Wireless LAN Notes

Wireless LAN Overview

  • Wireless LANs use wireless transmission mediums.
  • Initially faced challenges like high prices, low data rates, safety concerns, and licensing, but these issues have largely been resolved.
  • Wireless LANs have seen rapid growth in popularity.

Applications of Wireless LANs

  • LAN Extension:
    • Avoids the need for LAN cabling.
    • Simplifies relocation and network modifications.
    • Despite increasing reliance on twisted pair cabling (Cat 3 and Cat 5), wireless LANs haven't replaced wired LANs entirely.
    • Suitable for:
      • Large, open areas like manufacturing plants, stock exchanges, and warehouses.
      • Historical buildings.
      • Small offices where wired LANs are not cost-effective.
      • These environments may also have wired LANs for servers and stationary workstations.
  • Cross-Building Interconnect:
    • Connects LANs in nearby buildings via point-to-point wireless links.
    • Connects bridges or routers.
    • Considered a wireless LAN application.
  • Nomadic Access:
    • Links LAN hubs to mobile data terminals like laptops.
    • Enables data transfer from portable computers to servers.
    • Useful in extended environments like campuses where users need access to servers on a wired LAN while moving around with portable computers.
  • Infrastructure Wireless LAN:
    • Utilizes a high-speed backbone wired LAN.
    • Incorporates nomadic stations for wireless access.
  • Ad Hoc Networking:
    • Peer-to-peer networks set up temporarily for immediate needs.
    • Example: A group of employees with laptops in a meeting.

Wireless LAN Configuration

  • Single Cell Wireless LAN:
    • Includes user modules (UM) and a control module (CM).
    • Connects to Ethernet switches (10-Mbps and 100-Mbps).
    • Employs a bridge or router.
  • Multi-Cell Wireless LAN Configuration:
    • Utilizes multiple frequencies to avoid interference between cells.
    • Incorporates multiple UMs and CMs linked by a bridge or router and a 100-Mbps Ethernet switch.

Wireless LAN Requirements

  • General Requirements:
    • High capacity, short distances, full connectivity, and broadcast capability, similar to any LAN.
  • Specific Requirements:
    • Throughput: Efficient use of the wireless medium.
    • Number of Nodes: Support for hundreds of nodes across multiple cells.
    • Connection to Backbone LAN: Control modules connect to both types of LANs.
    • Service Area: 100 to 300 meters.
    • Low Power Consumption: Long battery life for mobile stations, without requiring frequent monitoring of access points.
    • Transmission Robustness and Security: Resistance to interference and eavesdropping.
    • Collocated Network Operation: Ability for two or more wireless LANs to operate in the same area.
    • License-Free Operation
    • Handoff/Roaming: Seamless transition between cells.
    • Dynamic Configuration: Easy addition, deletion, and relocation of end systems without disrupting users.

Wireless LAN Technologies

  • Infrared (IR) LANs:
    • Limited to a single room due to IR light's inability to penetrate opaque walls.
  • Spread Spectrum LANs:
    • Operate primarily in ISM (industrial, scientific, and medical) bands, requiring no FCC licensing in the USA.
  • Narrowband Microwave:
    • Use microwave frequencies, but not spread spectrum, and may require FCC licensing.

Infrared LANs: Strengths and Weaknesses

  • Strengths:
    • Virtually unlimited spectrum
    • Unregulated worldwide
    • High data rates.
    • Shares properties with visible light, such as diffuse reflection from light-colored objects.
    • More secure against eavesdropping than microwave.
    • Separate installation in each room without interference.
    • Inexpensive and simple, using intensity modulation.
  • Weaknesses:
    • Susceptible to background radiation (sunlight, indoor lighting).
    • Noise requires higher power, limiting range
    • Power limited by eye safety and power consumption concerns.

Infrared LAN Transmission Techniques

  • Directed-Beam IR:
    • Point-to-point links with a range dependent on power and focusing (up to kilometers).
    • Used for building interconnect within line of sight or indoor token ring LANs with IR transceivers positioned for data circulation.
  • Omnidirectional:
    • Single base station within line of sight of all other stations, typically ceiling-mounted, acting as a multiport repeater.
    • Other transceivers use directional beams aimed at the ceiling unit.
  • Diffused Configuration:
    • Transmitters are focused and aimed at a diffusely reflecting ceiling.

