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Chapter four: The Medium Access Control Sublayer sections: 4.1 – 4.2.5

Chapter four: The Medium Access Control Sublayer sections: 4.1 – 4.2.5. Shayla Cooke Armani Louden Monisiah Brockman Group 3. objective.

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Chapter four: The Medium Access Control Sublayer sections: 4.1 – 4.2.5

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  1. Chapter four: The Medium Access Control Sublayersections: 4.1 – 4.2.5 Shayla Cooke Armani Louden Monisiah Brockman Group 3

  2. objective • The main theme of the chapter is how to allocate a single broadcast channel (versus point to point) among competing users. Various approaches using static and dynamic channel allocation will be examined.

  3. The channel allocation problem • Main point: bursty nature of data communications. WANs not considered because its normally connected via a point to point channel. • Frequency Division Multiplexing (FDM) : Frequency of one channel divided (usually evenly) among (n) users. Each user appears to have full channel of full frequency/n. Wastes bandwidth when user has nothing to send or receive. Other users cannot take advantage of unused bandwidth.

  4. Continue… • Time Division Multiplexing - Time of one channel divided (usually evenly) among (n) users. Each user appears to have full channel for time/n. Same problems as FDM. • Analysis of Static Channel Allocation - Static allocation is a bad idea when considering that for an (n) divisions of a channel, any one user is limited to only 1/n channel bandwidth whether other users where accessing the channel or not. By limiting a user to only a fraction of the available channel, the delay to the user is increased over that if the entire channel were available.

  5. Dynamic Channel Allocation • Obviously static allocation of a multi-access channel is not generally desirable when overall channel usage is low. Channel use is bursty with long periods of inactivity punctuated by short bursts of activity.

  6. Dynamic Channel Allocation Assumptions • Station model - n independent stations each generating frames for transmission. One frame is generated and successfully transmitted at a time. • Single channel - A single channel is shared with other stations. • Collision - Overlapping transmissions destroy the frames, can be detected, and require retransmission. Collisions are the only errors. • Time • Continuous time - No master clock dividing time, transmissions can begin at any time. • Slotted time - Master clock, time is divided into discrete intervals (slots), transmissions can only begin at the start of a slot. Slots may contain 0 frames (idle), 1 (a successful transmission), or 2 (a collision). • Carrier • Carrier sense - Stations can detect whether the channel is in use prior to transmitting. If in use waits for channel to become available. • No carrier sense - Stations cannot detect whether the channel is in use prior to transmitting. Transmits and later determines whether successful.

  7. aloha • Radio broadcast on common frequency to transmit. The figure to the right: 4 independent stations each attempting to transmit at the same time. Because of the use of a common frequency, when more than one station broadcasts, the signal is effectively destroyed and must be retransmitted.

  8. Pure aloha • The first three assumptions Station model, Single Channel, and collision are valid along with: • Time • Continuous time - Each station can broadcast at any time. • Carrier • No carrier sense - Before transmitting, a station does not check for another transmitting.

  9. Slotted aloha • The assumption of continuous time is different from Pure ALOHA: • 1. Time • Slotted time - A station can broadcast only at the beginning of a slot. 2. Carrier • No carrier sense - A station does not detect another transmitting.

  10. Important points different from pure aloha • Station transmits whenever ready at the beginning of a slot. • Contention period - The time interval in which frames can overlap (i.e. collision can occur). • Assuming fixed size frames, for Slotted ALOHA the contention period begins when slot starts. • Because of a common reference at the slot start the contention period lasts for one frame time. Two stations that begin transmission simultaneously will overlap.

  11. Carrier Sense Multiple Access Protocols (CSMA) •Definition: Protocols in which stations listen for a carrier and act accordingly •These protocols help in improving performance and achieve a much better utilization.

