1 / 12

4.2.6 Wireless LAN Protocols

4.2.6 Wireless LAN Protocols. All radio transmitters have some fixed range Hidden station problem Fig. 4-11a Exposed station problem Fig 4-11b

huela
Download Presentation

4.2.6 Wireless LAN Protocols

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. 4.2.6 Wireless LAN Protocols • All radio transmitters have some fixed range • Hidden station problem Fig. 4-11a • Exposed station problem Fig 4-11b • MACA (Multiple access with collision avoidance): Fig 4.12: Sender A sends a short frame with length of the data frame to follow; the receiver B replies with a CTS; on receipt of CTS, A sends the data frame • MACAW: MACA for wireless: (i) An ACK frame is sent by receiver for each frame received (ii) CSMA is used to prevent two stations from transmitting RTS (iii) Backoff algorithm for each source-destination pair (iv) Stations exchange information about congestion

  2. Wireless LANs Sections 4.4-4.6

  3. Wireless LANs • 802.11 Protocol stack: Fig. 4-25 • 802.11 Physical layer: Infra red (TV remote control), FHSS (Frequency hopping spread spectrum)--- for random control and security, DSSS (Direct sequence spread spectrum), OFDM (Orthogonal frequency division multiplexing), HR-DSSS ( High-rate Direct sequence spread spectrum)

  4. 802.11 MAC Sublayer Protocol • How is it different from Ethernet? In Ethernet, a station waits until the ether goes silent and starts transmitting. If it does not receive a noise burst back within the first 64 bytes, the frame almost assuredly been delivered correctly. With wireless, this does not hold. • Hidden station problem: Fig. 4-26a: C is transmitting to B---but A does not know that B is busy; so it too transmits leading to collision at B • Exposed station problem: Fig. 4-26b: A is transmitting to D; B wants to transmits to C but wrongly concludes that C is busy. • Hence, 802.11 does not use Ethernet protocol

  5. Two modes of 802.11 operations • DCF (Distributed coordination function): CSMA/CA • Method 1: When a station wants to transmit, it senses the channel. If it is idle, it just starts transmitting. It does not sense the channel while transmitting. If the channel is busy, the sender defers until the channel goes idle. In case of a collision, it uses binary exponential backoff.

  6. DCF (Cont.) • Method 2: MCAW: Fig. 4-27: B&C are in the range of A; D is within the range of B but not within A’s range • A sends RTS to B; C also listens and declares a period of busy or NAV (Network Allocation Vector) • B sends CTS to A; this is also received by D which declares a period of NAV • On receiving CTS, A sends the data; • B sends an ACK.

  7. Handling unreliability through smaller fragments • 802.11 allows frames to fragmented into smaller pieces, each with its own checksum • Stop-and-wait protocol is used to send the individual fragments

  8. PCF • Base station polls other stations • No collisions occur due to centralized control • Base station broadcasts a beacon frame periodically • Beacon frame has system parameters such as hopping sequences and dwell times (time at each frequency), etc. It invites stations to join.

  9. 802.11 Data Frame Format • Fig. 4-30 • Frame classes: Data, control, management • Frame control • Duration: How long the channel will be busy (NAV) • Address1, 2—SRC and DST • Address 3,4---Base stations of SRC and DST

  10. Broadband Wireless • 802.16 is the standard for broadband wireless • Example: Erecting a big antenna on top of a hill just outside of town and let homes receive the transmissions • 802.16 receivers are wireless but not mobile; 802.11 primarily deals with individual mobile users with limited batter power • 802.16 operates in the 10-66 GHz range while 802.11 operates in the ISM band (2.4 GHz) • Summary: 802.11 is more like a mobile Ethernet; 802.16 is more like a stationary wireless cable television.

  11. BLUETOOTH • Short-range, low-power, inexpensive radios • Piconet—basic unit—Master node and up to 7 slave nodes within a distance of 10 meters; all communication is master-slave; not slave-slave • Piconert is a centralized TDM system, with the master controlling the clock and determining which devices get to communicate in which time slot---all communication is between master and slave only • Multiple piconets can coexist in the same room; they may be connected via bridge nodes forming a scatternet • Up to 255 parked nodes in the net

  12. Mobile IP • Every IP address contains a network address and a host number---not suitable for mobile computers • Goals for mobile IP: • Each mobile must be able use its home IP address anywhere • Software changes to the fixed hosts are not permitted • Changes to the router softwrae and tables are not permitted • Most packets for mobile hosts should not make detours on the way • No overhead should be incurred when a mobile host is at home • Solution: Every site that wants to allow its users to roam creates a home agent; every site

More Related