Flashcards
Channel Access
Cards in this deck (34)
- 1What does CSMA stand for?Carrier Sense Multiple Access
- 2What is the purpose of CSMA?To further minimise the chance of collisions occurring
- 3How does CSMA prevent collisions?Checks state of medium before transmitting
- 4CSMA: What are persistence methods?Persistence methods control what the station should when the channel is busy or idle
- 5What are the 3 persistence methods defined within CSMA?- 1-persistent
- Non-persistent
- p-persistent - 6Persistence Methods: How does 1-persistent work?- Transmitting station continuously senses the channel
- Once channel is idle, frame is sent immediately - 7Persistence Methods: How does non-persistent work?- Trans. station senses channel
- If busy, wait a random amount of time then check again
- If idle, send immediately - 8Persistence Methods: How does p-persistent work?- Used if channel has time slots w/ slot duration >= maximum propagation (transmission) time
- If busy wait a random number of time slots then sense again - 9What does CSMA/CD stand for and what is its purpose?Carrier Sense Multiple Access with Collision Detection
Detects collisions and provides a strategy for retransmission if a collision is detected - 10What is IEEE 802.3 and what persistence method does it use?Ethernet (wired) network
1-persistent CSMA/CD
- Aborts transmission if collision detected - 11Why does IEEE 802.3 use this persistence method?- non-persistent and p-efficient are inefficient
- 12What does IEEE 802.3 do if a collision is detected?Backoff for a period of time before trying again (binary exponential back off)
- 13How does binary exponential backoff work?- For 1st retransmission wait a random amount of time
- Next 9 attempts double mean val. of random delay
- Next 6 attempts back off(BO) time remains the same
- If still not transmitted, give up and report errorr - 14What are the 2 main benefits associated with Binary Exponential Backoff?- Low traffic: 1-persistence guarantees stations can send immediately
- High Traffic: BEB is at least as stable as other persistence methods - 15What is IEEE 802.11 and what are its versions and changing speeds?Wireless Networks
- IEEE 802.11a: 5GHz up to 54Mbit/s
- IEEE 802.11b: 2.4GHz up to 11Mbit/s
- IEEE 802.11g: improvement of b, up to 54MBit/s
- IEEE 802.11n: 2.4/5.0GHz: up to 600MBit/s - 16What are 802.11s two network configurations?- Basic Service Set (BSS)
. Ad-hoc networks
. Infrastructure networks
- Extended Service Set (ESS)
. Collection of BBS networks connect by distribution system
. Appears as a single network - 17What is CSMA/CD not good for and why?Not good for WLAN since radio signals typical half-duplex
- 182 802.11 alternates to CSMA/CD1. Distributed Coordination Function (DCF)
2. Point Coordination Function (PCF) - 19Characteristics of Distributed Coordination Function (DCF)?- Connection-based (random access)
- CSMA/CA
- No central control - 20Characteristics of Point Coordination Function (PCF)?- Permission based (scheduled access)
- Central control over when stations may transmit - 21How does DCF relate to CSMA?DCF is an extension of CSMA which defines a method for collision avoidance
- Cannot detect errors so we must try to avoid them - 22CSMA AVOIDS collisions by using...- Inter frame Space (IFS)
- Contention Windows
- Acknowledgements (ACK) - 23What is Interframe Space?- A period of time a station waits before transmitting after channel has been found idle.
- 24How does IFS work?- Waits time, if station is idle another station may have already started a transmission
- If station is still idle after IFS, station can send after waiting a period of time (contention time)
- High priority stations have shorter IFS - 25What is a Contention Window?After IFS, the length of time a station must wait before stating transmission (number of time slots)
- Number of slots changes according to exponential back off strategy
- Senses channel after waiting each time slot - 26How do acknowledgements (ACK) work?- After frame received, station uses FCS to verify the frame
- If correct ACK transmitted back to original station
- Discard frame otherwise
Sender
- ACK received, trans. successful
- Not ACK after time out period, transmission unsuccessful - 27What does DCF stand for and what does it specify?Distributed Coordination Function
- Stations must request access to the medium before transmitting - 28How does DCF work?- Broadcast request-to-send (RTS) frame specifying length of transmission
- Wait to receive the clear-to-send (CTS) frame from target station
- Send data on receipt of CTS packet
- Unsolicited RTS or CTS added to stations Network Allocation Vector (NAV) and remains idle for requested trans. time - 29What does PCF stand for?Point Coordination Function
- 30What is PCF and what are its properties?- A turn-based collision avoidance system
- Used for time-sensitive trans.
- Centralised and contention-free(?)
- No collisions ever occur - 31How does PCF work?- Access point (AP) polls stations one after another
- Any station with data to send waits for permission from AP before transmitting - 32Outline the hidden station problem- B wants to send data to A
- C wants to send data to A
- C out of Bs range so thinks A is free
- C sends data to A which causes a collision
- Stations B and C are hidden from each other - 33How do we solve the hidden station problem?Through the use of Request-to send (RTS) and Clear-to-Send(CTS) frames
- 34Outline the Exposed Station Problem- C wants to send data to D but doesn't as B is trans. to A
- Too conservative: D out of range so B can go ahead