Computer Networking Lecture Notes

1 November 2012 • Wireless Medium Access

---

Outline

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

Shared Channels

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

  

Wireless Interconnects

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.
• 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.

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 are C sending to B, C’s signal is better than A’s signal.

  

• C captures B.

Reservation

• Carrier sense is helpful but incomplete for wireless.
• 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.
• This is known as multiple access collision avoidance (MACA, also CSMA/CA).

CSMA/CA

• 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

Wi-Fi

• Some of the IEEE 802.11 (Wi-Fi) 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.

802.11 Topologies

Basic (ad hoc, mesh)		Extended

802.11 Architecture

• Now imagine the hosts moving around.

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

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.

---

This page last modified on 2011 October 9.