference is no longer an issue SCA provides more network information in order to make informed decisions
Protocol Analyzers
Specialized hardware of software products that capture frames to decode and analyze their contents Not used for the initial access point placement - useful for troubleshooting and traffic characterization Commonly used - Wireshark, Kismet, Commview, etc
Site Survey Analyzers
Specialized tools designed solely for wireless site surveys - can be full-featured or basic Capable of sending and receiving packets - with both packet types subject to through analysis Capable of collecting data in real time - with the user having to take much lower volume of notes Full-featured products allow for fine-tuning wireless communications used for the site survey Broadly used professional products include VisiWave, Ekahau, and AirMagnet Survey
Spectrum Analyzers
Specialized tools that scan the RF spectrum to locate potential sources of interference Passive receivers - only display the signal in a way that makes it easy for visual analysis Display unprocessed signal information - frequency, power, period, and the shape of the RF wave Need to be used multiple times - to evaluate the AP movements in order to minimize interference Commonly used spectrum analyzers - Chanalyzer, Cognio, Bantam, Rohde & Scharz, etc.
RTS/CTS
Stands for the Request-to-Send/Clear-to-Send - this protocol is also referred to as Virtual Carrier Sensing Used on top of CSMA/CA - provides collision-free transmissions and solves the hidden node problem Requires a node to reserve the channel before sending data by submitting a short RTS frame to the AP The node competes for the channel only when sending an RTS frame - and not when sending data afterwards A collision may occur on the RTS frame only - will be detected by the lack of a response to RTS submis
Variable Guard Interval
The default GI for IEEE 802.11a/g/n is 800 ns - results in the 3.2 + 0.8 = 4 µs symbol transmission time 802.11n or better APs add a shorter GI of 400 ns - drops symbol transmission time to 3.2 + 0.4 = 3.6 µs Such shorter GI should be used in case the environment does not result in substantial multipath behavior The reduced symbol transmission time results in higher data throughput per transmit radio
WLAN Service Sets
The term service set stands for all devices connected to an 802.11 wireless LAN Three different service set configurations are commonly used - basic service set, extended service set, and independent basic service set
Access Points
There are three main types of APs found in a controller-based architecture Lightweight APs Mesh APs Captive portal AP
Interframe Spacing (IFS)
To guarantee no collision when an ACK is sent, two IFS intervals - DIFS and SIFS - are used Node sending data have to wait for a Distributed IFS (DIFS) interval - in addition to the backoff interval Nodes sending ACKs have to wait for a Short IFS (SIFS) interval - a much shorter time than DIFS Due to the shorter waiting period, a receiver sending ACK always gets to the medium before other nodes Typical IFS interval duration is around 10μs and 50μs for SIFS and DIFS, respectively
Multiple-Channel Architecture vs. Single-Channel Architecture Models
Two architecture models for WLANs: Multiple-channel architecture Single-channel architecture Each approach has their own respective applications
Transmitting on a WLAN
Two procedures are defined by the 802.11 standard - DCF and PCF - that define channel access methods Distributed Coordination Function (DCF) is based on competition for the right to transmit (like Ethernet) Point Coordination Function (PCF) requires devices to transmit when their turn comes only (like Token Ring) Only one medium access method is used within every wired segment at once - competition or no competition IEEE enables WLANs to use both methods - with each network switching from DCF to PCF and back
Wireless Network Management Systems (WNMS
Typical features of a WNMS: Configuration management: new or updated configurations can be "pushed" out to wireless devices Firmware/Software distribution: can be distributed to all devices from a central management facility Intelligent scheduling: updates can be scheduled to occur late at night or on the weekends User and device monitoring: allows for monitoring historical information and use special diagnostic information to address problems
Typical Data Rate
Use of 64-QAM enables 6-bit wide subchannels, thus, with 108 data subchannels used (40 MHz): symbol size = (subchannel bit width) x (# subchannels) = = 6 x 108 = 648 bits The highest correction rate supported is 5/6 and thus the number of frame data bits in each symbol is: symbol data size = (symbol size) x (correction rate) = = 648 x 5/6 = 540 data bits
Highest Data Rate
Use of 64-QAM enables 6-bit wide subchannels, thus, with 48 data subchannels used: symbol size = (subchannel bit width) x (# subchannels) = = 6 x 48 = 288 bits The highest correction rate supported is 3/4 and thus the number of frame data bits in each symbol is: symbol data size = (symbol size) x (correction rate) = = 288 x 3/4 = 216 data bits
Lower Data Rate
Use of 64-QAM enables 6-bit wide subchannels, thus, with 52 data subchannels used (20 MHz): symbol size = (subchannel bit width) x (# subchannels) = = 6 x 52 = 312 bits The highest correction rate supported is 5/6 and thus the number of frame data bits in each symbol is: symbol data size = (symbol size) x (correction rate) = = 312 x 5/6 = 260 data bits
