Computer Networking Lecture Notes

2013 October 31 • Wireless Medium Access

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Outline

• Wireless
• Propagation
• Reservation
• IEEE 802.11, Wi-Fi.
  • Topologies and architecture
  • Frames

Shared Channels

• Shared (multiaccess, broadcast, random access) channels.

  

• What’s so special about wireless channels?
  • Why haven’t we solved this problem already?

Wireless Properties

• Three important wireless properties:
  1. Half duplex operation; a host can send or receive, not both.
  2. A time varying, complex channel.
  3. Frequent (1 in 10³ BER) and bursty errors.
• A wired connection is full duplex, consistent and simple, and exhibits low, random errors.
• Wireless MAC protocols are correspondingly more complicated.

Propagation

Assume radio signals propagate uniformly in all directions. Not even close to being true.

• Eventually signals become too weak to receive (fade).
  • Distance squared drop-off.

Hidden Senders

• Both A and C want to send to B.

  

• The hidden sender problem.

Good-Bye CSMA/CD

• Half-duplex transmission eliminates collision detection (CD).
  • No listening while sending.
• Carrier sense (CSMA) is still possible.
  • But hidden senders reduce its effectiveness.
  • This is not the propagation-delay collision window problem.
    • Although that problem exists too.

Exposed Senders

• A wants to send to B, C wants to send to D.

  

• The exposed sender problem.
  • Also the discontinuous LAN problem.

Capture

• A and C sending to B, C’s signal is stronger than A’s signal.

  C captures B.

• Sometimes capture is a problem, sometimes it’s a feature.

Other Factors

• Mobility and power are two more important factors.
• No wired tether leads to hosts that can move.
  • Network boundaries are not human friendly.
• Mobile devices tend to be small, and small batteries are not power dense.
  • Radio I-O requires power.

ALOHA

• What about ALOHA?
  • Pure ALOHA is a simple fully distributed algorithm.
  • Slotted ALOHA too, but less so.
• However, even under modest contention ALOHA isn’t efficient.
  • And it degrades rapidly under load.
  • Collisions are expensive in a mobile wireless environment.

Wireless MAC Protocols

• Particular services and infrastructure need particular MAC protocols.
  • Bluetooth (IEEE 802.15) - personal-area networks (PANs).
  • Wi-Fi (IEEE 802.11) - local-area networks (LANs).
  • WiMax (IEEE 802.16) - metropolitan area networks (MANs), wireless last miles.
• Other contenders include radio-frequency ID (RFID) and Iridium-like satellite networks.

Wi-Fi Protocols

• Some of the Wi-Fi IEEE 802.11 standards.

  802.11				GHz		Mbit/sec*
  original		1997		2.4		2
  a		1999		5		54
  b		1999		2.4		11
  g		2003		2.4		54
  n		2009		2.4 or 5		600

  • *Maximum; actual bit rates vary adaptively.
• Development continues.

Wi-Fi CSMA

• Carrier sense can be helpful, but avoiding collisions is even more helpful.
• Non-slotted p-persistent CSMA provides the starting point.
  • Don’t flip a coin on an idle channel, randomly backoff.
  • No collision detection, so expect acknowledgments.
• This is known as multiple access collision avoidance (MACA, also CSMA/CA).

CSMA/CA

• Backoff time is adjusted to channel characteristics.

Reservation

• Carrier sense is helpful, and reservation is even more helpful.
• Use small frames to reserve clear channels around sender and receiver.
  • Sender emits ready to send (RTS).
  • Receiver emits clear to send (CTS).
  • Data follows.
• RTS-CTS reservation is an optional CSMA/CA feature.

CSMA/CA Reservation

• RTS and CTS frames are small (10s bytes).
  • Less likely to collide, less lost when they do.
• Sender’s and receiver’s neighbors hear the reservation frames and mute to avoid collisions.
• RTS and CTS contain expected channel hold time.
  • Neighbors compute the mute-period size.

CSMA/CA Example

802.11 Topologies

• A collection of communicating systems is a basic service set (BSS)

  BSS		Independent BSS

Basic Service Systems

• An independent BSS is also known as an ad-hoc or mesh network.
  • Hosts communicate directly (or indirectly) with each other.
• All communication in a BSS goes through a distinguished system called the access point or base station.
  • All communication not with the access point is indirect.

802.11 Architecture

• Now imagine the hosts moving around.

Distribution Systems

• Two or more BSSs can be connected via a distribution system (DS) or a backbone.
  • The backbone is not part of 802.11, although it could be wireless.
  • The backbone can be connected to other networks too (the Internet, for example).
• An infrastructure is a collection of BSSs interconnected with distribution systems.

Finding APs

• A host scans for an access point:
  • Host broadcasts a Probe frame.
  • APs reply with a Probe Response frame.
  • Host chooses an AP and sends an Association Request frame.
  • The AP replies with an Association Response frame.
• A host scans for an AP when it doesn’t have one, or it wants a better one.

Scanning

• Host initiated scanning is called active scanning.
• Access points also provide passive scanning.
  • APs periodically broadcast Beacon frames.
  • A Beacon frame advertises an AP and its capabilities.
• A hand-off occurs when a host changes APs.

802.11 Frames

Other Wi-Fi Protocols

• There are a number of other Wi-Fi protocols in place or under development:
  • 802.11e - Quality of service specifications.
  • 802.11i - Security enhancements (replacing WEP).
  • 802.11k - Access-point selection.
  • 802.11r - Mobile management and hand-off.
  • 802.11s - Mesh (ad hoc) networks.

Bluetooth

• Bluetooth, a wireless technology to replace data cabling.
  • Also a 10th century Danish king.
• An example of a personal-area network (PAN).
  • Distance usually between 1 and 10 m.
  • Data rates between 1 and 3 Mbit/sec.
• Developed by an industrial consortium.
  • Standardized by IEEE 802.15.

Bluetooth Frequencies

• Bluetooth operates in an unlicensed ISM band from 2.4 to 2.4853 GHz.
  • Which conflicts with the lower Wi-Fi frequency band.
• The band is divided into 79 1 MHz channels and two approximately 2 MHz guard bands.
• The Bluetooth physical layer uses frequency hopping spread spectrum.
  • A form of frequency-division multiplexing.

Bluetooth Architectures

Piconet		Scatternet

• All hosts are mobile.

Masters and Slaves

• A Bluetooth host discovers nearby hosts and pages responders to form a piconet.
  • The discoverer is the piconet master, the responders are the slaves.
• A master has up to seven active slaves, and any number of parked slaves.
  • The master activates and parks slaves as necessary.
• Slaves can request master status.

Scatternets

• A host in more than one piconet is a bridge host.
  • The bridge is a master in at most one piconet.
• A bridge time-division multipexes among piconets.
• A group of hosts can form several piconets to get a scatternet.
  • This approximates peer-to-peer communication among hosts.

WiMAX

• WiMAX (Worldwide Interoperability for Microwave Access).
• An example of a metropolitan-area network (MAN).
  • Distance up to 10-30 miles.
  • Data rates between 30 and 40 Mbit/sec.
• A set of IEEE 802.16 standards.

Summary

• Carrier sense is less of an advantage for wireless.
  • And direct collision detection is impossible.
• IEEE 802.11 (Wi-Fi) is one of many wireless specifications.
  • Others include Bluetooth, WiMax, and G3-G4-LTE.
• There are many 802.11 specs, and more are on the way.

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This page last modified on 2011 October 9.