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Broadband Wireless Media Access Control From 802.11 to 802.16

Broadband Wireless Media Access Control From 802.11 to 802.16. Justin Comroe ASU CSE 535 Nov 27, 2007. WiFi Today at ASU. WiFi is used in-buildings as IEEE 802.11 was originally designed, as a wireless LAN

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Broadband Wireless Media Access Control From 802.11 to 802.16

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  1. Broadband WirelessMedia Access ControlFrom 802.11 to 802.16 Justin Comroe ASU CSE 535 Nov 27, 2007

  2. WiFi Today at ASU • WiFi is used in-buildings as IEEE 802.11 was originally designed, as a wireless LAN • WiFi is also being used outdoors as a MAN right now at ASU and many places around the nation • IEEE has completed development of a new standard for outdoor MAN • IEEE 802.16 has departed dramatically from the well-known & used 802.11 techniques for MAC So what’s wrong with the 802.11 MAC for outdoor applications, especially when everyone is using it and we already know it works?

  3. Collisions Kill • WiFi is basically a wireless Ethernet • The Medium is radio • A fundamental Medium Access Control (MAC) function is to prevent transmissions from colliding • when a station transmission overlaps another, one or both transmissions will often fail • Without collision control >81.6% of total airtime is wasted • Pure ALOHA maximum throughput is only 18.4%

  4. Collisions Kill Transmission 1 Transmission 2 Destructive interference at destination time Transmission 1 Transmission 2 x Wasted airtime

  5. IEEE 802.11 MAC • Stations listen to all transmissions and transmit when the channel appears idle • This is Carrier Sense Multiple Access, or CSMA • CSMA works a lot better than Aloha for all cases where all the stations can hear each other. Source for following material on 802.11: IEEE P802.11-96/49C “802.11 Tutorial”, from March 1996 http://grouper.ieee.org/groups/802/11/Tutorial/MAC.pdf This is IEEE’s official website for the 802.11 taskgroup

  6. Carrier Sense – Basic Principle Transmission 1 1 2 3 Transmission 2 Laptop 2 “hears” Laptop1 transmission time Transmission 1 [2]->[3] [1]->[3] Transmission 2 … and waits until done before transmitting

  7. IEEE 802.11 MAC • 802.11 further adds several methods meant for Collision Avoidance • IEEE 802.11 MAC is described as CSMA/CA • 802.11 has a MAC rule to preventing overlap, called the Distributed Coordination Function, or DCF

  8. Distributed / Short InterFrame Spaceand Backoff Interval A device wanting to transmit during Idle channel must first see a minimum DIFS interval The receiving unit gets to Acknowledge after an SIFS which is < DIFS time [1]->[3] [3]->[1] Transmission 1 Ack [2]->[3] SIFS DIFS Transmission 2 Deferred access DIFS A device wanting to transmit during a transmission-in-progress must wait an additional randomly selected number of slot times forming a Backoff Interval

  9. NAV • Under DCF, each unit places a Network Allocation Vector (NAV) in front of each transmission • identifies the length of its transmission Laptop 2 doesn’t “hears” end of Laptop1 transmission time Transmission 1 NAV Transmission 2 NAV … but still waits until done before transmitting because of NAV

  10. Hidden Node Transmission 1 2 1 Transmission 2 3 Presume [1] and [2] can both hear [3] and vice-versa, But [1] and [2] cannot hear each other. time [1]->[3] [2]->[3] Transmission 1 Transmission 2 x DIFS DIFS Destructive interference at [3]

  11. RequestToSend / ClearToSend Every transmission can begin with an RTS, which is responded to with a CTS. Transmission and Ack follow the RTS/CTS exchange. Further, the NAV values project enough time for the entire transaction to complete. time [1]->[3] [1]->[3] RTS Transmission 1 [3]->[1] [3]->[1] NAV NAV DIFS CTS Ack NAV NAV Even if unit [2] does not hear the RTS from [1]->[3], hearing the NAV preceding the CTS from [3] would defer access until the end of the entire transaction!

  12. 802.11 MAC Summary • All devices independently pick their own time to transmit • There is no central authority … DCF stands for “Distributed” • DIFS, SIFS, Backoff Intervals … and mainly collisions … all subtract from airtime that could successfully deliver payload • As more units compete, collisions increase • As collisions increase, capacity goes down • Therefore capacity goes down as more units compete

  13. This is not a Peer-Peer Network Units only transmit to Access Point The access point has a physically dominant location All units can see the access point Each unit might at best see only a few nearby units MAN Coverage

  14. MAN usage of 802.11 • Violates primary DCF assumptions • Units cannot hear other units • NAV cannot function when a peer user transmission is never heard • CSMA and CA both break down • Hidden Node CA doesn’t work either even if employing RTS/CTS • Mobile initiated RTS never heard by other mobiles • CSMA and CA both break down

  15. CSMA Breaks Down • When an access point has too many users, to the non-engineering user it simply appears to stop working • The engineering observer might presume that capacity is simply being divided • The truth is, when an access point has too many users, the capacity actually goes to zero, just as simple ALOHA when overloaded