Spread Spectrum LANs Configurations

  • Hub Configuration:
    • Uses multiple-cell arrangements with adjacent cells using different center frequencies.
    • Hubs are typically ceiling-mounted and connected to a wired LAN, facilitating connections between wired and wireless stations.
    • Hubs may control access using IEEE 802.11 point coordination function and act as multiport repeaters.
    • Stations transmit to and receive from the hub and may broadcast using omnidirectional antennas, implementing a logical bus configuration.
    • Hubs may support automatic handoff.
  • Peer-to-Peer Configuration:
    • No hub; uses MAC algorithms like CSMA to control access in ad hoc LANs.

Spread Spectrum LANs Transmission Issues

  • Licensing regulations vary by country.
  • In the USA, the FCC authorizes two unlicensed applications within the ISM band:
    • Spread spectrum: up to 1 watt.
    • Very low power systems: up to 0.5 watts.
    • Frequencies include 902-928 MHz, 2.4-2.4835 GHz, and 5.725-5.825 GHz.
  • 2.4 GHz is also used in Europe and Japan.
  • Higher frequency bands offer higher potential bandwidth but may face more interference.
    • 900 MHz band: interference from cordless phones, wireless microphones, and amateur radio.
    • 2.4 GHz band: interference from microwave ovens.
    • 5.8 GHz band: less competition.
  • Higher frequency bands typically require more expensive equipment.

Narrowband Microwave LANs

  • Use bandwidth just wide enough to accommodate the signal.
  • Historically, products used licensed bands, though some now operate in the ISM band.

Licensed Narrowband RF

  • Microwave frequencies for voice, data, and video are licensed within specific geographic areas to prevent interference.
  • License holders like Motorola, which holds 600 licenses in the 18-GHz range covering metropolitan areas in the USA, use cell configurations with nonoverlapping frequency bands in adjacent cells.
  • All transmissions are encrypted.
  • Licensed narrowband LAN guarantees interference-free communication, giving the license holder a legal right to an interference-free data channel.

Unlicensed Narrowband RF

  • RadioLAN introduced narrowband wireless LAN using the unlicensed ISM spectrum in 1995.
    • Operates at 10 Mbps in the 5.8-GHz band with low power (0.5 watts or less).
    • Range: 50 m in a semiopen office and 100 m in an open office.
    • Peer-to-peer configuration with dynamic master election based on location, interference, and signal strength.
    • Includes a dynamic relay function, allowing stations to act as repeaters to move data between stations out of range of each other.

IEEE 802.11 - BSS

  • Defines the MAC protocol and physical medium specifications for wireless LANs.
  • The basic building block is the basic service set (BSS), consisting of a number of stations using the same MAC protocol and competing for access to the same shared wireless medium.
  • BSSs may be isolated or connected to a backbone distribution system (DS) through an access point (AP), which functions as a bridge.
  • The MAC protocol may be distributed or controlled by a central coordination function in the AP.
  • A BSS generally corresponds to a cell.
  • The DS can be a switch, wired network, or wireless network.

BSS Configuration

  • In the simplest configuration, each station belongs to a single BSS and is within range only of other stations within that BSS.
  • Overlapping BSSs are possible, allowing a station to participate in more than one BSS.
  • The association between a station and a BSS is dynamic, with stations able to turn off, come within range, and go out of range.

Extended Service Set (ESS)

  • Two or more BSSs interconnected by a DS, typically a wired backbone, appearing as a single logical LAN to the LLC (Logical Link Control) layer.

Access Point (AP)

  • The logic within a station that provides access to the DS, offering DS services in addition to acting as a station.
  • To integrate IEEE 802.11 architecture with a wired LAN, a portal is used.
  • Portal logic is implemented in a device that is part of the wired LAN and attached to the DS, such as a bridge or router.

