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Ece453 – Introduction to Computer Networks Lecture 7 – Multiple Access Control (I)
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tarix | 11.07.2018 | ölçüsü | 0,77 Mb. | | #55451 |
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Lecture 7 – Multiple Access Control (I)
Broadcast vs. PPP Broadcast channel = multiaccess channel = random access channel Broadcast PPP - WAN (router-router lease line)
- Home user ISP
Delay Packet experiences delay from end to end Nodal delay = processing delay + queuing delay + transmission delay + propagation delay
The Channel Allocation Problem Static Channel Allocation in LANs and MANs - FDM or TDM
- Problems
- Fewer than N users
- A valuable chunk of time (TDM) or bandwidth (FDM) is wasted
- More than N users
- Some users are denied (even if another user is idle)
- Exactly N users
Dynamic Channel Allocation in LANs and MANs
Five Assumptions for Dynamic Channel Allocation Station Model - N independent stations generating frames
- Once a frame is generated, the station is blocked until the frame has been transmitted
Single Channel Assumption Collision Assumption - If two frames are transmitted simultaneously, they overlap in time and the resulting signal is garbled. This event is called a collision.
- All stations can detect collisions.
Time Assumption - Continuous time
- Slotted time
Carrier assumption - Carrier Sense (LAN)
- No Carrier Sense (Satellite)
ALOHA Carrier Sense Multiple Access Protocols Collision-Free Protocols Limited-Contention Protocols
Some Terminologies Contention systems - Systems in which multiple users share a common channel in a way that can lead to conflicts
Throughput - The maximum continuous traffic rate that a device can handle without dropping a single packet.
- Measured in terms of the number of frames per second at a given frame size
Frametime - Time used to transmit a frame (frame_size/data rate)
ALOHA Pure ALOHA - 1970, Norman Abramson, U. of Hawaii
- Users transmit whenever they have data
- When collision occurs, wait for a random amount of time and send again
Slotted ALOHA - 1972, Roberts
- Divide time into discrete intervals
- A computer is not permitted to send until the beginning of the next slot
- Doubles the maximum throughput
*ALOHA – Performance Analysis Poisson process (1837) - Simplest model for arrivals into a queue system
- the probability of an arrival in a small interval of time depends only on the size of the interval
- The probability that k frames are generated during a given frame time follows the Poisson distribution
CSMA – Carrier Sense Multiple Access (LAN) 1-persistent - When idle, transmit a frame
- When busy, continuously sensing
- When collision, wait for a random amount of time
Non-persistent - When idle, transmit a frame
- When busy, wait for a random amount of time
- When collision, wait for a random amount of time then re-sense
p-persistent - Slotted channels
- When idle, transmit with a probability p, defers until the next slot with probability 1-p
CSMA with collision detection (CSMA/CD)
A Worst Case Scenario
Collision-Free Protocols Assumption - N stations with a unique address from 0 to N-1
- Propagation delay is negligible
The bit-map protocol - A reservation protocol: the desire to transmit is broadcast before the actual transmission
Binary countdown
Limited-Contention Protocols ALOHA Collision-free protocols - Low load: high delay
- High load: improved efficiency
New protocol? – limited contention protocol - Uses contention at low load
- Uses collision-free at high load
- Adaptive tree walk protocol
Wireless LAN Protocols (a) The hidden station problem (C B) (b) the exposed station problem (C D) Activity around the receiver vs. activity around the sender
Wireless LAN Protocols MACA – Multiple Access with Collision Avoidance (Karn, 1990)
Multiple Access Protocols ALOHA Carrier Sense Multiple Access Protocols (CDMA) - 1-persistent
- Non-persistent
- p-persistent
- CDMA/CD (Ethernet)
Collision-Free Protocols - Bitmap protocol
- Binary countdown
Limited-Contention Protocols Wireless LAN Protocols
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