  12. 1-Persistent CSMA • This station transmits with a probability of 1 when it finds the channel idle • If idle, the data is sent • If busy data won’t send until station becomes idle • If a collision occurs, station waits then tries the process all over again. • Delay is small since always trying to send when channel becomes idle

  13. Nonpersistent CSMA • Similar to persistent • Difference: If the channels is busy it waits • Better channel utilization, longer delays.

  14. P-Persistent CSMA • Waits until the channel becomes idle and transmits with probability P • With probability 1-P wait a random time and listen again. • If busy process is repeated.

  15. CSMA with Collision detection (CSMA/CD) • If two or more stations decide to transmit simultaneously, there will be a collision. If a station detects a collision, it aborts its transmission, waits a random period of time, and then tries again. • Idle period occurs when all stations are quiet http://youtu.be/kpstNtIOBmQ http://youtu.be/cZKrtV2tbRA http://youtu.be/9A3jLps-G8M

  16. Collision Free Protocols • Basic bit-map method • Contention slot used to make reservation for transmitting by placing a 1 if station wants to transmit. Stations 1, 3, and 7 make reservation during contention slot; transmit in numerical order during appropriate data slot. All stations can see reservations broadcast by other stations. • Every station transmits a frame in turn in a predefined order.

  17. Collision Free Protocols • Token Bus • Token represents permission to send • Receive from one station and pass in another direction TOKEN STATION DIRECTION OF TRANSMISSION

  18. Collision Free Protocols • Binary Countdown • Reduces delay caused by contention slot period. Station wanting to send broadcasts address as bit string starting with high order bit. Addresses from all stations are ORed as arrive). Any station seeing a 1 in a position where their address is 0 drops out and must wait for next contention slot. After all address bits are broadcast the last remaining station transmits.

  19. Limited contention protocols • Collision-free technique at high load to provide good channel efficiency • Combination of the best properties of the contention and collision-free protocol is called limited-contention protocols. • Review of Systematic Protocols: • In systematic protocols each station attempts to acquire the channel with some probability with all stations using the same (p) • Overall performance can sometimes be improved by using a protocol that assigns different probabilities to different stations. • Probability of some station acquiring the channel can be increased only by decreasing the amount of competition, which is what the limited-contention protocols does.

  20. Continue…. • First divide the stations into groups • Only the members of group 0 are permitted to compete for slot 0 . If one of them succeeds, it acquires the channel and transmits its frame.

  21. Wireless lan protocols • A system of laptop computers that communicate by radio can be regarded as a wireless LAN • This type of LAN is an example of a broadcast channel. • There are somewhat a different properties than a wired LAN which leads to different MAC protocols.

  22. LAN continued…. • An office building with access points are placed around the building is a common configuration for a wireless LAN. • If the transmission power of the Aps and laptops is adjusted to have a range of tens of meters • Hereby rooms become like a single cell • Entire building becomes like the cellular telephony systems.

  23. Differences between wireless LANs and wire LANs • A station on a wireless LAN may not be able to transmit frames to or receive frames from all other stations. • In wired LANs, when one station sends a frame, all other stations receive it.

  24. Protocol is not really a good way to think about wireless because what matters for reception is interference at the receiver, not the sender. • A naïve approach to using a wireless LAN might be to try CSMA

  25. Because of these problems we want a MAC protocol that will prevent this kind of collision from happening because it wastes bandwidth. • The problem of a station not being able to detect a potential computer for the medium because the competitor is too far away is called Hidden Terminal Problem

  26. The exposed terminal problem • The Difficulties: • Before starting a transmission, a station really wants to know whether there is radio activity around the receiver. • CSMA merely tells it whether there is activity near the transmitter by sensing the carrier. • With wire all signals propagate to all stations, so this distinction does not exist.

  27. MACA • Multiple Access with Collision Avoidance is an early and influential protocol that tackles the problems for wireless LANs • The basic idea is for the sender to stimulate the receiver into outputting a short frame, so stations nearby can detect this transmission and avoid transmitting for the duration of the upcoming data frame.

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