IEEE 802.11 Architecture Components

  • Station (STA): A device with wireless capabilities.
  • Access Point (AP): Provides access to the Distribution System (DS) for STAs.
  • Basic Service Set (BSS): A group of STAs communicating within range of each other or through an AP.
  • Distribution System (DS): Connects APs to create an Extended Service Set (ESS).
  • Extended Service Set (ESS): A set of interconnected BSSs that appear as a single logical LAN.
  • Portal: Connects the 802.11 architecture to a wired LAN (IEEE 802.x LAN).

IEEE 802.11 Services

  • Station Services:
    • Authentication: Establishes the identity of stations to each other.
    • Deauthentication: Terminates an existing authentication.
    • Privacy: Prevents messages from being read by others.
    • MSDU Delivery: Handles the delivery of MAC service data units.
  • Distribution System Services:
    • Association: Establishes an initial association between a station and an AP.
    • Reassociation: Transfers an established association to another AP.
    • Disassociation: Terminates an association between a station and an AP.
    • Distribution: Used by stations to exchange MAC frames when the frame must traverse the DS.
    • Integration: Enables the transfer of data between a station on an 802.11 LAN and one on an integrated 802.x LAN.
    • Reassociation: Supports MSDU Delivery.

Categorizing Services

  • Station services are implemented in every 802.11 station, including AP stations.
  • Distribution services are provided between BSSs and may be implemented in APs or special-purpose devices.
  • Three services are used to control access and confidentiality, while six services support the delivery of MAC service data units (MSDUs) between stations.
  • If an MSDU is too large for a MAC frame, it is fragmented and transmitted in a series of frames.

Distribution of Messages Within a DS

  • Distribution is the primary service used by stations to exchange MAC frames when a frame must traverse the DS, such as from a station in one BSS to a station in another BSS.
  • Transport of a message through the DS is beyond the scope of 802.11.
  • If stations are within the same BSS, the distribution service logically goes through the single AP of that BSS.
  • Integration service enables the transfer of data between a station on an 802.11 LAN and one on an integrated 802.x LAN.
  • Integrated refers to a wired LAN physically connected to the DS.
  • Stations may be logically connected to an 802.11 LAN via the integration service.
  • The integration service handles address translation and media conversion.

Association Related Services

  • The purpose of the MAC layer is to transfer MSDUs between MAC entities, which is fulfilled by the distribution service (DS).
  • The DS requires information about stations within the ESS, provided by association-related services.
  • A station must be associated before communicating.
  • Three transition types based on mobility:
    • No transition: Stationary or moves within range of a single BSS.
    • BSS transition: Moves from one BSS to another within the same ESS, requiring addressing capability to recognize the new location.
    • ESS transition: Moves from a BSS in one ESS to a BSS in another ESS, only supported in the sense that the station can move, with no guarantee of maintaining upper-layer connections and likely disruption of service.

Station Location

  • The DS needs to know the location of the destination station to determine the identity of the AP to which the message should be delivered.
  • The station must maintain association with an AP within the current BSS.
  • Three services relate to this requirement:
    • Association: Establishes the initial association between a station and an AP to make the identity and address known.
    • Reassociation: Transfers an established association to another AP, allowing the station to move from one BSS to another.
    • Disassociation: Terminates an association, either from the station or AP, typically given before the station leaves the ESS or shuts down to protect the MAC management facility against stations that disappear without notification.

Access and Privacy Services - Authentication

  • On a wireless LAN, any station within radio range of other devices can transmit and receive.
  • Authentication is used to establish the identity of stations to each other, unlike wired LANs where access to a physical connection implies authority to connect to the LAN.
  • The authentication service is used to establish station identity.
  • 802.11 supports several authentication schemes and allows expansion of these schemes but does not mandate any particular scheme.
  • These schemes range from relatively insecure handshaking to public-key encryption schemes.
  • 802.11 requires mutually acceptable, successful authentication before association.

Access and Privacy Services - Deauthentication and Privacy

  • Deauthentication: Invoked whenever an existing authentication is to be terminated.
  • Privacy: Used to prevent messages from being read by others.
  • 802.11 provides for optional use of encryption.

Medium Access Control

  • The MAC layer covers three functional areas:
    • Reliable data delivery.
    • Access control.
    • Security.