MAC Frame Types Management
Used for establishing and maintaining connections between wireless devices, including the access point Required for locating an AP and accessing the WLAN, as well as for disconnecting from a wireless network Management (continued): Frame control - carries 802.11 protocol version, frame type, encryption, and other information Duration - number of microseconds needed to transmit the frame, varies depending upon transmission mode Address 1 and 2 - destination and source MAC addresses of receiving and sending nodes, respectively Sequence Control - carries frame's sequence number as well as its fragment number BSSID - physical address of the wireless access p
Documentation
Used to create a "hard copy" of the site survey results so they are available for future reference The form should include survey methods, RF frequency and channel plan, throughput findings, sources of interference, problem zones, facility drawings with access points shown, AP configuration, etc. Plain paper, a blueprint of the building, and a digital camera are useful documentation tools The overall format of the resultant form is voluntary - with survey results often stored in a database
Voice over WiFi Tools
VoIP traffic depends heavily on each frame arriving in sequence - intolerant to packet losses and delays Guideline - lost packets should comprise less than one percent of the total number of packets sent Guideline - each packet delay should be less than 50 ms to avoid an echo effect Guideline - a delay between packets (jitter) should be less than 5 ms Use of built-in tools in the VoWiFi handset is advised - provide true measurement of the RF environment
Other types of WLAN architectures:
WLAN arrays Cooperative control Mesh networks
Hidden Node Problem
WLAN client devices share the medium - use of an access point allows for longer distances between them RF signal attenuates faster as distance grows because of FSPL - clients may ignore each other's transmissions The channel will appear available to them - proceed with their transmissions seeing each other as noise There is no distance issue between an access point and clients - gets their transmissions at comparable power Transmissions collide in the access point's receiver - quickly detects damaged data in its buffer
RTS Threshold
When short data is sent, RTS and CTS frames result in latencies that are comparable to data transmission times RTS/CTS is beneficial most when large data frames are transmitted often over a busy wireless network RTS threshold is a configurable value (in bytes) on the AP that allows for sending short data without RTS/CTS Only data frames having size larger than this value are sent using RTS/CTS - with shorter ones sent without it Wise RTS threshold management may reduce RTS/CTS overhead upon the WLAN substantially
Wired LAN Review
Wired signals sent and received have comparable power levels - signals sent don't shadow out signals received Wired transceivers have ability to listen while they are transmitting - enables a collision detection mechanism A wired node that detects collision first can send special jamming code - notifies other nodes about the collision Stable power levels, multiple conductors, and ability to send jamming code result in reliable collision detection
Wireless Network Management Systems (WNMS)
Wireless network management system (WNMS): set of hardware/software that can be used to provide management of a wireless network Includes configuration management Deployment Troubleshooting
Start Frame Delimeter
a bit pattern that defines the beginning of the frame (1111001110100000
The lowest - Physical
layer manages transmissions over the specific network medium
The top - Application
layer protocols interact with the software providing network interface to the user
Operating systems and applications
must run on a WLAN without modification - all WLAN features are implemented at the OSI Data Link and Physical layers
Multiple BSS networks in an ESS
share information through the wired distribution system (DS) or a wireless distribution system (WDS) The DS decides if a frame exchange is needed between stations within the same BSS only or if an exchange with a wired network or a special device is necessary The DS media can be a wired network, a wireless radio, or a special purpose device that interconnects the APs
OFDM Math
symbol size = (subchannel bit width) x (# subchannels) symbol data size = (symbol size) x (correction rate) data rate per radio = (symbol data size) / (symbol interval) overall data rate = (data rate per radio) x (# radios)
Length
the length of the frame in bits
The physical area of RF coverage - Basic Service Area (BSA)
varies depending on the IEEE standard BSA for 802.11b/g networks is up to 107 m (350 ft) - while 802.11a BSA is up to 33 m (100 ft) The effective coverage range is shorter due to distance from the access point, number of users (15-25 per BSA), obstructions, and degree of network utilization Dynamic Rate Shifting makes the data rate to drop to the next lower value as distance from the AP increases - stations would slow down in favor of staying connected
Wireless Mesh Network (WMN)
when multiple mesh access points communicate between themselves Mesh access point does not have to be individually connected by a cable to an existing wired network Only one of the mesh access point in a WMN must be physically connected to the wired network
