Chapter 3 Multiple Access Protocols and Layer 2 Networks Flashcards
(67 cards)
FDMA (Frequency Division Multiple Access)
Gives each user a seperate frequency band
TDMA
gives each user separate slot assignment on a time slotted machine.
What are multiple access protocols used for?
To dynamically allocate shared communication channel among multiple competing users
How dynamic allocation different from FDMA/TDMA
Dynamic allocation adjusts to traffic in real TIME
What factors affect protocol design for multiple access
Prop Delay (short v long)
Wether Nodes can hear another
Ability to detect collisions
Three Main Categories of Multiple access protocols
- Contention-based (ALOHA, CSMA)
- Collision-free (Polling, token passing)
- Limited Contention (Hybrid protocols) low delay under light load
Pure ALOHA
user nodes shared same upstream radio frequency band
A protocol where users transmit whenever they want and retransmit after random delays if no ACK is recieved.
Used for IoT Applications
Pure ALOHA vulnerable period?
Vulnerable period for Frame Transmission is “2t”
Slotted ALOHA
Improvement upon ALOHA by requiring each user to transmit a frame at the beginning of a time slot. Reducing vunerablility period to t.
2 frame transmissions with either overlap entirely or not at all. Used in satellite channels, Cable, Cellular
CSMA (Carrier sense multiple access) protocol
“sense before send”, nodes can hear other nodes transmissions. Useful when propagation delay is small
nonpersitent CSMA
Listen to channel and transmit only if the channel is idle. If unsucessful send after random delay.
Vulnerable period = 2T
1-Persistent CSMA
Where channel is sensed busy; itll wait until the channel becomes idle then transmits. Doesn’t wait keeps sensing, Ethernet
Lower delay, but more collisions if load increases.
1-Persistent CSMA with Collision Detection (CSMA/CD)
Works best when able to listen while transmitting and abort is a collision is detected. Very good for small T/t but poorly when it gets too big.
used in Classic Ethernet
p-persistent CSMA
After sensing an idle channel, transmit with probability p or wait with 1-p using minislots.
Collision-free protocols
They are highly efficent under heavy load since no time is wasted on collisions. Utilize some form of subchannel. Cost of nodes having to incur delayes before transmission
Three collision free protocols
Polling - central controller
Token Bus
Token Ring
Limited Contention Protocols
Achieve both low delay under the load of the contention-based protoclols
- Capatanakis Adaptive Tree Walk
Capetanakis Tree Walk Protocol
Nodes “walk” a binary tree corresponding to the time interval in which any node and transmit. If a collision occurs the nodes are divided into two groups. Try to deperate transmitters. Traversal depth first.
What happens during a collision in the capetanakis protocol
The node group is divided and each subgroup is tested recusively to isolate transmitters
Two Enchancements to Capetanakis protocol
Skip Predictable Collisions: Skip subgroups known to have multiple nodes
Start at lower tree levels: Where load is likely high
IEEE 802.3
Working group produced physical layer and link later MAC standards for the eternet. Dominant wired LAN tech
Classic Ethernet
- Single cable to whcih all nodes attached.
- 1-Persistent CSMA/CD
- No ACK, but do retransmit if collision during transmission detected
What is binary exponential backoff?
An algorithm used to determine the duration of a random delay until attemp transmission again after a collison. used for 802.11 wifi.
After i-th colltiions node waits a chosen random time
Why is classic Ethernet not used anymore?
Breaks down at larger r/t ratios; point-to-point links replaced it