Reliable Data Delivery

  • 802.11 physical and MAC layers are subject to unreliability due to noise, interference, and other propagation effects that can result in the loss of frames.
  • Even with error-correction codes, frames may not be successfully received.
  • While this can be dealt with at a higher layer, such as TCP, it is more efficient to deal with errors at the MAC level because retransmission timers at higher layers are typically on the order of seconds.
  • 802.11 includes a frame exchange protocol in which a station receiving a frame returns an acknowledgment (ACK) frame.
  • The exchange is treated as an atomic unit and is not interrupted by any other station.
  • If no ACK is received within a short period of time, the frame is retransmitted.

Four Frame Exchange

  • Basic data transfer involves the exchange of two frames.
  • To further enhance reliability, a four-frame exchange may be used:
    • The source issues a Request to Send (RTS) frame to the destination.
    • The destination responds with a Clear to Send (CTS) frame.
    • After receiving the CTS, the source transmits the data.
    • The destination responds with an ACK frame.
  • The RTS alerts all stations within range of the source that an exchange is underway, and the CTS alerts all stations within range of the destination.
  • Stations refrain from transmission to avoid collision.
  • The RTS/CTS exchange is a required function of the MAC but may be disabled.

Media Access Control

  • Distributed Wireless Foundation MAC (DWFMAC):
    • A distributed access control mechanism with optional centralized control on top.
    • The lower sublayer is the distributed coordination function (DCF), which uses a contention algorithm to provide access to all traffic.
  • Point Coordination Function (PCF):
    • A centralized MAC algorithm that is contention-free, built on top of the DCF.

IEEE 802.11 Protocol Architecture

  • Logical Link Control (LLC) Layer
  • MAC Layer:
    • Contention-free service (Point Coordination Function - PCF)
    • Contention service (Distributed Coordination Function - DCF)
  • Physical Layer:
    • Various standards (IEEE 802.11, 802.11a, 802.11b, 802.11g) with different frequency bands, modulation techniques, and data rates.

Distributed Coordination Function

  • The DCF sublayer uses CSMA (Carrier Sense Multiple Access).
  • If a station has a frame to transmit, it listens to the medium.
  • If the medium is idle, the station may transmit; otherwise, it must wait until the current transmission is complete.
  • No collision detection is used because it is not practical on a wireless network due to the dynamic range of signals.
  • The DCF includes delays, known as interframe spaces (IFS), which amount to a priority scheme.

Interframe Space

  • A single delay is known as interframe space (IFS).
  • Using IFS, the rules for CSMA are:
    1. A station with a frame senses the medium; if it is idle, the station waits to see if it remains idle for one IFS and, if so, may transmit immediately.
    2. If the medium is busy (either initially or becomes busy during the IFS), the station defers transmission and continues to monitor until the current transmission is over.
    3. Once the current transmission is over, the station delays another IFS.
      • If the medium remains idle, the station backs off a random time and again senses the medium. If the medium is still idle, the station may transmit.
      • During the backoff time, if the medium becomes busy, the backoff timer is halted and resumes when the medium becomes idle.
  • To ensure stability, binary exponential backoff is used.

Priority

  • Three values are used for IFS:
    • SIFS (Short IFS): The shortest IFS, used for all immediate response actions.
    • PIFS (Point Coordination Function IFS): A midlength IFS, used by the centralized controller in the PCF scheme when issuing polls.
    • DIFS (Distributed Coordination Function IFS): The longest IFS, used as the minimum delay for asynchronous frames contending for access.

SIFS Use - ACK

  • A station using SIFS to determine transmission opportunity has the highest priority, in preference to a station waiting PIFS or DIFS time.
  • SIFS is used in the following circumstances:
    • Acknowledgment (ACK): A station responds with an ACK after waiting an SIFS gap.
    • Clear to Send (CTS): A station can ensure a data frame will get through by issuing an RTS. The destination station should immediately respond with a CTS if ready to receive.
    • Poll response.
  • SIFS provides efficient delivery of multiple frame LLC PDUs.

PIFS and DIFS

  • PIFS is used by the centralized controller when issuing polls and takes precedence over normal contention traffic.
  • Frames using SIFS have precedence over PCF polls.
  • DIFS is used for all ordinary asynchronous traffic.