  16. Full Disclosure • A handful of companies specifically create 802.11 based MAN products, but they often alter the 802.11 MAC (like Alvarion) • Virtually all 802.11 uses DCF, meaning Distributed • There is a relatively unutilized mode of 802.11 called PCF (Point Configuration Function) • PCF permits the Access Point to control access • PCF is useful for polling applications

  17. IEEE MAN Standard Takes a Radically Different Direction • IEEE 802.16 designed to be a MAN, not a LAN Source for following material on 802.16: IEEE C802.16-02/05 Ecklund, Marks, Stanwood, & Wang, “IEEE Standard 802.16: A Technical Overview of the WirelessMAN Air Interface for Broadband Wireless Access”, IEEE Communications Magazine, June 2002, pp.98-107 http://grouper.ieee.org/groups/802/16/docs/02/C80216-02_05.pdf This is IEEE’s official website for the 802.16 taskgroup

  18. IEEE 802.16 Fundamentals • Not a LAN … not Peer-Peer based • Instead: Access Point central control (point-multipoint) • No contention … for Payload • Instead: request/grant based scheduling

  19. 802.11 is like a meeting, where anyone can talk at any time DCF rules are like manners, recognizing who is currently speaking and being polite Contention MAC Too many people, and everyone tries to speak-up with people talking over each-other

  20. 802.16 Request/Grant is like a classroom controlled by a teacher Students are not allowed to talk until they raise their hand and are CALLED ON by the teacher Collisions can’t happen Scheduled MAC

  21. Requests – Almost The Only Contention in 802.16 • A regular time is set-aside every 0.5 msec to 2msec for any Student (user) to raise their hand (request access) • If the Teacher (Access Point) hears the request, they are called upon (sent a Grant) • If their request collides (no grant) they request again in a following slot • Users select a random position within the available request times, and the system can allocate enough request time to insure it’s never overloaded

  22. Students may ENTER or EXIT at any time Entering students must make their presence known Since students may exit without notice, all present students must periodically confirm their presence Registration – The Other Contention in 802.16 enter exit

  23. Framing • The channel is divided into repeated Uplink/Downlink time-frames by the Access Point • Frame rate is system defined to be 0.5msec, 1msec, or 2msec in length FDD Format DL DL DL DL DL DL TDD Format DL UL DL UL DL UL UL UL UL UL UL UL Single channel Separate UL & DL channels • Both TDD (single channel) and FDD (separate UL & DL frequencies) are standardized

  24. When / Where is the Contention? Initial Maintenance Opportunities Request Contention Opportunities • Contention is allocated pre-defined subframes within any/every UL Frame • Units randomly pick a sub-slot within an Initial Maintenance Opportunity slot to register • Units randomly pick a sub-slot within a Request Contention Opportunity to request service Scheduled Data slots IMO RCO SD SD SD Each Uplink Frame DL UL DL UL DL UL 802.16 Channel Frames

  25. Finding the Contention Opportunities Broadcast Control • All units must locate / lock onto the Preamble that starts every DL frame • The very next frame is Broadcast Control containing DL & UP maps • The UL map identifies the makeup of the next uplink frame DL MAP UL MAP Time Division Multiplex slots Pre BC TDM TDM TDM Each Downlink Frame DL UL DL UL DL UL 802.16 Channel Frames

  26. Registering • When a unit enters a cell, the unit locates the next opportunity from the preceeding frame UL-MAP • The unit can then transmit a registration on a randomly selected sub-slot in the next UL frame • The unit monitors for an Ack in a following DL frame Rx UL-MAP Rx Ack DL UL DL UL DL UL Tx Registration

  27. Sending Data • When there is inbound data to send, the unit locates the next opportunity from the preceeding frame UL-MAP • The unit can then transmit a request on a randomly selected sub-slot in the next UL frame • The unit monitors for a grant in a following DL frame, and using a new UL-MAP identifies an assigned scheduled data slot • The unit then monitors for an Ack in a following DL frame Rx UL-MAP and Grant Rx UL-MAP Rx Ack DL UL DL UL DL UL Tx Request Tx Data

  28. Bounding Contention • IEEE 802.16 limits contention to registration and service requests • This is a limited portion of UL Frames • Only the shortest control messages contend • Registrations & requests basically contain a unit identity and only a single argument • Everything else on the channel is scheduled • Scheduled means Contention Free

  29. Scheduling vs Capacity • Because contention is not permitted when transmitting payload, payload collision does not exist (cannot happen) • Payload capacity is guaranteed regardless of user load • Capacity trade-off for scheduling is bound by the amount of channel time allocated to request / grant contention

  30. Summary • 802.11 is a LAN, that is often used as a MAN • 802.11 uses contention with Carrier Sense and Collision Avoidance algorithms • Neither CS nor CA work properly as a MAN • When overloaded, 802.11 ceases to function when used as a MAN (capacity approaches zero) • A new standard has been developed for MAN applications (802.16) • 802.16 eliminates the basic shortcoming of 802.11 used as a MAN by eliminating virtually all contention

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