Point Coordination Function (PCF)

  • PCF is an alternative access method implemented on top of DCF, involving polling by a centralized polling master (point coordinator).
  • It uses PIFS when issuing polls, which is smaller than DIFS, allowing it to seize the medium and lock out all asynchronous traffic while it issues polls and receives responses.
  • For example, a wireless network configured so that the number of stations with time-sensitive traffic is controlled by the point coordinator.
  • The remaining traffic contends for access using CSMA.
  • The point coordinator polls in round-robin to stations configured for polling.
  • When a poll is issued, the polled station may respond using SIFS.
  • If the point coordinator receives a response, it issues another poll using PIFS; if no response is received during the expected turnaround time, the coordinator issues another poll.

Superframe

  • The point coordinator would lock out asynchronous traffic by issuing polls.
  • A superframe interval is defined.
  • During the first part of the superframe interval, the point coordinator polls round-robin to all stations configured for polling.
  • The point coordinator then idles for the remainder of the superframe, allowing a contention period for asynchronous access.
  • At the beginning of the superframe, the point coordinator may seize control and issue polls for a given period.
  • The rest of the superframe available for contention-based access.
  • At the end of the superframe interval, the point coordinator contends for access using PIFS.
    • If the medium is idle, the point coordinator gains immediate access and a full superframe period follows.
    • If the medium is busy, the point coordinator must wait for it to become idle, resulting in a foreshortened superframe period for the next cycle.

IEEE 802.11 MAC Frame Format

  • Frame Control (FC): 2 octets
  • Duration/Connection ID (D/I): 2 octets
  • Address 1: 6 octets
  • Address 2: 6 octets
  • Address 3: 6 octets
  • Sequence Control (SC): 2 octets
  • Address 4: 6 octets
  • Frame Body: 0 to 2312 octets
  • CRC: 4 octets

MAC Frame Fields (1)

  • Frame Control:
    • Indicates the type of frame (control, management, or data).
    • Provides control information, including whether the frame is to or from the DS, fragmentation information, and privacy information.
  • Duration/Connection ID:
    • If used as a duration field, indicates the time (in amuamus) the channel will be allocated for successful transmission of the MAC frame.
    • In some control frames, contains the association or connection identifier.
  • Addresses:
    • The number and meaning of address fields depend on the context.
    • Types include source, destination, transmitting station, and receiving station.

MAC Frame Fields (2)

  • Sequence Control:
    • A 4-bit fragment number subfield for fragmentation and reassembly.
    • A 12-bit sequence number to number frames between a given transmitter and receiver.
  • Frame Body:
    • Contains the MSDU (or a fragment of), LLC PDU, or MAC control information.
  • Frame Check Sequence:
    • A 32-bit cyclic redundancy check for error detection.

Control Frames

  • Used to assist in reliable data delivery.
    • Power Save-Poll (PS-Poll): Sent by any station to a station that includes an AP to request the AP transmit a frame buffered for this station while the station is in power-saving mode.
    • Request to Send (RTS): The first frame in a four-way frame exchange.
    • Clear to Send (CTS): The second frame in a four-way exchange.
    • Acknowledgment (ACK)
    • Contention-Free (CF)-end: Announces the end of the contention-free period part of PCF.
    • CF-End + CF-Ack: Acknowledges the CF-end, ends the contention-free period, and releases stations from associated restrictions.

Data Frames – Data Carrying

  • Eight data frame subtypes in two groups.
  • First four carry upper-level data from the source station to the destination station:
    • Data: The simplest data frame, which may be used in a contention or contention-free period.
    • Data + CF-Ack: Only sent during the contention-free period, carrying data and acknowledging previously received data.
    • Data + CF-Poll: Used by the point coordinator to deliver data and also to request the station send a data frame it may have buffered.
    • Data + CF-Ack + CF-Poll: Combines Data + CF-Ack and Data + CF-Poll.

Data Frames – Not Data Carrying

  • The remaining four data frames do not carry user data:
    • Null Function: Carries no data, polls, or acknowledgments, but carries a power management bit in the frame control field to the AP, indicating the station is changing to a low-power state.
    • The other three frames (CF-Ack, CF-Poll, and CF-Ack + CF-Poll) are the same as the corresponding frames in the preceding list (Data + CF-Ack, Data + CF-Poll, and Data + CF-Ack + CF-Poll) but without data.

Management Frames

  • Used to manage communications between stations and APs, such as the management of associations.
  • Include requests, responses, reassociation, dissociation, and authentication frames.

802.11 Physical Layer

  • Issued in four stages:
    • First part in 1997 (IEEE 802.11): Includes the MAC layer and three physical layer specifications (two in the 2.4-GHz band and one infrared), all operating at 1 and 2 Mbps.
    • Two additional parts in 1999:
      • IEEE 802.11a: The 5-GHz band up to 54 Mbps.
      • IEEE 802.11b: The 2.4-GHz band at 5.5 and 11 Mbps.
    • Most recent in 2002 (IEEE 802.g): Extends IEEE 802.11b to higher data rates.

Original 802.11 Physical Layer - DSSS

  • Three physical media:
    • Direct-sequence spread spectrum (DSSS).
      • 2. 4 GHz ISM band at 1 Mbps and 2 Mbps.
      • Up to seven channels, each 1 Mbps or 2 Mbps, can be used, depending on the bandwidth allocated by various national regulations.
      • Each channel bandwidth is 5 MHz.
      • Encoding scheme DBPSK for 1-Mbps and DQPSK for 2-Mbps.

Original 802.11 Physical Layer - FHSS

  • Frequency-hopping spread spectrum (FHSS)
    • 2. 4 GHz ISM band at 1 Mbps and 2 Mbps
    • Uses multiple channels with signal hopping from one channel to another based on a pseudonoise sequence.
    • 1-MHz channels are used.
    • The hopping scheme is adjustable.
      • Minimum hop rate for USA is 2.5 hops per second.
      • Minimum hop distance is 6 MHz in North America and most of Europe and 5 MHz in Japan.
    • Two-level Gaussian FSK modulation for 1-Mbps.
    • For 2 Mbps, four-level GFSK is used.

Original 802.11 Physical Layer – Infrared

  • Omnidirectional
  • Range up to 20 m
  • 1 Mbps used 16-PPM (pulse position modulation), where each group of 4 data bits is mapped into one of 16-PPM symbols, and each symbol is a string of 16 bits consisting of fifteen 0s and one binary 1.
  • For 2-Mbps, each group of 2 data bits is mapped into one of four 4-bit sequences, and each sequence consists of three 0s and one binary 1.
  • Intensity modulation is used where the presence of a signal corresponds to 1.

802.11a

  • 5-GHz band
  • Uses orthogonal frequency division multiplexing (OFDM), also called multicarrier modulation, which is not spread spectrum
  • Multiple carrier signals at different frequencies are used with some bits on each channel, similar to FDM but with all subchannels dedicated to a single source.
  • Data rates are 6, 9, 12, 18, 24, 36, 48, and 54 Mbps.
  • Up to 52 subcarriers are modulated using BPSK, QPSK, 16-QAM, or 64-QAM, depending on the rate.
  • Subcarrier frequency spacing is 0.3125 MHz.
  • Convolutional code provides forward error correction.

802.11b

  • Extension of the 802.11 DS-SS scheme
  • Data rates of 5.5 and 11 Mbps
  • Chipping rate of 11 MHz, the same as the original DS-SS scheme, resulting in the same occupied bandwidth.
  • Complementary code keying (CCK) modulation is used to achieve a higher data rate in the same bandwidth at the same chipping rate.
  • Input data is treated in blocks of 8 bits at 1.375 MHz:
    • 8bits/symbolamu1.375MHz=11Mbps8 bits/symbol amu 1.375 MHz = 11 Mbps
    • Six of these bits are mapped into one of 64 code sequences.
    • The output of mapping, plus two additional bits, forms the input to the QPSK modulator.

802.11g

  • Higher-speed extension to 802.11b
  • Combines physical layer encoding techniques used in 802.11a and 802.11b to provide service at a variety of data rates.